Cholesterol cell content modulates GTPase activity of G proteins in GH 4C 1 cell membranes

Abstract

Previous results from our laboratory showed that GH 4C 1 cells with low-cholesterol cell content had increased adenylyl cyclase (AC) activity with a parallel increase in G protein α subunits associated to the plasma membrane. This effect was directly related to mevalonate availability. In the present report, we characterized the high-affinity GTPase activity present in GH 4C 1 cell membranes and studied its regulation by cholesterol cell content. The high-affinity GTPase activity, measured as the [γ 32P]GTP hydrolysis rate, was both time-dependent and protein concentration-dependent. Cultured cells with lipoprotein-deficient serum (LPDS) showed decreased cholesterol cell content and decreased GTPase activity. The kinetic analysis, as interpreted by Lineweaver–Burk plots, indicated that low-cholesterol cell content had no effect on the apparent affinity for GTP, but resulted in a 47% decrease in the maximal velocity of the reaction. Addition of 25-hydroxycholesterol (25-HC), an inhibitor of the expression of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and synthetase to cells in LPDS, further decreased GTPase activity in a dose-dependent manner. This effect was reverted by exogenous cholesterol, but not by mevalonate. Studies with bacterial toxins revealed that neither cholera toxin (CTX) nor pertussis toxins (PTX) were able to revert the inhibition produced by low-cholesterol cell content. These results allowed us to postulate that cholesterol modulates GTPase activity in both Gs and Gi protein families. To analyse further the mechanism of modulation of GTPase activity by cholesterol cell content, [ 35S]GTPγS binding in membranes of GH 4C 1 cells was studied. Changes in cholesterol cell content did not have any effect on GTP binding. Data demonstrated that high-affinity GTPase activity in plasma membrane of GH 4C 1 cells is direct stimulated by cholesterol cell content and not by mevalonate availability. This example provides a mechanism by which cholesterol cell content can modulate signal transduction mediating by G proteins.