ERK and cell death: cadmium toxicity, sustained ERK activation and cell death

MINIREVIEW

ERK and cell death: cadmium toxicity, sustained ERKactivation and cell deathPatrick Martin and Philippe Pognonec

CNRS FRE3094, Universite de Nice Sophia Antipolis, Nice, France

Introduction

Extracellular signal-related kinase (ERK) is among the

most studied proteins, and is involved in many aspects

of cell physiology [1]. Discovered 25 years ago [2],

ERK actually consists of two closely related proteins

in vertebrates: ERK1 and ERK2. These proteins are

encoded by two related, but distinct, genes [3,4]. ERK

is commonly considered as a key player in mitogenic

signaling following growth factor stimulation [5,6]. The

functional specificities of ERK1 and ERK2 have been

investigated for many years, and despite earlier results

suggesting differences, it now appears that both are

functionally indistinguishable [7,8]. The originally

reported specificities turned out to be a reflection of

their relative abundances [7]. This is also supported by

the observation that some species harbor only one

ERK gene [7], and by the demonstration that ERK1

and -2 do functionally compensate for one another [9].

Nevertheless, thousands of reports (as of today a

Medline search with the term ‘ERK [ti]’ produces 3501

articles) have shown that ERK is a central kinase in

signal transduction pathways (reviewed in [10]). Its role

in cellular proliferation has been extensively investi-

gated, and its ability to protect cells against apoptosis

is also widely accepted. The kinetics of ERK activation

are biphasic, with a rapid and transient burst of activa-

tion (phosphorylation on Tyr and Thr residues) within

Keywords

cadmium; ERK; PKC; sustained activation;

ZIP8

Correspondence

P. Pognonec, Transcriptional Regulation and

Differentiation, CNRS FRE3094, Universite

de Nice, Parc Valrose, 06108 Nice cedex 2,

France

Tel ⁄ Fax: +33 492 07 64 13

E-mail: [email protected]

(Received 18 June 2009, revised 30 July

2009, accepted 15 August 2009)

doi:10.1111/j.1742-4658.2009.07369.x

Extracellular signal-related kinase (ERK) is a key player in cell signaling.

After 25 years of investigation, ERK has been associated with every major

aspect of cell physiology. Cell proliferation, cell transformation, protection

against apoptosis, among others, are influenced by ERK function. Surpris-

ingly, ERK has also been associated with two apparently opposing pro-

cesses. The involvement of ERK in cell proliferation has been extensively

described, as well as its function in postmitotic cells undergoing differentia-

tion. The analysis of these apparent discrepancies has led to a more precise

understanding of the multiple functions and regulations of ERK. More

recently, several groups have identified a new and unexpected role for

ERK. Although being accepted as an important player in the protection

against cell death by apoptosis, it is now clear that ERK can also be

directly linked to cell death signaling. Here, we review the role of ERK in

cell response to cadmium and its association with cell toxicity. In this sys-

tem, ERK is subjected to a continuous activation that can last for days,

which ultimately results in cell death. Cadmium entry into cells is responsi-

ble for this sustained ERK activation, probably via reactive oxygen species

production, and protein kinase C has a negative action on this cadmium-

dependent ERK activation by modulating cadmium entry into cells.

Abbreviations

ERK, extracellular signal-related kinase; MAPK, mitogen-activated protein kinase; PKC, protein kinase C; PMA, 4b-phorbol 12-myristate

13-acetate; ROS, reactive oxygen species.

FEBS Journal 277 (2010) 39–46 ª 2009 The Authors Journal compilation ª 2009 FEBS 39

minutes of stimulation, followed by a second and more

stable activation that can last for a few hours [11]. The

multiple facets of ERK functions have been linked to

the kinetics of activation [12], subcellular localization

[13] and scaffolding proteins [14].

Cadmium poisoning and sustained ERKactivation

Cadmium is a nonessential metal, recognized as a

potent environmental pollutant. Its very long biologi-

cal half life (15–30 years) makes it particularly toxic,

as it accumulates in different organs over time, ulti-

mately resulting in diseases such as kidney failure

and ⁄or bone injuries. The World Health Organization

[15] defines the critical cadmium concentration in the

kidney cortex at 200 mgÆkg–1 wet cortex, which

approximately corresponds to a 1 mm cadmium con-

centration. Above this threshold, kidney dysfunction

appears. It was recently discovered that in cells sub-

jected to a low-dose cadmium poisoning, ERK dis-

plays a very unconventional activation pattern. In all

cell types tested (primary mouse and rat fibroblasts,

kidney-derived epithelium cell lines, bone marrow-

derived macrophages, primary calvaria cells, HeLa

cells, HEK293 cells, C2C12 cells), a strong ERK acti-

vation was observed 16–24 h after the addition of

micromolar concentrations of cadmium. This activa-

tion remained high, for up to 6 days after cadmium

treatment, until the cells died [16]. ERK activation has

been associated with protection against apoptosis when

cells are subjected to stressful conditions [17]. Thus,

the cadmium-induced sustained activation of ERK

could be a response of the cells to survive following

cadmium poisoning. However, treatment of intoxicated

cells with U0126, a potent and specific mitogen-

activated protein kinase ⁄ERK kinase (MEK) inhibitor

that results in the complete inhibition of ERK activa-

tion, following cadmium exposure does not increase

cell death. Rather, a low level of cadmium is not as

toxic to cells in the presence of the MEK inhibitor as

it is in its absence. In recent years, a similar observa-

tion has been reported by several groups, in experi-

ments performed using relatively low concentrations of

cadmium (generally between 1 and 10 lm) [16,18–21].

This phenomenon is illustrated in Fig. 1 and has been

described in detail elsewhere [16]. It should be noted

that the decrease in cell number shown in Fig. 1 fol-

lowing cadmium treatment is the result of cell death

and not due to an inhibition of cell proliferation, as

determined by cell counting as well as quantification of

loss of cell integrity [22]. Although the ERK prodeath

effect discussed above has now been reported by

different groups in multiple cell types, it is intriguing

to note that other reports associated ERK with its

classical antiapoptotic function following cadmium

exposure, particularly in the human nonsmall cell lung

carcinoma CL3 cell line [23,24]. Intriguingly, others

did not see any effect (neither anti- nor proapoptotic)

of ERK activation in response to cadmium, for exam-

ple using Clara cells from rat lung [25], mouse brain

microvascular endothelial cells (bEnd.3) [26], the

human lymphoblastoid Boleth cell line [27] or the

human promonocytic U-937 cell line [28]. A key point

in explaining these discrepancies could be attributed to

the use of different conditions of cadmium treatments.

Most of the studies reporting a protective role for

A

B

Fig. 1. (A) Phenotypic illustration of the implication of ERK activa-

tion in cell death following cadmium treatment. HEK293 cells were

grown for 24 h in control medium (top row) or in the presence of

1 lM CdCl2 (bottom row). The cells were also treated with 5 lM

U0126, a potent MEK inhibitor (middle column), 20 lM U0126 (right

column) or no inhibitor (left column). This figure illustrates the toxic-

ity of cadmium on cells by comparing the two pictures in the left

column. In addition, the protective effect of blocking ERK activation

is shown in the lower row, where cadmium toxicity decreased in a

dose–response manner following U0126 addition. The top row indi-

cates that U0126 treatment alone had a cytostatic effect on cells,

as fewer cells were found after 24 h treatment with 20 lM U0126.

However, please note that with 20 lM U0126, cell density was

similar, independent of the presence of cadmium. This reflects the

protective effect of blocking ERK activation. (B) Western blot analy-

sis of HEK cell lysates prepared after no treatment (Ø), 24 h 2 lM

CdCl2 treatment (Cd), 24 h 10 lM U0126 treatment (U0), 24 h 2 lM

CdCl2 and 10 lM U0126 cotreatment (U0+Cd). The upper panel

displays the activated ERK (P-ERK), which was strongly expressed

following cadmium treatment, but almost undetectable when cells

were cotreated with cadmium and MEK inhibitor U0126. U0126

alone had no effect on ERK activation (U0). The lower panel repre-

sents the total amount of ERK proteins, whose level was not

affected by the different treatments.

ERK and cell death: cadmium toxicity P. Martin and P. Pognonec

40 FEBS Journal 277 (2010) 39–46 ª 2009 The Authors Journal compilation ª 2009 FEBS

ERK used either a higher cadmium concentration

and ⁄or shorter exposure times. Further studies will be

required to clarify the reasons for these apparent dis-

crepancies, which could also be cell type related.

ERK activation ⁄phosphorylation is the result of the

balance between its phosphorylation by MEK and its

dephosphorylation by dual-specificity phosphatases

[29], among them MKP3. Transiently transfected tetra-

cycline-inducible MKP3 phosphatase in HEK293 cells

results in diminished ERK activation. Under these con-

ditions, we found that HEK293 cells were more resistant

to cadmium poisoning as determined by quantifying

the number of adherent cells to those with a rounded

phenotype (P. Martin et al., unpublished results).

Taken together, the sustained activation of ERK

can thus be associated with, and is at least partly

responsible for, cell death signaling.

Reactive oxygen species (ROS) and ERKactivation

The mechanism responsible for this unusual sustained

activation of ERK remains unclear. It is well estab-

lished that cadmium results in a time- and dose-depen-

dent oxidative response of cells by the production of

ROS [30]. Several laboratories have found that pretreat-

ment with the N-acetyl cysteine (NAC) antioxidant effi-

ciently protects cells against cadmium toxicity and

abrogates concomitant ERK activation [31,32]. This

implication of ROS in ERK activation is strengthened

by a study using differentiated PC12 cells treated with

zinc cations. These authors reported a strong activation

of ERK leading to apoptosis that could be alleviated by

antioxidants [33]. It has also been demonstrated that

ROS are responsible for Ras activation, which in turn

stimulates its downstream transduction cascade, includ-

ing ERK [34–36]. However, the exact mechanisms by

which ROS activate mitogen-activated protein kinase

(MAPK) are still poorly understood. Taken together,

these results suggest that ROS production in response

to cadmium does participate in ERK activation.

Furthermore, recent data demonstrated that ROS

production also promotes MKP3 degradation via the

ubiquitination ⁄proteasome degradation pathway. This

MKP3 degradation correlates with a strong activation

of ERK [37]. In addition, phosphatases responsible for

kinase inactivation are also directly inactivated by oxi-

dation of cysteine thiols [38,39]. Therefore, the loss of

phosphatase activity is probably at least partly involved

in the sustained activation of ERK following cadmium

poisoning [33,38].

In conclusion, these data strongly suggest that

ROS production following cadmium poisoning results

in both the activation of the MAPK cascade leading

to ERK activation, and in the inactivation of

MKP3, which would stabilize ERK activation. These

two mechanisms could act together to induce the

delayed and sustained activation of ERK, which is

distinct from the canonical ERK activation observed

following growth factor stimulation. In this latter

case, an immediate response is observed following

ligand–receptor interaction, whereas in the case of

cadmium intoxication, the progressive accumulation

of ROS would result in the delayed and sustained

response of ERK. Further studies will be required to

validate this model.

Cadmium and calcium

In addition to the ROS pathway, other mechanisms

are known to be affected by cadmium and result

in ERK activation. Long-term exposure to 5 lM

cadmium leads to a rapid increase in internal calcium

concentration ([Ca2+]cyt) in NIH3T3 cells [40] and in

astrocytes. This increase in turn leads to an increase

in ROS and to mitochondrial impairment [41].

Pretreatment with a calcium chelator reduced ROS

production and increased cell survival, indicating that

cadmium-induced cell death resulted from disruption

of calcium homeostasis. Similarly, a rapid increase in

([Ca2+]cyt) is seen in mesangial cells exposed to

cadmium. These cells die by autophagy and apoptosis.

The use of different calcium inhibitors indicated that

the release of calcium from the endoplasmic reticulum

plays a crucial role in cadmium-induced cell death. In

addition, the cell-permeable calcium chelator 1,2-bis-

(o-aminophenoxy)-ethane-N,N,N¢,N¢ tetraacetic acid,

tetraacetoxymethyl ester (BAPTA-AM) eliminated

ERK activation as well as mitochondrial depolari-

zation and activation of caspases [42]. Together, these

results indicate that calcium plays an important role in

the cadmium response upstream of ROS production.

Sustained ERK activation, apoptosis,autophagy and necrosis

How this sustained ERK activation yields to the onset

of cell death is still controversial, despite indications

pointing towards the activation of caspases-3 and -8

[16]. It should be noted that although caspase activa-

tion has been associated with cadmium poisoning, our

recent unpublished results show that blocking caspases

with P35, a broad spectrum caspase inhibitor that is

very effective in protecting cells from a caspase-

8-induced apoptosis [43], does not protect cells from

cadmium toxicity, as cell phenotype and cell counts

P. Martin and P. Pognonec ERK and cell death: cadmium toxicity


remain indistinguishable from control cells. This obser-

vation has also been reported by other laboratories

showing that complete caspase-3 and -8 inhibition

results in blocking poly(ADP-ribose) polymerase cleav-

age, but not reducing cell death [27]. Similarly, treat-

ment of PC12 cells with zVAD-fmk, a pan-caspase

inhibitor, had only a limited effect on cadmium-

induced apoptosis [44]. Thus, cadmium, in addition to

caspase activation, induces parallel death pathways

that are P35-insensitive and that ultimately result in

cell death. High doses (350 lM) of cadmium result in

mitochondrial membrane depolarization within 1 h

and subsequent translocation of apoptosis-inducing

factor into the nucleus [27], which could account for

such a caspase-independent death pathway. Kim et al.

[21] also reported that inhibition of ERK activation

results in a better survival of murine J774A.1 macro-

phages through the strong attenuation of cadmium-

induced necrotic cell death, but not affecting caspase-3

activity and DNA fragmentation. Cell death following

cadmium intoxication is therefore not restricted to a

single process, but includes apoptosis, necrosis

and autophagy, depending upon the context of the

intoxication, such as cadmium concentration, as well

as the inducing pathway (lipid peroxidation, ROS

production, internal calcium concentration) [31,42,45].

Although several lines of evidence indicate that

cadmium can result in cell death via multiple path-

ways, the best indication that we have to date to

support the involvement of ERK sustained activation

in cadmium-induced cell death is an engineered cellu-

lar system (Raf-1:ER) in which Raf, an upstream

ERK kinase, is activated at will by the addition of

4-hydroxytamoxifen [46]. The induced activation of

Raf in this system results in sustained activations of

MEK and ERK. These activations in turn result in

the onset of apoptosis via the death receptor path-

way, but are independent of the mitochondrial path-

way. In this system, P35 protects cells from

apoptosis [46]. Together, these results show that con-

stitutive ERK activation per se is sufficient to induce

a cellular response that can ultimately lead to cell

death in certain systems, but that cadmium, in addi-

tion to caspase activation, triggers other parallel

pathways that also culminate in cell death. This

could explain why ERK inhibitors, although reported

by several groups as having a survival effect on cells

exposed to cadmium, are usually not sufficient to

completely protect cells. In addition, these different

death pathways are probably differentially regulated

depending upon the cell types considered and the

conditions of cadmium exposure [31,32].

Cadmium entry into cells is requiredfor sustained ERK activation andsubsequent cell death

Cadmium has long been shown to accumulate in cells.

However, being a nonessential element, no cadmium-

specific transport system exists. Cadmium appears to

‘hijack’ some other transport system(s) in cells. It is

reasonable to propose that other divalent metal cation

transporters could be used by cadmium for entry into

cells, and consequently, these other divalent cations

could compete with cadmium for cell entry and subse-

quently protect cells against cadmium poisoning.

Calcium, copper, magnesium, manganese and zinc

divalent metals have been tested as potential competi-

tors for cadmium entry. Zinc is considered effective in

protecting against cadmium poisoning [47]. In our

hands, although zinc has a significant, but somewhat

modest, effect on the different cell types tested, manga-

nese is by far the most effective competitor. This was

demonstrated by intracellular cadmium concentration

measurements using 109Cd. Adding a 10-fold manga-

nese molar excess compared with cadmium to the

culture medium resulted in a five-fold decrease in

intracellular cadmium concentration [48]. Under these

conditions, cells are phenotypically protected from

cadmium toxicity and ERK is not activated. This is a

strong indication that cadmium entry into cells is

necessary for sustained ERK activation, and that this

activation is tightly associated with cell death. Surpris-

ingly, the status of the Ras–Raf–MEK–ERK cascade

following cadmium treatment has not yet been pub-

lished. Our unpublished data indicate that in HEK293

cells exposed to 2 lM cadmium, c-Raf is maximally

phosphorylated after 8 h and then reaches undetect-

able levels after 48 h; MEK1 ⁄ 2 also reach a plateau

after 8 h and then the level remains unchanged; ERK

phosphorylation increases from 8 to 72 h following

cadmium addition (P. Martin et al., unpublished

results). This suggests that kinases upstream of ERK,

at least up to c-Raf, are also triggered by cadmium.

Rapid and transient activation of ERKby protein kinase C (PKC) prior tocadmium treatment protects cellsfrom cadmium intoxication

Sustained ERK activation following cadmium intoxica-

tion is linked to cell death. Could other ERK activa-

tors also play a role in the onset of cell toxicity

following cadmium treatment? Cells have been pre-

treated with the phorbol ester 4b-phorbol 12-myristate



13-acetate (PMA), which activates conventional, as

well as novel, PKCs. PKCs play a key role in a vast

array of cellular signaling, including the activation of

ERK [49]. PMA mimics classical cell stimulation by

growth factors, and PKC activation results in a strong

and transient activation of ERK [50]. Interestingly,

when HEK293 cells are pretreated with PMA, their

susceptibility to cadmium is substantially decreased.

The delayed and sustained cadmium-dependent ERK

activation is also strongly reduced under these condi-

tions. Prolonged PKC activation by phorbol esters is

known to downregulate PKC for at least 24 h [51].

However, it has been shown that the protective effect

of PMA on cadmium is not due to its downregulation,

but rather to a hit-and-run effect on PKC [22].

Accordingly, when GF 109203X, a broad-spectrum

PKC inhibitor, was applied before or together with

cadmium, ERK activation increased more than three-

fold and cells became more sensitive to cadmium, as

evidenced by a decrease in absolute cell number and a

more pronounced rounded phenotype [22]. Although

earlier work has reported that GF 109203X is also a

potent inhibitor of ribosomal S6 kinase (RSK) [52], a

kinase downstream of ERK potentially retro-inhibiting

the MAPK cascade, it is nevertheless reasonable to

assume that this GF 109203X effect, opposed to that

of PMA, indicates that PKC activation is not

responsible for the delayed and sustained activation of

ERK following cadmium treatment. Rather, PKC

appears to play a protective role in cells exposed to

cadmium, as its early activation by PMA reduces both

delayed ERK activation and cadmium toxicity,

whereas blocking its activity has the opposite effect

(Fig. 2) [22]. In short, PKC early activation has a pro-

tective effect on cadmium toxicity in HEK293 cells

[22,49,53]. It is intriguing to note that earlier work on

cells from rat pulmonary epithelium indicated that pre-

treatment with PMA has no significant effect on apop-

tosis after a 12 h exposure to 3 and 10 lM cadmium

[25]. Surprisingly, similar experiments on rat alveolar

epithelial cells showed that exposure to a relatively

high level of cadmium (20 lM for 24 h) resulted in

PKC activation, and that GF 109203X protected cells

from apoptosis [54]. However, in this cell system, PKC

activity is already high in control conditions, and

cadmium does not activate PKC further [25], or only

marginally with high cadmium doses [54]. In contrast,

PKC activity is low in HEK293 cells under normal

Fig. 2. PKC activity inhibits ERK activation in response to cadmium.

Exponentially growing HEK cells were used as a control or treated

for 24 h with 2 lM CdCl2. As seen by western blotting analysis on

the upper panel with an anti-ERK-P IgG, this treatment resulted in

the appearance of the activated ⁄ phosphorylated forms of ERK.

Cells were also cotreated with 100 ngÆmL–1 PMA and cadmium

(PMA ⁄ Cd 24h) or pretreated with PMA for 8 h and then treated

with cadmium for 24 h. In both cases, ERK activation was signifi-

cantly lower than for cadmium alone. Alternatively, cells were

cotreated with 2.5 lM of the PKC inhibitor GF 109203X and

100 ngÆmL–1 PMA and cadmium (GFX ⁄ PMA ⁄ Cd 24h) or pretreated

with GF 109203X and PMA for 8 h and then treated with cadmium

for 24 h. In both cases, ERK activation was significantly higher than

for cadmium alone. The lower panel displays the level of total ERK

proteins, which remained unchanged.

Fig. 3. Proposition of a schematic model for the ERK response to

cadmium poisoning. Cadmium enters cells through the ZIP8 trans-

porter, whose activity may be under the negative control of PKC

[22]. Once in the cells, cadmium accumulation results in a progres-

sive increase in [Ca2+]cyt and subsequently to production of ROS

[41]. ROS participate in both the activation of ERK [32] and in the

inhibition of dual-specificity phosphatases, including MKP3 [37].

This may participate in the constitutive and long-term activation of

ERK [22]. This activation ultimately results in the activation of casp-

ases-3 and -8 and in apoptosis [46]. ERK activation has also been

involved in necrotic cell death [21] and in autophagy [42]. Other

parallel pathways are also present, as ERK inhibition only partially

protects from cell death. In these pathways, PKC has been shown

to play a prodeath role [25,54]. The dashed lines represent indirect

pathways.



growth conditions, and is strongly activated following

a 24 h 2 lm cadmium treatment [22]. This could

explain the reasons for these discrepancies.

Cadmium transport

The zinc transporter, ZIP8, is the main transporter

hijacked by cadmium to enter cells [55]. Several lines of

evidence indicate that PKC activation could inhibit

ZIP8 activity. Indeed, PMA treatment of cells exposed

to cadmium, in addition to activation of PKC, results in

a lower intracellular concentration of cadmium and bet-

ter cell survival. An increased cell survival in the pres-

ence of cadmium is also obtained by ZIP8 knockdown

with specific small interfering RNAs [22]. Finally, three

potential sites that could be phosphorylated by PKC

have been identified on the ZIP8 amino acid sequence.

Together, these results are compatible with a model in

which PKC would negatively regulate ZIP8 transporter

activity via direct phosphorylation, and limit cadmium

entry into cells. Such a direct regulation of transporter

activities by PKC has already been reported [56,57]. Fur-

ther work will be required to validate this hypothesis.

Conclusion

Cadmium exposure results in a complex response of

cells. It involves internal calcium increases [40], oxida-

tive mechanisms [30], gene reprogramming [58], protein

degradation [59] and kinase activation [60]. Although

no definitive picture is currently available for ERK

function in response to cadmium, its sustained activa-

tion has been associated with cadmium-induced cell

death in several systems [16,18–21]. Particularly, phar-

macological inhibition of low-dose cadmium-induced

ERK activation diminishes cell death, whereas MKP3

exogenous expression diminishes cell death following

cadmium intoxication (P. Martin and P. Pognonec,

unpublished data). This sustained ERK activation is

probably the result of ROS-dependent inhibition of

ERK phosphatases, as well as upstream ERK activa-

tion. How ERK sustained activation ultimately results

in cell death remains poorly understood. Caspase-3

and -8 activation has been demonstrated following

cadmium exposure and sustained ERK activation,

reminiscent of the ERK-dependent caspase-3 and -8

activation reported in a synthetic system in which a

long-term activation of ERK can be induced [46].

Taken together with the other biological systems pre-

sented in this minireview series, cadmium poisoning is

a situation in which ERK activation, in addition to its

more traditionally described functions, appears to act

as a proapoptotic factor. Figure 3 is a schematic repre-

sentation of our current understanding of ERK

involvement in this process.

Acknowledgement

We thank Kim Boulukos for kindly accepting to

critically read this manuscript, despite her recent

career shift from a full-time scientist to a full-time

sculptor.

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ERK and cell death: cadmium toxicity, sustained ERK activation and cell death

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