Characterization of the structure and function of a new mitogen-activated protein kinase (p38beta)

Essential role for p38alpha mitogen-activated protein kinase in placental angiogenesis

The p38 family of mitogen-activated protein kinases (MAPKs) mediates signaling in response to environmental stresses and inflammatory cytokines, but the requirements for the p38 MAPK pathway in normal mammalian development have not been elucidated. Here, we show that targeted disruption of the p38alpha MAPK gene results in homozygous embryonic lethality because of severe defects in placental development. Although chorioallantoic placentation is initiated appropriately in p38alpha null homozygotes, placental defects are manifest at 10.5 days postcoitum as nearly complete loss of the labyrinth layer and significant reduction of the spongiotrophoblast. In particular, p38alpha mutant placentas display lack of vascularization of the labyrinth layer as well as increased rates of apoptosis, consistent with a defect in placental angiogenesis. Furthermore, p38alpha mutants display abnormal angiogenesis in the embryo proper as well as in the visceral yolk sac. Thus, our results indicate a requirement for p38alpha MAPK in diploid trophoblast development and placental vascularization and suggest a more general role for p38 MAPK signaling in embryonic angiogenesis.

Importance of the MKK6/p38 pathway for interleukin-12–induced STAT4 serine phosphorylation and transcriptional activity

Interleukin-12 (IL-12) is a key immunoregulatory cytokine that promotes Th1 differentiation and cell-mediated immune responses. The transcription factor STAT4 (signal transducer and activator of transcription 4) is an important element in mediating IL-12 signals, as evidenced by the fact that STAT4−/− mice display impaired responsiveness to IL-12 and deficient Th1 differentiation. STAT4 is inducibly phosphorylated on tyrosine and serine in response to IL-12, but the kinase(s) responsible for the latter event is unknown. Here we show that IL-12 induces STAT4 phosphorylation on serine 721 and that mutation of serine 721 interferes with STAT4 transcriptional activity. In addition, we show that mutation of tyrosine 693 abrogates IL-12–induced STAT4 tyrosine phosphorylation and transcriptional activity. Although the site surrounding serine 721 is an optimum consensus sequence for mitogen-activated family of protein kinases (MAPKs)-mediated phosphorylation, we demonstrate that IL-12 does not induce extracellular signal-regulated kinase (ERK) or c-Jun N-terminal kinase (JNK) activation in T and natural killer (NK) cells and that IL-12–induced STAT4 transcriptional activity is not affected by these kinases. Rather, we show that IL-12 induces p38 activation. Moreover, we demonstrate that p38α and its upstream activator, MKK6, phosphorylate STAT4 on serine 721, and are required for STAT4 full transcriptional activity induced by IL-12, establishing the MKK6/p38α/STAT4 pathway as an important mediator of IL-12 actions.

Importance of the MKK6/p38 pathway for interleukin-12–induced STAT4 serine phosphorylation and transcriptional activity

Interleukin-12 (IL-12) is a key immunoregulatory cytokine that promotes Th1 differentiation and cell-mediated immune responses. The transcription factor STAT4 (signal transducer and activator of transcription 4) is an important element in mediating IL-12 signals, as evidenced by the fact that STAT4−/− mice display impaired responsiveness to IL-12 and deficient Th1 differentiation. STAT4 is inducibly phosphorylated on tyrosine and serine in response to IL-12, but the kinase(s) responsible for the latter event is unknown. Here we show that IL-12 induces STAT4 phosphorylation on serine 721 and that mutation of serine 721 interferes with STAT4 transcriptional activity. In addition, we show that mutation of tyrosine 693 abrogates IL-12–induced STAT4 tyrosine phosphorylation and transcriptional activity. Although the site surrounding serine 721 is an optimum consensus sequence for mitogen-activated family of protein kinases (MAPKs)-mediated phosphorylation, we demonstrate that IL-12 does not induce extracellular signal-regulated kinase (ERK) or c-Jun N-terminal kinase (JNK) activation in T and natural killer (NK) cells and that IL-12–induced STAT4 transcriptional activity is not affected by these kinases. Rather, we show that IL-12 induces p38 activation. Moreover, we demonstrate that p38α and its upstream activator, MKK6, phosphorylate STAT4 on serine 721, and are required for STAT4 full transcriptional activity induced by IL-12, establishing the MKK6/p38α/STAT4 pathway as an important mediator of IL-12 actions.

p38 mitogen-activated protein kinase mediates bid cleavage, mitochondrial dysfunction, and caspase-3 activation during apoptosis induced by singlet oxygen but not by hydrogen peroxide

p38 mitogen-activated protein kinase is activated and involved in cleavage of caspase-3 during apoptosis induced by a number of stimuli. However, the signaling events triggered by p38 that result in caspase-3 activation are still unknown. In human leukemia cells, two reactive oxygen species, singlet oxygen and hydrogen peroxide (H(2)O(2)), selectively stimulated the phosphorylation of p38. Preincubation of cells with SB203580, a specific inhibitor of p38, dose dependently inhibited DNA fragmentation induced by singlet oxygen but not by H(2)O(2). Protection from apoptosis by SB203580 correlated with inhibition of caspase-3, and several events that are associated with caspase-3 activation, including Bid cleavage, decrease in mitochondrial transmembrane potential and release of cytochrome c from mitochondria, whereas caspase-8 cleavage was not affected by this inhibitor. In contrast, blockade of caspase-8 with Ile-Glu-Thr-Asp-fluoromethyl ketone is sufficient to prevent formation of DNA fragments and to inhibit all the above signaling events, with exception of p38 phosphorylation, in both singlet oxygen- and H(2)O(2)-treated cells. These data suggest that caspase-3 activation is regulated through redundant signaling pathways that involve p38 and caspase-8 acting upstream of Bid during singlet oxygen-induced apoptosis, whereas the activation of caspase-3 by H(2)O(2) is only governed by a caspase-8-mediated apoptotic pathway.

The cJun N-terminal kinase (JNK) signaling pathway mediates induction of urokinase-type plasminogen activator (uPA) by the alkylating agent MNNG

The monofunctional alkylating agent N-methyl-N-nitro-N-nitrosoguanidine (MNNG) is a widespread environmental carcinogen that causes DNA lesions, leading to cell death. However, MNNG can also trigger a cell-protective response by inducing the expression of DNA repair/transcription-related genes. We demonstrate that the urokinase-type plasminogen activator (uPA) gene product, a broad spectrum extracellular protease to which no DNA repair function has been assigned, is transcriptionally induced by MNNG in C2C12 and NIH3T3 cells. This induction required an AP1-enhancer element located at −2.4 kilobase (kb), because it was abrogated by deletion of this site. MNNG was found to induce the activation of JNK/SAPK and p38 mitogen-activated protein kinases (MAPKs). Accordingly, we attempted to assess the contribution of each of these MNNG-inducible MAPKs to uPA gene induction by this alkylating agent. Coexpression of dominant negative versions of kinases of the JNK pathway, such as catalytically inactive forms of MEKK1, MKK7, and JNKK, and of cytoplasmic JNK-inhibitor JIP-1, as well as treatment of cells with curcumin (which blocks JNK activation by MNNG), inhibited MNNG-induced uPA transcriptional activity. In contrast, neither dominant negative MKK6 nor SB203580, which specifically inhibit p38 MAP kinase activation, abrogated the MNNG-induced effect. Taken together, our results show that the JNK signaling pathway links external MNNG stimulation and AP1-dependent uPA gene expression, providing the first functional dissection of a transcription-coupled signal transduction pathway for MNNG.

The cJun N-terminal kinase (JNK) signaling pathway mediates induction of urokinase-type plasminogen activator (uPA) by the alkylating agent MNNG

The monofunctional alkylating agent N-methyl-N-nitro-N-nitrosoguanidine (MNNG) is a widespread environmental carcinogen that causes DNA lesions, leading to cell death. However, MNNG can also trigger a cell-protective response by inducing the expression of DNA repair/transcription-related genes. We demonstrate that the urokinase-type plasminogen activator (uPA) gene product, a broad spectrum extracellular protease to which no DNA repair function has been assigned, is transcriptionally induced by MNNG in C2C12 and NIH3T3 cells. This induction required an AP1-enhancer element located at −2.4 kilobase (kb), because it was abrogated by deletion of this site. MNNG was found to induce the activation of JNK/SAPK and p38 mitogen-activated protein kinases (MAPKs). Accordingly, we attempted to assess the contribution of each of these MNNG-inducible MAPKs to uPA gene induction by this alkylating agent. Coexpression of dominant negative versions of kinases of the JNK pathway, such as catalytically inactive forms of MEKK1, MKK7, and JNKK, and of cytoplasmic JNK-inhibitor JIP-1, as well as treatment of cells with curcumin (which blocks JNK activation by MNNG), inhibited MNNG-induced uPA transcriptional activity. In contrast, neither dominant negative MKK6 nor SB203580, which specifically inhibit p38 MAP kinase activation, abrogated the MNNG-induced effect. Taken together, our results show that the JNK signaling pathway links external MNNG stimulation and AP1-dependent uPA gene expression, providing the first functional dissection of a transcription-coupled signal transduction pathway for MNNG.

The differential role of extracellular signal-regulated kinases and p38 mitogen-activated protein kinase in eosinophil functions

The activation of eosinophils by cytokines is a major event in the pathogenesis of allergic diseases. We have investigated the activation of mitogen-activated protein (MAP) kinases and their functional relevance in eosinophil differentiation, survival, degranulation, and cytokine production. IL-5 induced phosphorylation and activation of extracellular signal-regulated kinases (ERK) and p38 MAP kinases in eosinophils. PD98059, a MAP/ERK kinase inhibitor, blocked phosphorylation of ERK1/2 in a dose-dependent manner. SB202190, a p38 inhibitor, blocked p38-dependent phosphorylation of activating transcription factor-2. To study the importance of the MAP kinases on eosinophil differentiation, we cultured mouse bone marrow cells with IL-3 and IL-5 in the presence of the inhibitors. SB202190 dramatically inhibited eosinophil differentiation by 71%. PD98059 was less potent and reduced eosinophil differentiation by 28%. Both inhibitors marginally inhibited eosinophil survival only at the highest doses. Prolonged incubation of eosinophils with IL-5 induced significant eosinophil-derived neurotoxin release. Both PD98059 and SB202190 nearly completely inhibited (87% and 100% inhibition, respectively) IL-5-stimulated eosinophil-derived neurotoxin release in a dose-dependent manner. Next, we examined the effect of the MAP kinase inhibitors on eosinophil production of the cytokine macrophage-inflammatory protein (MIP)-1alpha. PD98059 blocked C5a- but not ionomycin-induced MIP-1alpha production (59% inhibition at 50 microM concentration). In contrast, SB202190 nearly completely inhibited (99%) C5a-induced MIP-1alpha production. Further, it blocked ionomycin-stimulated production by 66%. Our results suggest that both p38 and ERK1/2 MAP kinases play an important role in eosinophil differentiation, cytokine production, and degranulation. The p38 MAP kinase plays a greater role than ERK1/2 in eosinophil differentiation and cytokine production.

Phosphorylation of the common neurotrophin receptor p75 by p38beta2 kinase affects NF-kappaB and AP-1 activities

The signaling pathways invoked by ligand binding to the common neurotrophin receptor p75NTR are incompletely understood. Using the yeast two-hybrid system, we identified the mitogen-activated protein (MAP) kinase p38beta2 as a specific interactor with the 5th and 6th alpha helices of the p75NTR intracytoplasmic region. The consequences of this interaction were studied, using primary cultures of Schwann cells and the 293T cell line. Phosphorylation of p75NTR by p38beta2 was induced in vitro and in vivo by MAP kinase kinases (MKK) 6 activation. This pathway demonstrated feedback in that nerve growth factor (NGF) binding increased p38beta2 activity, causing an increase of nuclear factor-kappaB (NF-kappaB) activation and a decrease of AP-1 activation. The mechanisms described explain at least in part why NGF binding to p75NTR increases cell survival in certain circumstances.

Regulation of skeletal myogenesis by association of the MEF2 transcription factor with class II histone deacetylases

Skeletal muscle differentiation is controlled by associations between myogenic basic-helix-loop-helix and MEF2 transcription factors. We show that chromatin associated with muscle genes regulated by these transcription factors becomes acetylated during myogenesis and that class II histone deacetylases (HDACs), which interact with MEF2, specifically suppress myoblast differentiation. These HDACs do not interact directly with MyoD, yet they suppress its myogenic activity through association with MEF2. Elevating the level of MyoD can override the repression imposed by HDACs on muscle genes. HDAC-mediated repression of myogenesis also can be overcome by CaM kinase and insulin-like growth factor (IGF) signaling. These findings reveal central roles for HDACs in chromatin remodeling during myogenesis and as intranuclear targets for signaling pathways controlled by IGF and CaM kinase.

The p38 mitogen-activated protein kinase is required for IL-12-induced IFN-gamma expression

IL-12 is a central immunoregulatory cytokine that promotes cell-mediated immune responses and the differentiation of naive CD4+ cells into Th1 cells. We and others have demonstrated that the Stat4 is critical for IFN-gamma production by activated T cells and Th1 cells. However, several studies have suggested that other pathways may be involved in IL-12-stimulated IFN-gamma expression. In this report we demonstrate that IL-12 activates mitogen-activated protein kinase kinase 3/6 (MKK) and p38 mitogen-activated protein kinase (MAPK), but not p44/42 (ERK) or stress-activated protein kinase/c-Jun N-terminal kinase MAPK. The activation of p38 MAPK is required for normal induction of IFN-gamma mRNA and IFN-gamma secretion by IL-12 in activated T cells and Th1 cells. Importantly, IL-12-stimulated p38 MAPK effector functions occur through a Stat4-independent mechanism and correlate with increased serine phosphorylation of activating transcription factor-2. The requirement for p38 MAPK in IL-12 function suggests that this pathway may be an important in vivo target for the anti-inflammatory actions of p38 MAPK inhibitors.

Marathon running transiently increases c-Jun NH2-terminal kinase and p38 activities in human skeletal muscle

We examined the pattern of activation and deactivation of the stress-activated protein kinase signalling molecules c-Jun NH2-terminal kinase (JNK) and p38 kinase in skeletal muscle in response to prolonged strenuous running exercise in human subjects. Male subjects (n = 14; age 32 +/- 2 years; VO2,max 60 +/- 2 ml kg-1 min-1) completed a 42.2 km marathon (mean race time 3 h 35 min). Muscle biopsies were obtained 10 days prior to the marathon, immediately following the race, and 1, 3 and 5 days after the race. The activation of JNK and p38, including both p38alpha and p38gamma, was measured with immune complex assays. The phosphorylation state of p38 (alpha and gamma) and the upstream regulators of JNK and p38, mitogen-activated protein kinase kinase 4 (MKK4) and mitogen-activated protein kinase kinase 6 (MKK6), were assessed using phosphospecific antibodies. JNK activity increased 7-fold over basal level immediately post-exercise, but decreased back to basal levels 1, 3 and 5 days after the exercise. p38gamma phosphorylation (4-fold) and activity (1.5-fold) increased immediately post-exercise and returned to basal levels at 1, 3 and 5 days following exercise. In contrast, p38alpha phosphorylation and activity did not change over the time course studied. MKK4 and MKK6 phosphorylation increased and decreased in a trend similar to that observed with JNK activity and p38gamma phosphorylation. Prolonged running exercise did not affect JNK, p38alpha, or p38gamma protein expression in the days following the race. This study demonstrates that both JNK and p38 intracellular signalling cascades are robustly, yet transiently increased following prolonged running exercise. The differential activation of the p38 isoforms with exercise in human skeletal muscle indicates that these proteins may have distinct functions in vivo.

Extracellular signals and scores of phosphatases: all roads lead to MAP kinase

MAP kinases function as key signal integration points for a vast number of external stimuli that affect the life and death of cells and are therefore subject to rigorous regulation. Here we review the numerous protein phosphatases that directly counteract MAP kinase activation. To simplify the complexity, we attempt to integrate the information into a 'sequential phosphatase model' of MAP kinase regulation.

p38 MAP kinase pathway regulates angiotensin II-induced contraction of rat vascular smooth muscle

Angiotensin II (ANG II) is a multifunctional hormone that exerts potent vasoconstrictor and hypertrophic effects on vascular smooth muscle. Here, we demonstrate that the p38 mitogen-activated protein (MAP) kinase pathway is involved in ANG II-induced vascular contraction. Addition of ANG II to rat aortic smooth muscle cells (SMC) caused a rapid and transient increase of p38 activity through activation of the AT(1) receptor subtype. This response to ANG II was strongly attenuated by pretreating cells with antioxidants and diphenylene iodonium and was mimicked by exposure of cells to H(2)O(2). Stimulation of p38 by ANG II resulted in the enzymatic activation of MAP kinase-activated protein (MAPKAP) kinase-2 and the phosphorylation of heat shock protein 27 (HSP27) in aortic SMC. Pretreatment of cells with the specific p38 MAP kinase inhibitor SB-203580 completely blocked the ANG II-dependent activation of MAPKAP kinase-2 and phosphorylation of HSP27. ANG II also caused a robust activation of MAPKAP kinase-2 in the intact rat aorta. Incubation with SB-203580 significantly decreased the potency of ANG II to induce contraction of rat aortic rings and depressed the maximal hormone response. These results suggest that the p38 MAP kinase pathway selectively modulates the vasoconstrictor action of ANG II in vascular smooth muscle.

Role of p38 mitogen-activated protein kinase in axotomy-induced apoptosis of rat retinal ganglion cells

p38 is a member of the mitogen-activated protein (MAP) kinase superfamily and mediates intracellular signal transduction. Recent studies suggest that p38 is involved in apoptotic signaling in several cell types, including neurons. In the mammalian retina, approximately 50% of the retinal ganglion cells (RGCs) die by apoptosis during development. Additionally, transection of the optic nerve close to the eye bulb causes apoptotic cell death of RGCs in adulthood. We investigated the role of p38 in axotomy-induced apoptosis of RGCs. One day after axotomy, activated (phosphorylated) p38 was visualized by immunocytochemistry in the nuclei of RGCs, but not in control retinas. Phosphorylated p38 was first detected on immunoblots 12 hr after axotomy, reached a maximum at 1 d, and then decreased. To investigate possible roles of p38 in RGC death, a p38 MAP kinase inhibitor, SB203580, was administered intravitreally at the time of axotomy and repeated at 5 and 10 d. Assayed 14 d after axotomy, SB203580 increased the number of surviving RGCs in a dose-dependent manner (the minimum effective concentration was 1.6 micrometer). Furthermore, MK801, a selective inhibitor of NMDA receptors, not only showed protective effects against RGC apoptosis but also attenuated p38 MAP kinase activation in a dose-dependent manner. Our findings imply that p38 is in the signaling pathway to RGC apoptosis mediated by glutamate neurotoxicity through NMDA receptors after damage to the optic nerve. p38 inhibitors could be potentially useful for the treatment of optic nerve trauma and neurodegenerative diseases that affect RGCs, such as glaucoma.

Lysophosphatidylcholine activates p38 and p42/44 mitogen-activated protein kinases in monocytic THP-1 cells, but only p38 activation is involved in its stimulated chemotaxis

Oxidized LDLs (OxLDLs) have been shown to be involved in recruitment of blood monocytes into the arterial subendothelial space, which is the earliest step in atherogenesis, but the underlying molecular mechanisms are poorly understood. The present study demonstrated that lysophosphatidylcholine (LPC), a major phospholipid component of OxLDL, strongly evoked phosphorylation and activation of p38 and p42/44 mitogen-activated protein kinases in monocytic cells. The stimulation of p38 and p42/44 occurred in a dose- and time-dependent manner, reaching the maximal activation at 25 microg/mL LPC within 5 minutes. Interestingly, inhibition of p38 activation by OxLDL or LPC, using its selective inhibitors (SB203580 and SKF86002), completely blocked OxLDL- or LPC-stimulated chemotaxis of THP-1 cells, which was measured in a transwell chemotaxis assay. In contrast, inhibition of p42/44 activation by its potent inhibitor (PD98059) did not block OxLDL- or LPC-stimulated chemotaxis. Moreover, expression of a p38 dominant-negative mutant (p38AF) reduced cell chemotaxis significantly. In addition, activation of p38 by LPC was apparently mediated neither by scavenger receptors nor by tyrosine kinase receptors. It was, however, effectively blocked by pertussis toxin and substantially reduced by phospholipase C inhibitor (U73122) and phosphatidylinositol 3-kinase inhibitors (wortmannin and LY294002). LPC also inhibited forskolin-stimulated cAMP accumulation in a pertussis toxin-sensitive manner, indicating that Gi/Go proteins likely mediated the effects of LPC. Our results suggested that OxLDL/LPC efficiently activated both p38 and p42/44, but only the activation of p38 was functionally associated with OxLDL-/LPC-induced chemotaxis in THP-1 cells.

p38 MAPK signalling cascades: ancient roles and new functions

p38 MAPKs are a conserved subfamily of MAPKs involved in the response to stress found in eukaryotic cells from yeast to mammals. The recent isolation of genes coding for members of this signalling cascade in Drosophila has provided us with the genetic tools to study their various biological roles and their regulatory interactions with other signalling pathways. This cascade participates in the immune response, a function that is remarkably conserved between flies and humans. Additionally, it appears to exert other fundamental roles during development, in cell fate specification in imaginal discs, and in cell polarity during oogenesis. These functions involve genetic and biochemical interactions with other signalling cascades, the decapentaplegic/TGFbeta, the wingless/Wnt and the torpedo/Ras-ERK pathways. In the near future, we can expect a flurry of information that will allow us to draw a comprehensive picture of the roles of signalling networks mediated by p38s during development.

Involvement of the MKK6-p38gamma cascade in gamma-radiation-induced cell cycle arrest

The p38 group of kinases belongs to the mitogen-activated protein (MAP) kinase superfamily with structural and functional characteristics distinguishable from those of the ERK, JNK (SAPK), and BMK (ERK5) kinases. Although there is a high degree of similarity among members of the p38 group in terms of structure and activation, each member appears to have a unique function. Here we show that activation of p38gamma (also known as ERK6 or SAPK3), but not the other p38 isoforms, is required for gamma-irradiation-induced G(2) arrest. Activation of the MKK6-p38gamma cascade is sufficient to induce G(2) arrest in cells, and expression of dominant negative alleles of MKK6 or p38gamma allows cells to escape the DNA damage-induce G(2) delay. Activation of p38gamma is dependent on ATM and leads to activation of Cds1 (also known as Chk2). These data suggest a model in which activation of ATM by gamma irradiation leads to the activation of MKK6, p38gamma, and Cds1 and that activation of both MKK6 and p38gamma is essential for the proper regulation of the G(2) checkpoint in mammalian cells.

Neuroprotective agent chlomethiazole attenuates c-fos, c-jun, and AP-1 activation through inhibition of p38 MAP kinase

Recent evidence suggests that stress-activated protein kinases expressed in glial cells have very important roles during cerebral ischemia. The neuroprotective agent chlomethiazole, which is known to enhance the conductance at the GABA(A) receptor complex, is presently in clinical trials for the treatment of severe stroke. Here the authors suggested that chlormethiazole has anti-inflammatory properties because it potently and selectively inhibited p38 mitogen-activated protein (MAP) kinase in primary cortical glial cultures. The inhibition of p38 MAP kinase resulted in the attenuation of the induction of c-fos and c-jun mRNA and AP-1 DNA binding by lipopolysaccharide (LPS). In addition, chlomethiazole inhibited the activation of an AP-1-dependent luciferase reporter plasmid in SK-N-MC human neuroblastoma cells in response to glutamate. Chlomethiazole inhibited the p38 MAP kinase activity as revealed by the decrease in the LPS-induced phosphorylation of the substrates ATF-2 and hsp27, whereas the phosphorylation status of the p38 MAP kinase itself was unaffected. Interestingly, chlomethiazole exhibited an IC(50) of approximately 2 micromol/L for inhibition of c-fos mRNA expression, indicating 25 to 75 times higher potency than reported EC(50) values for enhancing GABA(A) chloride currents. The results indicated a novel mechanism of action of chlomethiazole, and provided support for a distinctive role of p38 MAP kinase in cerebral ischemia.

Identification of a nuclear export signal in MKK6, an activator of the carp p38 mitogen-activated protein kinases

Carp homologues of p38 mitogen-activated protein kinase (MAPK) and its activator MAPK kinase 6 (MAPKK6, referred to as MKK6) were identified. There exist at least two distinct carp p38s, cp38a and cp38b, both of which consist of 361 amino acids. The transcript of c38a was exclusively expressed in the ovary, whereas that of cp38b was ubiquitously expressed. Western blot analysis with anti-(phosphorylated MAPK) Ig specific to the active p38 or JNK has shown that p38 was activated in response to hypertonic stress (1 M sorbitol) in epithelioma papilosum cyprini carp epithelial cells (EPC) and that the activation of p38 proceeded faster to the maximal level than that of JNK. Carp homologue (cMKK6) of p38 activator MKK6 consists of 404 amino acids. It was expressed ubiquitously but was most abundant in the ovary. An in vitro kinase assay demonstrated that cMKK6 is an upstream activator of cp38 and cp38b in carp because it specifically phosphorylated and activated cp38a and cp38b. Interestingly, we found that cMKK6 has a nuclear export signal (NES) sequence in its N-terminal region although upstream activators of stress-activated MAPKs, p38 and JNK, do not in other animals. The NES sequence facilitated nuclear export of cMKK6 and ovalbumin. Leucine residues in the sequence were crucial for the NES activity, as the activity was lost on replacement of the leucines to alanines. The existence of an NES in cMKK6 implies the requisite of strict regulation of the p38 MAPK pathway in carp. The abundance of these components for the stress-activated pathway in the ovary might be related to ectogenetic early development.

c-Jun NH2-terminal kinase targeting and phosphorylation of heat shock factor-1 suppress its transcriptional activity

The mammalian heat shock transcription factor HSF-1 regulates the expression of the heat shock proteins, molecular chaperones that are involved in cellular processes from higher order assembly to protein degradation. HSF-1 is a phosphorylated monomer under physiological growth conditions and is located mainly in the cytoplasm. Upon activation by a variety of environmental stresses, HSF-1 is translocated into the nucleus, forms trimers, acquires DNA binding activity, is hyperphosphorylated, appears as punctate granules, and increases transcriptional activity of target genes. As cells recover from stress, the punctate granules gradually disappear, and HSF-1 appears in a diffused staining pattern in the cytoplasm and nucleus. We have previously shown that the mitogen-activated protein kinase ERK phosphorylates and suppresses HSF-1-driven transcription. Here, we show that c-Jun NH(2)-terminal kinase (JNK) also phosphorylates and inactivates HSF-1. Overexpression of JNK facilitates the rapid disappearance of HSF-1 punctate granules after heat shock. Similar to ERK, JNK binds to HSF-1 in the conserved mitogen-activated protein kinases binding motifs and phosphorylates HSF-1 in the regulatory domain. The overexpression of an HSF-1-green fluorescent protein fusion construct lacking JNK phosphorylation sites causes this HSF-1 mutant to form nuclear granules that remain longer in the nucleus after heat shock. Taken together, these findings indicate that JNK phosphorylates HSF-1 and suppresses its transcriptional activity by rapidly clearing HSF-1 from the sites of transcription.

Regulation of TNF expression by multiple mitogen-activated protein kinase pathways

Stimulating macrophages with bacterial endotoxin (LPS) activates numerous intracellular signaling pathways that lead to the production of TNF. In this study, we show that four mitogen-activated protein (MAP) kinase pathways are activated in LPS-stimulated macrophages: the extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase/stress-activated protein kinase, p38, and Big MAP kinase (BMK)/ERK5 pathways. Although specific activation of a single MAP kinase pathway produces only a modest effect on TNF promoter activation, activation of each MAP kinase pathway is important for full induction of the TNF gene. Interestingly, a dramatic induction of TNF promoter-driven gene expression was observed when all of the four MAP kinase pathways were activated simultaneously, suggesting a cooperative effect among these kinases. Unexpectedly, cis elements known to be targeted by MAP kinases do not play a major role in multiple MAP kinase-induced TNF gene expression. Rather, a 40-bp sequence harboring the TATA box, is responsible for the gene up-regulation induced by MAP kinases. The proximity of the MAP kinase-responsive element to the transcriptional initiation site suggested that MAP kinases regulate the transcriptional initiation complex. Utilizing alpha-amanitin-resistant RNA polymerase II mutants with or without a C-terminal domain (CTD) deletion, we found that deleting the CTD to 31 tandem repeats (Delta31) led to >90% reduction in MAP kinase-mediated TNF production. Thus, our data demonstrate coordination of multiple MAP kinase pathways in TNF production and suggest that the CTD of RNA polymerase II is required to execute MAP kinase signaling in TNF expression.

Distinct carboxy-termini confer divergent characteristics to the mitogen-activated protein kinase p38alpha and its splice isoform Mxi2

The p38 family of mitogen-activated protein kinases is composed of several isoforms. Mxi2 is a splicing variant of p38alpha that harbors a unique carboxy-terminus. Here we show that this sole divergence results in remarkable differences between Mxi2 and p38alpha. Mxi2 is distinctively activated by stress stimuli and potently activated by mitogens. Mxi2 affinity for bona fide p38 substrates is remarkably diminished and Mxi2 activity is largely unaffected by the phosphatase CL100. Also, Mxi2 sensitivity to inhibition by SB203580 is greatly reduced. Interestingly, we show that the p38 C-terminus is involved in conferring sensitivity to this compound. Overall, our results point to the p38 carboxy-terminus as a key determinant of the biochemical properties of this family of kinases.

Constitutive activity and differential localization of p38alpha and p38beta MAPKs in adult mouse brain

To understand the roles of p38 mitogen-activated protein kinase (p38 MAPK) isoforms in adult mouse brain, in vivo activities and detailed expression patterns of two p38 isoforms, p38alpha and p38beta, were examined by using biochemical and immunohistochemical analyses. The result indicated that the activity of both p38alpha and p38b MAPKs in normal adult mouse brain was remarkably high, and the nuclear pool of the p38 isoforms was primarily responsible for most of the constitutive p38 MAPK activity in brain. Both p38alpha and p38beta were highly expressed in brain areas including cerebral cortex, hippocampus, cerebellum, and few nuclei of the brainstem. At the subcellular level, p38alpha was distributed in dendrites and in cytoplasmic and nuclear regions of cell body of neurons, which is in contrast to p38beta, since p38beta was preferentially expressed in nucleus of neurons. These results suggest that the p38 pathway may play an important role, not only in inflammation and neuronal cell death as previously suggested, but also in normal physiology of adult mouse brain.

Microtubule-interfering agents stimulate the transcription of cyclooxygenase-2. Evidence for involvement of ERK1/2 AND p38 mitogen-activated protein kinase pathways

We investigated whether microtubule-interfering agents (MIAs: taxol, colchicine, nocodazole, vinblastine, vincristine, 17-beta-estradiol, 2-methoxyestradiol) altered cyclooxygenase-2 (COX-2) expression in human mammary epithelial cells. MIAs enhanced prostaglandin E(2) synthesis and increased levels of COX-2 protein and mRNA. Nuclear run-off assays revealed increased rates of COX-2 transcription after treatment with MIAs. Calphostin C, an inhibitor of protein kinase C, blocked the induction of COX-2 by MIAs. The stimulation of COX-2 promoter activity by MIAs was inhibited by overexpressing dominant negative forms of Rho and Raf-1. MIAs stimulated ERK, JNK, and p38 mitogen-activated protein kinases (MAPK); pharmacological inhibitors of MAPK kinase and p38 MAPK blocked the induction of COX-2 by MIAs. Overexpressing dominant negative forms of ERK1 or p38 MAPK inhibited MIA-mediated activation of the COX-2 promoter. MIAs stimulated the binding of the activator protein-1 transcription factor complex to the cyclic AMP response element in the COX-2 promoter. A dominant negative form of c-Jun inhibited the activation of the COX-2 promoter by MIAs. Additionally, cytochalasin D, an agent that inhibits actin polymerization, stimulated COX-2 transcription by the same signaling pathway as MIAs. Thus, microtubule- or actin-interfering agents stimulated MAPK signaling and activator protein-1 activity. This led, in turn, to induction of COX-2 gene expression via the cyclic AMP response element site in the COX-2 promoter.

Nucleolin and YB-1 are required for JNK-mediated interleukin-2 mRNA stabilization during T-cell activation

Regulated mRNA turnover is a highly important process, but its mechanism is poorly understood. Using interleukin-2 (IL-2) mRNA as a model, we described a role for the JNK-signaling pathway in stabilization of IL-2 mRNA during T-cell activation, acting via a JNK response element (JRE) in the 5′ untranslated region (UTR). We have now identified two major RNA-binding proteins, nucleolin and YB-1, that specifically bind to the JRE. Binding of both proteins is required for IL-2 mRNA stabilization induced by T-cell activation signals and for JNK-induced stabilization in a cell-free system that duplicates essential features of regulated mRNA decay. Nucleolin and YB-1 are required for formation of an IL-2 mRNP complex that responds to specific mRNA stabilizing signals.

Suppression by metallothionein of doxorubicin-induced cardiomyocyte apoptosis through inhibition of p38 mitogen-activated protein kinases

Cardiomyopathy induced by doxorubicin (DOX) has long been a major impediment of clinical applications of this effective anticancer agent. Previous studies have shown that cardiac-specific metallothionein (MT)-overexpressing transgenic mice are highly resistant to DOX-induced cardiotoxicity. To investigate cellular and molecular mechanisms by which MT participates in this cytoprotection, transgenic mice containing high levels of cardiac MT and non-transgenic controls were treated intraperitoneally with DOX at a single dose of 15 mg/kg and sacrificed on the 4th day after treatment. Myocardial apoptosis was detected by a terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling assay and confirmed by electron microscopy of immunogold staining of apoptotic nuclei. Dual staining of cardiac alpha-sarcomeric actin using an immunohistochemical method further identified apoptotic myocytes. Apoptosis was significantly inhibited in the transgenic myocardium. The anti-apoptotic effect of MT was further revealed in primary cultures of neonatal mouse cardiomyocytes. Furthermore, DOX activated p38 mitogen-activated protein kinase (MAPK), which was critically involved in the apoptotic process, as demonstrated by inhibition of DOX-induced apoptosis by a p38-specific inhibitor, SB203580. Both DOX-induced p38 MAPK activation and apoptosis were dramatically inhibited in the transgenic cardiomyocytes. The results thus demonstrate that DOX induces apoptosis in cardiomyocytes both in vivo and in vitro and MT suppresses this effect through at least in part inhibition of p38 MAPK activation.

A role for the p38 MAP kinase pathway in the nuclear shuttling of NFATp

Calcium signals lead to the translocation of nuclear factor of activated T cells (NFAT) from the cytoplasm to the nucleus. This process is regulated by the calcium-activated phosphatase calcineurin, which can be cotransported with NFAT to the nucleus to maintain it transcriptionally active for the duration of calcium signaling. When the calcium signal ceases, NFAT is exported to the cytoplasm, and different NFAT kinases have been reported to oppose calcineurin activities and regulate the nuclear export of NFAT. Here we show that p38 MAPK phosphorylates in vitro and interacts in vivo with NFATp. Furthermore, the activation of this pathway in HeLa cells by cotransfection with activated MKK6 and p38 counteracts the calcium-induced nuclear accumulation of NFATp but not that of NFATc. By contrast, activation of JNK or ERK pathways failed to modify the nuclear shuttling of NFATp. Consistently, activation of p38, but not the JNK MAPK pathway, results in the inhibition of NFATp-driven transcription. In addition, the inhibition of the nuclear accumulation of NFATp by p38 appears to be mediated through the activation of NFATp nuclear export and takes place in a Leptomycin B-sensitive fashion, suggesting the involvement of the exportin CRM1 in this process. Thus, the p38 signal transduction pathway appears to play an important role in the regulation of the nuclear shuttling of NFATp and in cellular homeostasis.

JNK and p38 stresskinases--degenerative effectors of signal-transduction-cascades in the nervous system

The c-Jun N-terminal kinases (JNKs, also called stress activated protein kinases. SAPKs) and p38 kinases constitute together with extracellular signal-regulated kinases (ERKs) the family of MAP kinases. Whereas the functions of JNKs under physiological conditions are largely unknown, there is raising evidence that JNKs are potent effectors of apoptosis or degeneration of neurons in vitro and in the brain. The activation of the inducible transcription factor c-Jun by N-terminal phosphorylation is a central event in JNK-mediated degenerative processes that depend on de novo protein synthesis. At the post-translational level, cytoplasmic degenerative actions of JNKs might comprise inhibition of Bcl-2 and steroid hormone-receptor signaling or hyperphosphorylation of tau; and at transcriptional level, JNKs might trigger the induction of the apoptotic effectors p53 and Fas-Ligand by phosphorylation of c-Jun. The role of p38 is the nervous system is poorly understood, but its activation is also considered as part of the neuronal stress response. This review informs about the genetic processing, the regulation of activity and the biochemical actions of JNK and p38 isoforms in general. In the second part, we summarize the findings on expression and activation of JNKs and p38 under neurodegenerative condition. A particular focus is also put on the putative function of JNK under physiological conditions and for neuroprotection.

The JNK and P38 MAP kinase signaling pathways in T cell-mediated immune responses

The mitogen-activated protein (MAP) kinase family members, which include the extracellular response kinases (ERK), p38, and c-Jun amino terminal kinases (JNK), play a role in mediating signals triggered by cytokines, growth factors, and environmental stress. JNK and p38 MAP kinases have been involved in inflammatory processes induced by a variety of stimuli, such as oxidative stress. Here, we describe the role of the JNK and p38 MAP kinase signaling pathways in the development of T cells in the thymus, and activation and differentiation of T cells in the peripheral immune system.

Inhibition of p38 MAP kinase as a therapeutic strategy

Since the discovery of p38 MAP kinase in 1994, our understanding of its biology has progressed dramatically. The key advances include (1) identification of p38 MAP kinase homologs and protein kinases that act upstream and downstream from p38 MAP kinase, (2) identification of interesting and potentially important substrates, (3) elucidation of the role of p38 MAP kinase in cellular processes and (4) the establishment of the mechanism by which the pyridinylimidazole p38 MAP kinase inhibitors inhibit enzyme activity. It is now known that there are four members of the p38 MAP kinase family. They differ in their tissue distribution, regulation of kinase activation and subsequent phosphorylation of downstream substrates. They also differ in terms of their sensitivities toward the p38 MAP kinase inhibitors. The best-studied isoform is p38 alpha, whose activation has been observed in many hematopoietic and non-hematopoietic cell types upon treatment with appropriate stimuli. The pyridinylimidazole compounds, exemplified by SB 203580, were originally prepared as inflammatory cytokine synthesis inhibitors that subsequently were found to be selective inhibitors of p38 MAP kinase. SB 203580 inhibits the catalytic activity of p38 MAP kinase by competitive binding in the ATP pocket. X-ray crystallographic studies of the target enzyme complexed with inhibitor reinforce the observations made from site-directed mutagenesis studies, thereby providing a molecular basis for understanding the kinase selectivity of these inhibitors. The p38 MAP kinase inhibitors are efficacious in several disease models, including inflammation, arthritis and other joint diseases, septic shock, and myocardial injury. In all cases, p38 activation in key cell types correlated with disease initiation and progression. Treatment with p38 MAP kinase inhibitors attenuated both p38 activation and disease severity. Structurally diverse p38 MAP kinase inhibitors have been tested extensively in preclinical studies.

Signal-dependent activation of the MEF2 transcription factor by dissociation from histone deacetylases

Myocyte enhancer factor-2 (MEF2) transcription factors control muscle-specific and growth factor-inducible genes. We show that hypertrophic growth of cardiomyocytes in response to phenylephrine and serum is accompanied by activation of MEF2 through a posttranslational mechanism mediated by calcium, calmodulin-dependent protein kinase (CaMK), and mitogen-activated protein kinase (MAPK) signaling. CaMK stimulates MEF2 activity by dissociating class II histone deacetylases (HDACs) from the DNA-binding domain. MAPKs, which activate MEF2 by phosphorylation of the transcription activation domain, maximally stimulate MEF2 activity only when repression by HDACs is relieved by CaMK signaling to the DNA-binding domain. These findings identify MEF2 as an endpoint for hypertrophic stimuli in cardiomyocytes and demonstrate that MEF2 mediates synergistic transcriptional responses to the CaMK and MAPK signaling pathways by signal-dependent dissociation from HDACs.

Downregulation of JNK/SAPK activity is associated with the cross-resistance to P-glycoprotein-unrelated drugs in multidrug-resistant FM3A/M cells overexpressing P-glycoprotein

In the present study, cross-drug resistance in multidrug-resistant (MDR) cells, which overexpress P-glycoprotein (Pgp), a mdr1 gene product, against Pgp-unrelated drugs, and its relevance to c-Jun N-terminal kinase (JNK)/stress-activated protein kinase (SAPK) activity were examined. The multidrug-resistant FM3A/M cells overexpressing Pgp were resistant to apoptotic cell death induced either by Pgp-related drugs including vincristine and vinblastine, which are pumped out by Pgp, or by the Pgp-unrelated drugs including 5'-fluorouracil (5-FU) and bleomycin, which are not targets for Pgp, compared with the parental FM3A cells. Verapamil reversed the resistance of FM3A/M cells to apoptosis induced by the Pgp-related drugs but not that induced by the Pgp-unrelated drugs. Interestingly, FM3A/M cells have shown significantly lower basal and drug-stimulated JNK/SAPK activities than FM3A cells. After transfection with pEBG-SEK or pEBG-SAPK constructs, FM3A/M cells recovered the basal and Pgp-unrelated drug-stimulated activities of JNK/SAPK and the susceptibility to Pgp-unrelated drug-induced apoptotic cell death comparable to those of FM3A cells. Furthermore, FM3A cells became resistant to apoptotic cell death induced by vincristine and 5-FU after transfection with pEBG-SEK(K --> R), a dominant negative inhibitory mutant of SEK. These results suggest that downregulation of JNK/SAPK activity appears to confer on Pgp-associated FM3A/M cells a cross-resistance to Pgp-unrelated drugs.

Phosphorylation of eIF-4E on Ser 209 in response to mitogenic and inflammatory stimuli is faithfully detected by specific antibodies

Phosphorylation of Ser 209 is thought to modulate the activity of the cap-binding factor eIF-4E which is a crucial component in the initiation complex for cap-dependent translation of mRNA. We report here the full reconstitution of the p38 Map kinase cascade leading to phosphorylation of eIF-4E in vitro and the generation of antibodies specific for phospho-serine 209 in eIF-4E. These antibodies were used to probe the phosphorylation of eIF-4E in mammalian cells stimulated with mitogens and pro-inflammatory cytokines. Treatment of human dermal fibroblasts with FCS led to a transient hyperphosphorylation, followed by hypophosphorylation and return to normal state phosphorylation at 16 h after the initial stimulation. By using a potent small molecular weight inhibitor of Mnk1, the upstream kinase for eIF-4E, we observed a rapid dephosphorylation of eIF-4E within 45 min after addition of the inhibitor, suggesting a high turnover of phosphate on eIF-4E mediated by Mnk1 and a yet unidentified phosphatase.

MAP kinase pathways activated by stress: the p38 MAPK pathway

A stress-activated serine/threonine protein kinase, p38 mitogen-activated protein kinase (p38 MAPK), belongs to the MAP kinase superfamily. Diverse extracellular stimuli, including ultraviolet light, irradiation, heat shock, high osmotic stress, proinflammatory cytokines and certain mitogens, trigger a stress-regulated protein kinase cascade culminating in activation of p38 MAPK through phosphorylation on a TGY motif within the kinase activation loop. p38 MAPK appears to play a major role in apoptosis, cytokine production, transcriptional regulation, and cytoskeletal reorganization, and has been causally implicated in sepsis, ischemic heart disease, arthritis, human immunodeficiency virus infection, and Alzheimer's disease. The availability of specific inhibitors helps to clarify the role that p38 MAPK plays in these processes, and may ultimately offer therapeutic benefit for certain critically ill patients.

Contribution of MAP kinase pathways to the activation of ATF-2 in human neuroblastoma cells

Activated Transcription Factor-2 (ATF-2) is important during development of and during injury to the brain. Both Jun N-terminal Kinases (JNKs) and p38 Mitogen-Activated Protein Kinases (p38MAPKs) may phosphorylate ATF-2, but the contribution of these two pathways in cells has never been investigated. We have assayed endogenous p38MAPK activity in SK-N-MC and SH-SY5Y human neuroblastoma cells for activation of a GAL4/ATF-2 fusionprotein, by means of titrations of transfected expression plasmids and by using the p38MAPK inhibitor SB203580. It was found that basal activation of ATF-2 was independent of p38MAPK and that whereas MAPK kinase-3 (MKK3) was a weak inducer of ATF-2 activation, it was a potent activator of the stress activated transcription factor CHOP. In contrast, ATF-2 was very potently activated by the JNK pathway activator MAPK kinase kinase-1 (MEKK1). Thus, kinases downstream of MEKK1 appear relevant, but it is unlikely that p38MAPKs contribute quantitatively to activation of ATF-2 in these cells.

Conference highlight: do T cells care about the mitogen-activated protein kinase signalling pathways?

Mitogen-activated protein (MAP) kinases, which include the extracellular response kinases, p38 and c-Jun amino terminal kinases (JNK), play a significant role in mediating signals triggered by cytokines, growth factors and environmental stress. The JNK and p38 MAP kinases have been involved in growth, differentiation and cell death in different cell types. In the present paper, we describe how the JNK and p38 MAP kinase signalling pathways are regulated and their role during thymocyte development and the activation and differentiation of T cells in the peripheral immune system. The results from these studies demonstrate that the JNK and p38 MAP kinase signalling pathways regulate different aspects of T-cell mediated immune responses.

p38 MAP kinase activation by Clostridium difficile toxin A mediates monocyte necrosis, IL-8 production, and enteritis

Clostridium difficile toxin A causes acute neutrophil infiltration and intestinal mucosal injury. In cultured cells, toxin A inactivates Rho proteins by monoglucosylation. In monocytes, toxin A induces IL-8 production and necrosis by unknown mechanisms. We investigated the role of mitogen-activated protein (MAP) kinases in these events. In THP-1 monocytic cells, toxin A activated the 3 main MAP kinase cascades within 1 to 2 minutes. Activation of p38 was sustained, whereas stimulation of extracellular signal-regulated kinases and c-Jun NH(2)-terminal kinase was transient. Rho glucosylation became evident after 15 minutes. IL-8 gene expression was reduced by 70% by the MEK inhibitor PD98059 and abrogated by the p38 inhibitor SB203580 or by overexpression of dominant-negative mutants of the p38-activating kinases MKK3 and MKK6. SB203580 also blocked monocyte necrosis and IL-1beta release caused by toxin A but not by other toxins. Finally, in mouse ileum, SB203580 prevented toxin A-induced neutrophil recruitment by 92% and villous destruction by 90%. Thus, in monocytes exposed to toxin A, MAP kinase activation appears to precede Rho glucosylation and is required for IL-8 transcription and cell necrosis. p38 MAP kinase also mediates intestinal inflammation and mucosal damage induced by toxin A.

Mxi2, a splice variant of p38 stress-activated kinase, is a distal nephron protein regulated with kidney ischemia

Mxi2 is one of three known alternative spliced forms of the stress-activated mitogen-activated protein kinase p38 (CSBP). Mxi2 was originally identified as a Max-interacting protein and is the smallest member of the family of stress-activated kinases isolated to date. Mxi2 lacks most of the XI domain found in p38 and instead has a distinct COOH-terminal sequence of 17 amino acids. Here we present the genomic structure of the Mxi2/p38 locus on human chromosome 6q21.2/21.3 and establish the origin of the three spliced forms of p38. Using Mxi2-specific antibodies in mouse organs, we found the Mxi2 protein to be present exclusively in the kidney. Mxi2 is present predominantly in the distal tubule of the nephron and the level of the protein decreased during kidney ischemia-reperfusion. Stress signals or other known activators of the p38 pathway including MAP kinase-kinase 3 and MAP kinase-kinase 6 did not induce the kinase activity of Mxi2 using ATF-2 as a substrate. With the use of hybrid proteins encoding different portions of Mxi2 and p38 polypeptides, the different properties of Mxi2 can be assigned to its unique COOH terminus.

IL-4 regulation of IL-6 production involves Rac/Cdc42- and p38 MAPK-dependent pathways in keratinocytes

The stress-activated pathways leading to activation of p38 MAP kinase (p38 MAPK) and c-jun N-terminal kinases (JNK) have been shown to be activated by pro-inflammatory cytokines, physical and chemical stresses as well as a variety of hematopoietic growth factors. One exception is interleukin (IL)-4, which does not activate this pathway in hematopoietic cell. We report here that in A431, a keratinocytic cell line, IL-4 activates Rac and Cdc42 and their downstream effector p21-activated kinase (PAK). Rac and Cdc42 appear to regulate a protein kinase cascade initiated at the level of PAK and leading to activation of p38 MAPK, since IL-4 stimulates tyrosine phosphorylation of p38 MAPK and increases its catalytic activity. As A431 cells are able to produce IL-6 in response to IL-4 stimulation, we assessed the involvement of p38 MAPK in IL-6 gene expression. A pyrimidazole compound, SB203580, a specific inhibitor of p38 MAPK, inhibits production and gene expression of IL-6. SB203580 reduced significantly the stability of IL-6 mRNA. Here we provide evidence that p38 MAPK is activated in response to IL-4 and is involved in IL-6 synthesis by stabilizing IL-6 mRNA.

p62 functions as a p38 MAP kinase regulator

By screening a HeLa cDNA library to isolate genes encoding p38-regulating proteins, we have isolated two independent clones which encode the binding proteins to p38. We have found that both of these cDNA clones encode p62, first identified as a phosphorylation independent p56(lck) SH2 domain binding protein. Recent studies also indicate that p62 interacts with atypical PKCs to anchor them to intracellular membranes and with RIP to mediate signals to NF-kappaB through atypical PKCs. Moreover, p62 is shown to be involved in the transcriptional regulation via SV40 enhancer and to serve as a coactivator of an orphan nuclear hormone receptor. A coimmunoprecipitation assay shows its enhanced association in HeLa cells after stimulations such as sorbitol and anisomycin. An indirect immunofluorescence study indicates that p62 colocalizes with p38 in the nucleus in response to the stimulation. And in vitro kinase assays using MBP, but not ATF-2, as a substrate show that p62 enhances p38 activities in a dose-dependent manner. Together, these results demonstrate that p62 plays roles not only as an anchor but also as a regulator for the p38 kinase activity.

Molecular determinants that mediate selective activation of p38 MAP kinase isoforms

The p38 mitogen-activated protein kinase (MAPK) group is represented by four isoforms in mammals (p38alpha, p38beta2, p38gamma and p38delta). These p38 MAPK isoforms appear to mediate distinct functions in vivo due, in part, to differences in substrate phosphorylation by individual p38 MAPKs and also to selective activation by MAPK kinases (MAPKKs). Here we report the identification of two factors that contribute to the specificity of p38 MAPK activation. One mechanism of specificity is the selective formation of functional complexes between MAPKK and different p38 MAPKs. The formation of these complexes requires the presence of a MAPK docking site in the N-terminus of the MAPKK. The second mechanism that confers signaling specificity is the selective recognition of the activation loop (T-loop) of p38 MAPK isoforms. Together, these processes provide a mechanism that enables the selective activation of p38 MAPK in response to activated MAPKK.

Eotaxin induces degranulation and chemotaxis of eosinophils through the activation of ERK2 and p38 mitogen-activated protein kinases

Eotaxin and other CC chemokines acting via CC chemokine receptor-3 (CCR3) are believed to play an integral role in the development of eosinophilic inflammation in asthma and allergic inflammatory diseases. However, little is known about the intracellular events following agonist binding to CCR3 and the relationship of these events to the functional response of the cell. The objectives of this study were to investigate CCR3-mediated activation of the mitogen-activated protein (MAP) kinases extracellular signal-regulated kinase-2 (ERK2), p38, and c-jun N-terminal kinase (JNK) in eosinophils and to assess the requirement for MAP kinases in eotaxin-induced eosinophil cationic protein (ECP) release and chemotaxis. MAP kinase activation was studied in eotaxin-stimulated eosinophils (more than 97% purity) by Western blotting and immune-complex kinase assays. ECP release was measured by radioimmunoassay. Chemotaxis was assessed using Boyden microchambers. Eotaxin (10−11 to 10−7 mol/L) induced concentration-dependent phosphorylation of ERK2 and p38. Phosphorylation was detectable after 30 seconds, peaked at about 1 minute, and returned to baseline after 2 to 5 minutes. Phosphorylation of JNK above baseline could not be detected. The kinase activity of ERK2 and p38 paralleled phosphorylation. PD980 59, an inhibitor of the ERK2-activating enzyme MEK (MAP ERK kinase), blocked phosphorylation of ERK2 in a concentration-dependent manner. The functional relevance of ERK2 and p38 was studied using PD98 059 and the p38 inhibitor SB202 190. PD98 059 and SB202 190 both caused inhibition of eotaxin-induced ECP release and chemotaxis. We conclude that eotaxin induces a rapid concentration-dependent activation of ERK2 and p38 in eosinophils and that the activation of these MAP kinases is required for eotaxin-stimulated degranulation and directed locomotion.

Deficiency of the stress kinase p38alpha results in embryonic lethality: characterization of the kinase dependence of stress responses of enzyme-deficient embryonic stem cells

The mitogen-activated protein (MAP) kinase p38 is a key component of stress response pathways and the target of cytokine-suppressing antiinflammatory drugs (CSAIDs). A genetic approach was employed to inactivate the gene encoding one p38 isoform, p38alpha. Mice null for the p38alpha allele die during embryonic development. p38alpha(1/)- embryonic stem (ES) cells grown in the presence of high neomycin concentrations demonstrated conversion of the wild-type allele to a targeted allele. p38alpha(-/)- ES cells lacked p38alpha protein and failed to activate MAP kinase-activated protein (MAPKAP) kinase 2 in response to chemical stress inducers. In contrast, p38alpha(1/+) ES cells and primary embryonic fibroblasts responded to stress stimuli and phosphorylated p38alpha, and activated MAPKAP kinase 2. After in vitro differentiation, both wild-type and p38alpha(-/)- ES cells yielded cells that expressed the interleukin 1 receptor (IL-1R). p38alpha(1/+) but not p38alpha(-/)- IL-1R-positive cells responded to IL-1 activation to produce IL-6. Comparison of chemical-induced apoptosis processes revealed no significant difference between the p38alpha(1/+) and p38alpha(-/)- ES cells. Therefore, these studies demonstrate that p38alpha is a major upstream activator of MAPKAP kinase 2 and a key component of the IL-1 signaling pathway. However, p38alpha does not serve an indispensable role in apoptosis.

p38/SAPK2-dependent gene expression in Jurkat T cells

The stress-activated protein kinase p38/SAPK2 is known to regulate the activity of transcription factors and to control expression of several genes at the transcriptional or post-transcriptional level. In order to identify genes whose expression is under the control of p38/SAPK2 activity, we have compared the mRNA levels of a pattern of 588 genes between human Jurkat T cells with anisomycin-activated p38/SAPK2 and cells in which p38/SAPK2 was inhibited by the compound SB203580. Genes strongly expressed at the transcript level as a result of p38/SAPK2 activation are the transcription factors c-jun, fos-related antigen 1 (fra-1), the growth-arrest and DNA-damage gene gadd153 and early-growth-related gene 1 (egr-1) as well as the c-srk kinase csk and the nucleotide exchange factor ras-GRF. mRNAs significantly down-regulated include the insulin receptor IR, the adapter grb2, the transcription factor c-myc and the defender against apoptotic death, dad-1. For six selected genes, p38/SAPK2-regulated expression was confirmed and further analysed by Northern blot experiments, demonstrating a complex regulation of these genes under stress conditions.

Stress-activated protein kinases in the activation of rat hepatic stellate cells in culture

BACKGROUND/AIMS

The signal cascades involved in the activation of hepatic stellate cells (HSC) are largely unknown. Factors initiating activation include tumour necrosis factor (TNF)-alpha, transforming growth factor (TGF)-beta, endothelin, and oxidative stress. In other cell types some of these have been reported to stimulate p38 mitogen-activated protein (MAP) kinase and c-Jun N-terminal kinase (JNK). We have therefore investigated the role of these kinases in HSC activation.

METHODS

HSC were isolated from male Wistar rats. Quiescent experiments were performed on day 2 HSC and transformed experiments on day 15 passage 1 HSC. Kinase activities were determined by immunoprecipitation and phosphorylation of specific substrate proteins and alpha-smooth muscle actin (SMA) expression by immunoblotting.

RESULTS

The constitutive activity of p38 MAP kinase was higher in transformed versus quiescent cells. In quiescent cells TNFalpha stimulated p38 MAP kinase and JNK activities 12- and 4-fold respectively and this was halved by 2-mercaptoethanol, an indirect antioxidant. Endothelin-1 activated both kinases in quiescent cells via the endothelin-B receptor, while TGFbeta had no effect. Both 2-mercaptoethanol and a p38 inhibitor (SB202190) inhibited alpha-SMA expression by day 5 cells.

CONCLUSIONS

The activation of p38 MAP kinase and JNK by TNFalpha and endothelin, together with the inhibition of this activation by 2-mercaptoethanol, provides indirect evidence supporting their role in HSC transformation. Direct evidence for a role for p38 MAP kinase is provided by the observations that its constitutive activity is higher in transformed versus quiescent cells and that its inhibitor reduces HSC activation in culture as assessed by alpha-SMA expression.

Molecular mechanism of mechanical stress-induced cardiac hypertrophy

Mechanical stress is a major cause of cardiac hypertrophy. Although the mechanisms by which mechanical load induces cardiomyocyte hypertrophy have long been a subject of great interest for cardiologists, the lack of a good in vitro system has hampered the understanding of the biochemical mechanisms. For these past several years, however, an in vitro neonatal cardiocyte culture system has made it possible to examine the biochemical basis for the signal transduction of mechanical stress. Passive stretch of cardiac myocytes cultured on silicone membranes activates phosphorylation cascades of many protein kinases including protein kinase C, Raf-1 kinase and extracellular signal regulated kinases, and induces the expression of specific genes as well as an increase in protein synthesis. During that process, the secretion and production of vasoactive peptides such as angiotensin II and endothelin, are increased and they play critical roles in the induction of these hypertrophic responses. Although the involvement of vasoactive peptides in the development of cardiac hypertrophy is clinically important, the "mechanoreceptor" which receives the mechanical stress and converts it into intracellular biochemical signals remained unknown. We have recently obtained evidence suggesting that ion channels and integrins may be the "mechanoreceptor", the activation of which leads to cardiac hypertrophy.

Kinetic mechanism of the p38-alpha MAP kinase: phosphoryl transfer to synthetic peptides

p38 is a member of the mitogen-activated protein (MAP) kinase family. Activation (phosphorylation) of p38 acts as a switch for the transcriptional and translational regulation of a number of proteins, including the proinflammatory cytokines. Investigation of a set of small peptides revealed that, as with protein substrates, p38-alpha behaves as a proline-directed Ser/Thr MAP kinase for a peptide substrate, peptide 4 (IPTSPITTTYFFFKKK). We investigated the steady-state kinetic mechanism of the p38-alpha-catalyzed kinase reaction with EGF receptor peptide, peptide 1, as a substrate. Lineweaver-Burk analysis of the substrate kinetics yielded a family of lines intersecting to the left of the ordinate, with either ATP or peptide 1 as the varied substrate. Kinetic analysis in the presence of ADP yielded a competitive inhibition pattern when ATP was the varied substrate and a noncompetitive pattern if peptide 1 was the varied substrate. At saturating peptide substrate concentrations, inhibition by phosphopeptide product yielded an uncompetitive pattern when ATP was the varied substrate. These data are consistent with ordered binding with ATP as the initial substrate. We provide further evidence of the existence of a productive p38.ATP binary complex in that (a) activated p38-alpha has intrinsic ATPase activity, (b) ATPase and kinase activities are coupled, and (c) inhibitors of ATPase activity also inhibit the kinase activity with a similar inhibition constant. The k(cat) for the kinase reaction was lowered by 1.8-fold when ATP-gamma-S was used. Microviscosity linearly affected the k(cat) values of both the ATP and ATP-gamma-S reactions with a slope of about 0.8. These observations were interpreted to mean that the phosphoryl transfer step is not rate-limiting and that the release of product and/or enzyme isomerization is a possible rate-limiting step(s).

Molecular cloning and characterization of a novel human STE20-like kinase, hSLK

We have cloned a human counterpart to a guinea pig STE20-like kinase cDNA, designated human SLK (hSLK), from a human lung carcinomatous cell line A549 cDNA library. hSLK cDNA encodes a novel 1204 amino acid serine/threonine kinase for which the kinase domain located at the N-terminus shares considerable homology to that of the STE20-like kinase family. The C-terminal domain of hSLK includes both the coiled-coil structure and four Pro/Glu/Ser/Thr-rich (PEST) sequences, but not the GTPase-binding domain (GBD) that is characteristic of the p21-activated kinase (PAK) family, polyproline consensus binding sites, or the Leu-rich domain seen in the group I germinal center kinases (GCKs). Northern blot analysis indicated that hSLK was ubiquitously expressed. hSLK overexpressed in COS-7 cells phosphorylates itself as well as myelin basic protein used as a substrate. On the other hand, hSLK cannot activate any of the three well-characterized mitogen-activated protein kinase MAPK (ERK, JNK/SAPK and p38) pathways. Moreover, hSLK kinase activity is not upregulated by constitutive active forms of GTPases (RasV12, RacV12 and Cdc42V12). These structural and functional properties indicate that hSLK should be considered to be a new member of group II GCKs.

Thymocyte development past the CD4+CD8+stage requires an active p38 mitogen-activated protein kinase

Activation of the p38 mitogen-activated protein kinase (MAPK) pathway is important for some T-cell functions, but its role in intrathymic development is unclear. To investigate the function of p38 MAPK during the late stages of thymocyte differentiation, pharmacologic and genetic manipulations were used to inhibit p38 MAPK activity in developing thymocytes. Ligation of the T-cell antigen receptor (TCR) on either thymocytes or a thymocyte cell line resulted in p38 MAPK activation. Selective pharmacologic inhibition of p38 MAPK activity with the pyridinyl imidazole drug SB203580 severely impaired the development of mature CD4+ and CD8+ single positive (SP) thymocytes from their CD4+CD8+ double positive (DP) precursors in fetal thymic organ culture (FTOC). Further, pharmacologic or genetic suppression of p38 MAPK activity, the latter achieved by overexpressing a catalytically inactive p38 MAPK, resulted in a blockade of the DP-to-SP transition of a thymocyte cell line in a novel in vitro differentiation assay. Taken together, these data constitute the first demonstration that p38 MAPK plays a critical role in the DP-to-SP differentiation of thymocytes during late intrathymic development.

Oxidative stress and gene regulation

Reactive oxygen species are produced by all aerobic cells and are widely believed to play a pivotal role in aging as well as a number of degenerative diseases. The consequences of the generation of oxidants in cells does not appear to be limited to promotion of deleterious effects. Alterations in oxidative metabolism have long been known to occur during differentiation and development. Experimental perturbations in cellular redox state have been shown to exert a strong impact on these processes. The discovery of specific genes and pathways affected by oxidants led to the hypothesis that reactive oxygen species serve as subcellular messengers in gene regulatory and signal transduction pathways. Additionally, antioxidants can activate numerous genes and pathways. The burgeoning growth in the number of pathways shown to be dependent on oxidation or antioxidation has accelerated during the last decade. In the discussion presented here, we provide a tabular summary of many of the redox effects on gene expression and signaling pathways that are currently known to exist.