SPECIFIC INCREASE IN P85-ALPHA EXPRESSION IN RESPONSE TO DEXAMETHASONE IS ASSOCIATED WITH INHIBITION OF INSULIN-LIKE GROWTH-FACTOR-I STIMULATED PHOSPHATIDYLINOSITOL 3-KINASE ACTIVITY IN CULTURED MUSCLE-CELLS

The stimulation of phosphatidylinositol (PI) 3-kinase by insulin-like growth factor I (IGF-I) in L6 cultured skeletal muscle cells is inhibi ted by the glucocorticoid dexamethasone. The objective of this study w as to investigate the mechanism of dexamethasone action by determining its effects on the expression of the p85 alpha and p85 beta regulator y subunit isoforms of PI 3-kinase, their coupling with the p110 cataly tic subunit, and their association with insulin receptor substrate 1 ( IRS-1) in response to ICE-I stimulation. Dexamethasone induced a 300% increase in p85 alpha protein content in the L6 cultured myoblast cell line, whereas it increased p110 content by only 38% and had no effect on p85 beta. The increase in p85 alpha protein was associated with a coordinate increase in p85 alpha mRNA. Stimulation with IGF-I induced the association of p85 alpha and p85 beta with IRS-1, and this was acc ompanied by increased amounts of the p110 catalytic subunit and marked ly increased PI 3-kinase activity in IRS-1 immunoprecipitates. In cell s treated with dexamethasone, greater amounts of p85 alpha and lower a mounts of p85 beta, respectively, were found in IRS-l immunoprecipitat es, such that the alpha/beta ratio was markedly higher than in control cells. In spite of the increase in both total and IRS-1-associated p8 5 alpha following dexamethasone treatment, IRS-1-associated p110 catal ytic subunit and PI 3-kinase activity were decreased by approximately 50%. Thus, dexamethasone induces a specific increase in expression of the p85 alpha regulatory subunit that is not associated with a coordin ate increase in the p110 catalytic subunit of PI 3-kinase. As a conseq uence, in dexametbasone-treated cells, p85 alpha that is not coupled w ith p110 competes with both p85 alpha p110 and p85 beta p110 complexes for association with IRS-1, leading to increased p85 alpha but decrea sed p85 beta, p110, and PI 3-kinase activity in IRS-1 immunoprecipitat es.