Immunolocalization of aromatase, 17 alpha-hydroxylase and side-chain-cleavage cytochromes P-450 in the human ovary

Immunohistochemical localization of cholesterol side-chain-cleavage, 17 alpha-hydroxylase and aromtase cytochromes P-450 was performed in 35 morphologically normal human premenopausal ovaries by using specific antibodies against the enzymes. In well-developed ovarian follicles in the late stages of follicular growth, immunoreactivity of P-450AROM was only seen in granulosa cells while P-450(17 alpha) and P-450SCC activity was confined to theca interna cells, confirming that follicular oestrogen is produced in granulosa cells by the aromatization of androgens derived from the theca interna cells. In the corpus luteum, this functional differentiation is maintained, since immunoreactivity of P-450AROM was exclusively present in luteinized granulosa cells while that of P-450(17 alpha) was present in luteinized theca calls. Immunoreactivity of P-450SCC was present in both types of cells in the corpus luteum.

Regulation of steroid hydroxylase gene expression is multifactorial in nature

In summary, regulation of steroid hydroxylase gene expression is complex and multifactorial, involving cAMP-dependent and -independent mechanisms required for maintenance of optimal steroidogenic capacity, tissue-specific mechanisms which lead to different steroidogenic pathways in different tissues, and developmental mechanisms which lead to fetal imprinting of steroid hydroxylase expression and which probably overlap with both maintenance and tissue-specific mechanisms. Future studies will involve identification of the trans-acting factors associated with each of these aspects of the multifactorial regulation and characterization of the cis-regulatory elements to which they bind. Such studies will inevitably lead to the identification of genes encoding these trans-acting factors and investigation of their regulation. In this way, it will be possible to work outward from the steroid hydroxylase genes toward the cell surface receptors in order to elucidate the series of events which lead to cAMP-dependent and -independent regulation of steroid hydroxylase gene expression.

Isolation of a full-length cDNA insert encoding human aromatase system cytochrome P-450 and its expression in nonsteroidogenic cells

The isolation and cloning of a full-length cDNA insert complementary to mRNA encoding human aromatase system cytochrome P-450 is reported. The insert contains an open reading frame encoding a protein of 503 amino acids. This gene is clearly a member of the cytochrome P-450 gene superfamily, because the sequence contains regions of marked homology to those of other members, notably a putative membrane-spanning region, I helix, Ozols, and heme-binding regions. The cDNA was inserted into a modified pCMV vector and expressed in COS-1 monkey kidney tumor cells. The expressed protein was similar in size to human placental aromatase system cytochrome P-450, as detected by immunoblot analysis, and catalyzed the aromatization of androstenedione, testosterone, and 16 alpha-hydroxyandrostenedione. This activity was inhibited by the known aromatase inhibitors, 4-hydroxyandrostenedione and econazole. Thus the several steps involved in the aromatization reaction appear to be catalyzed by a single polypeptide chain, which can metabolize the three major physiological substrates.

The expression of a functional cDNA encoding the chicken cytochrome P-450arom (aromatase) that catalyzes the formation of estrogen from androgen

A complementary DNA (cDNA) copy of the aromatase P-450 has been isolated from a chicken ovary library using as probe a partial cDNA believed to encode the human placental aromatase. The predicted amino acid sequence of the chicken aromatase cDNA possesses regions of homology to that of its human counterpart, but only limited homology to other cytochrome P-450 enzymes. The introduction of the cDNA clone into COS-1 cells results in the production of high levels of aromatase activity. The chicken enzyme is targeted to the appropriate subcellular fraction in the transfected COS cells, and the apparent Km of the chicken aromatase activity, measured in microsomes prepared from the transfected cells, is similar to that of the enzyme prepared from chicken ovary microsomes. These findings establish that the cDNA clone encodes chicken ovarian aromatase and demonstrate that this protein can catalyze the three successive oxidation reactions necessary to form estrogen from androgen.

Combined 17 alpha-hydroxylase/17,20-lyase deficiency due to a stop codon in the N-terminal region of 17 alpha-hydroxylase cytochrome P-450

Steroid 17 alpha-hydroxylase (cytochrome P-450(17)alpha) mediates both 17 alpha-hydroxylase and 17,20-lyase activities. A relatively rare disease, 17 alpha-hydroxylase deficiency is characterized by defects in either or both of these activities. The molecular basis for variability of the defect is not well understood. We have determined the exonic sequence of the mutant P-450(17)alpha gene from one Japanese patient with combined 17 alpha-hydroxylase/17,20-lyase deficiencies. A stop codon (TGA) due to a single point mutation was found at the position of amino acid 17 in exon 1 of the P-450(17)alpha gene. The presence of a stop codon in the N-terminal region of this gene leads to the absence of a functional P-450(17)alpha protein in adrenal cortex and ovary, and consequently hypertension, primary amenorrhea and osteoporosis in this patient.

Effects of epidermal growth factor and insulin-like growth factor I on the levels of mRNA encoding aromatase cytochrome P-450 of human ovarian granulosa cells

The effects of growth factors to regulate the activity of aromatase, as well as the synthesis of aromatase cytochrome P-450 (P-450AROM) have been studied in human ovarian granulosa cells obtained from women undergoing oocyte retrieval. Insulin-like growth factor I (IGF-I) increased aromatase activity as well as the synthesis of P-450AROM, in a concentration-dependent fashion. The levels of hybridizable mRNA species encoding cytochrome P-450AROM were also increased with IGF-I treatment. By contrast, epidermal growth factor (EGF) had no effect on these parameters when added alone, but markedly inhibited the action of follicle-stimulating hormone (FSH) to stimulate aromatase activity, and the synthesis of cytochrome P-450AROM, as well as its ability to increase the levels of mRNA encoding the enzyme. It is concluded that these growth factors have opposite effects on aromatase activity, and that these actions reflect, in part, changes in the synthesis of cytochrome P-450AROM, which in turn are the consequence of changes in the levels of mRNA encoding this enzyme.

Secretion of progesterone and prostaglandins by cells of bovine corpora lutea from three stages of the luteal phase

The secretion of prostaglandins (PGs) by bovine corpora lutea was investigated. Corpora lutea from the early, early-mid and late-mid stages of the luteal phase were dissociated by collagenase treatment and cultured in monolayer in Dulbecco's modified Eagle's medium containing 10% (v/v) fetal calf serum. Treatment with either LH (100 ng/ml) or dibutyryl cyclic AMP (dbcAMP; 1 mmol/l) had no effect on progesterone secretion by early luteal phase cells but stimulated progesterone secretion two- to fourfold by cells from the latter stages. The secretion rates, per microgram cell protein, of 6-keto-PGF1 alpha, PGE2 and PGF2 alpha were substantially greater in cells from the early luteal phase than in those from the latter stages, however, all changes in PG secretion in response to treatments were qualitatively similar between cells from the three stages of the luteal phase. The secretion rate of 6-keto-PGF1 alpha was greater than that of PGE2 or PGF2 alpha and was inhibited by treatment with indomethacin (28 mumol/l) but unaltered by treatment with LH, dbcAMP or butyrate (1 mmol/l). Secretion of PGE2 was inhibited by indomethacin but stimulated two- to threefold by treatment with either dbcAMP or butyrate. Secretion of PGF2 alpha was minimal and not inhibited further by treatment with indomethacin, but was stimulated 10- to 40-fold with dbcAMP. Indomethacin treatment inhibited the stimulatory effect of dbcAMP; butyrate had no effect on PGF2 alpha secretion. Treatment with LH had no effect on any of the PGs measured. In these experiments the secretion of progesterone appeared unrelated to any changes in the secretion of PGs.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunoblot analysis of cholesterol side-chain cleavage cytochrome P-450 and adrenodoxin in corpora lutea of cyclic and late-pregnant sheep

The specific contents of cytochrome P-450scc and adrenodoxin in corpora lutea of late pregnant sheep were, respectively, 1/5 and 1/8 that of corpora lutea of the oestrous cycle, suggesting lower steroidogenic enzyme capacity in the former. The contents of Complex V proteins were also lower in the corpora lutea of late pregnancy. It was observed in the immunoblots of both Complex V and cytochrome P-450scc that immunoreactive bands of molecular weights lower than the native proteins were present in the samples from corpora lutea of late pregnancy, indicative of degradation of the native enzymes. It is concluded that corpora lutea of sheep during late pregnancy have a much lower enzyme capacity for steroidogenesis than do those of the oestrous cycle (mid-luteal phase) due to a reduction in the content of cytochrome P-450scc and adrenodoxin. The reduction in the levels of steroidogenic enzyme proteins appears to be unspecific and probably reflects an overall demise in mitochondrial functions.

Transcription of the bovine adrenodoxin gene produces two species of mRNA of which only one is translated into adrenodoxin

The 5'-end of the bovine adrenodoxin gene contains unique structural characteristics. Exons 1 and 2 appear to encode different presequences for this mitochondrial precursor protein. However, exon 1 contains a stop codon (TAA) in place of amino acid 15. Thus, while two species of bovine adrenodoxin mRNA arise from a single gene, only the abundant mRNA species (90%) is translated into the adrenodoxin precursor which contains the presequence encoded by exon 2. The minor mRNA species (10%) contains the sequence encoded by exon 1 and cannot be translated into an adrenodoxin precursor. Presumably the sequence within exon 2 is removed from this minor mRNA species by alternative splicing. Furthermore, the initiation of transcription of the major adrenodoxin mRNA (exon 2) lies within intron 1 of this unusual gene, while that for the other mRNA (exon 1) lies in the 5'-flanking region. Thus, the adrenodoxin gene is the first example of a gene encoding a mitochondrial protein which falls into the category of genes having alternate promoters utilizing a pattern of alternative splicing.

Structural characterization of normal and mutant human steroid 17 alpha-hydroxylase genes: molecular basis of one example of combined 17 alpha-hydroxylase/17,20 lyase deficiency

Steroid 17 alpha-hydroxylase (cytochrome P-450 17 alpha) catalyzes both 17 alpha-hydroxylation of pregnenolone and progesterone and 17,20-lysis of 17 alpha-hydroxypregnenolone and 17 alpha-hydroxyprogesterone. In the course of undertaking detailed investigation of the structure-function relationships which exist within this enzyme we have begun to elucidate the molecular basis of human deficiencies in either or both of these activities. Consequently we have determined the exonic structure of the human P-450 17 alpha gene as well as the sequences at the exon/intron boundaries and at the site of initiation of transcription. A single gene in the human genome encodes this protein, being the sole member of a unique gene family (P450XVII) within the P-450 supergene family. A protocol for exonic sequencing of the P-450 17 alpha gene has been established which permits structural analysis of the gene from patients having 17 alpha-hydroxylase and/or 17,20-lyase deficiency. This procedure has been applied to the mutant gene from one individual having combined 17 alpha-hydroxylase/17,20-lyase deficiencies. A four-base duplication is found in exon 8 producing a protein with an altered C-terminal amino acid sequence which results in loss of both enzymatic activities.

Simultaneous transfection of COS-1 cells with mitochondrial and microsomal steroid hydroxylases: incorporation of a steroidogenic pathway into nonsteroidogenic cells

Transfected, nonsteroidogenic COS-1 cells derived from monkey kidney are found to be capable of supporting the initial and rate-limiting step common to all steroidogenic pathways, the side-chain cleavage of cholesterol to produce pregnenolone. Endogenous COS-1 kidney cell renodoxin reductase and renodoxin are able to sustain low levels of this activity catalyzed by bovine cholesterol side-chain cleavage cytochrome P450 (P450scc) whose synthesis is directed by a transfected plasmid containing P450scc cDNA. Double transfection with both P450scc and adrenodoxin plasmids leads to greater pregnenolone production and indicates that adrenodoxin plays a role as a substrate for this reaction or that bovine adrenodoxin serves as a better electron donor than the endogenous iron-sulfur protein renodoxin. Also it is found that both the bovine adrenodoxin and P450scc precursor proteins are proteolytically processed upon their uptake by COS-1 cell mitochondria to forms having the same electrophoretic mobility as mature bovine adrenodoxin and P450scc. Following triple transfection of COS-1 cells with P450scc, adrenodoxin, and 17 alpha-hydroxylase cytochrome P450 plasmids, pregnenolone produced in mitochondria by the side-chain cleavage reaction can be further metabolized in the endoplasmic reticulum to 17 alpha-hydroxypregnenolone and dehydroepiandrosterone. Although this functional steroidogenic pathway can be incorporated into this nonsteroidogenic cell type, it is found to be nonresponsive to cAMP, a potent activator of steroid hormone biosynthesis in adrenal cortex, testis, and ovary. Thus the cellular mechanisms necessary to support both microsomal and mitochondrial steroid hydroxylase activities appear not to be tissue specific, whereas the acute cAMP-dependent regulation of steroidogenesis is not present in transformed kidney (COS-1) cells.

Tetradecanoyl phorbol acetate suppresses follicle-stimulating hormone-induced synthesis of the cholesterol side-chain cleavage enzyme complex in rat ovarian granulosa cells

The effect of tetradecanoylphorbol acetate (TPA) on follicle-stimulating hormone (FSH)-induced synthesis of the cholesterol side-chain cleavage (SCC) enzyme complex was studied in rat ovarian granulosa cells cultured for 48 h in serum-free medium. Cell proteins were radiolabeled with [35S]methionine, followed by immunoprecipitation of cholesterol side-chain cleavage cytochrome P-450 (P-450SCC) as well as the iron-sulfur protein adrenodoxin. Polyacrylamide gel electrophoresis and fluorography of the immunoprecipitates showed that TPA, when added in combination with FSH (50 ng/ml) or dibutyryl cAMP (Bt2cAMP; 1 mM), suppressed the stimulatory effects of these compounds on the synthesis of the SCC components in a concentration-dependent fashion. The effect of TPA was accompanied by decreased progesterone formation and decreased cAMP accumulation. The structural analog of TPA, phorbol-4 alpha-didecanoate, which does not activate protein kinase C (Ca2+/phospholipid-dependent enzyme), had no effect on the FSH- or Bt2cAMP-stimulated synthesis of SCC and progesterone or on cAMP formation. In addition to inhibiting the synthesis of these proteins, TPA greatly reduced the FSH- and Bt2cAMP-induced increase in levels of mRNA encoding the precursor form of P-450SCC. It is concluded that the effect of the phorbol ester TPA to inhibit FSH-stimulated progesterone formation in cultured ovarian granulosa cells of the rat involves decreased synthesis of the components of the SCC enzyme complex due to reduced levels of mRNA encoding the precursor forms of these proteins. The results are indicative that TPA not only inhibits FSH-mediated stimulation of cAMP formation but also may block cAMP-mediated induction of SCC synthesis. It is postulated that the effects of TPA may reflect the physiological role of protein kinase C in the regulation of ovarian steroidogenesis.

Immunohistochemical localization of aromatase cytochrome P-450 and estradiol dehydrogenase in the syncytiotrophoblast of the human placenta

Immunohistochemistry employing immunoglobulin G fractions raised against aromatase cytochrome P-450 and antiserum against 17 beta-estradiol dehydrogenase was used to localize these two steroid-converting enzymes in the human placenta. Immunostaining for both enzymes was found exclusively in the syncytiotrophoblast, while the underlying cytotrophoblast and the villus core did not stain. Ultrastructural examination of aromatase cytochrome P-450- and 17 beta-estradiol dehydrogenase-labeled sections disclosed immunoreactive product in the membranes of the endoplasmic reticulum; the nucleus, mitochondria, Golgi apparatus, and secretory granules were free of staining. These findings suggest that the syncytiotrophoblast is actively involved in the synthesis and metabolism of estrogens and in their role in placental endocrine function.

Sequencing of cDNA inserts encoding aromatase cytochrome P-450 (P-450AROM)

Two cDNA inserts complementary to mRNA encoding aromatase cytochrome P-450 (P-450AROM) have been isolated and characterized by restriction mapping and sequencing. The overlapping sequence encoded by these inserts is identical, and a putative heme-binding region has been identified. In addition, the open reading frame contains the sequences of all four cysteine-containing tryptic peptides isolated by Chen et al. (1986) from purified cytochrome P-450AROM. The inserts differ in the use of alternative poly A-addition signals, which is consistent with the presence of two major species of mRNA in human placenta, of 3.0 and 2.4 kb, which hybridize to these inserts. The identity of sequence between the two inserts and the likely presence of alternative poly A-addition signals, is suggestive that only one form of cytochrome P-450AROM is encoded by these mRNA species.

Changes in content of cytochrome P450(17)alpha, cytochrome P450scc, and 3-hydroxy-3-methylglutaryl CoA reductase in developing rat ovarian follicles and corpora lutea: correlation with theca cell steroidogenesis

The following study was undertaken to determine which hormones (luteinizing hormone, LH, and prolactin, PRL) and enzymes (cytochrome P450(17)alpha, nicotinamide adenine dinucleotide phosphate [NADPH]-cytochrome P450 reductase, 3-hydroxy-3-methylglutaryl [HMG] CoA reductase, cholesterol side-chain cleavage cytochrome P450 [P450scc], and adrenodoxin) were associated with the regulation of androgen biosynthesis by developing rat follicles and corpora lutea in vivo as well as by thecal explants maintained in culture. Immunoblots of soluble cell extracts of small antral (SA), preovulatory (PO), and luteinizing (PO + human chorionic gonadotropin [hCG], 7 h) follicles, newly formed corpora lutea (PO + hCG, 24 h), and corpora luteal isolated on Day 15 of pregnancy, demonstrated that cytochrome P450(17)alpha was low in SA follicles, selectively increased 4-fold in PO follicles, and decreased to less than 10% within 7 h after hCG. Filter hybridization assays using a 32P-labeled cytochrome P450(17)alpha cDNA probe demonstrated that changes in the content of P450(17)alpha mRNA exhibited a pattern similar to that of the enzyme. Conversely, immunoblots for other microsomal enzymes either exhibited no change (NADPH cytochrome P450 reductase) or a transient increase after the hCG surge (HMG CoA reductase), whereas the mitochondrial enzymes either increased markedly in association with luteinization (cytochrome P450scc) or were increased in a more transient manner (adrenodoxin). The LH-induced loss of cytochrome P450(17)alpha in vivo was not associated with loss of androgen biosynthesis when luteinizing theca were placed in culture in medium containing either LH or LH and PRL, suggesting that other hormones, or the presence of other cell types, are required to maintain the decrease in cytochrome P450(17)alpha in vivo. Conversely, the LH-induced increase in cytochrome P450scc in vivo was associated with the maintenance of elevated progesterone production by theca in culture, suggesting that cytochrome P450scc may be constitutively expressed in luteinized theca. Thus, thecal cell cytochrome P450(17)alpha and the regulation of its content and mRNA by LH are pivotal to the biosynthesis of androgens, the obligatory precursors for estradiol biosynthesis and the consequent development of preovulatory follicles. The molecular basis for the different effects of low versus elevated concentrations of LH on cytochrome P450(17)alpha, as well as cytochrome P450scc, remain to be determined.

Multiple species of bovine adrenodoxin mRNA. Occurrence of two different mitochondrial precursor sequences associated with the same mature sequence

Bovine adrenodoxin mRNA is found to consist of several distinct mRNA species which can be divided into two sets. Each set utilizes at least three of four separate poly(A)+ addition sites providing an explanation of the three sizes of adrenodoxin mRNA (1.75, 1.4, and 0.95 kilobases) previously observed in bovine adrenocortical RNA by this laboratory (Okamura, T., John, M.E., Zuber, M.X., Simpson, E.R., and Waterman, M.R. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 5705-5709). The two sets are distinguished from one another by a unique 5' sequence leading to two different amino acid sequences (approximately 10% homology) for the precursor portion of this nuclear encoded, mitochondrial protein. A common mature adrenodoxin sequence is encoded by both sets of mRNA. One set of RNAs is 10-fold more abundant than the other, but the levels of both sets can be induced by treatment of primary bovine adrenocortical cell cultures with adrenocorticotropin. The biological significance of these two types of adrenodoxin precursor sequences remains obscure.

Regulation by follicle-stimulating hormone of the synthesis of aromatase cytochrome P-450 in human granulosa cells

The effects of FSH to increase the activity of aromatase, as well as the synthesis of the components of the aromatase enzyme complex, have been studied in human ovarian granulosa cells obtained from women undergoing oocyte retrieval. FSH increased aromatase activity, as well as the synthesis of aromatase cytochrome P-450 (P-450AROM) in a time-dependent fashion, whereas in the absence of FSH, both activity and synthesis declined with duration of culture. The effect of FSH was mimicked by forskolin, an activator of adenylate cyclase. FSH also increased the synthesis of NADPH-cytochrome P-450 reductase, but to a relatively modest extent. The levels of hybridizable mRNA species encoding cytochrome P-450AROM of lengths 3.0, 2.4, and 1.6 kilobases were also increased with FSH treatment. It is concluded that the regulation of aromatase activity by FSH in human granulosa cells is mediated primarily by changes in the synthesis of cytochrome P-450AROM, that this action of FSH is mediated by cAMP, and that the changes in cytochrome P-450AROM synthesis are the consequences of changes in the levels of mRNA encoding this enzyme.

Vasoactive intestinal peptide regulates cholesterol side-chain cleavage cytochrome P-450 (P-450scc) gene expression in granulosa cells from immature rat ovaries

Vasoactive intestinal peptide (VIP), a neuropeptide present in ovarian nerves, has been previously shown to induce synthesis of the side-chain cleavage cytochrome P-450 enzyme which catalyzes the conversion of cholesterol to pregnenolone (the rate-limiting step in progesterone synthesis). In the present study we demonstrate, by means of a bovine 3'-specific P-450scc cDNA probe, that this VIP effect is exerted at least partially at the level of gene expression in cultured granulosa cells that were isolated from estrogen-primed, immature rats. The size and level of the 2.0 kilobase P-450scc mRNA species was assessed by Northern blot analysis, while the translatability of this mRNA was assayed by immunoisolation of the 35S-labeled P-450scc precursor protein translated from total RNA of control and stimulated granulosa cells. FSH was much more effective than VIP at increasing P-450scc mRNA concentrations in cultured granulosa cells, whereas secretin treatment was ineffective. The results suggest that, like FSH, the stimulatory effect of the neuropeptide VIP on ovarian progesterone secretion involves regulation of P-450scc gene expression during functional maturation of the prepubertal ovary.