Effect of lead on nitric oxide synthase expression in coronary endothelialcells - Role of superoxide

Chronic exposure to low levels of lead causes hypertension (HTN) in humans and animals. We have previously shown that increased reactive oxygen specie s (ROS) leads to enhanced NO inactivation, depressed NO bioavailability, an d compensatory upregulation of NO synthases (NOSs) in rats with lead-induce d HTN. We have further demonstrated increased ROS generation with lead expo sure in cultured endothelial cells. In the present study, we tested the eff ect of lead (medium containing lead acetate, 1 ppm) alone and with either t he superoxide dismutase-mimetic agent tempol or a potent antioxidant lazaro id compound (both at 10(-8) and 10(-7) mol/L) on endothelial NOS expression and NO production in cultured human coronary endothelial cells. Lead-treat ed cells showed a significant upregulation of endothelial NOS (eNOS) protei n abundance (P<0.002) and a significant increase in the production of NO me tabolites (NO2- +NO3- =NOx, P<0.01). Cotreatment with either tempol or laza roid abrogated the lead-induced upregulation of eNOS protein and NOx produc tion. In contrast, tempol and lazaroid had no effect on either eNOS protein expression or NOx production in the control cells. Thus, lead exposure upr egulated eNOS expression in vitro, simulating the results of our previous i n vivo studies. This phenomenon points to a direct as opposed to an indirec t (eg, HTN-mediated) effect of lead on NO metabolism. The reversal of lead effect by lazaroid and the cell-permeable superoxide dismutase-mimetic agen t tempol suggests that lead exposure increases generation and/or reduces di smutation of superoxide, which in turn promotes oxidative stress, enhances NO inactivation, and elicits a compensatory upregulation of eNOS whose expr ession is negatively regulated by NO.