Peroxynitrite, a marker of oxidative stress, is elevated in conditions associated with vascular endothelial cell dysfunction, such as atherosclerosis, preeclampsia, and diabetes. However, the effects of peroxynitrite on endothelial cell function are not clear. The endothelium-derived enzymes nitric oxide synthase (NOS) and prostaglandin H synthase (PGHS) mediate vascular reactivity and contain oxidant-sensitive isoforms (iNOS and PGHS-2) that can be induced by nuclear factor (NF)-kappaB activation. We investigated the effect(s) of peroxynitrite on NOS and PGHS pathways in endothelial cells. We hypothesized that peroxynitrite will increase levels of iNOS and PGHS-2 through activation of NF-kappaB. Western immunoblots of endothelial cells show that 3-morpholinosydnonimine (SIN-1; 0.5 mM), a peroxynitrite donor, increased iNOS protein mass, which can be inhibited by pyrroline dithiocarbamate (an NF-kappaB inhibitor) (167 +/- 24.2 vs. 78 +/- 19%, P < 0.05, n = 6). SIN-1 treatment also significantly increased NF-kappaB translocation into endothelial cell nuclei (135 +/- 10%, P < 0.05). Endothelial NOS, PGHS-1, and PGHS-2 protein levels were not altered by SIN-1. However, prostacyclin synthase protein mass, but not mRNA, was significantly reduced in SIN-1-treated endothelial cells (78 +/- 8.9%, P < 0.05). Our results illustrate novel mechanisms through which peroxynitrite may modulate vascular endothelial function.