Direct and indirect effects of growth hormone receptor ablation on liver expression of xenobiotic metabolizing genes.

Detoxification of ingested xenobiotic chemicals, and of potentially toxic endogenous metabolites, is carried out largely through a series of enzymes synthesized in the liver, sometimes called "xenobiotic metabolizing enzymes" (XME). Expression of these XME is sexually dimorphic in rodents and humans, with many of the XME expressed at higher levels in females. This expression pattern is thought to be regulated, in part, by the sex differences in circadian growth hormone (GH) pulsatility. We have evaluated mRNA, in the liver, for 52 XME genes in male and female mice of four mutant stocks, with diminished levels of GH receptor (GHR) either globally (GKO), or in liver (LKO), fat (FKO), or muscle (MKO) tissue specifically. The data show complex, sex-specific changes. For some XME, the expression pattern is consistent with direct control of hepatic mRNA by GHR in the liver. In contrast, other XME show evidence for indirect pathways in which hepatic XME expression is altered by GH signals in fat or skeletal muscle. The effects of GHR-null mutations on glucose control, responses to dietary interventions, steroid metabolism, detoxification pathways, and lifespan may depend on a mixture of direct hepatic effects and cross talk between different GH-responsive tissues.