Analysis of the promoter region of the gene encoding mouse cholesterol side-chain cleavage enzyme

Recombinant murine tumor necrosis factor-alpha inhibits cholesterol side-chain cleavage cytochrome P450 and insulin-like growth factor-I gene expression in rat Leydig cells

The purpose of the present study was to evaluate the effects of murine recombinant tumor necrosis factor-alpha (TNF-alpha) on rat Leydig cell function. In primary cultures of Leydig cells, we found that in the presence of hCG (10 ng/ml), testosterone levels were markedly elevated, 69.3 +/- 3.1 ng/10(6) cells/h (mean + SE). TNF-alpha in a concentration of 1 ng/ml markedly inhibited testosterone biosynthesis (a 69% reduction; p < 0.01) and 100 ng/ml of TNF-alpha almost completely inhibited testosterone formation (p < 0.001). TNF-alpha (10 ng/ml) inhibited hCG (0.1, 1 and 10 ng/ml)-induced testosterone formation by 63%, 67% and 61%, respectively. TNF-alpha (10 ng/ml) also markedly inhibited 8-bromo cAMP-induced testosterone formation from 76 +/- 9 ng/10(6) cells/h to 4.9 ng/10(6) cells/h. This indicates that the major effect of TNF-alpha is at steps beyond LH receptor site. To further evaluate the site(s) of action of TNF-alpha, we evaluated its effect on the conversion of precursor steroids to testosterone. We found that the addition of 20-hydroxy-cholesterol could not reverse inhibitory effects of TNF-alpha on hCG-induced testosterone formation. TNF-alpha had no effect on the conversions of pregnenolone, 17-OH-pregnenolone, DHEA and androstenedione to testosterone. This indicates that the major effect of TNF-alpha is at the key steroidogenic enzyme, P450scc. We reported previously that human recombinant TNF-alpha had no effect on hCG-induced testosterone formation but did enhance the inhibitory effects of human recombinant IL-1beta. In the present study, we demonstrated that both murine TNF-alpha and human IL-1beta were potent inhibitors of hCG-induced testosterone formation. IL-1beta alone in concentrations of 0.1, 1 and 10 ng/ml inhibited testosterone formation by 45%, 62% and 91%, respectively, in the presence of TNF-alpha (10 ng/ml), IL-1beta in a concentration as low as 0.1 ng/ml completely blocked hCG-induced testosterone formation. We next evaluated the effect of TNF-alpha on P450scc gene expression. There was no constitutively expressed P450scc mRNA in Leydig cells after 24 h in culture. In response to hCG, there was a 33-fold increase in the P450scc mRNA level. Both TNF-alpha and IL-1beta inhibited hCG-induced expression of P450scc mRNA. Finally, the effect of TNF-alpha on IGF-I gene expression was investigated since IGF-I enhances Leydig cell androgen formation and IGF-I gene is expressed in high levels in Leydig cells. TNF-alpha inhibited both large (7.4 kb) and small species (0.8-1.2 kb) IGF-I mRNA levels in a dose-dependent manner. In conclusion, murine TNF-alpha is a potent inhibitor of Leydig cell function. TNF-alpha inhibited both P450scc and IGF-I mRNA gene expression.

Differential regulation of the CYP11A1 (P450scc) and ferredoxin genes in adrenal and placental cells

The regulation of the genes encoding cholesterol side-chain cleavage enzyme (P450scc) and ferredoxin, two components in the first step of steroid synthetic pathways, was studied by RNA analyses of endogenous and transfected genes. cAMP rather than calcium was the major secondary messenger that stimulated expression of both P450scc and ferredoxin genes in human placental JEG-3 cells. The effect of cAMP on P450scc expression was abolished by cycloheximide in JEG-3 cells, but it was superinduced in mouse adrenal Y1 cells. For ferredoxin expression, both reagents have synergistic effect in Y1 and JEG-3 cells. To test the mechanism of regulation, DNA segments containing regulatory elements of the P450scc and ferredoxin genes were connected to reporter genes and analyzed in cotransfection experiments. The results showed that the proximal cAMP-responsive sequences of both P450scc and ferredoxin genes were stimulated by cAMP early in both Y1 and JEG-3 cells, requiring no new protein synthesis. This indicates a common mechanism for the regulated expression of both genes. P450scc possessed an additional upstream cAMP-responsive sequence that also responded to cAMP induction in a different manner from the proximal element. The presence of additional upstream regulatory elements makes it possible for the P450scc gene to be further regulated.

Effect of insulin-like growth factor-I on cholesterol side-chain cleavage cytochrome P450 messenger ribonucleic acid expression in ovarian theca-interstitial cells stimulated to differentiate in vitro

Currently available evidence supports the hypothesis that insulin-like growth factor-I (IGF-I) may play a role in stimulating ovarian theca-interstitial cell (TIC) differentiation in preantral follicles. The purpose of the present studies was to examine the potential role of IGF-I in TIC differentiation by determining the effects of IGF-I on cholesterol side-chain cleavage cytochrome P450 (P450SCC) mRNA expression in TIC stimulated to differentiate in vitro. TIC were isolated from the ovaries of hypophysectomized immature rats by Percoll gradient centrifugation and cultured in the presence and absence of LH and IGF-I up to 6 days. At various times cytoplasmic RNA was extracted from the TIC and P450SCC mRNA was measured by specific assay using reverse transcription followed by the polymerase chain reaction. Increasing concentrations of LH (0-1 microgram/ml) stimulated a dose-related increase in P450SCC mRNA (ED50 = 36.2 +/- 5.5 ng/ml) which reached maximal levels at 100 ng/ml of LH. Addition of IGF-I (30 ng/ml) caused a small increase in P450SCC mRNA over TIC treated with LH alone but did not alter the ED50 for LH stimulation. IGF-I alone also stimulated an increase in P450SCC mRNA which reached approximately 3-fold over unstimulated levels at 100 ng/ml. In the presence of LH, IGF-I stimulated a dose-related increase in P450SCC mRNA (ED50 = 1.2 +/- 0.05 ng/ml). Time-course studies revealed that expression of P450SCC mRNA was greatest at 2 days in TIC treated with IGF-I alone, LH alone or LH plus IGF-I and then declined at 4 and 6 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of CYP11A gene expression in bovine ovarian granulosa cells in primary culture by cAMP and phorbol esters is conferred by a common cis-acting element

Production and secretion of steroid hormones throughout the ovarian cycle occurs in a highly episodic and coordinated fashion that requires precise and finely tuned regulatory mechanisms. The regulation of ovarian steroidogenesis by the gonadotropin follicle stimulating hormone (FSH) and luteinizing hormone (LH) as well as by other factors occurs, at least in part, at the level of expression of the genes encoding steroidogenic enzymes. The present study is aimed at the elucidation of regulatory mechanisms by which cyclic adenosine monophosphate (cAMP) and protein kinase C regulate cytochrome P450scc (CYP11A) gene expression in bovine granulosa cells in primary culture. As a first step we characterized the bovine granulosa cell cultures with regard to regulation of P450scc activity and mRNA levels upon treatment with forskolin and/or the phorbol ester TPA. Forskolin, a potent stimulator of cAMP generation, increased both progesterone secretion and P450scc mRNA levels. In contrast, treatment with TPA alone decreased both basal progesterone production and P450scc mRNA accumulation. Co-treatment with forskolin and TPA decreased progesterone and P450scc mRNA levels as compared to forskolin treatment alone. The possibility that TPA interfered with the forskolin-stimulated cAMP production could be excluded because simultaneous treatment of granulosa cells with TPA and forskolin potentiated the formation of cAMP. In order to identify regulatory sequences within the 5' flanking region of the bovine CYP11A gene, chimeric DNA constructs comprizing regions of the CYP11A gene fused to a beta-globin-derived reporter gene were transfected into granulosa cells in primary culture.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of steroid hydroxylase gene expression: importance to physiology and disease

Steroid hydroxylase gene expression is multifactorial in nature, being regulated by tissue-specific, developmental, constitutive and signal transduction systems. The biochemistry of this complex pattern of regulation is not yet clearly elucidated, but studies in several laboratories have led to an understanding of specific aspects of regulation, particularly that involving signal transduction. The complexity of regulation appears to be necessary for normal human physiology because of the wide variety of steroid hormones produced by these enzymes. Genetic diseases associated with the steroid hydroxylases provide examples of how aberrant physiology can result from alterations in the multifactorial regulation of steroid hydroxylase gene expression.

Transcriptional regulation of the adrenal steroidogenic enzymes

Corticosteroid biosynthesis requires the concerted action of a related group of cytochrome P450 steroid hydroxylases. The genes encoding these steroid hydroxylases exhibit two distinct levels of transcriptional regulation: selective expression in steroidogenic cells and induction in response to trophic hormones. With respect to cell-selective expression, recent studies have identified a nuclear receptor protein expressed only in steroidogenic cells that is postulated to regulate the expression of all cytochrome P450 steroid hydroxylases through common promoter elements. In contrast, the coordinate responses of these genes to trophic hormones are not readily explained by a unifying mechanism, and their hormone responsive expression probably involves multiple promoter elements.

Promoter elements of the mouse 21-hydroxylase (Cyp-21) gene involved in cell-selective and cAMP-dependent gene expression

Cyp-21 (the mouse steroid 21-hydroxylase gene) is expressed exclusively in cells of the adrenal cortex, is induced by ACTH and cAMP, and is required for corticosteroid synthesis. This review examines the molecular basis for the regulated expression of Cyp-21 in the ACTH-responsive, mouse adrenocortical tumor cell line, Y1. We demonstrate that 330 bp of 5'-flanking DNA from the Cyp-21 gene are sufficient for cell-selective and ACTH-induced expression of Cyp-21, and that this promoter region comprises multiple, closely spaced enhancer elements each of which is required for promoter function. Within this promoter, we define three related elements that contain variations of an AGGTCA motif and that contribute to the cell-selective expression of Cyp-21. Variations of these same AGGTCA-bearing elements are also involved in the expression of Cyp 11a and Cyp 11b in Y1 adrenocortical cells. These elements interact with the same or closely related nuclear proteins found only in steroidogenic cell lines. Taken together, these results suggest that shared elements contribute to the adrenal cell-selective expression of at least three steroidogenic cytochrome P450 genes. The element at -170 and the related elements at -65, -140 and -210 in the Cyp-21 promoter are not active as enhancers in the mutant Y1 cell line, Kin-8. Kin-8 cells contain a mutation in the regulatory subunit of the type 1 cAMP-dependent protein kinase that renders the enzyme resistant to activation by cAMP. Therefore, these elements appear to be selectively dependent upon an intact cAMP-dependent protein kinase for enhancer function. Individually, none of these elements confer cAMP-dependence to a reporter gene driven by a heterologous promoter. On the basis of these observations, we suggest that ACTH- and cAMP-dependent expression of Cyp-21 requires the combined actions of the element at -170, and the related elements at -140, -210 and -65.

In vivo regulation of gene expression of enzymes controlling aldosterone synthesis in rat adrenal

We studied the effect of alterations in the intake of sodium and potassium as well as changes in circulating adrenocorticotropin (ACTH) on the expression of the two rate-limiting systems of aldosterone formation in the rat. Low sodium and high potassium intake promoted time-dependent increases in the zona glomerulosa cytochrome P450scc (P450scc) and cytochrome P450c11 (P450c11) protein and mRNA levels, but no changes were found in the zona fasciculata-reticularis. In addition, these responses were associated with markedly elevated transcriptional activities. To further define the contribution of P450c11 and P450c18 (aldosterone synthase) in response to these differing intakes, we evaluated their mRNA levels using gene-specific oligonucleotide probes. P450c18 mRNA was restricted to the zona glomerulosa, whereas P450c11 mRNA was detected in both zona glomerulosa and zona fasciculata-reticularis. Furthermore, only P450c18 mRNA was induced by both low sodium or high potassium intake, as P450c11 mRNA levels remained unchanged. Captopril, an inhibitor of angiotensin-I converting enzyme, abolished the enhancing effects of the low sodium regimen on P450scc and P450c18 mRNA levels. Captopril also suppressed the augmentation of P450c18 mRNA observed with potassium supplementation but had no effect on P450scc mRNA levels. When the hypocholesterolemic drug 4-aminopyrazolopyrimidine (4-APP) was administered to rats for 3 consecurive days, both the level of plasma ACTH and the adrenal content of mRNA encoding P450scc increased 24 h post final injection. The coadministration of dexamethasone with 4-APP prevented these increases. In contrast, the mRNA content of P450c11 remained at control levels. In conclusion, this work demonstrates that variations in the intake of sodium and potassium act on the expression of the CYP11B2 gene, but not on that of the CYP11B1 gene. Moreover angiotensin-II (A-II) is an important factor in this mechanism of action. Both ions also enhance the expression of the CYP11A1 gene. A-II appears to participate in the mechanism of action of the low sodium intake at this level. Another mechanism is postulated for the action of potassium supplementation since captopril did not prevent the increased expression of the CYP11A1 gene. In addition, the fact that 4-APP enhanced the mRNA level of P450scc but not that of P450c, also demonstrates different regulation of the P450s involved at the early and final steps of aldosteroone formation in the rat adrenal zona glomerulosa in vivo.

Hormonal regulation of steroidogenic enzyme gene expression in Leydig cells

In normal mouse Leydig cells, steady state levels of mRNA of CYP11A, 3β-hydroxysteroid dehydrogenase Δ⁵- >Δ⁴-isomerase (3βHSD), and CYP17 are differentially regulated. There is high basal expression of 3βHSD and CYP11A mRNA, while expression of CYP17 mRNA is absolutely dependent on cAMP stimulation. cAMP is required for maximal expression of all three enzymes. The expression of CYP11A in normal mouse Leydig cells is repressed by glucocorticoids. Glucocorticoids also repress both basal and cAMP-induced expression of 3βHSD mRNA, but do not repress the synthesis or mRNA levels of CYP17. cAMP induction of 3βHSD mRNA can be observed only when aminoglutethimide (AG), an inhibitor of cholesterol metabolism, is added to the Leydig cell cultures. The addition of AG also markedly increases cAMP induction of CYP17 mRNA levels. Addition of testosterone or the androgen agonist, mibolerone, to cAMP plus AG treated cultures reduced 3βHSD and CYP17 mRNA levels to levels comparable to those observed when cells were treated with cAMP only. These data indicate that testosterone acting via the androgen receptor represses expression of both CYP17 and 3βHSD. The role of protein synthesis in mediating the cAMP induction of 3βHSD, CYP17 and CYP11A was examined. The addition of cycloheximide, an inhibitor of protein synthesis, to cAMP treated cultures for 24 h completely suppressed both constitutive and cAMP-induced 3βHSD mRNA levels. Cycloheximide also repressed cAMP-induced levels of CYP17 to 12% of levels observed in the absence of cycloheximide. In sharp contrast, treatment for 24 h with cycloheximide did not suppress cAMP induction of CYP11A mRNA, but reduced basal levels by approx. 50%. These data indicate that newly synthesized protein(s) are required for cAMP induction of CYP17 and 3βHSD mRNA levels, but not for CYP11A mRNA. A mouse Cyp17 genomic clone containing the entire coding region plus 10 kb of 5' flanking region has been isolated. Fragments of 5' flanking sequences were subcloned into vectors containing the CAT reporter gene and transfected into MA-10 Leydig cells. Transfected cells were treated with cAMP and expression was determined by measuring CAT activity. A cAMP responsive element was identified in a region between -245 and -346 bp relative to the transcription initiation site of Cyp17. Cotransfection into MA-10 Leydig cells of constructs containing 4.5 kb of Cyp17 5' flanking sequences together with a mouse androgen receptor expression vector demonstrate a dose dependent repression of cAMP-induced Cyp17 transcription by the androgen receptor. Studies with the mouse Cyp11a gene demonstrate that the 5' flanking region of the gene contains sequences between 2.5 and 5 kb that are necessary for expression of mouse Cyp11a in Leydig cells but not in adrenal cells.

Regulation of the cholesterol side-chain cleavage cytochrome P-450 and adrenodoxin mRNAs in cultured choriocarcinoma cells

Cytochrome P-450scc (P-450scc) catalyzes the cholesterol side-chain cleavage reaction, a rate-limiting enzymatic step for progesterone synthesis in trophoblastic and other steroidogenic cells. Adrenodoxin is the iron/sulfur protein donating electrons to P-450scc during this reaction. We examined the effects of cholera toxin (CT), an activator of adenylate cyclase, and 12-O-tetradecanoylphorbol acetate (TPA), a phorbol ester protein kinase C activator, on the levels of mRNAs encoding P-450scc and adrenodoxin in JEG-3 choriocarcinoma cells. CT induced in a concentration- and time-dependent manner P-450scc and adrenodoxin mRNA levels to 8-fold and 1.5-fold above that of control, respectively. TPA also increased P-450scc and adrenodoxin mRNA levels about 3-fold and 1.5-fold above that of control, respectively. Epidermal growth factor (EGF) was found to weakly induce P-450scc mRNA accumulation with a maximal 20% stimulation above basal levels. The effects of CT and TPA were apparently additive on both mRNAs. The protein synthesis inhibitor cycloheximide diminished basal, CT-, TPA-, and EGF-stimulated P-450scc mRNA accumulation whereas the opposite was observed for the adrenodoxin mRNA. Insulin-like growth factor I (IGF-I) appeared to have no effect on either mRNA. These data indicate that: (1) the accumulation of P-450scc and adrenodoxin mRNAs is mainly controlled by the cyclic adenosine 3',5'-monophosphate (cAMP)-dependent pathway but their stimulation by TPA- and EGF-induced signals may also play a weaker synergistic role; (2) the protein synthesis inhibitor cycloheximide inhibits basal, CT-, TPA- and EGF-stimulated P-450scc mRNA levels while it increases the expression of adrenodoxin mRNA suggesting that in the malignant trophoblasts these two enzyme mRNAs are differentially controlled.

The role of transcriptional regulation in steroid hormone biosynthesis

The regulated expression of the genes encoding the various steroidogenic enzymes is a crucial component in the control of steroid hormone biosynthesis. Tissue-specific transcription of each of the steroidogenic enzyme genes determines the array of enzymes present within a steroidogenic tissue, and therefore the types of steroid hormones the tissue produces. Transcriptional regulation also determines developmental changes in the steroid hormones synthesized by steroidogenic tissues and for the quantitative regulation of steroid hormones necessary for reproduction and for maintaining physiological homeostasis. The molecular mechanisms governing transcriptional regulation of steroidogenic enzyme genes is now being studied. The results so far indicate that, like most other genes, transcription of steroidogenic enzyme genes is regulated by cis-elements in the 5' flanking DNA of the genes that bind trans-acting proteins found in the nucleus. Several types of cis-elements have been identified: elements responsible for basal transcription, for induction by cAMP, and for both basal and cAMP induction. Some of the basal cis-elements identified may have a role in tissue-specific transcription of certain steroidogenic enzyme genes in steroidogenic tissues. We have also identified regions in both the human P450scc and human P450c17 promoters that repress transcription when activated by the Ca2+/protein kinase C intracellular second messenger system used by angiotensin II. This review summarizes our current understanding of transcriptional regulation of the steroidogenic enzyme genes.