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 the levels of cellular phosphoinositides were investigated in 3T3-L1 adipocytes. Stimulation for 4 min with PMA (1 microM) or insulin (10 nM) increased levels of PtdIns(3,4,5)P3 approx. 2-fold and 6-fold respectively. PMA also had a small effect on the cellular levels of PtdIns4P, whereas insulin had no effect on PtdIns4P levels; levels of PtdIns(4,5)P2 and PtdIns3P were not significantly affected by either agent. Insulin 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 activity in anti-phosphotyrosine or anti-p85 immunoprecipitates. The stimulation of glucose transport by insulin or PMA was blocked by 100 nM wortmannin or 10 ng/ml LY294002, indicating that PI 3-kinase is essential for stimulation by both agents. In summary, these results demonstrate: (1) that PMA and insulin stimulate PtdIns(3,4,5)P3 production by distinct mechanisms in 3T3-L1 adipocytes, and (2) that stimulation of PtdIns(3,4,5)P3 production by PMA is likely to be important in signalling pathways leading from PMA stimulation to end-point responses such as glucose transport.