The primary structure of p38 gamma: a new member of p38 group of MAP kinases

Characterization of the structure and function of the fourth member of p38 group mitogen-activated protein kinases, p38delta

We have cloned and characterized a new member of the p38 group of mitogen-activated protein kinases here termed p38delta. Sequence comparisons revealed that p38delta is approximately 60% identical to the other three p38 isoforms but only 40-45% to the other mitogen-activated protein kinase family members. It contains the TGY dual phosphorylation site present in all p38 group members and is activated by a group of extracellular stimuli including cytokines and environmental stresses that also activate the other three known p38 isoforms. However, unlike the other p38 isoforms, the kinase activity of p38delta is not blocked by the pyridinyl imidazole, 4-(4-fluorophenyl)-2-2(4-hydroxyphenyl)-5-(4-pyridyl)-imidazole (identicalto SB202190). p38delta can be activated by MKK3 and MKK6, known activators of the other isoforms. Nonetheless, in-gel kinase assays provide evidence for additional activators. The data presented herein show that p38delta has many properties that are similar to those of other p38 group members. Nonetheless important differences exist among the four members of the p38 group of enzymes, and thus each may have highly specific, individual contributions to biologic events involving activation of the p38 pathways.

Selective activation of JNK/SAPK by interleukin-1 in rabbit liver is mediated by MKK7

Activation of jun N-terminal kinase (JNK)/stress-activated protein kinase (SAPK) by interleukin-1 (IL-1) has been reported in many cells and in rabbit liver. Here we report selective activation of JNK/SAPK, without activation of p38 or p42 mitogen-activated protein kinases (MAPKs), by IL-1 in rabbit liver. We identified an IL-1 regulated JNK/SAPK activator present in rabbit liver using S Sepharose chromatography. It was purified and immunoprecipitated by two antisera to MAP kinase kinase 7 (MKK7). It was not recognised by an antibody to MKK4. We conclude that MKK7 is the activator of JNK/SAPK activated by IL-1 in liver and that JNK/SAPK is the only MAPK activated by IL-1 in liver.

Novel homologues of CSBP/p38 MAP kinase: activation, substrate specificity and sensitivity to inhibition by pyridinyl imidazoles

A novel homologue of p38 MAP kinase, called SAPK4, has been cloned which shares 61% amino acid identity with p38 and is expressed predominantly in testes, pancreas and small intestine. We also cloned an alternative form of p38beta, termed p38beta2, which lacks the additional 8 amino acid insertion unique to p38beta. p38, p38beta, p38beta2, ERK6/p38gamma/SAPK3, and SAPK4 were characterized with respect to stimulus-dependent activation in transfected cells, substrate specificity, and sensitivity to inhibition by pyridinyl imidazoles. All homologues were stimulated, although to differing extents, by IL-1beta, TNF, sorbitol, and UV. Only SAPK3 and SAPK4 were stimulated significantly by PMA. p38beta showed the weakest activation overall. MBP, ATF-2, and both MAPKAP kinase-2 and kinase-3 were good substrates of p38 and p38beta in vitro. In contrast, only MBP, ATF2, and MAPKAP kinase-3 proved to be significant substrates of SAPK3 and SAPK4, and of these three, MAPKAP kinase-3 was by far the weakest. p38beta had very poor kinase activity for all substrates except MBP. While both p38 and p38beta2 were comparably inhibited by SB 203580 and SB 202190, neither SAPK3 nor SAPK4 were inhibited. p38beta was partially inhibited by both inhibitors. These data suggest that SAPK3 and SAPK4 form a distinct subset of the p38 MAP kinases with different expression pattern, response to stimuli, substrate specificity, and inhibitor sensitivity.

Apoptosis signaling pathway in T cells is composed of ICE/Ced-3 family proteases and MAP kinase kinase 6b

Fas/APO-1(CD95) ligation activates programmed cell death, a cellular process that plays an important role in the maturation of the host immune response. We show that activation of a specific MAP kinase kinase (MKK), MKK6b, is necessary and sufficient for Fas-induced apoptosis of Jurkat T cells. MKK6b activation occurs downstream of an interleukin-1 converting enzyme-like (ICE-like) protease(s), while execution of the apoptotic pathway by MKK6b requires both ICE- and CPP32-like proteases. Surprisingly, the p38 MAP kinase protein, a known substrate of MKK6b, does not participate in Fas/MKK6b-mediated apoptosis. These findings indicate a divergence of the MKK6b signaling pathways, one of which activates p38 and leads to regulation of gene expression, and one of which activates the ICE/Ced-3 family of proteases and leads to cell death. These studies represent a demonstration of an apoptotic pathway that is comprised of both the ICE/Ced-3 family of proteases and MAP kinase kinase 6.

The structural basis for the specificity of pyridinylimidazole inhibitors of p38 MAP kinase

BACKGROUND

The p38 mitogen-activated protein (MAP) kinase regulates signal transduction in response to environmental stress. Pyridinylimidazole compounds are specific inhibitors of p38 MAP kinase that block the production of the cytokines interleukin-1beta and tumor necrosis factor alpha, and they are effective in animal models of arthritis, bone resorption and endotoxin shock. These compounds have been useful probes for studying the physiological functions of the p38-mediated MAP kinase pathway.

RESULTS

We report the crystal structure of a novel pyridinylimidazole compound complexed with p38 MAP kinase, and we demonstrate that this compound binds to the same site on the kinase as does ATP. Mutagenesis showed that a single residue difference between p38 MAP kinase and other MAP kinases is sufficient to confer selectivity among pyridinylimidazole compounds.

CONCLUSIONS

Our results reveal how pyridinylimidazole compounds are potent and selective inhibitors of p38 MAP kinase but not other MAP kinases. It should now be possible to design other specific inhibitors of activated p38 MAP kinase using the structure of the nonphosphorylated enzyme.

Phosphorylation and activation of cytosolic phospholipase A2 by 38-kDa mitogen-activated protein kinase in collagen-stimulated human platelets

Phosphorylation and activation of cytosolic phospholipase A2 (PLA2) can occur independently of the activation of 42/44-kDa mitogen-activated protein (MAP) kinase in human platelets. We have investigated the hypothesis that the stress-activated p38 MAP kinase plays a role in the regulation of cytosolic PLA2. The specific inhibitor of p38 MAP kinase, SB 203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl) imidazole], completely blocked the collagen-stimulated phosphorylation of cytosolic PLA2 in the presence of a cyclooxygenase blocker, and reduced the release of [3H]arachidonic acid by low concentrations of collagen. Stimulation of platelets with collagen (100 microg/ml) enhanced in vitro PLA2 activity of platelet lysates twofold over basal levels. In vitro PLA2 activity was reduced to basal levels when platelets were stimulated in the presence of SB 203580, but not in the presence of an inhibitor of the kinase that activates p42/p44 MAP kinase. SB 203580 only partially inhibited phosphorylation of cytosolic PLA2 in platelets that had not been treated with a cyclooxygenase blocker indicating that secondary stimulation by thromboxane A2 induces cytosolic PLA2 phosphorylation, by kinase(s) other than p38 MAP kinase. Under these conditions, inhibition of p42/p44 MAP kinase did not result in a reduction of cytosolic PLA2 phosphorylation, which is in agreement with the results obtained in the presence of cyclooxygenase blockers. In contrast to collagen, both p38 MAP kinase and p42/p44 MAP kinase participated in the phosphorylation of cytosolic PLA2 in platelets stimulated by cross-linking of the low-affinity receptor for immune complexes, Fc gammaRIIA. The present results demonstrate an important role for p38 MAP kinase in the regulation of cytosolic PLA2 activity in collagen-stimulated human platelets.

A highly specific inhibitor of human p38 MAP kinase binds in the ATP pocket

The crystal structure of human p38 mitogen-activated protein (MAP) kinase in complex with a potent and highly specific pyridinyl-imidazole inhibitor has been determined at 2.0 A resolution. The structure of the kinase, which is in its unphosphorylated state, is similar to that of the closely-related ERK2. The inhibitor molecule is bound in the ATP pocket. A hydrogen bond is made between the pyridyl nitrogen of the inhibitor and the main chain amido nitrogen of residue 109, analogous to the interaction from the N1 atom of ATP. The crystal structure provides possible explanations for the specificity of this class of inhibitors. Other protein kinase inhibitors may achieve their specificity through a similar mechanism. The structure also reveals a possible second binding site for this inhibitor, with currently unknown function.