Inhibition of Gi proteins by low density lipoprotein attenuates bradykinin-stimulated release of endothelial-derived nitric oxide.

The mechanism(s) by which low density lipoprotein (LDL) attenuates the release of endothelial-derived nitric oxide (EDNO) is not known but may involve inhibition of membrane signal transduction. To test this hypothesis, we treated bovine aortic endothelial cells with LDL (0-500 micrograms/ml, 72 h) to determine its effect on G proteins (G(i) and Gq) which are known to couple the bradykinin receptor to EDNO release. Bradykinin-stimulated EDNO release was measured by perfusion bioassay and showed a maximal 56% decrease which was unaffected by cotreatment with pertussis toxin (PTX, 100 ng/ml, 16 h). Although radioligand binding studies and Western blotting did not detect any significant changes in the amount of bradykinin receptor, alpha i2, alpha i3, and alpha q, both agonist-stimulated GTPase activity and phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis were reduced by LDL treatment (77 and 70%, respectively). When G(i) and Gq function in LDL-treated membranes were assessed by PTX and COOH-terminal antisera to alpha i2 (P4), alpha i3 (JL14), and alpha q (Q17), only the Q17 antisera caused a further reduction in GTPase activity and PIP2 hydrolysis while treatment with PTX alone or in combination with the P4 and JL14 antisera had no further inhibitory effect. These results suggest that LDL attenuates EDNO release by preferentially inhibiting the function of Gi.