2 The Biology of Cancer Peter Curtin, M.D. Professor of
Clinical Medicine Clinical Director, Blood and Marrow
Transplantation Division September 12, 2013
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3 The Hallmarks of Cancer A conceptual framework for
understanding the biological diversity of cancer and to
understanding the multistep process of progression from normal
cells to a neoplastic state Hallmarks of Cancer are eight,
acquired, functional capabilities that allow cancer cells to
survive, proliferate and disseminate First proposed by Hanahan and
Weinberg in 2000 Updated and expanded by Hanahan and Weinberg
(Cell, 2011)
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4 The Hallmarks of Cancer 1. Sustaining proliferative signaling
2. Evading growth suppressors 3. Resisting cell death 4. Enabling
replicative immortality 5. Inducing angiogenesis 6. Activating
invasion and metastasis 7. Deregulating cellular energetics 8.
Avoiding immune destruction
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5 1. Sustaining proliferative signaling Cancer cells need to
sustain chronic proliferation Growth promoting signals are normally
conveyed by growth factors binding to cell surface receptors which
typically contain intracellular tyrosine kinase domains The
tyrosine kinase domain transmits signals via downstream signaling
protein pathways to control progression through the cell cycle,
cell growth and to influence cell survival and energy
metabolism
Slide 6
6 Growth factor receptor with tyrosine kinase
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7 Sustaining proliferative signaling Mechanisms to sustain
proliferative signaling: 1. Cancer cells produce growth factors
themselves (autocrine stimulation) 2. Stimulate nearby normal
(stromal) cells to produce growth factors 3. Increased levels of
cell surface receptor proteins 4. Structurally abnormal receptors,
active in the absence of growth factor 5. Constitutive activation
of signaling proteins downstream from the receptor
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8 2. Evading growth suppressors Cancer cells must overcome
programs that negatively regulate cell proliferation These programs
depend on the action of tumor suppressor genes that normally govern
the decision of cells to proliferate or to undergo program cell
death (apoptosis) Tumor suppressor genes have typically been
discovered when inactivation leads to the development of cancer The
retinoblastoma (RB) protein and p53 protein are two tumor
suppressors (among many described)
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9 Evading growth suppressors RB protein integrates
extracellular and intracellular signals to decide whether the cells
should proceed thru growth and division Loss of RB function (by
deletion or mutation) removes a gatekeeper of cell cycle
progression resulting in persistent cell proliferation p53 protein
senses intracellular stress and abnormality If DNA damage is
present or if growth promoting signals, oxygen or glucose are
suboptimal, p53 can stop cell cycle progression until these
conditions normalize If overwhelming or irreparable damage to
intracellular systems occurs, p53 can trigger program cell death
(apoptosis)
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10 3. Resisting cell death Programmed cell death by apoptosis
is a natural barrier to the development of cancer The apoptotic
machinery is composed of upstream regulator proteins and downstream
effector proteins Regulators Extracellular/extrinsic pathway: Fas
ligand and receptor, Tumor necrosis factor (TNF)/TNF receptor
Intracellular/intrinsic pathway: senses intracellular signals
Effectors: inactive proteases (caspases 8 and 9) are activated
initiating a proteolytic cascade leading to cellular disassembly
and consumption (apoptosis)
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11 Extrinsic & Intrinsic Apoptotic Pathways
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12 Resisting cell death: The Bcl-2 family The Bcl-2 family of
pro- and anti-apoptotic regulatory proteins: Bcl-2 (and relatives)
are inhibitors of apoptosis, acting by binding to and inactivating
two pro-apoptotic proteins, Bax and Bak, that live in the
mitochondrial membrane Bax and Bak, when released from Bcl-2
binding (and inhibition), disrupt the outer mitochondrial membrane,
releasing cytochrome c which activates the cascade of proteolytic
capsases leading to the cellular changes of apoptosis Bcl-2
interacts with Bax and Bak via BH3 interaction domains Other
proteins that sense cellular abnormalities contain BH3 domains
(BH3-only proteins) can activate apoptosis by interfering with
Bcl-2 or by activating Bax or Bak directly
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13 Resisting cell death Sensors that trigger apoptosis: DNA
damage sensor functions via p53 Insufficient survival factor
signaling (e.g. IL-3 for lymphocytes) Hyperactive signaling by some
oncoproteins (e.g. Myc) Each of these can activate BH3-only
proteins to induce apoptotic cascade
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14 Resisting cell death: Mechanisms to limit or circumvent
apoptosis: Loss of the p53 tumor suppressor function Increased
expression of anti-apoptotic regulators (e.g. Bcl-2) Increased
expression of survival signals (e.g. IL-3) Decreased expression of
pro-apoptotic regulators (e.g. Bax and Bak) Interrupting the
extrinsic apoptotic pathway
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15 4. Enabling replicative immortality Normal cells pass thru a
limited number of cell (division) cycles before becoming senescent
(alive but non-proliferative) or undergoing crisis leading to cell
death In the culture, repeated cycles of cell division lead to
senescence and then crisis, resulting in death of the majority of
cells. Rare cells that survive crisis exhibit unlimited replicative
potential and are said to be immortalized (a characteristic of
established cell lines) Telomeres, multiple tandem copies of 6
nucleotide repeats protecting the ends of chromosomes, are central
to limiting the number of division cycles that a normal cell can
undergo and, conversely, to the unlimited proliferation capacity of
malignant cells
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16 Enabling replicative immortality: Telomeres and Telomerase
In normal cells, telomeres progressively shorten with each cell
division Eventually lose the ability to protect the ends of
chromosomes from end-to-end fusions End-to-end chromosomal fusion
results in scrambling of the karyotype and leads to apoptosis The
length of telomeric DNA in a cell dictates how many cell
generations its offspring can pass through prior to telomere
erosion and cell death Telomerase is a specialized DNA polymerase
that adds telomere repeats to the ends of telomeres Telomerase is
almost absent in normal cells but is present in immortalized
cells
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17 Enabling replicative immortality: Telomeres and
Telomerase
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18 Enabling replicative immortality: Telomeres and Telomerase
Mechanisms to achieve replicative immortality: Increase expression
of telomerase Activation of an alternative, recombination based
telomere- maintenance mechanism (less common)
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19 5. Inducing angiogenesis Tumors require new blood vessel
formation to survive and grow In adult life, normal angiogenesis is
required as a part of wound healing and during the female
reproductive cycle During tumor development and progression the
angiogenic switch is activated and remains on to support crowding
of new vessels stained neoplastic growth The angiogenic switch is
controlled by factors that either induce or oppose angiogenesis
Vascular endothelial growth factor (VEGF) signals thru receptor
tyrosine kinases (VEGFR) to promote angiogenesis Fibroblast growth
factor (FGF) also is pro-angiogenic
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20 Inducing angiogenesis
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21 Inducing angiogenesis Thrombospondin-1 (TSP-1) and fragments
of Plasmin (angiostatin) and type 18 collagen (endostatin) are
endogenous inhibitors of angiogenesis These proteins serve as
regulators of normal transitory angiogenesis during healing and may
also act as barriers to cancer driven angiogenesis
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22 Inducing angiogenesis Mechanisms to induce angiogenesis:
Increased VEGF expression due to hypoxia or oncogene signaling (Ras
and Myc) Increased expression of FGF and other pro-angiogenic
molecules Leukocytes (macrophages, neutrophils, mast cells)
infiltrating pre- malignant and malignant lesions can activate the
angiogenic switch and sustain ongoing angiogenesis
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23 6. Activating invasion and metastasis Carcinomas developed
alterations in shape as well as in their attachment to other cells
and to the extracellular matrix E-cadherin is a cell-to-cell
adhesion molecule that helps to assemble epithelial cells into
sheets and to maintain quiescence E-cadherin is frequently down
regulated and occasionally inactivated by mutation in carcinomas
N-cadherin is normally expressed in migrating neurons and
mesenchymal cells during organogenesis N-cadherin is often
upregulated in invasive carcinoma cells
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24 Activating invasion and metastasis The invasion-metastasis
cascade: Local invasion followed by intravasation of cancer cells
into nearby blood and lymph vessels Transit via lymph and blood
followed by escape from vessels into distant parenchyma
(extravasation) Formation of small tumor nodules (micrometastases)
Growth into macroscopic tumors (colonization)
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25 Activating invasion and metastasis Epithelial-mesenchymal
transition (EMT) is a developmental regulatory program involved in
embryonic morphogenesis and wound healing that epithelial cancers
co-opt to acquired the ability to invade, resist apoptosis and
disseminate EMT and related migratory processes in embryogenesis
are controlled by a set of transcriptional factors (Snail, Slug,
Twist etc.) These factors are expressed widely in cancer and are
important in invasion and metastasis The factors induce loss of
adherans junctions, conversion from round to spindle shape,
expression of matrix-degrading enzymes, increased motility and
resistance to apoptosis The EMT program in embryogenesis and cancer
is influenced by signals from neighboring stromal cells
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26 Activating invasion and metastasis: EMT and Cancer
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27 Activating invasion and metastasis Crosstalk between cancer
cells and nearby stromal cells is involved in invasive growth and
metastasis: Mesenchymal stem cells produce CCL5 in response to
signals from cancer cells; CCL5 acts on the cancer cells to
stimulate invasion Cancer cells produce IL-4 to activate
macrophages to elaborate matrix-degrading enzymes to facilitate
invasion Tumor associated macrophages supply epidermal growth
factor to breast cancer cells; the cancer cells release CSF-1 to
stimulate the macrophages Colonization (growth into
macro-metastases) requires adaptation to a new environment
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28 7. Deregulating cellular energetics Chronic cell
proliferation requires adjustment of energy metabolism Normal cells
use glucose via glycolysis under anaerobic conditions but favor
oxidative phosphorylation under aerobic conditions Cancer cells can
reprogram glucose metabolism to favor glycolysis even under aerobic
conditions Glycolysis is much less efficient than oxidative
phosphorylation Cancer cells compensate by up-regulating glucose
transporters (GLUT1) to increase glucose uptake into the cell Use
of glycolysis is associated with activated oncogenes (RAS, MYC),
with mutant tumor suppressors (p53) and can be further increased in
the setting of hypoxia, present in many tumors
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29 Deregulating cellular energetics Why do cancer cells favor
glycolysis? Perhaps diversion of glycolytic intermediates into
biosynthetic pathways generating nucleosides and amino acids
facilitates biosynthesis of macromolecules required for
proliferation Similar metabolic changes are found in rapidly
dividing embryonic tissues suggesting a role in supporting active
cell proliferation Some tumors have two populations of cells, one
more hypoxic that use glycolysis and secrete lactate, another,
better oxygenated that imports lactate and uses it as an energy
source in oxidative phosphorylation Oxygenation in tumors
fluctuates, both in time and space, due to the instability and
disorganization of tumor vasculature
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30 8. Avoiding immune destruction Theory of Immune
Surveillance: Suggests that the immune system recognizes and
eliminates the vast majority of incipient cancer cells Tumors that
develop either manage to avoid immune detection or limit
immunologic killing Increases in certain cancers in
immunocompromised individuals supports this theory; however, most
of these cancers are virally induced, unlike the majority of
cancers Mice engineered to lack cytotoxic T lymphocytes (CTLs),
helper T cells or natural killer (NK) cells all developed more
carcinogen- induced tumors and more rapidly growing tumors Supports
the role of cellular immunity in tumor eradication
Slide 31
31 Avoiding immune destruction
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32 Avoiding immune destruction Transplant experiments showed
that tumors arising in immunodeficient mice can only be
successfully transplanted into immunodeficient mice while tumors
arising in immunocompetent mice can be successfully transplanted
into both types of mice Suggests that highly immunogenic cancers
are eliminated in immunocompetent mice (immunoediting), leaving
only weakly immunogenic cancers to survive and be transplanted into
both types of mice In immunodeficient mice, highly immunogenic
cancers survive and can be transplanted into immunodeficient mice
but will not survive in immunocompetent mice
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33 Avoiding immune destruction Clinical observations supporting
antitumor immune response: Patients with colon and ovarian tumors
that are heavily infiltrated with CTLs and NK cells have a better
prognosis Immunosupressed organ transplant recipients have
developed donor- derived cancers, suggesting that in the tumor-free
donor, the cancer was held in check by an intact immune system
Highly immunogenic tumors may have ways of disabling components of
the immune system by secreting TGF-beta or other immunosuppresive
factors or by recruiting regulatory T cells (T regs) or
myeloid-derived supressor cells which can both supress the function
of cytotoxic T lymphocytes
Slide 34
34 Two Enabling Characteristics of Cancer Genome Instability
and Mutation Tumor Promoting Inflammation
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35 Genome Instability and Mutation Acquisition of Hallmarks
depends on a succession of alterations in the genomes of neoplastic
cells Certain mutations offer a selective advantage leading to
outgrowth of the dominant clone Multistep tumor progression is a
series of clonal expansions each triggered by the chance
acquisition of an enabling mutation Cancer cells increase mutation
rate by: increased sensitivity to mutagenic agents thru breakdown
in the genomic maintenance machinery Mutation rate also increased
by compromising the systems that monitor genomic integrity and
force damaged cells into senescence or apoptosis
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36 Tumor-Promoting Inflammation Tumor cells are typically
infiltrated by cells of the immune system Immune cells aiming to
eradicate the tumor vs. enhancing tumor development and progression
Inflammatory cells can supply growth factors to sustain
proliferative signaling, survival factors limiting cell death,
pro-angiogenic factors, matrix-modifying enzymes that facilitate
angiogenesis, invasion and metastasis and induction signals leading
to activation of EMT Inflammation sometimes demonstrable at early
stages of tumor development and may foster development from
pre-malignant lesion to cancer Inflammatory cells can release
chemicals, particularly reactive oxygen species that are mutagenic,
hastening malignant progression Role of the tumor microenviroment
in cancer
Slide 37
37 The Cancer Stem Cell Cancers are comprised of heterogenous
populations of cells comprised of regions with various degrees of
differentiation, proliferation, vascularity, inflammation and
invasiveness Cancer stem cells represent a further degree of
cellular heterogeneity and are likely a common constituent of most
tumors Defined by their ability to efficiently seed new tumors in
recipient host mice Initially identified in hematologic
malignancies but subsequently found in solid tumors as well Cell
surface markers and gene transcription profiles similar to those of
normal tissue stem cells Cancer stems cells are likely to be
particularly resistant to conventional chemotherapy and may require
novel therapeutic approaches
Slide 38
38 The Cancer Stem Cell
Slide 39
39 The Cancer Genome Era The project to determine the sequence
of an entire human genome began in 1990 and was completed in 2003
Several large sequencing labs in the US and abroad participated The
cost of the Human Genome Project was $2.7 billion Subsequent
improvements in technology has allow faster and cheaper sequencing
Currently genome sequencing may take several weeks and cost $10,000
or less Identify important genetic mutations that are biologic
drivers for given cancers or subsets of cancers, improve
prognostication, provide additional targets for drug development,
provide foundation for cancer therapy that is tailored to each
cancer