Endothelin enhances lipopolysaccharide-induced expression of inducible nitric oxide synthase in rat glial cells

Lipopolysaccharide is known to stimulate production of nitrite via expression of inducible nitric oxide (NO) synthase in not only macrophages but also glial cells. We found that in glial cell cultures lipopolysaccharide-stimulated inducible NO synthase expression and nitrite accumulation were synergistically enhanced by pretreatment with endothelin, whereas endothelin itself did not induce these responses. Pretreatment with endothelin-1, endothelin-3, and the selective endothelin type B (ETB) receptor agonist IRL 1620 caused the same effect with similar potencies, suggesting that the synergism was mediated via the endothelin ETB receptor. A protein kinase C inhibitor, calphostin C, suppressed endothelin-3-enhanced inducible NO synthase expression. Pretreatment with either endothelin-3 or phorbol ester enhanced lipopolysaccharide-induced production of tumor necrosis factor-alpha (TNF-alpha). Simultaneous addition of TNF-alpha increased lipopolysaccharide-stimulated inducible NO synthase expression. These results suggest that the increase in inducible NO synthase expression by endothelin was due to the elevated TNF-alpha production via protein kinase C. Our findings present the possibility that endothelin is implicated in neurotoxicity via enhancement of inducible NO synthase expression.