MITOGEN-ACTIVATED PROTEIN-KINASES P42(MAPK) AND P44(MAPK) ARE REQUIRED FOR FIBROBLAST PROLIFERATION

The mitogen-activated protein kinases (MAP kinases) p42mapk and p44map k are serine/threonine kinases rapidly activated in cells stimulated w ith various extracellular signals by dual phosphorylation of tyrosine and threonine residues. They are thought to play a pivotal role in int egrating and transmitting transmembrane signals required for growth an d differentiation. Here we demonstrate that activation of these ubiqui tously expressed MAP kinases is essential for growth. To specifically suppress MAP kinase activation in fibroblasts, we transiently expresse d either the entire p44mapk antisense RNA or p44mapk kinase-deficient mutants (T192A or Y194F). As expected, and through independent mechani sms, both approaches strongly inhibited MAP kinase activation. The ant isense reduced the expression of endogenous p42mapk and p44mapk by 90% , whereas overexpression of the T192A mutant inhibited growth factor a ctivation of both endogenous MAP kinases by up to 70%. As a consequenc e, we found that the antisense as well as the T192A mutant of p44mapk inhibited growth factor-stimulated gene transcription (collagenase pro moter assay with chloramphenicol acetyltransferase reporter) and cell growth. These effects were proportional to the extent of MAP kinase in hibition and reversed by coexpression of the wild-type p44mapk. Theref ore we conclude that growth factor activation of p42mapk and p44mapk i s an absolute requirement for triggering the proliferative response.