Cytochrome P450 in livers of diabetic rats: regulation by growth hormone and insulin

The effects of pituitary and pancreatic hormones on the change in hepatic cytochrome P450s were studied in alloxan- or streptozotocin-induced male rats. In two major sex-specific forms, P450-male and P450(6 beta-1), the former was decreased in chronic (5 week) diabetes to only less than one-third of controls and the latter was also reduced in early (1 week) diabetes. In contrast, a main phenobarbital-inducible form, P450b, was enhanced 25- to 30-fold in these diabetic rats. 3-Methylcholanthrene-inducible P448H was also elevated 3-fold in alloxan-induced diabetes. These changes in hepatic contents of P450-male, P450-6 beta-1, and P450b, which are under the regulation of pituitary growth hormone, associated well with the reported results of time-dependent changes in growth hormone levels in diabetes (G.S. Tannenbaum (1981) Endocrinology 108, 76-82), suggesting that the change in growth hormone level is a factor responsible for alterations in hepatic cytochrome P450s. Normalizing effects of insulin on these forms were also studied. Treatment of diabetic rats with insulin reversed the decreased amounts of both P450-male protein and mRNA. Insulin also normalized hepatic contents of P450b, P4506 beta-1, and P448H. However, the treatment of hypophysectomized rats with insulin had no effect, and treatment of diabetic rats with growth hormone or a suppressing agent of somatostatin, cysteamine, showed trivial effects on P450-male and P450b. These results suggest that insulin does not act directly as a substitute of growth hormone, but exerts its effect indirectly through the normalization of a growth hormone-mediated process(es) in diabetic rats.     

The induction of a specific form of cytochrome P-450 (P-450j) by fasting

In previous work we have demonstrated that liver microsomal N-nitrosodimethylamine demethylase (NDMAd) activity is increased in rats by fasting, and we have postulated that this is due to the induction of a specific form of cytochrome P-450. This communication provides evidence for such a hypothesis. Fasting for 24 and 48 h caused 59 and 116% increases, respectively, in NDMAd activity in male rats, and fasting for 48 h caused a 63% increase in female rats. These increases were accompanied by corresponding increases of cytochrome P-450j (P-450ac) determined by immunoblotting. Fasting for 24 and 48 h also increased the mRNA for P-450j by 153 to 250%, as determined by hybridization with a cDNA probe of this cytochrome. The results suggest that fasting affects the gene expression of P-450j.     

Regulation of cytochrome P-450j, a high-affinity N-nitrosodimethylamine demethylase, in rat hepatic microsomes

Polyclonal antibodies were produced in rabbits against purified cytochrome P-450j isolated from isoniazid-treated adult male rats. The monospecificity of immunoadsorbed antibody to cytochrome P-450j was demonstrated by Ouchterlony double diffusion analyses, enzyme-linked immunosorbent assays, and immunoblots. Immunoquantitation results indicated that rat liver microsomal cytochrome P-450j content decreases between 3 and 6 weeks of age in both the male and female animal. Several xenobiotics, such as Aroclor 1254, mirex, and 3-methylcholanthrene, repressed cytochrome P-450j levels when administered to male rats. Isoniazid, dimethyl sulfoxide, pyrazole, 4-methylpyrazole, and ethanol were inducers of cytochrome P-450j in rat liver although these compounds showed different inducing potencies. Microsomes from adult male rats with chemically induced diabetes also contained elevated levels of cytochrome P-450j compared to untreated animals. Cytochrome P-450j levels were measurable in kidney, whereas this isozyme was barely detectable in lung, ovaries, and testes; however, extrahepatic cytochrome P-450j was inducible by isoniazid. Approximately 80-90% of microsomal N-nitrosodimethylamine demethylation was inhibited by antibody to cytochrome P-450j whether the microsomes were isolated from untreated rats or animals administered inducers or repressors of cytochrome P-450j. The residual catalytic activity resistant to antibody inhibition may be a reflection of the inaccessibility of a certain amount of cytochrome P-450j due to interference by NADPH-cytochrome P-450 reductase based on results obtained with the reconstituted system. There was a good correlation (r2 = 0.87) between cytochrome P-450j content and N-nitrosodimethylamine demethylase activity in microsomes from rats of different ages and treated with various xenobiotics. The evidence presented indicates that cytochrome P-450j is the primary, and perhaps sole, microsomal catalyst of N-nitrosodimethylamine demethylation at substrate concentrations relevant to hepatocarcinogenesis induced by N-nitrosodimethylamine.     

Effects of diabetes and recurrent hypoglycemia on the regulation of the sympathoadrenal system and hypothalamo-pituitary-adrenal axis

Epinephrine, norepinephrine, and corticosterone responses to hypoglycemia are impaired in diabetic rats. Recurrent hypoglycemia further diminishes epinephrine responses. This study examined the sympathoadrenal system and hypothalamo-pituitary-adrenal axis for molecular adaptations underlying these defects. Groups were normal (N) and diabetic (D) rats and diabetic rats exposed to 4 days of 2 episodes/day of hyperinsulinemic hypoglycemia (D-hypo) or hyperinsulinemic hyperglycemia (D-hyper). D-hypo and D-hyper rats differentiated effects of hypoglycemia and hyperinsulinemia. Adrenal tyrosine hydroxylase (TH) mRNA was reduced (P < 0.05 vs. N) 25% in all diabetic groups. Remarkably, mRNA for phenylethanolamine N-methyltransferase (PNMT), which converts norepinephrine to epinephrine, was reduced (P < 0.05 vs. all) 40% only in D-hypo rats. Paradoxically, dopamine beta-hydroxylase mRNA was elevated (P < 0.05 vs. D, D-hyper) in D-hypo rats. Hippocampal mineralocorticoid receptor (MR) mRNA was increased (P < 0.05 vs. N) in all diabetic groups. Hippocampal glucocorticoid receptor (GR), hypothalamic paraventricular nucleus (PVN) GR and corticotropin-releasing hormone (CRH), and pituitary GR and proopiomelanocortin (POMC) mRNA levels did not differ. We conclude that blunted corticosterone responses to hypoglycemia in diabetic rats are not due to altered basal expression of GR, CRH, and POMC in the hippocampus, PVN, and pituitary. The corticosterone defect also does not appear to be due to increased hippocampal MR, since we have reported normalized corticosterone responses in D-hypo and D-hyper rats. Furthermore, impaired epinephrine counterregulation in diabetes is associated with reduced adrenal TH mRNA, whereas the additional epinephrine defect after recurrent hypoglycemia is associated with decreases in both TH and PNMT mRNA.     

Developmental regulation of hepatic testosterone hydroxylases: simultaneous activation and repression of constitutively expressed cytochromes P450 in senescent rats

The aging process is generally associated with marked decreases in the activities of numerous enzymes as well as lower levels of sex hormones such as testosterone. We therefore examined testosterone metabolism in liver microsomes from individual 3- and 24-month-old male rats. Although the old rats exhibited lower 16 alpha-, 6 beta-, and 2 alpha-hydroxylase activities than the young rats, the old rats had a higher 7 alpha-hydroxylase activity. Immunoquantitation of P450a, a known 7 alpha-hydroxylase, showed that the level of this protein was elevated in the old rats, and was correlated with 7 alpha-hydroxylase activity. The mRNA for P450a was measured with a cDNA probe and its level was fivefold higher in the old rats, whereas levels of mRNA coding for a 6 beta-hydroxylase P450 were markedly decreased. The increased expression of cytochrome P450a demonstrates that the observed common decrease in cytochrome P450-catalyzed activities with senescence is not a universal phenomenon. Thus, constitutive expression of specific cytochrome P450 genes is repressed or activated in senescent rats.     

Ligand-dependent maintenance of ethanol-inducible cytochrome P-450 in primary rat hepatocyte cell cultures

Administration of ethanol, dimethylsulphoxide, 2-propanol or imidazole to rats caused 2-7-fold increases in the level of hepatic ethanol-inducible cytochrome P-450 (P-450j), without any concomitant enhancement of corresponding mRNA. All the compounds were able to stabilize P-450j in hepatocyte cultures for at least three days, whereas P-450j mRNA rapidly disappeared from the cultures. A correlation was reached between the concentration of Me2SO, ethanol and 2-propanol necessary to maintain P-450j in the cell cultures and their binding affinities to the enzyme. It is suggested that the ligand-bound form of P-450j in the hepatocytes is protected from degradation.     

Mechanism of induction of cytochrome P-450ac (P-450j) in chemically induced and spontaneously diabetic rats

Previous studies demonstrated that a microsomal high-affinity N-nitrosodimethylamine demethylase activity and cytochrome P-450ac (an acetone/ethanol-inducible form) were induced by streptozotocin-induced diabetes in rats. In the present work, the induction was studied in detail in two chemically induced (by streptozotocin and alloxan) diabetic rat models and one spontaneously (BB/Wor) diabetic rat model. All the diabetic conditions caused increases in three parameters: (a) microsomal N-nitrosodimethylamine demethylase activity which is known to be a good indicator of the level of P-450ac; (b) the levels of P-450ac as determined by immunoblot analysis; and (c) the levels of mRNA of P-450ac as determined by hybridization assays with a cDNA probe for this enzyme. These increases were abolished by treatment of the diabetic rats with insulin. The results suggest that the pathophysiological condition of diabetes is responsible for the induction of P-450ac and elevation of mRNA is involved in all of the three diabetic models investigated.     

Decrease in the levels of a constitutive cytochrome P-450 (RLM5) in hepatic microsomes of diabetic rats

Cytochrome P-450 dependent hydroxylation of testosterone has been measured in hepatic microsomes of control, diabetic and insulin-treated diabetic rats. The observed decrease in testosterone 16 alpha-hydroxylase activity in diabetes, an activity previously shown to be largely due to RLM5, was accompanied by a dramatic decrease in immunodetectable RLM5. Diabetic rats which received insulin had elevated testosterone 16 alpha-hydroxylase activity relative to the diabetic animals, which was accompanied by a corresponding increase in the levels of RLM5. These results provide evidence that specific constitutive cytochrome P-450 enzymes are altered in the diabetic state and that these changes are not permanent since they can be overcome, at least partially, by insulin replacement therapy.     

Hepatic P450 expression in hypothyroid rats: differential responsiveness of male-specific P450 forms 2a (IIIA2), 2c (IIC11), and RLM2 (IIA2) to thyroid hormone

Studies carried out in hypophysectomized adult rats have demonstrated that both thyroid hormone and GH can suppress hepatic expression of the steroid 6 beta-hydroxylase P450 2a (IIIA2). The present study further characterizes the influence of thyroid hormone on the expression of P450 2a and two other male-specific hepatic P450s, a steroid 2 alpha/16 alpha-hydroxylase, designated P450 2c (IIC11), and a steroid 15 alpha-hydroxylase, designated P450 RLM2 (IIA2). These studies were carried out in rats rendered hypothyroid by treatment with methimazole, which allows for the nonsurgical depletion of circulating T4, and in hypophysectomized rats. Hypothyroidism led to an increase in hepatic P450 2a (IIIA2) protein and mRNA in both male and female rats that was fully reversed by T4 replacement. In contrast, hypothyroidism decreased by 70-80% the expression of P450 2c (IIC11) activity and mRNA, but did not significantly alter the expression of P450 RLM2 (IIA2). The decrease in P450 2c (IIC11) was not reversed by T4 replacement, suggesting that it is a consequence of the loss of plasma GH pulses that occurs secondary to hypothyroidism. In agreement with these findings, T4 given to hypophysectomized rats partially suppressed the expression of P450 2a (IIIA2) mRNA, but not P450 2c (IIC11) or P450 RLM2 (IIA2) mRNA. A more complete suppression of P450 2a (IIIA2) mRNA as well as P450 2c (IIC11) mRNA was achieved when the hypophysectomized rats were treated with T3 at a supraphysiological, receptor-saturating dose. Although GH administered to intact male rats by continuous infusion fully suppressed all three male-specific P450 proteins and their mRNAs, the same treatment given to hypothyroid rats was only partially suppressive in the case of P450 2a (IIIA2) and P450 RLM2 (IIA2), unless combined with T4. In the case of P450 2c (IIC11), substantial suppression of the residual P450 present in hypothyroid rats was achieved by treatment with GH alone, despite persistent thyroid hormone deficiency. These studies demonstrate that while thyroid hormone is a negative regulator of P450 2a (IIIA2) expression and is required for the full suppression of that P450 and P450 RLM2 (IIA2) by the continuous plasma GH profiles associated with adult female rats, the suppression of P450 2c (IIC11) by continuous plasma GH is largely independent of the presence of thyroid hormone.     

Responses to insulin by two forms of rat hepatic microsomal cytochrome P-450 that undergo major (RLM6) and minor (RLM5b) elevations in diabetes

Total cytochrome P-450 levels rise in diabetic rats. Two specific forms of cytochrome P-450 that are elevated have been isolated from liver microsomes of streptozotocin-induced idabetic male rats. One enzyme, termed RLM6, metabolizes aniline and acetol, but not testosterone, in a reconstituted system with NADPH-cytochrome P-450 reductase. RLM6 is isolated as a high spin cytochrome with a minimum molecular weight of 53,500. It has a unique amino-terminal amino acid sequence lacking methionine at the amino-terminal position. Polyclonal antibodies to RLM6 recognized most other forms of cytochrome P-450 in Western blots, but could be made monospecific by adsorption to cross-reacting proteins coupled to Sepharose 4B. Using the monospecific antibodies, RLM6 was estimated to be present in microsomes of untreated male rats at 0.04 nmol/mg protein (5% of total P-450). In chronically diabetic rats this level rose to 0.35 nmol/mg protein and 24% of the P-450 content. Immunoreactive protein of molecular weight identical to RLM6 was elevated in microsomes of non-diabetic rats treated with ethanol, acetone, or isoniazid as well as in rats starved for 48 h. Insulin treatment of diabetic rats for 1 week lowered the immunologically detectable levels of RLM6 to levels found in the untreated rat. The other form of cytochrome P-450, RLM5b, does not metabolize aniline and only poorly metabolizes acetol and testosterone. This 52.5-kDa protein is isolated as a predominantly (60%) high spin enzyme. It has a unique NH2-terminal amino acid sequence with methionine as the terminal residue, and is present in untreated male rat liver microsomes at 0.16 nmol/mg protein. It is elevated in diabetes, like RLM6, but treatment with insulin for 1 week does not completely restore the microsomal content to that of the non-diabetic rat.     

Pregnenolone 16 alpha-carbonitrile-inducible P-450 gene family: gene conversion and differential regulation.

A cytochrome P-450 cDNA clone, designated pP450PCN2, homologous to the previously characterized pregnenolone 16 alpha-carbonitrile (PCN)-induced P-450 cDNA (pP450PCN1; F. J. Gonzalez, D. W. Nebert, J. P. Hardwick, and C. B. Kasper, J. Biol. Chem. 260:7435-7441), was isolated from a rat liver cDNA expression library by use of a polyclonal anti-P450PCN1 antibody. This P-450 cDNA contains 2,014 base pairs and yields an open reading frame of a protein consisting of 504 amino acids (Mr = 57,760). P450PCN2 cDNA and protein shared 90% nucleotide and 89% amino acid similarity with P450PCN1 cDNA and protein, respectively. The 5' untranslated, coding, and 3' untranslated regions between the two cDNAs share 94, 93, and 79% similarities, respectively. Nucleotide differences in the coding regions, however, are not evenly distributed. Complete homology exists between the two mRNAs for 425 nucleotides (positions 346 through 771). Other regions of 93 nucleotides containing only one difference and 147 nucleotides containing two differences exist toward the 3' end of the coding regions. These data suggest the possibility that a gene conversion event(s) have occurred subsequent to duplication of the ancestral P450PCN gene. Oligonucleotide probes unique for P450PCN1 and P450PCN2 cDNAs were used to examine the levels of their respective mRNAs in noninduced and PCN-induced liver cells and in male and female rats of various ages. P450PCN1 mRNA was not detectable in either male or female rats at any ages. In contrast, P450PCN2 mRNA was present at a low level in newborn rats and became elevated in both males and females at 1 week of age. Levels of p450PCN2 mRNA continued to increase in males until 12 weeks, whereas the mRNA in females reached peak levels at 2 weeks of age but declined continuously at the onset of puberty (between 4 and 12 weeks). These levels of P45PCN2 mRNA closely parallel the increases in testosterone 6 beta-hydroxylase activity and P450PCN2 protein level, as analyzed by Western blots. P450PCN1 mRNA was induced by PCN, dexamethasone, and phenobarbital in both male and female rats. P450PCN2 mRNA was not significantly induced by PCN or dexamethasone but was readily induced by phenobarbital. Testosterone 6 beta-hydroxylase activity was also induced severalfold by PCN, dexamethasone, and phenobarbital. These data demonstrate that P450PCN1 and P450PCN2 genes are differentially regulated during development and after administration of inducing compounds and furthermore suggest that both enzymes possess testosterone 6 beta-hydroxylase activity.     

Heme oxygenase-1 mRNA levels,metallothionein mRNA levels,lipid peroxidation and microsomal CYP1A activities in rats treated with 3,3-dichlorobenzidine and some other inducers of P450

The effect of 3,3-dichlorobenzidine (DCB), a potent inducer of CYP1A, on the levels of heme oxygenase-1 mRNA and metallothionein mRNAs was examined in the kidney, liver and lung of rats administered a single ip dose (157 μmol/kg) of the compound. DCB treatment increased heme oxygenase-I mRNA abundance in the kidney significantly from barely detectable levels in untreated animals; the maximum increase in the liver and lung was 24-fold and 4-fold, respectively. Hepatic microsomal heme oxygenase activity was also induced by DCB. In contrast with DCB, 2 other P450 inducers, β-naphthoflavone (β-NF) and phenobarbital did not elevate tissue HO-1 rnRNA levels. DCB pretreatment also elevated metallothionein mRNA levels in the kidney, liver and lung, with the effect in the lung being the least pronounced. In contrast with HO-1 mRNA, metallothionein mRNA was increased by the other P450 inducers examined. In vivo lipid peroxidation and in vitro NADPH-dependent microsomal lipid peroxidation were increased in the liver of DCB-treated rats but not in those of phenobarbital- or β-naphthoflavone-treated rats. Treatment with DCB or β-NF did not alter total hepatic microsomal P450 content, as measured spectrophotometrically, but induced the activity of CYP1A2. In contrast, the activity of CYP1A1 was induced to a lesser extent by DCB than by β-NF. The data show that DCB induces HO-1 as weD as P450 1A, confirm stimulation of lipid peroxidation by the compound, and suggest oxidative stress as a mechanism of HO-1 induction by the compound.     

Modulation of the rat hepatic cytochrome P450 composition by long-term streptozotocin-induced insulin-dependent diabetes

The effects of long-term insulin-dependent diabetes on the enzymatic activities of hepatic cytochrome P450 isozymes were determined in rats rendered diabetic by the administration of streptozotocin and killed 4, 8, and 12 weeks following treatment. The O-dealkylations of ethoxy-resorufin and pentoxyresorufin were elevated in the diabetic animals throughout the study, the extent of increase being similar at all three time points. p-Nitrophenol hydroxylase activity was induced in the diabetic animals 4 weeks following treatment with streptozotocin, but the extent of increase became less pronounced with the progress of the disease. A modest increase in ethylmorphine N-demethylase activity was also observed but only in the diabetic animals killed 4 weeks after the induction of diabetes. Finally, lauric acid hydroxylase activity was elevated in the diabetic animals 4 weeks following streptozotocin administration but then declined rapidly with the duration of the disease. It is concluded that the duration of diabetes modulates the hepatic cytochrome P450 profile, with the effect being isoenzyme specific. Mechanisms that may account for these changes are discussed.     

Divergent effects of cycloheximide on the induction of class II and class III cytochrome P450 mRNAs in cultures of adult rat hepatocytes

We have previously reported that when hepatocytes isolated from adult male rats are cultured in serum-free medium on matrigel, a reconstituted basement membrane gel, it is possible to elicit a stimulation of gene expression for both Class II cytochrome P450b/e and Class III cytochrome P450p by phenobarbital treatment (E.G. Schuetz et al., 1990 J. Biol. Chem. 265, 1188-1192). In the present study, an investigation of the requirement of protein synthesis for the rise in mRNAs for these cytochromes, pretreatment of the cells with cycloheximide prior to adding phenobarbital or "phenobarbital-like" inducers to the culture medium inhibited induction of P450b/e mRNA (46-90%), whereas the accumulation of P450p mRNA was enhanced (2- to 19-fold). Heme depletion did not appear to explain these observations because the inhibitory effects of cycloheximide on the induction of P450b/e mRNA were not overcome by supplementation of the medium with exogenous heme or with delta-aminolevulinic acid. Because Class IIIA P450s are regulated by gender as well as by phenobarbital, we examined the basal expression of P450p mRNA in cultures of hepatocytes derived from male rats and found that cycloheximide treatment was without effect. However, in cultures of hepatocytes isolated from female rats, where P450p mRNA is barely detectable, cycloheximide treatment greatly enhanced expression of P450p mRNA. As was observed in the cultured cells, the treatment of living female rats with cycloheximide also increased the amounts of P450p mRNA to levels comparable to those found in livers of untreated male rats. Analysis of Northern blots hybridized with oligonucleotides specific for P450PCN1(IIIA1) and P450PCN2(IIIA2), respectively, revealed that untreated male rat liver and cultures of hepatocytes prepared from these animals expressed readily detectable amounts of P450PCN1(IIIA1) mRNA. Such analyses confirmed that cycloheximide treatment selectively increased P450PCN1(IIIA1) mRNA in female rat liver, whereas the amount of mRNA for P450PCN2(IIIA2), a closely related male-specific family member, was unaffected. We conclude that the pathways for the induction of P450b/e and P450p by phenobarbital, and the pathways for the gender-specific basal expression of P450PCN1(IIIA1) and P450PCN2(IIIA2) are not the same and can be distinguished by their differential response to inhibition of ongoing protein synthesis.     

Suppression of male-specific cytochrome P450 2c and its mRNA by 3,4,5,3',4',5'-hexachlorobiphenyl in rat liver is not causally related to changes in serum testosterone

Rat cytochrome P450 2c (P450 gene IIC11) is a constitutive, male-specific hepatic enzyme which is suppressed greater than 90% by treatment with 3,4,5,3',4',5'-hexachlorobiphenyl (HCB) [H. N. Yeowell et al. (1987) Mol. Pharmacol. 32, 340-347]. HCB also decreases serum testosterone levels in adult male rats (greater than 98% loss). The present study assesses whether the suppression of P450 2c by HCB is a direct result of its effects on serum testosterone levels. Further, the site along the hypothalamic-pituitary-testicular axis at which HCB acts to depress testosterone secretion was examined. Administration of the synthetic androgen methyltrienolone to HCB-treated rats failed to prevent the suppression of P450 2c mRNA and its associated microsomal steroid 16 alpha-hydroxylase activity under conditions where it effectively reversed the large decrease in P450 2c mRNA and steroid 16 alpha-hydroxylase activity produced by castration. Hepatic steroid 6 beta-hydroxylase activity, which is catalyzed primarily by P450 2a (P450 gene IIIA2), was also suppressed by HCB and was not protected by methyltrienolone. Administration of either human chorionic gonadotropin, an analog of pituitary-derived luteinizing hormone, or the hypothalamic luteinizing hormone releasing hormone elevated serum testosterone levels to a much smaller extent in HCB-treated rats than in control rats. These results indicate that the effects of HCB on serum testosterone levels reflect its effects on testicular function rather than the pituitary or hypothalamus. However, the present study demonstrates that the consequential reduction in serum testosterone levels in HCB-treated rats is not causally related to the reduction in hepatic P450 2c levels. Thus, HCB must also act on some other regulatory mechanism involved in the expression of this protein.     

Thyroid hormone stimulation of NADPH P450 reductase expression in liver and extrahepatic tissues. Regulation by multiple mechanisms.

The role of thyroid hormone in regulating the expression of the flavoprotein NADPH cytochrome P450 reductase was studied in adult rats. Depletion of circulating thyroid hormone by hypophysectomy, or more selectively, by treatment with the anti-thyroid drug methimazole led to a 75-85% depletion of hepatic microsomal P450 reductase activity and protein in both male and female rats. Thyroxine substantially restored P450 reductase activity at a dose that rendered the thyroid-depleted rats euthyroid. Microsomal P450 reductase activity in several extrahepatic tissues was also dependent on thyroid hormone, but to a lesser extent than in liver (30-50% decrease in kidney, adrenal, lung, and heart but not in testis from hypothyroid rats). Hepatic P450 reductase mRNA levels were also decreased in the hypothyroid state, indicating that the loss of P450 reductase activity is not a consequence of the associated decreased availability of the FMN and FAD cofactors of P450 reductase. Parallel analysis of S14 mRNA, which has been studied extensively as a model thyroid-regulated liver gene product, indicated that P450 reductase and S14 mRNA respond similarly to these changes in thyroid state. In contrast, while the expression of S14 and several other thyroid hormone-dependent hepatic mRNAs is stimulated by feeding a high carbohydrate, fat-free diet, hepatic P450 reductase expression was not increased by this lipogenic diet. Injection of hypothyroid rats with T3 at a supraphysiologic, receptor-saturating dose stimulated a major induction of hepatic P450 reductase mRNA that was detectable 4 h after the T3 injection, and peaked at approximately 650% of euthyroid levels by 12 h. However, this same treatment stimulated a biphasic increase in P450 reductase protein and activity that required 3 days to reach normal euthyroid levels. T3 treatment of euthyroid rats also stimulated a major induction of P450 reductase mRNA that was maximal (12-fold increase) by 12 h, but in this case no major increase in P450 reductase protein or activity was detectable over a 3-day period. Together, these studies establish that thyroid hormone regulates P450 reductase expression by pretranslational mechanisms. They also suggest that other regulatory mechanisms, which may involve changes in P450 reductase protein stability and/or changes in the translational efficiency of its mRNA, are likely to occur.     

Suppression in the expression of a male-specific cytochrome P450, P450-male: difference in the effect of chemical inducers on P450-male mRNA and protein in rat livers

Hypophysectomy of male adult rats caused a 70% decrease in the hepatic level of mRNA hybridized to two specific oligonucleotide probes for the sequence of coding and 3'-noncoding regions of P450(M-1) (H. Yoshioka et al., (1987) J. Biol. Chem. 262, 1706-1711), which corresponds to P450-male. Treatment of hypophysectomized male and female rats with subcutaneous injection of human growth hormone twice a day for 7 days increased the mRNA to a level similar to that of normal male rats. In contrast, the mRNA was decreased by treatment with continuous infusion. These results correlated well with those on the amounts of P450-male protein, indicating that growth hormone regulates the hepatic level of P450-male protein mainly by acting at the pretranslational step. Treatment of adult male rats with phenobarbital (PB), dexamethasone (Dex), or 3-methylcholanthrene (MC) decreased the content of P450-male protein by 68, 36, and 46%, respectively. The content of P450-male protein was also decreased to 65% in Dex-treated hypophysectomized male rats, but was not changed by treatment of hypophysectomized male rats with PB or MC, suggesting that PB and MC decrease P450-male protein through a pituitary growth hormone-mediated process. However, the level of mRNA hybridizable to the P450-male oligonucleotide probe was not decreased, but rather it increased in PB- or Dex-treated hypophysectomized male rats. A similar inconsistent change in protein and mRNA was also observed in PB-treated normal rats. These results indicate that PB and Dex have an additional effect of increasing the hepatic level of the specific mRNA of P450-male/(M-1) or a closely related form. Noncoordinate changes in the level of P450-male protein and mRNA also suggest that the hepatic level of P450-male protein is regulated by plural mechanisms: pretranslational and translational regulation in which pituitary growth hormone and/or other endocrine factors are involved.