Sex differences exist in steroid and xenobiotic metabolism in the liver of a number of species. In the rat, the differences are regulated through the hypothalamo-pituitary axis. The previously postulated "feminizing factor" responsible for a female-type liver metabolism appears to be identical to growth hormone. The different effects of this peptide on hepatic metabolism in male and female rats may be related to the sexual dimorphism of the growth hormone secretory pattern; serum levels of growth hormone do not fluctuate as markedly in female as in male rats and may be simulated by administration of the hormone via osmotic minipumps, a procedure resulting in "feminization" of liver metabolism of male or hypophysectomized rats. This newly discovered system, the hypothalamo-pituitary-liver axis, represents a novel concept in endocrinology.

In rats, the onset of the sexually dimorphic pattern of growth hormone (GH) secretion and increased hepatic GH-binding capacity at puberty are temporally correlated with the sex-dependent expression of some hepatic cytochrome P450 enzymes involved in steroid metabolism. There are indications that the expression of the GH receptor gene itself is dependent on the sexually differentiated pattern of GH secretion. However, the molecular mechanisms by which a given pattern of GH secretion turns on a specific set of genes in the hepatocyte are not yet understood. Studies of the cytochrome P450 2C gene subfamily in hypophysectomized rats and isolated hepatocytes suggest that one major mechanism of GH action in the liver occurs through modulation of gene transcriptional initiation. The occurrence, in dwarf rats and in rats treated neonatally with monosodium glutamate, of sex differences in GH secretion and liver steroid metabolism typical of normal rats, in spite of a 95% reduction in pituitary GH levels, is compatible with the notion that extremely low levels of circulating GH are sufficient to regulate the expression of liver steroid-metabolizing enzymes. This, together with the fact that single daily subcutaneous injections of GH are sufficient to masculinize the liver of a hypophysectomized rat, indicates that neither the amplitude nor the frequency of the GH pulse is recognized as male or female by the hepatocyte, but rather the complete and prolonged suppression (in males) or the persistence (in females) of circulating GH during the trough period after a GH surge.

Growth hormone (GH) binding to its receptor modulates gene transcription by inf luencing the amount or activity of transcription factors. In the rat, GH exerts sexually dimorphic effects on liver gene transcription through its pattern of secretion which is intermittent in males and continuous in females. The expression of the CYP2C12 gene coding for the female-specific cytochrome P450 2C12 protein is dependent on the continuous exposure to GH. To identify the transcription factor(s) that mediate(s) this sex-dependent GH effect, we studied the interactions of the CYP2C12 promoter with liver nuclear proteins obtained from male and female rats and from hypophysectomized animals treated or not by continuous GH infusion. GH treatment induced the binding of a protein that we identified as hepatocyte nuclear factor (HNF) 6, the prototype of a novel class of homeodomain transcription factors. HNF-6 competed with HNF-3 for binding to the same site in the CYP2C12 promoter. This HNF-6͞ HNF-3 binding site conveyed both HNF-6-and HNF-3-stimulated transcription of a reporter gene construct in transient cotransfection experiments. Electrophoretic mobility shift assays showed more HNF-6 DNA-binding activity in female than in male liver nuclear extracts. Liver HNF-6 mRNA was barely detectable in the hypophysectomized rats and was restored to normal levels by GH treatment. This work provides an example of a homeodomain-containing transcription factor that is GH-regulated and also reports on the hormonal regulation of HNF-6.

The influence of endogenous sex steroids and exogenous testosterone treatment on pulsatile GH secretion, body weight, longitudinal bone growth, and hepatic steroid metabolism was studied in male and female adult rats. Blood samples were obtained from the tip of the tail, and maximum and minimum GH levels were determined in individual rats to evaluate pulse heights and baseline levels. Longitudinal bone growth was measured using the intravital marker tetracycline, and hepatic steroid metabolism was evaluated by determining the enzyme activities of 16 alpha-hydroxylase and 5 alpha-reductase. Neonatal, but not prepubertal, gonadectomy of male rats suppressed maximum and mean plasma GH levels during adult life. The body weight and the rate of longitudinal bone growth were also decreased. Testosterone treatment neonatally reversed all of these effects. Neonatal gonadectomy of male rats also caused an elevation of minimum plasma GH levels, an effect, however, which was not reversed by testosterone replacement during neonatal life. Neonatally gonadectomized females treated with testosterone neonatally or during adult life increased their maximum and decreased their minimum GH levels. Their longitudinal bone growth was increased. The body weight of these rats was increased by neonatal, but not adult, testosterone treatment. There was no effect of neonatal ovariectomy on plasma GH levels in 3- to 4-month-old female rats. However, neonatal ovariectomy did increase the maximum and mean plasma GH levels immediately postpubertally, suggesting that the effect of the ovaries on GH secretion differs among mature female rats of different ages. Prepubertal gonadectomy of male rats feminized their hepatic steroid metabolism by decreasing 16 alpha-hydroxylase and increasing 5 alpha-reductase activities. Neonatal gonadectomy caused an even more pronounced feminization, which was partly reversed in rats given testosterone replacement therapy neonatally. In neonatally gonadectomized female rats, treatment with testosterone during adult life increased 16 alpha-hydroxylase and decreased 5 alpha-reductase to levels seen in intact male rats. The present results indicate that neonatally secreted testicular androgens imprint the high amplitude pulses characteristic of GH secretion in adult male rats. Neonatal androgens also stimulate somatic growth and partially account for the masculinized hepatic steroid metabolism in the adult animal. It is proposed that imprinting of the GH secretory pattern contributes to the influence of neonatal testicular androgens on body growth and hepatic steroid-metabolizing enzymes.