STIMULATION OF G(S) AND INHIBITION OF G(I) PROTEIN FUNCTIONS BY MINIMALLY OXIDIZED LDL

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.