Role of Nitric Oxide in Downregulation of Cytochrome P450 1a1 and NADPH: Quinone Oxidoreductase 1 by Tumor Necrosis Factor‐α and Lipopolysaccharide

We previously demonstrated that tumor necrosis factor alpha (TNF‐α) and lipopolysaccharide (LPS) downregulate aryl hydrocarbon receptor (AhR)‐regulated genes, such as cytochrome P450 1a1 (Cyp1a1) and NADPH: quinone oxidoreductase 1 (Nqo1) gene expression, yet the mechanisms involved remain unknown. The correlation between the inflammation‐mediated suppression of AhR‐regulated genes and the TNF‐α or LPS‐induced nitric oxide (NO) production especially in murine hepatoma Hepa 1c1c7 cells has been questioned; therefore we investigated whether NO is involved in the modulation of Cyp1a1 and Nqo1 by TNF‐α or LPS in Hepa 1c1c7 cells. A significant dose‐dependent increase in the inducible nitric oxide synthase (NOS2) expression and NO production were observed by various concentrations of TNF‐α (1, 5, and 10 ng/mL) and LPS (1 and 5 µg/mL) which was completely inhibited by a NOS2 inhibitor, L‐N6‐(1‐iminoethyl) lysine (L‐NIL) (1 mM). Furthermore, TNF‐α and LPS significantly induced NOS2 expression. Both TNF‐α and LPS repressed the β‐naphthoflavone (βNF)‐mediated induction of Cyp1a1 and Nqo1 at mRNA and activity levels. The downregulation of Cyp1a1, but not Nqo1, was significantly prevented by L‐NIL. However, proxynitrite decomposer, iron tetrakis (N‐methyl‐4′‐pyridyl) porphyrinato (FeTMPyP) (5 µM) did not affect TNF‐α‐ and LPS‐mediated downregulation of Cyp1a1 and Nqo1 at mRNA and activity levels. These results show that NO, but not peroxynitrite, may be involved in TNF‐α‐ and LPS‐mediated downregulation of Cyp1a1 without affecting the downregulation of Nqo1. © 2007 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 96: 2795-2807, 2007