P38 alpha mitogen-activated protein kinase sensitizes cells to apoptosis induced by different stimuli

Gamma-tocotrienol-induced apoptosis in human gastric cancer SGC-7901 cells is associated with a suppression in mitogen-activated protein kinase signalling

Tocotrienols have been shown to inhibit proliferation and induce apoptosis in cancer cells. However, the molecular mechanisms involved in tocotrienol-induced apoptosis are still unclear. In the present study, gamma-tocotrienol induced apoptosis in human gastric adenocarcinoma SGC-7901 cell line through down regulation of the extracellular signal-regulated kinase (ERK) signalling pathway. Furthermore, gamma-tocotrienol-induced apoptosis was accompanied by down regulation of Bcl-2, up regulation of Bax, activation of caspase-3, and subsequent poly (ADP-ribose) polymerase cleavage. These results indicated that up or down regulation of Bcl-2 family proteins play a major role in the initiation of gamma-tocotrienol-induced apoptosis as an activator of caspase-3. Gamma-tocotrienol also down regulated the activation of the Raf-ERK signalling pathway, and down regulated c-Myc by decreasing the expressions of Raf-1 and p-ERK1/2 proteins. The results suggest that key regulators in tocotrienol-induced apoptosis may be Bcl-2 families and caspase-3 in SGC-7901 cells through down regulation of the Raf-ERK signalling pathway.

Increased TNFalpha and CCAAT/enhancer-binding protein homologous protein with aging predispose preadipocytes to resist adipogenesis

Fat depot sizes peak in middle age but decrease by advanced old age. This phenomenon is associated with ectopic fat deposition, decreased adipocyte size, impaired differentiation of preadipocytes into fat cells, decreased adipogenic transcription factor expression, and increased fat tissue inflammatory cytokine generation. To define the mechanisms contributing to impaired adipogenesis with aging, we examined the release of TNFalpha, which inhibits adipogenesis, and the expression of CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP), which blocks activity of adipogenic C/EBP family members, in preadipocytes cultured from young, middle-aged, and old rats. Medium conditioned by fat tissue, as well as preadipocytes, from old rats impeded lipid accumulation by preadipocytes from young animals. More TNFalpha was released by preadipocytes from old than young rats. Differences in TNFalpha-converting enzyme, TNFalpha degradation, or the presence of macrophages in cultures were not responsible. TNFalpha induced rat preadipocyte CHOP expression. CHOP was higher in undifferentiated preadipocytes from old than younger animals. Overexpression of CHOP in young rat preadipocytes inhibited lipid accumulation. TNFalpha short interference RNA reduced CHOP and partially restored lipid accumulation in old rat preadipocytes. CHOP normally increases during late differentiation, potentially modulating the process. This late increase in CHOP was not affected substantially by aging: CHOP was similar in differentiating preadipocytes and fat tissue from old and young animals. Hypoglycemia, which normally causes an adaptive increase in CHOP, was less effective in inducing CHOP in preadipocytes from old than younger animals. Thus increased TNFalpha release by undifferentiated preadipocytes with elevated basal CHOP contributes to impaired adipogenesis with aging.

Cyclin B1 proteolysis via p38 MAPK signaling participates in G2 checkpoint elicited by arsenite

Timely induction of cyclin B1 controls mitotic entry, whereas its proteolysis is essential for mitotic exit. By contrast, cyclin B1 transcription is repressed during G(2) arrest induced by DNA damage. The p38 mitogen-activated protein kinase is involved in the G(2) checkpoint; yet, its impact on cyclin B1 protein levels remains unclear. Here we show that untimely proteolysis of cyclin B1 following p38 activation contributes to G(2) checkpoint. Exposing early G(2) cells to arsenite impeded cyclin B1 protein accumulation, Cdk1 activation, and G(2)-to-M progression. Conversely, cyclin B1 was non-degradable in late G(2) and mitotic cells after arsenite. Cyclin B1 proteolysis was enhanced by arsenite in early G(2) and asynchronous cells. This rapid destruction of cyclin B1 was mediated via the ubiquitin-proteasome pathway probably in a Cdc20 and Cdh1 independent mechanism. Under arsenite, inhibition of p38 activation or depletion of p38alpha suppressed cyclin B1 ubiquitination and proteolysis, while forced expression of MKK6-p38 accelerated these events. Inactivation of p38 in arsenite-treated early G(2) cells allowed G(2)-to-M progression, blocked apoptosis, increased cell viability, and decreased micronucleus formation. Thus, p38 signaling pathway triggering cyclin B1 proteolysis after arsenite may play an important role in connecting G(2) arrest with apoptosis or genome instability.

Feedback regulation of p38 activity via ATF2 is essential for survival of embryonic liver cells

The ATF2 transcription factor is phosphorylated by the stress-activated mitogen-activated protein kinases (MAPKs) JNK and p38. We show that this phosphorylation is essential for ATF2 function in vivo, since a mouse carrying mutations in the critical phosphorylation sites has a strong phenotype identical to that seen upon deletion of the DNA-binding domain. In addition, combining this mutant with a knockout of the ATF2 homolog, ATF7, results in embryonic lethality with severe abnormalities in the developing liver and heart. The mutant fetal liver is characterized by high levels of apoptosis in developing hepatocytes and haematopoietic cells. Furthermore, we observe a significant increase in active p38 due to loss of a negative feedback loop involving the ATF2-dependent transcriptional activation of MAPK phosphatases. In embryonic liver cells, this increase drives apoptosis, since it can be suppressed by chemical inhibition of p38. Our findings demonstrate the importance of finely regulating the activities of MAPKs during development.

p38alpha MAPK can positively or negatively regulate Rac-1 activity depending on the presence of serum

The small GTP-ase Rac-1 can trigger p38 MAPK activation and, in turn, p38alpha can regulate signalling pathways that potentially impinge on Rac-1 activity. We have investigated the cross-talk between p38alpha and Rac-1 and found that p38alpha regulates the association between Rac-1 and caveolin-1 in serum-deprived cardiomyocytes. This interaction depends on cell attachment and correlates with higher levels of active Rac-1. Actin organization might regulate the formation of Rac-1-caveolin-1 complexes. In contrast, the Rac-1-caveolin-1 interaction is almost undetectable in the presence of serum, where Rac-1 activity is negatively regulated by p38alpha. Our results indicate that p38alpha can differentially contribute to Rac-1 activation depending on the presence of serum.

p38alpha MAP kinase mediates hypoxia-induced motor neuron cell death: a potential target of minocycline's neuroprotective action

Hypoxia-ischemia (HI) may play a significant role in motor neuron death associated with the pathology of spinal cord injury and, perhaps, amyotrophic lateral sclerosis. The present study employs an in vitro model of HI to investigate the role of a stress kinase pathway, i.e., p38 MAP kinase, in cell death signaling in a motor neuron cell line, i.e., NSC34, subjected to oxygen-glucose deprivation (OGD). Although the neurons were essentially tolerant to either hypoxia (0.2% O(2)) or low glucose (1 mM) alone, more than 60% of them died in response to combined low oxygen and low-glucose exposure. Minocycline, a semi-synthetic tetracycline known for its neuroprotective effects in models of neurodegeneration, afforded substantial (approximately 50%) protection against hypoxic cell death, assessed by lactate dehydrogenase release and flow cytometry, while suppressing OGD-induced p38 MAP kinase activation. An inhibitor of p38 kinase, SB203580, as well as siRNA-mediated down-regulation of p38 kinase elicited an almost complete blockade of OGD-induced cell death. The use of p38 isoform-specific siRNAs further revealed preferential involvement of the alpha over the beta isoform of p38 MAP kinase in hypoxic neuronal cell death in our model.

Nitrofen suppresses cell proliferation and promotes mitochondria-mediated apoptosis in type II pneumocytes

AIM

To characterize the molecular mechanisms of nitrofen-induced pulmonary hypoplasia.

METHODS

After administration of nitrofen to cultured type II A549 pneumocytes, cell proliferation and DNA synthesis were investigated by 3-(4,5- dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide colorimetry, colony formation assay, flow cytometry and [3H]-thymidine incorporation assay. Apoptosis was measured by terminal transferase-mediated dUTP nick-end-labeling, acridine orange-ethidium bromide staining and flow cytometry. Expression of proliferating cell nuclear antigen (PCNA) and apoptosis-related genes was assayed by immunofluorescence, RT-PCR and Western blot.

RESULTS

Nitrofen inhibited the cell proliferation of A549 cells in a dose- and time-dependent manner, accompanied by downregulation of PCNA. As a result, the DNA synthesis of nitrofentreated A549 cells decreased, while cell cycle was arrested at G0/G1 phase. Moreover, nitrofen induced apoptosis of A549 cells, which was not abolished by Z-Val-Ala- Asp(OCH3)- fluoromethylketone. In addition, nitrofen decreased the expression of Bcl-x( L), but not of Bcl-2, Bax, and Bak, resulting in a loss of mitochondrial membrane potential and the nuclear translocation of apoptosis-inducing factor (AIF). Meanwhile, nitrofen strongly activated the p38 mitogen-activated protein kinase (p38-MAPK). Pretreatment of cells with SB203580 (5 micromol/L) blocked nitrofen-induced phosphorylation of p38-MAPK and abolished nitrofen-induced AIF translocation and apoptosis in A549 cells.

CONCLUSION

Nitrofen suppresses the proliferation of cultured type II pneumocytes accompanied by the downregulation of PCNA, and induces mitochondria-mediated apoptosis involving the activation of p38-MAPK.

The p38α/β Mitogen-activated Protein Kinases Mediate Recruitment of CREB-binding Protein to Preserve Fast Myosin Heavy Chain IId/x Gene Activity in Myotubes

In skeletal muscle, the transformation of fast into slow fiber type is accompanied by shifts in fiber type-specific gene expression that includes down-regulation of the adult fast fiber myosin heavy chain IId/x (MyHCIId/x) gene. Here, we report that the mitogen-activated protein kinases (MAPKs) p38alpha/beta regulate MyHCIId/x gene expression. Electrical stimulation of rabbit skeletal muscle cells with a slow fiber type activity pattern and treatment of C2C12 myotubes with Ca(2+)-ionophore inhibited p38alpha/beta MAPKs and reduced fast fiber type MyHC protein expression and promoter activity. Pharmacological inhibition of p38alpha/beta also down-regulated MyHCII gene expression. In controls, binding of the myocyte enhancer factor-2 (MEF-2) isoforms C and D as a heterodimer to a proximal consensus site within the MyHCIId/x promoter and recruitment of a transcriptional coactivator, the CREB-binding protein CBP, were observed. Overexpression of wild type MEF-2C but not of a MEF-2C mutant that cannot be phosphorylated by p38 induced promoter activity. Mutation of the MEF-2-binding site decreased the inducing effect of overexpressed CBP. Inhibition of p38alpha/beta MAPKs abolished CBP binding, whereas enforced induction of p38 by activated MAPK kinase 6 (MKK6EE) enhanced binding of CBP and increased promoter activity. Furthermore, knockdown of endogenous CBP by RNA interference eliminated promoter activation by MEF-2C or MKK6EE. In electrical stimulated and Ca(2+)-ionophore-treated myotubes, CBP was absent in complex formation at that site. Taken together, the data indicate that p38alpha/beta MAPKs-mediated coactivator recruitment at a proximal MEF-2 site is important for MyHCIId/x gene regulation in skeletal muscle.

Genetic analysis of p38 MAP kinases in myogenesis: fundamental role of p38alpha in abrogating myoblast proliferation

The p38 mitogen-activated protein kinase (MAPK) pathway plays a critical role in skeletal muscle differentiation. However, the relative contribution of the four p38 MAPKs (p38alpha, p38beta, p38gamma and p38delta) to this process is unknown. Here we show that myoblasts lacking p38alpha, but not those lacking p38beta or p38delta, are unable to differentiate and form multinucleated myotubes, whereas p38gamma-deficient myoblasts exhibit an attenuated fusion capacity. The defective myogenesis in the absence of p38alpha is caused by delayed cell-cycle exit and continuous proliferation in differentiation-promoting conditions. Indeed, activation of JNK/cJun was enhanced in p38alpha-deficient myoblasts leading to increased cyclin D1 transcription, whereas inhibition of JNK activity rescued the proliferation phenotype. Thus, p38alpha controls myogenesis by antagonizing the activation of the JNK proliferation-promoting pathway, before its direct effect on muscle differentiation-specific gene transcription. More importantly, in agreement with the defective myogenesis of cultured p38alpha(Delta/Delta) myoblasts, neonatal muscle deficient in p38alpha shows cellular hyperproliferation and delayed maturation. This study provides novel evidence of a fundamental role of p38alpha in muscle formation in vitro and in vivo.

p38alpha MAP kinase as a sensor of reactive oxygen species in tumorigenesis

p38alpha is a stress-activated protein kinase that negatively regulates malignant transformation induced by oncogenic H-Ras, although the mechanisms involved are not fully understood. Here, we show that p38alpha is not a general inhibitor of oncogenic signaling, but that it specifically modulates transformation induced by oncogenes that produce reactive oxygen species (ROS). This inhibitory effect is due to the ROS-induced activation of p38alpha early in the process of transformation, which induces apoptosis and prevents the accumulation of ROS and their carcinogenic effects. Accordingly, highly tumorigenic cancer cell lines have developed a mechanism to uncouple p38alpha activation from ROS production. Our results indicate that oxidative stress sensing plays a key role in the inhibition of tumor initiation by p38alpha.

Altered cell adhesion and cell viability in a p38alpha mitogen-activated protein kinase-deficient mouse embryonic stem cell line

p38 mitogen-activated protein (MAP) kinase alpha (p38alpha) is a broadly expressed protein kinase that regulates growth and development. Most studies of p38alpha have been in somatic cells. Little is known about its function in embryonic stem (ES) cells. Using a ES cell line isolated from p38alpha knockout mouse embryos (p38alpha (-/-) ES cells), we investigated roles of p38alpha in the regulation of ES cell activities. p38alpha (-/-) ES cells displayed several altered features different from wild-type cells. The major findings are that p38alpha (-/-) ES cells have significantly increased cell adhesion to several extracelluar matrix proteins, correlating with elevated phosphorylation of focal adhesion kinase and paxillin. p38alpha (-/-) ES cells also showed increased cell viability, correlating with increased expression of survivin and activation of AKT (protein kinase B), two molecules that are known to improve cell viability. p38alpha (-/-) ES cells reach confluence faster than wild-type cells in routine cell culture. However, this is not due to a higher cell proliferation rate in p38alpha (-/-) ES cells, but rather is likely a result of improved cell adhesion and/or cell viability. Together our results indicated that p38alpha may negatively regulate mouse ES cell adhesion and viability.

Immunohistochemical study of phospho-Stat3-ser727 expression in feline mammary gland tumours

We describe the expression of pStat3-ser727 (signal transducer and activator of transcription 3 phosphorylated on serine 727) in normal, hyperplastic and neoplastic feline mammary gland tissue assessed by immunohistochemistry in 56 cats. The samples included 4 normal mammary non-lactating tissues, 13 hyperplastic lesions (9 lobular and 4 fibroepithelial) and 39 tumours (6 benign and 33 carcinomas). For immunohistochemistry, tissue sections were incubated with anti-pStat3-ser727 monoclonal antibodies and visualized with EnVision-DAB polymer. pStat3-ser727 positivity was quantified in a semi-quantitative manner, differentiating cytoplasmic and nuclear localization. Intense anti-pStat3-ser727 immunoreactivity was detected in epithelial neoplastic cells and in the fibro-component in two fibroepithelial hyperplastic lesions. The immunostaining was dot-like in the cytoplasm and homogeneous in the nuclei in both benign and malignant lesions. Statistically significant relations were observed between nuclear expression of pStat3-ser727 and the pleomorphism score (p = 0.006), mitotic activity (p < 0.0001), and histological grade (p < 0.0001). In contrast, no significant correlations were observed for cytoplasmic pStat3-ser727. These findings add new and interesting information on the potential role of the phosphorylated form of Stat3 in malignant lesions.

BIRB 796 enhances cytotoxicity triggered by bortezomib, heat shock protein (Hsp) 90 inhibitor, and dexamethasone via inhibition of p38 mitogen-activated protein kinase/Hsp27 pathway in multiple myeloma cell lines and inhibits paracrine tumour growth

We have previously shown that heat shock protein (Hsp) 27 or its upstream activator p38 mitogen-activated protein kinase (MAPK) confers resistance to bortezomib and dexamethasone (Dex) in multiple myeloma (MM) cells. This study examined anti-MM activity of a novel p38 MAPK inhibitor, BIRB 796, alone and in combination with conventional and novel therapeutic agents. BIRB 796 blocked baseline and bortezomib-triggered upregulation of p38 MAPK and Hsp27 phosphorylation, thereby enhancing cytotoxicity and caspase activation. The Hsp90 inhibitor 17-allylamino-17-demethoxy-geldanamycin (17-AAG) upregulated protein expression and phosphorylation of Hsp27; conversely, BIRB 796 inhibited this phosphorylation and enhanced 17-AAG-induced cytotoxicity. Importantly, BIRB 796 inhibited Hsp27 phosphorylation induced by 17-AAG plus bortezomib, thereby enhancing cytotoxicity. In bone marrow stromal cells (BMSC), BIRB 796 inhibited phosphorylation of p38 MAPK and secretion of interleukin-6 (IL-6) and vascular endothelial growth factor triggered by either tumour necrosis factor-alpha or tumour growth factor-beta1. BIRB 796 also inhibited IL-6 secretion induced in BMSCs by adherence to MM cells, thereby inhibiting tumour cell proliferation. These studies therefore suggest that BIRB 796 overcomes drug-resistance in the BM microenvironment, providing the framework for clinical trials of a p38 MAPK inhibitor, alone and in combination with bortezomib, Hsp90 inhibitor, or Dex, to improve patient outcome in MM.

Hyperactive variants of p38alpha induce, whereas hyperactive variants of p38gamma suppress, activating protein 1-mediated transcription

The p38 family of kinases is a subgroup of the mitogen-activated protein kinase family. It is composed of four isoforms and is involved in critical biological processes as well as in inflammatory diseases. The exact unique role of each p38 isoform in these processes is not understood well. To approach this question we have been developing intrinsically active variants of p38s. Recently we described a series of mutants of the human p38alpha, which were spontaneously active as recombinant proteins purified from Escherichia coli cells. We show here that some of these mutants are spontaneously active in several mammalian cells in culture. The spontaneous activity of some mutants is higher than the activity of the fully activated wild type counterpart. We further produced mutants of the other p38 isoforms and found that p38beta(D176A), p38gamma(D179A), p38delta(D176A), and p38delta(F324S) are spontaneously active in vivo. The active mutants are also spontaneously phosphorylated. To test whether the mutants actually fulfill downstream duties of p38 proteins, we tested their effect on activating protein 1(AP-1)-mediated transcription. Active mutants of p38alpha induced AP-1-driven reporter genes, as well as the c-jun and c-fos promoters. An active variant of p38gamma suppressed AP-1-mediated transcription. When active variants of p38alpha and p38gamma were co-expressed, AP-1 activity was not induced, showing that p38gamma is dominant over p38alpha with respect to AP-1 activation. Thus, intrinsically active variants that are spontaneously active in vivo have been obtained for all p38 isoforms. These variants have disclosed different effects of each isoform on AP-1 activity.

Swimming stress in DN 14-3-3 mice triggers maladaptive cardiac remodeling: role of p38 MAPK

It is generally believed that a mechanical signal initiates a cascade of biological events leading to coordinated cardiac remodeling. 14-3-3 family members are dimeric phosphoserine-binding proteins that regulate signal transduction, apoptotic, and checkpoint control pathways. To evaluate the molecular mechanism underlying swimming stress-induced cardiac remodeling, we examined the role of 14-3-3 protein and MAPK pathway by pharmacological and genetic means using transgenic mice with cardiac-specific expression of dominant-negative (DN) mutants of 14-3-3 (DN 14-3-3/TG) and p38alpha/beta MAPK (DNp38alpha and DNp38beta) mice. p38 MAPK activation was earlier, more marked, and longer in the myocardium of the TG group compared with that of the nontransgenic (NTG) group after swimming stress, whereas JNK activation was detected on day 5 and decreased afterward. In contrast, ERK1/2 was not activated after swimming stress in either group. Cardiomyocyte apoptosis, cardiac hypertrophy, and fibrosis were greatly increased in the TG group compared with those in the NTG group. Moreover, we found a significant correlation between p38 MAPK activation and apoptosis in the TG group. Furthermore, DN 14-3-3 hearts showed enhanced atrial natriuretic peptide expression. In contrast, DNp38alpha and DNp38beta mice exhibited reduced mortality and increased resistance to cardiac remodeling after 28 days of swimming stress compared with TG and NTG mice. Besides, treatment with a p38 MAPK inhibitor, FR-167653, resulted in regression of cardiac hypertrophy and fibrosis and improvement in the survival rate in the TG group. These results indicate for the first time that 14-3-3 protein along with p38 MAPK plays a crucial role in left ventricular remodeling associated with swimming stress.

Diabetes alters vascular mechanotransduction: pressure-induced regulation of mitogen activated protein kinases in the rat inferior vena cava

Background

Diabetes mellitus is an important risk factor for increased vein graft failure after bypass surgery. However, the cellular and molecular mechanism(s) underlying vessel attrition in this population remain largely unexplored. Recent reports have suggested that the pathological remodeling of vein grafts may be mediated by mechanically-induced activation of the mitogen activated protein kinase (MAPK) signaling pathways and the MAPK-related induction of caspase-3 activity. On the basis of these findings, we hypothesized that diabetes may be associated with alterations in how veins "sense" and "respond" to altered mechanical loading.

Methods

Inferior venae cavae (IVC) from the non-diabetic lean (LNZ) and the diabetic obese (OSXZ) Zucker rats were isolated and incubated ex vivo under basal or pressurized conditions (120 mmHg). Protein expression, basal activation and the ability of increased pressure to activate MAPK pathways and apoptosis-related signaling was evaluated by immunoblot analysis.

Results

Immunoblot analyses revealed differential expression and activation of extracellular signal-regulated kinase (ERK1/2), p38 and c-Jun NH2-terminal kinase (JNK) MAPKs in the IVCs of diabetic rats as compared to non-diabetic rats. In particular, the expression and basal phosphorylation of p38β- (52.3 ± 11.8%; 45.8 ± 18.2%), JNK 1- (21.5 ± 9.3%; 19.4 ± 11.6%) and JNK3-MAPK (16.8 ± 3.3%; 29.5 ± 17.6%) were significantly higher (P < 0.05) in the diabetic vena cava. An acute increase in IVC intraluminal pressure failed to increase the phosphorylation of ERK1-, JNK-2, or any of the p38-MAPKs in the diabetic obese Zucker rats. Also, IVC loading in the LNZ led to a 276.0 ± 36.0% and 85.8 ± 25.1% (P < 0.05) increase in the cleavage of caspase-3 and caspase-9, respectively, with no effect on these molecules in the OSXZ. No differences were found in the regulation of Bax and Bcl-2 between groups. However, basal expression levels of Akt, phospho-Akt, PTEN, phospho-PTEN and phospho-Bad were higher in the diabetic venae cavae (P < 0.05).

Conclusion

These data suggest that diabetes is associated with significant alteration in the ability of the vena cava to activate MAPK- and apoptosis-related signaling. Whether these changes are associated with the increased vein graft attrition seen in the diabetic population will require further investigation.

Triptolide induces Bcl-2 cleavage and mitochondria dependent apoptosis in p53-deficient HL-60 cells

Triptolide, a bioactive component of the Chinese medicinal herb Tripterygium wilfordii Hook F., induces p53-mediated apoptosis in cancer cells. This study demonstrated that triptolide activated an alternative p53-independent apoptotic pathway in HL-60 cells. In the absence of an intact p53 and without changing Bax level, at nM range triptolide induced apoptosis with concomitant DNA fragmentation, S phase cell cycle arrest, mitochondrial cytochrome c release and the activation of caspases. Besides, both caspases 8 and 9 were activated and the simultaneous inhibition of both was required to completely block triptolide's apoptotic effect. Importantly, triptolide induced the appearance of a truncated 23kD Bcl-2 which was inhibited by the general caspase inhibitor Z-VAD-FMK. In the MCF-7 cells that possessed the wild type p53 but lacked caspases 3, triptolide induced cell death with an increase in p53 but Bcl-2 remained unaltered. On the other hand, transfected cells overexpressing the 28kD Bcl-2 became more resistant to triptolide and upon triptolide treatment accumulated in the G(1) instead of S phase. After 36h treatment, triptolide activated JNK pathways, at the same time inactivated the ERK and p38 pathways. However, SP600125, a specific JNK inhibitor, could not inhibit the triptolide-mediated cleavage of caspase 3, indicated that activation of JNK might not be related to the apoptotic effects of triptolide. Our data suggest that in the absence of an intact p53 and without altering Bax level triptolide induces apoptosis activates a positive amplification loop involving caspase-mediated Bcl-2 cleavage/activation, mitochondrial cytochrome c release and further activation of caspases.

Analysis of Properties of Small Heat Shock Protein Hsp25 in MAPK-activated Protein Kinase 2 (MK2)-deficient Cells

Small heat shock proteins (sHsps) exist in dynamic oligomeric complexes and display diverse biological functions ranging from chaperone properties to modulator of apoptosis. So far, the role of stress-dependent phosphorylation of mammalian sHsps for its structure and function has been analyzed by using various phosphorylation site mutants overexpressed in different cell types as well as by non-exclusive inhibitors of the p38 MAPK cascade. Here we investigate the role of phosphorylation of endogenous sHsp in a genetic model lacking the major Hsp25 kinase, the MAP kinase-activated protein kinase MK2. We demonstrate that in MK2-deficient fibroblasts, where no stress-dependent phosphorylation of Hsp25 at Ser86 and no in vitro binding to 14-3-3 was detectable, stress-dependent disaggregation of endogenous Hsp25 complexes is impared and kinetics of arsenite-dependent, H2O2-dependent, and sublethal heat shock-induced insolubilization of Hsp25 is delayed. Similarly, green fluorescent protein-tagged Hsp25 shows retarded subcellular accumulation into stress granules in MK2-deficient cells after arsenite treatment. Decreased insolubilization of Hsp25 in MK2-deficient cells correlates with increased resistance against arsenite, H2O2, and sublethal heat shock treatment and with decreased apoptosis. In contrast, after severe, lethal heat shock MK2-deficient embryonic fibroblasts cells show fast and complete insolubilization of Hsp25 independent of MK2 and no increased stress resistance. Hence, MK2-dependent formation of insoluble stress granules and irreversible cell damage by oxidative stresses and sublethal heat shock correlate and only upon severe, lethal heat shock MK2-independent processes could determine insolubilization of Hsp25 and are more relevant for cellular stress damage.

Negative regulation of Akt activity by p38alpha MAP kinase in cardiomyocytes involves membrane localization of PP2A through interaction with caveolin-1

Cardiomyocyte-derived cell lines deficient in p38alpha are more resistant to apoptosis owing to lower expression of the pro-apoptotic proteins Bax and Fas and upregulation of the ERK survival pathway. Here, we show that increased Akt activity also contributes to the enhanced survival of p38alpha-deficient cardiomyocytes. We found that the serine/threonine phosphatase PP2A can be targeted to caveolae through interaction with caveolin-1 in a p38alpha-dependent manner. In agreement with this, PP2A activity associated with caveolin-1 was higher in wild type than in p38alpha-deficient cells. Akt was also present in caveolae and incubation of wild-type cells with the PP2A inhibitor okadaic acid increases the levels of Akt activity. Thus, p38alpha-induced re-localization of PP2A to caveolae can lead to dephosphorylation and inhibition of Akt, which in turn would contribute to the decreased survival observed in wild type cells. However, cell detachment impairs the formation of the PP2A/caveolin-1 complex and, as a consequence, phospho-Akt levels and survival are no longer regulated by p38alpha in detached wild type cardiomyocytes. Our results suggest that p38alpha can negatively modulate Akt activity, independently of PI3K, by regulating the interaction between caveolin-1 and PP2A through a mechanism dependent on cell attachment.

MAP-quest: could we produce constitutively active variants of MAP kinases?

Constitutively active mutants that acquired intrinsic activity and escaped regulation, serve as powerful tools for revealing the biochemical, biological and pathological functions of proteins. Such mutants are not available for mitogen-activated protein kinases (MAPKs). It is not known how to mimic the unusual mode of MAPK activation and to enforce, by mutations, their active conformation. In this review we describe the strategies employed in attempts to overcome this obstacle. We focus on a recent breakthrough with the p38 family that suggests that active variants of all MAPKs will soon be available.

Interferons limit inflammatory responses by induction of tristetraprolin

Interferons (IFNs) are cytokines with pronounced proinflammatory properties. Here we provide evidence that IFNs also play a key role in decline of inflammation by inducing expression of tristetraprolin (Ttp). TTP is an RNA-binding protein that destabilizes several AU-rich element-containing mRNAs including TNFalpha. By promoting mRNA decay, TTP significantly contributes to cytokine homeostasis. Now we report that IFNs strongly stimulate expression of TTP if a costimulatory stress signal is provided. IFN-induced expression of Ttp depends on the IFN-activated transcription factor STAT1, and the costimulatory stress signal requires p38 MAPK. Within the Ttp promoter we have identified a functional gamma interferon-activated sequence that recruits STAT1. Consistently, STAT1 is required for full expression of Ttp in response to LPS that stimulates both p38 MAPK and, indirectly, interferon signaling. We demonstrate that in macrophages IFN-induced TTP protein limits LPS-stimulated expression of several proinflammatory genes, such as TNFalpha, IL-6, Ccl2, and Ccl3. Thus, our findings establish a link between interferon responses and TTP-mediated mRNA decay during inflammation, and propose a novel immunomodulatory role of IFNs.

TEF-1 and C/EBPbeta are major p38alpha MAPK-regulated transcription factors in proliferating cardiomyocytes

p38 MAPKs (mitogen-activated protein kinases) play important roles in the regulation of cellular responses to environmental stress. Recently, this signalling pathway has also been implicated in the regulation of processes unrelated to stress, for example, in T lymphocytes and cardiomyocytes. In order to identify molecular targets responsible for the housekeeping functions of p38 MAPKs, we have analysed the differences in the transcriptomes of normally proliferating wild-type and p38alpha knockout immortalized embryonic cardiomyocytes. Interestingly, many potential components of the myocardium extracellular matrix were found to be upregulated in the absence of p38alpha. Further analysis of the microarray data identified TEF-1 (transcriptional enhancer factor-1), a known regulator of heart-specific gene expression, and C/EBPbeta (CCAAT/enhancer-binding protein beta), as the two transcription factors the binding sites of which were most enriched in the promoters of p38alpha-regulated genes. We have focused on the study of the extracellular matrix component COL1A1 (alpha1 chain of type I collagen) and found evidence for the involvement of both TEF-1 and C/EBPbeta in the p38alpha-dependent inhibition of COL1A1 transcription. Our data therefore show that p38 MAPKs regulate TEF-1 and C/EBPbeta transcriptional activity in the absence of environmental stress and suggests a role for p38alpha in the expression of extracellular matrix components that maintain organ architecture.

Bcl-2 Phosphorylation by p38 MAPK: identification of target sites and biologic consequences

The antiapoptotic role of Bcl-2 can be regulated by its phosphorylation in serine and threonine residues located in a nonstructured loop that links BH3 and BH4 domains. p38 MAPK has been identified as one of the kinases able to mediate such phosphorylation, through direct interaction with Bcl-2 protein in the mitochondrial compartment. In this study, we identify, by using mass spectrometry techniques and specific anti-phosphopeptide antibodies, Ser(87) and Thr(56) as the Bcl-2 residues phosphorylated by p38 MAPK and show that phosphorylation of these residues is always associated with a decrease in the antiapoptotic potential of Bcl-2 protein. Furthermore, we obtained evidence that p38 MAPK-induced Bcl-2 phosphorylation plays a key role in the early events following serum deprivation in embryonic fibroblasts. Both cytochrome c release and caspase activation triggered by p38 MAPK activation and Bcl-2 phosphorylation are absent in embryonic fibroblasts from p38alpha knock-out mice (p38alpha(-/-) MEF), whereas they occur within 12 h of serum withdrawal in p38alpha(+/+) MEF; moreover, they can be prevented by p38 MAPK inhibitors and are not associated with the synthesis of the proapoptotic proteins Bax and Fas. Thus, Bcl-2 phosphorylation by activated p38 MAPK is a key event in the early induction of apoptosis under conditions of cellular stress.

Proteomic analysis of p38α mitogen-activated protein kinase-regulated changes in membrane fractions of RAS-transformed fibroblasts

Oncogenic Ras signaling has been long known to play an important role in tumorigenesis and human cancer. In this report, we have used the sensitive 2-D-DIGE coupled to MS for the identification of proteins differentially expressed at the cell membrane level between oncogenic H-RasV12-transformed wild-type and p38alpha-deficient mouse embryo fibroblasts (MEFs). Following trifluoroethanol solubilization, 76 proteins were found to be differentially regulated. After PMF, 63 spots containing 42 different proteins were unequivocally identified by MALDI-TOF MS coupled with database interrogation. As expected, many of them were membrane proteins. Six proteins were selected for further validation studies based on their potential functional link with malignant transformation and signal transduction. These were prohibitin (PHB), protein disulfide isomerase 3 (PDIA3), focal adhesion kinase 2 (FAK2), c-GMP dependent protein kinase 2 (KGP2), NADH-ubiquinone oxidoreductase 30 kDa subunit (NUGM) and translationally controlled tumor protein (TCTP). All these proteins were up-regulated in the membranes of H-RasV12-transformed p38alpha-/-cells, except for prohibitin, which was down-regulated. An excellent correlation was found between DIGE results and Western blot studies, indicating the reliability of the 2-D-DIGE analysis. The available evidence about the putative function of the identified proteins supports the emerging role of p38alpha as a negative regulator of tumorigenesis. Further studies are in progress to elucidate the implications of these findings in the regulation of H-Ras-induced transformation by p38alpha signaling.

Poly(ADP-ribose) Polymerase-1 Signaling to Mitochondria in Necrotic Cell Death Requires RIP1/TRAF2-mediated JNK1 Activation

Poly(ADP-ribose) polymerase-1 (PARP-1) hyperactivation-induced necrosis has been implicated in several pathophysiological conditions. Although mitochondrial dysfunction and apoptosis-inducing factor translocation from the mitochondria to the nucleus have been suggested to play very important roles in PARP-1-mediated cell death, the signaling events downstream of PARP-1 activation in initiating mitochondria dysfunction are not clear. Here we used the DNA alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine, a potent PARP-1 activator, to study PARP-1 activation-mediated cell death. We found, based on genetic knockouts and pharmacological inhibition, that c-Jun N-terminal kinase (JNK), especially JNK1, but not the other groups of mitogen-activated protein kinase, is required for PARP-1-induced mitochondrial dysfunction, apoptosis-inducing factor translocation, and subsequent cell death. We reveal that receptor-interacting protein 1 (RIP1) and tumor necrosis factor receptor-associated factor 2 (TRAF2), are upstream of JNK in PARP-1 hyperactivated cells, because PARP-1-induced JNK activation was attenuated in RIP1-/- and TRAF2-/- mouse embryonic fibroblast cells. Consistently, knockouts of RIP1 and TRAF2 caused a resistance to PARP-1-induced cell death. Therefore, our study uncovers that RIP1, TRAF2, and JNK comprise a pathway to mediate the signaling from PARP-1 overactivation to mitochondrial dysfunction.

Signaling-transduction pathways required for ex vivo expansion of human limbal explants on intact amniotic membrane

PURPOSE

Ex vivo expansion of limbal epithelial progenitor cells on amniotic membrane (AM) without 3T3 fibroblasts is a new surgical approach to treat limbal stem cell deficiency. Such expansion requires NGF-TrkA-mediated signaling, and this study was conducted to delineate the downstream signaling pathways.

METHODS

The human corneolimbal ring was cut into explants and cultured on intact human AM. At day 0 or 10, low-molecular-weight inhibitors were added, whereas the control group received dimethyl sulfoxide (DMSO). The epithelial outgrowth rate was monitored for 17 days, and the epithelial cells were collected for Western blot analysis.

RESULTS

In the control, most expansion of human limbal epithelial cells started from the limbus from days 5 to 7 and reached approximately 80% confluence at day 17. Compared with the control, the outgrowth was completely inhibited by 50 microM LY294002 or 50 microM SR13668 and was significantly suppressed by 10 microM U0126, but was not affected by 10 microM of either SB203580 or JNK inhibitor 1. The inhibition of outgrowth by LY294002, SR13668, and U0126 was reversible. Western blot analysis showed that phosphorylation of Akt and FKHRL1was abolished by LY294002 and SR13668, but downregulated by U0126, which also abolished phosphorylation of p44/42 mitogen-activated protein kinase (MAPK). The phosphorylation of p38 and JNK MAPK were downregulated or abolished during ex vivo expansion.

CONCLUSIONS

Ex vivo expansion of human limbal epithelial progenitor cells on intact AM is mediated by the survival signaling pathway mediated by PI3K-Akt-FKHRL1 and by the mitogenic MAPK pathway mediated by p44/42 at the expense of p38 and JNK MAPK.

2-Deoxy-D-ribose inhibits hypoxia-induced apoptosis by suppressing the phosphorylation of p38 MAPK

An angiogenic factor, platelet-derived endothelial cell growth factor/thymidine phosphorylase (TP), stimulates the chemotaxis of endothelial cells and confers resistance to apoptosis induced by hypoxia. 2-Deoxy-d-ribose, a degradation product of thymidine generated by TP enzymatic activity, partially prevented hypoxia-induced apoptosis. 2-Deoxy-d-ribose inhibited hypoxia-induced phosphorylation of p38 mitogen-activated protein kinase (MAPK) but not c-jun NH(2)-terminal kinase/stress-activated protein kinase in human leukemia HL-60 cells. 2-Deoxy-d-ribose also suppressed the levels of Bax attached to mitochondria under hypoxic conditions. SB203580, a specific inhibitor of the p38 MAPK, suppressed the hypoxia-induced apoptosis of HL-60 cells. These findings suggest that one of the molecular bases for resistance to hypoxia-induced apoptosis conferred by 2-deoxy-d-ribose is the inhibition of the p38 signaling pathway. The expression levels of TP are elevated in many malignant solid tumors and thus the 2-deoxy-d-ribose generated by TP in these tumors may play an important role in tumor progression by preventing hypoxia-induced apoptosis.

MSKs are required for the transcription of the nuclear orphan receptors Nur77, Nurr1 and Nor1 downstream of MAPK signalling

MSK (mitogen- and stress-activated protein kinase) 1 and MSK2 are kinases activated downstream of either the ERK (extracellular-signal-regulated kinase) 1/2 or p38 MAPK (mitogen-activated protein kinase) pathways in vivo and are required for the phosphorylation of CREB (cAMP response element-binding protein) and histone H3. Here we show that the MSKs are involved in regulating the transcription of the immediate early gene Nur77. Stimulation of mouse embryonic fibroblasts with PMA, EGF (epidermal growth factor), TNF (tumour necrosis factor) or anisomycin resulted in induction of the Nur77 mRNA. The induction of Nur77 by TNF and anisomycin was abolished in MSK1/2 double-knockout cells, whereas induction was significantly reduced in response to PMA or EGF. The MSK responsive elements were mapped to two AP (activator protein)-1-like elements in the Nur77 promoter. The induction of Nur77 was also blocked by A-CREB, suggesting that MSKs control Nur77 transcription by phosphorylating CREB bound to the two AP-1-like elements. Consistent with the decrease in Nur77 mRNA levels in the MSK1/2-knockout cells, it was also found that MSKs were required for the induction of Nur77 protein by PMA and TNF. MSKs were also found to be required for the transcription of two genes related to Nur77, Nurr1 and Nor1, which were also transcribed in a CREB- or ATF1 (activating transcription factor-1)-dependent manner. Downstream of anisomycin signalling, a second ERK-dependent pathway, independent of MSK and CREB, was also required for the transcription of Nurr1 and Nor1.

Acute thermal stress and various heavy metals induce tissue-specific pro-or anti-apoptotic events via the p38-MAPK signal transduction pathway in Mytilus galloprovincialis (Lam.)

We investigated the effects of various heavy metals such as copper, zinc and cadmium, as well as acute thermal stress, on the signalling mechanisms involved in the protection and/or apoptosis of Mytilus galloprovincialis mantle and gill tissues. The results of our studies revealed that mantle and gill tissues differentially respond to the stressful stimuli examined. In the mantle tissue, 1 μmol l–1Cu2+ and 50 μmol l–1 Zn2+ induced a transient p38-MAPK activation, whereas 1 μmol l–1Cd2+ induced a biphasic profile of the kinase phosphorylation with maximal values at 15 and 120 min of treatment, respectively. Furthermore, 1μmol l–1 SB203580 abolished the Cu2+-induced kinase phosphorylation. In gills, both Cu2+ and Zn2+induced a considerably higher p38-MAPK activation, which remained elevated for at least 60 min, whereas Cd2+ induced a maximal kinase activation within 60 min of treatment. Hypothermia (4°C) induced a moderate kinase phosphorylation (maximised at 30 min), whereas hyperthermia (30°C) induced a rapid (within 15 min) p38-MAPK phosphorylation that remained considerably above basal levels for at least 2 h. Our studies on the synergistic effect of hyperthermia and Cu2+ revealed that these two stressful stimuli are additive in the mantle tissue, inducing an almost double p38-MAPK activation. Further studies on the involvement of the p38-MAPK signalling pathway in tissue-specific pro- or anti-apoptotic events revealed that identical stressful stimuli possibly lead to apoptotic death via the caspase-3 activation in the mantle tissue and to anti-apoptotic events possibly via the induction of Hsp70 overexpression in the gill tissue.

Ischemic neoangiogenesis enhanced by beta2-adrenergic receptor overexpression: a novel role for the endothelial adrenergic system

Beta2-adrenergic receptors (beta2ARs) are widely expressed, although their physiological relevance in many tissues is not yet fully understood. In vascular endothelial cells, they regulate NO release and vessel tone. Here we provide novel evidence that beta2ARs can regulate neoangiogenesis in response to chronic ischemia. We used in vivo adenoviral-mediated gene transfer of the human beta2AR to the endothelium of the rat femoral artery and increased beta2AR signaling resulting in ameliorated angiographic blood flow and hindlimb perfusion after chronic ischemia. Histological analysis confirmed that beta2AR overexpression also produced benefits on capillary density. The same maneuver partially rescued impaired angiogenesis in spontaneously hypertensive rats (SHR), whereas gene delivery of the G-protein-coupling defective mutant Ile164 beta2AR failed to provide ameliorations. Stimulation of endogenous and overexpressed beta2AR on endothelial cells in vitro was found to regulate cell number by inducing proliferation and [3H]-thymidine incorporation through means of extracellular receptor-activated kinase and vascular endothelial growth factor. The beta2AR also has novel effects on endothelial cell number through stimulation of proapoptosis and antiapoptosis pathways involving p38 mitogen-activated protein kinase and PI3-kinase/Akt activation. Therefore, beta2ARs play a critical role in endothelial cell proliferation and function including revascularization, suggesting a novel and physiologically relevant role in neoangiogenesis in response to ischemia.

PAK4 functions in tumor necrosis factor (TNF) alpha-induced survival pathways by facilitating TRADD binding to the TNF receptor

PAK4 is a member of the group B family of p21-activated kinases. Its expression is elevated in many cancer cell lines, and activated PAK4 is highly transforming, suggesting that it plays an important role in tumorigenesis. Although most previous work was carried out with overexpressed PAK4, here we used RNA interference to knock down endogenous PAK4 in cancer cells. By studying PAK4 knockdown HeLa cells, we demonstrated that endogenous PAK4 is required for anchorage-independent growth. Because cell survival is a key part of tumorigenesis and anchorage-independent growth, we studied whether PAK4 has a role in protecting cells from cell death. To address this, we studied the role for PAK4 downstream to the tumor necrosis factor (TNF) alpha receptor. Although overexpressed PAK4 was previously shown to abrogate proapoptotic pathways, here we demonstrate that endogenous PAK4 is required for the full activation of prosurvival pathways induced by TNFalpha. Our results indicate that PAK4 is required for optimal binding of the scaffold protein TRADD to the activated TNFalpha receptor through both kinase-dependent and kinase-independent mechanisms. Consequently, activation of several prosurvival pathways, including the NFkappaB and ERK pathways, is reduced in the absence of PAK4. Interestingly, constitutive activation of the NFkappaB and ERK pathways could compensate for the lack of PAK4, indicating that these pathways function downstream to PAK4. The role for PAK4 in regulating prosurvival pathways is a completely new function for this protein, and the connection between PAK4 and cell survival under stress helps explain its role in tumorigenesis and development.

p38 mitogen-activated protein kinase plays a key role in regulating MAPKAPK2 expression

One of three major families of the mitogen-activated kinases (MAPK), p38 as well as JNK, has been shown to transduce extracellular stress stimuli into cellular responses by phospho-relay cascades. Among p38 families, p38alpha is a widely characterized isoform and the biological phenomena are explained by its kinase activity regulating functions of its downstream substrates. However, its specific contributions to each phenomenon are yet not fully elucidated. For better understanding of the role of MAPKs, especially p38alpha, we utilized newly established mouse fibroblast cell lines originated from a p38alpha null mouse, namely, a parental cell line without p38alpha gene locus, knockout of p38alpha (KOP), Zeosin-resistant (ZKOP), revertant of p38alpha (RKOP), and Exip revertant (EKOP). EKOP is smaller in size but grows faster than the others. Although comparable amounts of ERK and JNK are expressed in each cell line, ERK is highly phosphorylated in EKOP even in normal culture conditions. Serum stimulation after serum starvation led to ERK phosphorylation in RKOP and ZKOP, but not in EKOP as much. On the contrary, relative phosphorylation level of JNK to total JNK in response to UV was low in RKOP. And its phosphorylation as well as total JNK is slightly lower in EKOP. RKOP is less sensitive to UV irradiation as judged by the survival rate. Stress response upon UV or sorbitol stimuli, leading to mitogen activate protein kinase activated kinase 2 (MAPKAPK2) phosphorylation, was only observed in RKOP. Further experiments reveal that MAPKAPK2 expression is largely suppressed in ZKOP and EKOP. Its expression was recovered by re-introduction of p38alpha. The loss of MAPKAPK2 expression accompanied by the defect of p38alpha is confirmed in an embryonic extract prepared from p38alpha null mice. These data demonstrate that p38 signal pathway is regulated not only by phosphorylation but also by modulation of the expression of its component. Together, we have established cell lines that can be used in analyzing the functions of MAPKs, especially p38alpha, and show that p38 is indispensable for MAPKAPK2 expression.

Phosphorylation of EEA1 by p38 MAP kinase regulates mu opioid receptor endocytosis

Morphine analgesic properties and side effects such as tolerance are mediated by the mu opioid receptor (MOR) whose endocytosis is considered of primary importance for opioid pharmacological effects. Here, we show that p38 mitogen-activated protein kinase (MAPK) activation is required for MOR endocytosis and sufficient to trigger its constitutive internalization in the absence of agonist. Further studies established a functional link between p38 MAPK and the small GTPase Rab5, a key regulator of endocytosis. Expression of an activated mutant of Rab5 stimulated endocytosis of MOR ligand-independently in wild-type but not in p38alpha-/- cells. We found that p38alpha can phosphorylate the Rab5 effectors EEA1 and Rabenosyn-5 on Thr-1392 and Ser-215, respectively, and these phosphorylation events regulate the recruitment of EEA1 and Rabenosyn-5 to membranes. Moreover, phosphomimetic mutation of Thr-1392 in EEA1 can bypass the requirement for p38alpha in MOR endocytosis. Our results highlight a novel mechanism whereby p38 MAPK regulates receptor endocytosis under physiological conditions via phosphorylation of Rab5 effectors.

Epidermal growth factor stimulation of trophoblast differentiation requires MAPK11/14 (p38 MAP kinase) activation

Cultured human term villous cytotrophoblasts (CT) have been reported to be nonproliferating but differentiate when exposed to epidermal growth factor (EGF). Here we show that CT differentiate into chorionic gonadotropin (beta-hCG/CGB)-expressing cells when cultured with medium alone. The addition of EGF decreases CGB secretion and prolongs production for up to 13 days. EGF stimulates the phosphorylation (activation) of the signaling intermediate p38 (MAPK11/14), and blocking phosphorylation pharmacologically with either SB203580 or SB202190 strongly inhibited spontaneous and EGF-stimulated secretion of CGB. In addition, EGF-stimulated fusion of cytotrophoblasts into syncytial units was strongly inhibited by SB203580. EGF upregulated trophoblast proliferation (measured by bromodeoxyuridine uptake) and SB203580 increased this proliferation after 5 days. In agreement with these observations, EGF and SB203580 increased expression of the G1-phase-specific gene cyclin-D1 (CCND1) and SB203580 downmodulated its inhibitor p21 (CDKN1A). When added to villous explant cultures, EGF did nothing to the pattern of CGB secretion, but addition of SB203580 prevented the normal surge in secretion during syncytial regeneration over Days 3-7. These data support the hypothesis that EGF-stimulated cytotrophoblast differentiation to syncytium requires MAPK11/14 activation, and that cytotrophoblast proliferation can be stimulated in culture by EGF and enhanced by MAPK11/14 inhibition with a consequent reduction of differentiation.

Mitochondrial reactive oxygen species activation of p38 mitogen-activated protein kinase is required for hypoxia signaling

Mammalian cells have the ability to sense low oxygen levels (hypoxia). An adaptive response to hypoxia involves the induction of the transcription factor hypoxia-inducible factor 1 (HIF-1). The intracellular signaling pathways that regulate HIF-1 activation during hypoxia remain unknown. Here, we demonstrate that p38alpha-/- cells fail to activate HIF-1 under hypoxic conditions. Cells deficient in Mkk3 and Mkk6, the upstream regulators of p38alpha, also fail to activate HIF-1 under hypoxic conditions. The p38alpha-/- cells are able to activate HIF-1 in response to anoxia or iron chelators during normoxia. Furthermore, the hypoxic activation of p38alpha and HIF-1 was abolished by myxothiazol, a mitochondrial complex III inhibitor, and glutathione peroxidase 1 (GPX1), a scavenger of hydrogen peroxide. Thus, the activation of p38alpha and HIF-1 is dependent on the generation of mitochondrial reactive oxygen species. These results provide genetic evidence that p38 mitogen-activated protein kinase signaling is essential for HIF-1 activation.

Sensitivity of human cord blood cells to tetrachloroethylene: cellular and molecular endpoints

The International Agency for Research on Cancer (IARC) currently lists tetrachloroethylene [perchloroethylene (PCE)] as being carcinogenic in animals. PCE is listed as possibly carcinogenic to humans upon occupational exposure. Human exposure to PCE can produce oesophageal cancer, cervical cancer, non-Hodgkin's lymphoma, urinary bladder cancer and leukemia. This work shows that PCE modulates the expression of some genes implicated in cancer induction, cell differentiation, cell-cycle progression, and the survival and clonogenic potential of human cord blood cells. After exposure to the compound, the modulated genes were involved in inflammatory responses as with the mitogen-activated protein kinase 14 (MPK 14), or in tumor and metastasis progression as with the matrix metalloproteinase 17 (MMP 17), in cell proliferation as with c-jun and c-fos, and moreover in the apoptotic process as with interferon alpha-inducible protein (IFI), BAX and BCL-2. Analysis of cord blood cells via flow cytometry showed that PCE treatment induced a statistically significant increase in necrosis after 24 h, while the clonogenicity of Human Colony-Forming Unit-Granulocyte/Macrophage (CFU-GM) and Burst-Forming Unit-Erythrocyte (BFU-E) progenitors did not change. In conclusion, our data showed that PCE affected various pathways involved in cancer induction, but its action on cell proliferation and differentiation is not yet clearly understood.

Survival versus apoptotic 17beta-estradiol effect: role of ER alpha and ER beta activated non-genomic signaling

The capability of 17beta-estradiol (E2) to induce the non-genomic activities of its receptors (ER alpha and ER beta) and to evoke different signaling pathways committed to the regulation of cell proliferation has been analyzed in different cell cancer lines containing transfected (HeLa) or endogenous (HepG2, DLD1) ER alpha or ER beta. In these cell lines, E2 induced different effects on cell growth/apoptosis in dependence of ER isoforms present. The E2-ER alpha complex rapidly activated multiple signal transduction pathways (i.e., ERK/MAPK, PI3K/AKT) committed to both cell cycle progression and apoptotic cascade prevention. On the other hand, the E2-ER beta complex induced the rapid and persistent phosphorylation of p38/MAPK which, in turn, was involved in caspase-3 activation and cleavage of poly(ADP-ribose)polymerase, driving cells into the apoptotic cycle. In addition, the E2-ER beta complex did not activate any of the E2-ER alpha-activated signal molecules involved in cell growth. Taken together, these results demonstrate the ability of ER beta isoform to activate specific signal transduction pathways starting from plasma membrane that may justify the effect of E2 in inducing cell proliferation or apoptosis in cancer cells. In particular this hormone promotes cell survival through ER alpha non-genomic signaling and cell death through ER beta non-genomic signaling.

Essential role for mitogen-activated protein (MAP) kinase phosphatase-1 in stress-responsive MAP kinase and cell survival signaling

Mitogen-activated protein kinase (MAPK) phosphatases (MKPs) constitute a family of 11 dual-specificity phosphatases that inactivate the MAPKs by dephosphorylation. Although the contribution of MAPKs to cell growth and cell death has been examined extensively, it remains unclear whether MKPs play an essential role in the regulation of these processes. To clarify the role of MKP-1, we determined the effects on the MAPKs and cell growth and death in primary fibroblasts derived from mice lacking MKP-1. Here we have shown that MKP-1 is critical for the inactivation of p38 MAPK and JNK following stimulation with serum, anisomycin, and osmotic stress. In addition, MKP-1 was identified as a critical negative regulator of the cAMP-mediated p38 MAPK pathway. MKP-1-deficient mouse embryonic fibroblasts (MEFs) displayed enhanced p38 MAPK activity and cAMP-response element-dependent transcriptional activation in response to forskolin. Surprisingly, MKP-1-deficient fibroblasts exhibited reduced cell growth compared with wild type MEFs as a result of enhanced cell death. The enhanced level of cell death in MKP-1-deficient MEFs was rescued by SB203580, an inhibitor of p38 MAPK. MKP-1-deficient MEFs were also sensitive to anisomycin-induced apoptosis. Collectively, these data demonstrate that MKP-1 promotes cell survival by attenuating stress-responsive MAPK-mediated apoptosis.

Splicing DNA-damage responses to tumour cell death

The ability of a tumour cell to evade programmed cell death (apoptosis) is crucial in the development of cancer. The process of apoptosis is complex and involves the careful interplay of a host of signalling molecules. Cellular stresses, such as DNA-damage, can initiate apoptosis through multiple pathways, all of which eventually lead to eradication of damaged cells that may otherwise go on to form a tumour. Moreover, the relevance of this to combating cancer is very strong since several therapeutic agents used to treat malignant disease utilize the cells' apoptotic machinery. The purpose of this review is to provide an insight into what we know about how apoptosis is initiated by DNA-damaging agents, how pro- and anti-apoptotic signals converge in the execution of cell death, and how such mechanisms can be perturbed in cancer.

Interaction between Smad7 and beta-catenin: importance for transforming growth factor beta-induced apoptosis

Members of the transforming growth factor beta (TGF-beta) and Wnt/wingless superfamilies regulate cell fate during development and tissue maintenance. Here we report that Smad7 interacts with beta-catenin and lymphoid enhancer binding factor 1/T-cell-specific factor (LEF1/TCF), transcriptional regulators in Wnt signaling, in a TGF-beta-dependent manner. Smad7 was found to be required for TGF-beta1-induced accumulation of beta-catenin and LEF1 in human prostate cancer (PC-3U) cells as well as in human keratinocytes (HaCaT cells). Moreover, when the endogenous Smad7 was repressed by specific small interfering RNA, TGF-beta-induced increase of activated p38, Akt phosphorylated on Ser473, glycogen synthase kinase 3beta phosphorylated on Ser9 was prevented, as well as the TGF-beta-induced association between beta-catenin and LEF1. Notably, the observed physical association of Smad7 and beta-catenin was found to be important for TGF-beta-induced apoptosis, since suppression of beta-catenin expression by small interfering RNA decreased the apoptotic response to TGF-beta.

Stress kinase signaling in cancer: fact or fiction?

Cancer results from genetic alterations in intracellular signaling pathways, which normally orchestrate the execution of developmental programs and the organismic response to extrinsic factors. Mutations in upstream activators and components of the cytoplasmic (Ras-Raf MEK-ERK) cascade frequently occur in tumors. In vitro and in vivo studies have shown that isolated activation of this pathway is both, necessary and sufficient for transformation. During the last years two new groups of related kinases have joined the ranks of mitogen-activated protein kinases, stress-activated protein kinases/Jun N-terminal kinases and p38. Their activation not only occurs during cellular responses to unphysiological stimuli but also downstream of cytokine and pathogen receptors and has been observed in tumors. In this article we will review the role of stress kinases in cancer, and discuss the mechanisms through which they regulate the transformation process.

p38alpha, but not p38beta, inhibits the phosphorylation and presence of c-FLIPS in DISC to potentiate Fas-mediated caspase-8 activation and type I apoptotic signaling

Pharmacological inhibitors of JNK (SP600125) and p38 (PD169316) sensitize tumor cells to Fas-mediated apoptosis. PD169316 is less potent than SP600125 and diminishes its effect when present together. Because the p38 isoforms that promote (p38alpha) or inhibit (p38beta) apoptosis are both suppressed by PD169316, we investigated their regulatory involvement in Fas-signaling. We report here, that p38alpha, but not p38beta, exerts its proapoptotic effect by inhibiting the phosphorylation and presence of c-FLIPS, but not c-FLIPL, in the DISC to promote caspase-8 activation and type I signaling in Fas-activated Jurkat cells. Its effect was enhanced by enforced expression of Flag-tagged p38alpha and was attenuated by its inactive mutant (p38alpha-AGF) or by translational silencing. By contrast, type II signaling was facilitated by p38alpha-dependent mitochondrial presence of tBid and inhibition of Bcl-2 (Ser70) phosphorylation as well as by p38alpha/beta-dependent mitochondrial localization of Bax and inhibition of phosphorylation of Bad (Ser112/Ser155). Potentiation of Fas-mediated apoptosis by the inhibition of JNK1/2 correlated with the loss of Bad (Ser136) phosphorylation and was dependent on the stimulatory effect of p38alpha on DISC and the downstream effects of both p38alpha and p38beta. These data underscore the need to reassess the findings obtained with pan-p38 inhibitors and suggest that activation of p38alpha coupled with targeted inhibition of p38beta and JNK1/2 should optimally sensitize tumor cells to Fas-mediated apoptosis.

Activation of p38 MAPK is required for Bax translocation to mitochondria, cytochrome c release and apoptosis induced by UVB irradiation in human keratinocytes

This study establishes that activation of p38 MAPK by UVB represents a crucial signal required for the conformational change and translocation of Bax to the mitochondria in human keratinocytes. UVB-induced Bax translocation and mitochondrial cytochrome c release, which precede caspase activation and other endpoints of the apoptotic program such as chromatin fragmentation and loss of mitochondrial transmembrane potential, are blocked by genetic or pharmacological inhibition of the p38alpha MAPK. Inhibition of p38 MAPK strongly reduces the UVB-induced formation of sunburn cells and blocks Bax conformational change both in cultured human keratinocytes and in human skin, providing clear evidence for the physiological role of the p38 MAPK-Bax pathway in the removal of precancerous, UVB-damaged keratinocytes. Furthermore, we show that Bcl-2 overexpression, but not the pan-caspase inhibitor zVAD-fmk, blocks Bax conformational change and its subsequent translocation downstream of p38 MAPK. These data indicate that the activation of p38 MAPK by UVB engages a caspase-independent death signal leading to mitochondrial membrane permeabilization and apoptosis in human keratinocytes and suggest that p38 MAPK might have a preventive role in the process of photocarcinogenesis.

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).

Mitogen-activated protein kinases in apoptosis regulation

Cells are continuously exposed to a variety of environmental stresses and have to decide 'to be or not to be' depending on the types and strength of stress. Among the many signaling pathways that respond to stress, mitogen-activated protein kinase (MAPK) family members are crucial for the maintenance of cells. Three subfamilies of MAPKs have been identified: extracellular signal-regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs), and p38-MAPKs. It has been originally shown that ERKs are important for cell survival, whereas JNKs and p38-MAPKs were deemed stress responsive and thus involved in apoptosis. However, the regulation of apoptosis by MAPKs is more complex than initially thought and often controversial. In this review, we discuss MAPKs in apoptosis regulation with attention to mouse genetic models and critically point out the multiple roles of MAPKs.