Stress-induced inhibition of ERK1 and ERK2 by direct interaction with p38 MAP kinase

We have identified a direct physical interaction between the stress signaling p38alpha MAP kinase and the mitogen-activated protein kinases ERK1 and ERK2 by affinity chromatography and coimmunoprecipitation studies. Phosphorylation and activation of p38alpha enhanced its interaction with ERK1/2, and this correlated with inhibition of ERK1/2 phosphotransferase activity. The loss of epidermal growth factor-induced activation and phosphorylation of ERK1/2 but not of their direct activator MEK1 in HeLa cells transfected with the p38alpha activator MKK6(E) indicated that activated p38alpha may sequester ERK1/2 and sterically block their phosphorylation by MEK1.     

Pyrazoloheteroaryls: novel p38alpha MAP kinase inhibiting scaffolds with oral activity

A test library with three novel p38alpha inhibitory scaffolds and a narrow set of substituents was prepared. Appropriate combination of substituent and scaffold generated potent p38alpha inhibitors, for example, pyrazolo[3,4-b]pyridine 9, pyrazolo[3,4-d]pyrimidine 18a and pyrazolo[3,4-b]pyrazine 23b with potent in vivo activity upon oral administration in animal models of rheumatoid arthritis.     

Specific regulation of cytokine-dependent p38 MAP kinase activation by p62/SQSTM1

We have previously shown that p62/SQSTM1 binds to p38. In this study, we identified two association domains of p62 to p38 by conducting co-immunoprecipitation experiments. One domain comprises the amino acids 173-182, named N-terminal p38 interaction (NPI) domain, and the other domain comprises the amino acids 335-344, named C-terminal p38 interaction (CPI) domain. An aspartic acid tripeptide located at 335-337 was required for their association. However, the direct interaction was only observed between the recombinant p38 and the peptide of the NPI domain, but not that of the CPI domain in the surface plasmon resonance analyses. These results suggest that the CPI domain may serve to form a certain conformation suitable for the association with p38. Furthermore, we showed that knockdown of p62 expression by siRNA led to impaired p38 phosphorylation only when HeLa cells were stimulated by cytokine. The critical role of p62 in cytokine-dependent p38 signalling pathway was further confirmed by measuring IL-8 mRNA. Cytokine mRNA is often stabilized via p38 pathway. In the absence of p62, IL-8 mRNA induced by IL-1beta became more fragile. These data show that p62 specifically regulates cytokine-dependent p38 signalling pathway.     

Negative feedback regulation of MKK6 mRNA stability by p38alpha mitogen-activated protein kinase

p38 mitogen-activated protein (MAP) kinases play an important role in the regulation of cellular responses to all kinds of stresses. The most abundant and broadly expressed p38 MAP kinase is p38alpha, which can also control the proliferation, differentiation, and survival of several cell types. Here we show that the absence of p38alpha correlates with the up-regulation of one of its upstream activators, the MAP kinase kinase MKK6, in p38alpha(-/-) knockout mice and in cultured cells derived from them. In contrast, the expression levels of the p38 activators MKK3 and MKK4 are not affected in p38alpha-deficient cells. The increase in MKK6 protein concentration correlates with increased amounts of MKK6 mRNA in the p38alpha(-/-) cells. Pharmacological inhibition of p38alpha also up-regulates MKK6 mRNA levels in HEK293 cells. Conversely, reintroduction of p38alpha into p38alpha(-/-) cells reduces the levels of MKK6 protein and mRNA to the normal levels found in wild-type cells. Moreover, we show that the MKK6 mRNA is more stable in p38alpha(-/-) cells and that the 3'untranslated region of this mRNA can differentially regulate the stability of the lacZ reporter gene in a p38alpha-dependent manner. Our data indicate that p38alpha can negatively regulate the stability of the MKK6 mRNA and thus control the steady-state concentration of one of its upstream activators.     

SAR of benzoylpyridines and benzophenones as p38alpha MAP kinase inhibitors with oral activity

Benzoylpyridines and benzophenones were synthesized and evaluated in vitro as p38alpha inhibitors and in vivo in several models of rheumatoid arthritis. Oral activity was found to depend upon substitution: 1,1-dimethylpropynylamine substituted benzophenone 10b (IC50: 14 nM) and pyridinoyl substituted benzimidazole 17b (IC50: 21 nM) showed highest efficacy and selectivity with ED50s of 9.5 and 8.6 mg/kg p.o. in CIA.     

TAB-1 modulates intracellular localization of p38 MAP kinase and downstream signaling

Stress-activated mitogen-activated protein (MAP) kinase p38 mediates stress signaling in mammalian cells via threonine and tyrosine phosphorylation in its conserved TGY motif by upstream MAP kinase kinases (MKKs). In addition, p38 MAP kinase can also be activated by an MKK-independent mechanism involving TAB-1 (TAK-1-binding protein)-mediated autophosphorylation. Although TAB-1-mediated p38 activation has been implicated in ischemic heart, the biological consequences and downstream signaling of TAB-1-mediated p38 activation in cardiomyocytes is largely unknown. We show here that TAB-1 expression leads to a significant induction of p38 autophosphorylation and consequent kinase activation in cultured neonatal cardiomyocytes. In contrast to MKK3-induced p38 kinase downstream effects, TAB-1-induced p38 kinase activation does not induce expression of pro-inflammatory genes, cardiac marker gene expression, or changes in cellular morphology. Rather, TAB-1 binds to p38 and prevents p38 nuclear localization. Furthermore, TAB-1 disrupts p38 interaction with MKK3 and redirects p38 localization in the cytosol. Consequently, TAB-1 expression antagonizes the downstream activity of p38 kinase induced by MKK3 and attenuates interleukin-1beta-induced inflammatory gene induction in cardiomyocytes. These data suggest that TAB-1 can mediate MKK-independent p38 kinase activation while negatively modulating MKK-dependent p38 function. Our study not only redefines the functional role of TAB-1 in p38 kinase-mediated signaling pathways but also provides the first evidence that intracellular localization of p38 kinase and complex interaction dictates its downstream effects. These results suggest a previously unknown mechanism for stress-MAP kinase regulation in mammalian cells.     

The subcellular localization control of integrin linked kinase 1 through its protein-protein interaction with caveolin-1

Integrin linked kinase 1 (ILK1), a member of the serine/threonine kinases, has been shown to be crucial for the cell survival, differentiation, and Wnt signaling. Firstly, by using a confocal microscopy and a transfection approach, we obtained the evidence that ILK1 interacts physically with caveolin-1, a 22-kDa integral membrane protein, which is the principal structural and regulatory component of caveolae membranes. By ILK1 deletion mutant analysis, we characterized the caveolin-1-binding domain in the kinase domain of ILK1. In addition, we found that native ILK1 is associated with endogenous caveolin-1 in COS-1 cells. Secondly, transient transfection assays showed that a reduction in caveolin-1 binding leads to a substantial increase in the serine/threonine phosphorylation of ILK1. Thirdly, caveolin-1 and its scaffolding peptide (amino acids 82-101) functionally suppressed the auto-kinase activity of purified recombinant ILK1 protein. Fourthly, the association of ILK1 with caveolin-1 regulated its cytoplasmic retention; if it was not associated with caveolin-1, it was transported to the nucleus. Fifthly, we also noticed the putative nuclear localization sequences (nls) in ILK1 near the caveolin-1-binding domain. Thus, our data indicate that caveolin-1 regulates ILK1 auto-phosphorylation activity and its subcellular localization via a specific protein-protein interaction through blocking the exposure of its putative nls motif.     

A novel lipid binding site formed by the MAP kinase insert in p38 alpha

The p38 mitogen-activated protein (MAP) kinases function as signaling molecules essential for many cellular processes, particularly mediating stress response. The activity of p38 MAP kinases is meticulously regulated to reach the desired cellular phenotype. Several alternative activation and attenuation mechanisms have been characterized recently which include new phosphorylation sites. Here we present the crystal structure of p38α MAP kinase in complex with n-octyl-β-glucopyranoside detergent. The complex unveils a novel lipid-binding site formed by a local conformational change of the MAP kinase insert. This binding is the first attribution for a possible role of the MAP kinase insert in p38. The binding site can accommodate a large selection of lipidic molecules. In addition, we also show via biophysical methods that arachidonic acid and its derivatives bind p38α in vitro. Based on our analysis we propose that the binding of lipids could fine-tune p38α catalytic activity towards a preferred phenotype.     

Modification of collagen by 3-deoxyglucosone alters wound healing through differential regulation of p38 MAP kinase

Background

Wound healing is a highly dynamic process that requires signaling from the extracellular matrix to the fibroblasts for migration and proliferation, and closure of the wound. This rate of wound closure is impaired in diabetes, which may be due to the increased levels of the precursor for advanced glycation end products, 3-deoxyglucosone (3DG). Previous studies suggest a differential role for p38 mitogen-activated kinase (MAPK) during wound healing; whereby, p38 MAPK acts as a growth kinase during normal wound healing, but acts as a stress kinase during diabetic wound repair. Therefore, we investigated the signaling cross-talk by which p38 MAPK mediates wound healing in fibroblasts cultured on native collagen and 3DG-collagen.

Methodology/Principal Findings

Using human dermal fibroblasts cultured on 3DG-collagen as a model of diabetic wounds, we demonstrated that p38 MAPK can promote either cell growth or cell death, and this was dependent on the activation of AKT and ERK1/2. Wound closure on native collagen was dependent on p38 MAPK phosphorylation of AKT and ERK1/2. Furthermore, proliferation and collagen production in fibroblasts cultured on native collagen was dependent on p38 MAPK regulation of AKT and ERK1/2. In contrast, 3DG-collagen decreased fibroblast migration, proliferation, and collagen expression through ERK1/2 and AKT downregulation via p38 MAPK.

Conclusions/Significance

Taken together, the present study shows that p38 MAPK is a key signaling molecule that plays a significantly opposite role during times of cellular growth and cellular stress, which may account for the differing rates of wound closure seen in diabetic populations.     

Prevention of MKK6-dependent activation by binding to p38alpha MAP kinase

Inhibition of p38alpha MAP kinase is a potential approach for the treatment of inflammatory disorders. MKK6-dependent phosphorylation on the activation loop of p38alpha increases its catalytic activity and affinity for ATP. An inhibitor, BIRB796, binds at a site used by the purine moiety of ATP and extends into a "selectivity pocket", which is not used by ATP. It displaces the Asp168-Phe169-Gly170 motif at the start of the activation loop, promoting a "DFG-out" conformation. Some other inhibitors bind only in the purine site, with p38alpha remaining in a "DFG-in" conformation. We now demonstrate that selectivity pocket compounds prevent MKK6-dependent activation of p38alpha in addition to inhibiting catalysis by activated p38alpha. Inhibitors using only the purine site do not prevent MKK6-dependent activation. We present kinetic analyses of seven inhibitors, whose crystal structures as complexes with p38alpha have been determined. This work includes four new crystal structures and a novel assay to measure K(d) for nonactivated p38alpha. Selectivity pocket compounds associate with p38alpha over 30-fold more slowly than purine site compounds, apparently due to low abundance of the DFG-out conformation. At concentrations that inhibit cellular production of an inflammatory cytokine, TNFalpha, selectivity pocket compounds decrease levels of phosphorylated p38alpha and beta. Stabilization of a DFG-out conformation appears to interfere with recognition of p38alpha as a substrate by MKK6. ATP competes less effectively for prevention of activation than for inhibition of catalysis. By binding to a different conformation of the enzyme, compounds that prevent activation offer an alternative approach to modulation of p38alpha.     

Sphingosine 1-phosphate amplifies phosphoinositide hydrolysis stimulated by prostaglandin f2 alpha in osteoblasts: involvement of p38MAP kinase

We previously showed that sphingosine 1-phosphate phosphorylates p42/p44 mitogen-activated protein (MAP) kinase and p38 MAP kinase in osteoblast-like MC3T3-E1 cells. In the present study, we investigated the effect of sphingosine 1-phosphate on phospholipase C-catalyzing phosphoinositide hydrolysis induced by prostaglandin F2alpha (PGF2 alpha) in these cells. Sphingosine 1-phosphate significantly amplified the inositol phosphates formation by PGF2 alpha. Sphingosine 1-phosphate did not enhance the formation induced by NaF, a direct activator of heterotrimeric GTP-binding proteins. PD98059, an inhibitor of the kinase that activates p42/p44 MAP kinase, had little effect on the amplification by sphingosine 1-phosphate. SB203580, an inhibitor of p38 MAP kinase, reduced the effect of sphingosine 1-phosphate on the formation of inositol phosphates by PGF2 alpha. The phosphorylation of p42/p44 MAP kinase by PGF alpha was attenuated by PD98059. SB203580 suppressed the phosphorylation of p38 MAP kinase by PGF2 alpha. Tumor necrosis factor-alpha enhanced the PGF2 alpha-stimulated formation of inositol phosphates. These results strongly suggest that sphingosine 1-phosphate amplifies PGF2 alpha-induced phosphoinositide hydrolysis by phospholipase C through p38 MAP kinase in osteoblasts.     

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

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

Involvement of ASK1 in Ca2+-induced p38 MAP kinase activation

The mammalian mitogen-activated protein (MAP) kinase kinase kinase apoptosis signal-regulating kinase 1 (ASK1) is a pivotal component in cytokine- and stress-induced apoptosis. It also regulates cell differentiation and survival through p38 MAP kinase activation. Here we show that Ca²⁺ signalling regulates the ASK1–p38 MAP kinase cascade. Ca²⁺ influx evoked by membrane depolarization in primary neurons and synaptosomes induced activation of p38, which was impaired in those derived from ASK1-deficient mice. Ca²⁺/calmodulin-dependent protein kinase type II (CaMKII) activated ASK1 by phosphorylation. Moreover, p38 activation induced by the expression of constitutively active CaMKII required endogenous ASK1. Thus, ASK1 is a critical intermediate of Ca²⁺ signalling between CaMKII and p38 MAP kinase.     

Regulated expression of galectin-1 during T-cell activation involves Lck and Fyn kinases and signaling through MEK1/ERK,p38 MAP kinase and p70S6 kinase

It has been shown that staphylococcal enterotoxin A (SEA) acts through human peripheral blood mononuclear cells (PBMC) to stimulate synthesis or release of pyrogenic cytokines. Nuclear factor-kappa B (NF-kappaB) is thought to play an important role in inflammatory responses through the regulation of genes encoding pro-inflammatory cytokines. The purpose of the present study was to determine whether the NF-kappaB mechanisms in human PBMC are involved in SEA-induced fever. Western blot evaluation revealed SEA was able to induce nuclear translocation of NF-kappaB from cytosol to nucleus in PBMC, which could be abolished by a NF-kappaB inhibitor such as pyrrolidine dithiocarbamate (PDTC), sodium pyrithione (Pyri), N-acetyl-L-cysteine (NAC), or curcumin (Cur). Electrophoretic mobility shift assay also showed that the NF-kappaB DNA-binding activity was increased in the SEA-treated PBMC. Again, the SEA-induced increased NF-kappaB binding activity was significantly attenuated by either PDTC, Pyri, NAC or Cur. The pyrogenic responses to supernatant fluids obtained from human PBMC stimulated with SEA were associated with increased levels of interleukin 1-beta (IL-1beta), IL-6, and tumor necrosis factor-alpha (TNF-alpha) in the supernatant fluids. Both the fever and the increased levels of IL-1beta, IL-6, and TNF-alpha in supernatant fluids obtained from the SEA-stimulated PBMC were decreased by incubating SEA-PBMC with either PDTC, Pyri, NAC, or Cur. Furthermore, the fever induced by systemic or central administration of SEA in rabbits were attenuated by pre-treatment with an systemic or central dose of either PDTC, Pyri, NAC, or Cur. The data indicate that inhibition of NF-kappaB prevents SEA-induced fever.     

Regulation of Drosophila p38 activation by specific MAP2 kinase and MAP3 kinase in response to different stimuli

The p38 mitogen-activated protein kinase (MAPK) signaling pathway plays an important role in cellular responses to inflammatory stimuli and environmental stress. Activation of p38 is mediated through phosphorylation by upstream MAPKK, which in turn is activated by MAPKKK. However, the mechanism of how different upstream MAP2Ks and MAP3Ks specifically contribute to p38 activation in response to different stimuli is still not clearly understood. By using double-stranded RNA-mediated interference (RNAi) in Drosophila cells, we demonstrate that D-MKK3 is a major MAP2K responsible for D-p38 activation by UV, heat shock, NaCl or peptiodglycan (PGN). Stimulation of UV and PGN activates D-p38 through D-MEKK1, heat shock-induced activation of D-p38 signals through both D-MEKK1 and D-ASK1. On the other hand, maximal activation of D-p38 by NaCl requires the expression of four MAP3Ks.     

Basic fibroblast growth factor upregulates cyclooxygenase-2 in I407 cells through p38 MAP kinase

The intestinal cell line I407 responds to basic fibroblast growth factor (bFGF) by upregulating cyclooxygenase-2 (COX-2) mRNA and protein expression and increasing PGE(2) production. bFGF treatment of I407 cells results in phosphorylation of p38, and the p38 inhibitor SB-203580 abrogates bFGF-induced PGE(2) synthesis. Wild-type p38alpha (p38alphaWT) and dominant-negative p38alpha (p38alphaDN) stable transfectant clones of I407 cells were used to examine the role of the p38 MAP kinase pathway in the events controlling PGE(2) synthesis after treatment with bFGF. Treatment of p38alphaWT clones with bFGF resulted in increased COX-2 protein levels and PGE(2) synthesis similar to those seen in bFGF-treated control-transfected cells. In contrast, the p38alphaDN clones failed to upregulate COX-2 protein or increase PGE(2) synthesis when treated with bFGF. Exogenous arachidonate did not restore PGE(2) synthesis by p38alphaDN cells. bFGF treatment increased COX-2 mRNA stability, and the p38 inhibitor SB-203580 attenuated COX-2 mRNA stability in bFGF-treated I407 cells. These data demonstrate a crucial role for p38alpha in growth factor-induced PGE(2) synthesis by intestinal cells. Furthermore, they indicate that p38 activity is required at a step distal to arachidonate release, most likely COX-2 upregulation, because exogenous arachidonate did not restore PGE(2) synthesis.