INACTIVATION OF GLYCOGEN-SYNTHASE KINASE-3 BY EPIDERMAL GROWTH-FACTORIS MEDIATED BY MITOGEN-ACTIVATED PROTEIN KINASE P90 RIBOSOMAL-PROTEINS6 KINASE SIGNALING PATHWAY IN NIH/3T3 CELLS/

The role of the p90 ribosomal protein S6 kinase/mitogen-activated prot ein kinase (RSK/MAPK) signaling pathway in regulating glycogen synthas e kinase-3 (GSK-3) activity was investigated. In vitro studies showed that GSK-3 was inactivated by 50% upon incubation with RSK purified fr om epidermal growth factor (EGF)-stimulated NIH/3T3 cells. Subsequentl y, the effect of EGF on GSK-3 activity was measured in NIH/3T3 cells t hat stably overexpressed mutated forms of MAPK kinase (MAPKK). The act ivation of RSK by EGF was markedly decreased in cell lines expressing the dominant negative MAPKK mutants S222A and K97A and was increased i n cells expressing the S222E mutant as compared with control cell line s. EGF induced a rapid decrease in GSK-3 beta activity (50%) in contro l and S222E cells; however, only 25 and 10% inhibition in GSK-3 beta a ctivity was observed in cell lines expressing the dominant negative mu tants K97A and S222A, respectively, suggesting that inhibition of GSK- 3 was partially blocked in these cells. Taken together, these results suggest that the action of EGF on GSK-3 inactivation is mediated by th e RSK/MAPK signaling pathway in NIH/3T3 cells and provide evidence for a mechanism regulating GSK-3 activity in intact cells.