Endothelin-1 activates p38 mitogen-activated protein kinase and cytosolic phospholipase A(2) in cat iris sphincter smooth muscle cells

We have shown previously that cytosolic phospholipase A(2) (cPLA(2)) is res ponsible for endothelin-1-induced release of arachidonic acid for prostagla ndin synthesis in cat iris sphincter smooth muscle (CISM) cells [Husain and Abdel-Latif (1998) Biochim. Biophys. Acta 1392, 127-144]. Here we show tha t p38 mitogen-activated protein (MAP) kinase, but not p42/p44 MAP kinases, plays an important role in the phosphorylation and activation of cPLA(2) in endothelin-1-stimulated CISM cells. This conclusion is supported by the fo llowing findings. Both p38 MAP kinase and p42/p44 MAP kinases were present in the CISM cells and both were activated by endothelin-1. SB203580, a pote nt specific inhibitor of p38 MAP kinase, but not the p42/p44 MAP kinases sp ecific inhibitor, PD98059, markedly suppressed endothelin-1-enhanced cPLA(2 ) phosphorylation, cPLA(2) activity and arachidonic acid release. The addit ion of endothelin-1 resulted in the phosphorylation and activation of cPLA( 2). Endothelin-1 stimulated p38 MAP kinase activity in a time- and concentr ation-dependent manner, and these effects were mediated through the endothe lin-A receptor subtype. The protein kinase C (PKC) inhibitor, RO 31-8220, h ad no inhibitory effect on endothelin-1-induced p38 MAP kinase activation, suggesting that endothelin-1 activation of p38 MAP kinase is independent of PKC. Pertussis toxin inhibited both endothelin-l and mastoparan stimulatio n of p38 MAP kinase activity and arachidonic acid release. The inhibitory e ffects of pertussis toxin are not mediated through cAMP formation. Mastopar an-stimulated [H-3]arachidonic acid release and cPLA(2) activation was inhi bited by SB203580, but not by RO 31-8220. These data suggest that endotheli n-1 binds to the endothelin-a receptor to activate the Gi-protein which, th rough a series of kinases, leads to the activation of p38 MAP kinase and su bsequently to phosphorylation and activation of cPLA(2). Activation of cPLA (2) leads to the liberation of arachidonic acid from membrane phospholipids . The ability of the activated endothelin-a receptor, which is coupled to b oth Gq- and Gi-proteins, to recruit and activate this complex signal transd uction pathway remains to be elucidated. Further studies on the mechanism o f these relation ships could provide important information about the functi ons of p38 MAP kinase in smooth muscle.