p38 targets various transcription factors via intermediate proteins

A novel ribosomal S6 kinase (RSK) family member, RSK-B, has been identified in a p38alphaMAPK-baited intracellular interaction screen. RSK-B presents two catalytic domains typical for the RSK family. The protein kinase C-like N-terminal and the calcium/calmodulin kinase-like C-terminal domains both contain conserved ATP-binding and activation consensus sequences. RSK-B is a p38alphaMAPK substrate, and activated by p38alphaMAPK and, more weakly, by ERK1. RSK-B phosphorylates the cAMP response element-binding protein (CREB) and c-Fos peptides. In intracellular assays, RSK-B drives cAMP response element- and AP1-dependent reporter expression. RSK-B locates to the cell nucleus and co-translocates p38alphaMAPK. In conclusion, RSK-B is a novel CREB kinase under dominant p38alphaMAPK control, also phosphorylating additional substrates (Pierrat, 1998).

The p38/stress-activated protein kinase2 (p38/SAPK2) is activated by cellular stress and proinflammatory cytokines. In human Jurkat T-cells, induction of the early growth response gene-1 (egr-1) by anisomycin is completely inhibited by SB203580, a specific inhibitor of p38/SAPK2a and p38/SAPK2b. Northern blot and reporter gene experiments indicate that this block is at the level of mRNA biosynthesis. Using mutants of the egr-1 promoter, it has been demonstrated that a distal cAMP-responsive element (CRE; nucleotides -134 to -126) is necessary to control egr-1 induction by p38/SAPK2. Pull-down assays indicate that phospho-CRE binding protein (CREB) and phospho-activating transcription factor-1 (ATF1) bind to this element in a p38/SAPK2-dependent manner. In response to anisomycin, two known CREB kinases downstream of p38/SAPK2 [MAPKAP kinase 2 (MK2) and mitogen- and stress-activated kinase 1 (MSK1)] show increased activity. However, in MK2 -/- fibroblasts derived from mice carrying a disruption of the MK2 gene, the phosphorylation of CREB and ATF1 and the expression of egr-1 reach levels comparable with wild type cells. This finding excludes MK2 as an involved enzyme. It is concluded that egr-1 induction by anisomycin is mediated by p38/SAPK2 and probably by MSK1. Phosphorylated CREB and ATF1 then bind to the CRE of the egr-1 promoter and cause a stress-dependent transcriptional activation of this gene (Rolli, 1999).

The mechanisms by which growth factor-induced signals are propagated to the nucleus, leading to the activation of the transcription factor CREB, have been characterized. Nerve growth factor (NGF) activates multiple signaling pathways that mediate the phosphorylation of CREB at the critical regulatory site, serine 133 (Ser-133). NGF activates the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinases (MAPKs), which in turn activate the pp90 ribosomal S6 kinase (RSK) family of Ser/Thr kinases, all three members of which were found to catalyze CREB Ser-133 phosphorylation in vitro and in vivo. In addition to the ERK/RSK pathway, NGF was found to activate the p38 MAPK and its downstream effector, MAPK-activated protein kinase 2 (MAPKAP kinase 2), resulting in phosphorylation of CREB at Ser-133. Inhibition of either the ERK/RSK or the p38/MAPKAP kinase 2 pathway only partially blocks NGF-induced CREB Ser-133 phosphorylation, suggesting that either pathway alone is sufficient for coupling the NGF signal to CREB activation. However, inhibition of both the ERK/RSK and the p38/MAPKAP kinase 2 pathways completely abolishes NGF-induced CREB Ser-133 phosphorylation. These findings indicate that NGF activates two distinct MAPK pathways, both of which contribute to the phosphorylation of the transcription factor CREB and the activation of immediate-early genes (Xing, 1998).

Several growth factor- and calcium-regulated kinases such as pp90(rsk) or CaM kinase IV can phosphorylate the transcription factor serum response factor (SRF) at serine 103 (Ser-103). However, it is unknown whether stress-regulated kinases can also phosphorylate SRF. Treatment of cells with anisomycin, arsenite, sodium fluoride, or tetrafluoroaluminate induces phosphorylation of SRF at Ser-103 in both HeLa and NIH3T3 cells. This phosphorylation is dependent on the kinase p38/SAPK2 and correlates with the activation of MAPKAP kinase 2 (MK2). MK2 phosphorylates SRF in vitro at Ser-103 with similar efficiency as the small heat shock protein Hsp25 and significantly better than CREB. Comparison of wild type murine fibroblasts with those derived from MK2-deficient mice [Mk(-/-)] reveals MK2 as the major SRF kinase induced by arsenite. These results demonstrate that SRF is targeted by several signal transduction pathways within cells and establishes SRF as a nuclear target for MAPKAP kinase 2 (Heidenreich, 1999).

Insulin upstream factor 1 (IUF1), a transcription factor present in pancreatic beta-cells, binds to the sequence C(C/T)TAATG present at several sites within the human insulin promoter. cDNA encoding human IUF1 was isolated and sequenced and this cDNA was used to identify the signal transduction pathway by which glucose triggers IUF1 activation. In human islets, or in the mouse beta-cell line MIN6, high glucose induces the binding of IUF1 to DNA, an effect mimicked by serine/threonine phosphatase inhibitors, indicating that DNA binding is induced by a phosphorylation mechanism. The glucose-stimulated binding of IUF1 to DNA and IUF1-dependent gene transcription are both prevented by SB 203580, a specific inhibitor of stress-activated protein kinase 2 (SAPK2) but not by several other protein kinase inhibitors. Consistent with this finding, high glucose activates mitogen-activated protein kinase-activated protein kinase 2 (MAPKAP kinase-2) (a downstream target of SAPK2) in MIN6 cells, an effect that is also blocked by SB 203580. Cellular stresses that trigger the activation of SAPK2 and MAPKAP kinase-2 (arsenite, heat shock) also stimulate IUF1 binding to DNA and IUF1-dependent gene transcription, and these effects are also prevented by SB 203580. IUF1 expressed in Escherichia coli is unable to bind to DNA, but binding is induced by incubation with MgATP, SAPK2, and a MIN6 cell extract, which results in the conversion of IUF1 to a slower migrating form. SAPK2 cannot be replaced by p42 MAP kinase, MAPKAP kinase-2, or MAPKAP kinase-3. The glucose-stimulated activation of IUF1 DNA binding and MAPKAP kinase-2 (but not the arsenite-induced activation of these proteins) is prevented by wortmannin and LY 294002 at concentrations similar to those that inhibit phosphatidylinositide 3-kinase. These results indicate that high glucose (a cellular stress) activates SAPK2 by a novel mechanism in which a wortmannin/LY 294002-sensitive component plays an essential role. SAPK2 then activates IUF1 indirectly by activating a novel IUF1-activating enzyme (Macfarlane, 1997).

The nucleosomal response refers to the rapid phosphorylation of histone H3 on serine 10 and HMG-14 on serine 6 that occurs concomitantly with immediate-early (IE) gene induction in response to a wide variety of stimuli. Using antibodies against the phosphorylated residues, it has been shown that H3 and HMG-14 phosphorylation is mediated via different MAP kinase (MAPK) cascades, depending on the stimulus. The nucleosomal response elicited by TPA is ERK-dependent, whereas that elicited by anisomycin is p38 MAPK-dependent. In intact cells, the nucleosomal response can be selectively inhibited using the protein kinase inhibitor H89. MAPK activation and phosphorylation of transcription factors are largely unaffected by H89, whereas induction of IE genes is inhibited and its characteristics markedly altered. MSK1 (Drosophila homolog: JIL-1) is considered the most likely kinase to mediate this response because (1) it is activated by both ERK and p38 MAPKs; (2) it is an extremely efficient kinase for HMG-14 and H3, utilizing the physiologically relevant sites; and (3) its activity towards H3/HMG-14 is uniquely sensitive to H89 inhibition. Thus, the nucleosomal response is an invariable consequence of ERK and p38 but not JNK/SAPK activation, and MSK1 potentially provides a link to complete the circuit between cell surface and nucleosome (Thomson, 1999).

p38 regulates the localization of SAP97

Activation of the p38 MAP kinase pathways is crucial for the adaptation of mammalian cells to changes in the osmolarity of the environment. SAP97/hDlg, the mammalian homologue of the Drosophila tumour suppressor Dlg, has been identified as a physiological substrate for the p38gamma MAP kinase (SAPK3/p38gamma) isoform. SAP97/hDlg is a scaffold protein that forms multiprotein complexes with a variety of proteins and is targeted to the cytoskeleton by its association with the protein guanylate kinase-associated protein (GKAP). The SAPK3/p38gamma-catalysed phosphorylation of SAP97/hDlg triggers its dissociation from GKAP and therefore releases it from the cytoskeleton. This is likely to regulate the integrity of intercellular-junctional complexes, and cell shape and volume in response to osmotic stress (Sabio, 2005).

p38 and the cell cycle

Basic fibroblast growth factor (FGF-2) is a member of a family of polypeptides that have roles in a wide range of biological processes. To determine why different cell types show distinct responses to treatment with FGF-2, the array of FGF receptors present on the surface of a cell that differentiates in response to FGF-2 (PC12 cells) was compared with that present on the surface of a cell that proliferates in response to FGF-2 (Swiss 3T3 fibroblasts). Both cell types express exclusively FGFR1, suggesting that there are cell type-specific FGFR1 signaling pathways. Since mitogen-activated protein kinases function as mediators of cellular responses to a variety of stimuli, the roles of these proteins in FGF-mediated responses were examined. FGF-2 activates extracellular signal-regulated kinases with similar kinetics in both fibroblasts and PC12 cells, and a specific inhibitor of extracellular signal-regulated kinase activation blocks differentiation but has little effect on proliferation. In contrast, while p38 mitogen-activated protein kinase is activated weakly and transiently in PC12 cells treated with FGF-2, a much stronger and sustained activation of this kinase is seen in FGF-2-treated fibroblasts. Furthermore, specific inhibitors of this kinase block proliferation but have no effect on differentiation. This effect on proliferation is specific for FGF-2 since the same concentrations of inhibitors have little or no effect on proliferation induced by serum (Maher, 1999).

Antimitogenic stimuli such as environmental or genotoxic stress, transforming growth factor-beta, and the inflammatory cytokines tumor necrosis factor and interleukin-1 activate two extracellular signal-regulated kinase (ERK)-based signaling pathways: the stress-activated protein kinase (SAPK/JNK) pathway and the p38 pathway. Activated p38 phosphorylates transcription factors important in the regulation of cell growth and apoptosis, including activating transcription factor 2 (ATF2), Max, and cAMP response element-binding protein-homologous protein/growth arrest DNA damage 153 (CHDP/GADD153). In turn, p38 lies downstream of the Rho family GTPases Cdc42Hs and Rac1, as well as at least three mitogen-activated protein kinase (MAPK)/ERK-kinases (MEKs): MAPK kinases-3, -6, and SAPK/ERK-kinase-1. Although many of the stimuli that activate p38 can also inhibit cell cycle progression, a clear-cut role for the p38 pathway in cell cycle regulation has not been established. Using a quantitative microinjection approach, it has been shown that Cdc42Hs, but not Rac1 or RhoA, can inhibit cell cycle progression at G1/S through a mechanism requiring activation of p38. These results suggest a novel role for Cdc42Hs in cell cycle inhibition. Furthermore, these results suggest that although both Cdc42Hs and Rac1 can activate p38 in situ, the effects of Cdc42Hs and Rac1 on cell cycle progression are, in fact, quite distinct (Molnar, 1997).

Adult mammalian cardiomyocytes are considered terminally differentiated and incapable of proliferation. Consequently, acutely injured mammalian hearts do not regenerate -- they scar. Adult mammalian cardiomyocytes can divide. One important mechanism used by mammalian cardiomyocytes to control cell cycle is p38 MAP kinase activity. p38 regulates expression of genes required for mitosis in cardiomyocytes, including cyclin A and cyclin B. p38 activity is inversely correlated with cardiac growth during development, and its overexpression blocks fetal cardiomyocyte proliferation. Activation of p38 in vivo by MKK3bE reduces BrdU incorporation in fetal cardiomyocytes by 17.6%. In contrast, cardiac-specific p38alpha knockout mice show a 92.3% increase in neonatal cardiomyocyte mitoses. Furthermore, inhibition of p38 in adult cardiomyocytes promotes cytokinesis. Finally, mitosis in adult cardiomyocytes is associated with transient dedifferentiation of the contractile apparatus. These findings establish p38 as a key negative regulator of cardiomyocyte proliferation and indicate that adult cardiomyocytes can divide (Engel, 2005).

p38 and ectodomain shedding of transmembrane proteins

A variety of transmembrane proteins, such as transforming growth factor-alpha (TGF-alpha), tumor necrosis factor-alpha (TNF-alpha) and L-selectin, undergo shedding, i.e. cleavage of the ectodomain, resulting in release of a soluble protein. Although the physiological relevance of ectodomain shedding is well recognized, little is known about the signaling mechanisms activating this process. Growth factor activation of cell surface tyrosine kinase receptors induces ectodomain cleavage of transmembrane TGF-alpha through activation of the Erk MAP kinase signaling cascade, without the need for new protein synthesis. In addition, expression of constitutively activated MEK1 or its downstream target Erk2 MAP kinase is sufficient to stimulate TGF-alpha shedding. The basal cleavage level in the absence of exogenous growth factor stimulation is due to p38 MAP kinase signaling. Accordingly, a constitutively activated MKK6, a p38 activator, activates TGF-alpha shedding in the absence of exogenous stimuli. In addition to TGF-alpha shedding, these mechanisms also mediate L-selectin and TNF-alpha cleavage. Thus, L-selectin shedding by neutrophils, induced by N-formylmethionyl-leucyl-phenylalanine, is strongly inhibited by inhibitors of Erk MAP kinase or p38 MAP kinase signaling. These results indicate that activation of Erk and p38 signaling pathways may represent a general physiological mechanism to induce shedding of a variety of transmembrane proteins. Considering the nature of the p38 and Erk MAP kinase pathways, it is tempting to speculate that a proximal phosphorylation event will play a key role in the activation of the protease(s) in response to the very diverse extracellular signals (Fan, 1999).

p38 and membrane trafficking

Early endocytic membrane traffic is regulated by the small GTPase Rab5, which cycles between GTP- and GDP-bound states as well as between membrane and cytosol. The latter cycle depends on GDI, which functions as a Rab vehicle in the aqueous environment of the cytosol. Formation of the GDI:Rab5 complex is stimulated by a cytosolic factor that was purified and then identified as p38 MAPK. p38 regulates GDI in the cytosolic cycle of Rab5 and modulates endocytosis in vivo. These observations reveal the existence of a cross-talk between endocytosis and the p38-dependent stress response, thus providing molecular evidence that endocytosis can be regulated by the environment. It is attractive to speculate that the stressed-induced increase in endocytosis allows more efficient internalization of cell surface components for repair, storage, or degradation (Cavalli, 2001).

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. 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, a significant increase was observed 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. These findings demonstrate the importance of finely regulating the activities of MAPKs during development (Breitwieser, 2007).

Regulation of PKD by the MAPK p38delta in insulin secretion and glucose homeostasis

Dysfunction and loss of insulin-producing pancreatic beta cells represent hallmarks of diabetes mellitus. This study shows that mice lacking the mitogen-activated protein kinase (MAPK) p38delta display improved glucose tolerance due to enhanced insulin secretion from pancreatic beta cells. Deletion of p38delta results in pronounced activation of protein kinase D (PKD), the latter of which was identified as a pivotal regulator of stimulated insulin exocytosis. p38delta catalyzes an inhibitory phosphorylation of PKD1, thereby attenuating stimulated insulin secretion. In addition, p38delta null mice are protected against high-fat-feeding-induced insulin resistance and oxidative stress-mediated beta cell failure. Inhibition of PKD1 reverses enhanced insulin secretion from p38delta-deficient islets and glucose tolerance in p38delta null mice as well as their susceptibility to oxidative stress. In conclusion, the p38delta-PKD pathway integrates regulation of the insulin secretory capacity and survival of pancreatic beta cells, pointing to a pivotal role for this pathway in the development of overt diabetes mellitus (Sumara, 2009).

p38 development and differentiation

Cleavage is one of the initial steps of embryogenesis, and is characterized by a series of symmetric and synchronous cell divisions. p38 MAP kinase is asymmetrically activated on one side of the blastodisc during the early cleavage period in zebrafish (Danio rerio) embryos. When a dominant negative (DN) form of p38 is uniformly expressed, blastomere cleavage is impaired on one side of the blastodisc, resulting in the formation of blastomeres with a large mass of cytoplasm and an enlarged nucleus on the affected side. The area affected by the DN-p38 expression does not correlate with the initial cleavage plane, but coincides with the side where dharma/bozozok, a dorsal-specific zygotic gene, is expressed. Furthermore, UV irradiation and removal of the vegetal yolk mass before the first cleavage, both of which inhibit the initiation of the dorsalizing signals, abolishes the asymmetric p38 activation. These findings suggest that asymmetric p38 activation is required for symmetric and synchronous cleavage, and may be regulated by the same machinery that controls the initiation of dorsalizing signals (Fujii, 2000).

Transcripts of the earliest dorsal marker dharma were always detected in the enlarged blastomeres induced by DN-p38a. Furthermore, disruption of microtubules by UV irradiation or depletion of the vegetal yolk material (two processes known to abolish the dorsalizing signals) inhibits the asymmetric activation of p38 during the early cleavage period. Nonetheless, the asymmetry in both p38 activation and in the cleavage defects elicited by DN-p38a does not correlate with the initial cleavage plane. This result is consistent with previous reports indicating that the orientation of the dorso-ventral axis is apparently random with respect to the first or second cleavage plane in zebrafish. Taken together, these results indicate a striking link between the machinery that induces asymmetric activation of p38 and the signals that determine the future dorsal side. This raises the possibility that a putative p38 activator 'X',which is present at the vegetal pole, is transported together with (or without) dorsal determinants via the microtubule array (Fujii, 2000).

The differentiation of C2C12 myoblasts to myotubes is accompanied by a strong activation of p70 S6 kinase and the mitogen-activated protein kinase (MAPK) family member SAPK2/p38, without significant activation of p42 MAPK and only slight activation of SAPK1/JNK and protein kinase Balpha. Consistent with these findings, SB 203580 (a specific inhibitor of SAPK2/p38) or rapamycin (which blocks the activation of p70 S6 kinase) prevents the formation of multinucleated myotubes, as well as the expression of muscle-specific proteins, which include SAPK3 (another MAPK family member). PD 098059 (which prevents the activation of p42 MAPK) has no effect on myotube formation. Surprisingly, the slow activation of p70 S6 kinase during differentiation is not only prevented by rapamycin but also by SB 203580, and the activation of MAPKAP kinase-2 (an in vivo substrate of SAPK2/p38) is not only prevented by SB 203580 but also by rapamycin. In contrast, the acute activation of p70 S6 kinase in C2C12 myoblasts induced by phorbol esters is unaffected by SB 203580 and the acute activation of MAPKAP kinase-2 induced by anisomycin is unaffected by rapamycin. These results show for the first time that SAPK2/p38 plays an essential role in C2C12 cell differentiation (Cuenda, 1999).

p38 MAP kinase (p38) and JNK have been described as playing a critical role in the response to a variety of environmental stresses and proinflammatory cytokines. Hematopoietic cytokines activate not only classical MAP kinases (ERK), but also p38 and JNK. However, the physiological function of these kinases in hematopoiesis remains obscure. All MAP kinases examined, ERK1, ERK2, p38, JNK1, and JNK2, were rapidly and transiently activated by erythropoietin (Epo) stimulation in SKT6 cells, which can be induced to differentiate into hemoglobinized cells in response to Epo. Furthermore, p38-specific inhibitor SB203580 but not MEK-specific inhibitor PD98059 significantly suppresses Epo-induced differentiation and antisense oligonucleotides of p38, JNK1, and JNK2, but neither ERK1 nor ERK2 clearly inhibit Epo-induced hemoglobinization. However, in Epo-dependent FD-EPO cells, inhibition of either ERKs, p38, or JNKs suppresses cell growth. Furthermore, forced expression of a gain-of-function MKK6 mutant, which specifically activates p38, induces hemoglobinization of SKT6 cells without Epo. These results indicate that activation of p38 and JNKs but not of ERKs is required for Epo-induced erythroid differentiation of SKT6 cells, whereas all of these kinases are involved in Epo-induced mitogenesis of FD-EPO cells (Nagata, 1998).

The p38 MAPK pathway and the small heat-shock protein HSP25 are involved in sequential steps necessary for cardiogenesis, but it is unlikely that HSP25 is a direct target of the p38 pathway. HSP25 is expressed in the heart early during development; although multiple roles for HSP25 have been proposed, its specific role during development and differentiation is not known. P19 is an embryonal carcinoma cell line that can be induced to differentiate in vitro into either cardiomyocytes or neurons. P19 was used to examine the role of HSP25 in differentiation. HSP25 expression is strongly increased in P19 cardiomyocytes. Antisense HSP25 expression reduces the extent of cardiomyocyte differentiation and results in reduced expression of cardiac actin and the intermediate filament desmin and reduced level of cardiac mRNAs. Thus, HSP25 is necessary for differentiation of P19 into cardiomyocytes. In contrast, P19 neurons do not express HSP25 and antisense HSP25 expression has no effect on neuronal differentiation. The phosphorylation of HSP25 by the p38/SAPK2 pathway is known to be important for certain of its functions. Inhibition of this pathway by the specific inhibitor SB203580 prevents cardiomyocyte differentiation of P19 cells. In contrast, PD90589, which inhibits the ERK1/2 pathway, has no effect. Surprisingly, cardiogenesis is only sensitive to SB203580 during the first two days of differentiation, before HSP25 expression increases. In contrast to the effect of antisense HSP25, SB203580 reduces the level of expression of the mesodermal marker Brachyury-T during differentiation. Therefore, it is proposed that the p38 pathway acts on an essential target during early cardiogenesis. Once this initial step is complete, HSP25 is necessary for the functional differentiation of P19 cardiomyocytes, but its phosphorylation by p38/SAPK2 is not required (Davidson, 2000).

MyoD inhibits cell proliferation and promotes muscle differentiation. A paradoxical feature of rhabdomyosarcoma (RMS), a tumor arising from muscle precursors, is the block of the differentiation program and the deregulated proliferation despite MyoD expression. A deficiency in RMS of a factor required for MyoD activity has been implicated by previous studies. p38 MAP kinase (MAPK) activation, which is essential for muscle differentiation, is deficient in RMS cells. Enforced induction of p38 MAPK by an activated MAPK kinase 6 (MKK6EE) restores MyoD function and enhances MEF2 activity in RMS deficient for p38 MAPK activation, leading to growth arrest and terminal differentiation. Stress and cytokines can activate the p38 MAPK in RMS cells, however, these stimuli do not promote differentiation, possibly because they activate p38 MAPK only transiently and they also activate JNK, which can antagonize differentiation. The data indicate that p38 MAPK stimulates MyoD activity by a mechanism that does not involves the interaction with the p38 MAPK substrate MEF2. Moreover, a p38 MAPK-dependent activation of MyoD through direct phosphorylation could not be detected. Thus, p38 MAPK might indirectly activate MyoD by targeting bHLH-interacting proteins or cofactors like p300 and PCAF It is concluded that selective and sustained p38 MAPK activation, which is distinct from the stress-activated response, is required for differentiation and can be disrupted in human tumors (Puri, 2000).

Activity of the p38alpha MAP kinase is stimulated by various stresses and hematopoietic growth factors. A role for p38alpha in mouse development and physiology was investigated by targeted disruption of the p38alpha locus. Whereas some p38alpha-/- embryos die between embryonic days 11.5 and 12.5, those that develop past this stage have normal morphology but are anemic owing to failed definitive erythropoiesis, caused by diminished erythropoietin (Epo) gene expression. As p38alpha-deficient hematopoietic stem cells reconstitute lethally irradiated hosts, p38alpha function is not required downstream of Epo receptor. Inhibition of p38 activity also interferes with stabilization of Epo mRNA in human hepatoma cells undergoing hypoxic stress. The p38alpha MAP kinase plays a critical role linking developmental and stress-induced erythropoiesis through regulation of Epo expression (Tamura, 2000).

Mammalian preimplantation development involves several crucial events, such as compaction and blastocyst formation, but little is known about essential genes that regulate this developmental process. This study focuses on MAP kinase signaling pathways as potential regulatory pathways for the process. The results show that inhibition of the JNK pathway or of the p38 MAP kinase pathway, but not of the ERK pathway, results in inhibition of cavity formation, and that JNK and p38 are active during mouse preimplantation development. Subsequent microarray analyses shows that, of about 39,000 transcripts analyzed, the number of those genes whose expression level is sensitive to the inhibition of the JNK or the p38 pathway, but insensitive to the inhibition of the ERK pathway, is only 156. Moreover, of the 156 genes, expression of 10 genes (two genes upregulated and eight genes downregulated) is sensitive to either inhibition of the JNK or p38 pathways. These 10 genes include several genes known for their function in axis and pattern formation. Downregulation of some of the 10 genes simultaneously using siRNA leads to abnormality in cavity formation. Thus, this study has successfully narrowed down candidate genes of interest, detailed analysis of which will probably lead to elucidation of the molecular mechanism of preimplantation development (Maekawa, 2005).

Ectodermal Smad4 and p38 MAPK are functionally redundant in mediating TGF-beta/BMP signaling during tooth and palate development

Smad4 is a central intracellular effector of TGF-beta signaling. Smad-independent TGF-beta pathways, such as those mediated by p38 MAPK, have been identified in cell culture systems, but their in vivo functional mechanisms remain unclear. This study investigated the role of TGF-beta signaling in tooth and palate development and noted that conditional inactivation of Smad4 in oral epithelium results in much milder phenotypes than those seen with the corresponding receptor mutants, Bmpr1a and Tgfbr2, respectively. Perturbed p38 function in these tissues likewise has no effect by itself; however, when both Smad4 and p38 functions are compromised, dramatic recapitulation of the receptor mutant phenotypes results. Thus, this study demonstrates that p38 and Smad4 are functionally redundant in mediating TGF-beta signaling in diverse contexts during embryonic organogenesis. The ability of epithelium to utilize both pathways illustrates the complicated nature of TGF-beta signaling mechanisms in development and disease (Xu, 2008).

This study shows that p38 MAPK functions redundantly with Smad4 to mediate BMP signaling during tooth development, and that tooth development can be arrested at the bud stage only by blocking both Smad4 and p38 MAPK. This functional redundancy is in sharp contrast to the result of loss of Smad4 in the cranial neural crest-derived dental mesenchyme, in which tooth development is retarded at the dental laminar stage (prior to the bud stage). Thus, there is an absolute requirement for Smad4 in the cranial neural crest-derived dental mesenchyme. In addition, Smad4-mediated TGF-β/BMP signaling is required for the homeobox gene patterning of oral/aboral and proximal/distal domains within the first branchial arch. Therefore, in the CNC-derived mesenchyme, TGF-β/BMP signals rely on Smad4-dependent pathways to mediate epithelial-mesenchymal interactions that control craniofacial organogenesis. Previous studies have shown that BMP4 signaling is critical for mediating cell death in the enamel knot, whereas Msx1-mediated BMP signaling is critical for cell proliferation in the dental mesenchyme. Taken together, the temporal and tissue-specific activation of Smad4-dependent or -independent BMP signaling pathways may regulate different downstream target genes and contribute to the diverse functional outcomes of BMP signaling in regulating the fate of dental epithelial and cranial neural crest-derived mesenchymal cells during tooth development (Xu, 2008).

Local caspase activation interacts with Slit-Robo signaling to restrict axonal arborization

In addition to being critical for apoptosis, components of the apoptotic pathway, such as caspases, are involved in other physiological processes in many types of cells, including neurons. However, very little is known about their role in dynamic, nonphysically destructive processes, such as axonal arborization and synaptogenesis. This study shows that caspases are locally active in vivo at the branch points of young, dynamic retinal ganglion cell axonal arbors but not in the cell body or in stable mature arbors. Caspase activation, dependent on Caspase-3, Caspase-9, and p38 mitogen-activated protein kinase (MAPK), rapidly increased at branch points corresponding with branch tip addition. Time-lapse imaging revealed that knockdown of Caspase-3 and Caspase-9 led to more stable arbors and presynaptic sites. Genetic analysis showed that Caspase-3, Caspase-9, and p38 MAPK interacted with Slit1a-Robo2 signaling, suggesting that localized activation of caspases lie downstream of a ligand receptor system, acting as key promoters of axonal branch tip and synaptic dynamics to restrict arbor growth in vivo in the central nervous system (Campbell, 2013).

p38b Evolutionary homologs back to part 1/3 | part 2/3 |

p38b: Biological Overview | Regulation | Developmental Biology | References

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