Effects of insulin, insulin-like growth factor-I, and phorbol esters on neutral cholesteryl esterase activity in cultured rat vascular smooth muscle cells.

We investigated the effects of insulin, insulin-like growth factor-I (IGF-I), and phorbol 12-myristate 13-acetate (PMA) on neutral cholesteryl esterase activity in cultured rat vascular smooth muscle cells. Insulin and IGF-I at concentrations between 10(-9) mol/L and 10(-6) mol/L significantly decreased neutral cholesteryl esterase activity in growth-arrested vascular smooth muscle cells in a dose-dependent manner but with no influences on the intracellular concentration of 3',5'-adenosine monophosphate (cyclic AMP). Treatment of cells with KT5720 (10(-7) mol/L to 10(-5) mol/L), a specific inhibitor of cyclic AMP-dependent protein kinase, significantly decreased neutral cholesteryl esterase activity in a dose-dependent manner. Incubation of cells for 6 to 12 hours with PMA (10(-9) mol/L to 10(-6) mol/L), an activator of protein kinase C, significantly increased neutral cholesteryl esterase activity in a dose-dependent manner. However, down-regulation of protein kinase C activity by long-term incubation (18 to 48 hours) with PMA resulted in a significant decrease in neutral cholesteryl esterase activity. Treatment of cells with UCN-01 (10(-7) mol/L to 10(-5) mol/L), a specific protein kinase C inhibitor, decreased the enzyme activity in a dose-dependent manner and completely blocked the activation of the enzyme by PMA. When insulin or IGF-I at a concentration of 10(-6) mol/L was present in the medium containing CL 277,082--an inhibitor of acyl coenzyme A:cholesterol acyltransferase--cellular cholesteryl ester content of the cells significantly increased. In contrast, after the treatment with PMA at a concentration of 10(-6) mol/L in the presence of CL 277,082, the net cholesteryl ester content of the cells significantly declined. These data suggest that both insulin and IGF-I may increase cholesteryl ester accumulation in arterial smooth muscle cells by decreasing arterial cholesteryl ester hydrolysis. The data also suggest that neutral cholesteryl esterase is activated not only by cyclic AMP-dependent protein kinase but also by protein kinase C. Thus growth factors may exert their antilipolytic or lipolytic actions specifically by modulating neutral cholesteryl esterase activity in vascular smooth muscle cells. Neutral cholesteryl esterase of vascular smooth muscle cells may be regulated by recholesteryl esterase of vascular smooth muscle cells may be regulated by reversible phosphorylation, with the phosphorylated form being the active form.