Down-regulation of phenobarbital-induced cytochrome P4502B mRNAs and proteins by endotoxin in mice: independence from nitric oxide production by inducible nitric oxide synthase.

Multiple hepatic cytochrome P450 enzymes are down-regulated at the mRNA and protein levels during inflammation and infection. A body of evidence suggests that nitric oxide (NO) produced from inducible NO synthase (NOS2) is responsible for some of these effects. The current study was designed to examine the NO dependencies of the down-regulation of phenobarbital-induced CYP2B mRNAs and proteins by bacterial endotoxin (lipopolysaccharide, LPS) treatment in vivo, using an NOS2-null mouse model. Treatment of C57/BL6 mice with 0.3 mg/kg of LPS maximally suppressed phenobarbital-induced CYP2B9 and 2B10 mRNAs measured 12 hr after injection, whereas 1-10 mg/kg of LPS was required to elevate NO production. Down-regulation of CYP2B mRNAs by 1 mg/kg of LPS was equivalent in wild-type and NOS2-null mice. No effect of LPS in the dose range of 0.3 to 10 mg/kg was observed on microsomal CYP2B protein levels measured 12 hr after treatment, whereas 1 mg/kg of LPS suppressed CYP2B proteins 24 hr after treatment in both wild-type and NOS2-null mice. We conclude that the main mechanism for the down-regulation of CYP2B proteins in mouse liver following moderate- or high-dose LPS treatment is via NO-independent suppression of CYP2B9 and 2B10 mRNAs. Unlike rat hepatocytes, the contribution of a rapid, NO-dependent mechanism of CYP2B protein suppression in mouse liver appears to be minor or non-existent.