Mitochrondrial Gateways to Cancer

5/22/2018 Mitochrondrial Gateways to Cancer

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Review

Mitochondrial gateways to cancer

Lorenzo Galluzzi a,b,c,1, Eugenia Morselli a,b,c,1, Oliver Kepp a,b,c,1, Ilio Vitale a,b,c, Alice Rigoni a,b,c,Erika Vacchelli b,c,d, Mickael Michaud a,b,c, Hans Zischka e, Maria Castedo a,b,c, Guido Kroemer a,b,c,*

a INSERM, U848, Institut Gustave Roussy, PR1, 39, rue Camille Desmoulins, F-94805 Villejuif, Franceb Institut Gustave Roussy, F-94805 Villejuif, Francec Universit Paris-Sud XI, F-94805 Villejuif, Franced INSERM, U805, F-94805 Villejuif, Francee Institute of Toxicology, Helmholtz Center Munich, German Research Center for Environmental Health, D-85764 Oberschleissheim, Germany

a r t i c l e i n f o

Article history:

Received 26 May 2009

Received in revised form 12 August 2009

Accepted 13 August 2009

Keywords:

Bcl-2

Caspases

Mitochondrial transmembrane potential

Oncoproteins

p53Tumor suppressors

a b s t r a c t

Mitochondria are required for cellular survival, yet can also orchestrate cell death. The

peculiar biochemical properties of these organelles, which are intimately linked to their

compartmentalized ultrastructure, provide an optimal microenvironment for multiple bio-

synthetic and bioenergetic pathways. Most intracellular ATP is generated by mitochondrial

respiration, which also represents the most relevant source of intracellular reactive oxygen

species. Mitochondria participate in a plethora of anabolic pathways, including cholesterol,

cardiolipin, heme and nucleotide biosynthesis. Moreover, mitochondria integrate numer-

ous pro-survival and pro-death signals, thereby exerting a decisive control over several

biochemical cascades leading to cell death, in particular the intrinsic pathway of apoptosis.

Therefore, it is not surprising that cancer cells often manifest the deregulation of one or

several mitochondrial functions. The six classical hallmarks of cancer (i.e., limitless replica-tion, self-provision of proliferative stimuli, insensitivity to antiproliferative signals, dis-

abled apoptosis, sustained angiogenesis, invasiveness/metastatic potential), as well as

other common features of tumors (i.e., avoidance of the immune response, enhanced ana-

bolic metabolism, disabled autophagy) may directly or indirectly implicate deregulated

mitochondria. In this review, we discuss several mechanisms by which mitochondria can

contribute to malignant transformation and tumor progression.

2009 Elsevier Ltd. All rights reserved.

0098-2997/$ - see front matter 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.mam.2009.08.002

Abbreviations: ACL, ATP citrate lyase; AIF, apoptosis inducing factor; AML, acute myeloid leukemia; ANT, adenine nucleotide translocase; BH, Bcl-2

homology domain; CRT, calreticulin; CypD, cyclophilin D; Cytc, cytochromec;Dwm, mitochondrial transmembrane potential; DCA, dichloroacetate; ECM,extracellular matrix; EMT, epithelialmesenchymal transition; EndoG, endonuclease G; ER, endoplasmic reticulum; FASN, fatty acid synthase; FH, fumarate

hydratase; HIF-1, hypoxia-inducible factor 1; HK, hexokinase; HSP, heat-shock protein; IjBa, inhibitor ofjB a subunit; IjKb, IjB kinase b subunit; IM,mitochondrial inner membrane; IMS, mitochondrial intermembrane space; IP3R, inositol 1,4,5-trisphosphate receptor; MAPK, mitogen-activated protein

kinase; MEFs, mouse embryonic fibroblasts; MMP, mitochondrial membrane permeabilization; MnSOD, manganese superoxide dismutase; MOMP,

mitochondrial outer membrane permeabilization; MPT, mitochondrial permeability transition; mtCK, mitochondrial creatine kinase; OM, mitochondrial

outer membrane; PBR, peripheral benzodiazepine receptor; PDGF, platelet-derived growth factor; PDH, pyruvate dehydrogenase; PDK1, PDH kinase 1; PET,

positron emission tomography; PFK2, 6-phosphofructo-2-kinase; PI3K, phosphatidylinositol-3 kinase; PINK1, PTEN induced putative kinase 1; PKM2,

pyruvate kinase M2 isoform; PTPC, permeability transition pore complex; RNAi, RNA interference; ROS, reactive oxygen species; SDH, succinate

dehydrogenase; siRNA, small interfering RNA; Stat, signal transducer and activator of transcription; TCA, tricarboxylic acid; TGF b, transforming growth

factor b; TSPO, translocator protein of 18 kDa; VDAC, voltage-dependent anion channel.

* Corresponding author. Address: INSERM, U848, Institut Gustave Roussy, PR1, 39, rue Camille Desmoulins, F-94805 Villejuif, France. Tel.: +33 1 4211

6046; fax: +33 1 4211 6047.

E-mail address: [email protected](G. Kroemer).1 These authors equally contributed to the article.

Molecular Aspects of Medicine 31 (2010) 120

Contents lists available at ScienceDirect

Molecular Aspects of Medicine

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c at e / m a m
http://dx.doi.org/10.1016/j.mam.2009.08.002mailto:[email protected]://www.sciencedirect.com/science/journal/00982997http://www.elsevier.com/locate/mamhttp://www.elsevier.com/locate/mamhttp://www.sciencedirect.com/science/journal/00982997mailto:[email protected]://dx.doi.org/10.1016/j.mam.2009.08.002


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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1. Mitochondria in cell life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2. Mitochondria in cell death . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. General implication of mitochondria in tumorigenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Limitless proliferative potential and mitochondria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.1. Immortalization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2. Self-sufficient growth signaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.3. Insensitivity to antiproliferative signals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.4. Disabled cell death . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4. Angiogenesis, invasiveness, metastatic potential and mitochondria. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

5. Avoidance of the immune response and mitochondria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

6. Metabolic reprogramming and mitochondria. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

7. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1. Introduction

Mitochondria are indispensable for the survival of higher eukaryotic cells, including cancer cells. At the same time, theseorganelles hide the weapons of cellular suicide, meaning that they control cell death. The scope of this introduction is tobriefly review the quintessential vital and lethal functions of mitochondria.

1.1. Mitochondria in cell life

The essential participation of mitochondria in cellular functions has been highlighted more than one century ago. Orig-inally described as bioplasts (Altmann, 1890), these organelles were soon renamed due to their threadlike appearance dur-ing spermatogenesis (mitos = thread, chondros = granule) (Benda, 1898). The first attempt to isolate mitochondria waslaunched in the thirties (Ernster and Schatz, 1981), approximately at the same time as Warburg described the fundamentalsof cancer cell metabolism (Warburg et al., 1930). Around and after Wold War II, the basic biochemical reactions underlyingcellular respiration were revealed, culminating with Mitchells chemiosmosis hypothesis on the conservation of energy inthe form of a proton gradient across the mitochondrial inner membrane (IM) ( Mitchell, 1961; Ernster and Schatz, 1981).

The most important biochemical cascade confined to mitochondria is oxidative phosphorylation, a coordinated series ofredox reactions catalyzed by five multi-subunit enzymatic activities embedded in the IM (i.e., respiratory complexes IIV andthe F1FO-ATP synthase) and two soluble factors (i.e., cytochromec, Cytc; coenzyme Q10), that function as electron shuttleswithin the mitochondrial intermembrane space (IMS). Oxidative phosphorylation results in the generation of an electro-chemical gradient across the IM that is dissipated in a controlled fashion by the F1FO-ATP synthase to generate the bulkof intracellular ATP stores (Mitchell, 1961; Reichert and Neupert, 2004). In a healthy eukaryotic cell, around 2.000 mitochon-dria produce more than 90% of intracellular ATP (Voet and Voet, 1995; Pedersen, 2007). Additional metabolic pathways thatare located within mitochondria include the Krebs cycle, heme biosynthesis, b-oxidation of fatty acids, steroidogenesis, themetabolism of certain amino acids as well as the formation of Fe/S clusters (Reichert and Neupert, 2004). Moreover, mito-chondria host the initial steps of ammonium detoxification (by the urea cycle, also known as KrebsHenseleit cycle) as wellas reactions belonging to gluconeogenesis and ketogenesis (Michal, 1999)(Fig. 1).

Beside their role as ubiquitous power plants, mitochondria serve in a highly specialized fashion the needs of their host

tissue cells, which results in significant tissue-dependent variations of their activity (including Ca

2+

homeostasis) (Campa-nella et al., 2004; Rizzuto and Pozzan, 2006) and molecular composition (Mootha et al., 2003). A comparative proteome anal-ysis of mitochondria obtained from different murine organs (i.e., liver, brain, heart and kidney) revealed that only half ofbona

fidemitochondrial proteins are omnipresent, while the other half have 50% probability to occur in a given tissue ( Moothaet al., 2003). This implies that current estimates of the total number of mitochondrial proteins might have to be revised. Cur-rently, the MITOP2 database for human mitochondria lists a reference set that includes around 900 proteins with a verifiedmitochondrial localization. Importantly, depletion and/or malfunction of almost 200 of these proteins have been linked tohuman disease (http://www.mitop.de:8080/mitop2/). We surmise that this is just the tip of the iceberg and that system biol-ogy approaches will unravel the existence of additional mitochondrial components playing important roles in health anddisease.

1.2. Mitochondria in cell death

Mitochondrial membrane permeabilization (MMP) is widely considered as the point of no return in the cascade of eventsleading to cell death via intrinsic apoptosis (Kroemer et al., 2007), as well as through non-apoptotic cell death subroutines

2 L. Galluzzi et al./ Molecular Aspects of Medicine 31 (2010) 120
http://www.mitop.de:8080/mitop2/http://www.mitop.de:8080/mitop2/


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(Galluzzi et al., 2007, 2009; Kroemer et al., 2009). Under conditions of stress, lethal and vital signals are opposing each otherat the level of mitochondrial membranes. If pro-death stimuli predominate, MMP takes place and leads to a series of cata-strophic consequences that eventually seal the cells fate. These include (but are not limited to) the dissipation of the mito-chondrial transmembrane potential (Dwm), which immediately results in the arrest of mitochondrial ATP synthesis and ofseveral other biosynthetic pathways, as well as the mitochondrio-cytosolic translocation of numerous proteins that are nor-mally confined within the IMS, where they exert vital functions (Kroemer et al., 2007; Galluzzi et al., 2008a). These factorscan be subdivided into three broad categories: direct caspase activators (e.g., Cytc), indirect caspase activators (e.g., Smac/Diablo; Omi/HtrA2) and caspase-independent cell death effectors (e.g., apoptosis inducing factor, AIF; endonuclease G,

Fig. 1. Mitochondria in cell life.Through oxidative phosphorylation, mitochondria produce the bulk of intracellular ATP, and hence are considered the cells

power plants. In addition, mitochondria regulate Ca

2+

homeostasis and host (at least some steps of) several other metabolic circuitries including (but notlimited to) the Krebs cycle, the urea cycle, gluconeogenesis, ketogenesis, heme biosynthesis, fatty acid b-oxidation, steroidogenesis, the metabolism of

certain amino acids and the formation of Fe/S clusters. ER, endoplasmic reticulum; PM, plasma membrane.

Fig. 2. Mitochondria in cell death.Upon activation by BH3-only proteins, pro-apoptotic multidomain members of the Bcl-2 family (e.g., Bax, Bak) fully insert

into the mitochondrial outer membrane, oligomerize and form protein-permeable channels, thereby inducing mitochondrial outer membrane

permeabilization (MOMP, upper left panel). In other circumstances (e.g., under oxidative stress), mitochondrial permeability transition (MPT, lower left

panel) is initiated at the inner mitochondrial membrane, due to the opening of the supramolecular protein complex known as permeability transition pore

complex (PTPC). Both MOMP and MPT eventually lead to the cytosolic spillage of proteins that normally reside within the mitochondrial intermembrane

space. This ignites caspase-dependent and -independent biochemical cascades that execute cell death (right panel). Please refer to the main text for furtherdetails. AIF, apoptosis inducing factor; Cyt c, cytochromec; Dwm, mitochondrial transmembrane potential; EndoG, endonuclease G.

L. Galluzzi et al./ Molecular Aspects of Medicine 31 (2010) 120 3


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EndoG). Once in the cytosol, IMS proteins activate caspase-dependent and -independent mechanisms that altogether medi-ate the execution of cell death (Kroemer et al., 2007)(Fig. 2).

MMP can result from two distinct, yet partially overlapping mechanisms (Kroemer et al., 2007). Upon activation, multi-domain pro-apoptotic members of the Bcl-2 protein family may form protein-permeable pores in the mitochondrial outermembrane (OM), a process known as mitochondrial outer membrane permeabilization (MOMP). MOMP results in the releaseof IMS proteins into the cytosol, among which soluble components of the respiratory chain, whose depletion (eventually)leads to respiratory chain uncoupling and Dwmloss (Fig. 2). Anti-apoptotic proteins from the Bcl-2 family (e.g., Bcl-2, Bcl-

XL) also (but not only) act by sequestering their pro-apoptotic counterparts into inactive complexes, which can be disruptedby pro-apoptotic BH3-only proteins (Willis and Adams, 2005).Alternatively, MMP can be ignited at the mitochondrial inner membrane (IM), due to the opening of a supramolecular

entity that is assembled at the junctions between the OM and the IM and that is known as the permeability transition porecomplex (PTPC). The PTPC normally ensures the exchange of metabolites between the cytosol and the mitochondrial matrix,but can also mediate the so-called mitochondrial permeability transition (MPT). This process consists in an abrupt loss ofthe impermeability of the IM to solutes, immediately followed by Dwmdissipation and osmotic swelling of the mitochon-drial matrix, which eventually leads to OM breakdown and spillage of IMS proteins into the cytosol (Fig. 2). The PTPC hasbeen described as a very dynamic structure, and its precise molecular composition is still debated. However, some consensusexists on the components that would constitute its backbone, including the voltage-dependent anion channel (VDAC) in theOM, the adenine nucleotide translocase (ANT) in the IM, and cyclophilin D (CypD) in the mitochondrial matrix. Importantly,both pro- and anti-apoptotic members of the Bcl-2 protein family have been shown to interact with the PTPC (Marzo et al.,1998a,b), suggesting that MOMP- and MPT-driven MMP are not completely independent from each other.

2. General implication of mitochondria in tumorigenesis

As mitochondria play a critical role in numerous bioenergetic, anabolic and cell death-inducing biochemical pathways, itis not surprising that mitochondrial dysfunction contributes to the development of a plethora of human diseases, whichrange from highly tissue-specific conditions to generalized whole-body disorders including cancer (Taylor and Turnbull,2005). Several common features of established tumor cells can directly or indirectly result from mitochondrial deregulation(see below). Moreover, mitochondria may be implicated in early tumorigenesis, as cancer progenitor cells appear, replicateand progressively acquire a malignant phenotype.

A number of conditions can lead to the intramitochondrial overgeneration of reactive oxygen species (ROS), which favoroxidative damage-dependent mutagenesis and hence promote tumorigenesis (Zhou et al., 2007). Mutations of the nuclear ormitochondrial DNA (mtDNA) may affect components of the respiratory chain (e.g., cytochrome b) and favor uncoupling,which results in increased electron leakage and ROS overproduction (Dasgupta et al., 2008). Such mutations are found in

a wide variety of cancer types (Modica-Napolitano and Singh, 2004). Old or damaged mitochondria produce high levels ofROS, but are normally degraded through mitophagy, a specialized branch of macroautophagy that selectively targets theseorganelles to lysosome-mediated disposal (Tolkovsky, 2009). As the molecular machineries for mitophagy and macroauto-phagy partially (but not completely) overlap (for a review see (Klionsky et al., 2007) a n d(Xie and Klionsky, 2007)), a reducedmitophagic flow may derive either from defects in general autophagic modulators (e.g., Beclin 1) (Yue et al., 2003) or frommitophagy-specific deficits (Schweers et al., 2007). Irrespective of the underlying mechanisms, insufficient clearance of old/damaged mitochondria may favor malignant transformation (Morselli et al., 2009).

It has been postulated that a mitochondrial damage checkpoint (mitocheckpoint) would be turned on by mitochondrialderangement, thereby preventing cell cycle progression until the restoration of mitochondrial functions ( Singh, 2006). Inconditions of excessive mitochondrial injury, the mitocheckpoint would activate senescence, thereby acting as a bone fidetumor suppressor mechanism (Singh, 2006). Although the precise molecular mechanisms remain unidentified, it has beensuggested that mitocheckpoint-triggered senescence would mimic the pathway of senescence triggered by telomere attri-tion (Campisi, 2005; Singh et al., 2005). This implies that mitochondrial dysfunction would favor tumorigenesis only when

the senescence program has been inactivated.In a milestone article published in 2000, Hanahan and Weinberg enumerated six hallmarks that characterize most human

cancers: limitless proliferative potential, self-sufficient growth signaling, insensitivity to antiproliferative signals, disabledapoptosis, sustained angiogenesis and invasiveness/metastatic potential (Hanahan and Weinberg, 2000). More recently,other common features of tumor cells have been proposed, including enhanced anabolism (Kroemer and Pouyssegur,2008), avoidance of the immunosurveillance (Zitvogel et al., 2008), and suppressed autophagy (Morselli et al., 2009). As dis-cusses in the following sections, several among these characteristics may be directly/indirectly linked to mitochondria.

3. Limitless proliferative potential and mitochondria

Tumor cells of distinct origin share the ability to proliferate in an uncontrollable fashion, both in vitro andin vivo. Thisproliferative potential results from multiple molecular defects. First, in contrast to normal cells, tumor cells are immortal-

ized, meaning that they can replicate in a limitless fashion without undergoing senescence. Frequently, this involves theacquisition of constitutive telomerase expression, which prevents the erosion of telomeres that normally occurs with aging



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(Dong et al., 2005). Second, cancer cell proliferation often does not depend on extrinsic signals. Gain-of-function mutationsthat affect growth factor-activated signaling cascades and/or the non-physiological autocrine/paracrine production ofgrowth factors constitute two of the possible mechanisms by which tumor cells emancipate from the proliferative controlnormally exerted by the local microenvironment (Gschwind et al., 2004). Finally, as compared to their normal counterparts,tumor cells are resistant to antiproliferative signals, as well as to multiple stress conditions that usually trigger cell death(Igney and Krammer, 2002; Gatenby and Gillies, 2004). Loss-of-function mutations in genes that encode cell cycle arresting(e.g., p21Cip1) and pro-apoptotic (e.g., Bax, caspases) proteins, as well as the genetic/epigenetic upregulation of cell cycle pro-

moting (e.g., cyclin D1) and anti-apoptotic (e.g., Bcl-2) modulators are found in a relevant fraction of human cancers ( Bauret al., 1999; Nakamura et al., 2001; Sakuragi et al., 2002; Rassidakis et al., 2003; Soung et al., 2005; Tashiro et al., 2007 ).

3.1. Immortalization

hTERT, which is overexpressed in most tumor cells, is the catalytic subunit of telomerase and is required for the mainte-nance of telomere length (Masutomi et al., 2003). Therefore, hTERT is commonly believed to act in the nucleus. However, dueto a bona fide mitochondrial targeting sequence, hTERT is also found in mitochondria, where it maintains its catalytic activity(Santos et al., 2004). Early studies based on the overexpression of catalytically-proficient hTERT variants selectively targetedto mitochondria or the nucleus suggested that mitochondrial hTERT would enhance oxidative stress-mediated mtDNA dam-age and cell death, whereas nuclear hTERT would indirectly exert cytoprotective effects (Santos et al., 2006). These observa-tions are in contrast with recent results demonstrating that endogenous hTERT translocates to mitochondria in conditions ofoxidative stress, thereby binding mtDNA, increasing respiratory chain activity and preserving Dwm (Ahmed et al., 2008;

Haendeler et al., 2009). Moreover, the unselective depletion of hTERT via RNA interference (RNAi) has been shown to sen-sitize cervical and colon carcinoma cells to multiple cell death triggers (including oxidative stress) ( Massard et al., 2006; Xiet al., 2006). As this chemosensitizing effect was observed within 48 h after transfection of hTERT-specific small interferingRNAs (siRNAs), it cannot be attributed to the attrition of telomeres (Massard et al., 2006). Finally, there are many examples inwhich the overexpression of wild-type hTERT resulted in increased resistance to mitochondrial apoptosis (Zhang et al.,2003b; Xi et al., 2006; Ahmed et al., 2008). Altogether, these observations suggest that hTERT protects mitochondria againstMMP, thereby inhibiting the intrinsic pathway of apoptosis. The precise mechanisms accounting for the MMP-inhibitoryfunction of hTERT have not yet been elucidated.

Expression-profiling of primary breast tumor cultures pre- and post-hTERT transduction, and spontaneously immortal-ized breast cancer cell lines has been employed to identify an ensemble of molecular features characteristic of tumor cellimmortalization (Dairkee et al., 2007). Such an immortalization signature was characterized by the overexpression of oxi-doreductase genes that affected multiple mitochondrial parameters (including Dwmand ROS production), and could be re-versed by hTERT silencing. In clinical breast cancer samples, the expression of components of the immortalization signature

was inversely correlated with patient survival. These results suggest that hTERT activates transcriptional programs that favorcancer-associated mitochondrial alterations (Dairkee et al., 2007).Mortalin (which is also known as mtHsp70, Hsp75, GRP75, and PBP74) is a constitutively expressed protein from the

heat-shock protein (HSP) family that was first remarked for its differential localization in mortal vs.immortal cells (Wadhwaet al., 1993). Thus, while mortal mouse embryonic fibroblasts (MEFs) contained mortalin in a cytosolic localization, theirimmortalized counterparts presented mortalin in perinuclear clusters (Wadhwa et al., 1993). Mortalin is mainly found inmitochondria but also in extra-mitochondrial compartments including the endoplasmic reticulum (ER) (Ran et al., 2000).Depending on its subcellular distribution and on its binding partners, numerous functions have been ascribed to mortalin.For instance, cytoplasmic mortalin is able to sequester the tumor suppressor protein p53, thereby inhibiting both its nuclear(cell cycle arresting and/or pro-apoptotic) and extranuclear (pro-apoptotic) functions (Wadhwa et al., 2002). Moreover,mortalin has been shown to indirectly affect the Ras-Raf signaling pathway (Wadhwa et al., 2003), which exerts a major con-trol on cell proliferation and is often deregulated in human tumors (Schubbert et al., 2007), and to be phosphorylated by thePTEN induced putative kinase 1 (PINK1), thereby mediating cytoprotective effects (Pridgeon et al., 2007). Finally, mortalin

reportedly antagonizes oxidative stress-induced cell death in a number of distinct experimental settings (Hua et al.,2007; Voloboueva et al., 2008). In line with these observations, mortalin overexpression has been shown to significantly en-hance the tumorigenic potential of immortalized human embryonic fibroblasts,in vivo (Wadhwa et al., 2006), and has alsobeen identified as an independent negative prognostic variable in colorectal cancer patients (Dundas et al., 2005). In spite ofthe multifaceted implication of mortalin in tumorigenesis and tumor progression, it remains unclear whether this mitochon-drial HSP also exertsbona fideimmortalizing effects.

3.2. Self-sufficient growth signaling

GRIM-19 is a mitochondrial protein originally identified by virtue of its interaction with viral factors that modulate inter-feron b- and retinoic acid-induced cell death (Seo et al., 2002). Subsequent yeast two-hybrid screenings suggested thatGRIM-19 can interact with signal transducer and activator of transcription 3 (Stat3), a latent cytoplasmic transcription factorthat is activated by several cytokines and growth signals (Lufei et al., 2003). Upon interaction with GRIM-19, Stat3 clustered

in perinuclear aggregates but could not translocate to the nucleus, thereby losing its function as a transcription factor ( Lufeiet al., 2003). GRIM-19 has been shown to inhibit Stat3- (but not Stat1-) driven transcription (Zhang et al., 2003a), and to



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suppress the growth of Src-transformed and Stat3-overexpressing murine fibroblasts (Lufei et al., 2003). Stat3 promotes cellsurvival by transactivating a large array of anti-apoptotic genes, and is constitutively activated in a variety of pre-malignantand malignant lesions (Buettner et al., 2002). The discovery of somatic and germline mutations that affect the GRIM-19-encoding gene (ndufa13) in different types of human cancers has formally assigned GRIM-19 to the group of tumor suppres-sors (Fusco et al., 2005; Maximo et al., 2005). Of note, GRIM-19 acts as a tumor suppressor not only by inhibiting Stat3, butalso by modulating other mitochondrial and extramitochondrial processes that are involved in tumorigenesis (see below).Finally, as demonstrated by the generation ofndufa13/ mice, GRIM-19 is essential for early embryonic development

due to its crucial role in the assembly and function of the respiratory chain complex I (Huang et al., 2004; Lu and Cao,2008). Thus, GRIM-19 provides an interesting example of how mitochondria can be involved in self-sufficient growth signal-ing (as well as in other features, see below) of cancer cells.

Constitutive NF-jB activation is common to different types of hematopoietic and solid malignancies. Depending on thecell type and NF-jB-activating stimulus, members of the NF-jB family can orchestrate transcriptional programs affectingprocesses as diverse as proliferation, differentiation, inflammation, angiogenesis and cell death (Van Waes, 2007). Althoughthe NF-jB system is most frequently implicated in tumorigenesis by virtue of its anti-apoptotic effects (see below), severalstudies indicate that NF-jB may also promote the emancipation of cancer cells from extrinsic growth signals. The promoterregion of multiple genes that control cell cycle progression (e.g.,ccnb1,ccnd1,ccne1, coding for cyclin B1, D1 and E1, respec-tively) contain NF-jB-responsive elements. Moreover, NF-jB has been shown to mediate the proliferation of glioblastomacells triggered by platelet-derived growth factor (PDGF) (Smith et al., 2008), thereby actingde factoas an effector of growthsignaling cascades. The NF-jB system and mitochondria are interconnected at multiple levels. To mention a few examples: apool of NF-jB is localized to the IMS thanks to the interaction between IjBa(inhibitor ofjBa subunit) and ANT (Bottero

et al., 2001); NF-jB can be activated by mitochondrial stress-induced alterations of Ca

2+

homeostasis, which results in nu-clear gene expression and phenotypic modifications (Biswas et al., 2005); and, in multiple experimental settings, NF-jB-dependent gene transactivation requires mitochondrial ROS (Hughes et al., 2005; Lluis et al., 2007). Finally, mitochondrialROS have been suggested to mediate estrogen-induced cell cycle progression by activating NF-jB (and other transcriptionfactors) (Felty and Roy, 2005), which points to the existence of a specific functional interplay between mitochondria andNF-jB-dependent proliferation. Intriguingly, both mitochondrial uncoupling and mtDNA depletion have been shown to acti-vate NF-jB via a specific pathway that involves calcineurin and the IjB kinaseb subunit (IjKb), suggesting that mitochon-dria employ preferential mechanisms to propagate signals of stress to the NF-jB system (Biswas et al., 2005, 2008).

Several proteins that are known to transduce growth signals, such as members of the mitogen-activated protein kinase(MAPK) family, have been shown to localize to mitochondria and to regulate tumorigenesis-related processes includingmitophagy (see above) (Dagda et al., 2008) and steroidogenesis (see below) (Poderoso et al., 2008). Moreover, the subcellularlocalization of MAPK family members reportedly affects the sensitivity of cancer cells to antiproliferative signals (see below)(Galli et al., 2008). Altogether, these examples suggest that the interplay between components of multiple growth signaling

cascades and mitochondria may have tumorigenic outcomes, either by providing cells with self-sufficient proliferative stim-uli or by modulating mitochondrial processes that are linked to oncogenesis and/or tumor progression.

3.3. Insensitivity to antiproliferative signals

Nonmalignant cells exposed to adverse conditions (e.g., chemotherapeutic agents) or intrinsic stress (e.g., oncogenicstress) normally block the advancement of their cell cycle and activate mechanisms of repair and adaptation to stress. If cellsfail to recover, the cell cycle arrest becomes permanent (senescence) and may finally, after a latency period, trigger cell death(Campisi and dAdda di Fagagna, 2007). A plethora of human cancers are characterized by the genetic and/or epigenetic inac-tivation of cell cycle arresting proteins, including (but not limited to) p14ARF, p16INK4a, p21Cip1, p27Kip1 and pRb (Baur et al.,1999; Nakamura et al., 2001), thereby becoming insensitive to extrinsic/intrinsic antiproliferative signals.

Manganese superoxide dismutase (MnSOD), which catalyzes the transformation of superoxide to molecular oxygen (O2)and hydrogen peroxide (H2O2), represents (one of) the major antioxidant enzyme in mitochondria. MnSOD is encoded by a

nuclear gene mapping to locus 6q25.3, and its function is often lost in tumor cells due to deletions and/or point mutations(Foulkes et al., 1993; Re et al., 2003). The overexpression of catalytically active MnSOD reportedly inhibits the proliferation ofa wide variety of cancer types (Oberley, 2005), strongly pointing to MnSOD as a (mitochondrial) tumor suppressor protein.However, the precise molecular mechanisms that underlie MnSOD-dependent cell cycle control have not yet been eluci-dated, and contrasting observations have been published. While in some normal and immortalized cell lines the overexpres-sion of MnSOD led to the transactivation of p21 (Zhong et al., 2004; Sarsour et al., 2005), in other experimental settingsMnSOD triggered senescence though a p21-independent pathway (Takada et al., 2002; Behrend et al., 2005). Steady-statelevels of H2O2reportedly inhibit cell growth by controlling the activation status as well as the mitochondrio-nuclear redis-tribution of multiple members of the MAPK family ( i.e., ERK1/2, p38, JNK1/2) (Galli et al., 2008). Irrespective of the above-mentioned controversy, these observations suggest that the absence of MnSOD might contribute to tumorigenesis byfavoring the insensitivity of cancer cells to antiproliferative signals, most likely via indirect redox circuitries. In line with thishypothesis, high levels of MnSOD have been associated with poor survival in multiple malignancies including glioblastoma(Ria et al., 2001), gastric cancer (Kim et al., 2002) and colorectal carcinoma (Nozoe et al., 2003).

An intriguing link between the mitochondrial fission/fusion machinery and senescence has recently been pointed out (Leeet al., 2007). In particular, RNAi-mediated depletion of the mitochondrial protein FIS1 (which is required for fission due to its



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ability to recruit the mitochondrial membrane segregating factor DRP1) (Perfettini et al., 2005), has been shown to inducethe appearance of morphological and biochemical hallmarks of senescence, which could be prevented by the concomitantdepletion of OPA1, a component of the apparatus for mitochondrial fusion ( Lee et al., 2007). Moreover, high nuclear levelsof DRP1 have been associated with increased chemoresistance and poor prognosis in lung cancer (Chiang et al., 2009). Takentogether, these observations suggest that the expression levels and subcellular distribution of proteins that regulate mito-chondrial dynamics may affect tumorigenesis by favoring immortalization and chemoresistance.

3.4. Disabled cell death

As mitochondria occupy a central position in multiple subroutines of cell death (including but not limited to intrinsic/extrinsic apoptosis and necrosis) (Kroemer et al., 2009) and as cancer cells are more resistant to death induction than theirnormal counterparts, it is not surprising that numerous proteins localized to (or directly interacting with) mitochondria mayexert important roles in oncosuppression, oncogenesis, tumor progression and chemotherapy resistance. So far, dozens if nothundreds of molecular pathways that link the mitochondrial control of cell death to malignant transformation have beencharacterized (Ferri and Kroemer, 2001; Green and Kroemer, 2004). The precise description of these biochemical mecha-nisms largely exceeds the scope of the present article (for a review see (Kroemer et al., 2007)). Here, we will list a few proteinfamilies that are particularly relevant for the mitochondrial control of cell death in tumors.

The function of several pro-apoptotic proteins from the Bcl-2 family is lost via genetic and/or epigenetic mechanisms in awide variety of human cancers. This applies to both multidomain (e.g., Bax, Bak) and BH3-only (e.g., Bid, Bad) pro-apoptotic

members of the Bcl-2 family (Gutierrez et al., 1999; Sakuragi et al., 2002; Sturm et al., 2006). As an alternative, tumor cellsoverexpress anti-apoptotic Bcl-2-like proteins (e.g., Bcl-2, Bcl-XL, Mcl-1) (Sakuragi et al., 2002; Rassidakis et al., 2003). Thiscan derive from a variety of mechanisms including transcriptional upregulation, gene amplification and reciprocal translo-cation. Thebcl-2gene itself has been first identified due to its overexpression in B-cell follicular lymphoma (Pegoraro et al.,1984). This resulted from a reciprocal chromosomal translocation (t14:18) that placed the Bcl-2 coding sequence in frameunder the control of the immunoglobulin heavy chain promoter (Pegoraro et al., 1984).

The backbone components and interactors of the PTPC function at the crossroad between cell death regulation and bio-energetic metabolism, and some of them exhibit altered expression or function in pre-malignant and malignant lesions(Brenner and Grimm, 2006). Among others, this applies to: ANT1, which is highly expressed in terminally differentiated tis-sues but poorly in highly proliferating (cancer) cells (Jang et al., 2008); ANT2, which is up-regulated in several hormone-dependent malignancies (Le Bras et al., 2006); hexokinase 2 (HK2), which may determine the high glycolytic phenotype thatcharacterizes many cancers known as the Warburg effect (see below) (Pedersen et al., 2002; Rho et al., 2007); peripheralbenzodiazepine receptor (PBR, also known as TSPO, i.e., translocator protein of 18 kDa), which is implicated in cholesterol

metabolism and whose overexpression has been shown to correlate with aggressive phenotype in breast, colorectal andprostate cancer (Papadopoulos, 2003); and mitochondrial creatine kinase (mtCK), which may exert tumor suppressing func-tions in oral squamous cell carcinoma (Onda et al., 2006). Specific VDAC isoforms are known to control mitochondrial celldeath by acting either as pro-death (e.g., VDAC1) (Shimizu et al., 2000, 2001) or pro-survival (e.g., VDAC2) (Chandra et al.,2005) modulators. However, mutation or altered expression of VDAC has not been formally demonstrated in human cancer(Torres-Cabala et al., 2006; Rho et al., 2007). Whether this reflects the existence of multiple VDAC isoforms and/or of proteinsthat may substitute for VDAC during tumorigenesis remains to be determined (Baines et al., 2007; Galluzzi and Kroemer,2007).

Upon MMP, the translocation of IMS proteins from mitochondria to the cytoplasm activates the caspase cascade as well ascaspaseindependent cell death mechanisms (Kroemer et al., 2007). However, as demonstrated by gene knockout studies,most IMS proteins contribute to cellular homeostasis and survival as they are present in mitochondria (Galluzzi et al.,2008a). It is therefore not surprising that the levels of some IMS proteins are increased in specific tumor types (e.g., AIFand EndoG in colorectal and gastric carcinomas) (Lee et al., 2006; Yoo et al., 2008) and decreased in others (e.g., AIF in acute

myeloid leukemia (AML) and EndoG in hepatocellular carcinoma) (Hess et al., 2007; Ahn et al., 2008). This presumably re-flects the fact that the relative contribution of each IMS protein to pro-survival and pro-death mechanisms may be highlyvariable in different tissues. In AML patients, reduced expression of AIF has a negative prognostic impact ( Hess et al.,2007), underscoring the general importance of the mitochondrial cell death pathway for the pathophysiology of this devas-tating disease (Del Poeta et al., 2008).

A final example of the multifaceted relationship between disabled cell death and mitochondria in tumor cells is providedby p53, which is mutated and/or inactivated by epigenetic mechanisms in more than 50% of all human cancers (Vousden andLane, 2007). p53 has first been characterized for its ability to accumulate upon stress and to orchestrate the transactivationof pro-apoptotic (e.g., Bax, Puma), cell-cycle arresting (e.g., p21Cip1), and autophagy-inducing (e.g., DRAM) proteins (Vogel-stein et al., 2000; Vousden and Lane, 2007). However, p53 has recently been found to exert a plethora of extranuclear activ-ities. In particular, p53 can trigger MMP by physically interacting with Bcl-2 family proteins ( Galluzzi et al., 2008c;Morselliet al., 2008a), meaning that the cytoplasmic pool of p53 can cooperate with its nuclear counterpart in the activation of apop-totic programs. Moreover, cytoplasmic p53 inhibits autophagy (Morselli et al., 2008b; Tasdemir et al., 2008), thereby sub-

verting the mechanism that removes damaged mitochondria, and as a consequence further reducing the threshold atwhich MMP causes cell death.



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Dozens, if not hundreds, of cancer-associated p53 mutations have been identified so far, each of which results in specificfunctional changes at the protein level (Soussi, 2007). Several mutations that disrupt the capacity of the p53 protein to bindDNA also affect its interaction with Bcl-2 family proteins (Moll et al., 2005; Tomita et al., 2006), suggesting that such muta-tions can act as dual hits and abolish both the nuclear and the extranuclear functions of p53.

4. Angiogenesis, invasiveness, metastatic potential and mitochondria

As solid tumors proliferate, cells that are localized in the lesion core are confronted with unfavorable environmental con-ditions, including low levels of oxygen and glucose. The response to hypoxic and metabolic stress is orchestrated by the tran-scription factor hypoxia-inducing factor 1 (HIF-1), which is composed by a constitutive (b) and an oxygen-sensitive (a)subunit (Semenza, 2007). HIF-1 transactivates a consistent number of genes whose products are implicated in metabolism(e.g., pyruvate dehydrogenase kinase 1, PDK1, see below) and angiogenesis (e.g., vascularendothelial growth factor 1, VEGF-1) (Hirota and Semenza, 2006; Pouyssegur et al., 2006). The most prominent pathway for HIF-1 activation requires mito-chondrial ROS, and reportedly depends on the presence and function of the respiratory complex III ( Klimova and Chandel,2008). HIF-1 regulation is altered by genetic and/or epigenetic mechanisms in many types of human cancer ( Pouysseguret al., 2006). Recently, a chemical library screen has lead to the identification of alkyliminophenylacetate compounds thatpotently inhibit HIF-1-dependent transcription (Lin et al., 2008). These agents as well as nonalkyliminophenylacetateHIF-1 inhibitors identified were found to block mitochondrial ROS generation, further highlighting the essential role of mito-chondria in the HIF-1 mediated response to hypoxic stress (Lin et al., 2008). By favoring the loss of E-cadherin, HIF-1 alsopromotes the so-called epithelialmesenchymal transition (EMT), which is a prerequisite for the metastatic behavior of solid

tumors (see below) (Esteban et al., 2006; Pouyssegur et al., 2006). Both endothelial and tumor cells ectopically express themitochondrial F1FO-ATP synthase at the cell surface, where it may contribute to angiogenesis by favoring extracellular acid-ification (Chi et al., 2007). Inhibition of the F1FO-ATP synthase at the plasma membrane by endogenous angiostatin or by aspecific antibody has been associated with intracellular acidification that correlated with anti-angiogenic effects (Chi et al.,2007).

Invasion and metastasis require dramatic biochemical modifications including cytoskeletal rearrangements, loss of inte-grin-dependent and -independent anchorage, enhanced motility and degradation of the extracellular matrix (ECM) (Joyceand Pollard, 2009; Nguyen et al., 2009). Normal cells that detach from the basal membrane activate a cell death mechanismknown as anoikis, which is mostly mediated by apoptotic executioners (Simpson et al., 2008). Thus, disabled cell death (seeabove) not only promotes early tumorigenesis and chemotherapy resistance, but also sustains the invasive/metastatic atti-tude of tumor cells. However, this is not the sole link between mitochondria and the invasive/metastatic attitude of cancercells. Mitochondrial and extramitochondrial ROS deeply affect integrin signaling (Svineng et al., 2008), as well as the expres-sion and activation of matrix metalloproteinases (Nelson and Melendez, 2004; Kumar et al., 2008). According to one recent

report, ROS would also favor the upregulation of CXCL14, a chemokine that promotes cell motility by binding to the inositol1,4,5-trisphosphate receptor (IP3R) on the ER and hence elevating cytosolic Ca

2+ (Pelicano et al., 2009). Gene expression ar-rays have demonstrated that oxidative phosphorylation-impaired osteosarcoma cells exhibit an altered expression of severalextracellular matrix remodeling factors including, but not limited to, metalloproteinases and their tissue inhibitors (vanWaveren et al., 2006). In non-small cell lung cancer A549 cells, partial mtDNA depletion and treatment with mitochondrialinhibitors reportedly induce an invasive phenotype characterized by the upregulation of cathepsin L and transforminggrowth factor b (TGFb), increased cytosolic Ca2+ concentrations, and consequent activation of calcineurin and Ca2+-depen-dent MAPK kinases (Amuthan et al., 2002). In liver metastases of colorectal cancer, the a and dsubunits of the mitochondrialF1FO-ATP synthase are expressed at higher levels than in primary tumors, and siRNA-mediated downregulation of these pro-teins could decrease the in vitro invasiveness of human colon cancer cells (Chang et al., 2007). Of note, GRIM-19 has beenshown to inhibit V-Src-induced cell motility by preventing the cytoskeletal rearrangements that are required for podosomeformation, thereby exerting oncosuppressing functions also via Stat3-independent mechanisms (see above) (Sun et al.,2009). Smac/DIABLO reportedly inhibits motility and migration of SH-EP neuroblastoma cells in the absence of any pro-

apoptotic stimulus (Vogler et al., 2005). Moreover, in colorectal cancer patients, the expression levels of Smac/DIABLO inver-sely correlate with the incidence of metastasis and survival, pointing to Smac/DIABLO as a valuable independent prognosticfactor, at least in some clinical settings (Endo et al., 2009). Altogether, these examples support the notion that mitochondrialalterations contribute to the angiogenetic, invasive and metastatic behavior of cancer cells.

5. Avoidance of the immune response and mitochondria

During the last decade, great attention has been paid to the role of the immune system in the control of tumorigenesis. Inparticular, there is growing consensus on the fact that malignant transformation is associated with an acquired capacity oflive (and dying) cancer cells to avoid the immune response ( Zitvogel et al., 2008).

The Bcl-2 family members Bax and Bak do not only play a critical role in the regulation of mitochondrial apoptosis (seeabove) (Kroemer et al., 2007), but also participate in the molecular machinery that allow for the exposure of the ER-sessile

protein calreticulin (CRT) on the plasma membrane of dying tumor cells ( Panaretakis et al., 2009). Some chemotherapeuticagents (including anthracyclins and ionizing irradiation) are particularly effective in killing cancer cells while triggering CRT



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exposure, which is required for the elicitation of a protective anticancer immune response in vivo (Obeid et al., 2007). Re-cently, the molecular pathway that mediates pre-apoptotic CRT exposure has been shown to include an apoptotic modulethat relies on the essential contribution of Bax, Bak and caspase-8 ( Panaretakis et al., 2009). This apoptotic module wouldoperate downstream of an ER stress response that is necessary (but not sufficient) to elicit the immune response againstanthracyclin-treated tumor cells. Accordingly, depletion of Bax and Bak (as well as of caspase-8 and several additional pro-teins that participate in the ER stress response) abolished CRT exposure induced by anthracyclins, thereby reducing theimmunogenicity of dying cancer cells injected into mice (Panaretakis et al., 2009). At present it is not known whether

Bax and Bak contribute to CRT exposure by virtue of their capacity to regulate Ca

2+

fluxes (Scorrano et al., 2003), whichare required for CRT exposure (Tufi et al., 2008), or whether they operate at the level of mitochondria. Irrespective of thisopen question, the inactivation the Bax/Bak system may provide tumors with a dual advantage (i) by reducing the numberof cancer cells succumbing to therapeutic regimens, and (ii) by blocking CRT exposure and thus inhibiting the anticancer im-mune response elicited by dying tumor cells (Zitvogel et al., 2008).

Mitochondrial and extra-mitochondrial members of the HSP family (e.g., HSP70) exposed at the cell surface or released bydying cancer cells transduce an endogenous danger signal that can increase the immunogenicity of tumors (Jeannin et al.,2008). Nevertheless, HSPs are often upregulated in malignant cells, likewise because most of these proteins confer relevantcytoprotection in response to adverse conditions including oxygen/glucose shortage, oxidative stress and chemotherapy(Calderwood and Ciocca, 2008). Thus, high HSP levels may exert bona fide oncogenic functions by protecting cancer cellsfrom stress, while at the same time exposing tumors to more intense immune responses. The fact that (at least in specificcases) the expression levels of some HSP (e.g., HSP20) inversely correlate with tumor stage indicates that the immunogenicfunction of selected HSPs may be more important than their cytoprotective roles during the selection process that occur

throughout tumor progression (Noda et al., 2007).

6. Metabolic reprogramming and mitochondria

The most impressive metabolic alteration of cancer cells consists in the so-called Warburg phenomenon, that is an in-creased flow through glycolysis in spite of high oxygen tension (aerobic glycolysis), which leads to enhanced lactate gener-ation (Brahimi-Horn et al., 2007). The Warburg phenomenon arises from both mitochondrial and extra-mitochondrialalterations, and it is sufficiently diffuse among different types of cancer that it is exploited for clinical tumor imaging bymeans of a glucose derivative coupled to positron emission tomography (PET) (Mankoff et al., 2007). Besides the Warburgeffect, most tumors present extensive metabolic reprogramming, which frequently implicates mitochondria. The mecha-nisms that account for such alterations are deeply intertwined (Kroemer and Pouyssegur, 2008), and their description is be-yond the aim of the present review. Intriguingly, most molecular changes that favor oncogenesis also have a direct or indirect

metabolic outcome. Here, a few examples will be provided to highlight the critical contribution of mitochondria to yet an-other feature of tumor cells.Often, mitochondria from tumor cells are relatively small, present evident ultrastructural alterations, are deficient in the

b-F1ATP synthase subunit, and are characterized by an increased Dwm(Kim et al., 2007; Lopez-Rios et al., 2007), altogetherpointing to primary defects in the respiratory chain that may contribute to aerobic glycolysis. Impaired oxidative phosphor-ylation favoring the Warburg phenomenon may result from several oncogenic mechanisms. Defects in the p53 system leadto impaired transactivation of SCO2, a mitochondrial protein required for the assembly of cytochrome c oxidase ( Matobaet al., 2006), and of TIGAR, an isoform of 6-phosphofructo-2-kinase (PFK2) whose expression reportedly inhibits ROS gener-ation, thereby exerting tumor suppressive functions (Bensaad et al., 2006). Constitutive activation of phosphatidylinositol-3kinase (PI3K) leads to the transcriptional repression of carnitine palmitoyltransferase 1A, an enzyme of the OM that normallyinitiates the mitochondrial import of fatty acids destined to b-oxidation (Deberardinis et al., 2006). HIF-1 is often hyperac-tivated in cancer cells (see above) and controls the transcription of pyruvate dehydrogenase kinase 1 (PDK1) ( Kim et al.,2006). PDK1-dependent inhibition of pyruvate dehydrogenase (PDH) results in decreased conversion of pyruvate to acet-

yl-CoA, which represents the rate-limiting reaction of the tricarboxylic acid (TCA) cycle, and hence deficient oxidative phos-phorylation (Papandreou et al., 2006). Pharmacological PDK1 inhibition with dichloroacetate (DCA, a metabolic modulatorused for decades in humans to treat lactic acidosis) reportedly reactivates PDH and corrects mitochondrial hyperpolarization,thereby triggering apoptosis in several tumor cell lines (Bonnet et al., 2007). Intriguingly, tumorigenic germline mutations oftwo TCA cycle enzymes (i.e., fumarate hydratase, FH, and succinate dehydrogenase, SDH) promote HIF-1 induction via theaccumulation of intermediate metabolites (i.e., fumarate and succinate) that inhibit the oxygen-dependent pathway thatnormally targets HIF-1ato degradation (Gottlieb and Tomlinson, 2005).

In malignant cells, both HK1 and HK2 are more tightly associated with VDAC at the OM than in normal cells, which allowsthem to gain direct access to mitochondrial ATP exported into the cytosol (Pastorino et al., 2005) Increased HK/VDAC bindingmay result from constitutive signaling through the PI3K-Akt axis by at least two distinct mechanisms. First, the interaction ofHK2 with mitochondria is enhanced by direct Akt-dependent HK2 phosphorylation (Miyamoto et al., 2008). Second, Akt mayinterfere with the GSK3b-dependent phosphorylation of VDAC, which reportedly disrupts the HK2/VDAC interaction(Pastorino et al., 2005). In tumor cells characterized by inefficient oxidative phosphorylation, VDAC-bound HK may couple

residual ATP production in mitochondria with the rate-limiting step of glycolysis, thereby providing the basis for theWarburg phenomenon. Mitochondrially-bound HK inhibits MMP, presumably by interfering with the opening of the PTPC



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(which also involves VDAC) (Abu-Hamad et al., 2008). In line with this notion, peptides (Chiara et al., 2008) as well as smallmolecules (Galluzzi et al., 2008b; Goldin et al., 2008) that disrupt the interaction between HK and mitochondria can selec-tively kill tumor cells, both in vitro andin vivo. Intriguingly, recent work suggests that the detachment of HK2 from mito-chondria would trigger PTPC-dependent MMP independently of VDAC (Chiara et al., 2008), consistent with the notionthat VDAC is dispensable for MMP in several models of cell death ( Galluzzi and Kroemer, 2007).

A generalized increase in anabolism characterizes nearly all cancer types (Kroemer and Pouyssegur, 2008). Sustained sig-naling though the anabolic PI3K-Akt axis enhances fatty acid biosynthesis and steroidogenesis through the activation of ATP

citrate lyase (ACL), the cytosolic enzyme that catalyzes the ATP-dependent cleavage of citrate into acetyl-CoA and oxalace-tate (Manning and Cantley, 2007), and by transactivating fatty acid synthase (FASN), which is upregulated in many cancers(Wang et al., 2005). PBR, one of the putative components of the PTPC (see above), controls the rate-limiting step of steroi-dogenesis by regulating cholesterol transport from the OM to the IM (Papadopoulos, 2003), and is overexpressed in numer-ous hormone-dependent and independent cancers. In line with the notion that membrane biogenesis (which requirescholesterol) is considerably increased in highly proliferating cells, PBR overexpression has been found to correlate withthe aggressive phenotype of breast, colorectal and prostate cancer (Hardwick et al., 1999; Papadopoulos, 2003). Finally,oncogenic tyrosine kinases can stimulate anabolic reactions in cancer cells by partially inhibiting the cancer-specific isoformof pyruvate kinase PKM2, which catalyzes the last step of glycolysis leading to pyruvate (Christofk et al., 2008). In conditionsof elevated glycolytic flux, this would result in the deviation of metabolic intermediates toward anabolic reactions, andconcomitantly avoid an excessive production of pyruvate.

As it stands, mitochondrial alterations lying at the crossroad between metabolism and cell death regulation representpromising targets for the development of novel therapies for the treatment of cancer. Molecules that simultaneously revert

the hyperglycolytic state of malignant cells and sensitize them to death induction may turn out to be particularly efficientanticancer drugs. DCA and HK-targeted strategies have provided a first proof-of-principle for this approach.

7. Concluding remarks

As discussed above, a wide array of human malignancies share molecular alterations that directly or indirectly linked tomitochondria (Fig. 3,Table 1). Since mitochondria occupy at a strategic position between bioenergetic/biosynthetic metab-olism and cell death regulation, they are emerging as privileged targets for the development of novel chemotherapeuticagents (Gogvadze et al., 2008). During the last decade, numerous approaches that selectively target cancer cells by virtueof their mitochondrial defects have been shown to exert antitumor effects (Galluzzi et al., 2006). These include, but arenot limited to: (i) mitochondriotoxic agents that preferentially accumulate in cancer cells due to mitochondrial hyperpolar-ization (e.g., F16) (Fantin et al., 2002); (ii) pharmacological modulators of the Bcl-2 protein family (e.g., ABT-737, HA14-1)(Manero et al., 2006; Mason et al., 2008); (iii) compounds that bind to putative PTPC subunits (e.g., PK11195) (Decaudin

Fig. 3. Mitochondrial gateways to cancer.Most if not all the common features of cancer cells, as first proposed by Hanahan and Weinberg (2000) and later by

Kroemer and Pouyssegur (2008), Zitvogel et al. (2008), and Morselli et al. (2009) , implicate either directly or in an indirect fashion mitochondria. Thus,

mitochondria represent promising targets for the development of novel anticancer strategies. In this context, the most valuable compounds would be those

that simultaneously hit multiple mitochondrial processes, thereby mimicking the efficacy of combination therapies yet presenting the side effects of a

single agent. Please refer to the text for further details. HIF-1, hypoxia-inducing factor 1; MnSOD, manganese superoxide dismutase; PBR, peripheralbenzodiazepine receptor; PDK1, pyruvate dehydrogenase kinase 1.



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Table 1

Examples of the multifaceted implication of mitochondria in cancer.

Protein/process Localization Physiological function(s) Implication(s) in cancer References

AIF IMS Upon MMP, goes to the nucleus and

mediate DNA degradation

Required for the assembly/maintenance of

complex I

Overexpressed in gastric

carcinoma cells as

compared to normal

mucosal cells

Lowexpression is a negative

prognostic factor in AML

Lee et al. (2006), Kroemer

et al. (2007), Hess et al.

(2007), Galluzzi et al.

(2008a)

ANTs IM Backbone PTPC subunits

Normally mediate the exchange of

adenine nucleotides across IM

ANT1 is downregulated in

highly proliferating (cancer)

cells

ANT2 is anti-apoptotic and

upregulated in many

cancers

Le Bras et al. (2006), Jang

et al. (2008)

Bak/Bax Cytoplasm

OM

Promote MMP and cell death by multiple

mechanisms

Required for CRT exposure on the surface

of dying cancer cells

Inactivated/lost in several

types of human cancer

Gutierrez et al. (1999),

Sakuragi et al. (2002),

Kroemer et al. (2007)

Bcl-2/Bcl-XL ER

OM

Block MMP, thereby enhancing the

resistance of cancer cells to a plethora of

cell death inducers

Overexpressed in many

hematopoietic cancers, due

to amplification or

translocation

Rassidakis et al. (2003),


BH3-only proteins Cytoplasm

Cytoskeleton

OM

Sense stress and activate cell death-

inducing mechanisms

Inactivated/lost in several

types of human cancer

Sturm et al. (2006), Kroemer

et al. (2007)

CPT1A OM Initiates the mitochondrial import of fatty

acids destined to b-oxidation

Repressed by constitutive

PI3K activation, which

favors the Warburg

phenomenon

Deberardinis et al. (2006)

Dwm IM Required for mitochondrial ATP synthesisvia F1FO-ATP synthase

Many types of cancers

exhibit increased Dwm,often coupled to inefficient

OXPHOS

Kim et al. (2007), Lopez-

Rios et al. (2007)

DRP1 Cytoplasm

Mitochondria

Nucleus

Required for mitochondrial membrane

segregation during mitochondrial fission

Elevated nuclear expression

of DRP1 has been associated

with poor prognosis in lung

cancer

Chiang et al. (2009)

EndoG IMS Upon MMP, goes to the nucleus and

mediate DNA degradation

Involved in recombination-dependent

DNA repair

Altered expression in

human tumors of distinct

origin

Required for the survival of

tetraploid colon cancer cells

Ahn et al. (2008), Galluzzi

et al. (2008a), Yoo et al.

(2008)

F1FO-ATP synthase IM Normally ensures mitochondrial ATP

synthesis driven by DwmFavor extracellular acidification, thereby

promoting angiogenesis; inhibited by the

endogenous anti-angiogenic factor

angiostatin

Overexpressed in colorectal

cancer metastatic lesions as

compared to primary

tumors

Ectopically expressed at the

plasma membrane surface

in endothelial and tumor

cells

Chang et al. (2007), Chi et al.

(2007)

Fatty acidbiosynthesis

CytoplasmMitochondria

Supplies cells with fatty acids Enhanced in cancer cells byPI3K-dependent activation

of ACL and FASN

Manning and Cantley(2007), Wang et al. (2005)

FIS1 OM Recruits Drp1, required for mitochondrial

fission

Prevents senescence Lee et al. (2007)

GRIM-19 Mitochondria Sequesters and inhibits Stat3

Prevents cytoskeletal rearrangements

required for podosome formation

Required for complex I assembly

Mutated in a wide array of

human neoplasms

Lufei et al. (2003), Zhang

et al. (2003a), Fusco et al.

(2005), Maximo et al.

(2005), Sun et al. (2009)

HIF-1 Nucleus Controls the transcription of genes

implicated in invasion, angiogenesis and

metabolism

HIF-1 activation requires ROS, and

depends on complex III

Deregulated by a plethora of

molecular mechanisms in

several types of cancer

Hirota and Semenza (2006),

Pouyssegur et al. (2006),

Klimova and Chandel

(2008)

(continued on next page)



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Table 1(continued)


HKs Cytoplasm Catalyze the first and rate-limiting step of

glycolysis

Putative PTPC subunits

In cancer cells, HKs are

tightly associated with

VDAC, in turn favoring the

Warburg effect

Pastorino et al. (2005),

Galluzzi et al. (2008b),

Miyamoto et al. (2008)

HSPs Cytoplasm

MitochondriaNucleus

Molecular chaperones that exert

cytoprotective effects in response toadverse conditions

Exposed at the surface or released by

dying cancer cells

Upregulated in multiple

solid and hematologicaltumors

HSP20 expression levels

inversely correlate with

hepatocellular tumor stage

Noda et al. (2007), Jeannin

et al. (2008), Calderwoodand Ciocca (2008)

hTERT Mitochondria

Nucleus

Sustains immortalization by preserving

telomere length

Binds to mtDNA and exerts MMP-

inhibitory effects

In vitro, depletion of hTERT

sensitizes cancer cells to

death, while hTERT

overexpression exerts

cytoprotective effects

Massard et al. (2006),

Santos et al. (2006), Xi et al.

(2006), Ahmed et al. (2008),

Haendeler et al. (2009)

MAPKs Cytoplasm

Mitochondria

Nucleus

Mediate several growth signaling cascades

converging on the activation of

transcription factors

From mitochondria, they

regulate tumorigenesis-

related processes and they

sustain insensitivity to

antiproliferative signals

Dagda et al. (2008), Galli

et al. (2008), Poderoso et al.

(2008)

Mitophagy Cytoplasm Impaired mitophagy promote ROSgeneration

Autophagy/mitophagy-relevant proteins are often

lost in cancer

Yue et al. (2003)

MnSOD Mitochondria Can trigger senescence via p21-dependent

or -independent mechanisms, presumably

by maintaining high H2O2levels

Inactivated by genetic or

epigenetic mechanisms in a

variety of tumors

High expression is a

negative prognostic factor

in glioblastoma, gastric and

colorectal cancer

Ria et al. (2001), Kim et al.

(2002), Nozoe et al. (2003),

Re et al. (2003), Zhong et al.

(2004), Behrend et al.

(2005), Sarsour et al. (2005)

Mortalin Cytoplasm

ER

Mitochondria

Binds to and inactivate p53

Influences the Ras-Raf-MAPK signaling

pathway

Exerts antioxidant effects upon

phosphorylation by PINK1

Enhances tumorigenicity of

HEFs, in vivoNegative prognostic factor

in colorectal cancer

Wadhwa et al. (2002,2003),

Dundas et al. (2005),

Wadhwa et al. (2006), Hua

et al. (2007), Pridgeon et al.

(2007)

NF-jB CytoplasmIMS

Nucleus

May promote malignancy in response to

mitochondrial stress

NF-jB activation requires mitochondrialROS

Constitutively activated in

multiple human tumors,

transduces pro-proliferative

and anti-apoptotic signals

Hughes et al. (2005)), Lluis

et al. (2007), Biswas et al.

(2005)

OXPHOS IM Generates the Dwmthat is required forATP synthesis by F1FO-ATP synthase

Impaired in many human

cancers, in turn favoring the

Warburg effect

Brahimi-Horn et al. (2007),

Dasgupta et al. (2008),

Kroemer and Pouyssegur

(2008)

p53 Nucleus Master regulator of cell death, cell cycle

and autophagy

Controls the expression of multiple

enzymes implicated in bioenergetic/redox

metabolism

Inactivated by genetic and/

or epigenetic mechanisms

in more than 50% of human

cancers

Vogelstein et al. (2000),

Vousden and Lane (2007),

Galluzzi et al., 2008c),

Morselli et al. (2008a)

OM Directly promotes MMP

PBR OM Regulates cholesterol import from the OMto the IM

Putative PTPC subunit

Overexpressed in hormone-dependent cancers, which

correlates with malignancy

Hardwick et al. (1999),Papadopoulos (2003),


PKM2 Cytoplasm Catalyzes the last step of glycolysis

leading to pyruvate generation

Inhibited by constitutively

active RTKs, resulting in the

deviation of glycolytic

intermediates to anabolic

pathways

Christofk et al. (2008)

PDK1 Mitochondria Normally inhibits PDH, resulting in

decreased conversion of pyruvate to

acetyl-CoA

Upregulated by HIF-1 in

cancer cells, thus promoting

the Warburg phenotype

Papandreou et al. (2006),

Bonnet et al. (2007)

ROS Cytoplasm

ECM

IM

Promote mutagenesis

Favor invasion/metastasis

Promote HIF-1 and NF-jB activation

Extra-mitochondrial

sources of ROS may also

contribute to oncogenesis

Nelson and Melendez

(2004), Zhou et al. (2007),

Kumar et al. (2008), Svineng

et al. (2008), Pelicano et al.

(2009)



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Table 1 (continued)


SCO2 IM Required for COX assembly Underexpressed in p53-

deficient cancer cells

Matoba et al. (2006)

Smac/DIABLO IMS Upon release into the cytosol, indirectly

promotes the activation of the caspase

cascade

Inhibits neuroblastoma cell

migration,in vitro

Expression levels inversely

correlate with survival incolorectal cancer patients

Vogler et al. (2005),

Kroemer et al. (2007), Endo

et al. (2009)

TCA cycle Mitochondria Produces the reducing equivalents

required for OXPHOS

FH and SDH mutations

indirectly promote HIF-1

induction

Gottlieb and Tomlinson

(2005)

TIGAR Cytoplasm p53-regulated PFK2 isoform that reduces

ROS generation

Expression reduced in p53-

deficient cancer cells

Bensaad et al. (2006)

VDACs OM Backbone PTPC subunits

Mediate the exchange of small

metabolites between the cytosol and the

mitochondrial matrix

VDAC1 is pro-apoptotic

VDAC2 is anti-apoptotic by

binding to/inhibiting Bak

Shimizu et al. (2000),

Shimizu et al. (2001),

Chandra et al. (2005)

Abbreviations: ACL, ATP citrate lyase; AML, acute myeloid leukemia; AIF, apoptosis inducing factor; ANTs, adenine nucleotide translocases; BH3, Bcl-2

homology domain; Dwm, mitochondrial transmembrane potential; COX, cytochrome coxidase; CPT1A, carnitine palmitoyltransferase 1A; CRT, calreticulin;DRP1, dynamin-related protein 1; ECM, extracelluar matrix; EndoG, endonuclease G; ER, endoplasmic reticulum; FASN, fatty acid synthase; FH, fumarate

hydratase; HEFs, human embryonic fibroblasts; HIF-1, hypoxia-inducible factor 1; HKs, hexokinases; HSP, heat-shock protein; hTERT, human telomerasereverse transcriptase; IM, mitochondrial inner membrane; IMS, mitochondrial intermembrane space; MAPK, mitogen-activated protein kinase; MMP,

mitochondrial membrane permeabilization; MsSOD, manganese superoxide dismutase; OM, mitochondrial outer membrane; OXPHOS, oxidative phos-

phorylation; PBR, peripheral benzodiazepine receptor; PDH, pyruvate dehydrogenase; PDK1, PDH kinase 1; PFK2, 6-phosphofructo-2-kinase; PI3K, phos-

phatidyolinositol-3 kinase; PINK1, PTEN induced putative kinase 1; PKM2, M2 isoform of pyruvate kinase; PTPC, permeability transition pore complex; ROS,

reactive oxygen species; RTKs, receptor tyrosine kinases; SCO2, synthesis of cytochrome coxidase 2; SDH, succinate dehydrogenase; TCA, tricarboxylic acid;

TIGAR, TP53-induced glycolysis and apoptosis regulator; VDACs, voltage-dependent anion channels.

Table 2

Examples of mitochondrially-targeted compounds with anticancer properties.

Agent Target/mode of action Antitumor activity References

Modulators of the Bcl-2 protein family

A-385358 Bcl-XL-specific inhibitor Synergized with paclitaxel in NSCLC

A549 cells xenograft models

Shoemaker et al. (2006)

ABT-263 Bcl-2, Bcl-XLand Bcl-w inhibitor Undergoing phase I/II clinical trials

for the therapy of solid and

hematological malignancies

(Tse et al. (2008), Kang and

Reynolds (2009)

ABT-737 Bcl-2, Bcl-XLand Mcl-1 inhibitor In vivo, in murine xenograft models of

lymphoma and glioblastoma

Oltersdorf et al. (2005), Kang and

Reynolds (2009)

AT-101 Bcl-2, Bcl-XLand Mcl-1 inhibitor Undergoing phase I/II clinical trials

for the treatment several solid

tumors

Tomillero and Moral (2008), Kang

and Reynolds (2009)

GX15-070 Mcl-1-specific inhibitor Undergoing phase I/II clinical trials

for the therapy of several solid and

hematological cancers

Nguyen et al. (2007), Kang and

Reynolds (2009)

HA14-1 Bcl-2 ligand In vitro, in multiple cancer cell lines,

and in vivo, in glioblastoma

xenografts

Manero et al. (2006)

Oblimersen sodium Bcl-2-specific antisense

oligonucleotide

Undergoing phase II/III clinical trials

for the treatment of melanoma, CLL

and NHL

Tomillero and Moral (2008), Kang

and Reynolds (2009)

SAHB peptide Direct Bax activator In vitro, in a panel of leukemic cell

lines and in vivo, ina murine modelof

leukemia

Walensky et al. (2004), Gavathiotis

et al. (2008)

Lipophilic cationic agents

F16 Direct inducer of MPT In vitro, in a variety of mouse and

human tumor cell lines

Fantin et al. (2002), Fantin and

Leder (2004)

(KLAKKLAK)2peptide Exerts direct mitochondrion-

permeabilizing effects

In vitro, in KS and breast cancer cells

and in vivo, in models of breast

carcinoma

Ellerby et al. (1999)

(continued on next page)



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et al., 2002); (iv) redox active agents that trigger cell death by provoking futile redox cycles in mitochondria (e.g., b-phen-ylethyl isothiocyanate, arsenic trioxide) (Trachootham et al., 2006; Toogood, 2008); and retinoid-related molecules that in-duce MPT independently of retinoid receptors (e.g., CD437, ST1926) (Marchetti et al., 1999; Fontana and Rishi, 2002;Garattini et al., 2004)(Table 2). Hypothetically, the most efficient mitochondrially-targeted therapies would be those thataffect mitochondrial processes linked to several features of the neoplastic phenotype. As an example, compounds that dis-rupt the interaction between HK and VDAC might display consistent antitumor effects by virtue of a dual effect: uncouplingof aerobic glycolysis from residual ATP synthesis occurring in mitochondria and sensitization to PTPC-dependent cell death(Goldin et al., 2008). Similar approaches may hit cancer cells though multiple simultaneous mechanisms, thereby acting de

factoas combination therapies while bringing about the side effects of single agents, with obvious benefits. In conclusion, weexpect that the ever accumulating knowledge on tumor-associated mitochondrial defects will lead to the design of effective

Table 2(continued)

Agent Target/mode of action Antitumor activity References

L-t-C6-Pyr-Cer Ceramide-derivative, accumulates in

mitochondria driven by Dwm

In vitroand in vivo, in HNSCC cell

lines and xenografts

Senkal et al. (2006)

PTPC interactors

Clodronate Inhibitor of ANT-mediated ATP/ADP

exchange

Employed as an adjuvant treatment

for the reduction of skeletal

metastases

Lehenkari et al. (2002), Diel et al.

(2008)

FGIN-1-27 PBR ligand In vivo, in mice xenografted with

human HT29 colorectal cancer cells

Shoukrun et al. (2008)

Methyl jasmonate Disrupts the HK-VDAC interaction at

the OM

In vivo, in mouse models of leukemia,

melanoma and colorectal carcinoma

Galluzzi et al.(2008b), Goldin et al.,

2008)

PK11195 Acts both as a PBR ligand and via

PBR-independent pathways

In vitro, in multiple cancer cell lines,

andin vivo, in murine models of SCLC

Decaudin et al. (2002),Gonzalez-Polo

et al. (2005)

Redox-active agents

5-ALA Metabolized to the light-sensitive

ROS producer protoporphyrin IX

Currently used for PTD of pre-

cancerous actinic keratosis and some

types of skin lymphoma

Gupta and Ryder (2003)

Arsenite trioxide Triggers ROS overgeneration,

oxidative damage and MPT

Currently in use for the treatment of

PML and multiple myeloma

Chen et al. (1998), Larochette et al.

(1999), Toogood (2008)

Benzyl isothiocyanate Ignites ROS overgeneration byinhibiting

complex III

In vivo, in models of leukemia andpancreatic cancer

Sahu and Srivastava (2009), Tsouet al. (in press)

PEITC Disables the glutathione system,

thereby triggering ROS overload

In vivo, in models of xenografted and

chemically-induced lung and

prostate cancer

Trachootham et al. (2006), Khor et al.

(2006)

Retinoid-related molecules

4-HPR Activates ROS-dependent MPT by

elevating ceramide levels

Undergoing phase I/II clinical trials

for the therapy of several solid and

hematological cancers

Hail and Lotan (2001), Lovat et al.

(2004)

ATRA Can bind ANT and hence trigger MPT

independently of RAR

Currently employed for the treatment

of promyelocytic leukemia

Notario et al. (2003)

Betulinic acid Triggers PTPC-dependent MPT In vivo, in mouse models of

melanoma and neuroectodermal

tumors

Fulda and Debatin (2000), Mullauer

et al. (2009)

CD437 Impairs Ca2+ homeostasis, thereby

mediating MPT

In vivo, in murine models of APL,

melanoma and teratocarcinoma

Marchetti et al. (1999), Fontana and

Rishi (2002)

ST1926 Impairs Ca2+ homeostasis, thereby

mediating MPT

Undergoing phase I clinical trials for

the treatment of leukemia

Garattini et al. (2004), Parrella et al.

(2006)

Abbreviations: 4-HPR, N-(4-hydroxyphenyl)retinamide; 5-ALA, 5-aminolevulinic acid; A-385358, 4-(3-dimethylamino-1-phenylsulfanylmethylpropyla-

mino)-N-(4-(4,4-dimethylpiperidin-1-yl)benzoyl)-3-nitrobenzenesulfonamide; ANT, adenine nucleotide translocase; APL, acute promyelocytic leukemia;

AT-101, R-()-gossypol acetic acid; ATRA, all-trans-retinoic acid; CD437, 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid; CLL,

chronic lymphocytic leukemia, Dwm, mitochondrial transmembrane potential; FGIN-1-27, 2-aryl-3-indoleacetamide; GX15-070, obatoclax; HK, hexoki-nase; HNSCC, head and neck squamous cell carcinoma; KS, Kaposis sarcoma; L-t-C6-Pyr-Cer, L-threo-C6-pyridinium-ceramide-bromide; MCL, mantle cell

lymphoma; MPT, mitochondrial permeability transition; NHL, non-Hodgkin lymphoma; NSCLC, non-small cell lung cancer; OM, mitochondrial outer

membrane; PBR, peripheral benzodiazepine receptor; PEITC, b-phenylethyl isothiocyanate; PK11195, 1-(2-chlorophenyl-N-methylpropyl)-3-isoquinolin-

ecarboxamide; PML, promyelocytic leukemia; PTD, photodynamic therapy; PTPC, permeability transition pore complex; RAR, retinoic acid receptor; ROS,

reactive oxygen species; SAHB, stabilized alpha-helix of Bcl-2 domains; SCLC, small-cell lung cancer; ST1926 (E)-3-(40-hydroxy-30-adamantylbiphenyl-4-

yl)acrylic acid; VDAC, voltage-dependent anion channel.



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anticancer therapies. On similar grounds, chemopreventive therapies might aim at closing the mitochondrial gateways toneoplasia.

Acknowledgments

The authors declare no conflicting financial interests. This work was supported by grants from Ligue Nationale contre leCancer (LNC), Agence Nationale de Recherche (ANR), Agence Nationale de Recherches sur le SIDA (ANRS), Institut National du

Cancer (INCa), Cancrople Ile-de-France, Fondation pour la Recherche Mdicale (FRM), Sidaction and the European Com-mission (Active p53, Apo-Sys, ApopTrain, TransDeath, RIGHT). O.K. is the recipient of an EMBO Ph.D. fellowship. E.M. isfunded by an ApopTrain Ph.D. student fellowship.

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