Effect of insulin on farnesyltransferase activity in 3T3-L1 adipocytes.

Activation of p21(ras) by GTP loading is a critical step in a cascade of intracellular insulin signaling. Farnesylation of p21(ras) protein is an obligatory event that facilitates Ras migration to the plasma membrane and subsequent activation. Farnesyltransferase (FTase) is a ubiquitous enzyme that catalyzes the lipid modification of p21(ras) by the addition of farnesyl to the C-terminal "CAAX" motif. In vitro and in vivo FTase activities were studied in 3T3-L1 adipocytes in response to insulin challenge. Insulin exerted a biphasic stimulatory effect on FTase activity measured in vitro with a 31% increase at 5 min and a 130% increase at 60 min. Insulin-stimulated farnesylation of p21(ras) pools in vivo correlated with FTase activity seen in vitro by displaying an increase in farnesylated p21(ras) from 40% of total cellular Ras in control cells to 63% by 5 min and 80% by 60 min (p < 0.05) in insulin-treated cells. Insulin challenge of 3T3-L1 adipocytes increased incorporation of tritiated mevalonic acid in p21(ras) in a dose-dependent manner and stimulated a 2-fold increase in phosphorylation of the alpha-subunit of FTase at 5 min and a 4-fold increase at 60 min.