Stimulation of Gs and inhibition of Gi 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 of 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 Gs complexes. Studies with pertussis toxin-treated membranes in which Gi was completely inhibited were used to directly address the effect of MM-LDL on the Gs 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 Gs stimulation. These results also show that MM-LDL increases cAMP levels by inhibiting Gi. MM-LDL inhibited ADP ribosylation of Gi by about 30% and completely abolished the ability of serotonin to interact with Gi complexes, whereas direct activation of Gi by mastoparan was not inhibited. This observation suggests that MM-LDL interferes with the interaction of Gi 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 Gi and inhibiting Gi complexes.
Parhami
F
F
Department of Medicine, UCLA School of Medicine, USA.
Fang
Z T
ZT
Yang
B
B
Fogelman
A M
AM
Berliner
J A
JA
D000262:Adenylyl Cyclases / Q000378:metabolism*; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000378:metabolism; D004730:Endothelium, Vascular / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006801:Humans; D015227:Lipid Peroxidation; D008077:Lipoproteins, LDL / Q000378:metabolism* / Q000494:pharmacology; D015398:Signal Transduction
D008077:Lipoproteins, LDL; D000242:Cyclic AMP; D019204:GTP-Binding Proteins; D000262:Adenylyl Cyclases