DIFFERENTIAL INDUCTION OF NITRIC-OXIDE SYNTHASE IN HEPATOCYTES DURINGENDOTOXEMIA AND THE ACUTE-PHASE RESPONSE

Objective: Nitric oxide (NO) is a potent biologic mediator produced by hepatocytes following exposure to cytokines and lipopolysaccharide (L PS). These cytokines are also known to regulate induction of the hepat ic acute-phase response. The objective of this study was to determine whether inducible nitric oxide synthase (iNOS), the enzyme that produc es NO, is expressed as part of the hepatic acute-phase response. Desig n: The gene expression for inducible NOS (iNOS) as well as alpha(1)-ac id glycoprotein (AGP), an established acute-phase reactant, was measur ed by Northern blot analysis in rat hepatocytes in vivo during endotox emia (LPS injection) and during the acute phase response produced by h indlimb turpentine injection. Hepatocyte iNOS messenger RNA (mRNA) lev els were correlated with iNOS activity and circulating plasma nitrite and nitrate levels. In vitro, iNOS and AGP mRNA levels were determined in cultured hepatocytes stimulated with interleukin 6 (IL-6), interle ukin 1 beta (IL-1 beta), tumor necrosis factor alpha (TNF-alpha), or d examethasone. Results: The AGP mRNA levels were increased in vivo foll owing both LPS and turpentine injection, while iNOS expression was ind uced only by LPS injection. Hepatocyte iNOS activity and plasma nitrit e and nitrate levels also increased after LPS treatment. In vitro, the cytokine combination IL-6, IL-1 beta, and TNF-alpha induced hepatocyt e iNOS expression but had minimal effects on AGP in the absence of dex amethasone. Addition of dexamethasone alone markedly increased AGP mRN A levels, with further increases seen with TNF-alpha or IL-1 beta addi tion. In contrast, dexamethasone decreased iNOS expression. Conclusion : The results show that hepatocyte iNOS expression is not part of the acute-phase response induced by remote inflammation and indicates that iNOS is differentially regulated from the acute-phase reactant, AGP.