Pretranslational hormonal control of male-specific cytochrome P-450 16 alpha in rat liver

Characterization of the proximal promoter and two silencer elements in the CYP2C11 gene expressed in rat liver

The cytochrome P450 gene CYP2C11, expressed in the liver of male rats, is transcriptionally regulated in a dual fashion by the sexually dimorphic secretion pattern of growth hormone. To enable analysis of transcriptional regulatory DNA elements, rat genomic sequences were cloned. DNase I hypersensitivity analysis of rat liver nuclei revealed the existence of two hypersensitive sites whose presence in the vicinity of the transcription start site correlates to high transcriptional activity of the gene. Deletion mutants of the 5' flank were fused to reporter genes and transiently transfected into HepG2 cells or into primary adult rat hypatocytes. Transfection experiments in combination with DNase I footprinting analysis in vitro led to the identification of two negative regulatory regions spanning nucleotides -1,230 to -1,188 and -409 to -368 and designated (SIL1200) and (SIL400), respectively. When placed in front of the heterologous thymidine kinase promoter, SIL1200 and SIL400 reduced the activity of the chloramphenicol acetyl transferase reporter gene to 13% and 23% of the control value, respectively. No sex-dependent binding of liver nuclear extracts to the two silencers could be detected by in vitro footprinting or gel retardation assays. However, a sex-dependent footprint consistently stronger with male liver nuclear extracts than with female extracts was observed in the -320 to -294 region. A significant level of identity was found between the DNA sequence corresponding to this footprint and that of orphan steroid receptor elements as well as with that of a basal transcription element common to several CYP2C genes. However, the identity of a potential trans-acting factor binding between -320 and -294 or response of this element to growth hormone is as yet unknown. A sex- and GH secretory profile-dependent protein-DNA interaction in vitro was observed in the -107 to -95 region. In spite of the sequence similarity that exists between this region and the consensus binding site for HNF-1, this region does not bind HNF-1 alpha. This element acted as a repressor on the heterologous thymidine kinase promoter. To date, the two silencer elements and possibly also the HNF-1-like element are the only functional elements defined in the CYP2C11 gene, and it is conceivable that induction of the gene involves derepression of the silencer elements.

Sex change in cytochrome P-450 phenotype by growth hormone treatment of adult rat hepatocytes maintained in a culture system on matrigel

Results of studies of hypophysectomized rats suggest that growth hormone serves as a final common mediator through which gonadal steroids and other modifiers of pituitary function alter the expression of gender-specific liver genes such as the sexually dimorphic pair of cytochrome P-450 isozymes, male-specific P-450h and female-specific P-450i. We tested the effects of growth hormone in a system for primary monolayer culture of adult rat hepatocytes on a laminin-rich extracellular matrix (matrigel), which permits sustained expression of both constitutive and inducible liver genes in a chemically defined medium. Cultures of freshly isolated hepatocytes prepared from untreated male rats and samples of the intact donor liver contained readily detectable quantities of immunoreactive P-450h protein (measured on immunoblots of cell microsomes) and P-450h mRNA (measured on Northern blots of cellular RNA). Neither P-450i immunoreactive protein nor P-450i mRNA were present. Addition of physiologic concentrations of human or bovine growth hormone, but not of prolactin, to culture medium lacking insulin or other hormones resulted in prompt induction of P-450i immunoreactive protein and P-450i mRNA. Induction of P-450i mRNA in male hepatocyte cultures was dependent on the concentration of growth hormone, required as little as 24 hr of exposure, and was markedly attenuated in cultures maintained on type I collagen rather than on matrigel. Growth hormone treatment also induced the level of mRNA for insulin-like growth factor I, whereas the amount of mRNA for the male-specific urinary protein alpha 2 mu-globulin was unaffected. Cultures of hepatocytes derived from untreated adult female rats retained high levels of P-450i mRNA but only if the culture medium contained growth hormone. None of the tested treatments with estrogens, androgens, glucocorticoids, or growth hormone induced P-450h mRNA or P-450h immunoreactive protein in cultures of female hepatocytes. We conclude that the somatogenic effects of growth hormone acting alone and directly on the hepatocyte in culture are sufficient to "feminize" the cytochrome P-450 phenotype. The present culture system offers a way to explore the molecular basis for hormonal control of liver gene expression.

cDNA cloning, sequence, and regulation of a major female-specific and growth hormone-inducible rat liver cytochrome P-450 active in 15 beta-hydroxylation of steroid sulfates

cDNA clones encompassing the complete coding sequence for the female-specific rat liver microsomal 15 beta-hydroxylase have been isolated and sequenced. This cytochrome P-450 species (P-450 15 beta) has a total of 490 amino acid residues, and its deduced amino acid sequence as well as direct sequencing of the amino-terminal portion of the purified protein revealed its identity with P-450 i. P-450 15 beta was found to be 66% similar to the male-specific rat liver microsomal 16 alpha-hydroxylase, P-450 16 alpha. Furthermore, P-450 15 beta is 47% similar to the major phenobarbital-inducible cytochrome P-450 in rat liver, P-450 b, while its structural similarity to P-450 c, P-450 pregnenolone-16a-carbonitrile, and P-450 lauric acid omega-hydroxylase is less than 30%. However, the developmentally regulated cytochromes P-450 f and P-450 PB-1 both have 68% similarity to P-450 15 beta. It is therefore concluded that P-450 15 beta belongs to the same gene subfamily (P450IIC) that also includes cytochromes P-450 f, PB-1, and 16 alpha. The plasma pattern of growth hormone (GH) is known to be a major determinant for the sex-specific expression of P-450 15 beta and P-450 16 alpha. Continuous infusion of GH to hypophysectomized male rats causes an increase in P-450 15 beta mRNA after 6 days of treatment, while intermittent injections have no effect. On the other hand, no effect on P-450 16 alpha mRNA is observed after continuous administration of GH, while intermittent injections cause an increase after 2 days of treatment. This relatively long period before P-450 15 beta and P-450 16 alpha mRNA induction is seen might indicate that protein factors mediating GH action are involved.