CHARACTERIZATION OF THE MITOGEN-ACTIVATED PROTEIN-KINASE 90-KILODALTON RIBOSOMAL-PROTEIN SB KINASE SIGNALING PATHWAY IN 3T3-L1 ADIPOCYTES AND ITS ROLE IN INSULIN-STIMULATED GLUCOSE-TRANSPORT

Insulin exerts diverse effects on mitogenesis, metabolism, gene expression, and protein synthesis depending on the target cell type. A variety of extracellular serine/threonine kinases, including the ribosomal protein S6 kinases pp70-ribosomal S6 kinase (pp70-S6K) and pp90-ribosomal S6 kinase (pp90(rsk)) and the erk-encoded mitogen-activated protein (MAP) kinases pp44(mapk)/ERK-1 and pp42(mapk)/ERK-2, have been postulated as mediators of insulin action. In this study, we have investigated the role of the MAP kinase/pp90(rsk) signaling pathway in insulin-stimulated glucose transport in 3T3-L1 adipocytes. Differentiation of 3T3-L1 fibroblasts into adipocyte-like cells was accompanied by a marked increase in the capacity of insulin to activate pp90(rsk) and pp44(mapk). Whereas the maximal insulin-stimulated pp90(rsk) and pp44(mapk) activities were only similar to 30% of the serum-stimulated activities in preadipocytes, the insulin-stimulated kinase activities in adipocytes were equal to or greater than the serum-stimulated activities. The increase in hormone receptor number accompanying differentiation accounted for the greater sensitivity, as overexpression of human insulin receptors in NIH-3T3 cells also conferred insulin-stimulatable kinase activity. In 3T3-L1 adipocytes, the stimulation of pp90(rsk) and pp44(mapk) activities was sufficiently rapid and hormone sensitive to convey a signal for increased hexose uptake. However, epidermal growth factor and fetal bovine serum were equipotent with insulin in stimulating pp90(rsk) and pp44(mapk) activities in adipocytes, but were without effect on hexose uptake. These data indicate that activation of these enzymes is not sufficient for the acute stimulation of transport.