Functional role of p21 during the cellular response to stress

A wide range of stress stimuli, including oxidants, genotoxins, metabolic deficiencies, and irradiation, have been shown to induce expression of the cyclin-dependent kinase inhibitor p21. Among the best characterized mediators of p21 induction by stress is the tumor suppressor gene p53, which acts as a transcriptional activator to enhance the expression of the p21 gene. However, many other mechanisms involving transcriptional and posttranscriptional events have been found to participate in the elevation of p21 levels by stressful agents. The significance of the stress-mediated elevation in p21 expression is not fully understood, but it is clear that alterations in p21 expression impact on the ability of the cell to survive the insult. Although a large number of reports have demonstrated correlations between the expression of p21 and cellular outcome, this review will focus only on those reports where the role of p21 in a given stress paradigm has been investigated directly, through use of different strategies to manipulate p21 expression followed by assessment of the consequences of altered p21 expression on cell survival. The majority of such studies have revealed that p21 exerts a protective function against stress, and this property appears to rely, at least in part, on the ability of p21 to suppress cell proliferation. A few exceptions to this universal protective influence of p21 have also been observed and will be discussed.

Serum withdrawal and etoposide induce apoptosis in human lung carcinoma cell line A549 via distinct pathways

The molecular events associated with apoptosis induced by two distinct triggers (1) serum withdrawal and (2) etoposide treatment were investigated in the human lung carcinoma cell line A549. Although both serum withdrawal and etoposide treatment resulted in internucleosomal DNA fragmentation, the morphologic features were distinct. Serum deprived apoptotic cells appeared small, round and refractile, with little evidence of nuclear fragmentation; etoposide-induced apoptotic cells appeared enlarged and flattened and displayed prominent nuclear fragmentation. p53 and p21/waf1 protein levels were elevated in etoposide-treated cells, but not in cells subjected to serum with-drawal. Apoptosis induced by both treatments was accompanied by a significant reduction in Rb protein levels. However, etoposide treatment led to hypo-phosphorylation of Rb, while serum withdrawal did not alter the Rb phosphorylation pattern. Serum withdrawal-induced apoptosis was correlated with activation of JNK and suppression of ERK activities, while both JNK and ERK activities were slightly elevated during etoposid- induced apoptosis. Together, these results support the hypothesis that apoptosis induced by serum withdrawal and etoposide treatment occurs through different pathways and involves distinct mediators.

Involvement of the MAP kinase pathways in induction of GADD45 following UV radiation

The p53-regulated stress-inducible gene GADD45 has been shown to participate in cellular response to DNA damage, including cell cycle checkpoint, apoptosis, and DNA repair. However, the regulation of GADD45 expression is complex and may involve both p53-dependent and -independent pathways. Recent findings have demonstrated that the p53-independent induction of GADD45 is mainly regulated by the transcription factors Oct-1 and NF-YA, which directly bind to their consensus motifs located at the GADD45 promoter region. Here, we report that mitogen-activated protein (MAP) kinases are involved in the induction of the GADD45 promoter after DNA damage. Inhibition of JNK1 and ERK kinase activities either by expression of the dominant negative mutant JNK1 or by treatment with a selective chemical inhibitor of ERK (PD098059) substantially abrogates the UV induction of the GADD45 promoter. In contrast, a p38 kinase inhibitor (SB203580) has little effect on GADD45 induction by UV. In addition, the GADD45 promoter is strongly activated following expression of JNK1; Raf-1, which is an upstream activator of the ERK pathway; or MEK1, an upstream activator of both the ERK and the JNK pathways. Activation of the GADD45 promoter by MAP kinases does not require normal p53 function. Interestingly, the MAP kinase-regulatory effect appears to be mediated via OCT-1 and CAAT motifs since disruption of these sites abrogates activation of the GADD45 promoter by MAP kinases. Therefore, these findings indicate that the MAP kinase pathways are involved in the regulation of the p53-independent induction of the GADD45 promoter, probably via interaction with transcription factors that directly bind to OCT-1 and CAAT motifs.

Loss of HuR is linked to reduced expression of proliferative genes during replicative senescence

Cellular aging is accompanied by alterations in gene expression patterns. Here, using two models of replicative senescence, we describe the influence of the RNA-binding protein HuR in regulating the expression of several genes whose expression decreases during senescence. We demonstrate that HuR levels, HuR binding to target mRNAs encoding proliferative genes, and the half-lives of such mRNAs are lower in senescent cells. Importantly, overexpression of HuR in senescent cells restored a "younger" phenotype, while a reduction in HuR expression accentuated the senescent phenotype. Our studies highlight a critical role for HuR during the process of replicative senescence.

Protective role for c-Jun in the cellular response to DNA damage

c-Jun, a member of the activation protein 1 (AP-1) family of transcription factors, has been implicated in the regulation of many important biological processes including cell cycle progression, transformation, differentiation, and apoptosis. Accordingly, its expression and function are upregulated in response to diverse stimuli including mitogens and a wide range of stresses. Transcriptional activation of the c-Jun protein is dependent on its phosphorylation at Ser-63 and Ser-73, a process mediated by c-Jun N-terminal kinase. Active c-Jun is required for AP-1 transactivation and c-Jun-mediated transformation, but its role during stress remains unclear as both pro-apoptotic and pro-survival effects of c-Jun have been observed. Here we investigated the importance of c-Jun N-terminal phosphorylation in influencing the sensitivity of human T98G glioblastoma cells to a variety of cytotoxic agents. Stable expression of a nonphosphorylatable dominant negative protein c-Jun(S63A,S73A) markedly inhibited the activation of AP-1-driven transcription and greatly increased the cytotoxic effects of DNA-damaging agents associated with enhanced apoptosis. However, the same cells expressing the mutant Jun protein did not differ from parental cells in their sensitivity to several non-DNA-damaging cytotoxic agents. Our results suggest that activated c-Jun has a selective role in protecting human tumor cells from apoptosis induced by DNA damage.

Oxidative stress-induced phospholipase C-gamma 1 activation enhances cell survival

Phospholipase C-gamma1 (PLC-gamma1) is rapidly activated in response to growth factor stimulation and plays an important role in regulating cell proliferation and differentiation through the generation of the second messengers diacylglycerol and inositol 1,4,5-trisphosphate, leading to the activation of protein kinase C (PKC) and increased levels of intracellular calcium, respectively. Given the existing overlap between signaling pathways that are activated in response to oxidant injury and those involved in responding to proliferative stimuli, we investigated the role of PLC-gamma1 during the cellular response to oxidative stress. Treatment of normal mouse embryonic fibroblasts (MEF) with H2O2 resulted in time- and concentration-dependent tyrosine phosphorylation of PLC-gamma1. Phosphorylation could be blocked by pharmacological inhibitors of Src family tyrosine kinases or the epidermal growth factor receptor tyrosine kinase, but not by inhibitors of the platelet-derived growth factor receptor or phosphatidylinositol 3-kinase. To investigate the physiologic relevance of H2O2-induced tyrosine phosphorylation of PLC-gamma1, we compared survival of normal MEF and PLC-gamma1-deficient MEF following exposure to H2O2. Treatment of PLC-gamma1-deficient MEF with H2O2 resulted in rapid cell death, whereas normal MEF were resistant to the stress. Pretreatment of normal MEF with a selective pharmacological inhibitor of PLC-gamma1, or inhibitors of inositol trisphosphate receptors and PKC, increased their sensitivity to H2O2, whereas treatment of PLC-gamma1-deficient MEF with agents capable of directly activating PKC and enhancing calcium mobilization significantly improved their survival. Finally, reconstitution of PLC-gamma1 protein expression in PLC-gamma1-deficient MEF restored cell survival following H2O2 treatment. These findings suggest an important protective function for PLC-gamma1 activation during the cellular response to oxidative stress.

Jun NH2-terminal kinase phosphorylation of p53 on Thr-81 is important for p53 stabilization and transcriptional activities in response to stress

The p53 tumor suppressor protein plays a key role in the regulation of stress-mediated growth arrest and apoptosis. Stress-induced phosphorylation of p53 tightly regulates its stability and transcriptional activities. Mass spectrometry analysis of p53 phosphorylated in 293T cells by active Jun NH2-terminal kinase (JNK) identified T81 as the JNK phosphorylation site. JNK phosphorylated p53 at T81 in response to DNA damage and stress-inducing agents, as determined by phospho-specific antibodies to T81. Unlike wild-type p53, in response to JNK stimuli p53 mutated on T81 (T81A) did not exhibit increased expression or concomitant activation of transcriptional activity, growth inhibition, and apoptosis. Forced expression of MKP5, a JNK phosphatase, in JNK kinase-expressing cells decreased T81 phosphorylation while reducing p53 transcriptional activity and p53-mediated apoptosis. Similarly transfection of antisense JNK 1 and -2 decreased T81 phosphorylation in response to UV irradiation. More than 180 human tumors have been reported to contain p53 with mutations within the region that encompasses T81 and the JNK binding site (amino acids 81 to 116). Our studies identify an additional mechanism for the regulation of p53 stability and functional activities in response to stress.

Gadd153 sensitizes cells to endoplasmic reticulum stress by down-regulating Bcl2 and perturbing the cellular redox state

gadd153, also known as chop, is a highly stress-inducible gene that is robustly expressed following disruption of homeostasis in the endoplasmic reticulum (ER) (so-called ER stress). Although all reported types of ER stress induce expression of Gadd153, its role in the stress response has remained largely undefined. Several studies have correlated Gadd153 expression with cell death, but a mechanistic link between Gadd153 and apoptosis has never been demonstrated. To address this issue we employed a cell model system in which Gadd153 is constitutively overexpressed, as well as two cell lines in which Gadd153 expression is conditional. In all cell lines, overexpression of Gadd153 sensitized cells to ER stress. Investigation of the mechanisms contributing to this effect revealed that elevated Gadd153 expression results in the down-regulation of Bcl2 expression, depletion of cellular glutathione, and exaggerated production of reactive oxygen species. Restoration of Bcl2 expression in Gadd153-overexpressing cells led to replenishment of glutathione and a reduction in levels of reactive oxygen species, and it protected cells from ER stress-induced cell death. We conclude that Gadd153 sensitizes cells to ER stress through mechanisms that involve down-regulation of Bcl2 and enhanced oxidant injury.

Requirement for ERK activation in cisplatin-induced apoptosis

Cisplatin activates multiple signal transduction pathways involved in coordinating cellular responses to stress. Here we demonstrate a requirement for extracellular signal-regulated protein kinase (ERK), a member of the mitogen-activated protein kinase family in mediating cisplatin-induced apoptosis of human cervical carcinoma HeLa cells. Cisplatin treatment resulted in dose- and time- dependent activation of ERK. That elevated ERK activity contributed to cell death by cisplatin was supported by several observations: 1) PD98059 and U0126, chemical inhibitors of the MEK/ERK signaling pathway, prevented apoptosis; 2) pretreatment of cells with TPA, an activator of the ERK pathway, enhanced their sensitivity to cisplatin; 3) suramin, a growth factor receptor antagonist that greatly suppressed ERK activation, likewise inhibited cisplatin-induced apoptosis; and, finally, 4) HeLa cell variants selected for cisplatin resistance showed reduced activation of ERK following cisplatin treatment. Cisplatin-induced apoptosis was associated with cytochrome c release and subsequent caspase-3 activation, both of which could be prevented by treatment with the MEK inhibitors. However, the caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone protected HeLa cells against apoptosis without affecting ERK activation. Taken together, our findings suggest that ERK activation plays an active role in mediating cisplatin-induced apoptosis of HeLa cells and functions upstream of caspase activation to initiate the apoptotic signal.

Enhanced sensitivity and long-term G2 arrest in hydrogen peroxide-treated Ku80-null cells are unrelated to DNA repair defects

While the Ku complex, comprised of Ku70 and Ku80, is primarily involved in the repair of DNA double-strand breaks, it is also believed to participate in additional cellular processes. Here, treatment of embryo fibroblasts (MEFs) derived from either wild-type or Ku80-null (Ku80(-/-)) mice with various stress agents revealed that hydrogen peroxide (H(2)O(2)) was markedly more cytotoxic for Ku80(-/-) MEFs and led to their long-term accumulation in the G2 phase. This differential response was not due to differences in DNA repair, since H(2)O(2)-triggered DNA damage was repaired with comparable efficiency in both Wt and Ku80(-/-) MEFs, but was associated with differences in the expression of important cell cycle regulatory genes. Our results support the notion that Ku80-mediated cytoprotection and G2-progression are not only dependent on the cell's DNA repair but also may reflect Ku80's influence on additional cellular processes such as gene expression.

Peroxynitrite activates the phosphoinositide 3-kinase/Akt pathway in human skin primary fibroblasts

Peroxynitrite is a potent oxidizing and nitrating species formed in a diffusion-limited reaction between nitrogen monoxide and superoxide. It induces apoptosis through unknown mechanisms and is believed to interfere with receptor tyrosine kinase signalling through nitration of tyrosine residues. One pathway emanating from receptor tyrosine kinases is that leading to activation of the anti-apoptotic kinase Akt. In the present study we provide evidence that peroxynitrite, administered to cells using two different delivery systems, results in the dose- and time-dependent activation of Akt. Akt activation is rapid and followed by phosphorylation of glycogen synthase kinase-3, an established substrate of Akt. Akt activation is inhibited in the presence of the phosphoinositide 3-kinase (PI-3K) inhibitors wortmannin and LY294002, and by treatment with the platelet-derived growth factor (PDGF) receptor (PDGFR) inhibitor AG1295, indicating a requirement for PDGFR and PI-3K in mediating peroxynitrite-induced Akt activation. Accordingly, the PDGFR-A and PDGFR-B isoforms were shown to undergo rapid tyrosine phosphorylation on treatment with peroxynitrite. Prior exposure of cells to peroxynitrite interferes with PDGF-induced Akt phosphorylation. Our findings suggest that Akt activation occurs as an acute response to peroxynitrite treatment and could play an important role in influencing cell survival and/or alter the cellular response to other growth regulatory signals.

Oxidants, oxidative stress and the biology of ageing

Living in an oxygenated environment has required the evolution of effective cellular strategies to detect and detoxify metabolites of molecular oxygen known as reactive oxygen species. Here we review evidence that the appropriate and inappropriate production of oxidants, together with the ability of organisms to respond to oxidative stress, is intricately connected to ageing and life span.

Down-regulation of cyclin D1 expression by prostaglandin A(2) is mediated by enhanced cyclin D1 mRNA turnover

Prostaglandin A(2) (PGA(2)), an experimental chemotherapeutic agent, causes growth arrest associated with decreased cyclin D1 expression in several cancer cell lines. Here, using human non-small-cell lung carcinoma H1299 cells, we investigated the mechanisms whereby PGA(2) down-regulates cyclin D1 expression. Transcription rates of the cyclin D1 gene, studied using a cyclin D1 promoter-luciferase construct and nuclear run-on assays, were not affected by PGA(2) treatment. Instead, the cyclin D1 mRNA was rendered unstable after exposure to PGA(2). Since the stability of labile mRNA is modulated through binding of proteins to specific mRNA sequences, we sought to identify protein(s) recognizing the cyclin D1 mRNA. In electrophoretic mobility-shift assays using radiolabeled RNA probes derived from different regions of cyclin D1 mRNA, we observed that (i) lysates prepared from PGA(2)-treated cells exhibited enhanced protein-cyclin D1 RNA complex formation; (ii) the kinetics of complex formation correlated closely with that of cyclin D1 mRNA loss; and (iii) binding occurred within a 390-base cyclin D1 3' untranslated region (UTR) (K12). This binding activity could be cross-linked, revealing proteins ranging from 30 to 47 kDa. The RNA-binding protein AUF1, previously associated with the degradation of target mRNAs, bound cyclin D1 mRNA, because anti-AUF1 antibodies were capable of supershifting or immunoprecipitating cyclin D1 mRNA-protein complexes. Finally, insertion of K12 in the 3'UTR of reporter genes markedly reduced the expression and half-life of the resulting chimeric mRNAs in transfected, PGA(2)-treated cells. Our data demonstrate that PGA(2) down-regulates cyclin D1 expression by decreasing cyclin D1 mRNA stability and implicates a 390-base element in the 3'UTR in this regulation.

c-Jun N-terminal kinase is essential for growth of human T98G glioblastoma cells

The c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) pathway is activated by numerous cellular stresses. Although it has been implicated in mediating apoptosis and growth factor signaling, its role in regulating cell growth is not yet clear. Here, the influence of JNK on basal (unstimulated) growth of human tumor glioblastoma T98G cells was investigated using highly specific JNK antisense oligonucleotides to inhibit JNK expression. Transient depletion of either JNK1 or JNK2 suppressed cell growth associated with an inhibition of DNA synthesis and cell cycle arrest in S phase. The growth-inhibitory potency of JNK2 antisense ((JNK)2 IC(50) = 0.14 micrometer) was greater than that of JNK1 antisense ((JNK)1 IC(50) = 0.37 micrometer), suggesting that JNK2 plays a dominant role in regulating growth of T98G cells. Indeed, JNK2 antisense-treated populations exhibited greater inhibition of DNA synthesis and accumulation of S-phase cells than did the JNK1 antisense-treated cultures, with a significant proportion of these cells detaching from the tissue culture plate. JNK2 (but not JNK1) antisense-treated cultures exhibited marked elevation in the expression of the cyclin-dependent kinase inhibitor p21(cip1/waf1) accompanied by inhibition of Cdk2/Cdc2 kinase activities. Taken together, these results indicate that JNK is required for growth of T98G cells in nonstress conditions and that p21(cip1/waf1) may contribute to the sustained growth arrest of JNK2-depleted T98G cultures.

Transforming growth factor-beta 1 suppresses serum deprivation-induced death of A549 cells through differential effects on c-Jun and JNK activities

Transforming growth factor (TGF)-beta1, a pleiotropic cytokine involved in regulating growth and differentiation, can exert both pro-apoptotic and anti-apoptotic effects depending on the cell type or circumstances. We observed that TGF-beta1 blocked apoptosis resulting from serum withdrawal in A549 human lung carcinoma cells. This was associated with suppression of JNK activation that occurs concomitant with the onset of apoptosis in the absence of TGF-beta1, suggesting that JNK plays an active role in the death process and that TGF-beta1 exerts its protective influence by altering JNK activity. Overexpression of a dominant negative mutant form of SEK1, an upstream activator of JNK, likewise suppressed JNK activation and inhibited apoptosis. Investigation of early events following TGF-beta1 treatment revealed an early induction and phosphorylation of c-Jun that was absent in cells subjected to serum withdrawal alone. That TGF-beta1-induced expression of c-Jun is important for survival was supported by the finding that overexpression of non-phosphosphorylatable dominant negative mutant c-Jun, c-Jun(S73A), attenuated the protective influence of TGF-beta1. Our findings suggest that JNK activation is a late but essential event in serum deprivation-induced apoptosis in A549 cells. TGF-beta1 prevents apoptosis, in part, through the early induction and phosphorylation of c-Jun, which in turn results in attenuated JNK activation.

Epidermal growth factor receptor-dependent Akt activation by oxidative stress enhances cell survival

The serine/threonine kinase Akt (also known as protein kinase B) is activated in response to various stimuli by a mechanism involving phosphoinositide 3-kinase (PI3-K). Akt provides a survival signal that protects cells from apoptosis induced by growth factor withdrawal, but its function in other forms of stress is less clear. Here we investigated the role of PI3-K/Akt during the cellular response to oxidant injury. H(2)O(2) treatment elevated Akt activity in multiple cell types in a time- (5-30 min) and dose (400 microM-2 mm)-dependent manner. Expression of a dominant negative mutant of p85 (regulatory component of PI3-K) and treatment with inhibitors of PI3-K (wortmannin and LY294002) prevented H(2)O(2)-induced Akt activation. Akt activation by H(2)O(2) also depended on epidermal growth factor receptor (EGFR) signaling; H(2)O(2) treatment led to EGFR phosphorylation, and inhibition of EGFR activation prevented Akt activation by H(2)O(2). As H(2)O(2) causes apoptosis of HeLa cells, we investigated whether alterations of PI3-K/Akt signaling would affect this response. Wortmannin and LY294002 treatment significantly enhanced H(2)O(2)-induced apoptosis, whereas expression of exogenous myristoylated Akt (an activated form) inhibited cell death. Constitutive expression of v-Akt likewise enhanced survival of H(2)O(2)-treated NIH3T3 cells. These results suggest that H(2)O(2) activates Akt via an EGFR/PI3-K-dependent pathway and that elevated Akt activity confers protection against oxidative stress-induced apoptosis.

Age-related decline in Ras/ERK mitogen-activated protein kinase cascade is linked to a reduced association between Shc and EGF receptor

Numerous studies have demonstrated that the proliferative capacity of cells declines with age. Using rat primary hepatocytes as a model system, we recently demonstrated that this age-related decline in the proliferative response to mitogenic stimulation is associated with decreased activities of both extracellular signal-regulated kinase (ERK) and p70 S6 kinase (p70(S6k)). To unravel the molecular basis for age-related defects in the ERK pathway, we have now characterized the upstream signaling events that occur after epidermal growth factor (EGF) stimulation in young and aged hepatocytes. As previously noted for ERK, the activities of both MEK (the kinase immediately upstream of ERK) and Ras following EGF stimulation were significantly lower in aged hepatocytes. An examination of the EGF receptor (EGFR) revealed a similar amount of EGFR in the two age groups. Likewise, EGFR and Shc, an adaptor protein that plays a crucial role in linking EGFR to Ras activation, underwent tyrosine phosphorylation to a similar degree in both young and aged hepatocytes. However, in aged cells Shc was unable to form stable complexes with EGFR after EGF stimulation. Our results suggest that a decrease in the association between Shc and EGFR in aged cells underlies the age-related declines in the ERK signaling cascade and in proliferative capacity.

Inhibition of c-Jun N-terminal kinase 2 expression suppresses growth and induces apoptosis of human tumor cells in a p53-dependent manner

c-Jun N-terminal kinase (JNK) plays a critical role in coordinating the cellular response to stress and has been implicated in regulating cell growth and transformation. To investigate the growth-regulatory functions of JNK1 and JNK2, we used specific antisense oligonucleotides (AS) to inhibit their expression. A survey of several human tumor cell lines revealed that JNKAS treatment markedly inhibited the growth of cells with mutant p53 status but not that of cells with normal p53 function. To further examine the influence of p53 on cell sensitivity to JNKAS treatment, we compared the responsiveness of RKO, MCF-7, and HCT116 cells with normal p53 function to that of RKO E6, MCF-7 E6, and HCT116 p53(-/-), which were rendered p53 deficient by different methods. Inhibition of JNK2 (and to a lesser extent JNK1) expression dramatically reduced the growth of p53-deficient cells but not that of their normal counterparts. JNK2AS-induced growth inhibition was correlated with significant apoptosis. JNK2AS treatment induced the expression of the cyclin-dependent kinase inhibitor p21(Cip1/Waf1) in parental MCF-7, RKO, and HCT116 cells but not in the p53-deficient derivatives. That p21(Cip1/Waf1) expression contributes to the survival of JNK2AS-treated cells was supported by additional experiments demonstrating that p21(Cip1/Waf1) deficiency in HCT116 cells also results in heightened sensitivity to JNKAS treatment. Our results indicate that perturbation of JNK2 expression adversely affects the growth of otherwise nonstressed cells. p53 and its downstream effector p21(Cip1/Waf1) are important in counteracting these detrimental effects and promoting cell survival.

Activation of the cholesterol pathway and Ras maturation in response to stress

All cells depend on sterols and isoprenoids derived from mevalonate (MVA) for growth, differentiation, and maintenance of homeostatic functions. In plants, environmental insults like heat and sunlight trigger the synthesis of isoprene, also derived from MVA, and this phenomenon has been associated with enhanced tolerance to heat. Here, we show that in human prostate adenocarcinoma PC-3M cells heat shock leads to activation of the MVA pathway. This is characterized by a dose- and time-dependent elevation in 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) activity, enhanced sterol and isoprenoid synthesis, and increased protein prenylation. Furthermore, prenylation and subsequent membrane localization of Ras, a central player in cell signaling, was rapidly induced following heat stress. These effects were dose-dependent, augmented with repeated insults, and were prevented by culturing cells in the presence of lovastatin, a competitive inhibitor of HMGR. Enhanced Ras maturation by heat stress was also associated with a heightened activation of extracellular signal-regulated kinase (ERK), a key mediator of both mitogenic and stress signaling pathways, in response to subsequent growth factor stimulation. Thus, activation of the MVA pathway may constitute an important adaptive host response to stress, and have significant implications to carcinogenesis.

gadd45 is not required for activation of c-Jun N-terminal kinase or p38 during acute stress

Cells respond to environmental stress with activation of c-Jun N-terminal kinase (JNK) and p38. Recent studies have implicated Gadd45 and two related proteins, MyD118/Gadd45beta and CR6/Gadd45gamma, as initiators of JNK/p38 signaling via their interaction with an upstream kinase MTK1. It was proposed that stress-induced expression of the Gadd45-related proteins leads to MTK1 activation and subsequent JNK/p38 activation. Using embryo fibroblasts from gadd45-null mice, we have addressed the requirement for Gadd45 in mediating JNK/p38 activation during acute stress. Comparison of JNK/p38 activities in response to methyl methanesulfonate, hydrogen peroxide, UVC irradiation, sorbitol, and anisomycin treatment of gadd45(+/+) and gadd45(-/-) fibroblasts revealed no deficiency in JNK/p38 activation in gadd45(-/-) fibroblasts. In addition, in wild type cells, JNK and p38 activation significantly preceded gadd45 induction with all stresses. Examination of myd118/gadd45beta and cr6/gadd45gamma expression in gadd45(+/+) and gadd45(-/-) fibroblasts revealed similar induction patterns in the two cell types, which, like gadd45 expression, was delayed relative to JNK/p38 activation. We conclude that gadd45 expression is not required for activation of JNK/p38 by environmental stresses, nor are stress-induced increases in myd118/gadd45beta and cr6/gadd45gamma expression necessary for kinase activation in response to such insults.

The antiapoptotic decoy receptor TRID/TRAIL-R3 is a p53-regulated DNA damage-inducible gene that is overexpressed in primary tumors of the gastrointestinal tract

Both DR4 and DR5 have recently been identified as membrane death receptors that are activated by their ligand TRAIL to engage the intracellular apoptotic machinery. TRID (also named as TRAIL-R3) is an antagonist decoy receptor and lacks the cytoplasmic death domain. TRID protects from TRAIL-induced apoptosis by competing with DR4 and DR5 for binding to TRAIL. TRID has been shown to be overexpressed in normal human tissues but not in malignantly transformed cell lines. DR5 is a p53-regulated gene and we have recently reported that DR5 expression is induced in response to genotoxic stress in both a p53-dependent and independent manner (Sheikh et al., 1998). In the current study, we demonstrate that TRID gene expression is also induced by the genotoxic agents ionizing radiation and methyl methanesulfonate (MMS) in predominantly p53 wild-type cells, whereas UV-irradiation does not induce TRID gene expression. Consistent with these results, exogenous wild-type p53 also upregulates the expression of endogenous TRID in p53-null cells. Thus, TRID appears to be a p53 target gene that is regulated by genotoxic stress in a p53-dependent manner. Using primary gastrointestinal tract (GIT) tumors and their matching normal tissue, we also demonstrate for the first time that TRID expression is enhanced in primary tumors of the GIT. It is, therefore, possible that TRID overexpressing GIT tumors may gain a selective growth advantage by escaping from TRAIL-induced apoptosis.

Serine protease inhibitor TPCK prevents Taxol-induced cell death and blocks c-Raf-1 and Bcl-2 phosphorylation in human breast carcinoma cells

The mechanism of Taxol-induced apoptosis was investigated in MCF-7 human breast carcinoma cells. Taxol-induced apoptosis was associated with phosphorylation of both c-Raf-1 and Bcl-2 and activation of ERK and JNK MAP kinases. The serine protease inhibitor N-tosyl-L-phenylalanine chloromethyl ketone (TPCK) effectively blocked apoptosis, but N-p-tosyl-L-lysine chloromethyl ketone (TLCK), another serine protease inhibitor, was without effect. TPCK treatment also prevented phosphorylation of c-Raf-1 and Bcl-2 in response to Taxol treatment. The serine protease inhibitor did not alter JNK activity, but it enhanced Taxol-induced activation of ERK1/2. Treatment of cells with the inhibitor of MEK activation, PD98059, prevented Taxol-induced ERK activation both in the presence and absence of TPCK, but did not influence survival of either Taxol- or Taxol plus TPCK-treated cells. In addition, PD98059 had no effect on c-Raf-1 or Bcl-2 phosphorylation. Thus, while the Taxol-induced phosphorylations of c-Raf-1 and Bcl-2 proteins appear to be coupled, these events can be disassociated from ERK1/2 activation. In summary, these findings suggest that phosphorylation of c-Raf-1 and Bcl-2, but not ERK1/2, are important signaling events in Taxol-induced apoptosis of MCF-7 breast cancer cells and that a TPCK inhibitable protease(s) is required for these processes.

Phorbol ester-induced mononuclear cell differentiation is blocked by the mitogen-activated protein kinase kinase (MEK) inhibitor PD98059

The purpose of this study was to evaluate whether the mitogen-activated protein kinase (MAPK) signaling pathway contributes to 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced mononuclear differentiation in the human myeloblastic leukemia ML-1 cells. Upon TPA treatment, the activity of ERK1 and ERK2 rapidly increased, with maximal induction between 1 and 3 h, while ERK2 protein levels remained constant. The activity of JNK1 was also significantly induced, with JNK1 protein levels increasing moderately during exposure to TPA. Treatment of cells with PD98059, a specific inhibitor of mitogen-activated protein kinase kinase (MEK), inhibited TPA-induced ERK2 activity. Furthermore, PD98059 completely blocked the TPA-induced differentiation of ML-1 cells, as assessed by a number of features associated with mononuclear differentiation including changes in morphology, nonspecific esterase activity, phagocytic ability, NADPH oxidase activity, mitochondrial respiration, and c-jun mRNA inducibility. We conclude that activation of the MEK/ERK signaling pathway is necessary for TPA-induced mononuclear cell differentiation.

Complexes containing activating transcription factor (ATF)/cAMP-responsive-element-binding protein (CREB) interact with the CCAAT/enhancer-binding protein (C/EBP)-ATF composite site to regulate Gadd153 expression during the stress response

Gadd153, also known as chop, encodes a member of the CCAAT/enhancer-binding protein (C/EBP) transcription factor family and is transcriptionally activated by cellular stress signals. We recently demonstrated that arsenite treatment of rat pheochromocytoma PC12 cells results in the biphasic induction of Gadd153 mRNA expression, controlled in part through binding of C/EBPbeta and two uncharacterized protein complexes to the C/EBP-ATF (activating transcription factor) composite site in the Gadd153 promoter. In this report, we identified components of these additional complexes as two ATF/CREB (cAMP-responsive-element-binding protein) transcription factors having differential binding activities dependent upon the time of arsenite exposure. During arsenite treatment of PC12 cells, we observed enhanced binding of ATF4 to the C/EBP-ATF site at 2 h as Gadd153 mRNA levels increased, and enhanced binding of ATF3 complexes at 6 h as Gadd153 expression declined. We further demonstrated that ATF4 activates, while ATF3 represses, Gadd153 promoter activity through the C/EBP-ATF site. ATF3 also repressed ATF4-mediated transactivation and arsenite-induced activation of the Gadd153 promoter. Our results suggest that numerous members of the ATF/CREB family are involved in the cellular stress response, and that regulation of stress-induced biphasic Gadd153 expression in PC12 cells involves the ordered, sequential binding of multiple transcription factor complexes to the C/EBP-ATF composite site.

Protective function of von Hippel-Lindau protein against impaired protein processing in renal carcinoma cells

The absence of functional von Hippel-Lindau (VHL) tumor suppressor gene leads to the development of neoplasias characteristic of VHL disease, including renal cell carcinoma (RCC). Here, we compared the sensitivity of RCC cells lacking VHL gene function with that of RCC cells expressing the wild-type VHL gene (wtVHL) after exposure to various stresses. While the response to most treatments was not affected by the VHL gene status, glucose deprivation was found to be much more cytotoxic for RCC cells lacking VHL gene function than for wtVHL-expressing cells. The heightened sensitivity of VHL-deficient cells was not attributed to dissimilar energy requirements or to differences in glucose uptake, but more likely reflects a lesser ability of VHL-deficient cells to handle abnormally processed proteins arising from impaired glycosylation. In support of this hypothesis, other treatments which act through different mechanisms to interfere with protein processing (i.e., tunicamycin, brefeldin A, and azetidine) were also found to be much more toxic for VHL-deficient cells. Furthermore, ubiquitination of cellular proteins was elevated in VHL-deficient cells, particularly after glucose deprivation, supporting a role for the VHL gene in ubiquitin-mediated proteolysis. Accordingly, the rate of elimination of abnormal proteins was lower in cells lacking a functional VHL gene than in wtVHL-expressing cells. Thus, pVHL appears to participate in the elimination of misprocessed proteins, such as those arising in the cell due to the unavailability of glucose or to other stresses.

Tumor promoter arsenite activates extracellular signal-regulated kinase through a signaling pathway mediated by epidermal growth factor receptor and Shc

Although arsenite is an established carcinogen, the mechanisms underlying its tumor-promoting properties are poorly understood. Previously, we reported that arsenite treatment leads to the activation of the extracellular signal-regulated kinase (ERK) in rat PC12 cells through a Ras-dependent pathway. To identify potential mediators of the upstream signaling cascade, we examined the tyrosine phosphorylation profile in cells exposed to arsenite. Arsenite treatment rapidly stimulated tyrosine phosphorylation of several proteins in a Ras-independent manner, with a pattern similar to that seen in response to epidermal growth factor (EGF) treatment. Among these phosphorylated proteins were three isoforms of the proto-oncoprotein Shc as well as the EGF receptor (EGFR). Tyrosine phosphorylation of Shc allowed for enhanced interactions between Shc and Grb2 as identified by coimmunoprecipitation experiments. The arsenite-induced tyrosine phosphorylation of Shc, enhancement of Shc and Grb2 interactions, and activation of ERK were all drastically reduced by treatment of cells with either the general growth factor receptor poison suramin or the EGFR-selective inhibitor tyrphostin AG1478. Down-regulation of EGFR expression through pretreatment of cells with EGF also attenuated ERK activation and Shc tyrosine phosphorylation in response to arsenite treatment. These results demonstrate that the EGFR and Shc are critical mediators in the activation of the Ras/ERK signaling cascade by arsenite and suggest that arsenite acts as a tumor promoter largely by usurping this growth factor signaling pathway.

Age-related changes in activation of mitogen-activated protein kinase cascades by oxidative stress

Oxidative stress is thought to play a critical role in aging and the pathogenesis of human disease. Molecular studies of both the physiologic function of oxidants and the deleterious consequences of exposure to oxidative stress have suggested that signal transduction cascades may be targeted by oxidants. Here, we review recent studies from this laboratory examining the molecular basis for the activation of mitogen-activated protein kinases by oxidative stress and the influence of these pathways on cellular fate. We examine the association between constitutive activation of extracellular signal-regulated kinase (ERK) and cancer, and discuss how such mechanisms may contribute to oxidant-induced skin carcinogenesis. We also address the relationship between a decline in activation of this same pathway and the aged phenotype. In this regard, we review evidence that a decrease in activation of ERK by growth factor correlates with a reduced proliferative capacity in the isolated rat hepatocyte model, and we provide new data indicating that the activation of the ERK pathway in response to oxidant stimuli is also decreased with age. Further evidence demonstrates that this alteration is associated with both a reduced mitogenic response and a decline in hepatocyte cell survival in response to oxidative stress. Finally, we provide perspective on how modulations in ERK signaling may interplay with other changes in signal transduction cascades in the aging process.

The cellular response to oxidative stress: influences of mitogen-activated protein kinase signalling pathways on cell survival

The mammalian response to stress is complex, often involving multiple signalling pathways that act in concert to influence cell fate. To examine potential interactions between the signalling cascades, we have focused on the effects of a model oxidant stress in a single cell type through an examination of the relative influences of mitogen-activated protein kinases (MAPKs) as well as two proposed apoptosis regulators, nuclear factor kappaB (NF-kappaB) and Bcl-2, in determining cell survival. Treatment of HeLa cells with H2O2 resulted in a time- and dose-dependent induction of apoptosis accompanied by sustained activation of all three MAPK subfamilies: extracellular signal-regulated protein kinase (ERK), c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) and p38. This H2O2-induced apoptosis was markedly enhanced when ERK2 activation was selectively inhibited by PD098059. Apoptosis decreased when JNK/SAPK activation was inhibited by expression of a dominant negative mutant form of SAPK/ERK kinase 1. Inhibition of the p38 kinase activity with p38-specific inhibitors SB202190 and SB203580 had no effect on cell survival. Because NF-kappaB activation by H2O2 is potentially related to both the ERK and JNK/SAPK signalling pathways, we examined the effects of inhibiting the activation of NF-kappaB; this interference had no effect on the cellular response to H2O2. Overexpression of the anti-apoptotic protein Bcl-2 significantly decreased the apoptosis seen after treatment with H2O2 without altering ERK or JNK/SAPK activities. Our results suggest that ERK and JNK/SAPK act in opposition to influence cell survival in response to oxidative stress, whereas neither p38 nor NF-kappaB affects the outcome. Bcl-2 acts independently and downstream of ERK and JNK/SAPK to enhance the survival of H2O2-treated cells.

Impairments in both p70 S6 kinase and extracellular signal-regulated kinase signaling pathways contribute to the decline in proliferative capacity of aged hepatocytes

Treatment of primary cultured hepatocytes from adult (6-month-old) rats with epidermal growth factor (EGF) results in a marked elevation in DNA synthesis, a response that is markedly attenuated in cells of aged (24-month-old) animals. Recently we demonstrated that this age-related attenuation is associated with reduced activation of extracellular signal-regulated kinase (ERK) in response to EGF treatment. In order to gain further insight into the mechanisms responsible for the age-related decline in this proliferative response, we investigated the expression and/or activities of several other regulatory molecules important for G1 to S phase progression in EGF-stimulated young and aged hepatocytes. Induction of cyclin D1 and activation of cyclin-dependent kinase 2 (CDK2) by EGF were found to be diminished in the aged cells. In young cells, prior treatment with rapamycin inhibited the induction of DNA synthesis and activation of CDK2 to levels similar to those seen in aged cells without inhibiting ERK activity and cyclin D1 expression. This suggested that a distinct, ERK-independent, rapamycin-sensitive pathway might also contribute to the proliferative response in hepatocytes and be subject to age-related alterations. Further studies demonstrated that activation of p70 S6 kinase (p70S6k), a rapamycin-sensitive event, following EGF treatment was 40% lower in aged hepatocytes relative to young cells, although the kinetics of activation did not differ in the two age groups. Western blot analysis for p70S6k expression revealed similar levels of proteins in young and aged cells. From these findings, we conclude that deficiencies in both the ERK and p70S6k signaling pathways contribute to the age-related decline in the proliferative response of hepatocytes.

p53-dependent elevation of p21Waf1 expression by UV light is mediated through mRNA stabilization and involves a vanadate-sensitive regulatory system

Exposure of mammalian cells to adverse stimuli triggers the expression of numerous stress response genes, many of which are presumed to enhance cell survival. In this study, we examined the mechanisms contributing to the induction of p21Waf1 by stress and its influence on the survival of cells subjected to short-wavelength UVC irradiation. UVC was found to elevate p21Waf1 mRNA expression in mouse embryonal fibroblasts (MEFs) and human colorectal carcinoma (RKO) cells in a p53-dependent manner. The lack of p21Waf1 induction in p53-deficient MEFs and RKO cells correlated with diminished cell survival following UVC irradiation. Unexpectedly, UVC treatment was also found to block the induction of p21Waf1 by various stress-inducing agents such as mimosine in the p53-deficient cells. Additional studies indicated that induction of p21Waf1 by UVC occurs primarily through enhanced mRNA stability rather than increased transcription; in p53-/- MEFs, failure to elevate p21Waf1 after treatment with UVC appears to be due to their inability to stabilize the p21Waf1 transcripts. Treatment of the p53-/- MEFs with the protein tyrosine phosphatase inhibitor vanadate reversed the UVC-induced block on p21Waf1 induction and resulted in their enhanced survival following irradiation. Thus, in cells bearing normal p53, UVC augments p21Waf1 expression by increasing the half-life of p21Waf1 mRNA; without p53, p21Waf1 mRNA remains unstable after UVC, apparently due to a pathway involving tyrosine phosphatase activity.

On the feasibility of MRI-guided focused ultrasound for local induction of gene expression

Gene therapy is a promising approach to the treatment of many forms of disease, including cancer. Of critical concern in its implementation is the ability to control the location, duration, and level of expression of the therapeutic gene. Here, we propose the use of local heat in combination with a heat-sensitive promoter to help accomplish this. Certain members of the family of heat shock protein (hsp) promoters display a regulation that depends strongly on temperature. We present a study of natural hsp70 induction in rat leg by MRI-guided focused ultrasound to investigate the hsp70 promoter as a possible candidate for use in control of gene expression with local heat. A temperature increase of 5-8 degrees C in the focal region for 45 minutes led to a differential expression of the hsp70 mRNA between the focal region and the surrounding tissue ranging from a factor of 3 to 67.

p27Kip1 overexpression causes apoptotic death of mammalian cells

p27KiP1, a member of the Cip/Kip family of cyclin-dependent kinase (cdk) inhibitors, has been implicated in mediating G1 arrest in response to a variety of growth inhibitory signals. Its importance in regulating cell growth is emphasized by the fact that mice lacking p27Kip1 are abnormally large and display hyperplasia of multiple tissues. However, these mice retain the ability to undergo G1 arrest in response to growth inhibitory signals, suggesting that p27KiP1 may serve other functions important for controlling tissue growth. In the present study, we utilized an adenoviral vector-based expression system to examine the consequences of p27Kip1 overexpression in the human carcinoma cell lines A549, HeLa and RKO, in human melanoma SK-MEL-110 cells, in human lung fibroblasts IMR90 and in the rat fibroblast line Rat1. We demonstrate that overexpression of p27Kip1 leads to apoptotic cell death in all cell types, and further show that ectopic expression of Bcl-2 can protect HeLa cells from apoptosis mediated by p27Kip1 overexpression. To our knowledge, this is the first study demonstrating that p27Kip1 can induce apoptosis. Our findings provide new insight into the possible functions of this growth regulatory protein, and support the potential utility of gene therapeutic approaches aimed at elevating p27Kip1 expression for treatment of human cancers.

gadd153/Chop10, a potential target gene of the transcriptional repressor ATF3

Recently, we demonstrated that the function of ATF3, a stress-inducible transcriptional repressor, is negatively regulated by a bZip protein, gadd153/Chop10. In this report, we present evidence that ATF3 can repress the expression of its own inhibitor, gadd153/Chop10. First, ATF3 represses a chloramphenicol acetyltransferase reporter gene driven by the gadd153/Chop10 promoter when assayed by a transfection assay in vivo and a transcription assay in vitro. Second, the gadd153/Chop10 promoter contains two functionally important binding sites for ATF3: an AP-1 site and a C/EBP-ATF composite site, a previously unidentified binding site for ATF3. The absence of either site reduces the ability of ATF3 to repress the promoter. Third, overexpression of ATF3 by transient transfection results in a reduction of the endogenous gadd153/Chop10 mRNA level. Fourth, as described previously, ATF3 is induced in the liver upon CCl4 treatment. Intriguingly, we show in this report that gadd153/Chop10 mRNA is not present in areas where ATF3 is induced. Taken together, these results strongly suggest that ATF3 represses the expression of gadd153/Chop10. The mutual negative regulation between ATF3 and gadd153/Chop10 is discussed.

Induction of mitogen-activated protein kinase phosphatase-1 during acute hypertension

Recently, we demonstrated that elevated blood pressure activates mitogen-activated protein (MAP) kinases in rat aorta. Here we provide evidence that the vascular response to acute hypertension also includes induction of MAP kinase phosphatase-1 (MKP-1), which has been shown to function in the dephosphorylation and inactivation of MAP kinases. Restraint or immobilization stress, which leads to a rapid rise in blood pressure, resulted in a rapid and transient induction of MKP-1 mRNA followed by elevated MKP-1 protein expression in rat aorta. That the induction of MKP-1 by restraint was due to the rise in blood pressure was supported by the finding that several different hypertensive agents (phenylephrine, vasopressin, and angiotensin II) were likewise capable of eliciting the response, and sodium nitroprusside, a nonspecific vasodilator agent that prevented the acute rise in blood pressure in response to the hypertensive agents, abrogated MKP-1 mRNA induction. The in vivo effects could not be mimicked by treatment of cultured aortic smooth muscle cells with similar doses of the hypertensive agents. These findings support a role for MKP-1 in the in vivo regulation of MAP kinase activity during hemodynamic stress.

Potentiation of heat stress-induced hsp70 expression in vivo by aspirin

Studies in cultured cells have demonstrated that non-steroidal anti-inflammatory agents can potentiate heat-induced hsp70 expression through activation of HSF1 to a DNA binding state. We investigated the influence of aspirin on hsp70 expression in intact rats subjected to heat stress. Rats were injected intraperitoneally either with aspirin (100 mg/kg) or vehicle alone, 60 min prior to their placement at 37 degrees C or room temperature for 30 min. hsp70 mRNA expression was analyzed in lung, liver and kidney isolated from animals assigned to one of four different treatment paradigms; untreated controls, heat, aspirin, and aspirin-plus-heat. Comparison of hsp70 expression in the treatment groups revealed that in all tissues examined, aspirin-plus-heat treatment resulted in 3-4 fold higher levels of hsp70 mRNA relative to those seen with heat treatment alone. Little or no hsp70 mRNA expression was detected in the unheated groups, regardless of aspirin treatment. In keeping with the mRNA expression, Hsp70 protein levels were also elevated in aspirin-plus-heat treated animals. Aspirin treatment did not alter hsp70 protein expression in the absence of heat. In contrast to in vitro observations, aspirin treatment in vivo did not alter HSF1 DNA binding properties. Core body temperature measurements revealed that aspirin pretreatment enhanced the rise in body temperature seen in response to heat treatment. This increased hyperthermic response to heat stress probably accounts for the potentiation of hsp70 expression observed in aspirin-plus-heat treated rats. Given the widespread use of aspirin in humans within a dose range comparable to that used here, our findings are likely to have important physiological consequences.

The molecular response to reductive stress in LLC-PK1 renal epithelial cells: coordinate transcriptional regulation of gadd153 and grp78 genes by thiols

Organic thiols are toxic to eukaryotic cells. Treatment of cells with thiols activates expression of grp78, but it is not known if, like other forms of stress, there is a battery of stress response genes that are induced by thiols. In LLC-PK1 renal epithelial cells, mRNAs for both grp78 and gadd153 were induced by thiols with similar time, concentration and structure-activity dependence. Dithiothreitol (DTT) was the most potent reductant and inducer of gene expression among the thiols tested. Nuclear run-on assays demonstrated that DTT activated both grp78 and gadd153 genes transcriptionally. A hamster gadd153 promoter construct which contains enhancer elements necessary for gadd153 activation was stably integrated into the LLC-PK1 cell genome and was activated by DTT. Although auto-oxidation of thiols can generate active oxygen species, transcriptional activation of the gadd153 promoter was not due to formation of hydrogen peroxide or superoxide since neither catalase nor superoxide dismutase prevented activation of the gadd153 promoter by DTT. The concentration dependence for activation of the gadd153 promoter correlated with inhibition of dome formation and protein synthesis, two toxic effects of DTT in LLC-PK1 cells. Thus, both grp78 and gadd153 are members of a gene battery which is responsive to reductive stress. There appears to be considerable, but not complete, overlap between the upstream signaling pathways for activation of both genes.

p21(Waf1/Cip1) protects against p53-mediated apoptosis of human melanoma cells

The tumor suppressive effect of p53 is believed to be rooted in its two primary functions: the implementation of cellular growth arrest and the execution of apoptotic cell death. While p53-regulated expression of the cyclin-dependent kinase inhibitor p21(Waf1/Cip1) appears to be central for the implementation of G1 arrest, the participation of p21(Waf1/Cip1) in p53-triggered cell death remains controversial. In the present study, overexpression of p53 in human melanoma SK-MEL-110 cells through use of an adenoviral expression vector (AdCMV.p53) was found to result in apoptosis, while similar infection of primary vascular smooth muscle cells (VSMC) instead resulted in a moderate inhibition of growth. Expression of p21(Waf1/Cip1) was strongly elevated in VSMC, but showed little change in SK-MEL-110 cells, although expression of another p53-regulated gene (GADD45) was comparable in both AdCMV.p53-infected cell types. Evidence that p21(Waf1/Cip1) expression may be required for surviving p53-induced cell death was further supported by the finding that p53 overexpression was highly toxic for p21-deficient mouse embryonal fibroblasts (p21-/- MEFs). In both SK-MEL-110 and p21-/- MEFs, adenovirus-driven ectopic expression of p21(Waf1/Cip1) resulted in a substantial protection against p53-induced apoptosis, indicating that p21(Waf1/Cip1) rescued cells from a path of programmed cell death to one of enhanced survival.

Up-regulation and functional role of p21Waf1/Cip1 during growth arrest of human breast carcinoma MCF-7 cells by phenylacetate

Phenylacetate (PA) and related aromatic fatty acids constitute a novel class of relatively nontoxic antineoplastic agents. These compounds induce tumor cytostasis and growth inhibition and differentiation of cancer cells, but little is known regarding the molecular events mediating these biological effects. Using human breast carcinoma MCF-7 cells as a model, we show here that PA-induced growth arrest is associated with enhanced expression of the cyclin-dependent kinase inhibitor p21Waf1/Cip1 and dephosphorylation of the retinoblastoma protein (pRB). The induction of p21WAF1/CIP1 mRNA by PA was independent of the cellular p53 status. To directly assess the contribution of p21Waf1/Cip1 to PA-mediated cytostasis, we compared the effects of PA in parental MCF-7 cells and cells expressing reduced levels of p21Waf1/Cip1 protein (clones AS.3 and AS.4), accomplished through constitutive expression of antisense p21Waf1/Cip1 transcripts. In contrast to parental cells, AS.3 and AS.4 cells did not show reduced pRB phosphorylation following PA treatment, indicating that p21Waf1/Cip1 induction by PA is required for dephosphorylation (inactivation) of pRB, a known mediator of cell cycle control. A prominent role for p21Waf1/Cip1 in mediating PA-induced growth arrest was further supported by the demonstration that embryonal fibroblasts derived from a p21WAF1/CIP1 knockout mouse (p21-/- mouse embryonal fibroblasts) did not growth arrest following PA treatment, whereas PA effectively induced p21WAF1/CIP1 mRNA and growth inhibition of the wild-type mouse embryonal fibroblasts. Taken together, our findings strongly support a role for p21Waf1/Cip1 in the PA-mediated inhibition of cell growth.

Protective role of p21(Waf1/Cip1) against prostaglandin A2-mediated apoptosis of human colorectal carcinoma cells

Prostaglandin A2 (PGA2) suppresses tumor growth in vivo, is potently antiproliferative in vitro, and is a model drug for the study of the mammalian stress response. Our previous studies using breast carcinoma MCF-7 cells suggested that p21(Waf1/Cip1) induction enabled cells to survive PGA2 exposure. Indeed, the marked sensitivity of human colorectal carcinoma RKO cells to the cytotoxicity of PGA2 is known to be associated with a lack of a PGA2-mediated increase in p21(Waf1/Cip1) expression, inhibition of cyclin-dependent kinase activity, and growth arrest. To determine if cell death following exposure to PGA2 could be prevented by forcing the expression of p21(Waf1/Cip1) in RKO cells, we utilized an adenoviral vector-based expression system. We demonstrate that ectopic expression of p21(Waf1/Cip1) largely rescued RKO cells from PGA2-induced apoptotic cell death, directly implicating p21(Waf1/Cip1) as a determinant of the cellular outcome (survival versus death) following exposure to PGA2. To discern whether p21(Waf1/Cip1)-mediated protection operates through the implementation of cellular growth arrest, other growth-inhibitory treatments were studied for the ability to attenuate PGA2-induced cell death. Neither serum depletion nor suramin (a growth factor receptor antagonist) protected RKO cells against PGA2 cytotoxicity, and neither induced p21(Waf1/Cip1) expression. Mimosine, however, enhanced p21(Waf1/Cip1) expression, completely inhibited RKO cell proliferation, and exerted marked protection against a subsequent PGA2 challenge. Taken together, our results directly demonstrate a protective role for p21(Waf1/Cip1) during PGA2 cellular stress and provide strong evidence that the implementation of cellular growth arrest contributes to this protective influence.

Effects of perturbations of the hypothalamic-pituitary-adrenal axis on the acute phase response: altered C/EBP and acute phase response gene expression in lipopolysaccharide-treated rats

In this study, we investigated the influence of long term perturbations of the hypothalamicpituitary-adrenal axis on the acute phase response elicited following lipopolysaccharide (LPS) challenge in rats. Specifically, we examined the effects of either long term absence of glucocorticoids (adrenalectomized rats treated with placebo chronic release pellets) or extended exposure to pharmacologic levels of glucocorticoids (adrenalectomized rats treated with dexamethasone chronic release pellets) on the expression of selected acute phase proteins and various members of the CCAAT/enhancer-binding protein (C/EBP) family of transcription factors. Both hypothalamic-pituitary-adrenal axis manipulations resulted in a reduction of the acute phase response as assessed by the LPS-mediated induction of acute phase proteins and C/EBP gene expression, with dexamethasone treatment exhibiting a greater inhibitory effect than adrenalectomy. Induction of hemopexin, alpha 1-acid glycoprotein, alpha 2-macroglobulin, GADD153, C/EBP beta, and C/EBP delta mRNAs by LPS were all abolished in dexamethasone-treated rats. These findings have direct implications for patients undergoing chronic high dose glucocorticoid therapy.

Mitogen-activated protein kinase (MAPK) activation by butylated hydroxytoluene hydroperoxide: implications for cellular survival and tumor promotion

The mitogen-activated protein kinase (MAPK) cascade plays an important role in carcinogenic development. Herein, we show that the skin tumor promoter butylated hydroxytoluene hydroperoxide (BHTOOH) stimulates a rapid and potent (14- to 20-fold) activation of extracellular signal-regulated kinase (ERK) in vivo and in cultured mouse keratinocytes. BHTOOH also moderately (5-fold) activated c-jun-N-terminal kinase, and 38-kDa MAPK-related protein in these same cells. N-acetylcysteine and o-phenanthroline abolished ERK activation by BHTOOH, consistent with a requirement for metal-dependent formation of reactive intermediates. Indeed, 4-CD3-BHTOOH, an analogue that generates less of the metabolite BHT-quinone methide (2,6-di-tert-butyl-4-methylene-2,5-cyclohexadienone) and fewer tumors in vivo, accordingly exhibited diminished potency for activating ERK. ERK activation by BHTOOH was inhibited by suramin, and by expression of dominant-negative Ras-N-17 in PC12 cells, suggesting overlap between the pathways for BHTOOH and growth factor signaling. Induction of MAPK-dependent genes c-fos and MAPK phosphatase-1 by BHTOOH was also blocked by Ras-N-17 expression. Moreover, expression of Ras-N-17 or kinase-defective MAPK kinase (MEK) diminished cell survival following BHTOOH exposure. Similarly, pretreatment with suramin or the MEK inhibitor PD098059 also potentiated the toxicity of BHTOOH. On the other hand, expression of constitutively active MEK enhanced cell survival. Thus, we demonstrate that the MAPK cascade is critical to the cellular response to BHTOOH. This study suggests a functional role for MAPK activation in tumor promotion stimulated by oxidants and other agents.

Activation of heat shock transcription factor 1 in rat aorta in response to high blood pressure

We have previously demonstrated that acute hypertension induces heat shock protein gene expression in rat arterial wall. Here we provide evidence that this induction is mediated through the activation of heat shock transcription factor 1 in response to high blood pressure. Rats subjected to restraint or immobilization stress displayed an acute elevation in systolic pressure accompanied by an increase in heat shock protein 70 mRNA expression. Consistent with the rapid time course of mRNA induction, an increase in binding activity to an oligonucleotide encompassing a consensus heat shock element sequence was seen in protein extracts from aorta of restrained rats as assessed with gel mobility shift assays. A similar increase in DNA binding activity was also observed in aortic extracts from rats treated with various hypertensive agents, including phenylephrine, angiotensin II, and vasopressin. That the DNA binding activity was attributed to heat shock factor 1 was shown through use of antibodies to the transcription factor that retarded the DNA-protein complexes in gel mobility supershift assays. Western blot analysis of heat shock factor 1 protein expression in aortic extracts showed a slower mobility form of the protein in hypertensive rats, indicative of an activated, presumably phosphorylated, form of the transcription factor. These findings support the view that heat shock factor 1 is responsible for induction of heat shock protein 70 in the arterial wall during acute hypertension, a response that is likely to play an important role in protecting arteries during hemodynamic stress.

Physical and functional association between GADD153 and CCAAT/enhancer-binding protein beta during cellular stress

GADD153, a ubiquitously expressed member of the CCAAT/enhancer-binding protein (C/EBP) family is induced by a wide variety of growth-arresting and DNA-damaging agents. Functionally, GADD153 has been postulated to act as a dominant-negative regulator of C/EBPs. Therefore we sought to gain evidence for interactions between GADD153 and other C/EBPs during cellular responses to stress. In this report we have demonstrated that treatment of rat pheochromocytoma PC12 cells with sodium arsenite leads to enhanced expression of C/EBP-beta and GADD153 (growth arrest and DNA damage inducible gene 153) but not other C/EBPs. Coimmunoprecipitation experiments provided evidence for the formation of endogenous GADD153-C/EBP-beta complexes in arsenite-treated cells. Additional experiments were performed to determine the role of such complexes in regulating GADD153 expression. Previous studies in our laboratory demonstrated that the GADD153 promoter contains a C/EBP binding site through which other C/EBPs interact to transactivate GADD153 expression in liver hepatoma cells. Here, we demonstrate that extracts prepared from arsenite-treated PC12 cells likewise show increased amounts of factors capable of binding to the GADD153-C/EBP site and that these complexes are comprised at least in part of C/EBP-beta. Forced expression of C/EBP-beta was found to be capable of transactivating the GADD153 promoter in PC12 cells cotransfected with plasmids expressing a GADD153 reporter gene and C/EBP-beta protein. However, overexpression of GADD153 inhibited the transactivation of the GADD153 promoter by C/EBP-beta. These findings provide evidence for an autoregulatory loop in which stress-induced GADD153 feeds back to attenuate GADD153 expression during the cellular response to stress.

Induction of the mammalian stress response gene GADD153 by oxidative stress: role of AP-1 element

GADD153 is a CCAAT/enhancer-binding-protein-related gene that may function to control cellular growth in response to stress signals. In this study, a variety of oxidant treatments were shown to stimulate endogenous GADD153 mRNA expression and to transcriptionally activate a GADD153 promoter-reporter gene construct in transfected HeLa cells. Both commonalities and distinctions in the induction of GADD153 by H2O2 and the thiol-reactive compound arsenite were demonstrated. GADD153 mRNA induction by both H2O2 and arsenite was potentiated by GSH depletion, and completely inhibited by N-acetyl-cysteine. o-Phenanthroline and mannitol blocked GADD153 induction by H2O2, indicating that iron-generated hydroxyl radical mediates this induction. Concordantly, GSH peroxidase overexpression in WI38 cells attenuated GADD153 mRNA induction by H2O2. However, GADD153 induction by arsenite was only modestly reduced in the same cells, suggesting a lesser contribution of peroxides to gene activation by arsenite. We also demonstrated that oxidative stress participates in the induction of GADD153 by UVC (254 nm) irradiation. Finally, both promoter-deletion analysis and point mutation of the AP-1 site in an otherwise intact promoter support a significant role for AP-1 in transcriptional activation of GADD153 by UVC or oxidant treatment. Indeed, exposure of cells to oxidants or UVC stimulated binding of Fos and Jun to the GADD153 AP-1 element. Together, these results demonstrate that both free-radical generation and thiol modification can transcriptionally activate GADD153, and that AP-1 is critical to oxidative regulation of this gene. This study further supports a role for the GADD153 gene product in the cellular response to oxidant injury.

Inhibition of G1 cyclin-dependent kinase activity during growth arrest of human breast carcinoma cells by prostaglandin A2

Prostaglandin A2 (PGA2) potently inhibits cell proliferation and suppresses tumor growth in vivo, but little is known regarding the molecular mechanisms mediating these effects. Here we demonstrate that treatment of breast carcinoma MCF-7 cells with PGA2 leads to G1 arrest associated with a dramatic decrease in the levels of cyclin D1 and cyclin-dependent kinase 4 (cdk4) and accompanied by an increase in the expression of p21. We further show that these effects occur independent of cellular p53 status. The decline in cyclin D and cdk4 protein levels is correlated with loss in cdk4 kinase activity, cdk2 activity is also significantly inhibited in PGA2-treated cells, an effect closely associated with the upregulation of p21. Immunoprecipitation experiments verified that p21 was indeed complexed with cdk2 in PGA2-treated cells. Additional experiments with synchronized MCF-7 cultures stimulated with serum revealed that treatment with PGA2 prevents the progression of cells from G1 to S. Accordingly, the kinase activity associated with cdk4, cyclin E, and cdk2 immunocomplexes, which normally increases following serum addition, was unchanged in PGA2-treated cells. Furthermore, the retinoblastoma protein (Rb), a substrate of cdk4 and cdk2 whose phosphorylation is necessary for cell cycle progression, remains underphosphorylated in PGA2-treated serum-stimulated cells. These findings indicate that PGA2 exerts its growth-inhibitory effects through modulation of the expression and/or activity of several key G1 regulatory proteins. Our results highlight the chemotherapeutic potential of PGA2, particularly for suppressing growth of tumors lacking p53 function.

Activation of mitogen-activated protein kinase by H2O2. Role in cell survival following oxidant injury

The mitogen-activated protein kinase (MAPK) family is comprised of key regulatory proteins that control the cellular response to both proliferation and stress signals. In this study we investigated the factors controlling MAPK activation by H2O2 and explored the impact of altering the pathways to kinase activation on cell survival following H2O2 exposure. Potent activation (10-20-fold) of extracellular signal-regulated protein kinase (ERK2) occurred within 10 min of H2O2 treatment, whereupon rapid inactivation ensued. H2O2 activated ERK2 in several cell types and also moderately activated (3-5-fold) both c-Jun N-terminal kinase and p38/RK/CSBP. Additionally, H2O2 increased the mRNA expression of MAPK-dependent genes c-jun, c-fos, and MAPK phosphatase-1. Suramin pretreatment completely inhibited H2O2 stimulation of ERK2, highlighting a role for growth factor receptors in this activation. Further, ERK2 activation by H2O2 was blocked by pretreatment with either N-acetyl-cysteine, o-phenanthroline, or mannitol, indicating that metal-catalyzed free radical formation mediates the initiation of signal transduction by H2O2. H2O2-stimulated activation of ERK2 was abolished in PC12 cells by inducible or constitutive expression of the dominant negative Ras-N-17 allele. Interestingly, PC12/Ras-N-17 cells were more sensitive than wild-type PC12 cells to H2O2 toxicity. Moreover, NIH 3T3 cells expressing constitutively active MAPK kinase (MEK, the immediate upstream regulator of ERK) were more resistant to H2O2 toxicity, while those expressing kinase-defective MEK were more sensitive, than cells expressing wild-type MEK. Taken together, these studies provide insight into mechanisms of MAPK regulation by H2O2 and suggest that ERK plays a critical role in cell survival following oxidant injury.

Age-related decline in mitogen-activated protein kinase activity in epidermal growth factor-stimulated rat hepatocytes

A number of studies have demonstrated that the proliferative capacity of cells declines with aging. In particular, epidermal growth factor (EGF)-stimulated DNA synthesis is reduced in hepatocytes from aged rats relative to young rats. Growth factor stimulation activates a genetic program in large part regulated by a family of mitogen-activated protein kinases (MAPK) that phosphorylate and thereby activate transcription factors involved in controlling the expression of proliferation-associated genes. In the present study, we compared the activation of the extracellular signal-regulated kinase 2 (ERK2) and c-Jun N-terminal kinase 1 (JNK1) MAPK in EGF-stimulated hepatocytes derived from young (6-month) and aged (24-month) rats. JNK activity was not appreciably altered by EGF treatment of cells from either age group. In contrast, ERK2 was highly activated by EGF treatment, but the magnitude of activation was significantly lower in hepatocytes of aged animals compared to those of young animals (7-fold versus 20-fold, respectively). The reduced ERK2 activity in response to EGF was associated with decreased c-fos and c-jun mRNA expression and lower levels of AP-1 transcription factor DNA binding activity in the aged hepatocytes. Finally, the basal expression of MAPK phosphatase 1, a MAPK-regulated gene involved in regulating MAPK activity, was higher in aged hepatocytes. Taken together, these findings suggest that an alteration in the balance between MAP kinase-phosphatase activities could contribute to the age-related decline in proliferative capacity.

Role of p21 in prostaglandin A2-mediated cellular arrest and death

Prostaglandin A2 (PGA2) treatment induces growth arrest of most cells, and we have recently shown that, for breast carcinoma MCF-7 cells, this is correlated with an induction of the cyclin-dependent kinase inhibitor p21 and reduced cyclin-dependent kinase 2 activity. In this study, we demonstrate that, in RKO cells, PGA2 treatment fails to induce growth arrest, but rather results in significant cell death. These effects are correlated with lack of p21 induction and enhanced cyclin-dependent kinase 2 activity. Reduction of endogenous p21 expression in MCF-7 cells through expression of antisense p21 resulted in a response pattern approaching that seen in RKO cells, characterized by diminished growth arrest and increased death. These findings support a role for p21 in PGA2-mediated growth arrest, which we propose serves to prevent cell death caused by inappropriate cell cycle progression.

Acute hypertension activates mitogen-activated protein kinases in arterial wall

Mitogen-activated protein (MAP) kinases are rapidly activated in cells stimulated with various extracellular signals by dual phosphorylation of tyrosine and threonine residues. They are thought to play a pivotal role in transmitting transmembrane signals required for cell growth and differentiation. Herein we provide evidence that two distinct classes of MAP kinases, the extracellular signal-regulated kinases (ERK) and the c-Jun NH2-terminal kinases (JNK), are transiently activated in rat arteries (aorta, carotid and femoral arteries) in response to an acute elevation in blood pressure induced by either restraint or administration of hypertensive agents (i.e., phenylephrine and angiotensin II). Kinase activation is followed by an increase in c-fos and c-jun gene expression and enhanced activating protein 1 (AP-1) DNA-binding activity. Activation of ERK and JNK could contribute to smooth muscle cell hypertrophy/hyperplasia during arterial remodeling due to frequent and/or persistent elevations in blood pressure.