Regulation of pp90^rsk Phosphorylation and S6 Phosphotransferase Activity in Swiss 3T3 Cells by Growth Factor-, Phorbol Ester-, and Cyclic AMP-Mediated Signal Transduction

Abstract

Somatic cell homologs to the Xenopus laevis S6 protein kinases (referred to collectively as pp90^rsk) have recently been identified and partially characterized. Here we examine alterations in pp90^rsk phosphorylation and S6 phosphotransferase activity in response to regulators of multiple signal transduction systems: purified growth factors, phorbol ester, changes in cyclic AMP (cAMP) levels, and sodium vanadate. All reagents tested increased pp90^rsk serine and threonine phosphorylation, but only those agents that regulate cell proliferation and sodium vanadate activated its S6 kinase activity. In addition to the cAMP-stimulated phosphorylation of pp90^rsk, a simple correlation between the extent of growth-regulated pp90^rsk phosphorylation and S6 phosphotransferase activity was not observed. Quantitative phosphorylation of pp90^rsk continued to increase after its S6 kinase activity began its return towards basal levels. However, a close correlation between the appearance and disappearance of a slow-mobility form of phosphorylated pp90^rsk (by electrophoresis) and pp90^rsk activity was observed. In addition, pp90^rsk was regulated by both protein kinase C-independent and -dependent signaling mechanisms. The extent of protein kinase C participation, however, varied depending on which growth factor receptor was activated. Furthermore, growth factor-specific differences in the temporal regulation of pp90^rsk S6 phosphotransferase activity were also observed. These results support the notion that the complex regulation of the rsk gene product constitutes one of the primary responses of animal cells to mitogenic signals.