A dual role of growth hormone as a feminizing and masculinizing factor in the control of sex-specific cytochrome P-450 isozymes in rat liver

Constitutively Active STAT5b Feminizes Mouse Liver Gene Expression

STAT5 is an essential transcriptional regulator of the sex-biased actions of GH in the liver. Delivery of constitutively active STAT5 (STAT5CA) to male mouse liver using an engineered adeno-associated virus with high tropism for the liver is shown to induce widespread feminization of the liver, with extensive induction of female-biased genes and repression of male-biased genes, largely mimicking results obtained when male mice are given GH as a continuous infusion. Many of the STAT5CA-responding genes were associated with nearby (< 50 kb) sites of STAT5 binding to liver chromatin, supporting the proposed direct role of persistently active STAT5 in continuous GH-induced liver feminization. The feminizing effects of STAT5CA were dose-dependent; moreover, at higher levels, STAT5CA overexpression resulted in some histopathology, including hepatocyte hyperplasia, and increased karyomegaly and multinuclear hepatocytes. These findings establish that the persistent activation of STAT5 by GH that characterizes female liver is by itself sufficient to account for the sex-dependent expression of a majority of hepatic sex-biased genes. Moreover, histological changes seen when STAT5CA is overexpressed highlight the importance of carefully evaluating such effects before considering STAT5 derivatives for therapeutic use in treating liver disease.

Broad variability in pharmacokinetics of GH following rhGH injections in children

OBJECTIVE

Daily subcutaneous self-injection of GH is used worldwide to treat short stature in childhood; longitudinal data on the impact of this regimen on GH-uptake are lacking.

DESIGN

Children with/without GH-deficiency participating in clinical trials were followed prospectively (≤8 times). Blood was sampled pre-GH-injection (dose GH³³/GH⁶⁷ μg/kg) and either every 30 min thereafter for 24 h (Experimental-setting; 59 GH-curves/15 children); or every 2 h thereafter for 16 h (Clinical-setting; 429 GH-curves/117 children). Pharmacokinetics were estimated by time T_max (h) of maximal GH-concentration (C_max, mU/L) and area under the curve for 16 h (AUC, mU/L ∗ h).

RESULTS

In the Clinical-setting, median C_max was 71 mU/L and AUC was 534 mU/L ∗ h, with coefficients of variation for intra-individual variation of 39% and 36%, respectively, and inter-individual variation of 44% and 42%, respectively. 43% of C_max and AUC variability was explained by GH-dose and proxies for injection depth (baseline GH-level, GH_peakwidth, BMI_SDS). In the Experimental- versus Clinical-setting, 85% and 40% of GH-curves, respectively, reached zero-levels within 24 h. A longer duration was found following a more superficial GH-injection. Spontaneous GH-peaks were identified already 6 h after the GH-injection in about half of the curves of both GHD and non-GHD patients.

CONCLUSION

Very broad intra-individual and inter-individual variability was found. A high GH-peak will optimize growth effects; the highest C_max was found after a deep injection of GH at the higher dose and concentration. In as many as 60% of the children, GH remained detectable in serum after 24 h; a constant GH-level will promote IGF-I and metabolic effects.

17β-Estradiol in the systemic circulation derives mainly from the parietal cells in cholestatic female rats

PURPOSE

Estrogenic symptoms of liver disease patients including biliary tract disorder with high frequency is observed in clinical cases. However, the origin of 17β-estradiol which is abundant enough to cause symptoms remains uncertain. In male rats, it has been reported that the parietal cells which have an abundance of aromatase-synthesized 17β-estradiol, and a part of 17β-estradiol secreted into the portal vein, may flow into the systemic circulation under a pathophysiological condition of the liver including bile duct ligation (BDL). The aim of this study is to reveal the origin of 17β-estradiol increment in female rats and to investigate the effect of BDL on the ovary during the estrus cycle.

METHODS

Wistar female rats were used, and the common bile duct was ligated twice and transected completely at 7 days before termination. Serum portal venous and arterial 17β-estradiol levels, Cyp19a1 expressions, aromatase protein levels, and estrogen receptor (ER) α levels in the liver were measured during the estrus cycle.

RESULTS

Both arterial and portal venous 17β-estradiol levels increased 2.9 times at proestrus and maintained constant levels during the cycle. The expression of Cyp19a1 and aromatase protein in the stomach maintained constant levels, and significantly decreased during the estrus cycle in the ovary. Hepatic ERα protein and Esr1 expressions decrease by BDL in all stages.

CONCLUSIONS

These results suggest that the increment of serum 17β-estradiol levels in obstructive cholestasis induced by BDL is derived from 17β-estradiol secreted from the parietal cells in females as well as males.

Gastric estradiol-17β (E2) and liver ERα correlate with serum E2 in the cholestatic male rat

Cholestasis is associated with changes in hepatic cholesterol metabolism and serum estrogen levels. Ueyama and colleagues reported that the gastric estradiol-17β (E2) level in the portal vein is several times higher than that in the artery. This study aimed to clarify the relationships between gastric E2, hepatic estrogen receptor (ER) α and cholesterol metabolism in cholestatic male rats induced by bile duct ligation (BDL). After BDL, serum E2 levels in the portal vein and artery were measured by ELISA. The gene expression of gastric estrogen-synthesizing enzymes and various hepatic enzymes for cholesterol metabolism were measured by real-time RT-PCR, and gastric aromatase and hepatic ERα proteins were determined by immunohistochemistry and western blotting. Portal E2 levels increased by 4.9, 5.0, and 3.6 times that of controls at 2 days after BDL (BDL2d), BDL4d, and BDL7d respectively. The change in arterial E2 levels was positively correlated with that in the portal vein. Under these conditions, the expression of hepatic Ers1 (ERα) mRNA and protein was significantly reduced in a negative correlation with serum E2 levels in the portal vein after BDL. The expression of hepatic male-specific cytochrome P450 (CYP) genes Cyp2c55 and Cyp3a2 decreased and female-specific Cyp2c12 increased after BDL. It is postulated that the increase in gastric E2 levels, which occurs after BDL, results in the reduction of hepatic ERα, the elevation of arterial E2 level and leads to cholesterol metabolism becoming sex steroid dependent.

Sex and the Liver – A Journey Through Five Decades

Metabolism of steroids and drugs in rodents is sexually differentiated. The reason for this turned out to be the sexually differentiated growth hormone (GH) secretory pattern regulating the expression of a number of hepatic cytochrome P-450 genes. Although not fully resolved, it is clear that several signaling pathways and transcription factors are involved in mediating the effects of GH. It may be argued that such a well-controlled physiological system should have an important biological role and we speculate that the demands of a robust hepatic steroid metabolism during pregnancy has led to the development of this sexually differentiated hypothalamo-pituitary-liver axis.

Effects of the synthetic liver X receptor agonist T0901317 on the growth hormone and thyroid hormone axes in male rats

Liver X receptors (LXRs), activated by oxysterols, play an important role in the regulation of lipid and glucose metabolism, which is also markedly dependent on thyroid hormone and growth hormone (GH) status. Here, we investigated how a 1-week exposure to the synthetic LXR agonist T0901317 affected GH secretion and thyroid hormone status in male rats. While the pulse frequency of GH secretion was marginally affected there was a highly significant decrease in the triiodo-L-thyronine/thyroxine (T3/T4) ratio in plasma. This effect was associated with decreased expression of deiodinase 1 (DIO1) and 2 (DIO2) mRNA in the liver and thyroid gland, respectively. Expression of sterol regulatory element binding protein-1c (SREBP-1c), the hallmark of stimulated lipogenesis, was markedly increased in both thyroid and pituitary implying that protracted pharmacological LXR activation may promote lipid accumulation in these endocrine tissues. These findings suggest that attention must be given to pituitary hormone dependent axes when developing therapeutic strategies based on agonism of the LXRs, e.g. for treatment of atherosclerosis.

Role of signalling systems in the effects of dietary factors on the expression of mammalian CYPs

Changes in mammalian diets alter the hepatic expression of CYP drug-metabolising enzymes and endobiotic oxidases. Thus, dietary constituents may significantly influence the duration of action of chemicals in tissues. Recent improvements in the mechanistic information on the regulation of constitutive and inducible expression of CYPs has facilitated our understanding as to how dietary factors modulate expression. Altered regulation appears to occur either by direct activation of transcription factors or by indirect modulation of signal transduction pathways. For example, dietary lipid directly activates PPAR-alpha, or other nuclear hormone receptors, to elicit CYP induction, and vitamin A deficiency downregulates the growth hormone-responsive CYP2C11 by perturbing Janus kinase-signal transducers and activators of transcription signalling. This article focuses on the present understanding of the regulation of CYP genes by dietary nutrients.

Enhanced bioavailability of a new thiazolidine derivative FPFS-410, an antidiabetic and lipid-lowering drug, after oral administration of its hydroxypropyl-β-cyclodextrin complex to bile duct-cannulated rats

The effect of bile acids on bioavailability of FPFS-410 (2-(N-Cyanoimino)-5-{(E)-4-styrylbenzylidene}-4-oxothiazolidine) after oral administration of the drug and its 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CyD) complex was investigated. The complexation with HP-beta-CyD increased the oral bioavailability of FPFS-410 in normal rats in a HP-beta-CyD concentration-dependent manner, compared with that of drug alone. In bile duct-cannulated rats, bile acid concentrations in pylic serum and biliary were decreased to 18% and 14% of sham-operated rats, respectively. After oral administration of the HP-beta-CyD complex, the plasma levels of FPFS-410 were lower in bile duct-cannulated rats than in sham-operated rats up to 1 h, however, this order reversed from 2 to 12 h. The plasma levels of M1, a dominant metabolite of FPFS-410 in rats, significantly decreased until 2 h after administration of the complex in bile duct-cannulated rats, compared with in sham-operated rats. Bioconversion of FPFS-410 to M1 and CYP3A2 expression in the liver was markedly lowered by bile duct-cannulation. Bile duct-cannulation did not, however, affect the serum levels of estradiol. These results suggest that bile acids have a pivotal role for bioavailability of FPFS-410 after oral administration of the FPFS-410 complex with HP-beta-CyD through CYP3A2 activity in liver of rats.

Activation of the glucocorticoid receptor or liver X receptors interferes with growth hormone-induced akr1b7 gene expression in rat hepatocytes

The akr1b7 gene encodes an aldo-keto reductase involved in detoxification of isocaproaldehyde, the product from side chain cleavage of cholesterol, and of 4-hydroxynonenal (4-HNE) formed by lipid peroxidation and cleavage. Here we show that the expression of akr1b7 mRNA in rat liver is sexually differentiated, expressed in females but not in males, and regulated by the sexually dimorphic secretion pattern of GH. A GH dose-dependent induction of akr1b7 was demonstrated in cultured primary rat hepatocytes, which was sensitive to cycloheximide. Activation of the glucocorticoid receptor (GR) or liver X receptors (LXR) by dexamethasone (Dex) and T1317, respectively, attenuated the GH-induced expression of akr1b7 and CYP2C12, the prototypical rat hepatic gene dependent on the female-characteristic secretion pattern of GH. In contrast, neither Dex nor T1317 had any repressive effect on the GH induction of IGF-I mRNA. A common mechanism for LXR- and GR-mediated repressive actions on gene transcription is inhibition of nuclear factor (NF)-kappaB; however, EMSAs and pharmacological interference with NF-kappaB signaling provided no evidence for the involvement of NF-kappaB in the repressive action of Dex and T1317 on GH-induced akr1b7 expression.

Neuroendocrine facets of human puberty

The unfolding of pubertal growth and maturation entails multisystem collaboration. Most notably, the outflow of gonadotropins and growth hormone (GH) proceeds both independently and jointly. The current update highlights this unique dependency in the human.

Cytochrome P450 3A4 activity after surgical stress

OBJECTIVE

To evaluate the relationship between the acute inflammatory response after surgical trauma and changes in hepatic cytochrome P450 3A4 activity, compare changes in cytochrome P450 3A4 activity after procedures with varying degrees of surgical stress, and to explore the time course of any potential drug-cytokine interaction after surgery.

DESIGN

Prospective, open-label study with each patient serving as his or her own control.

SETTING

University-affiliated, acute care, general hospital.

PATIENTS

A total of 16 patients scheduled for elective repair of an abdominal aortic aneurysm (n = 5), complete or partial colectomy (n = 6), or peripheral vascular surgery with graft (n = 5).

INTERVENTIONS

Cytochrome P450 3A4 activity was estimated using the carbon-14 [14C]erythromycin breath test (ERMBT) before surgery and 24, 48, and 72 hrs after surgery. Abdominal aortic aneurysm and colectomy patients also had an ERMBT performed at discharge. Blood samples were obtained before surgery, immediately after surgery, and 6, 24, 32, 48, and 72 hrs after surgery for determination of plasma concentrations of interleukin-6, interleukin-1beta, and tumor necrosis factor-alpha. Clinical markers of surgical stress that were collected included duration of surgery, estimated blood loss, and volume of fluids administered in the operating room.

MEASUREMENTS AND MAIN RESULTS

ERMBT results significantly declined in all three surgical groups, with the lowest value at the time of the 72-hr study in all three groups. There was a trend toward differences in ERMBT results among groups that did not reach statistical significance (p =.06). The nadir ERMBT result was significantly and negatively correlated with both peak interleukin-6 concentration (r(s) = -.541, p =.03) and log interleukin-6 area under the curve from 0 to 72 hrs (r(s) = -.597, p =.014). Subjects with a peak interleukin-6 of >100 pg/mL had a significantly lower nadir ERMBT compared with subjects with a peak interleukin-6 of <100 pg/mL (35.5% +/- 5.2% vs. 74.7% +/- 5.1%, p <.001).

CONCLUSIONS

Acute inflammation after elective surgery was associated with a significant decline in cytochrome P450 3A4 activity, which is predictive of clinically important changes in the metabolism of commonly used drugs that are substrates for this enzyme.

Sexual dimorphic expression of ADH in rat liver: importance of the hypothalamic-pituitary-liver axis

Hepatic alcohol dehydrogenase (ADH) activity is higher in female than in male rats. Although sex steroids, thyroid, and growth hormone (GH) have been shown to regulate hepatic ADH, the mechanism(s) for sexual dimorphic expression is unclear. We tested the possibility that the GH secretory pattern determined differential expression of ADH. Gonadectomized and hypophysectomized male and female rats were examined. Hepatic ADH activity was 2.1-fold greater in females. Because protein and mRNA content were also 1.7- and 2.4-fold greater, results indicated that activity differences were due to pretranslational mechanisms. Estradiol increased ADH selectively in males, and testosterone selectively decreased activity and mRNA levels in females. Effect of sex steroids on ADH was lost after hypophysectomy; infusion of GH in males increased ADH to basal female levels, supporting a role of the pituitary-liver axis. However, GH and L-thyroxine (T4) replacements alone in hypophysectomized rats did not restore dimorphic differences for either ADH activity or mRNA levels. On the other hand, T4 in combination with intermittent administration of GH reduced ADH activity and mRNA to basal male values, whereas T4 plus GH infusion replicated female levels. These results indicate that the intermittent male pattern of GH secretion combined with T4 is the principal determinant of low ADH activity in male liver.

Growth hormone regulation of rat liver gene expression assessed by SSH and microarray

The sexually dimorphic secretion of growth hormone (GH) that prevails in the rat leads to a sex-differentiated expression of GH target genes, particularly in the liver. We have used subtractive suppressive hybridization (SSH) to search for new target genes induced by the female-characteristic, near continuous, pattern of GH secretion. Microarrays and dot-blot hybridizations were used in an attempt to confirm differential ratios of expression of obtained SSH clones. Out of 173 unique SSH clones, 41 could be verified as differentially expressed. Among these, we identified 17 known genes not previously recognized as differentially regulated by the sex-specific GH pattern. Additional SSH clones may also represent genes subjected to sex-specific GH regulation since only transcripts abundantly expressed could be verified. Optimized analyses, specific for each gene, are required to fully characterize the degree of differential expression.

Expression of rat aldehyde reductase AKR7A1: influence of age and sex and tissue-specific inducibility

The regulation of the aldo-keto reductase AKR7A1 was examined in the livers of male and female rats during development by using Western blots, and its contribution to carbonyl metabolism was assessed by using enzyme assays. Hepatic levels of AKR7A1 are low in fetal rats and rise to a peak at around 6 weeks of age in animals of both sexes. Higher levels of the enzyme are found in adult male rat liver than in adult female rat liver. The reductase, therefore, appears to be subject to sex-specific regulation. The effect of growth hormone in mediating this difference in expression was examined by using hypophysectomized animals whose serum growth hormone levels had been feminized by continuous administration. Results demonstrate that such treatment leads to a reduction in AKR7A1 expression. AKR7A1 was found to be constitutively expressed in rat tissues such as liver, kidney, small intestine, and testis, but it was not detected in nasal mucosa, skeletal muscle, heart, adrenal gland, brain, or spleen. However, AKR7A1 was inducible by the synthetic antioxidant ethoxyquin in liver, kidney, and small intestine, but not in the other tissues examined. These results show that levels of this important detoxication enzyme vary considerably according to age and sex and that dietary antioxidants can also influence its level in several tissues.

Cloning of a novel growth hormone-regulated rat complementary deoxyribonucleic acid with homology to the human alpha1B-glycoprotein, characterizing a new protein family

A sex-specific secretion of GH prevails in the rat. This has bearings on the expression of target genes, particularly in the liver. We have used suppressive subtractive hybridization to search for genes expressed in response to the female-characteristic, near-continuous secretion of GH. One sequence was particularly abundant among the obtained clones. After isolation of the corresponding full-length complementary DNA using rapid amplification of complementary DNA ends, it was found to be homologous to the human alpha1B-glycoprotein. Sequence comparisons suggest that the human alpha1B-glycoprotein and the rat homolog are members of a new family of proteins, of which at least four additional forms were found in the databases of human and mouse expressed sequence tags. In situ hybridization confirmed the female-specific expression, and by RNase protection analysis a liver-specific expression was indicated. Up-regulation of the messenger RNA by continuous exposure to GH, but not to the male-characteristic intermittent exposure, was demonstrated in hypophysectomized rats and in cultured primary hepatocytes.

Effects of bile duct ligation on hepatic expression of female-specific CYP2C12 in male and female rats

Gender differences in hepatic sex steroid and drug metabolism result from hormonal regulation of specific cytochrome P450 genes (CYP). In male rats, bile duct ligation (BDL) is associated with down-regulation of the male-specific genes, CYP2C11 and CYP3A2, together with a decrease in serum testosterone levels and a two- to three-fold increase in serum estradiol concentrations. We anticipated that if estrogen is responsible for down-regulation of male-specific CYPs in BDL male rats, the female-specific CYP2C12, which is not normally present in adult male rat liver, should be up-regulated. We examined this proposal by determining the profile of hepatic cytochrome P450 enzymes in female rats subjected to BDL, and by seeking evidence for expression of CYP2C12 in male rats that do not normally express this gene. In female rats killed 5 days after BDL, total cytochrome P450 content and NADPH-cytochrome P450-reductase (P450-reductase) were decreased to 74% and 58% of control, respectively. Microsomal enzyme activities attributable to CYP2A1, CYP2C6, and CYP2E1 were 50% to 60% of control, but ethylmorphine N-demethylase, which in female liver is catalyzed by CYP2C12 and to a lesser extent CYP2C6, was significantly less affected (81% of control). Likewise, levels of CYP2C6 and P450-reductase proteins were decreased in proportion to the corresponding enzyme activities (50% to 60%), while CYP2C12 protein (and mRNA levels) were not altered in BDL female rat liver. In sham-operated male rats, transcripts for CYP2C12 were rarely detected, but mRNA levels rose to appreciable levels within 24 hours of BDL, and CYP2C12 protein was expressed in hepatic microsomes of BDL male rats. Administration of estradiol to male rats produced a similar elevation of CYP2C12 mRNA, to values approximately 40% of female rats. It is concluded that CYP2C12 is up-regulated in male rats with cholestasis caused by BDL, while CYP2C12 protein is preserved in female rats when other microsomal proteins are decreased. These changes may be related to the increase in serum estradiol levels that result from altered hepatic steroid metabolism. The results demonstrate that activities of individual drug-metabolizing enzymes in liver disease can be determined by dysregulation of the constitutive expression of hepatic CYP genes.

Growth hormone- and testosterone-dependent regulation of glutathione transferase subunit A5 in rat liver

The class Alpha glutathione S-transferase (GST) subunit A5 is expressed in the livers of young male and female rats. After sexual maturation, this protein is no longer detectable in the livers of male rats, but is still expressed in female rats. We have previously demonstrated that the sexually dimorphic secretion of growth hormone regulates the levels of certain class Mu GSTs in rat liver, and this study was designed to investigate the hormonal regulation of GSTA5. Control and hypophysectomized rats of both sexes were used to study the role of growth hormone in the regulation of hepatic GSTA5; and the influence of testosterone on the expression of this same subunit was investigated in intact females and castrated males. Liver cytosols were subjected to SDS/PAGE and immunoblotting using antibodies directed towards rat (r)GSTA5, and to affinity purification on glutathione-Sepharose followed by reverse-phase HPLC in order to quantify the relative levels of rGSTA1, A2, A3, A4, M1 and M2 subunits. These analyses revealed that the expression of rGSTA5 is, indeed, regulated by both growth hormone and testosterone.

Estrogen antagonism on T3 and growth hormone control of the liver microsomal low-affinity glucocorticoid binding site (LAGS)

Male rat liver microsomes contain a low-affinity glucocorticoid binding site (LAGS) capable of binding all natural glucocorticoids and progesterone with a Kd from 20 to 100 nM. The LAGS level is under endocrine control by T3, glucocorticoids and GH. These hormones act synergistically at physiological concentrations to increase the LAGS level. Since female rats show a LAGS level that is much lower than the males (0.15 vs 23 pmol/mg protein, respectively), here we investigated whether estradiol could decrease the LAGS in the male rat. Orchiectomized (OX) male rats showed a higher LAGS level than intact rats. This effect was reversed by implanting a Sylastic capsule containing testosterone. When the OX rats were implanted for 20 days with estrogen capsules that provided an estradiol level in serum of 40 pg/ml, their LAGS level decreased from 23 to 0.2 pmol/mg protein. This effect was not observed in intact male rats and can be partially reversed by testosterone implants into OX rats. Both hypophysectomized male rats and hypothyroid-orchiectomized male rats showed very low levels of LAGS. Administration of physiological doses of GH and/or T3 to these rats greatly increased their LAGS level (from 0.3 to 15 and 16 pmol/mg protein, respectively). Implantation of estrogen capsules to these rats two weeks prior to starting treatment completely inhibited the increase in the LAGS level in response to T3, and significantly decreased the response to hGH, and to a combination of hGH and T3. These results suggest that physiological estradiol levels can antagonize the LAGS induction by T3 and hGH in the male rat, and could be responsible for the low level of LAGS in the female rat. Moreover, estrogen capsules also inhibited the increase in the body and hepatic weights observed after hGH treatment, which suggests a powerful inhibitory effect of low estradiol levels on the male rat liver functions under regulation by T3 and/or GH.

Pesticides: multiple mechanisms of demasculinization

Many pesticides are known to produce reproductive and developmental effects in chronically exposed non-target organisms, including humans. Recent evidence suggests that demasculinization may be an important mechanism responsible for some of these effects. Some pesticides have been shown to interact with the androgen receptor and to act as antagonists, while others have been shown to interact with the estrogen receptor and function as estrogens in both in vitro and in vivo. Many pesticides can also lower serum androgen levels by altering rates of synthesis or metabolism. Given the ubiquity of pesticides in the environment and the multiple mechanisms whereby they can elicit demasculinizing effects, synergy between such compounds may produce clinical endocrine dysfunction at current human exposure levels.

Sex-specific effects of growth hormone on hepatic 11beta-hydroxysteroid dehydrogenase activity and gene expression in hypothyroid rats

To investigate the effects of growth hormone (GH) on 11beta-HSD1, we determined changes in hepatic 11beta-HSD1 activity in hypothyroid rats following treatment with subcutaneous (s.c) injection of GH for periods ranging from 24 h to 7 days. In male rats, hypothyroidism markedly reduced the hepatic 11beta-HSD1 activity and serum testosterone levels (p < 0.01). Subcutaneous injection of GH once daily to male hypothyroid rats for 48 h inhibited hepatic 11beta-HSD1 activity. However, the same daily dose of GH administered to male hypothyroid rats for 7 days, resulted in a marked increase in hepatic 11beta-HSD1 activity and gene expression (p < 0.01). Furthermore, daily s.c injections of GH to castrated male hypothyroid rats for 7 days reduced hepatic 11beta-HSD1 activity rather than inducing it, the same response seen in hypothyroid female rats. In addition, the treatment of castrated male hypothyroid rats with testosterone for 7 days significantly increased this enzyme activity (p < 0.01). The changes in hepatic 11beta-HSD1 were demonstrated to be associated with the testes in hypothyroid male rats following treatment with GH for 7 days. Moreover, the prolonged exposure to GH required to induce hepatic 11beta-HSD1 in intact hypothyroid male rats and the lack of a similar effect in castrated male hypothyroid rats suggests that this action is indirect and that it may be mediated by androgen production from Leydig cells of the testes and induced by the daily injections of GH. Treatment of hypothyroid male rats with GH at 6-h intervals, however, feminized the hepatic 11beta-HSD1 gene expression.

Effects of metyrapone on expression of CYPs 2C11, 3A2, and other 3A genes in rat hepatocytes cultured on matrigel

Hepatocytes cultured on matrigel express many liver-specific functions, but the levels and activities of the predominant male-specific rat hepatic CYPs, 3A2 and 2C11, decline rapidly in culture. Metyrapone maintains the level of total cytochrome P450 of rat hepatocytes in primary culture, but the mechanism underlying this effect has not been completely elucidated. The present study sought to determine whether metyrapone acts solely to stabilise CYP proteins in rat hepatocytes cultured on matrigel, or whether it also influences mRNA levels of the encoding genes. Metyrapone maintained the level of total cytochrome P450 in cultured hepatocytes so that values were > 200% of those found in untreated control cells 24 hr after isolation. At this time, CYP3A2-mediated testosterone 6 beta-hydroxylation was approximately 7-fold higher in hepatocytes cultured in the presence of metyrapone than in control cells, and CYP2C11-dependent testosterone 2 alpha- and 16 alpha-hydroxylation activities were between 2 and 3-fold greater. The results inferred from catalytic activities were supported by immunoquantitation of CYP3A and 2C11 proteins. The trend of increased CYP protein levels in metyrapone-treated cells continued throughout the 48-hr culture period. In control cells, CYP3A2 and 2C11 mRNA levels fell abruptly in culture to reach values at 24 hr that were < 30% of those in freshly isolated cells; addition of metyrapone failed to arrest this fall. However, treatment of cells with metyrapone considerably elevated levels of one or more CYP3A subfamily mRNA species, as detected by a riboprobe based on the cDNA for CYP3A1 ("CYP3A1-like mRNA') that were demonstrated, by another riboprobe, not to be CYP3A2 or RNCYP3AM. RT-PCR of mRNA prepared from cultured hepatocytes, followed by restriction mapping of the cloned cDNAs was used to characterise the CYP3A induced by metyrapone. This revealed that elevated levels of the CYP3A1-like mRNA were attributable to induction of RL33/cDEX mRNA; there were no CYP3A1 cDNAs isolated from these cells. These data are interpreted as indicating that metyrapone stabilises the expression of cytochrome P450 in culture by both pre- and posttranslational mechanisms. The particular mechanism employed is gene-specific, whereby even the highly homologous genes CYP3A2, RL33/cDEX and, possibly, RNCYP3AM are subject to different types of regulation in the presence of metyrapone.

Hormonal regulation of the female enriched GH receptor/binding protein mRNA in rat liver

At least two classes of mRNA for the GH receptor (GHR) and GH binding protein (GH BP) with different 5' untranslated first exons exist in the rat. One such class, the GHR1 is predominantly expressed in the liver of female rats. The hepatic expression of the GHR1 mRNA in normal and hypophsectomized rats of both sexes was studied by employing an RNase protection/solution hybridization assay. Normal females expressed 10-fold more GHR1 mRNA than males, hypophysectomy of female rats decreased the GHR1 level to that observed in male rats. Continuous GH treatment of hypophysectomized male and female rats for 6 days increased the expression of GHR1 mRNA to levels found in normal females, whereas intermittent GH treatment without effect. Bovine GH(bGH) induced the GHR1 expression in a time- and dose-dependent manner in primary cultures of adult rat hepatocytes as determined by solution hybridization. Maximal induction was achieved after 72 h of treatment with 50 ng bGH/ml medium. Female enriched expression of receptor and binding protein mRNAs raises the possibility that they participate in determining the ability of the liver to respond differently to the male and female GH secretory patterns. Our in vitro model utilizing cultures of primary adult rat hepatocytes could be used to address this issue as well as explore a hormonal interplay in regulation of GHR1 expression.

Influence of growth hormone on the hepatic mixed function oxidase and transferase systems of rainbow trout

The effect of GH treatment on hepatic cytochrome P450 content, aryl hydrocarbon hydroxylase (AHH), aminopyrine-N-demethylase (AND), testosterone hydroxylase, testosterone 5α- and 5β-reductase, UDP-glucuronyl transferase (UDPGT) and glutathione S-transferase (GST) activities in immature rainbow trout were investigated. Hepatic cytochrome P450 content, AHH and GST activities were measured in both GH implanted and GH injected animals whereas other activities were assayed in GH implanted trout only.GH implants significantly decreased cytochrome P450 content at 15 days compared to the control but no significant effect was observed at 15 or 30 d when GH was injected biweekly. In both cases, AHH activity was significantly decreased by GH treatment compared to the control whereas GST remained unchanged. Compared to the control, GH implanted fish exhibited a pronounced inhibition of AND, a decreased 6β and 16β-testosterone hydroxylation, an inhibition of UDPGT with testosterone as substrate and an enhanced 17β-testosterone oxidation.

Growth hormone signaling leading to CYP2C12 gene expression in rat hepatocytes involves phospholipase A2

The expression of CYP2C12 is liver-specific and regulated at the transcriptional level by growth hormone (GH). In attempts to elucidate the nature of signaling molecules mediating the GH regulation of this gene in rat hepatocytes, a role for phospholipase A2 (PLA2) as a transducer of GH-induced levels of P4502C12 mRNA was investigated. GH was shown to induce tyrosyl-phosphorylation of p42 and p44 microtubule-associated protein (MAP) kinases and to reduce the electrophoretic mobility of a 100-kDa protein, immunologically related to cPLA2. These events were observed in parallel with GH-stimulated release of [3H]arachidonic acid ([3H]AA) from cellular phospholipids of rat hepatocytes labeled with [3H]AA. These rapid effects of GH action, as well as the GH-induced expression of CYP2C12, were inhibited in cells treated with the tyrosine kinase inhibitor herbimycin A. Similarly, when the GH-induced liberation of [3H]AA was blocked by the PLA2 inhibitor mepacrine or the Ca2+ channel blocker verapamil, GH-induced accumulation of P4502C12 mRNA was absent. These results suggest a correlation between PLA2 activity and GH regulation of the CYP2C12 gene. The inhibitory effect of mepacrine on GH induction of P4502C12 mRNA was reversed by AA addition, further supporting a role for eicosanoids in the regulation of CYP2C12. Finally, inhibitors of P450-mediated AA metabolism, SKF-525A and ketoconazole as well as eicosatetraynoic acid, blocked the GH-mediated induction of P4502C12 mRNA, whereas more specific inhibitors of cyclooxygenase or lipoxygenase metabolism did not. Based on these results, we suggest that GH signaling in rat hepatocytes, leading to increased expression of CYP2C12, involves PLA2 activation and subsequent P450-catalyzed formation of an active AA metabolite.

Neonatal exposure to xenobiotics alters adult hepatic protein kinase C alpha levels and testosterone metabolism: differential effects by diethylstilbestrol and phenobarbital

Hepatic enzymes that metabolize endogenous and xenobiotic compounds have been shown to be altered in adult rats that had been exposed to xenobiotics as neonates. Protein kinase C (PKC) is important in intracellular signaling and has been implicated in the regulation of hepatic monooxygenases. Therefore, we examined the effects of neonatal exposure to diethylstilbestrol (DES) and phenobarbital (PB) on hepatic microsomal testosterone metabolism and on the alpha form of protein kinase C (PKC alpha) in adult rats. In adult males, neonatal exposure to DES altered adult testosterone metabolism such that 7 alpha-hydroxylation was increased by 58% but 2 alpha-, 16 alpha-, and 6 beta-hydroxylations and conversion to androstenedione were decreased 31-44%. In contrast, adult males neonatally exposed to PB showed increased (20-27%) testosterone 2 alpha- and 16 alpha-hydroxylations and androstenedione formation, but no effect was observed in the rate of 6 beta- or 7 alpha-hydroxylations. Western blot analyses indicated that cytosolic PKC alpha levels in male rats neonatally exposed to PB were decreased by approximately 63% relative to the vehicle control group but were not significantly altered in the DES males. The PKC alpha levels generally correlated (r = -.75) with 16 alpha-hydroxytestosterone formation in all samples. These results show that neonatal treatment with DES or PB differentially alters hepatic monooxygenase enzyme activities and PKC alpha levels in adult rats.

Alterations in cytochrome P-450 levels in adult rats following neonatal exposure to xenobiotics

Neonatal exposure to certain xenobiotics has been shown to alter hepatic metabolism in adult rats in a manner that indicates long-term changes in enzyme regulation. Previously, we have observed changes in adult testosterone metabolism and in cytochrome P-450 (P-450) mRNA levels in animals neonatally exposed to phenobarbital (PB) or diethylstilbestrol (DES). In order to test for other enzyme alterations, we used Western blot procedures for specific P-450s to analyze hepatic microsomes from adult rats (24 wk old) that had been exposed neonatally to DES, PB, 7,12-dimethylbenz[a]anthracene (DMBA), or pregnenolone 16 alpha-carbonitrile (PCN). The most striking effects were observed in the DES-treated males: P-4502C6 and an immunologically similar protein were increased 60 and 90%, respectively, relative to control values, but P-4503A2 was decreased by 44%. No changes were observed in the DES-treated males in levels of P-4502E1, P-4502B, or the male-specific P-4502C13. Adult males neonatally treated with PB had 150% increase in levels of anti-P4502B-reactive protein without significant changes in the other enzymes. The DES- and DMBA-treated females had increased levels of the female-specific P-4502C12 of 38 and 48%, respectively, but no other observed alterations. The results confirm that neonatal exposure to DES or PB can cause alterations in adult hepatic cytochrome P-450 levels but show that these chemicals act on different enzymes. Neonatal DMBA resulted in changes in adult females similar to those produced by the synthetic estrogen DES, but did so at about two-thirds lower dose.

The role and mechanism of growth hormone in the regulation of sexually dimorphic P450 enzymes in rat liver

The determination of sexually dimorphic hepatic steroid metabolism in rat liver has been shown to involve growth hormone. However, the mechanisms by which growth hormone controls the cytochrome P450 enzymes responsible for this dimorphic steroid metabolism is largely unknown. In this review we discuss different levels of growth hormone signal transduction, including receptor binding, signal transduction and activation of target genes by growth hormone.

Regulation by growth hormone and glucocorticoid of testosterone metabolism in long-term cultures of hepatocytes from male and female rats

The activities of 2-, 6 beta-, 7 alpha- and 16 alpha-testosterone hydroxylase and 5 alpha-testosterone reductase were measured in intact hepatocytes from male and female rats cultured for 8 days in a modified Waymouth medium supplemented with 0.1 or 1.0 microM dexamethasone with or without addition of 1 microgram/mL growth hormone. During culture of hepatocytes from female rats the activity of the male-specific 16 alpha-testosterone hydroxylase increased. This increase was significantly inhibited at day 8 by 1 microM dexamethasone as well as by growth hormone. Furthermore, in cultures of hepatocytes from male rats, the activity of the constitutive 16 alpha-testosterone hydroxylase was decreased by 1 microM dexamethasone as well as by growth hormone. The induction of 6 beta-testosterone hydroxylase by dexamethasone was suppressed by growth hormone in hepatocytes from both male and female rats, while the 7 alpha-testosterone hydroxylase activity was unaffected by culture time, hormone additions and gender. The decrease in female-specific 5 alpha-reductase activity with culture time in hepatocytes from female rats was significantly attenuated by growth hormone at 0.1 microM dexamethasone. The effects of growth hormone on testosterone hydroxylase activities in hepatocyte cultures from male and female rats are in accordance with the concept of growth hormone as a "feminization signal". The results suggest that the glucocorticoid-dependent expression of the male constitutive 16 alpha-hydroxylase requires periods of low levels of growth hormone.

Constitutive expression and hormonal regulation of male sexually differentiated cytochromes P450 in primary cultured rat hepatocytes

Experiments, predominantly performed in vivo, have shown that the pattern of growth hormone (GH) release from the pituitary gland is a major regulator of sex-specific cytochromes P450 (P450) in rats and other rodents. However, difficulty in constitutively expressing male-specific forms of P450 using in vitro models, such as primary cell culture, has impeded efforts to examine the direct actions of hormones on these enzyme forms. In the present study mRNA species for the male-specific P450 2C11 and 2C13, but not 3A2, were successfully expressed in primary hepatocytes cultured on a laminin-rich extracellular matrix (matrigel) in a serum-free, chemically defined medium containing insulin as the only hormone. When cells were exposed to GH (100 ng/ml), 2C11 mRNA expression was virtually abolished and 2C13 expression decreased to approximately 50% of control values, demonstrating that the negative regulation of these P450 forms by GH is a direct action on hepatocytes. Dexamethasone (DEX, 10(-8) M) increased the expression of 2C11 to 195% while decreasing 2C13 expression to 25% of control values. When GH and DEX were administered concurrently 2C11 was downregulated, indicating that GH is the dominant regulatory hormone for this form. L-Triiodothyronine (T3) (10(-9) M) suppressed 2C11 (46% of control) but had no effect on 2C13 mRNA expression. The positive regulatory effect of glucocorticoids on 2C11 was also found to occur in vivo and demonstrated to operate predominantly at the transcriptional level. This study demonstrates that primary cultures of hypatocytes are a suitable in vitro model for studies on regulation of some male-specific P450 forms and that GH, DEX, and T3 act directly on hepatocytes at a pretranslational level to regulate these forms.

Somatotropic and lactotropic receptors in transgenic mice expressing human or bovine growth hormone genes

The somatotropic and lactotropic receptors were studied in liver microsomal preparations from transgenic mice carrying the human growth hormone (hGH) or bovine growth hormone (bGH) gene fused to mouse metallothionein-I (MT) or phosphoenolpyruvate carboxykinase promoter/regulator (PEPCK). Specificity studies indicated that, similarly to normal mice, liver microsomes from the transgenic animals possess a mixed population of somatotropic and lactotropic binding sites. In transgenic animals of both sexes, the binding capacity of somatotropic receptors was significantly increased without corresponding changes in affinity. Expression of the MT-hGH hybrid gene was associated with the induction of somatotropic receptors which was approximately twice as great as that measured in animals expressing the MT-bGH hybrid gene. The binding capacity of lactotropic receptors in liver microsomes (quantitated by the use of labelled ovine prolactin) was increased 2-3 fold in transgenic females and approximately 10-fold in transgenic males as compared to the respective normal controls. We conclude that lifelong excess of GH up-regulates hepatic GH and prolactin receptors, and that lactogenic activity of GH is not essential for induction of prolactin receptors in the liver of transgenic mice.

Further characterization of hormonal regulation of glutathione transferase in rat liver and adrenal glands. Sex differences and demonstration that growth hormone regulates the hepatic levels

Immunoblot experiments and reverse-phase h.p.l.c. were used to study the levels of glutathione transferase subunits 1, 2, 3, 4, 6, 7 and 8 in the liver and adrenal of intact and hypophysectomized male and female Sprague-Dawley rats. A sexual dimorphism in the levels of several of these isoenzymes and in their responses to hypophysectomy was demonstrated. In the liver of sham-operated females and males there are differences in glutathione transferase activities and isoenzyme pattern. H.p.l.c. analysis showed higher levels of subunits 1, 3 and 4 in male rats compared with females. In contrast with the pronounced sex differences in sham-operated rats, the isoenzyme patterns of hypophysectomized males and females were very similar. In the adrenal glands, however, a sexual dimorphism became apparent only after hypophysectomy, when the level of subunit 4 was increased 14-fold in the female, whereas the corresponding increase in the male rat was only 2.7-fold. The hepatic pattern of glutathione transferase subunits could be altered by continuous infusion of growth hormone to both sham-operated and hypophysectomized rats of both sexes. This treatment feminized the isoenzyme pattern in sham-operated males and a similar effect was obtained upon treating hypophysectomized rats with thyroxine, cortisone acetate and a continuous infusion of growth hormone.

Growth hormone regulation of hepatic cytochrome P450 expression in the rat

GH by means of its sexually differentiated secretory pattern is the predominant regulator of the expression of cytochrome P450 enzymes responsible for a sexual dimorphism of hepatic steroid metabolism. Other hormones, such as gonadal, thyroid and glucocorticoid hormones, as well as insulin appear to modulate the sexually differentiated expression of these enzymes. The major constitutively expressed sex specific forms of P450, belonging to the P4502C-subfamily, have been shown to be regulated by GH at the level of transcription. However, the GH postreceptor events leading to increased or decreased transcriptional activity are essentially unknown. Neither is the functional role of the soluble GH binding protein yet resolved. On-going protein synthesis is a prerequisite for GH transcriptional activation of the female specific P4502C12 but not for all GH effects in the hepatocyte. With regard to signalling mechanisms PKC activity appears to be permissive for the GH induction of P4502C12 but some as yet unidentified factor/kinase(s) may also be activated. The transcriptional control exerted on the rat P4502C-gene subfamily by the pattern of GH secretion offers a versatile tool to elucidate the molecular mechanisms of GH regulation of cytochrome P450 expression.

Regulations of male-dominant P-450Md mRNA in rat liver by hormonal factors and xenobiotics

Male-dominant P-450Md mRNA was undetectable in the livers of newborn rats. In female rat livers, the mRNA appeared at 17 days of age and then decreased to very low levels in the adult periods. The level of P-450Md mRNA in female rats was increased by phenobarbital or dexamethasone treatment, whereas the level in the males was depressed by methylcholanthrene. Hypophysectomy decreased the level of P-450Md mRNA in male rat livers, and continuous infusion or twice-daily injections of growth hormone to hypophysectomized rats caused further suppression or clear restoration, respectively, of the mRNA level.

Evidence from dwarf rats that growth hormone may not regulate the sexual differentiation of liver cytochrome P450 enzymes and steroid 5 alpha-reductase

Differences in the pattern of growth hormone (GH) secretion in mature rats (i.e., "continuous" secretion in females versus "pulsatile" secretion in males) are thought to be the underlying cause of sex-dependent differences in a subpopulation of liver microsomal P450 enzymes and steroid 5 alpha-reductase. A new strain of dwarf rats (NIMR/AS) has recently been shown to have low or undetectable levels of circulating GH due to a selective defect in pituitary GH synthesis. We have measured the levels and/or activity of IIA1 (P450a), IIA2 (P450m), IIC11 (P450h), IIC12 (P450i), IIIA2 (a P450p isozyme), and steroid 5 alpha-reductase in liver microsomes from male and female dwarf rats, to test the hypothesis that the expression of these sexually dimorphic enzymes is regulated by GH. In mature rats, the levels of liver microsomal IIA2, IIC11, and IIIA2 were higher in male than in female dwarf rats, whereas the levels of activity of IIA1, IIC12, and steroid 5 alpha-reductase were greater in female than in male dwarf rats. These sex differences resulted from age-related changes in either male dwarf rats (i.e., an increase in IIC11 and IIA2 and a decrease in IIA1) or female dwarf rats (i.e., an increase in IIC12 and 5 alpha-reductase and a decrease in IIIA2). The magnitudes of these sex-dependent, age-related changes were essentially indistinguishable from those observed in normal rats. These unexpected results suggest that GH is not the pituitary factor responsible for regulating the levels of sexually dimorphic, steroid-metabolizing enzymes in rat liver. Alternatively, it is possible that these enzymes are regulated by extremely low levels of GH. In either case, the current model of how steroid-metabolizing enzymes are regulated in rats must be revised to account for the normal sexual differentiation of these enzymes in dwarf rats.

Developmental expression of rabbit cytochrome P450 CYP1A1, CYP1A2 and CYP3A6 genes. Effect of weaning and rifampicin

Developmental expression of CYP1A1, CYP1A2 and CYP3A6 in the rabbit have been studied. Cytochromes P450IA1, P450IA2 and P450IIIA6 exhibited comparable patterns of developmental expression. Present at low level (less than 0.05 nmol/mg) in the new born animal up to week 3, these proteins sharply accumulated between weeks 3 and 4 to reach a maximum by week 4 (P450IA1, 0.2 nmol/mg; P450IA2, 0.8 nmol/mg; P450IIIA6, 0.12 nmol/mg) and decreased in the adult (P450IA1, 0.2 nmol/mg; P450IA2, 0.4 nmol/mg; P450IIIA6, 0.09 nmol/mg). Cytochromes P450IA1 and P450IA2 were not expressed in the untreated fetus. Onset of CYP3A6 gene expression occurred at day 30 of gestation and both transcription and mRNA accumulation were transplacentally inducible by rifampicin only shortly before birth, i.e. after treatment of the females between days 28 and 30 of gestation. Both long (1.85 kb) and short (1.7 kb) mRNA transcripts were expressed in untreated or rifampicin-treated fetuses. CYP3A6 gene expression was also induced by rifampicin in 1-week-old and 2-week-old animals. Developmental expression of CYP1A1 and CYP1A2 genes was shown to be closely related to the diet change accompanying weaning which occurs at weeks 3-4. In animals subjected to either delayed (week 6) or early (week 2) weaning, sharp accumulation of messages, proteins and related activities were delayed or anticipated accordingly with respect to normal weaning. Artificially scheduled weaning gave similar results when repeated with biological-grade lucern (grown in the absence of chemical fertilizers, pesticides, etc.), the main constituent of commercial rabbit chow. While CYP3A6 gene expression could be brought forward by early weaning at week 2, both message and protein did not exhibit increased accumulation after delayed weaning at week 6, and remained at the low level of the new born animal. Treatment of 1-week-old and 2-week-old animals with triiodothyronine or of 3-week-old animals with propylthiouracil, an antithyroid factor, did not modify the normal pattern of developmental expression of genes CYP1A1, CYP1A2 and CYP3A6. It is concluded that (a) the onset of CYP3A6 gene expression in the fetus occurs at day 30 of gestation, (b) expression of this gene may be induced transplacentally by rifampicin, (c) CYP1A1, CYP1A2 and CYP3A6 gene expression is sharply activated at weaning, and (d) thyroid hormones appear not to be responsible for the pattern of developmental expression of these genes in the rabbit.

Regulation of the expression of the sex-specific isoforms of cytochrome P-450 in rat liver

The hepatic metabolism of steroid hormones and of xenobiotics frequently depends on the expression of the sex-specific isoforms of cytochrome P-450 and on differences in sex hormones. Following biochemical, immunological and molecular biological investigations, it was shown that in adult rat liver there exist at least four male-specific and one female-specific isoforms of cytochrome P-450. The designation of these sex-specific genes is IIC11, IIIA2, IIC13 and IIA2 in males, and IIC12 in females. The irreversible programming of the expression of these isoforms of cytochrome P-450 in adulthood occurs during the perinatal period of life, and is named enzyme imprinting. One of the main factors that regulates the expression of the sex-specific isoforms of cytochrome P-450 is the level of androgens in the blood. Castration of adult rats decreased the level of the male isoforms of cytochrome P-450 and the activity of the monooxygenase enzyme system that remained higher than in intact females. The mechanism of enzyme imprinting can be explained as follows: neonatal androgens program the secretion of hypothalamic hormones, somatostatin and growth-hormone-releasing factor. These factors determine the type of growth hormone secretion in adult rats, and this controls the type of sex-specific isoforms of cytochrome P-450 expressed in adulthood. Metabolic regulation similar to that outlined above was shown to occur for several metabolism-dependent chemical carcinogens. Such a pathway may explain the different sensitivity displayed by male and female rats to treatment with these carcinogenic agents. One possible way of modulating the expression of some isoforms of cytochrome P-450 in adult rats is by treating neonates with specific xenobiotics that change the constitutive expression of neonatal androgens. It appears that this enzyme imprinting plays an important role in determining the individual sensitivity to the carcinogenic effects of chemicals.

Effects of neonatal sexual differentiation, growth hormone and testosterone on thymic weights and thymosin-beta 4 in hypophysectomized rats

Experiments were conducted to analyze the effects of growth hormone and testosterone in conjunction with the effects of neonatal sexual differentiation (via castration of males at days 2 or 11 of age and androgenization of females at day 3 of age) on thymic weight and thymosin-beta 4 concentrations in hypophysectomized rats (day 30 of age). Ten days post-hypophysectomy, hormonal treatments were initiated on males, male castrates, females, and androgenized females. Growth hormone (25 micrograms daily), testosterone propionate (100 micrograms/day), and the combination of the two hormonal treatments were administered for seven days, then thymic weights and blood samples were collected. Administration of growth hormone to hypophysectomized rats increased thymosin-beta 4 concentration in a dose-dependent manner, but injection of testosterone had no effect on thymosin-beta 4 concentrations. Testosterone treatment decreased thymic weights whereas growth hormone increased thymic weights. Hypophysectomized males had increased thymosin-beta 4 concentrations compared with female and neonatally-androgenized female rats. However, hypophysectomy eliminated any thymic weight differences between males and females. The data support a possible endocrine role for the thymus gland and thymic peptides in that they are integrated into the control and support of other endocrine systems. Although differences in thymosin-beta 4 concentrations were noted between males and females, sexual differentiation of the immune system was unaltered by neonatal castration of males or testosterone treatment of females and may indicate sexual differences in immune function are established in utero.

Structural and regulatory analysis of a cytochrome P450 gene (CYP2C12) expressed predominantly in female rat liver

Cytochrome P450 15 beta is a female-specific gene product that catalyzes the hydroxylation of steroid sulfates at the 15 beta position. Isolation and analysis of the gene for rat P450 15 beta reveals nine coding exons and encompasses more than 35 kb of chromosomal DNA. The intron-exon junctions are at similar positions with the P450 genes of the II family, whose gene structure has been determined. Sequencing of about 2 kb of the 5'-flanking region indicates the presence of the Alu-like R.dre.1 repetitive sequence, a GA-rich stretch that is also found in the 5'-flanking DNA of the male-specific cytochrome P450 16 alpha gene, the enhancer octamer sequence ATGCAAAT, and three CAAAGTT repeats just upstream from the TATA box. Primer extension reveals a major and a minor transcription start site located 22 and 26 bases 5' to the translation initiation codon, respectively. This gene is developmentally regulated and transcriptional activation accounts, at least partly, for the sexually differentiated expression of cytochrome P450 15 beta.

Interactions of insulin, thyroxine and dexamethasone with growth hormone in the control of steroid metabolism by isolated rat hepatocytes

In vivo studies have established that continuous infusion of male rats with growth hormone (GH) results in the feminisation of hepatic steroid metabolism. In this study, however, no feminising effects were observed in response to GH treatment alone in isolated hepatocytes. Combined incubations with insulin and GH or with dexamethasone and GH showed that GH antagonised the stimulatory effects of insulin and dexamethasone respectively. Pretreatment of hepatocytes over 30 min with thyroxine and GH also resulted in a decrease in enzyme activity as compared to the response with thyroxine alone. Thus no feminisation effects were seen with any of the combinations of hormones noted above. Preincubation of hepatocytes with dexamethasone, thyroxine and GH, however, resulted in the expression of the feminisation effect noted in vivo. It therefore appears from this study that dexamethasone and thyroxine may play a permissive role in mediating the feminising effects of GH in isolated hepatocytes, and that GH may be interacting with these hormones in vivo to produce such effects.