Oxidative stress disrupts nitric oxide synthase activation in liver endothelial cells

Oxidative stress may mediate vascular disruption associated with a loss of endothelial nitric oxide synthase (eNOS) activity and a hypersensitivity to the constrictor effects of endothelin-1 (ET-1). We hypothesize that this is due, in part, to uncoupling of ETB receptors from eNOS activation. Thus, we tested whether oxidative stress (OS) affects liver vascular relaxation by reducing basal and ET-1-induced NO production. Primary sinusoidal endothelial cell cultures were pretreated with H2O2 (25 μM) for 1 or 6 h before a 10-min ET-1 stimulation. OS resulted in a significant basal and ET-1-induced decrease in NO production. Acute OS increased the monomeric form of the inhibitory protein caveolin-1 (1.2 ± 0.05 vs 0.9 ± 0.02, p < 0.01) and increased the eNOS–caveolin association as determined by coimmunoprecipitation (1.24 ± 0.04 vs 0.97 ± 0.04, p < 0.05). ET-1 stimulation further exacerbated these effects. Subacute OS inhibited ET-1-induced eNOS phosphorylation of serine 1177 (activation residue) (1 ± 0.07 vs 1.6 ± 0.04, p < 0.05) and dephosphorylation of the inhibitory residue threonine 495 (1.5 ± 0.08 vs 0.7 ± 0.02, p < 0.01). Additionally subacute OS resulted in dissociation of eNOS from ETB (0.8 ± 0.09 vs 1.2 ± 0.06, p < 0.05). Our findings indicate that acute and subacute oxidative stress result in the inhibition of induced nitric oxide synthase activity through distinct mechanisms dependent on caveolin-1 inhibition, ETB dissociation, and eNOS phosphorylation.