Effect of lead on nitric oxide synthase expression in coronary endothelial cells: 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 species (ROS) leads to enhanced NO inactivation, depressed NO bioavailability, and compensatory upregulation of NO synthases (NOSs) in rats with lead-induced HTN. We have further demonstrated increased ROS generation with lead exposure in cultured endothelial cells. In the present study, we tested the effect of lead (medium containing lead acetate, 1 ppm) alone and with either the superoxide dismutase-mimetic agent tempol or a potent antioxidant lazaroid compound (both at 10(-8) and 10(-7) mol/L) on endothelial NOS expression and NO production in cultured human coronary endothelial cells. Lead-treated cells showed a significant upregulation of endothelial NOS (eNOS) protein abundance (P:<0.002) and a significant increase in the production of NO metabolites (NO(2)(-) +NO(3)(-)=NOx, P:<0.01). Cotreatment with either tempol or lazaroid abrogated the lead-induced upregulation of eNOS protein and NO(x) production. In contrast, tempol and lazaroid had no effect on either eNOS protein expression or NO(x) production in the control cells. Thus, lead exposure upregulated eNOS expression in vitro, simulating the results of our previous in vivo studies. This phenomenon points to a direct as opposed to an indirect (eg, HTN-mediated) effect of lead on NO metabolism. The reversal of lead effect by lazaroid and the cell-permeable superoxide dismutase-mimetic agent tempol suggests that lead exposure increases generation and/or reduces dismutation of superoxide, which in turn promotes oxidative stress, enhances NO inactivation, and elicits a compensatory upregulation of eNOS whose expression is negatively regulated by NO.