Neurosteroid hydroxylase CYP7B: Vivid reporter activity in dentate gyrus of gene-targeted mice and abolition of a widespread pathway of steroid and oxysterol hydroxylation

The major adrenal steroid dehydroepiandrosterone (DHEA) enhances memory and immune function but has no known dedicated receptor; local metabolism may govern its activity. We described a cytochrome P450 expressed in brain and other tissues, CYP7B, that catalyzes the 7α-hydroxylation of oxysterols and 3β-hydroxysteroids including DHEA. We report here that CYP7B mRNA and 7α-hydroxylation activity are widespread in rat tissues. However, steroids related to DHEA are reported to be modified at positions other than 7α, exemplified by prominent 6α-hydroxylation of 5α-androstane-3β,17β-diol (A/anediol) in some rodent tissues including brain. To determine whether CYP7B is responsible for these and other activities we disrupted the mouse Cyp7b gene by targeted insertion of an IRES-lacZ reporter cassette, placing reporter enzyme activity (β-galactosidase) under Cyp7b promoter control. In heterozygous mouse brain, chromogenic detection of reporter activity was strikingly restricted to the dentate gyrus. Staining did not exactly reproduce the in situ hybridization expression pattern; post-transcriptional control is inferred. Lower level staining was detected in cerebellum, liver, and kidney, and which largely paralleled mRNA distribution. Liver and kidney expression was sexually dimorphic. Mice homozygous for the insertion are viable and superficially normal, but ex vivo metabolism of DHEA to 7α-hydroxy-DHEA was abolished in brain, spleen, thymus, heart, lung, prostate, uterus, and mammary gland; lower abundance metabolites were also eliminated. 7α-Hydroxylation of 25-hydroxycholesterol and related substrates was also abolished, as was presumed 6α-hydroxylation of A/anediol. These different enzyme activities therefore derive from the Cyp7b gene. CYP7B is thus a major extrahepatic steroid and oxysterol hydroxylase and provides the predominant route for local metabolism of DHEA and related molecules in brain and other tissues.