Simultaneous activation of p90rsk and p70s6k S6 kinases by growth hormone in 3T3-F442A preadipocytes.

Growth hormone promotes the differentiation of 3T3-F442A preadipocytes via unknown mechanisms. The ability of growth hormone to enhance the phosphorylation of ribosomal protein S6 through the activation of S6 kinases in this cell line was investigated. Growth hormone rapidly stimulated an S6 phosphotransferase activity measured in unpurified extracts. Using specific antisera, this activity was resolved into two components, comprising the p90rsk and p70s6k S6 kinases. Activation of these enzymes occurred simultaneously within minutes but proceeded with distinct time courses. p90rsk activation was transient, down-regulating within 60 min, whereas p70s6k activation was sustained beyond this time. The degree of activation of both S6 kinases by growth hormone closely paralleled their apparent phosphorylation status, with multi-site phosphorylation associated with full activation. Pretreatment of cells with a selective inhibitor of protein kinase C prevented activation of both S6 kinases by growth hormone but not by epidermal growth factor.