PHORBOL ESTERS STIMULATE PHOSPHATIDYLINOSITOL 3,4,5-TRISPHOSPHATE PRODUCTION IN 3T3-L1 ADIPOCYTES - IMPLICATIONS FOR STIMULATION OF GLUCOSE-TRANSPORT

The effects of insulin and phorbol 12-myristate 13-acetate (PMA) on th e levels of cellular phosphoinositides were investigated in 3T3-L1 adi pocytes. Stimulation for 4 min with PMA (1 mu M) or insulin (10 nM) in creased levels of PtdIns(3,4,5)P-3 approx. 2-fold and 6-fold respectiv ely. PMA also had a small effect on the cellular levels of PtdIns4P, w hereas insulin had no effect on PtdIns4P levels; levels of PtdIns(4,5) P-2 and PtdIns3P were not significantly affected by either agent. Insu lin increased the levels of the p85 alpha subunit of phosphoinositide (PI) 3-kinase associated with membranes, whereas PMA decreased levels of membrane-associated p85 alpha. PMA did not increase PI 3-kinase act ivity in anti-phosphotyrosine or anti-p85 immunoprecipitates. The stim ulation of glucose transport by insulin or PMA was blocked by 100 nM w ortmannin or 10 ng/ml LY294002, indicating that PI 3-kinase is essenti al for stimulation by both agents. In summary, these results demonstra te: (1) that PMA and insulin stimulate PtdIns(3,4,5)P-3 production by distinct mechanisms in 3T3-L1 adipocytes, and (2) that stimulation of PtdIns(3,4,5)P-3 production by PMA is likely to be important in signal ling pathways leading from PMA stimulation to end-point responses such as glucose transport.