No abstract

Bacterial lipopolysaccharide (LPS) and a diverse array of other immunostimulants and cytokines suppress the metaboism of endogenous and exogenous substances by reducing activity of the hepatic cytochrome P450 mixedfunction oxidase system. Although this effect of immunostimulants was first described almost 40 yr ago, the mechanism is obscure. Immunostimulants are now known to cause NO overproduction by celis via induction of nitric oxide synthase.We have investigated whether NO overproduction is involved in suppressing hepatic metabolism by LPS. In vitro treatment of hepatic microsomes with NO, produced by chemical decomposition of 3-morpholinosydnonimnue or by nitric oxide synthase, substantially suppressed cytochrome P450-dependent oxygenation reactions. This effect of NO was seen with hepatic microsomes prepared from two species (rat and chicken) and after exposure to chemicais that induce distinct molecular isoforms of cytochromes P450 (-naphthoflavone, 3-methylcholanthrene, and phenobarbital). Spectral studies indicate that NO reacts in vitro with both Fe2+-and Fe3+-hemes in microsomal cytochromes P450. In vivo, LPS diminished the phenobarbital-induced dealkylation of 7-pentoxyresorufin by rat liver microsomes and reduced the apparent P450 content as measured by CO binding. These LPS effects were associated with induction of NO synthesis; LPS-induced NO synthesis showed a strong positive correlation with the severity of cytochrome P450 inhibition. The decrease in both hepatic microsomal P450 activity and CO binding caused by LPS was largely prevented by the selective NO synthase inhibitor N"-nitro-L-arginine methyl ester. Our rmdings implicate NO overproduction as a major factor mediating the suppression of hepatic metabolism by immunostimulants such as LPS.NO is a secretory product of mammalian cells that is important in the regulation of vascular tone, platelet function, neurotransmission, and host-defense mechanisms (1-3). These physiological actions are attributable to the oxidation by NO of heme and nonheme iron and iron-sulfur complexes in the active sites of key metabolic enzymes. Thus, through its interaction with iron, NO modulates the activity of target proteins. Although NO is normally produced in relatively small quantities and presumably by particular cell types (e.g., endothelial cells and neurons), NO (PB) (CYP2B1/2), (-naphthoflavone (3-NF), and 3-methylcholanthrene (3-MC) (CYPlA1/2) (4-7).Immunological stimuli depress cytochrome P450-mediated hepatic metabolism of a variety of drugs and endogenous substances. Indeed, attenuated cytochrome P450 activity has been seen in animals after infection with bacteria and viruses or after treatment with cytokines (e.g., interleukin 1; interferon 'y, tumor necrosis factor a) and immunostimulants (e.g., LPS and single-stranded RNA) (8, 9). Despite wide acceptance that the immune system can inhibit hepatic drug metabolism, the mechanism of this effect is largely unknown. Our study reveals that NO, produced by immunoactivated cells, mediates sup...

The environmental toxin TCDD (2,3,7,8-tetrachlorodibenzop-dioxin, dioxin) produces diverse toxic effects including a lethal wasting syndrome whose hallmark is suppressed hepatic gluconeogenesis. All TCDD toxicities require activation of the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor. Whereas the mechanism for AHR induction of target genes is well understood, it is not known how AHR activation produces any TCDD toxicity. This report identifies for the first time an AHR target gene, TiPARP (TCDD-inducible poly(ADPribose) polymerase, PARP7) that can mediate a TCDD toxicity, i.e. suppression of hepatic gluconeogenesis. TCDD suppressed hepatic glucose production, expression of key gluconeogenic genes, phosphoenolpyruvate carboxykinase (PEPCK), and glucose-6-phosphatase (G6Pase), and NAD ؉ levels, and increased PARP activity and

Photooxidized tryptophan (TRP) in tissue culture medium elicits a transient cytochrome P450 (CYP1) induction response in cultured cells. We show here that exposure of TRP to window sunlight (aTRP) greatly increased the potency, efficacy, and duration of CYP1A induction by TRP in primary chick embryo hepatocytes and in vivo. Aqueous TRP exposed to sunlight for 7 days exhibited a 100-fold or greater increase in potency over TRP in medium. The induction response was sustained for at least 48 h and was comparable in efficacy to 2,3,7,8-tetrachlorodibenzo-p-dioxin. In hepatocytes, increases in mRNAs for CYP1A4 and CYP1A5, chick orthologs of mammalian CYP1A1 and 1A2, preceded increases in CYP1A proteins and enzyme activities, 7-ethoxyresorufin deethylase (EROD) for CYP1A4 and arachidonic acid epoxygenation for CYP1A5, consistent with a transcriptional mechanism. Aryl hydrocarbon receptor (AhR) dependence was evidenced by aTRP induction of EROD in wild-type Hepa1c1c7 cells but not in AhR-defective (c35) mutants. Preparations of aTRP were stable for many months at 4 degrees C and were relatively resistant to metabolism by hepatocytes or liver microsomes. Fractionation of aTRP by HPLC analysis coupled with EROD assays showed that aTRP contained multiple photoproducts and CYP1A inducing components, which varied in sensitivity to metabolism by hepatocytes. The previously identified TRP photoproduct, 6-formylindolo[3,2-b]carbazole (FICZ), was one component, but FICZ was not required for CYP1A induction by the aTRP mixture. These findings identify the indoor environment, and window sunlight in particular, as a new source of CYP1A inducers. Further, the evidence that biologically active metabolites of an endogenous substrate, arachidonic acid, are formed by aTRP-induced CYP1A provides a pathway by which TRP photoproducts, like toxic xenobiotics, could have significant physiologic effects.

No abstract