Stimulation of Gsand Inhibition of GiProtein Functions by Minimally Oxidized LDL

Abstract

We have previously shown that treatment of aortic endothelial cells with minimally oxidized LDL (MM-LDL) induces their interaction with monocytes but not neutrophils and that these induced responses are associated with increased cAMP levels. Here we studied the mechanism by which MM-LDL elevates cAMP levels. Treatment of human aortic endothelial cells with MM-LDL resulted in a saturable dose-dependent increase in cAMP levels. Studies using a combination of pertussis toxin and MM-LDL suggested that part of the cAMP increase was due to the stimulation of G_s complexes. Studies with pertussis toxin–treated membranes in which G_i was completely inhibited were used to directly address the effect of MM-LDL on the G_s pathway. MM-LDL and an oxidized lipid (palmitoyl arachidonyl phosphatidylcholine), the effects of which mimic those of MM-LDL, caused a 40% to 100% increase in cAMP levels in these isolated membranes that was augmented by GTP, thus showing G_s stimulation. These results also show that MM-LDL increases cAMP levels by inhibiting G_i. MM-LDL inhibited ADP ribosylation of G_i by about 30% and completely abolished the ability of serotonin to interact with G_i complexes, whereas direct activation of G_i by mastoparan was not inhibited. This observation suggests that MM-LDL interferes with the interaction of G_i molecules with inhibitory receptors. There was no direct effect of MM-LDL on adenylate cyclase. Overall, these studies show that MM-LDL increases cAMP levels both by stimulating G_s and inhibiting G_i complexes.