MITOGEN-ACTIVATED PROTEIN-KINASE AND P70 RIBOSOMAL-PROTEIN S6 KINASE ARE NOT INVOLVED IN THE INSULIN-DEPENDENT STIMULATION OF CAMP-PHOSPHODIESTERASE KINASE IN RAT ADIPOCYTES

To elucidate the mechanism of anti-lipolytic action of insulin in rat epididymal adipocytes, we explored the potential mechanism that might be involved in the hormone-dependent stimulation of cAMP phosphodieste rase (PDE) kinase. PDE kinase was assayed in a cell-free system. Both wortmannin and LY294002: highly specific inhibitors of phosphatidylino sitol 3-kinase, almost completely blocked the hormonal effect not only on PDE kinase but also on mitogen-activated protein (MAP) kinase. Nei ther PD98059, a specific inhibitor of MAP kinase, nor rapamycin. a pot ent inhibitor of insulin-dependent stimulation of p70 ribosomal protei n S6 kinase (p70(S6K)), had inhibitory effect on that of PDE kinase. T hese results are consistent with the view that (i) insulin-activated P DE kinase as well as MAP kinase and p70(S6K) are localized downstream of phosphatidylinositol 3-kinase, (ii) PDE kinase is distinct from eit her MAP kinase or p70(S6K) and (iii) PDE kinase does not exist downstr eam of either MAP kinase or p70(S6K). It is suggested that PDE kinase and MAP kinase or p70(S6K) may be localized in sepal ate branches of t he cascade of insulin action. The branching point of the cascade could be either at or below the level of phosphatidylinositol 3-kinase. (C) 1998 Elsevier Science B.V.