Effects of insulin and phorbol esters on MARCKS (myristoylated alanine-rich C-kinase substrate) phosphorylation (and other parameters of protein kinase C activation) in rat adipocytes, rat soleus muscle and BC3H-1 myocytes.

To evaluate the question of whether or not insulin activates protein kinase C (PKC), we compared the effects of insulin and phorbol esters on the phosphorylation of the PKC substrate, i.e. myristoylated alanine-rich C-kinase substrate (MARCKS). In rat adipocytes, rat soleus muscle and BC3H-1 myocytes, maximally effective concentrations of insulin and phorbol esters provoked comparable, rapid, 2-fold (on average), non-additive increases in the phosphorylation of immunoprecipitable MARCKS. These effects of insulin and phorbol esters on MARCKS phosphorylation in intact adipocytes and soleus muscles were paralleled by similar increases in the phosphorylation of an exogenous, soluble, 85 kDa PKC substrate (apparently a MARCKS protein) during incubation of post-nuclear membrane fractions in vitro. Increases in the phosphorylation of this 85 kDa PKC substrate in vitro were also observed in assays of both plasma membranes and microsomes obtained from rat adipocytes that had been treated with insulin or phorbol esters. These insulin-induced increases in PKC-dependent phosphorylating activities of adipocyte plasma membrane and microsomes were associated with increases in membrane contents of diacylglycerol, PKC-beta1 and PKC-beta2. Our findings suggest that insulin both translocates and activates PKC in rat adipocytes, rat soleus muscles and BC3H-1 myocytes.