The p38 signal transduction pathway: activation and function

Restraint of proinflammatory cytokine biosynthesis by mitogen-activated protein kinase phosphatase-1 in lipopolysaccharide-stimulated macrophages

Exposure of macrophages to LPS elicits the production of proinflammatory cytokines, such as TNF-alpha, through complex signaling mechanisms. Mitogen-activated protein (MAP) kinases play a critical role in this process. In the present study, we have addressed the role of MAP kinase phosphatase-1 (MKP-1) in regulating proinflammatory cytokine production using RAW264.7 macrophages. Analysis of MAP kinase activity revealed a transient activation of c-Jun N-terminal kinase (JNK) and p38 after LPS stimulation. Interestingly, MKP-1 was induced concurrently with the inactivation of JNK and p38, whereas blocking MKP-1 induction by triptolide prevented this inactivation. Ectopic expression of MKP-1 accelerated JNK and p38 inactivation and substantially inhibited the production of TNF-alpha and IL-6. Induction of MKP-1 by LPS was found to be extracellular signal-regulated kinase dependent and involved enhanced gene expression and increased protein stability. Finally, MKP-1 expression was also induced by glucocorticoids as well as cholera toxin B subunit, an agent capable of preventing autoimmune diseases in animal models. These findings highlight MKP-1 as a critical negative regulator of the macrophage inflammatory response, underscoring its premise as a potential target for developing novel anti-inflammatory drugs.

Neurotoxicity of pneumolysin, a major pneumococcal virulence factor, involves calcium influx and depends on activation of p38 mitogen-activated protein kinase

Neuronal injury in bacterial meningitis is caused by the interplay of host inflammatory responses and direct bacterial toxicity. We investigated the mechanisms by which pneumolysin, a cytosolic pneumococcal protein, induces damage to neurons. The toxicity after exposure of human SH-SY5Y neuroblastoma cells and hippocampal organotypic cultures to pneumolysin was time- and dose-dependent. Pneumolysin led to a strong calcium influx apparently mediated by pores on the cell membrane formed by the toxin itself and not by voltage-gated calcium channels. Buffering of intracellular calcium with BAPTA-AM [1, 2-bis (o-aminophenoxy) ethane N, N, N', N'-tetraacetic acid tetra(acetomethoxyl) ester] improved survival of neuronal cells following challenge with pneumolysin. Western blotting revealed increased phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK) as early as 30 min after challenge with pneumolysin. SB 203580, a potent and selective inhibitor of p38 MAPK, rescued human neuronal cells from pneumolysin-induced death. Inhibition of the mitochondrial permeability transition pore using bongkrekate and caspase inhibition also improved survival following challenge with the toxin. Modulation of cell death pathways activated by pneumolysin may influence the outcome of pneumococcal meningitis.

A novel regulatory mechanism of the mitochondrial Ca2+ uniporter revealed by the p38 mitogen-activated protein kinase inhibitor SB202190

It is widely acknowledged that mitochondrial Ca2+ uptake modulates the cytosolic [Ca2+] ([Ca2+]c) acting as a transient Ca2+ buffer. In addition, mitochondrial [Ca2+] ([Ca2+]M) regulates the rate of respiration and may trigger opening of the permeability transition pore and start apoptosis. However, no mechanism for the physiological regulation of mitochondrial Ca2+ uptake has been described. We show here that SB202190, an inhibitor of p38 mitogen-activated protein (MAP) kinase, strongly stimulates ruthenium red-sensitive mitochondrial Ca2+ uptake, both in intact and in permeabilized HeLa cells. The [Ca2+]M peak induced by agonists was increased about fourfold in the presence of the inhibitor, with a concomitant reduction in the [Ca2+]c peak. The stimulation occurred fast and was rapidly reversible. In addition, experiments in permeabilized cells perfused with controlled [Ca2+] showed that SB202190 stimulated mitochondrial Ca2+ uptake by more than 10-fold, but only in the physiological [Ca2+]c range (1-4 mM). Other structurally related p38 MAP kinase inhibitors (SB203580, PD169316, or SB220025) produced little or no effect. Our data suggest that in HeLa cells, a protein kinase sensitive to SB202190 tonically inhibits the mitochondrial Ca2+ uniporter. This novel regulatory mechanism may be of paramount importance to modulate mitochondrial Ca2+ uptake under different physiopathological conditions.

RAGE blockade stabilizes established atherosclerosis in diabetic apolipoprotein E-null mice

BACKGROUND

Previous studies suggested that blockade of RAGE in diabetic apolipoprotein (apo) E-null mice suppressed early acceleration of atherosclerosis. A critical test of the potential applicability of RAGE blockade to clinical settings was its ability to impact established vascular disease. In this study, we tested the hypothesis that RAGE contributed to lesion progression in established atherosclerosis in diabetic apoE-null mice.

METHODS AND RESULTS

Male apoE-null mice, age 6 weeks, were rendered diabetic with streptozotocin or treated with citrate buffer. At age 14 weeks, certain mice were killed or treated with once-daily murine soluble RAGE or albumin; all mice were killed at age 20 weeks. Compared with diabetic mice at age 14 weeks, albumin-treated animals displayed increased atherosclerotic lesion area and complexity. In diabetic mice treated with sRAGE from age 14 to 20 weeks, lesion area and complexity were significantly reduced and not statistically different from those observed in diabetic mice at age 14 weeks. In parallel, decreased parameters of inflammation and mononuclear phagocyte and smooth muscle cell activation were observed.

CONCLUSIONS

RAGE contributes not only to accelerated lesion formation in diabetic apoE-null mice but also to lesion progression. Blockade of RAGE may be a novel strategy to stabilize atherosclerosis and vascular inflammation in established diabetes.

Functional regulation of TEL by p38-induced phosphorylation

TEL is a nuclear phosphoprotein that belongs to a member of the ETS family transcription factors. TEL acts as a tumor suppressor and is essential for establishing hematopoiesis in neonatal bone marrow. Because TEL possesses multiple putative mitogen-activated protein (MAP) kinase phosphorylation sites, we here investigated functional regulation of TEL via stress signaling pathways. We showed that TEL becomes phosphorylated in vivo by activated p38 but not by JNK1. The constitutive and inducible phosphorylation sites were found to be Ser(22) and Ser(257), respectively. TEL bound to p38 and was directly phosphorylated in vitro by p38. In vivo p38-dependent phosphorylation reduced trans-repressional abilities of TEL through ETS-binding consensus site. These data indicate that TEL's functions are potentially regulated by p38 which is activated by various kinds of stresses. TEL could be a constituent downstream of the specific MAP kinase in the signal transduction system.

Activation of Rac1 and the p38 mitogen-activated protein kinase pathway in response to arsenic trioxide

Arsenic trioxide induces differentiation and apoptosis of malignant cells in vitro and in vivo, but the mechanisms by which such effects occur have not been elucidated. In the present study we provide evidence that arsenic trioxide induces activation of the small G-protein Rac1 and the alpha and beta isoforms of the p38 mitogen-activated protein (MAP) kinase in several leukemia cell lines. Such activation of Rac1 and p38-isoforms results in downstream engagement of the MAP kinase-activated protein kinase-2 and is enhanced by pre-treatment of cells with ascorbic acid. Interestingly, pharmacological inhibition of p38 potentiates arsenic-dependent apoptosis and suppression of growth of leukemia cell lines, suggesting that this signaling cascade negatively regulates induction of antileukemic responses by arsenic trioxide. Consistent with this, overexpression of a dominant-negative p38 mutant (p38betaAGF) enhances the antiproliferative effects of arsenic trioxide on target cells. To further define the relevance of activation of the Rac1/p38 MAP kinase pathway in the induction of arsenic-dependent antileukemic effects, studies were performed using bone marrows from patients with chronic myelogenous leukemia. Arsenic trioxide suppressed the growth of leukemic myeloid (CFU-GM) progenitors from such patients, whereas concomitant pharmacological inhibition of the p38 pathway enhanced its growth-suppressive effects. Altogether, these data provide evidence for a novel function of the p38 MAP kinase pathway, acting as a negative regulator of arsenic trioxide-induced apoptosis and inhibition of malignant cell growth.

Regulation of growth-related genes by interleukin-6 in murine myeloma cells

Interleukin-6 (IL-6), a pleiotropic cytokine with effects on several hematopoietic and other normal cells, is also important for the growth and survival of tumor cells such as murine plasmacytomas and human myelomas. Exploiting the 11A3 hybridoma cell line for its IL-6 requirement to proliferate in vitro, we used subtractive suppression hybridization (SSH) to identify genes whose expression is stimulated and/or repressed in response to IL-6. Northern blot analysis of 100 arbitrarily picked subtracted cDNA clones revealed that expression of 11 mRNAs were IL-6-modulated. Among these, eight were genes known to encode a variety of proteins such as enzymes (PCK, MTDNI), structural proteins (Tropoelastin), transcriptional regulators (BRG1) and proteins involved in cell division control (Cyclin A, OAZi) or cell signaling (PIX, TOPK/PBK). The recently identified MAPKK-like protein kinase TOPK/PBK gene represents a likely candidate IL-6 target gene as suggested by its significant up-regulated expression in hybridoma cells induced to grow by a brief IL-6 pulse. The diversity of growth-related genes identified in this study further emphasizes the central role of IL-6 in the regulation of myeloma cell expansion in addition to its previously demonstrated role in the inhibition of apoptosis.

Estrogen Receptor-dependent and Estrogen Receptor-independent Pathways for Tamoxifen and 4-Hydroxytamoxifen-induced Programmed Cell Death

The therapeutic efficacy of tamoxifen (TAM) in cancer therapy is thought to arise primarily from its ability to compete with estrogens for binding to the estrogen receptor (ER). We show that TAM and its active metabolite, 4-hydroxytamoxifen (OHT), can actively induce programmed cell death through distinct ER-dependent and ER-independent pathways. The ER-independent pathway is activated by 10-20 microm TAM and OHT and by 10-20 microm 17beta-estradiol and raloxifene, and occurs in ER-negative cells. The ER dependence of a second pathway, caused by submicromolar concentrations of TAM and OHT, was demonstrated by the ability of the ER ligands 17beta-estradiol, raloxifene, and ICI 182,780 to effectively block the cell death-inducing effects of TAM and OHT. Because the p38-specific inhibitor SB203580 blocks OHT.ER-induced cell death, stress kinase pathways are likely involved. ER-independent cell death triggers classic caspase-dependent apoptosis. However, although OHT.ER triggers some hallmarks of apoptosis, including Bax translocation and cytochrome c release, the absence of poly(ADP-ribose) polymerase cleavage or DNA laddering indicates that the death pathway involved is caspase-independent. The OHT.ER-dependent cell death pathway appears to diverge from classical apoptosis at the level of caspase 9 activation. The ability to promote ER-dependent programmed cell death represents a novel activity of TAM and OHT.

DLPC decreases TGF-beta1-induced collagen mRNA by inhibiting p38 MAPK in hepatic stellate cells

Dilinoleoylphosphatidylcholine (DLPC), the active component of polyenylphosphatidylcholine extracted from soybeans, decreases collagen accumulation induced by TGF-beta1 in cultured hepatic stellate cells (HSCs). Because DLPC exerts antioxidant effects and TGF-beta1 generates oxidative stress, we evaluated whether the antifibrogenic effect of DLPC is linked to its antioxidant action. In passage 1 culture of rat HSCs, TGF-beta1 induced a concentration-dependent increase in procollagen-alpha(1)(I) mRNA levels and enhanced intracellular H(2)O(2) and superoxide anion formation and lipid peroxidation but decreased GSH levels. These changes were prevented by DLPC. Upregulation of collagen mRNA by TGF-beta1 was likewise inhibited by catalase and p38 MAPK inhibitor SB-203580, suggesting involvement of H(2)O(2) and p38 MAPK signaling in this process. TGF-beta1 or addition of H(2)O(2) to HSCs activated p38 MAPK with a rise in procollagen mRNA level; these changes were blocked by catalase and SB-203580 and likewise by DLPC. alpha-Smooth muscle actin abundance in HSCs was not altered by TGF-beta1 treatment (with or without DLPC), indicating that downregulation of procollagen mRNA by DLPC was not due to alteration in HSC activation. These results demonstrate that DLPC prevents TGF-beta1-induced increase in collagen mRNA by inhibiting generation of oxidative stress and associated H(2)O(2)-dependent p38 MAPK activation, which explains its antifibrogenic effect. DLPC, an innocuous phospholipid, may be considered for prevention and treatment of liver fibrosis.

Role of p38 and p44/42 mitogen-activated protein kinases in microglia

Although microglial cells are thought to play a beneficial role in the regeneration and plasticity of the central nervous system (CNS), recent studies have indicated that at least some molecules released by microglia may be harmful in acute brain insults and neurodegenerative diseases. Therefore, the pathways mediating the synthesis and release of these neurotoxic compounds are of importance. p38 and p44/42 families of mitogen-activated protein kinases (MAPKs) in microglia respond strongly to various extracellular stimuli, such as ATP, thrombin, and beta-amyloid, a peptide thought to be responsible for the neuropathology in Alzheimer's disease. In this review we describe in vivo evidence implicating that p38 and p44/42 MAPKs may play a critical role in harmful microglial activation in acute brain injury, such as stroke, and in more chronic neurodegenerative diseases, such as Alzheimer's disease. We also clarify the extracellular signals responsible for activation of p38 and p44/42 MAPK in microglia and review the responses so far reported to be mediated by these kinases.

Neuronal p38 MAPK signalling: an emerging regulator of cell fate and function in the nervous system

p38 mitogen-activated protein kinases (MAPKs), together with extracellular signal-regulated kinases (ERKs) and c-Jun N-terminal kinases (JNKs), constitute the MAPK family. Multiple intracellular signalling pathways that converge on MAPKs exist in all eukaryotic cells and play pivotal roles in a wide variety of cellular functions. p38 MAPKs and JNKs, also termed stress-activated protein kinases (SAPKs), are preferentially activated by various cytotoxic stresses and cytokines and appear to be potent regulators of stress-induced apoptosis. Whereas JNKs have been shown to play pivotal roles in the regulation of neuronal apoptosis, the role of p38 MAPKs in the nervous system is poorly understood. However, accumulating evidence from mammalian cell culture systems and the strong genetic tool C. elegans suggests that neuronal p38 signalling has diverse functions beyond the control of cell death and survival. This review focuses on possible roles for the p38 pathway in the nervous system, with principal emphasis placed on the roles in neuronal cell fate decision and function.

Porphyromonas gingivalis lipopolysaccharide interferes with salivary mucin synthesis through inducible nitric oxide synthase activation by ERK and p38 kinase

Porphyromonas gingivalis is a Gram-negative periodontopathic bacterium colonizing the oral cavity and its lipopolysaccharide (LPS) is a key factor in the development of periodontitis. We investigated the effect of P. gingivalis LPS on the cellular responses associated with mucin synthesis in sublingual salivary gland acinar cells. Exposure of the acinar cells to the LPS led to a dose-dependent decrease in mucin synthesis and was accompanied by a massive induction in inducible nitric oxide synthase (NOS-2) activity and the increase in NO production, caspase-3 activity and apoptosis. Inhibition of extracellular signal-regulated kinase (ERK) with PD98059 accelerated the LPS-induced decrease in the glycoprotein synthesis and caused further increase in apoptosis and NOS-2 activity, while the blockade of p38 mitogen-activated kinase (MAPK) with SB203580 countered the LPS-induced reduction in the glycoprotein synthesis and obviated the induced increases in NOS-2 and apoptosis. Introduction of NOS-2 inhibitor, L-NAME, not only countered the LPS-induced increase in NO generation, caspase-3 activity and apoptosis, but caused the impedance of the LPS inhibition on mucin synthesis. The findings point to the upregulation in NOS-2 expression by P. gingivalis LPS as a key detrimental culprit affecting salivary mucin synthesis.

Structure of mitogen-activated protein kinase-activated protein (MAPKAP) kinase 2 suggests a bifunctional switch that couples kinase activation with nuclear export

MAPK-activated protein kinase 2 (MAPKAPK2), one of several kinases directly phosphorylated and activated by p38 MAPK, plays a central role in the inflammatory response. The activated MAPKAPK2 phosphorylates its nuclear targets CREB/ATF1, serum response factor, and E2A protein E47 and its cytoplasmic targets HSP25/27, LSP-1, 5-lipoxygenase, glycogen synthase, and tyrosine hydroxylase. The crystal structure of unphosphorylated MAPKAPK2, determined at 2.8 A resolution, includes the kinase domain and the C-terminal regulatory domain. Although the protein is inactive, the kinase domain adopts an active conformation with aspartate 366 mimicking the missing phosphorylated threonine 222 in the activation loop. The C-terminal regulatory domain forms a helix-turn-helix plus a long strand. Phosphorylation of threonine 334, which is located between the kinase domain and the C-terminal regulatory domain, may serve as a switch for MAPKAPK2 nuclear import and export. Phosphorylated MAPKAPK2 masks the nuclear localization signal at its C terminus by binding to p38. It unmasks the nuclear export signal, which is part of the second C-terminal helix packed along the surface of kinase domain C-lobe, and thereby carries p38 to the cytoplasm.

Osmotic stress sensitizes naturally resistant cells to TNF-alpha-induced apoptosis

Most cells are naturally resistant to TNF-alpha-induced cell death and become sensitized when NF-kappaB transactivation is blocked or in the presence of protein synthesis inhibitors that prevent the expression of anti-apoptotic genes. In this report we analyzed the role of osmotic stress on TNF-alpha-induced cell death. We found that it sensitizes the naturally resistant HeLa cells to TNF-alpha-induced apoptosis, with the involvement of an increase in the activity of several kinases, the inhibition of Bcl-2 expression, and a late increase on NF-kappaB activation. Cell death occurs regardless of the enhanced NF-kappaB activity, whose inhibition produces an increase in apoptosis. The inhibition of p38 kinase, also involved in NF-kappaB activation, significantly increases the effect of osmotic stress on TNF-alpha-induced cell death.

In the cellular garden of forking paths: how p38 MAPKs signal for downstream assistance

Mitogen-activated protein kinases (MAPKs) are evolutionarily conserved enzymes which connect cell-surface receptors to regulatory targets within cells and convert receptor signals into various outputs. In mammalian cells, four distinct MAPKs have been identified: the extracellular signal-related kinases (ERK)-1/2, the c-jun N-terminal kinases or stress-activated protein kinases 1 (JNK1/2/3, or SAPK1s), the p38 MAPKs (p38 alpha/beta/gamma/delta, or SAPK2s), and the ERK5 or big MAP kinase 1 (BMK1). The p38 MAPK cascade is activated by stress or cytokines and leads to phosphorylation of its central elements, the p38 MAPKs. Downstream of p38 MAPKs there is a diversification and extensive branching of signalling pathways. For that reason, we will focus in this review on the different signalling events that are triggered by p38 activity, and analyse how these events contribute to specific gene expression and cellular responses.

Chromatin structure and control of beta-like globin gene switching

The human beta-globin locus is a complex genetic system widely used for analysis of eukaryotic gene expression. The locus consists of five functional beta-like globin genes, epsilon, (G)gamma, (A)gamma, delta, and beta, arrayed on the chromosome in the order that they are expressed during ontogeny. Globin gene expression is regulated, in part, by the locus control region, which physically consists of five DNaseI-hypersensitive sites located 6-22 Kb upstream of the epsilon -globin gene. During ontogeny two switches occur in beta-globin gene expression that reflect the changing oxygen requirements of the fetus. The first switch from embryonic epsilon - to fetal gamma-globin occurs at six weeks of gestation. The second switch from gamma- to adult delta- and beta-globin occurs shortly after birth. Throughout the locus, cis-acting elements exist that are dynamically bound by trans-acting proteins, including transcription factors, co-activators, repressors, and chromatin modifiers. Discovery of novel erythroid-specific transcription factors and a role for chromatin structure in gene expression have enhanced our understanding of the mechanism of globin gene switching. However, the hierarchy of events regulating gene expression during development, from extracellular signaling to transcriptional activation or repression, is complex. In this review we attempt to unify the current knowledge regarding the interplay of cis-acting elements, transcription factors, and chromatin modifiers into a comprehensive overview of globin gene switching.

Peroxynitrite signaling: receptor tyrosine kinases and activation of stress-responsive pathways

Peroxynitrite, generated for example in inflammatory processes, is capable of nitrating and oxidizing biomolecules, implying a considerable impact on the integrity of cellular structures. Cells respond to stressful conditions by the activation of signaling pathways, including receptor tyrosine kinase-dependent pathways such as mitogen-activated protein kinases and the phosphoinositide-3-kinase/Akt pathway. Peroxynitrite affects signaling pathways by nitration as well as by oxidation: while nitration of tyrosine residues by peroxynitrite modulates signaling processes relying on tyrosine phosphorylation and dephosphorylation, oxidation of phosphotyrosine phosphatases may lead to an alteration in the tyrosine phosphorylation/dephosphorylation balance. The flavanol (-)-epicatechin is a potent inhibitor of tyrosine nitration and may be employed as a tool to distinguish signaling effects due to tyrosine nitration from those that are due to oxidation reactions.

Motoneuron death triggered by a specific pathway downstream of Fas. potentiation by ALS-linked SOD1 mutations

Death pathways restricted to specific neuronal classes could potentially allow for precise control of developmental neuronal death and also underlie the selectivity of neuronal loss in neurodegenerative disease. We show that Fas-triggered death of normal embryonic motoneurons requires transcriptional upregulation of neuronal NOS and involves Daxx, ASK1, and p38 together with the classical FADD/caspase-8 cascade. No evidence for involvement of this pathway was found in cells other than motoneurons. Motoneurons from transgenic mice overexpressing ALS-linked SOD1 mutants (G37R, G85R, or G93A) displayed increased susceptibility to activation of this pathway: they were more sensitive to Fas- or NO-triggered cell death but not to trophic deprivation or excitotoxic stimulation. Thus, triggering of a motoneuron-restricted cell death pathway by neighboring cells might contribute to motoneuron loss in ALS.

The effect of aging on p38 signaling pathway activity in the mouse liver and in response to ROS generated by 3-nitropropionic acid

Since mitochondrial dysfunction is a major source of oxidative stress in aged tissues, we asked whether the basal activities of stress response signaling pathway(s) are indicative of oxidative stress in aged tissues. To address this issue we asked whether: (a). aging affects the basal activity of the p38 MAPK stress signaling pathway; (b). the p38 MAPK pathway is activated by 3-nitropropionic acid (3-NPA), an inhibitor of complex II (succinic dehydrogenase) and generator of reactive oxygen species (ROS); (c). aging affects the response of the p38 alpha signaling pathway to 3-NPA. Our studies have shown that the basal kinase activities of p38 alpha, its upstream activator, MKK3, and its downstream substrate, ATF-2, are elevated in livers of aged C57BL/6 male mice and that these kinase activities, which are induced by 3-NPA in young livers, do not occur in aged livers. Furthermore, although aging does not affect their protein pool levels there are specific increases in phosphorylation of threonine residues in the p38 alpha and ATF-2 catalytic sites that might account for the increased basal level kinase activities in the aged livers. Our studies suggest that failure to respond to 3-NPA may be a factor in the susceptibility of aged tissue to oxidative damage, and support our hypothesis that aged tissues (especially liver) develop a state of chronic stress even in the absence of a challenge.

Activation of p38 mitogen-activated protein kinase attenuates Leishmania donovani infection in macrophages

Leishmania-induced macrophage dysfunctions have been correlated with altered signaling events. In this work, we report that SB203580, a specific inhibitor of p38 mitogen-activated protein kinases (MAPK), increases Leishmania donovani survival in human peripheral blood mononuclear macrophages. Consistent with this finding, activation of p38 and c-jun N-terminal kinase (JNK) MAPK signaling pathways by anisomycin significantly reduced parasite survival within these cells. However, the majority of the effect was seen in a 50% reduction in the percentage of macrophages infected, with little effect on the highly infected macrophages. The observed effect was likely to be due to the p38 MAPK pathway since SB203580 was able to completely reverse the effect of anisomycin. These findings suggest that the previously reported p38 MAPK inhibition by Leishmania infection may be partially overcome by anisomycin. Similar effects were observed in pretreated macrophages or in treatment of infected macrophages. These results suggests that p38 MAPK activation may have a potential therapeutic value in the treatment of visceral leishmaniasis.

Novel protein kinase C isoforms regulate human keratinocyte differentiation by activating a p38 delta mitogen-activated protein kinase cascade that targets CCAAT/enhancer-binding protein alpha

The novel protein kinase C (nPKC) isoforms are important regulators of human involucrin (hINV) gene expression during keratinocyte differentiation (Efimova, T., and Eckert, R. L. (2000) J. Biol. Chem. 275, 1601-1607). Although the regulatory mechanism involves mitogen-activated protein kinase (MAPK) activation, the role of individual MAPK isoforms has not been elucidated. We therefore examined the effects of individual nPKCs on MAPK activation. We observe unique changes whereby nPKC expression simultaneously increases p38 activity and decreases ERK1 and ERK2 activity. Although p38 alpha, p38 beta, and p38 delta are expressed in keratinocytes, only a single isoform, p38 delta, accounts for the increased p38 activity. Parallel studies indicate that this isoform is also activated by treatment with the keratinocyte regulatory agents, 12-O-tetradecanoylphorbol-13-acetate, calcium, and okadaic acid. These changes in MAPK activity are associated with increased C/EBP alpha transcription factor expression and DNA binding to the hINV promoter and increased hINV gene expression. Expression of PKC delta, PKC epsilon, or PKC eta causes a 10-fold increase in hINV promoter activity, whereas C/EBP alpha expression produces a 25-fold increase. However, simultaneous expression of both proteins causes a synergistic 100-fold increase in promoter activity. These responses are eliminated by the dominant-negative C/EBP isoform, GADD153, and are also inhibited by dominant-negative forms of Ras, MEKK1, MEK3, and p38. These results suggest that the nPKC isoforms produce a unique shift in MAPK activity via a Ras, MEKK1, MEK3 pathway, to increase p38 delta and inhibit ERK1/2 and ultimately increase C/EBP alpha binding to the hINV promoter and hINV gene expression.

p38 mitogen-activated protein kinase dephosphorylation is regulated by protein phosphatase 2A in human platelets activated by collagen

Collagen and the cross-linked collagen-related peptide (CRP-XL) each induced platelet p38 mitogen-activated protein kinase (p38) phosphorylation after 2 min. Subsequent dephosphorylation occurred in platelets activated with collagen, but not with CRP-XL, demonstrating glycoprotein VI-independent regulation of p38. Okadaic acid and fostriecin, inhibitors specific for protein phosphatase 2A (PP2A), blocked p38 dephosphorylation, and PP2A co-immunoprecipitated with phospho-p38. In addition, use of phenylarsine oxide suggested that tyrosine phosphatases and PP2A may act in concert to dephosphorylate p38. Finally, regulation of p38 in collagen-stimulated Glanzmann's platelets was indistinguishable from that in normal platelets, showing that p38 regulation is independent of integrin alphaIIbbeta3.

p38 MAPK-mediated transcriptional activation of inducible nitric-oxide synthase in glial cells. Roles of nuclear factors, nuclear factor kappa B, cAMP response element-binding protein, CCAAT/enhancer-binding protein-beta, and activating transcription factor-2

Previous studies have shown that mitogen-activated protein kinase (MAPK) cascades signal the induction of inducible nitric-oxide synthase (iNOS) in glial cells (Bhat, N. R., Zhang, P., Lee, J. C., and Hogan E. L. (1998) J. Neurosci. 18, 1633-1641; Bhat, N. R., Zhang, P., and Bhat, A. N. (1999) J. Neurochem. 72, 472-478). This study further investigates the role of p38 MAPK in the transcriptional activation of the iNOS gene by transient transfection with constitutively active upstream kinases in the pathway (i.e. MAPK kinase 3 (MKK3b(E)) and MAPK kinase 6 (MKK6b(E)). Expression in C-6 glial cells of either MKK3b(E) or MKK6b(E) resulted in an induction of the activity of a cotransfected rat iNOS promoter-reporter (iNOS-luciferase (Luc)) gene and an enhancement of cytokine-induced expression of iNOS mRNA, both of which were inhibitable by the p38 MAPK inhibitor SB203580. The MKK constructs also induced cAMP response element-mediated (CRE-Luc) and nuclear factor kappa B-dependent (nuclear factor kappa B-Luc) transcriptional activities. Transfection with dominant negative (dn) forms of CRE-binding protein (CREB) and CCAAT/enhancer-binding protein (C/EBP), the two CRE-binding transcription factors targeted by the p38 MAPK pathway, resulted in opposite effects; dnCREB enhanced and dnC/EBP inhibited iNOS-Luc parallel to their effects on CRE-Luc. In addition, the induction, by MKK3b(E) and MKK6b(E), of iNOS promoter activity was enhanced by a wild-type activating transcription factor (ATF-2), whereas a phosphorylation-defective form of ATF-2 had a suppressive effect. The results of these molecular studies provide evidence for an important role for the p38 MAPK pathway in the transcriptional activation of the iNOS gene in rat glial cells involving the transcription factors nuclear factor kappa B, C/EBP, and ATF-2.

Dealing with osmostress through MAP kinase activation

In response to changes in the extracellular environment, cells coordinate intracellular activities to maximize their probability of survival and proliferation. Eukaryotic cells, from yeast to mammals, transduce diverse extracellular stimuli through the cell by multiple mitogen-activated protein kinase (MAPK) cascades. Exposure of cells to increases in extracellular osmolarity results in rapid activation of a highly conserved family of MAPKs, known as stress-activated MAPKs (SAPKs). Activation of SAPKs is essential for the induction of adaptive responses required for cell survival upon osmostress. Recent studies have begun to shed light on the broad effects of SAPK activation in the modulation of several aspects of cell physiology, ranging from the control of gene expression to the regulation of cell division.

Activation of GCN2 in UV-irradiated cells inhibits translation

BACKGROUND

Mammalian cells subjected to ultraviolet (UV) irradiation actively repress DNA replication, transcription, and mRNA translation. While the effects of UV irradiation on DNA replication and transcription have been extensively studied, the mechanism(s) responsible for translational repression are poorly understood.

RESULTS

Here, we demonstrate that UV irradiation elicits phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2alpha) by activating the kinase GCN2 in a manner that does not require SAPK/JNK or p38 MAP kinase. GCN2-/- cells, and cells expressing nonphosphorylatable eIF2alpha as their only source of eIF2alpha protein, fail to repress translation in response to UV irradiation.

CONCLUSIONS

These results provide a mechanism for translation inhibition by UV irradiation and identify a hitherto unrecognized role for mammalian GCN2 as a mediator of the cellular response to UV stress.

Leptin signalling

The identification of leptin as the product of the obesity (ob) gene has been followed by extensive research identifying a wide spectrum of physiological effects elicited by this adipose-derived hormone. These effects are mediated via a family of cytokine-like receptor isoforms distributed in both the central nervous system and periphery. The signal transduction pathways regulated by leptin are diverse and include those characteristic of both cytokine and growth factor receptor signalling. This review describes the structure and function of leptin receptors and summarizes recent progress that has been made in characterizing the increasing number of signal transduction pathways regulated by leptin.

Stress kinase p38 mediates EGFR transactivation by hyperosmolar concentrations of sorbitol

Activation of the epidermal growth factor receptor (EGFR) has been shown to occur by ligand-dependent and ligand-independent mechanisms. Different molecular mechanisms have been found to be responsible for ligand-independent receptor transactivation. Here, we show that hyperosmolar concentrations of sorbitol activate the EGFR in human keratinocytes. Experiments using specific inhibitors of EGFR phosphorylation show that the increased amount of activated receptors is the result of a decreased rate of dephosphorylation. Furthermore, sorbitol treatment results in a strong activation of stress kinase p38. Treatment of the cells with SB203580, a known inhibitor of p38 alpha and beta kinases, results in impairment of receptor activation, indicating that the stress kinase is involved in receptor activation modulation. This is further reinforced by experiments showing that addition of Toxin B, known to be an inhibitor of the small Rho GTPases rac1, cdc42, and Rho A/B, to the cells results in a strong induction of EGFR activation. Our results point, therefore, to a mechanism by which osmotic shock activates EGFR through the small Rho GTPases-p38 stress kinase pathway.

Yeast go the whole HOG for the hyperosmotic response

An evolutionarily conserved mitogen-activated protein kinase pathway--the high osmolarity glycerol (HOG) pathway--mediates the hyperosmotic response in Saccharomyces cerevisiae. A variety of powerful approaches has generated a comprehensive picture of how cells respond to this stress condition. Several presumptive osmosensors on the cell surface recruit and activate downstream signaling components, which regulate the activity of transcription factors to control gene expression.

Temporal gradients in shear stimulate osteoblastic proliferation via ERK1/2 and retinoblastoma protein

Bone cells are subject to interstitial fluid flow (IFF) driven by venous pressure and mechanical loading. Rapid dynamic changes in mechanical loading cause transient gradients in IFF. The effects of pulsatile flow (temporal gradients in fluid shear) on rat UMR106 cells and rat primary osteoblastic cells were studied. Pulsatile flow induced a 95% increase in S-phase UMR106 cells compared with static controls. In contrast, ramped steady flow stimulated only a 3% increase. Similar patterns of S-phase induction were also observed in rat primary osteoblastic cells. Pulsatile flow significantly increased relative UMR106 cell number by 37 and 62% at 1.5 and 24 h, respectively. Pulsatile flow also significantly increased extracellular signal-regulated kinase (ERK1/2) phosphorylation by 418%, whereas ramped steady flow reduced ERK1/2 activation to 17% of control. Correspondingly, retinoblastoma protein was significantly phosphorylated by pulsatile fluid flow. Inhibition of mitogen-activated protein (MAP)/ERK kinase (MEK)1/2 by U0126 (a specific MEK1/2 inhibitor) reduced shear-induced ERK1/2 phosphorylation and cell proliferation. These findings suggest that temporal gradients in fluid shear stress are potent stimuli of bone cell proliferation.

The p38 and JNK pathways cooperate to trans-activate vitamin D receptor via c-Jun/AP-1 and sensitize human breast cancer cells to vitamin D(3)-induced growth inhibition

The signaling connection between mitogen-activated protein kinases(MAPKs) and nuclear steroid receptors is complex and remains mostly unexplored. Here we report that stress-activated protein kinases p38 and JNK trans-activate nuclear steroid vitamin D receptor (VDR) gene and increase vitamin D(3)-dependent growth inhibition in human breast cancer cells. Activation of p38 and JNK by an active MAPK kinase 6 stimulates VDR promoter activity independently of the ligand vitamin D(3) and estrogen receptor expression. Moreover, stimulation of the endogenous stress pathways by adenovirus-mediated delivery of recombinant MAPK kinase 6 also activates VDR and sensitizes MCF-7 cells to vitamin D(3)-dependent growth inhibition. Both the p38 and JNK MAPK pathways and the downstream transcription factor c-Jun/AP-1 are required for the VDR stimulation, as revealed by application of their dominant negatives, the specific p38 inhibitor SB203580, and site-directed mutagenesis of the AP-1 element in the VDR promoter. The essential role of the p38 and JNK stress pathways in up-regulation of VDR expression is further confirmed by using the chemical stimulator arsenite. These results establish a signaling connection between the stress MAPK pathways and steroid hormone receptor VDR expression and thereby offer new insights into regulation of cell growth by the MAPK pathways through regulation of vitamin D(3)/VDR activity.

TGF-beta receptor-activated p38 MAP kinase mediates Smad-independent TGF-beta responses

Through the action of its membrane-bound type I receptors, transforming growth factor-beta (TGF-beta) elicits a wide range of cellular responses that regulate cell proliferation, differentiation and apoptosis. Many of the signaling responses induced by TGF-beta are mediated by Smad proteins, but certain evidence has suggested that TGF-beta can also signal independently of Smads. We found in mouse mammary epithelial (NMuMG) cells, which respond to TGF-beta treatment in multiple ways, that TGF-beta-induced activation of p38 MAP kinase is required for TGF-beta-induced apoptosis, epithelial-to-mesenchymal transition (EMT), but not growth arrest. We further demonstrated that activation of p38 is independent of Smads using a mutant type I receptor, which is incapable of activating Smads but still retains the kinase activity. This mutant receptor is sufficient to activate p38 and cause NMuMG cells to undergo apoptosis. However, it is not sufficient to induce EMT. These results indicate that TGF-beta receptor signals through multiple intracellular pathways and provide first-hand biochemical evidence for the existence of Smad-independent TGF-beta receptor signaling.

Cell adhesion differentially regulates the nucleocytoplasmic distribution of active MAP kinases

Cells decide whether to undergo processes, such as proliferation,differentiation and apoptosis, based upon the cues they receive from both circulating factors and integrin-mediated adhesion to the extracellular matrix. Integrins control the activation of the early signaling pathways. For example, growth factor activation of the ERK cascade is enhanced when cells are adherent. In addition, adhesion receptors oversee the cellular localization of critical signaling components. We have recently shown that ERK signaling to the nucleus is regulated by cell adhesion at the level of nucleocytoplasmic trafficking. Since the ERKs are only one class of MAP kinase, we extended these studies to include both JNK and p38 MAP kinases. We have rendered JNK and p38 activation in NIH 3T3 fibroblasts anchorage-independent either by treatment with anisomycin or by expression of upstream activators. Under conditions whereby JNK activation is anchorage-independent, we show that localization of JNK to the nucleus and JNK-mediated phosphorylation of c-Jun and Elk-1 is not altered by loss of adhesion. Likewise, the ability of activated p38 to accumulate in the nucleus was similar in suspended and adherent cells. Finally, we show that expression of a form of ERK, which is activated and resistant to nuclear export, reverses the adhesion-dependency of ERK phosphorylation of Elk-1. Thus, adhesion differentially regulates the nucleocytoplasmic distribution of MAP kinase members; ERK accumulation in the nucleus occurs more efficiently in adherent cells, whereas nuclear accumulation of active p38 and active JNK are unaffected by changes in adhesion.

Hepatic ischemic preconditioning in mice is associated with activation of NF-kappaB, p38 kinase, and cell cycle entry

A brief period of hepatic ischemia protects the liver against subsequent ischemia-reperfusion (IR) injury, but the mechanism of such preconditioning is poorly understood. We examined whether preconditioning activated nuclear factor kappa B (NF-kappaB), the stress-activated protein kinases (SAPK), c-Jun N-terminal kinase-1 (JNK-1) and p38, and entry into the cell cycle. We used a murine model of partial hepatic ischemia. Preconditioning was performed by clamping the vasculature for 2 to 20 minutes, and allowing reperfusion for 10 minutes before 90-minute ischemia or IR. As assessed by serum alanine aminotransferase (ALT) levels and liver histology, preconditioning periods of 5 and 10 minutes were highly protective against IR injury, whereas 2-, 15-, and 20-minute intervals were ineffective. Preconditioning was associated with entry of hepatocytes into the cell cycle within 2 hours of subsequent IR, as indicated by proliferating cell nuclear antigen (PCNA) nuclear staining, induction of cyclin D1 and numerous mitotic figures; in the absence of preconditioning, such changes were not seen until 24 hours. Preconditioning increased nuclear binding of NF-kappaB within 30 minutes of the subsequent ischemic interval, paralleled by degradation of inhibitory (binding) protein for NF-kappaB (IkappaBalpha). Ischemic preconditioning also activated p38 kinase and JNK-1, which are known to converge on cyclin D1 regulation. The protective effect of the preconditioning regimen was more closely associated with p38 kinase than JNK-1 activation. In conclusion, the hepatoprotective effects of ischemic preconditioning are associated with activation of NF-kappaB and SAPKs that are associated with entry of hepatocytes into the cell cycle, a critical biological effect that favors survival of the liver against ischemic and IR injury.

Green tea polyphenol blocks h(2)o(2)-induced interleukin-8 production from human alveolar epithelial cells

Reactive oxygen species (ROS) play crucial roles in ischemia-reperfusion (IR) injury of lung transplants. Reactive oxygen species may stimulate the production of neutrophil chemotactic factors such as interleukin-8 (IL-8), from alveolar epithelial cells, causing recruitment and activation of neutrophils in the reperfused tissue. Green tea polyphenol has potent anti-oxidative activities and anti-inflammatory effects by decreasing cytokine production. In the present study, we found that green tea polyphenol significantly inhibited IL-8 production induced by hydrogen peroxide (H(2)O(2)) in human lung alveolar epithelial cells (A549 line). It has been shown that mitogen activated protein kinases, such as Jun N-terminal kinase (JNK), p38 and p44/42, could mediate IL-8 production from a variety of cell types. We further investigated the effect of green tea polyphenol on these protein kinases, and demonstrated that H(2)O(2)-induced phosphorylation of JNK and p38 but not p44/42 was inhibited by green tea polyphenol in A549 cells. We speculate that green tea polyphenol may inhibit H(2)O(2)-induced IL-8 production from A549 cells through inactivation of JNK and p38.

Disruption in gastric mucin synthesis by Helicobacter pylori lipopolysaccharide involves ERK and p38 mitogen-activated protein kinase participation

Helicobacter pylori is a primary factor in the etiology of gastric disease, and its early pathogenic effects are manifested by up-regulation of inflammatory processes and the loss of mucus coat continuity. We investigated the role of extracellular signal-regulated kinase (ERK) and p38 mitogen activated protein kinase (MAPK) in the disturbances in gastric mucin synthesis and apoptotic processes evoked by H. pylori lipopolysaccharide (LPS). Exposure of gastric mucosal cells to the LPS led to a dose-dependent decrease (up to 59.5%) in mucin synthesis, accompanied by a marked increase in caspase-3 activity and apoptosis. Inhibition of ERK with PD98059 accelerated (up to 36.1%) the LPS-induced decrease in mucin synthesis, and caused further enhancement in caspase-3 activity and apoptosis. Blockade of p38 kinase with SB203580 produced reversal in the LPS-induced reduction in mucin synthesis, and substantially countered the LPS-induced increases in caspas-3 activity and apoptosis. Moreover, inhibition of caspase-3 blocked the LPS-induced increase in caspse-3 activity and produced an increase in mucin synthesis. Thus the detrimental influence of H. pylori LPS on gastric mucin synthesis is closely linked to caspase-3 activation and apoptosis, and involves ERK and p38 kinase participation.

A novel beta-lactam antibiotic activates tumor cell apoptotic program by inducing DNA damage

Many of the anticancer drugs in current use are toxic and thus limited in their efficacy. It therefore becomes essential to develop novel chemotherapeutic agents with lower levels of toxicity. The beta-lactam antibiotics have been used for many years to treat bacterial infections with limited or no toxicity. Until now, it has never been shown that beta-lactams could kill tumor cells. Here, for the first time, we have discovered and characterized the apoptosis-inducing properties of a family of novel beta-lactam antibiotics against human leukemia, breast, prostate, and head-and-neck cancer cells. We found that one particular lead compound (lactam 1) with an N-methylthio group was able to induce DNA damage and inhibit DNA replication in Jurkat T cells within a 2-h treatment. This was followed by p38 mitogen-activated protein kinase activation, S phase arrest, and apoptotic cell death. p38 was found to be a central player in beta-lactam-induced apoptosis and resided downstream of DNA damage but upstream of caspase activation. Accompanying caspase-8 activation was cleavage of the pro-apoptotic Bcl-2 family protein Bid, and release of the mitochondrial cytochrome c. This was also associated with activation of caspase-9 and -3. Analogs of lactam 1 in which the N-methylthio group was replaced with other organothio chains exhibited progressive decreased potencies to induce DNA damage, p38 kinase activation, S phase arrest, and apoptosis, demonstrating requirement of the N-methylthio group. Because of the ease of synthesis and structural manipulation, we believe these beta-lactams may have the potential to be developed into anticancer agents.

Putative role of 67 kDa elastin-laminin receptor in tumor invasion

Cellular regulatory mechanisms normally maintain a delicate balance between cell proliferation, quiescence and death. The imbalance between these functions resulting from molecular intracellular changes is a key factor in tumorigenesis. Tumor cells detaching from the primary tumor possess a propension for invasion and metastasis formation. These tumor cells can attach, migrate, proliferate and grow in host tissue. The surrounding extracellular matrix (ECM) modulates these functions. It is now widely accepted that cell-matrix interactions play an important role in these processes. Most investigators concentrated their attention on the role of integrins in the above processes. There are, however, only scant data on the role of elastin and its receptors in tumor invasion. Nevertheless, experimental evidence indicates that the 67 kDa elastin-laminin receptor (ELR) subunit plays an important role in tumor invasion by mediating essential tumor cell functions leading to metastases. In this review we will concentrate on the putative role of the 67 kDa ELR subunit in tumor invasion.

Type II keratins are phosphorylated on a unique motif during stress and mitosis in tissues and cultured cells

Epithelial cell keratins make up the type I (K9-K20) and type II (K1-K8) intermediate filament proteins. In glandular epithelia, K8 becomes phosphorylated on S73 ((71)LLpSPL) in human cultured cells and tissues during stress, apoptosis, and mitosis. Of all known proteins, the context of the K8 S73 motif (LLS/TPL) is unique to type II keratins and is conserved in epidermal K5/K6, esophageal K4, and type II hair keratins, except that serine is replaced by threonine. Because knowledge regarding epidermal and esophageal keratin regulation is limited, we tested whether K4-K6 are phosphorylated on the LLTPL motif. K5 and K6 become phosphorylated in vitro on threonine by the stress-activated kinase p38. Site-specific anti-phosphokeratin antibodies to LLpTPL were generated, which demonstrated negligible basal K4-K6 phosphorylation. In contrast, treatment of primary keratinocytes and other cultured cells, and ex vivo skin and esophagus cultures, with serine/threonine phosphatase inhibitors causes a dramatic increase in K4-K6 LLpTPL phosphorylation. This phosphorylation is accompanied by keratin solubilization, filament reorganization, and collapse. K5/K6 LLTPL phosphorylation occurs in vivo during mitosis and apoptosis induced by UV light or anisomycin, and in human psoriatic skin and squamous cell carcinoma. In conclusion, type II keratins of proliferating epithelia undergo phosphorylation at a unique and conserved motif as part of physiological mitotic and stress-related signals.

In vivo identification of the mitogen-activated protein kinase cascade as a central pathogenic pathway in experimental mesangioproliferative glomerulonephritis

Evidence was recently provided for the activation of extracellular signal-regulated kinase (ERK), the best characterized mitogen-activated protein kinase, as an intracellular convergence point for mitogenic stimuli in animal models of glomerulonephritis (GN). In this study, in vivo ERK activity was blocked, with a pharmacologic inhibitor (U0126) of the ERK-activating kinase, in rats with mesangioproliferative GN. After injection of the monoclonal anti-Thy1.1 antibody (OX-7), the rats were treated (days 3 to 6) with low (10 mg/kg body wt) or high (100 mg/kg body wt) doses of U0126 administered intraperitoneally twice daily. On day 6 after induction of the disease, whole cortical tissue and isolated glomeruli were examined by using kinase activity assays, Western blot analyses, and immunohistochemical assays. Treatment with U0126 significantly reduced glomerular stimulation of ERK in anti-Thy1 GN. In the high dose-treated group, this downregulation was accompanied by a reduction in the number of glomerular mitotic figures, back to healthy control levels, and significant decreases in the numbers of total (P < 0.05) and 5-bromo-2'-deoxyuridine-positive (P < 0.05) glomerular cells. Immunohistochemical double-staining of renal sections demonstrated that mesangial cells were the major glomerular targets of U0126 in anti-Thy1 GN. These observations point to ERK as a putative intracellular mediator of the proliferative response in GN and suggest that pharmacologic treatments that interfere with the activation of ERK may be of potential therapeutic interest.

Murine coronavirus replication-induced p38 mitogen-activated protein kinase activation promotes interleukin-6 production and virus replication in cultured cells

Analyses of mitogen-activated protein kinases (MAPKs) in a mouse hepatitis virus (MHV)-infected macrophage-derived J774.1 cell line showed activation of two MAPKs, p38 MAPK and c-Jun N-terminal kinase (JNK), but not of extracellular signal-regulated kinase (ERK). Activation of MAPKs was evident by 6 h postinfection. However, UV-irradiated MHV failed to activate MAPKs, which demonstrated that MHV replication was necessary for their activation. Several other MHV-permissive cell lines also showed activation of both p38 MAPK and JNK, which indicated that the MHV-induced stress-kinase activation was not restricted to any particular cell type. The upstream kinase responsible for activating MHV-induced p38 MAPK was the MAPK kinase 3. Experiments with a specific inhibitor of p38 MAPK, SB 203580, demonstrated that MHV-induced p38 MAPK activation resulted in the accumulation of interleukin-6 (IL-6) mRNAs and an increase in the production of IL-6, regardless of MHV-induced general host protein synthesis inhibition. Furthermore, MHV production was suppressed in SB 203580-treated cells, demonstrating that activated p38 MAPK played a role in MHV replication. The reduced MHV production in SB 203580-treated cells was, at least in part, due to a decrease in virus-specific protein synthesis and virus-specific mRNA accumulation. Interestingly, there was a transient increase in the amount of phosphorylation of the translation initiation factor 4E (eIF4E) in infected cells, and this eIF4E phosphorylation was p38 MAPK dependent; it is known that phosphorylated eIF4E enhances translation rates of cap-containing mRNAs. Furthermore, the upstream kinase responsible for eIF4E phosphorylation, MAPK-interacting kinase 1, was also phosphorylated and activated in response to MHV infection. Our data suggested that host cells, in response to MHV replication, activated p38 MAPK, which subsequently phosphorylated eIF4E to efficiently translate certain host proteins, including IL-6, during virus-induced severe host protein synthesis inhibition. MHV utilized this p38 MAPK-dependent increase in eIF4E phosphorylation to promote virus-specific protein synthesis and subsequent progeny virus production. Enhancement of virus-specific protein synthesis through virus-induced eIF4E activation has not been reported in any other viruses.

Role of MAP kinases and their cross-talk in TGF-beta1-induced apoptosis in FaO rat hepatoma cell line

Transforming growth factor (TGF) beta1 is a potent inducer of apoptosis in the liver. During TGF-beta1-induced apoptosis, 3 mitogen-activated protein (MAP) kinases (extracellular signal-regulated kinase [ERK], c-Jun N-terminal kinase [JNK], and p38 kinase) showed simultaneously sustained activation in FaO rat hepatoma cells. TGF-beta1-induced apoptosis was markedly enhanced when ERK activation was selectively inhibited by the mitogen-activated protein kinase/extracellular signal-regulated kinase kinase inhibitor PD98059. In contrast, both interfering with p38 activity by overexpression of the dominant negative (DN) MKK6 mutant and inhibition of the JNK pathway by overexpression of the DN SEK1 mutant resulted in suppression of mitochondrial cytochrome c release, abrogating TGF-beta1-induced apoptosis. In addition, antiapoptotic Bcl-2 blocked mitochondrial cytochrome c release, suppressing TGF-beta1-induced activation of JNK and p38. Inhibition of ERK activity enhanced TGF-beta1-induced p38 and JNK activation. However, inhibition of the JNK pathway suppressed p38 but induced transient ERK activation. Similarly, interfering with the p38 pathway also attenuated JNK activation but generated transient ERK activation in response to TGF-beta1. These results indicate that disrupting one MAP kinase pathway affects the TGF-beta1-induced activation of other MAP kinases, suggesting cross-talk among MAP kinase pathways. In conclusion, we propose that the balance and integration of MAP kinase signaling may regulate commitment to TGF-beta1-induced apoptosis modulating the release of cytochrome c from mitochondria.

Glucocorticoids synergistically enhance nontypeable Haemophilus influenzae-induced Toll-like receptor 2 expression via a negative cross-talk with p38 MAP kinase

The recognition of invading microbes followed by the induction of effective innate immune response is crucial for host survival. Human surface epithelial cells are situated at host-environment boundaries and thus act as the first line of host defense against invading microbes. They recognize the microbial ligands via Toll-like receptors (TLRs) expressed on the surface of epithelial cells. TLR2 has gained importance as a major receptor for a variety of microbial ligands. In contrast to its high expression in lymphoid tissues, TLR2 is expressed at low level in epithelial cells. Thus, it remains unclear whether the low amount of TLR2 expressed in epithelial cells is sufficient for mediating bacteria-induced host defense and immune response and whether TLR2 expression can be up-regulated by bacteria during infection. Here, we show that TLR2, although expressed at very low level in unstimulated human epithelial cells, is greatly up-regulated by nontypeable Hemophilus influenzae (NTHi), an important human bacterial pathogen causing otitis media and chronic obstructive pulmonary diseases. Activation of an IKKbeta-IkappaBalpha-dependent NF-kappaB pathway is required for TLR2 induction, whereas inhibition of the MKK3/6-p38alpha/beta pathway leads to enhancement of NTHi-induced TLR2 up-regulation. Surprisingly, glucocorticoids, well known potent anti-inflammatory agents, synergistically enhance NTHi-induced TLR2 up-regulation likely via a negative cross-talk with the p38 MAP kinase pathway. These studies may bring new insights into the role of bacteria and glucocorticoids in regulating host defense and immune response and lead to novel therapeutic strategies for modulating innate immune and inflammatory responses for otitis media and chronic obstructive pulmonary diseases.

Japanese encephalitis virus infection initiates endoplasmic reticulum stress and an unfolded protein response

The malfunctioning of the endoplasmic reticulum (ER) of cells in hosts ranging from yeast to mammals can trigger an unfolded protein response (UPR). Such malfunctioning can result from a variety of ER stresses, including the inhibition of protein glycosylation and calcium imbalance. To cope with ER stresses, cells may rely on the UPR to send a signal(s) from the ER to the nucleus to stimulate appropriate cellular responses, including induction of chaperone expression. During Japanese encephalitis virus (JEV) infection, the lumen of the ER rapidly accumulates substantial amounts of viral proteins for virus progeny production. In the present study, we demonstrate that as evidenced by certain chaperone inductions, JEV infection triggers the UPR in fibroblast BHK-21 cells and in neuronal N18 and NT-2 cells, in which JEV results in apoptotic cell death. By contrast, no UPR was observed in apoptosis-resistant K562 cells infected by JEV. JEV infection also activates expression of CHOP/GADD153, a distinctive transcription factor often induced by the UPR, and appears to trigger activation of p38 mitogen-activated protein kinase, a posttranslational activator of CHOP. Ectopic enforcement of CHOP expression enhanced JEV-induced apoptosis, whereas treatment with a p38-specific inhibitor, SB203580, partially blocked JEV-induced apoptosis. Interestingly, bcl-2 overexpression and treatment with a pancaspase inhibitor, z-VAD-fmk, inhibited CHOP induction and diminished JEV-induced apoptosis, suggesting that Bcl-2 and caspases could be the upstream regulators of CHOP. Our results thus suggest that virus-induced ER stress may participate, via p38-dependent and CHOP-mediated pathways, in the apoptotic process triggered by JEV infection.

Non-thermal activation of the hsp27/p38MAPK stress pathway by mobile phone radiation in human endothelial cells: molecular mechanism for cancer- and blood-brain barrier-related effects

We have examined whether non-thermal exposures of cultures of the human endothelial cell line EA.hy926 to 900 MHz GSM mobile phone microwave radiation could activate stress response. Results obtained demonstrate that 1-hour non-thermal exposure of EA.hy926 cells changes the phosphorylation status of numerous, yet largely unidentified, proteins. One of the affected proteins was identified as heat shock protein-27 (hsp27). Mobile phone exposure caused a transient increase in phosphorylation of hsp27, an effect which was prevented by SB203580, a specific inhibitor of p38 mitogen-activated protein kinase (p38MAPK). Also, mobile phone exposure caused transient changes in the protein expression levels of hsp27 and p38MAPK. All these changes were non-thermal effects because, as determined using temperature probes, irradiation did not alter the temperature of cell cultures, which remained throughout the irradiation period at 37 +/- 0.3 degrees C. Changes in the overall pattern of protein phosphorylation suggest that mobile phone radiation activates a variety of cellular signal transduction pathways, among them the hsp27/p38MAPK stress response pathway. Based on the known functions of hsp27, we put forward the hypothesis that mobile phone radiation-induced activation of hsp27 may (i) facilitate the development of brain cancer by inhibiting the cytochrome c/caspase-3 apoptotic pathway and (ii) cause an increase in blood-brain barrier permeability through stabilization of endothelial cell stress fibers. We postulate that these events, when occurring repeatedly over a long period of time, might become a health hazard because of the possible accumulation of brain tissue damage. Furthermore, our hypothesis suggests that other brain damaging factors may co-participate in mobile phone radiation-induced effects.

Cell signaling in aging and apoptosis

Alterations in apoptotic potential, due to perturbations in cell signaling cascades, could underlie age-related organ-specific cellular degeneration and death. While increased apoptosis could lead to cell loss, as in neuronal degeneration, loss of apoptosis competence might well result in the loss of phenotypic fidelity of somatic cells, which could explain to some extent, the age-related increase in cancer incidence. Results from our laboratory indicate that after subjecting young and old rats to genotoxic stress in the form of methyl methanesulfonate (MMS), an apoptotic response is quickly mounted in the liver of the young animals but virtually absent in the same organ of old animals (Nature Med. 8 (2002) 3). To address the possible molecular signaling defect(s) responsible for the age-related dysfunction of apoptosis in response to MMS, mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinases (ERKs), c-Jun NH(2)-terminal kinases (JNKs) and p38 MAPKs, were evaluated in the liver of young and old rats after MMS treatment. The results demonstrated distinct age-specific patterns of MMS-induced MAPKs activation, suggesting that the balance between cell survival and apoptosis after genotoxic stress may be impaired during aging. These results are discussed in terms of the relative importance in aging of biological redundancy, a concept put forward by the late Bernard Strehler, and cellular fidelity.

Phospholipase C-beta 2 interacts with mitogen-activated protein kinase kinase 3

Phospholipase C (PLC)-beta enzymes (isoenzymes beta 1-beta 4) are activated by G protein subunits, leading to the generation of intracellular messengers which mobilize calcium and activate protein kinase C. It has recently been recognized that these enzymes interact with and are regulated by proteins other than G proteins. Using the yeast two-hybrid technique to screen a leukocyte library we identified mitogen-activated protein kinase kinase 3 (MKK3) as a partner of PLC-beta 2. The interaction was confirmed by co-immunoprecipitation assays which indicated that MKK3 interacts with PLC-beta 2, but not with other PLC-betas. PLC-beta 2 interacted weakly with MKK6, which is related to MKK3, but not with the other MKK3 tested. The region of PLC-beta 2 involved in the interaction with MKK3 was mapped to the C-terminus of PLC-beta 2. p38MAPK also co-immunoprecipitated with PLC-beta 2. The data suggest that PLC-beta 2 serves an unappreciated role assembling components of the p38MAPK signaling module.

Requirement of c-Jun N terminal kinase and P38 mitogen-activated protein kinase for anti-IgM-induced reduction in mitochondrial membrane potential in CH31 B lymphoma cells

CH31 cells have been used for analysis of B cell tolerance, since engagement of membrane immunoglobulin (mIg) results in loss in mitochondrial membrane potential (DeltaPsim), followed by cell death. We have reported that the dominant-negative (dn) form of c-Jun N-terminal kinase (JNK) substantially prevented a loss of DeltaPsim at 24 h, with partial protection around 48 h after anti-IgM stimulation. In this study, we demonstrate that anti-IgM induced a sustained activation of p38 mitogen-activated protein (MAP) kinase. The p38MAP kinase inhibitor SB203580 substantially prevented loss of DeltaPsim at 14 h, with partial prevention (18-24 h) after anti-IgM stimulation. The dnJNK1-mediated prevention of anti-IgM-induced mitochondrial dissipation was enhanced by SB203580 at 42 h, but not 24 h, after stimulation, suggesting that activation of either p38 MAP kinase or JNK may be sufficient for the initiation of early phase of anti-IgM-induced loss of DeltaPsim while both may be necessary in the late phase.

Anti-inflammatory effects of a p38 mitogen-activated protein kinase inhibitor during human endotoxemia

The p38 mitogen-activated protein kinase (MAPK) participates in intracellular signaling cascades resulting in inflammatory responses. Therefore, inhibition of the p38 MAPK pathway may form the basis of a new strategy for treatment of inflammatory diseases. However, p38 MAPK activation during systemic inflammation in humans has not yet been shown, and its functional significance in vivo remains unclear. Hence, we exposed 24 healthy male subjects to an i.v. dose of LPS (4 ng/kg), preceded 3 h earlier by orally administered 600 or 50 mg BIRB 796 BS (an in vitro p38 MAPK inhibitor) or placebo. Both doses of BIRB 796 BS significantly inhibited LPS-induced p38 MAPK activation in the leukocyte fraction of the volunteers. Cytokine production (TNF-alpha, IL-6, IL-10, and IL-1R antagonist) was strongly inhibited by both low and high dose p38 MAPK inhibitor. In addition, p38 MAPK inhibition diminished leukocyte responses, including neutrophilia, release of elastase-alpha(1)-antitrypsin complexes, and up-regulation of CD11b with down-regulation of L-selectin. Finally, blocking p38 MAPK decreased C-reactive protein release. These data identify p38 MAPK as a principal mediator of the inflammatory response to LPS in humans. Furthermore, the anti-inflammatory potential of an oral p38 MAPK inhibitor in humans in vivo suggests that p38 MAPK inhibitors may provide a new therapeutic option in the treatment of inflammatory diseases.

Modulation of endothelial cell apoptosis by heme oxygenase-1-derived carbon monoxide

It is well established that expression of heme oxygenase-1 (HO-1) acts in a cytoprotective manner in a variety of cell types, including in endothelial cells (EC). We have recently shown that HO-1 expression protects EC from undergoing apoptosis. We have also shown that the antiapoptotic effect of HO-1 is mediated through heme catabolism into the gas carbon monoxide (CO). In this review, we discuss the possible molecular mechanisms by which HO-1-derived CO suppresses EC apoptosis. We will review data suggesting that the antiapoptotic effect of CO acts through the activation of the p38 mitogen-activated protein kinase signal transduction pathway and requires the activation of the transcription factor nuclear factor-kappa B (NF-kappa B), as well as the expression of a subset of NF-kappa B-dependent antiapoptotic genes.

Role of p38 MAPK in transforming growth factor beta stimulation of collagen production by scleroderma and healthy dermal fibroblasts

Transforming growth factor beta has been implicated as a mediator of excessive extracellular matrix deposition in scar tissue and fibrosis, including systemic sclerosis. To further characterize the mechanism of collagen gene expression in systemic sclerosis and healthy skin fibroblasts, we examined the role of p38 MAPK signaling in collagen gene regulation by transforming growth factor beta. Treatment of dermal fibroblasts with transforming growth factor beta resulted in a prolonged activation of p38 MAPK. Furthermore, a specific inhibitor of p38 suppressed transforming growth factor beta stimulation of collagen type I mRNA and the alpha2(I) collagen promoter activity. To further probe the role of p38 in collagen regulation by transforming growth factor beta, we utilized an expression vector containing p38alpha cDNA. Ectopic expression of p38alpha enhanced COL1A2 promoter activity and potentiated transforming growth factor beta stimulation of this promoter. The p38 response element in the COL1A2 promoter overlapped with the previously characterized transforming growth factor beta response element. Consistent with these observations, collagen type I mRNA and protein levels were increased in transforming-growth-factor-beta-stimulated fibroblasts transduced with an adenoviral vector expressing p38alpha. To determine the possible role of p38 in abnormal collagen production by systemic sclerosis fibroblasts, p38 protein levels were compared in systemic sclerosis and healthy skin fibroblasts. Both cell types exhibited similar total levels of p38 MAPK and similar kinetics of p38 activation in response to transforming growth factor beta. In conclusion, this study demonstrates a costimulatory role for p38 MAPK in transforming growth factor beta induction of the collagen type I gene. Expression levels and activation status of p38 are not consistently elevated in systemic sclerosis fibroblasts suggesting that the p38 MAPK pathway is not dysregulated in systemic sclerosis fibroblasts.