INHIBITION OF MAP KINASE BY PROSTAGLANDIN E(2) AND FORSKOLIN IN RAT RENAL MESANGIAL CELLS

Activation of the mitogen-activated protein (MAP) kinase pathway is believed to play a critical role in normal and pathophysiological proliferation of mesangial cells. Recent studies have shown that MAP kinase activation by growth factors in other cell types involves activation of the low-molecular-weight G protein Ras and the protooncogene serine kinase c-Raf-1. In this study, the role of this pathway in rat renal mesangial cells was assessed. Platelet-derived growth factor (PDGF), epidermal growth factor (EGF), as well. as phorbol esters (PMA) rapidly activated MAP kinase three- to fourfold in these cells. PDGF and EGF, but not PMA, were able to activate c-Raf-1 and Ras activity. Stimulation of mesangial cells with the inflammatory mediator prostaglandin E(2) (PGE(2)) or elevation of intracellular adenosine 3',5'-cyclic monophosphate (cAMP) by treatment with forskolin markedly blunted activation of MAP kinase induced by PDGF and EGF, but not by PMA. Consistent with this observation, PGE(2) abolished growth factor-induced activation of c-Raf-l. However, Ras activation induced by growth factors was not affected by PGE(2) and forskolin. These results suggest that MAP kinase activation can occur by at least two separate pathways in mesangial cells. Tyrosine kinase receptors activate MAP kinase through activation of Ras and Raf. This pathway can be blocked by PGE(2) and elevation of cAMP, presumably by interfering with the ability of Ras to activate Raf In addition, activation of protein kinase C by phorbol esters can activate MAP kinase in a Ras/Raf-independent manner. This pathway is not sensitive to inhibition by PGE(2) or cAMP. It is likely that activation of each of these pathways, while both resulting in a stimulated MAP kinase, will have different physiological consequences in mesangial cells.