SELECTIVE MODULATION OF MAP KINASE IN EMBRYONIC PALATE CELLS

Murine embryonic palate mesenchyme (MEPM) cells are responsive to a nu mber of endogenous factors found in the local embryonic tissue environ ment. Recently, it was shown that activation of the cyclic AMP (cAMP) or the transforming growth factor beta (TGF beta) signal transduction pathways modulates the proliferative response of MEPM cells to epiderm al growth factor (EGF). Since the mitogen-activated protein kinase (MA PK) cascade is a signal transduction pathway that mediates cellular re sponsiveness to EGF, we examined the possibility that several signalin g pathways which abrogate ECF-stimulated proliferation do so via the p 42/p44 MAPK signaling pathway. We demonstrate that EGF stimulates MAPK phosphorylation and activity in MEPM cells maximally at 5 minutes. Ty rosine phosphorylation and activation of MAPK was unaffected by treatm ent of MEPM cells with TGF beta or cholera toxin. Similarly, TGF beta altered neither ECF-induced MAPK tyrosine phosphorylation nor activity . However, the calcium ionophore, A23187, significantly increased MAPK phosphorylation which was further increased in the presence of EGF, a lthough calcium mobilization reduced EGF-induced proliferation. Despit e the increase in phosphorylation, we could not demonstrate induction of MAPK activity by A23187. Like EGF, phorbol ester, under conditions which activate PKC isozymes in MEPM cells, increased MAPK phosphorylat ion and activity but was also growth inhibitory to MEPM cells. The MEK inhibitor, PD098059, only partially abrogated ECF-induced phosphoryla tion. Likewise, depletion of PKC isozymes partially abrogated ECF-indu ced MAPK phosphorylation. Inhibition of both MEK and PKC isozymes resu lted in a marked decrease in MAPK activity, confirming that EGF uses m ultiple pathways to stimulate MAPK activity. These data indicate that the MAPK cascade does not mediate signal transduction of several agent s that inhibit growth in MEPM cells, and that there is a dissociation of the proliferative response and MAP kinase activation. Furthermore, other signaling pathways known to play significant roles in differenti ation of palatal tissue converge with the MAPK cascade and may use thi s pathway in the regulation of alternative cellular processes. (C) 199 8 Wiley-Liss, Inc.