Role of nitric oxide in the cytokine-mediated regulation of cytochrome P-450.

We explored the effects of cytokines on cytochrome P-450 (CYP) in rat hepatocyte primary cultures. CYP content and several CYP protein levels were assessed in hepatocytes treated with a cytokine combination consisting of tumor necrosis factor-alpha (TNF-alpha), interleukin-1 beta (IL-1 beta), and interferon-gamma (IFN gamma). The combination was found to depress CYP content by 69 +/- 6%. Protein levels of CYP forms 1A2, 2C11, 2B1/2, and 3A2 were assessed with immunoblotting. Treatment with the cytokine combination resulted in a decrease in each CYP enzyme, with CYP2B1/2 exhibiting the greatest loss, to 33 +/- 9% of untreated cells. The addition of inhibitors of nitric oxide synthase (NOS) significantly prevented the cytokine-mediated decrease in each CYP protein, indicating a role for nitric oxide (NO) in the down-regulation. Treatment of hepatocytes with the NO donor 1-hydroxy-2-oxo-3,3-bis(3-aminoethyl)-1-triazene (300 microM) caused a decrease in each CYP apoprotein, with CYP2B1/2 exhibiting the greatest decrease, to 33 +/- 8% of untreated cells. Decreases in CYP protein levels were observed in response to treatment with TNF alpha, IL-1 beta, or IL-6 alone. With IL-1 beta treatment, increased levels of NO production were accompanied by decreased levels of each CYP protein. With TNF alpha treatment, increased levels of NO production were accompanied by decreased levels of CYP2B1/2 and CYP3A2. The effects of IL-1 beta and TNF alpha were blocked by the inclusion of the NOS inhibitors. Conversely, IL-6 caused a decrease in each of the CYP enzymes but did not affect NO production. The results indicate a dissociation in vitro between NOS induction and CYP down-regulation for IL-6 treatment, whereas the down-regulation of CYP by TNF alpha and IL-1 beta in vitro is directly associated with NO production.