p38 kinase mediates UV-induced phosphorylation of p53 protein at serine 389

Involvement of ERKs, RSK2 and PKR in UVA-induced signal transduction toward phosphorylation of eIF2alpha (Ser(51))

Double-stranded RNA-dependent protein kinase R (PKR) has been implicated in anti-viral (antitumor) and apoptotic responses. PKR is activated by extracellular stresses and phosphorylates the alpha subunit of protein synthesis initiation factor eIF2, thereby inhibiting protein synthesis and impeding virus multiplication. Phosphorylation of eIF2alpha in mammalian cells has been shown to be increased after ultraviolet (UV) stress and to be required for UV-induced repression of protein translation. UVA is an important etiological factor in skin carcinogenesis and we observed that UVA induced phosphorylation of PKR (Thr(451)) and eIF2alpha (Ser(51)) in mouse skin epidermal JB6 Cl41 cells. The induction was suppressed by the MEK1 inhibitor, PD 98059. UVA stimulation of PKR and eIF2alpha phosphorylation was also inhibited by a dominant-negative mutant (DNM) of ERK2- or RSK2-deficient cells (RSK2(-)). An inhibitor of p38, SB 202190 or a DNM of p38alpha kinase (DNM-p38alpha) suppressed UVA-induced phosphorylation of eIF2alpha (Ser(51)) but had no effect on phosphorylation of PKR (Thr(451)). Our data indicated that phosphorylation of PKR at Thr(451) is mediated through ERK2 and RSK2, but not through p38 kinase, and is involved in the regulation of Ser(51) phosphorylation of eIF2alpha in UVA-irradiated JB6 cells. In vitro and in vivo kinase assays indicated that phosphorylation of eIF2alpha at Ser(51) occurred indirectly through ERK2, RSK2 or p38 kinase in the cellular response to UVA. These data may lead to the use of these signaling molecules as targets to develop more effective chemopreventive agents with fewer side effects to control UV-induced skin cancer.

p38 MAPK in regulating cellular responses to ultraviolet radiation

Solar ultraviolet (UV) radiation is a major environmental factor that causes DNA damage, inflammation, erythema, sunburn, immunosuppression, photoaging, gene mutations, and skin cancer. p38 mitogen activated protein kinase (MAPK) are strongly activated by UV radiation, and play important roles in regulating cellular responses to UV. In this review, we examine the role played by p38 MAPK in mediating UV-induced cell cycle, apoptosis, inflammation, and skin tanning response. We review the role played by p38 MAPK in transcriptional regulation of key downstream genes that have been implicated in the regulation of cellular responses to UV radiation. Understanding this will undoubtedly help in the prevention and control of UV-induced damage and the development of novel therapeutic strategies.

Delayed expression of apoptotic and cell-cycle control genes in carcinogen-exposed bladders of mice lacking p53.S389 phosphorylation

Mice with non-phosphorylated serine 389 in p53 are susceptible for bladder tumors induced by 2-acetylaminofluorene (2-AAF). Since p53 is a transcription factor, this might well be preceded by differences in the regulation of gene expression. Microarray analysis was used to determine early transcriptional changes that might underlie this cancer-prone phenotype. Interestingly, lack of Ser389 phosphorylation led to endogenously different gene expression levels. The number of genes affected was, however, rather small. Conversely, after short-term exposure to 2-AAF, wild-type and p53.S389A bladders demonstrated a significant number of differentially expressed genes. Differences between wild-type and p53.S389A could mainly be attributed to a delayed, rather than complete absence of, transcriptional response of a group of genes, including well-known p53 target genes involved in apoptosis and cell-cycle control like Bax, Perp and P21. An analysis of differentially expressed genes in non-tumorigenic tissue and bladder tumors of p53.S389A after long-term exposure to 2-AAF revealed 319 genes. Comparison of these with those found after short-term exposure resulted in 23 transcripts. These possible marker genes might be useful for the early prediction of bladder tumor development. In conclusion, our data indicate that lack of Ser389 phosphorylation results in aberrant expression of genes needed to execute vital responses to DNA damage. Post-translational modifications, like Ser389 phosphorylation, seem crucial for fine-tuning the transcription of a specific set of genes and do not appear to give rise to major changes in transcription patterns. As such, Ser389 phosphorylation is needed for some, but certainly not all, p53 functions.

Posttranslational phosphorylation of mutant p53 protein in tumor development

p53 has been called the "cellular gatekeeper" and the "genome guard," because in response to exposure to DNA-damaging agents, it induces cell-cycle arrest in G1 or apoptosis and also directly affects DNA replication. Multiple mechanisms regulate p53 activity and posttranslational modification, including multisite phosphorylation of wild-type p53, in particular. Normal functions of wild-type p53 are abrogated by mutation of this gene, and oncogenic studies have revealed that p53 mutation is among the most common genetic alteration in human cancers. It is generally accepted that mutant p53 protein may not only lose the tumor suppressor functions of wild-type p53 but also acquire additional tumorigenetic roles, including dominant-negative effects and gain of function. Although many studies have revealed such aberrant functions of mutant p53, less is known about the posttranslational phosphorylation status of mutant p53 and novel biological functions of phosphorylation in carcinogenesis.

Sulindac-derived reactive oxygen species induce apoptosis of human multiple myeloma cells via p38 mitogen activated protein kinase-induced mitochondrial dysfunction

Non-steroidal anti-inflammatory drugs are well known to induce apoptosis of cancer cells independent of their ability to inhibit cyclooxygenase-2, but the molecular mechanism for this effect has not yet been fully elucidated. The purpose of this study was to elucidate the potential signaling components underlying sulindac-induced apoptosis in human multiple myeloma (MM) cells. We found that sulindac induces apoptosis by promoting ROS generation, accompanied by opening of mitochondrial permeability transition pores, release of cytochrome c and apoptosis inducing factor from mitochondria, followed by caspase activation. Bcl-2 cleavage and down-regulation of the inhibitor of apoptosis proteins (IAPs) family including cIAP-1/2, XIAP, and survivin, occurred downstream of ROS production during sulindac-induced apoptosis. Forced expression of survivin and Bcl-2 blocked sulindac-induced apoptosis. Most importantly, sulindac-derived ROS activated p38 mitogen-activated protein kinase and p53. SB203580, a p38 mitogen-activated protein kinase inhibitor, and RNA inhibition of p53 inhibited the sulindac-induced apoptosis. Furthermore, p53, Bax, and Bak accumulated in mitochondria during sulindac-induced apoptosis. All of these events were significantly suppressed by SB203580. Our results demonstrate a novel mechanism of sulindac-induced apoptosis in human MM cells, namely, accumulation of p53, Bax, and Bak in mitochondria mediated by p38 MAPK activation downstream of ROS production.

Sphingosine-1-phosphate lyase potentiates apoptosis via p53- and p38-dependent pathways and is down-regulated in colon cancer

Sphingolipid metabolites such as sphingosine-1-phosphate (S1P) and ceramide modulate apoptosis during development and in response to stress. In general, ceramide promotes apoptosis, whereas S1P stimulates cell proliferation and protects against apoptosis. S1P is irreversibly degraded by the enzyme S1P lyase (SPL). In this study, we show a crucial role for SPL in mediating cellular responses to stress. SPL expression in HEK293 cells potentiated apoptosis in response to stressful stimuli including DNA damage. This effect seemed to be independent of ceramide generation but required SPL enzymatic activity and the actions of p38 MAP kinase, p53, p53-inducible death domain protein (PIDD), and caspase-2 as shown by molecular and chemical inhibition of each of these targets. Further, SPL expression led to constitutive activation of p38. Endogenous SPL expression was induced by DNA damage in WT cells, whereas SPL knockdown diminished apoptotic responses. Importantly, SPL expression was significantly down-regulated in human colon cancer tissues in comparison with normal adjacent tissues, as determined by quantitative real-time PCR (Q-PCR) and immunohistochemical analysis. Down-regulation of S1P phosphatases was also observed, suggesting that colon cancer cells manifest a block in S1P catabolism. In addition, SPL expression and activity were down-regulated in adenomatous lesions of the Min mouse model of intestinal tumorigenesis. Taken together, these results indicate that endogenous SPL may play a physiological role in stress-induced apoptosis and provide an example of altered SPL expression in a human tumor. Our findings suggest that genetic or epigenetic changes affecting intestinal S1P metabolism may correlate with and potentially contribute to carcinogenesis.

Expression Profiling of UVB Response in Melanocytes Identifies a Set of p53-Target Genes

Epidermal melanocytes execute specific physiological programs in response to UV radiation (UVR) at the cutaneous interface. Many melanocytic responses, including increased dendrite formation, enhanced melanogenesis/melanization, and cell cycle arrest impact the ability of melanocytes to survive and to attenuate the UVR insult. Although some of the molecules that underlie these UVR programs are known, a coherent view of UVR-induced transcriptional changes is lacking. Using primary melanocyte cultures, we assessed for UVR-mediated alterations in over 47,000 transcripts using Affymetrix Human Genome U133 Plus 2.0 microarrays. From the 100 most statistically robust changes in transcript level, there were 84 genes that were suppressed >2.0-fold by UVR; among these transcripts, the identities of 48 of these genes were known. Similarly, there were 99 genes that were induced >2.0-fold by UVR; the identity of 57 of these genes were known. We then subjected these top 100 changes to the Ingenuity Pathway analysis program and identified a group of p53 targets including the cell cycle regulator CDKN1A (p21CIP), the WNT pathway regulator DKK1 (dickkopf homolog 1), the receptor tyrosine kinase EPHA2, growth factor GDF15, ferrodoxin reductase (FDXR), p53-inducible protein TP53I3, transcription factor ATF3, DNA repair enzyme DDB2, and the beta-adrenergic receptor ADBR2. These genes were also found to be consistently elevated by UVR in six independent melanocyte lines, although there were interindividual variations in magnitude. WWOX, whose protein product interacts and regulates p53 and p73, was found to be consistently suppressed by UVR. There was also a subgroup of neurite/axonal developmental genes that were altered in response to UVR, suggesting that melanocytic and neuronal arborization may share similar mechanisms. When compared to melanomas, the basal levels of many of these p53-responsive genes were greatly dysregulated. Three genes--CDKN1A, DDB2 and ADRB2--exhibited a trend towards loss of expression in melanomas thereby raising the possibility of a linked role in tumorigenesis. These expression data provide a global view of UVR-induced changes in melanocytes and, more importantly, generate novel hypotheses regarding melanocyte physiology.

Topical p38MAPK inhibition reduces dermal inflammation and epithelial apoptosis in burn wounds

Thermal injury induces dermal inflammatory and proapoptotic signaling. These phenomena extend burn wound size and trigger a systemic inflammatory response, factors known to adversely affect outcomes. p38MAPK is known to trigger inflammatory responses and induce epithelial proapoptotic genes. We hypothesize that topical p38MAPK inhibition will attenuate excessive inflammatory and apoptotic signaling and reduce dermal tissue loss. Rats were given a 30% total body surface area partial thickness burn or sham injury. Some of the animals were treated with a p38MAPK inhibitor or vehicle, which was applied directly to the wound. Dermal inflammation was investigated with enzyme-linked immunosorbent assay, reverse transcriptase polymerase chain reaction, myeloperoxidase assay, and Evans blue extravasation. Apoptotic changes were detected using terminal deoxynucleotidyl transferase dUTP nick-end labeling assay and Caspase-3 in situ staining. Burn injury activated dermal p38MAPK and induced a significant rise in dermal IL-6, TNF-alpha, and IL-1beta expression. Neutrophil sequestration, microvascular damage, and hair follicle apoptosis were significantly elevated after injury. Topical p38MAPK inhibition significantly attenuated downstream dermal p38MAPK targets, proinflammatory cytokine expression, neutrophil sequestration, and microvascular injury. A significant reduction in hair follicle apoptosis was seen. This study demonstrates the attenuation of burn-induced cellular stress by topical application of p38MAPK inhibitors. Blunting early excessive inflammatory signaling may be an efficient strategy to improve patient outcomes after burn injury.

Inhibition of centrosome protein assembly leads to p53-dependent exit from the cell cycle

Previous evidence has indicated that an intact centrosome is essential for cell cycle progress and that elimination of the centrosome or depletion of individual centrosome proteins prevents the entry into S phase. To investigate the molecular mechanisms of centrosome-dependent cell cycle progress, we performed RNA silencing experiments of two centrosome-associated proteins, pericentriolar material 1 (PCM-1) and pericentrin, in primary human fibroblasts. We found that cells depleted of PCM-1 or pericentrin show lower levels of markers for S phase and cell proliferation, including cyclin A, Ki-67, proliferating cell nuclear antigen, minichromosome maintenance deficient 3, and phosphorylated retinoblastoma protein. Also, the percentage of cells undergoing DNA replication was reduced by >50%. At the same time, levels of p53 and p21 increased in these cells, and cells were predisposed to undergo senescence. Conversely, depletion of centrosome proteins in cells lacking p53 did not cause any cell cycle arrest. Inhibition of p38 mitogen-activated protein kinase rescued cell cycle activity after centrosome protein depletion, indicating that p53 is activated by the p38 stress pathway.

The impact of altered annexin I protein levels on apoptosis and signal transduction pathways in prostate cancer cells

BACKGROUND

Although reduced expression levels of annexin I (ANX I) protein is a common finding in all stages of prostate cancer a causative relationship between ANX I dysregulation and prostate cancer development has yet to be established.

METHODS

Annexin I expression was restored in LNCaP and MDA PCa 2b that normally express low or undetectable levels of ANX I protein. The impact of restoring ANX I expression on cell viability, colony formation in soft agar, apoptosis, and extracellular signal-regulated kinases (ERK), p38, c-Jun N-terminal kinases (JNK) activation was examined.

RESULTS

Restoring ANX I expression reduced cell viability, colony formation, in addition to inducing apoptosis. The proliferative response of epidermal growth factor was blocked by restoring ANX I expression. Furthermore, increasing basal and induced levels of phosphorylated p38 and JNK were observed in prostate cancer cells following restoration of ANX I expression.

CONCLUSIONS

Annexin I may have tumor suppressor functions in prostate cancer. The pro-apoptotic effect of ANX I involves the activation of p38 and JNK, which appears to shift the balance of signal transduction away from proliferation and toward apoptosis.

Modulation of p53 transcriptional activity by PRIMA-1 and Pifithrin-alpha on staurosporine-induced apoptosis of wild-type and mutated p53 epithelial cells

We recently argued for a major role of p53 in staurosporine(ST)-induced apoptosis of immortalized epithelial cells, depending on their p53 status. Here, we studied the effects of PRIMA-1 (p53 reactivation and induction of massive apoptosis) and Pifithrin-alpha (p fifty-three inhibitor) in combination with ST to reinforce our previous results by respectively restoring or inhibiting the p53 transcriptional activity in different cell lines.PRIMA-1 does modify neither expression of apoptosis-related proteins nor the percentage of wild-type p53 HeLa and CaSki cells with [symbol: see text]delta psi m and DNA cleavage, whilst it increases by 45% Bax expression and apoptosis of mutated p53 C33A cells. Pifithrin-alpha, does modify neither Bax expression nor apoptosis level of C33A cells, but readily inhibits both [symbol: see text]delta psi m and DNA fragmentation of p53wt cells with decreasing Bax expression. These data support the evidence that PRIMA-1 could be a good candidate, as an anti-cancer drug targeting mutant p53, in order to increase ST efficiency. Moreover, Pifithrin-alpha could be used in combination with ST and PRIMA-1 to prevent side effects of anti-tumor therapies in cells expressing mutant P53.

c-Jun/AP-1 controls liver regeneration by repressing p53/p21 and p38 MAPK activity

The AP-1 transcription factor c-Jun is a key regulator of hepatocyte proliferation. Mice lacking c-Jun in the liver (c-jun (Deltali*)) display impaired liver regeneration after partial hepatectomy (PH). This phenotype correlates with increased protein levels of the cdk-inhibitor p21 in the liver. We performed PH experiments in several double-knockout mouse models to genetically identify the signaling events regulated by c-Jun. Inactivation of p53 in c-jun (Deltali*) mice abrogated both hepatocyte cell cycle block and increased p21 protein expression. Consistently, liver regeneration was rescued in c-jun (Deltali*) p21 (-/-) double-mutant mice. This indicated that c-Jun controls hepatocyte proliferation by a p53/p21-dependent mechanism. Analyses of p21 mRNA and protein expression in livers of c-jun (Deltali*) mice after PH revealed that the accumulation of p21 protein is due to a post-transcriptional/post-translational mechanism. We have investigated several candidate pathways implicated in the regulation of p21 expression, and observed increased activity of the stress kinase p38 in regenerating livers of c-jun (Deltali*) mice. Importantly, conditional deletion of p38alpha in livers of c-jun (Deltali*) mice fully restored hepatocyte proliferation and attenuated increased p21 protein levels after PH. These data demonstrate that c-Jun/AP-1 regulates liver regeneration through a novel molecular pathway that involves p53, p21, and the stress kinase p38alpha.

Protein serine/threonine phosphatase-1 dephosphorylates p53 at Ser-15 and Ser-37 to modulate its transcriptional and apoptotic activities

We have previously demonstrated that the serine/threonine protein phosphatase-1 (PP-1) plays an important role in promoting cell survival. However, the molecular mechanisms by which PP-1 promotes survival remain largely unknown. In the present study, we provide evidence to show that PP-1 can directly dephosphorylate a master regulator of apoptosis, p53, to negatively modulate its transcriptional and apoptotic activities, and thus to promote cell survival. As a transcriptional factor, the function of p53 can be greatly regulated by phosphorylation and dephosphorylation. While the kinases responsible for phosphorylation of the 17 serine/threonine sites have been identified, the dephosphorylation of these sites remains largely unknown. In the present study, we demonstrate that PP-1 can dephosphorylate p53 at Ser-15 and Ser-37 through co-immunoprecipitation, in vitro and in vivo dephosphorylation assays, overexpression and silence of the gene encoding the catalytic subunit for PP-1. We further show that mutations mimicking constitutive dephosphorylation or phosphorylation of p53 at these sites attenuate or enhance its transcriptional activity, respectively. As a result of the changed p53 activity, expression of the downstream apoptosis-related genes such as bcl-2 and bax is accordingly altered and the apoptotic events are either largely abrogated or enhanced. Thus, our results demonstrate that PP-1 directly dephosphorylates p53, and dephosphorylation of p53 has as important impact on its functions as phosphorylation does. In addition, our results reveal that one of the molecular mechanisms by which PP-1 promotes cell survival is to dephosphorylate p53, and thus negatively regulate p53-dependent death pathway.

Multiple actions of pifithrin-alpha on doxorubicin-induced apoptosis in rat myoblastic H9c2 cells

Doxorubicin (Dox) is a chemotherapeutic agent that causes significant cardiotoxicity. We showed previously that Dox activates p53 and induces apoptosis in mouse hearts. This study was designed to elucidate the molecular events that lead to p53 stabilization, to examine the pathways involved in Dox-induced apoptosis, and to evaluate the effectiveness of pifithrin-alpha (PFT-alpha), a p53 inhibitor, in blocking apoptosis of rat H9c2 myoblasts. H9c2 cells that were exposed to 5 muM Dox had elevated levels of p53 and phosphorylated p53 at Ser15. Dox also triggered a transient activation of p38, p42/p44ERK, and p46/p54JNK MAP kinases. Caspase activity assays and Western blot analysis showed that H9c2 cells treated with Dox for 16 h had marked increase in the levels of caspases-2, -3, -8, -9, -12, Fas, and cleaved poly(ADP ribose) polymerase (PARP). There was a concomitant increase in p53 binding activity, cytochrome c release, and apoptosis. These results suggest that Dox can trigger intrinsic, extrinsic, and endoplasmic reticulum-associated apoptotic pathways. Pretreatment of cells with PFT-alpha followed by Dox administration attenuated Dox-induced increases in p53 levels and p53 binding activity and partially blocked the activation of p46/p54JNK and p42/p44ERK. PFT-alpha also led to decreased levels of caspases-2, -3, -8, -9, -12, Fas, PARP, cytochrome c release, and apoptosis. Our results suggest that p53 stabilization is a focal point of Dox-induced apoptosis and that PFT-alpha interferes with multiple steps of Dox-induced apoptosis.

Effects of MAP kinase inhibitors on epidermal growth factor-induced neoplastic transformation of human keratinocytes

We previously reported data regarding the mechanism of neoplastic transformation in JB6 Cl41 mouse skin epidermal cells. However, experimental in vitro models for studying neoplastic transformation of human cells could provide further insight into the mechanisms of human cancer development. In this study, we have established a neoplastic transformation model with HaCaT cells, a human keratinocyte cell line, and showed the usefulness of this cell line for studying the mechanisms of neoplastic transformation. Epidermal growth factor (EGF) treatment induced a dose-dependent anchorage-independent cell transformation in HaCaT cells. Furthermore, PD98059, a mitogen-activated protein (MAP) kinase/ERK kinase (MEK) inhibitor, or SP600125, c-Jun N-terminal kinase (JNK) inhibitor, decreased cell growth, EGF-induced DNA synthesis and transformation. Unlike observations in the JB6 mouse epidermal cell model, SB203580, a stress-activated protein kinase-2/p38 alpha and beta (p38) inhibitor, increased EGF-induced transformation in HaCaT cells. These results suggest that extracellular-signal regulated kinase (ERK), JNK, or p38 are implicated in EGF-induced neoplastic transformation of human cells.

Hindlimb unloading increases muscle content of cytosolic but not nuclear Id2 and p53 proteins in young adult and aged rats

This study tested the hypothesis that inhibitor of differentiation-2 (Id2), p53, and heat shock proteins (HSP) are responsive to suspension-induced muscle atrophy. Fourteen days of hindlimb suspension were used to unload the hindlimbs and induce atrophy in gastrocnemius muscles of young adult and aged rats. Following suspension, medial gastrocnemius muscle wet weight was reduced by approximately 30%, and the muscle wet weight normalized to the animal body weight decreased by 11 and 15% in young adult and aged animals, respectively. mRNA abundances of Id2, p53, HSP70-2, and HSP27 did not change with suspension, whereas HSP70-1 mRNA content was lower in the suspended muscle compared with the control muscle in both young adult and aged animals. Our immunoblot analyses indicated that protein expressions of HSP70 and HSP60 were not different between suspended and control muscles in both ages, whereas HSP27 protein content was increased in suspended muscle relative to control muscle only in young adult animals. Id2 and p53 protein contents were elevated in the cytosolic fraction of suspended muscle compared with the control muscle in both young and aged animals, but these changes were not found in the nuclear protein fraction. Furthermore, compared with young adult, aged muscles had a lower HSP70-1 mRNA content but higher HSP70-2 mRNA content and protein contents of Id2, p53, HSP70, and HSP27. These findings are consistent with the hypothesis that Id2 and p53 are responsive to unloading-induced muscle atrophy. Moreover, our data indicate that aging is accompanied with altered abundances of HSP70-1 and HSP70-2 mRNA, in addition to Id2, p53, HSP70, and HSP27 protein in rat gastrocnemius muscle.

Activation of ERK1/2 and p38 kinases by polycyclic aromatic hydrocarbons in rat liver epithelial cells is associated with induction of apoptosis

Deregulation of various signaling pathways, linked either to induction of cell proliferation or to modulation of cellular differentiation and apoptosis, has been proposed to contribute to carcinogenicity of polycyclic aromatic hydrocarbons (PAHs). In the present study, we investigated effects of the PAHs previously shown to induce cell proliferation and/or apoptosis in contact-inhibited rat liver epithelial WB-F344 cells, with an aim to define the role of mitogen-activated protein kinases in both events. We found that only strong genotoxin dibenzo[a,l]pyrene (DBalP) activated extracellular signal-regulated kinases 1 and 2 (ERK1/2) and p38 kinase, but not c-Jun N-terminal kinases (JNKs), at concentrations inducing both apoptosis and phosphorylation of p53 tumor suppressor at serine 15 residue. In contrast, the PAHs stimulating cell proliferation in WB-F344 cell line had no effect on activation of ERK1/2, p38 or JNKs. Synthetic inhibitors of ERK1/2 activation (U0126) or p38 kinase activity (SB203580) prevented both apoptosis and induction of p53 phosphorylation by DBalP. Pifithrin-alpha, inhibitor of p53 transcriptional activity, prevented induction of apoptosis and activation of ERK1/2 and p38. Taken together, our data suggest that both ERK1/2 and p38 are activated in response to DBalP and that they might be involved in regulation of cellular response to DNA damage induced by DBalP, while neither kinase is involved in the release from contact inhibition induced by PAHs.

The p38 mitogen-activated protein kinase signaling cascade in CD4 T cells

Since the identification of the p38 mitogen-activated protein kinase (MAPK) as a key signal-transducing molecule in the expression of the proinflammatory cytokine tumor necrosis factor (TNF) more than 10 years ago, huge efforts have been made to develop inhibitors of p38 MAPK with the intent to modulate unwanted TNF activity in diseases such as autoimmune diseases or sepsis. However, despite some anti-inflammatory effects in animal models, no p38 MAPK inhibitor has yet demonstrated clinical efficacy in human autoimmune disorders. One possible reason for this paradox might relate to the fact that the p38 MAPK signaling cascade is involved in the functional regulation of several different cell types that all contribute to the complex pathogenesis of human autoimmune diseases. In particular, p38 MAPK has a multifaceted role in CD4 T cells that have been implicated in initiating and driving sustained inflammation in autoimmune diseases, such as rheumatoid arthritis or systemic vasculitis. Here we review recent advances in the understanding of the role of the p38 MAPK signaling cascade in CD4 T cells and the consequences that its inhibition provokes in T cell functions in vitro and in vivo. These new data suggest that p38 MAPK inhibitors may elicit several unwanted effects in human autoimmune diseases but may be useful for the treatment of allergic disorders.

Activation of p38 MAP kinase by DNA double-strand breaks in V(D)J recombination induces a G2/M cell cycle checkpoint

Delay of cell cycle progression in response to double-strand DNA breaks (DSBs) is critical to allow time for DNA repair and prevent cellular transformation. Here, we show that the p38 mitogen-activated protein (MAP) kinase signaling pathway is activated in immature thymocytes along with TcRbeta gene V(D)J recombination. Active p38 MAP kinase promotes a G2/M cell cycle checkpoint through the phosphorylation and activation of p53 in these cells in vivo. Inactivation of p38 MAP kinase and p53 is required for DN3 thymocytes to exit the G2/M checkpoint, progress through mitosis and further differentiate. We propose that p38 MAP kinase is activated by V(D)J-mediated DSBs and induces a p53-mediated G2/M checkpoint to allow DNA repair and prevent cellular transformation.

Up-regulation of PTEN (phosphatase and tensin homolog deleted on chromosome ten) mediates p38 MAPK stress signal-induced inhibition of insulin signaling. A cross-talk between stress signaling and insulin signaling in resistin-treated human endothelial cells

The key feature of metabolic syndrome, a cluster of metabolic and cardiovascular disorders, is systemic insulin resistance, which is associated with dysregulated endothelial nitric-oxide synthase (eNOS). Stress signaling induced by inflammation can inhibit insulin signaling. However, molecular mechanisms for the cross-talk between stress signaling and insulin resistance are only partially understood. Resistin, an adipokine/cytokine, is involved in inflammatory processes that could lead to insulin resistance status and vascular diseases. In the current study, we observed that resistin inhibited insulin signaling and eNOS activation in endothelial cells. Up-regulation of PTEN (phosphatase and tensin homolog deleted on chromosome ten) expression by resistin may mediate the inhibitory effects. Activated stress signaling p38 MAPK, but not JNK, is involved in PTEN up-regulation. We further found that p38 target transcriptional factor activating transcription factor-2 (ATF-2) bound to ATF sites in the PTEN promoter. The phosphorylation/activation of ATF-2 and its binding to PTEN promoter were increased by resistin treatment. In summary, up-regulation of PTEN is involved in the inhibitory effects of resistin on insulin signaling and eNOS activation in endothelial cells. Resistin induces PTEN expression by activating stress signaling p38 pathway, which may activate target transcription factor ATF-2, which in turn induces PTEN expression. Our findings suggest that resistin-mediated inhibition of insulin signaling and eNOS activation may contribute to cardiovascular diseases.

Respiratory burst: role in signal transduction in alveolar macrophages

Alveolar macrophages play an important role in defense against airborne pathogens and particles. These macrophages respond through both the adaptive and acquired immune responses, and through the activation of a multitude of signaling pathways. One major macrophage defense mechanism is respiratory burst, the production of reactive oxygen species (ROS). While the ROS produced may act directly in pathogen killing, they may also be involved as secondary signaling messengers. This review focuses on the activation of four main signaling pathways following the production of reactive oxygen species. These pathways include the nuclear factor kappa beta (NFkB), activating protein-1 (AP-1), mitogen-activating protein kinase (MAPK), and phosphotidyl inositol-3 kinase (PI3K) pathways. This review also briefly examines the role of ROS in DNA damage, in particular looking at the base excision repair pathway (BER), the main pathway involved in repair of oxidative DNA damage. This review highlights many of the studies in the field of ROS, signal transduction, and DNA damage; however, work still remains to further elucidate the role of ROS in disease.

Cdk9 phosphorylates p53 on serine 392 independently of CKII

The tumor suppressor p53 is an important cellular protein, which controls cell cycle progression. Phosphorylation is one of the mechanisms by which p53 is regulated. Here we report the interaction of p53 with another key regulator, cdk9, which together with cyclin T1 forms the positive transcription elongation complex, p-TEFb. This complex cooperates with the HIV-1 Tat protein to cause the phosphorylation of the carboxyl terminal domain (CTD) of RNA polymerase II and this facilitates the elongation of HIV-1 transcription. We demonstrate that cdk9 phosphorylates p53 on serine 392 through their direct physical interaction. Results from protein-protein interaction assays revealed that cdk9 interacts with the C-terminal domain (aa 361-393) of p53, while p53 interacts with the N-terminal domain of cdk9. Transfection and protein binding assays (EMSA and ChIP) demonstrated the ability of p53 to bind and activate the cdk9 promoter. Interestingly, cdk9 phosphorylates serine 392 of p53, which could be also phosphorylated by casein kinase II. Kinase assays demonstrated that cdk9 phosphorylates p53 independently of CKII. These studies demonstrate the existence of a feedback-loop between p53 and cdk9, pinpointing a novel mechanism by which p53 regulates the basal transcriptional machinery.

G-protein-coupled OX1 orexin/hcrtr-1 hypocretin receptors induce caspase-dependent and -independent cell death through p38 mitogen-/stress-activated protein kinase

We have investigated the signaling of OX(1) receptors to cell death using Chinese hamster ovary cells as a model system. OX(1) receptor stimulation with orexin-A caused a delayed cell death independently of cytosolic Ca(2+) elevation. The classical mitogen-activated protein kinase (MAPK) pathways, ERK and p38, were strongly activated by orexin-A. p38 was essential for induction of cell death, whereas the ERK pathway appeared protective. A pathway often implicated in the p38-mediated cell death, activation of p53, did not mediate the cell death, as there was no stabilization of p53 or increase in p53-dependent transcriptional activity, and dominant-negative p53 constructs did not inhibit cell demise. Under basal conditions, orexin-A-induced cell death was associated with compact chromatin condensation and it required de novo gene transcription and protein synthesis, the classical hallmarks of programmed (apoptotic) cell death. However, though the pan-caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-(O-methyl)fluoromethyl ketone (Z-VAD-fmk) fully inhibited the caspase activity, it did not rescue the cells from orexin-A-induced death. In the presence of Z-VAD-fmk, orexin-A-induced cell death was still dependent on p38 and de novo protein synthesis, but it no longer required gene transcription. Thus, caspase inhibition causes activation of alternative, gene transcription-independent death pathway. In summary, the present study points out mechanisms for orexin receptor-mediated cell death and adds to our general understanding of the role of G-protein-coupled receptor signaling in cell death by suggesting a pathway from G-protein-coupled receptors to cell death via p38 mitogen-/stress-activated protein kinase independent of p53 and caspase activation.

p38 MAP kinase regulates benzo(a)pyrene-induced apoptosis through the regulation of p53 activation

Polycyclic aromatic hydrocarbons, such as benzo(a)pyrene (BaP), are widespread in the environment and cause untoward effects, including carcinogenesis, in mammalian cells. However, the molecular mechanism of apoptosis by BaP is remained to be elusive. Pharmacological inhibition of p38 kinase markedly inhibited the BaP-induced cytotoxicity, which was proven as apoptosis characterized by an increase in sub-G(0)/G(1) fraction of DNA content, ladder-pattern fragmentation of genomic DNA, and catalytic activation of caspase-3 with PARP cleavage. Our data also demonstrated that activation of caspase-3 was accompanied with activation of caspase-9 and mitochondrial dysfunction, which was also apparently suppressed by pretreatment with p38 kinase inhibitors. Also, pharmacological inhibition of p38 markedly inhibited the phosphorylation, accumulated expression, and transactivation activity of p53 in BaP-treated cells. Adenoviral overexpression of human p53 (wild-type) further augmented in increase of PARP cleavage and the sub-G(0)/G(1) fraction of DNA content. Furthermore, p53 mediated apoptotic activity in BaP-treated cells was inhibited by p38 kinase inhibitor. The current data collectively indicate that BaP induces apoptosis of Hepa1c1c7 cells via activation of p53-related signaling, which was, in part, regulated by p38 kinase.

Attenuation of BPDE-induced p53 accumulation by TPA is associated with a decrease in stability and phosphorylation of p53 and downregulation of NFkappaB activation: role of p38 MAP kinase

DNA damage caused by benzo[a]pyrene (B[a]P) or other polynuclear hydrocarbons (PAHs) induce p53 protein as a protective measure to eliminate the possibility of mutagenic fixation of the DNA damage. 12-O-tetradecanoylphorbol-13-acetate (TPA) inhibits p53 response induced by B[a]P and other DNA-damaging agents and may cause tumor promotion. The molecular mechanism of attenuation of B[a]P-induced p53 response by TPA is not known. We investigated the effect of TPA on p53 response in (+/-)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE)-treated mouse epidermal JB6(P(+)) Cl 41 cells. BPDE treatment induced p53 accumulation which was attenuated significantly by TPA. Cells treated with BPDE and TPA showed increased ratio of Mdm2 to p53 proteins in p53 immunoprecipitate and decreased p53 life span compared to BPDE-treated cells indicating p53 destabilization by TPA. TPA also inhibited BPDE-induced p53 phosphorylation at serine15. Activation of both ERKs and p38 MAPK by BPDE and attenuation of BPDE-induced p53 accumulation by U0126 or SB202190, specific inhibitor of MEK1/2 or p38 MAPK, indicate the role of ERKs and p38 MAPK in p53 accumulation. Interestingly, TPA potentiated BPDE-induced activation of ERKs whereas p38 MAPK activation was significantly inhibited by TPA, suggesting that inhibition of p38 MAPK is involved in p53 attenuation by TPA. Furthermore, SB202190 treatment caused decreased p53 stability and inhibition of phosphorylation of p53 at serine15 in BPDE-treated cells. We also observed that TPA or SB202190 attenuated BPDE-induced nuclear factor kappa B (NFkappaB) activation in JB6 Cl 41 cells harboring NFkappaB reporter plasmid. To our knowledge this is the first report that TPA inhibits chemical carcinogen-induced NFkappaB activation. Interference of TPA with BPDE-induced NFkappaB activation implicates abrogation of p53 function which has been discussed. Overall, our data suggest that abrogation of BPDE-induced p53 response and of NFkappaB activation by TPA is mediated by impairment of the signaling pathway involving p38 MAPK.

An evaluation of the ability of pifithrin-α and -β to inhibit p53 function in two wild-type p53 human tumor cell lines

The small-molecule compound pifithrin-alpha (PFT-alpha) has been reported to inhibit p53 function and protect against a variety of genotoxic agents. We show here that PFT-alpha is unstable in tissue culture medium and is rapidly converted to its condensation product PFT-beta. Both compounds showed limited solubility with PFT-alpha precipitating out of tissue culture medium at concentrations >30 micromol/L. PFT-alpha and -beta exhibited cytotoxic effects in vitro towards two human wild-type p53-expressing tumor cell lines, A2780 ovarian and HCT116 colon (IC(50) values for both cell lines were 21.3 +/- 8.1 micromol/L for PFT-alpha and 90.3 +/- 15.5 micromol/L for PFT-beta, mean +/- SD, n = 4). There was no evidence of protection by clonogenic assay with either compound in combination with ionizing radiation. Indeed, there was some evidence that PFT-alpha enhanced cytotoxicity, particularly at higher concentrations of PFT-alpha. Neither compound had any effect on p53, p21, or MDM-2 protein expression following ionizing radiation exposure and there was no evidence of any abrogation of p53-dependent, ionizing radiation-induced cell cycle arrest. Similarly, there was no evidence of cellular protection, or of effects on p53-dependent gene transcription, or on translation of MDM-2 or p21 following UV treatment of these human tumor cell lines. In addition, there was no effect on p53 or p21 gene transactivation or p38 phosphorylation after UV irradiation of NIH-3T3 mouse fibroblasts. In conclusion, neither PFT-alpha nor -beta can be regarded as a ubiquitous inhibitor of p53 function, and caution should be exercised in the use of these agents as specific p53 inhibitors.

Puckered, a Drosophila MAPK phosphatase, ensures cell viability by antagonizing JNK-induced apoptosis

MAPK phosphatases (MKPs) are important negative regulators of MAPKs in vivo, but ascertaining the role of specific MKPs is hindered by functional redundancy in vertebrates. Thus, we characterized MKP function by examining the function of Puckered (Puc), the sole Drosophila Jun N-terminal kinase (JNK)-specific MKP, during embryonic and imaginal disc development. We demonstrate that Puc is a key anti-apoptotic factor that prevents apoptosis in epithelial cells by restraining basal JNK signaling. Furthermore, we demonstrate that JNK signaling plays an important role in gamma-irradiation-induced apoptosis, and examine how JNK signaling fits into the circuitry regulating this process. Radiation upregulates both JNK activity and puc expression in a p53-dependent manner, and apoptosis induced by loss of Puc can be suppressed by p53 inactivation. JNK signaling acts upstream of both Reaper and effector caspases. Finally, we demonstrate that JNK signaling directs normal developmentally regulated apoptotic events. However, if cell death is prevented, JNK activation can trigger tissue overgrowth. Thus, MKPs are key regulators of the delicate balance between proliferation, differentiation and apoptosis during development.

Loss of Tumor Suppressor p53 Decreases PTEN Expression and Enhances Signaling Pathways Leading to Activation of Activator Protein 1 and Nuclear Factor κB Induced by UV Radiation

Transcription factor p53 and phosphatase PTEN are two tumor suppressors that play essential roles in suppression of carcinogenesis. However, the mechanisms by which p53 mediates anticancer activity and the relationship between p53 and PTEN are not well understood. In the present study, we found that pretreatment of mouse epidermal Cl41 cells with pifithrin-alpha, an inhibitor for p53-dependent transcriptional activation, resulted in a marked increase in UV-induced activation of activator protein 1 (AP-1) and nuclear factor kappaB (NF-kappaB). Consistent with activation of AP-1 and NF-kappaB, pifithrin-alpha was also able to enhance the UV-induced phosphorylation of c-Jun-NH2-kinases (JNK) and p38 kinase, whereas it did not show any effect on phosphorylation of extracellular signal-regulated kinases. Furthermore, the UV-induced signal activation, including phosphorylation of JNK, p38 kinase, Akt, and p70S6K, was significantly enhanced in p53-deficient cells (p53-/-), which can be reversed by p53 reconstitution. In addition, knockdown of p53 expression by its small interfering RNA also caused the elevation of AP-1 activation and Akt phosphorylation induced by UV radiation. These results show that p53 has a suppressive activity on the cell signaling pathways leading to activation of AP-1 and NF-kappaB in cell response to UV radiation. More importantly, deficiency of p53 expression resulted in a decrease in PTEN protein expression, suggesting that p53 plays a critical role in the regulation of PTEN expression. In addition, overexpression of wild-type PTEN resulted in inhibition of UV-induced AP-1 activity. Because PTEN is a well-known phosphatase involved in the regulation of phosphatidylinositol 3-kinase (PI-3K)/Akt signaling pathway, taken together with the evidence that PI-3K/Akt plays an important role in the activation of AP-1 and NF-kappaB during tumor development, we anticipate that inhibition of AP-1 and NF-kappaB by tumor suppressor p53 seems to be mediated via PTEN, which may be a novel mechanism involved in anticancer activity of p53 protein.

Lack of p53 Ser389 phosphorylation predisposes mice to develop 2-acetylaminofluorene-induced bladder tumors but not ionizing radiation-induced lymphomas

Cellular activity of the tumor suppressor protein p53 is primarily regulated by posttranslational modifications. Phosphorylation of the COOH terminus, including Ser389, is thought to result in a conformational change of the p53 protein, enhancing DNA binding and transcriptional activity. In vitro studies presented here show that, in addition to UV radiation, Ser389 is phosphorylated upon exposure to 2-acetylaminofluorene (2-AAF). Both agents induce bulky DNA adducts repaired by nucleotide excision repair (NER). In contrast, ionizing radiation, known to induce DNA damage not repaired by NER, does not result in Ser389 phosphorylation. Previously, we have shown that p53.S389A mutant mice, lacking the Ser389 phosphorylation site, are sensitive to developing UV-induced skin tumors. Here, we show that p53.S389A mice are also prone to developing 2-AAF-induced urinary bladder tumors, whereas no increased tumor response was found upon ionizing irradiation. These results provide evidence for our hypothesis that phosphorylation of Ser389 is important for activation of p53 to exert its function as a tumor suppressor not exclusively upon the presence of UV-induced DNA damage, but also upon exposure to other bulky adduct-inducing agents. Analysis of 2-AAF- and UV-induced tumors from p53.S389A mice revealed the presence of additional p53 mutations, indicating that lack of Ser389 phosphorylation by itself is not sufficient to abrogate p53 function in tumor suppression. In addition, analyses of skin tumors of p53.S389A mice revealed an interesting hotspot mutation previously found exclusively in NER-deficient mice and patients.

The p53 pathway is synergized by p38 MAPK signaling to mediate 11,11′-dideoxyverticillin-induced G2/M arrest

The phytochemical 11,11'-dideoxyverticillin, derived from the fungus Shiraia bambusicola, has been shown to possess potent anticancer activity in vitro and in vivo. Here, we investigated the effect of 11,11'-dideoxyverticillin on cell cycle progression, and explored the potential mechanisms for this effect. A concentration- and time-dependent cell cycle blockade at G2/M phase was observed in human colon cancer cells (HCT-116) following 11,11'-dideoxyverticillin treatment and was associated with marked increases in levels of p53, phospho-p53(ser20) and phospho-Chk2(Thr 68). When wild type p53 expression was specifically inhibited by RNA interference, HCT-116 cells treated with 11,11'-dideoxyverticillin failed to arrest in G2/M and did not show increased phospho-Chk2(Thr 68). On the other hand, 11,11'-dideoxyverticillin treatment also elicited p38 MAP kinase activity and expression of phospho-p38 MAPK. Treatment with a specific p38 MAPK inhibitor (SB203580) successfully inhibited p38 MAPK and delayed the onset of G2/M arrest induced by 0.5 microM 11,11'-dideoxyverticillin after approximately 6 h, but did not abolish the induction of G2/M arrest. Additionally, SB203580 did not alter the levels of p53, phospho-p53 (ser20), or phospho-Chk2 (Thr68) proteins in 11,11'-dideoxyverticillin-treated cells. Together, these findings indicate that p53-mediated phosphorylation of Chk2 maybe plays a vital role in 11,11'-dideoxyverticillin-induced G2/M arrest, and that p38 MAPK might accelerate this progression. Our work suggests a new possibility of interactions among p53, Chk2 and p38 MAPK signaling in G2/M arrest.

Activation and signaling of the p38 MAP kinase pathway

The family members of the mitogen-activated protein (MAP) kinases mediate a wide variety of cellular behaviors in response to extracellular stimuli. One of the four main sub-groups, the p38 group of MAP kinases, serve as a nexus for signal transduction and play a vital role in numerous biological processes. In this review, we highlight the known characteristics and components of the p38 pathway along with the mechanism and consequences of p38 activation. We focus on the role of p38 as a signal transduction mediator and examine the evidence linking p38 to inflammation, cell cycle, cell death, development, cell differentiation, senescence and tumorigenesis in specific cell types. Upstream and downstream components of p38 are described and questions remaining to be answered are posed. Finally, we propose several directions for future research on p38.

The PI3K inhibitor LY294002 prevents p53 induction by DNA damage and attenuates chemotherapy-induced apoptosis

The p53 tumor suppressor plays a key role in the natural protection against cancer. Activation of p53 by DNA-damaging agents can contribute to successful elimination of cancer cells via chemotherapy-induced apoptosis. The phosphatidylinositol-3 kinase (PI3K) pathway, triggered in normal cells upon exposure to growth factors, regulates a cascade of proliferation and survival signals. The PI3K pathway is abnormally active in many cancers, thus making it an attractive target for inactivation in an attempt to achieve better cancer therapy. We report here that exposure to LY294002, a potent PI3K inhibitor, aborts the activation of p53 by several drugs commonly used in cancer chemotherapy. Concomitantly, LY294002 attenuates p53-dependent, chemotherapy-induced apoptosis of cancer cells. These findings invoke an unexpected positive role for PI3K in p53 activation by anticancer agents, and suggest that the efficacy of PI3K inhibitors in cancer therapy may be greatly affected by the tumor p53 status.

Id2 and p53 participate in apoptosis during unloading-induced muscle atrophy

Apoptotic signaling was examined in the patagialis (PAT) muscles of young adult and old quail. One wing was loaded for 14 days to induce hypertrophy and then unloaded for 7 or 14 days to induce muscle atrophy. Although the nuclear Id2 protein content was not different between unloaded and control muscles in either age group, cytoplasmic Id2 protein content of unloaded muscles was higher than that in contralateral control muscles after 7 days of unloading in young quails. Nuclear and cytoplasmic p53 contents and the p53 nuclear index of the unloaded muscles were higher than those in control muscles after 7 days of unloading in young quails, whereas in aged quails, the p53 and Id2 contents and p53 nuclear index of the unloaded muscles were not altered by unloading. Immunofluorescent staining indicated that myonuclei and activated satellite cell nuclei contributed to the increased number of p53-positive nuclei. Conversely, unloading in either young adult or aged PAT muscles did not alter c-Myc protein content. Although Cu-Zn-SOD content was not different in unloaded and control muscles, Mn-SOD content increased in PAT muscles after 7 days of unloading in young quails, suggesting that unloading induced an oxidative disturbance in these muscles. Moderate correlational relationships existed among Id2, p53, c-Myc, SOD, apoptosis-regulatory factors, and TdT-mediated dUTP nick end labeling index. These data indicate that Id2 and p53 are involved in the apoptotic responses during unloading-induced muscle atrophy after hypertrophy in young adult birds. Furthermore, our data suggest that there is an aging-dependent regulation of Id2 and p53 during unloading of previously hypertrophied muscles.

Phosphorylation of Ser28 in histone H3 mediated by mixed lineage kinase-like mitogen-activated protein triple kinase alpha

The mitogen-activated protein kinase cascades elicit modification of chromatin proteins such as histone H3 by phosphorylation concomitant with gene activation. Here, we demonstrate for the first time that the mixed lineage kinase-like mitogen-activated protein triple kinase (MLTK)-alpha phosphorylates histone H3 at Ser28. MLTK-alpha but neither a kinase-negative mutant of MLTK-alpha nor MLTK-beta interacted with and phosphorylated histone H3 in vivo and in vitro. When overexpressed in 293T or JB6 Cl41 cells, MLTK-alpha phosphorylated histone H3 at Ser28 but not at Ser10. The interaction between MLTK-alpha and histone H3 was enhanced by stimulation with ultraviolet B light (UVB) or epidermal growth factor (EGF), which resulted in the accumulation of MLTK-alpha in the nucleus. UVB- or EGF-induced phosphorylation of histone H3 at Ser28 was not affected by PD 98059, a MEK inhibitor, or SB 202190, a p38 kinase inhibitor, in MLTK-alpha-overexpressing JB6 Cl41 cells. Significantly, UVB- or EGF-induced phosphorylation of histone H3 at Ser28 was blocked by small interfering RNA of MLTK-alpha. The inhibition of histone H3 phosphorylation at Ser28 in the MLTK-alpha knock-down JB6 Cl41 cells was not due to a defect in mitogen- and stress-activated protein kinase 1 or 90-kDa ribosomal S6 kinase (p90RSK) activity. In summary, these results illustrate that MLTK-alpha plays a key role in the UVB- and EGF-induced phosphorylation of histone H3 at Ser28, suggesting that MLTK-alpha might be a new histone H3 kinase at the level of mitogen-activated protein kinase kinase kinases.

Increased sensitivity to UV radiation in mice with a p53 point mutation at Ser389

Phosphorylation is important for p53 protein stabilization and activation after DNA damage. Serine 389 of p53 is specifically phosphorylated after UV irradiation, whereas gamma radiation activates p53 through a different pathway. To study the in vivo significance of p53 phosphorylation at serine 389, we generated a physiological mouse model in which p53 phosphorylation at serine 389 is abolished by alanine substitution. Homozygous mutant p53.S389A mice are viable and have an apparently normal phenotype. However, cells isolated from these mice are partly compromised in transcriptional activation of p53 target genes and apoptosis after UV irradiation, whereas gamma radiation-induced responses are not affected. Moreover, p53.S389A mice show increased sensitivity to UV-induced skin tumor development, signifying the importance of serine 389 phosphorylation for the tumor-suppressive function of p53.

Sustained activation of p38 mitogen-activated protein kinase and c-Jun N-terminal kinase pathways by hepatitis B virus X protein mediates apoptosis via induction of Fas/FasL and tumor necrosis factor (TNF) receptor 1/TNF-alpha expression

Activation of the cellular stress pathways (c-Jun N-terminal kinase [JNK] and p38 mitogen-activated protein [MAP] kinase) is linked to apoptosis. However, whether both pathways are required for apoptosis remains unresolved. Hepatitis B virus X protein (pX) activates p38 MAP kinase and JNK pathways and, in response to weak apoptotic signals, sensitizes hepatocytes to apoptosis. Employing hepatocyte cell lines expressing pX, which was regulated by tetracycline, we investigated the mechanism of apoptosis by p38 MAP kinase and JNK pathway activation. Inhibition of the p38 MAP kinase pathway rescues by 80% the initiation of pX-mediated apoptosis, whereas subsequent apoptotic events involve both pathways. pX-mediated activation of p38 MAP kinase and JNK pathways is sustained, inducing the transcription of the death receptor family genes encoding Fas/FasL and tumor necrosis factor receptor 1 (TNFR1)/TNF-alpha and the p53-regulated Bax and Noxa genes. The pX-dependent expression of Fas/FasL and TNFR1/TNF-alpha mediates caspase 8 activation, resulting in Bid cleavage. In turn, activated Bid, acting with pX-induced Bax and Noxa, mediates the mitochondrial release of cytochrome c, resulting in the activation of caspase 9 and apoptosis. Combined antibody neutralization of FasL and TNF-alpha reduces by 70% the initiation of pX-mediated apoptosis. These results support the importance of the pX-dependent activation of both the p38 MAP kinase and JNK pathways in pX-mediated apoptosis and suggest that this mechanism of apoptosis occurs in vivo in response to weak apoptotic signals.

TNFalpha induces sequential activation of Cdc42- and p38/p53-dependent pathways that antagonistically regulate filopodia formation

Cell migration is an essential function in various physiological processes, including tissue repair and tumour invasion. Repair of tissue damage requires the recruitment of fibroblasts to sites of tissue injury, which is mediated in part by the cytokine tumour necrosis factor alpha (TNFalpha). As dynamic rearrangements of actin cytoskeleton control cell locomotion, this implicates that TNFalpha is a potent coordinator of cellular actin changes. We have investigated the role of TNFalpha in regulating the cortical actin-containing structures essential for cell locomotion called filopodia. Kinetic analysis of TNFalpha-treated mouse embryonic fibroblasts (MEFs) revealed a dual effect on filopodia formation: a rapid and transient induction mediated by Cdc42 GTPase that is then counteracted by a subsequent sustained inhibition requiring activation of the mitogen-activated protein kinase p38 but not Cdc42 activity. This inhibition also involves the tumour suppressor p53, given that it is activated in response to TNFalpha following the same time course as the decrease of filopodia formation. This functional activation of p53, measured by transcription induction of its target p21WAF1(p21), is also associated with p38 kinase-dependent phosphorylation of p53 at serine 18. Furthermore, TNFalpha did not inhibit filopodia formation in MEFs treated with the transcription inhibitor actinomycin D, in p53-deficient MEFs, or MEFs expressing p53 mutants H273 or H175, which supports a role for the transcriptional activity of p53 in mediating TNFalpha-dependent filopodia inhibition. Our data delineate a novel inhibitory pathway in which TNFalpha prevents filopodia formation and cell migration through the activation of the mitogen-activated protein kinase (MAPK) p38, which in turn activates p53. This shows that TNFalpha on its own initiates antagonistic signals that modulate events linked to cell migration.

Essential role of p53 in silica-induced apoptosis

Occupational exposure to mineral dusts, such as silica, has been associated with progressive pulmonary inflammation, lung cancer, and fibrosis. However, the mechanisms involved in this process are poorly understood. Because p53 is a key transcription factor regulating many important apoptosis-related genes, we hypothesized that p53 may play a key role in silica-induced apoptosis and that abnormal regulation of p53 by silica may contribute to development of lung cancer as well as silicosis. We used both in vitro and in vivo studies to test this hypothesis. Treatment of JB6 cells carrying a p53-luciferase reporter plasmid with silica caused dose-dependent p53 transactivation. Western blot indicates that silica not only stimulated p53 protein expression but also caused p53 phosphorylation at Ser392. TUNEL and DNA fragmentation analysis show that silica caused apoptosis in both JB6 cells and wild-type p53 (p53+/+) fibroblasts but not in p53-deficient (p53-/-) fibroblasts. Similar results were obtained by in vivo studies. Intratracheal instillation of mice with silica induced apoptosis in the lung of p53+/+ mice, whereas this induction was significantly inhibited in p53-/- mice. Confocal image analysis indicates that most apoptotic cells induced by silica were alveolar macrophages. These results demonstrate for the first time that silica induces p53 transactivation via induction of p53 protein expression and phosphorylation of p53 protein and that p53 plays a crucial role in the signal transduction pathways of silica-induced apoptosis. This finding may provide an important link in understanding the molecular mechanisms of silica-induced carcinogenesis and pathogenesis in the lung.

Intracellular Abeta42 activates p53 promoter: a pathway to neurodegeneration in Alzheimer's disease

The amyloid beta-protein (Abeta) ending at 42 plays a pivotal role in Alzheimer's disease (AD). We have reported previously that intracellular Abeta42 is associated with neuronal apoptosis in vitro and in vivo. Here, we show that intracellular Abeta42 directly activated the p53 promoter, resulting in p53-dependent apoptosis, and that intracellular Abeta40 had a similar but lesser effect. Moreover, oxidative DNA damage induced nuclear localization of Abeta42 with p53 mRNA elevation in guinea-pig primary neurons. Also, p53 expression was elevated in brain of sporadic AD and transgenic mice carrying mutant familial AD genes. Remarkably, accumulation of both Abeta42 and p53 was found in some degenerating-shape neurons in both transgenic mice and human AD cases. Thus, the intracellular Abeta42/p53 pathway may be directly relevant to neuronal loss in AD. Although neurotoxicity of extracellular Abeta is well known and synaptic/mitochondrial dysfunction by intracellular Abeta42 has recently been suggested, intracellular Abeta42 may cause p53-dependent neuronal apoptosis through activation of the p53 promoter; thus demonstrating an alternative pathogenesis in AD.

Post-translational modification of p53 in tumorigenesis

Interest in the tumour suppressor p53 has generated much information regarding the complexity of its function and regulation in carcinogenesis. However, gaps still exist in our knowledge regarding the role of p53 post-translational modifications in carcinogenesis and cancer prevention. A thorough understanding of p53 will be extremely useful in the development of new strategies for treating and preventing cancer, including restoration of p53 function and selective killing of tumours with mutant TP53.

Oxidative stress, radiation-adaptive responses, and aging

Organisms living in an aerobic environment were forced to evolve effective cellular strategies to detoxify reactive oxygen species. Besides diverse antioxidant enzymes and compounds, DNA repair enzymes, and disassembly systems, which remove damaged proteins, regulation systems that control transcription, translation, and activation have also been developed. The adaptive responses, especially those to radiation, are defensive regulation mechanisms by which oxidative stress (conditioning irradiation) elicits a response against damage because of subsequent stress (challenging irradiation). Although many researchers have investigated these molecular mechanisms, they remain obscure because of their complex signaling pathways and the involvement of various proteins. This article reviews the factors concerned with radiation-adaptive response, the signaling pathways activated by conditioning irradiation, and the effects of aging on radiation-adaptive response. The proteomics approach is also introduced, which is a useful method for studying stress response in cells.

Basic fibroblast growth factor-induced cell death is effected through sustained activation of p38MAPK and up-regulation of the death receptor p75NTR

Basic fibroblast growth factor (bFGF) induces cell death in cells of the Ewing's sarcoma family of tumors in vivo and in vitro. In this study we demonstrate that this is dependent on the rapid and sustained activation of p38(MAPK), in contrast to the transient activation of p38(MAPK) associated with bFGF-induced cell proliferation. Stem cell factor-induced survival of TC-32 cells was also associated with transient activation of p38(MAPK). Inhibition of p38(MAPK) by SB202190 and p38(MAPK) small interfering RNA reduces bFGF-induced death in TC-32 cells, consistent with the hypothesis that activation of p38(MAPK) is essential for induction of death by bFGF. This appears to be dependent on sustained activation of p38(MAPK), demonstrated by inhibition of bFGF-induced cell death following addition of SB202190 to TC-32 cells 5 min after exposure to bFGF (20 ng/ml) and activation of p38(MAPK). Prolonged activation of p38(MAPK) is accompanied by a rapid and sustained phosphorylation of Ras and ERK; inhibition of ERK phosphorylation using the MEK-1 inhibitor PD98059 rescued approximately 30% of cells from bFGF-induced death suggesting ERK plays a secondary role in the induction of death. This hypothesis is supported by observations in the A673 cell line; bFGF induced sustained activation of ERK and transient activation of p38(MAPK), which was not associated with cell death. These data demonstrate that sustained activation of p38(MAPK) is essential for activation of the death cascade following exposure of Ewing's sarcoma family of tumors cells to bFGF and provide evidence that activation of p38(MAPK) results in an up-regulation of the death receptor p75(NTR).

p38 Mitogen-Activated Protein Kinase Mediates Dual Role of Ultraviolet B Radiation in Induction of Maturation and Apoptosis of Monocyte-Derived Dendritic Cells

Although ultraviolet B (UVB) induces apoptosis and functional perturbations in dendritic cells (DC), for example, Langerhans cells (LC), it also stimulates some LC into maturation after irradiation in vivo. To analyze its reciprocal effects on DC, we elucidated the direct effect of UVB on DC in vitro using human monocyte-derived DC (MoDC). UVB from 50 to 200 J per m2 stimulated the maturation of MoDC with (1) augmented expression of CD86 and HLA-DR, (2) enhanced production of IL-1beta, IL-6, IL-8, and TNF-alpha at both the mRNA and protein levels, and (3) enhanced allostimulatory capacity on a per-cell basis, whereas the exceeded doses induced apoptotic cell death. Western-blot analysis of MoDC after UVB demonstrated a concentration-dependent phosphorylation of p38- and c-JUN N-terminal kinase (JNK)-mitogen-activated protein kinases (MAPK), but not that of extracellular signal-regulated kinases. p38 MAPK-inhibitor, SB203580, inhibited both UVB-induced maturation and apoptosis of MoDC. Interestingly, MoDC that had undergone apoptosis exhibited an augmented expression of HLA-DR without upregulation of CD86 antigen, suggesting their tolerogenic phenotype. Thus, our study revealed a dual effect of UVB, to stimulate maturation or to induce apoptosis in MoDC, depending on the dosage, via p38 MAPK pathway.

Roles of p38 and JNK mitogen-activated protein kinase pathways during cantharidin-induced apoptosis in U937 cells

Cantharidin is an active compound from blister beetles traditionally used for the treatment of cancer. It is known to exert its antitumor activity by inducing apoptosis in cancer cells. However, its signaling pathway still remains unclear. Therefore, we investigated the roles of the mitogen-activated protein kinases (MAPKs) and the tumor suppressor gene, p53, during cantharidin-induced apoptosis in U937 human leukemic cells. Cantharidin effectively activated ERK-1/2, p38 and JNK in U937 cells in a time- and dose-dependent manner. Cantharidin also exhibited a strong cytotoxicity and induced apoptosis in U937 cells. For the evaluation of the role of MAPKs, PD98059, SB202190 and SP600125 were used as MAPK inhibitors for ERK-1/2, p38 and JNK. PD98059 did not affect cantharidin-induced cytotoxicity and apoptosis, whereas SB202190 and SP600125 significantly interfered with cytotoxic and apoptotic activities induced by cantharidin. Cantharidin alone induced the apoptosis by phosphorylation of p53, up-regulation of downstream target genes, MDM2 and p21 and also cleaved caspase-3, whereas SB202190 and SP600125 caused the down-regulation of p53, MDM-2, p21 and cleaved caspase-3 after a co-treatment with cantharidin. Similarly, SB202190 and SP600125 significantly disturbed the caspase-3 activity after a co-treatment with cantharidin by colorimetric assay. Taken together, these results suggest that cantharidin can induce apoptosis by activation of p38 and JNK MAP kinase pathways associated with p53 and caspase-3.

Phosphorylation of p53 on Thr55 by ERK2 is necessary for doxorubicin-induced p53 activation and cell death

We recently reported that exposure of human cervical carcinoma cells to doxorubicin results in extracellular signal-regulated kinase (ERK)2 activation, which in turn phosphorylates p53 on a previously uncharacterized site, Thr55. This study sought to clarify the biological significance of doxorubicin-induced Thr55 phosphorylation. In breast carcinoma MCF7 cells, doxorubicin (300 nM) activated ERK2 and induced phosphorylation of p53 on Thr55 residues. Pretreatment of MCF7 cells with an ERK2 chemical inhibitor, PD98059 or U0126, blocked doxorubicin-induced p53 activation and suppressed phosphorylation of p53Thr55. MCF55a cells were established by transfection of full-length p53 carrying Thr55 mutation (Thr to Ala) into MCF7 cells. Doxorubicin (500 nM) could not induce p53 activation in MCF55a cells, which showed significantly increased drug resistance toward doxorubicin. While the expression of the apoptotic protein, Bax, showed no difference between MCF7 and MCF55a cells, Bcl-2, an antiapoptotic protein, was constitutively expressed in MCF55a cells. The increase of Bcl-2 protein and/or Bcl-2/Bax ratio might at least partly contribute to the drug resistance of MCF55a cells. In summary, our results suggest that phosphorylation of p53Thr55 by ERK2 is important for doxorubicin-induced p53 activation and cell death.

Bcl-xL and E1B-19K proteins inhibit p53-induced irreversible growth arrest and senescence by preventing reactive oxygen species-dependent p38 activation

In this study, we describe novel functions of the anti-apoptotic Bcl-2 family proteins. Bcl-x(L) and E1B-19K were found to inhibit p53-induced irreversible growth arrest and senescence, but not to inhibit transient growth arrest, implying that Bcl-x(L) and E1B-19K are specifically involved in senescence without participating in growth arrest. We provide several lines of evidences showing that the functions of Bcl-x(L) and E1B-19K to prevent generation of reactive oxygen species (ROS) are important to inhibit senescence induction. First, we found that that ROS are increased during p53-induced senescence. Moreover, Bcl-x(L) and E1B-19K inhibit this p53-induced ROS generation. Second, antioxidants prevent the induction of senescence and ROS by p53, but not the persistence of the senescence phenotype. Third, the anti-senescence functions of Bcl-x(L) and E1B-19K were suppressed by adding exogenous ROS. These results suggest that Bcl-x(L) and E1B-19K inhibit senescence induction by preventing ROS generation. Furthermore, p38 kinase was found to be activated during p53-induced senescence, but not in cells expressing Bcl-x(L) or E1B-19K, or in cells treated with anti-oxidants. Consistently, a chemical inhibitor of p38 kinase, SB203580, was found to inhibit p53-induced senescence, but only when treated before the cellular commitment to senescence, implying that p38 kinase is necessary for senescence induction. Therefore, Bcl-x(L) and E1B-19K inhibit p53-induced senescence by preventing ROS generation, which in turn leads to the activation of p38 kinase. These results also suggest that the oncogenic potential of Bcl-2 is due to its ability to inhibit senescence as well as apoptosis.

Neurotoxicity of platinum compounds: comparison of the effects of cisplatin and oxaliplatin on the human neuroblastoma cell line SH-SY5Y

The main dose-limiting side effect of cancer treatment with platinum compounds is peripheral neurotoxicity. To investigate the intracellular mechanisms of platinum drugs neurotoxicity we have studied the effects of cisplatin and oxaliplatin on the human neuroblastoma cell line SH-SY5Y. Both platinum compounds are toxic causing cellular death by inducing apoptosis but oxaliplatin is less neurotoxic than cisplatin. The study of the proteins involved in the intracellular transduction pathways that may cause apoptotic death, revealed a very similar pattern of changes after exposure to cisplatin or oxaliplatin. In particular, as demonstrated by densitometric analysis, after exposure to both platinum compounds the total amount of the anti-apoptotic protein Bcl-2 was significantly reduced. Conversely, the amount of the pro-apoptotic protein p53 significantly increased. Caspases 3 and 7 were activated, but their activation was a late event, indicating a secondary role in the apoptotic process. Among the mitogen activated protein kinases, only the p38 protein was activated (phosphorylated) early enough to have a possible role in inducing apoptosis, possibly through p53 stabilization. The results of the present study and the data of the literature demonstrate that the ways in which cisplatin and oxaliplatin are neurotoxic are very similar and include not only DNA damage, but also the modulation of specific molecules involved in regulating the cellular equilibrium between apoptotic death and the cell cycle.

Involvement of tumor suppressor protein p53 and p38 MAPK in caffeic acid phenethyl ester-induced apoptosis of C6 glioma cells

Caffeic acid phenethyl ester (CAPE), an active component of propolis, has many biological and pharmacological activities including antioxidant, anti-inflammation, antiviral action, and anticancer effect. Our previous studies showed that CAPE exhibited significant cytotoxicity in oral cancer cells. Herein we further investigated the cytotoxicity potential of CAPE and the mechanism of its action in C6 glioma cells. The data exhibited that C6 glioma cells underwent internucleosomal DNA fragmentation 24 hr after the treatment of CAPE (50 microM). The proportion of C6 glioma cells with hypodiploid nuclei was increased to 24% at 36 hr after the exposure. Further results showed that CAPE induced the release of cytochrome c from mitochondria into cytosol, and the activation of CPP32. CAPE application also enhanced the expression of p53, Bax, and Bak. Finally, the potential signaling components underlying CAPE induction of apoptosis were elucidated. We found that CAPE activated extracellular signal-regulated kinase (ERKs) and p38 mitogen-activated protein kinase (p38 MAPK) in C6 glioma cells. More importantly, p38 kinase formed a complex with p53 after the treatment of CAPE for 0.5 hr. The expression of p53, phospho-serine 15 of p53, and Bax, and inactivate form of CPP32 was suppressed by a pretreatment of a specific p38 MAPK inhibitor, SB203580. The resultant data suggest that p38 MAPK mediated the CAPE-induced p53-dependent apoptosis in C6 glioma cells.

Cytochrome c Release From Oridonin-Treated Apoptotic A375-S2 Cells Is Dependent on p53 and Extracellular Signal-Regulated Kinase Activation

We have reported that oridonin isolated from Rabdosia rubescens induces apoptosis of human melanoma A375-S2 cells within 12 h. In this study, TUNEL assay and flow cytometric analysis also indicate that one of the causes of A375-S2 cell death induced by oridonin was apoptosis. The cell death was preceded by the release of cytochrome c from the mitochondria. Twelve hours after treatment with oridonin, the ratio of Bax/Bcl-xL protein expression was increased and release of cytochrome c was decreased by an extracellular signal-regulated kinase (ERK) MAPK inhibitor (PD98059) and a phosphoinositide 3-kinases (PI3-K) inhibitor (wortmannin). A mitochondrial permeability transition (MPT) inhibitor, decylubiquinone, suppressed the release of cytochrome c without affecting Bax expression. The activation of p53 by oridonin was also blocked by wortmannin. In addtion, PD98059 and wortmannin significantly decreased oridonin-induced DNA fragmentation, but the p38 MAPK inhibitor (SB203580) did not after DNA fragmentation. Oridonin induced A375-S2 cell apoptosis by activating parallel p53 and ERK pathways, increasing the ratio of Bax/Bcl-xL protein expression, and promoting the release of cytochrome c into the cytosol, resulting in apoptotic cell death.