PROTEIN PHOSPHATASE-1 AND INSULIN ACTION

Protein Phosphatase-1 (PP-1) appears to be the key component of the in sulin signalling pathway which is responsible for bridging the initial insulin-simulated phosphorylation cascade with the ultimate dephospho rylation of insulin sensitive substrates. Dephosphorylations catalyzed by PP-1 activate glycogen synthase (GS) and simultaneously inactivate phosphorylase a and phosphorylase kinase promoting glycogen synthesis . Our in vivo studies using L6 rat skeletal muscle cells and freshly i solated adipocytes indicate that insulin stimulates PP-1 by increasing the phosphorylation status of its regulatory subunit (PP-1(G)). PP-1 activation is accompanied by an inactivation of Protein Phosphatase-2A (PP-2A) activity. To gain insight into the upstream kinases that medi ate insulin-stimulated PP-1(G) phosphorylation, we employed inhibitors of the ras/MAPK, PI3-kinase, and PKC signalling pathways. These inhib itor studies suggest that PP-1, phosphorylation is mediated via a comp lex, cell type specific mechanism involving PI3-kinase/PKC/PKB and/or the ras/MAP kinase/Rsk kinase cascade. CAMP agonists such as SpcAMP (v ia PKA) and TNF-alpha (recently identified as endogenous inhibitor of insulin action via ceramide) block insulin-stimulated PP-1(G) phosphor ylation with a parallel decrease of PP-1 activity, presumably due to t he dissociation of the PP-1 catalytic subunit from the regulatory G-su bunit. It appears that any agent or condition which interferes with th e insulin-induced phosphorylation and activation of PP-1, will decreas e the magnitude of insulin's effect on downstream metabolic processes. Therefore, regulation of the PP-1(G) subunit by site-specific phospho rylation plays an important role in insulin signal transduction in tar get cells. Mechanistic and functional studies with cell lines expressi ng PP-1(G) subunit site-specific mutations will help clarify the exact role and regulation of PP-1(G) site-specific phosphorylations on PP-1 catalytic function.