Role of hepatocyte nuclear factors in transcriptional regulation of male-specific CYP2A2

Cytochrome P450 2A2 (CYP2A2) is an adult male-specific rat liver steroid hydroxylase whose sex-dependent expression is regulated at the transcriptional level by sexually dimorphic pituitary growth hormone (GH) secretory patterns. In contrast to CYP2C11 and other male-specific, plasma GH pulse-inducible liver genes, CYP2A2 is highly expressed in hypophysectomized rat liver, despite the absence of GH stimulation. CYP2A2 promoter fragments 0.9-6.2 kb long exhibited unusually high basal promoter activity when transfected into the liver cell line HepG2. A further similar to2.5-fold increase in activity was obtained by cotransfection of hepatocyte nuclear factor (HNF) 3gamma or HNF4alpha. CYP2A2 promoter activity was inhibited similar to85% by transfection of HNF3beta or HNF6, both of which are more highly expressed in female than male liver and can strongly trans-activate the female-specific CYP2C12 promoter. The male GH pulse-activated transcription factor STAT5b had no effect on CYP2A2 promoter activity, either alone or in combination with HNF3gamma and HNF4alpha, consistent with the GH pulse-independence of CYP2A2 expression. By contrast, STAT5b synergistically enhanced the transcriptional activity of HNF4alpha toward two other male-specific liver target genes, Cyp2d9 and CYP8B1. Furthermore, STAT5b in combination with the HNF4alpha coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha strongly enhanced the transcriptional activity of HNF4alpha toward CYP8B1 but not toward CYP2A2. These findings support the hypothesis that sex-dependent HNFs contribute to the sexually dimorphic expression of CYP2A2 and other liver CYPs and highlight the ability of STAT5b to act in concert with HNF4alpha to regulate select male-specific liver CYP genes.