Isolation and characterization of the cDNA encoding the spiny dogfish shark (Squalus acanthias) form of cytochrome P450c17

Changes in Sex Steroids and Ovarian Steroidogenic Enzyme mRNA Levels in Artificially Maturing Japanese Eel (Anguilla japonica) and Naturally Maturing New Zealand Longfin Eel (Anguilla dieffenbachii) during Vitellogenesis

Repeated hormone injections are routinely used to induce advanced stages of oogenesis in freshwater eels, but this approach may result in aberrant germ cell development. To investigate the underlying causes, levels of sex steroids (testosterone, T; estradiol-17β, E2) and ovarian steroidogenic enzyme mRNAs were compared between artificially maturing Japanese eels and wild-caught, spontaneously maturing New Zealand longfin eels. The latter were employed as reference, as wild Japanese eels in advanced stages of oogenesis are near-impossible to catch. Serum T levels in artificially maturing Japanese eel changed with stage in a pattern that was comparable to that in longfin eels. Likewise, ovarian mRNA levels of most steroidogenic enzyme genes were not qualitatively dissimilar between both eel species when taking developmental stage into account. However, aromatase (cyp19a) mRNA levels, together with serum E2 levels, rapidly increased in artificially maturing Japanese eels in mid-late stages of oogenesis (gonadosomatic index, GSI = 13.8%), whereas no such increase was evident in longfin eels (GSI ~ 6.9%). In addition, sex steroid and target gene mRNA levels fluctuated drastically with each hormone injection. We contend that expression of most target genes, possibly even that of cyp19a, during induced oogenesis could be “normal”, with the drastic fluctuations due most likely to hormone delivery through repeated injections. The effects of these fluctuations on gamete quality remain unknown and resolving this issue may prove fruitful in the future to further artificial propagation of anguillid eels.

Molecular cloning, characterization expression of P450c17-I and P450c17-II and their functions analysis during the reproductive cycle in males of barfin flounder (Verasper moseri)

P450c17, a key steroidogenic enzyme, plays important roles in the production of sex steroid and cortisol. In teleost, there are two types of P450c17, P450c17-I possessing 17α-hydroxylase and 17, 20-lyase activities, and P450c17-II only possessing 17α-hydroxylase activity. This work describes the molecular cloning of the cDNA encoding the barfin flounder (Verasper moseri) P450c17-I and P450c17-II by means of RT-PCR and 5' and 3' rapid amplification of cDNA ends (RACE) analyses and mRNA expression profiles analyzing by semiquantitative RT-PCR. Respectively, P450c17-I and P450c17-II mRNA levels in the testes correlated with serum testosterone (T) level, as well as gonadosomatic index (GSI) of males during specific stages of spermatogenesis. P450c17-I and P450c17-II mRNA were expressed in the testis and ovary, suggesting that both of them participate in the production of sex steroid in barfin flounder gonads. P450c17-I mRNA was undetectable; in contrast, P450c17-II mRNA was detected at the highest level in the head kidney, meaning that only P450c17-II is involved in the production of cortisol in barfin flounder. The results demonstrated that both of P450c17-I and P450c17-II participate in the production of sex steroid in male barfin flounder gonads.

Cloning and expression of P450c17-I (17α-hydroxylase/17,20-lyase) in brain and ovary during gonad development in Cynoglossus semilaevis

Cytochrome P450c17 (CYP17, 17a-hydroxylase/17,20-lyase) is a critical enzyme in the production of androgens and estrogens in vertebrates. A 2,469 bp full length cDNA of P450c17-I (CYP17A1) has been isolated from the ovary of half-smooth tongue sole, Cynoglossus semilaevis which encodes 509 amino acids. Additionally, a relatively shorter cDNA (1,742 bp), a likely result of polyadenylation, was also found. The putative P450c17-I enzyme shares high sequence identity with that of the fathead minnow (73%), zebrafish (71%), the Japanese eel (70%), catfish (70%), tilapia (79%), three-spined stickleback (81%), medaka (79%), dogfish (60%), chicken (65%), rat (47%), and human (49%). Semi-quantitative RT-PCR analysis of spatial expression showed the enzyme was predominantly expressed in the ovaries and the brain. P450c17-I was also detected in the stomach, intestine, gill, spleen, kidney, and head kidney, albeit weakly. Further examination of temporal expression pattern of P450c17-I in ovary and brain revealed developmental stage-dependency. In addition to this our data on T and E2 levels further endorse the critical role of P450c17-I during shift in steroidogenesis. Based on the present study we indicate an important role for P450c17-I during ovarian development. However, further studies are needed at transcriptional regulation level for deeper insights into the physiological functions of P450c17-I.

A role for cytochrome P450 17alpha-hydroxylase/c17-20 lyase during shift in steroidogenesis occurring in ovarian follicles prior to oocyte maturation

Cytochrome P450 17alpha-hydroxylase/c17-20 lyase (P450c17) is regarded as one of the key enzymes involved in the steroidogenic shift that occurs prior to oocyte maturation in teleosts. Role of P450c17 in the shift in steroidogenesis during oocyte maturation is a contentious issue even after identification of a novel type of P450c17 that lacks lyase activity. To understand the role of P450c17 in steroidogenic shift explicitly, a full length cDNA encoding p450c17 from ovary of air-breathing catfish, Clarias gariepinus was cloned. p450c17 transiently expressed in COS-7 cells converted progesterone to androstenedione through 17alpha-hydroxyprogesterone and catfish p450c17 was found to be expressed ubiquitously with relatively higher levels in gonads, brain, kidney and gills. Immunocytochemical analysis revealed the presence of P450C17 in follicular layer of ovarian follicle, interstitial cells and spermatocytes of testis. p450c17 expression and ratio of lyase to hydroxylase was high in preparatory and pre-spawning phases of ovary and low in spawning phase. Expression of p450c17 correlated well with testicular recrudescence with maximum expression in preparatory and spawning phases. Neither protein expression nor lyase/hydroxylase activity changed significantly during hCG-induced oocyte maturation, in vitro and in vivo though mRNA levels increased. These results tend to suggest that the ovarian follicles attains capacity to produce maximum precursor steroid levels before spawning that might contribute to the shift in steroidogenesis.

Comparison of the 17α-Hydroxylase/C17,20-Lyase Activities of Porcine, Guinea Pig and Bovine P450c17 Using Purified Recombinant Fusion Proteins Containing P450c17 Linked to NADPH-P450 Reductase

The cDNAs for cytochrome P450c17 (P450c17) of three species, pig, guinea pig, and cow, representing three families of mammals (suidae, procaviidae, and bovidae, respectively) were each engineered into an expression plasmid (pCWori+). The P450c17 domain of the coding sequence was connected to a truncated form of rat NADPH-P450 reductase by a linker sequence encoding two amino acids (SerThr). These fusion proteins were expressed in E. coli and purified for use in enzymatic assays to determine similarities and differences in 17 alpha-hydroxylase and lyase activities. The fusion proteins were found to catalyze both the 17 alpha-hydroxylation of progesterone (P4) and pregnenolone (P5) to 17 alpha-hydroxylated P4 and P5 (17 alpha-OH P4 and 17 alpha-OH P5) followed by the C17,20-lyase reaction for the conversion of these C(21)-17 alpha-hydroxylated steroids to C(19)-steroids (the C17,20-lyase reaction). These in vitro studies show that (a) porcine P450c17 possesses cytochrome b(5) (b(5))-stimulated C17,20-lyase activity that converts 17 alpha OH-P4 to androstenedione (AD) but also converts 17 alpha-OHP5 to dehydroepiandrosterone (DHEA); (b) guinea pig P450c17 possesses a b(5)-stimulated C17,20-lyase activity that converts 17 alpha-OH P4 to AD but does not convert 17 alpha-OH P5 to DHEA., and (c) bovine P450c17 possesses a b(5)-stimulated C17,20-lyase activity that converts 17 alpha-OH P5 to DHEA but does not convert 17 alpha-OH P4 to AD. Thus, the P450c17 of each species differs in its ability to catalyze in vitro the conversion of C(21)-steroids to C(19)-steroids. In addition, each P450c17 is capable of catalyzing additional hydroxylation reactions leading to low levels of 2 alpha-, 6 beta-, 16- and 21-hydroxy-metabolites. Porcine P450c17 also catalyzes the b(5)-dependent synthesis of andien-beta (androsta-5,16-dien-3beta-ol) from P5. When the amino acid sequences of the three P450c17s were aligned there was an approximate 50% variation in the alignment identity (227 differences in the sequences of 509 amino acids). Alignment did not permit the assignment of specific amino acids or domains to the observed differences in enzymatic activities.

Elevated expression of P450c17 (CYP17) during testicular formation in the frog

Sex steroids play decisive roles in gonadal differentiation in many species of vertebrates. The sex can be changed by sex steroids in some species of amphibians, but the mechanism of the sex-reversal is largely unknown. In this study, we cloned and characterized 3 cDNAs encoding sex steroid-synthesizing enzymes, i.e., CYP11A1, CYP17 and 3beta-HSD from the frog Rana rugosa. RT-PCR analysis showed that the CYP17 expression was much higher in male gonads than in female ones during sex determination in R. rugosa, whereas CYP11A1 and 3beta-HSD showed no sexually dimorphic expression. When testosterone was injected into tadpoles for female-to-male sex reversal, CYP17 expression appeared to be very strong in the gonad at days 16 and 24 after injection of testosterone. CYP11A1 was also transcribed higher at day 16, but its expression was weaker when compared with that of CYP17. The expression of 3beta-HSD did not change during the sex reversal. In addition, in situ hybridization analysis revealed that CYP17 was expressed in somatic cells of the indifferent male gonad and in those of the testis. Positive signals of CYP17 were also produced in somatic cells of a female-to-male sex-reversed gonad (testis) at days 16 and 24 post testosterone injection, but not in the ovary. Taken together, the results suggest that CYP17 is very involved in testicular differentiation of the gonad in R. rugosa.

Characterization, expression and transcriptional regulation of P450c17-I and -II in the medaka, Oryzias latipes

Recently we reported the isolation of two types of cytochrome P450c17s (steroid 17alpha-hydroxylase/C17, 20 lyase) encoded by two different genes, from genomes of teleost fish. In this study, we characterized the expression profile, enzymatic activity and transcriptional regulation of P450c17-I and -II in the medaka ovary. Similar to tilapia, medaka P450c17-I possessed both hydroxylase and lyase activities, while P450c17-II possessed only the hydroxylase activity. In situ hybridization and gene expression profiles during 48h prior to spawning indicated that P450c17-I is responsible for the production of estradiol-17beta during oocyte growth, while P450c17-II for the production of 17alpha, 20beta-dihydroxy-4-pregnen-3-one during oocyte maturation and cortisol production in the head kidney. Luciferase assays and expression profiles of transcription factors as revealed by real time PCR suggested that P450c17-I and -II expression are tightly controlled by Ad4BP/SF-1, Lrh-1, Foxl2, and Dax1 during the 48 h prior to spawning.

A novel type of P450c17 lacking the lyase activity is responsible for C21-steroid biosynthesis in the fish ovary and head kidney

Cytochrome P450c17 is the single enzyme that mediates the 17 alpha-hydroxylase and 17, 20 lyase activities during the biosynthesis of steroid hormones in the gonads and adrenal gland. However, the mechanism underlying its dual action continues to be a controversy in the field of steroidogenesis in fish. In an attempt to resolve this issue, we identified a novel type of P450c17 (P450c17-II) by an in silico analysis from the genomes of six fish species. We cloned P450c17-II from tilapia and medaka, and comparison with the conventional P450c17-I revealed that they differ in gene structure and enzymatic activity. Enzymatic assays by thin-layer chromatography revealed that P450c17-II possesses only the 17 alpha-hydroxylase activity without any 17, 20 lyase activity, unlike P450c17-I, which has both these activities. In testis, both P450c17-I and -II express in the interstitial cells. Remarkable differences, revealed by in situ hybridization, in the expression patterns of the P450c17-I and -II in the ovary and head kidney of tilapia during various stages of development strongly suggest that P450c17-I is responsible for the synthesis of estradiol-17beta in the ovary, whereas P450c17-II is required for the production of C21 steroids such as cortisol in the head kidney. More interestingly, a temporally controlled switching is observable in the expression of these two genes during the steroidogenic shift from estradiol-17beta to the C21 steroid, 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (maturation-inducing hormone of fish oocytes) in the fish ovary, revealing a role for P450c17-II in the production of hormones that induce oocyte maturation in fish.

Functional expression and characterisation of human cytochrome P45017α in Pichia pastoris

Human cytochrome P45017alpha (CYP17), present in mammalian adrenal and gonadal tissues, catalyses both steroid 17-hydroxylation and C17,20 lyase reactions, producing intermediates for the glucocorticoid and androgenic pathways, respectively. The characterisation of this complex enzyme was initially hampered due to low level in vivo expression of CYP17. Heterologous expression systems have contributed greatly to our current knowledge of CYP17's dual catalytic activity. However, due to the hydrophobic nature of this membrane-bound protein, primarily truncated and modified forms of CYP17 are currently being expressed heterologously. Although the N-terminally modified enzyme has been well characterised, protein structure and function studies still necessitate the expression of unmodified, wild-type CYP17. We report here the expression of a catalytically active, unmodified human CYP17 in the industrial methylotrophic yeast, Pichia pastoris. A typical P450 carbon monoxide difference spectrum, with an absorption maximum at 448nm and a substrate-induced type I spectrum were recorded using a detergent-solubilised cellular fraction containing CYP17. The expressed enzyme catalysed the conversion of progesterone to 17-hydroxyprogesterone as well as 16-hydroxyprogesterone, a product unique to human and chimpanzee CYP17. This is the first report showing the heterologous expression of a fully functional human steroidogenic cytochrome P450 enzyme in P. pastoris.

Cloning of the zebra finch androgen synthetic enzyme CYP17: a study of its neural expression throughout posthatch development

Male zebra finches develop a robust neural song system that supports singing, but females have a minimal song circuit and do not sing. Estrogens masculinize the song circuit and are especially potent during the first 3 weeks of posthatch development. The gonads do not seem to supply the masculinizing steroids, implying that another tissue synthesizes steroids. Evidence suggests that the brain is capable of synthesizing neurosteroids, which in developing zebra finches may be required for song system differentiation. Aromatase, the enzyme that synthesizes estrogen from androgen, is equally abundant in male and female brains. To investigate further the potential for neurosteroidogenesis in the zebra finch brain, we cloned and examined the expression of 17alpha-hydroxylase/17,20 lyase (CYP17), the enzyme that synthesizes the androgenic substrate for aromatase. We used Northern blots, reverse transcription-polymerase chain reaction, and in situ hybridization to show that CYP17 is transcribed in developing and adult brains. CYP17 is transcribed at developmental stages and in brain areas potentially important to aspects of the developing song system, although no sex difference was detected in mRNA levels. Our results support the hypothesis that neurosteroids may act to influence brain organization and function in the zebra finch.

Cloning of zebrafish ovarian P450c17 (CYP17, 17alpha-hydroxylase/17, 20-lyase) and characterization of its expression in gonadal and extra-gonadal tissues

Cytochrome P450c17 (CYP17, 17alpha-hydroxylase/17, 20-lyase) has been viewed as a critical enzyme for the biosynthesis of gonadal and adrenal steroids in vertebrates. A full-length cDNA coding for P450c17 has been cloned from the zebrafish ovary. It encodes 519 amino acids and shares high sequence identity with that of fathead minnow (92%), rainbow trout (74%), chicken (64%), rat (47%), and human (48%). Northern blot analysis demonstrated that it is predominantly expressed in the ovary and testis; however, a weak hybridization signal was also detected in the whole kidney of the zebrafish, suggesting a role for P450c17 in the steroidogenesis of both gonads and renal/interrenal tissues. We further examined the expression of P450c17 in the developing ovarian follicles. Northern analysis and RT-PCR revealed abundant expression of the enzyme in the follicles of all major stages including pre-vitellogenic, vitellogenic and post-vitellogenic full-grown follicles; however, unlike P450c17 in the rainbow trout and eel, the expression of zebrafish P450c17 did not show significant temporal difference among different developmental stages, similar to the finding in the fathead minnow. The constitutive nature of P450c17 expression during zebrafish follicle development may partly explain the poor transcriptional response of the enzyme to the treatments with gonadotropin or activin, both of which enhance oocyte maturation in this species. It is also of interest to note that when assayed by RT-PCR, the enzyme P450c17 also seemed to be expressed in several other non-gonadal tissues including the brain, gill, liver and intestine, and its expression did not show significant sexual dimorphism in the brain of adult zebrafish. The physiological relevance of these findings remains to be further elucidated.

Alternative splicing and differential expression of P450c17 (CYP17) in gonads during sex transformation in the rice field eel

Several mechanisms were used in determination of the development of the male or female of vertebrates. The genes for determination of sequential hermaphrodite sex are unknown. Here, we reported cloning, alternative splicing, and expression patterns of the CYP17 gene of the rice field eel, a teleost fish with a characteristic of nature sex reversal. The CYP17 gene of the rice field eel was clustered into the CYP17 gene group of all the other vertebrates, especially into the fish subgroup. Four isoforms of the CYP17 were generated in gonads by alternative splicing and polyadenylation. Alternative splicing events of all these isoforms occurred in 3(') regions, which encoded three different sizes (517, 512, and 159aa) of proteins. RT-PCR results indicate specific expression in gonads of these isoforms. Northern blot analysis shows that expression patterns of the CYP17 (dominantly expressed in testis, less in ovary, and the least in ovotestis) are consistent with the sex reversal process of the rice field eel. In situ hybridization further shows its specific expression in germinal lamellae, the gonadal epithelium of the gonads. These findings indicate that CYP17 is differentially regulated in a sex- and developmentally specific manner, suggesting that the CYP17 potentially has important roles in gonad differentiation during sex reversal of the rice field eel.

Cloning and gene expression of P450 17alpha-hydroxylase,17,20-lyase cDNA in the gonads and brain of the fathead minnow Pimephales promelas

P450 17alpha-hydroxylase,17,20-lyase (P450c17) is a key steroidogenic enzyme in the production of androgens and, therefore, is also indispensable for the production of oestrogens (that are produced from the aromatisation of androgens). In this study, P450c17 cDNA was cloned from the ovary of the fathead minnow (FHM) and its gene expression was examined in the gonads and brains of male and female FHM at different stages of gonadal development with a view to developing an understanding of its involvement in the reproductive physiology in this species. The FHM-P450c17 cDNA sequence cloned was 1812 bp in length, with an open reading frame of 1554 nucleotides encoding a protein of 518 amino acids. Amino acid identity of FHM-P450c17 with P450c17s in other animals was up to 81.8% in other teleosts (channel catfish), 62% in elasmobranches (spiny dogfish), 64% in birds (chicken), and up to 48.8% in mammals (human). FHM-P450c17 gene expression occurred in the ovary, testis, and also in the brain (both male and female) at all stages of sexual development studied. Expression in the brain was at least 30-fold lower than in the gonads, but consistent in all fish life stages studied. In the testis, FHM-P450c17 gene expression was negatively correlated with gonadal development, but there was no obvious association between P450c17 gene expression and sexual development in the ovary, or brain (in both males and females). The results from this study demonstrate the expression of P450c17 in the brain for the first time in fish. Enzymatic studies are now needed to investigate the possible role of P450c17 in neurosteroid production in teleosts.

Changes in the expression of genes encoding steroidogenic enzymes in the channel catfish (Ictalurus punctatus) ovary throughout a reproductive cycle

In vertebrates, the growth and maturation of the ovarian follicle is dependent on the appropriate dynamics of sex steroid secretion, which is dictated by gene expression of the steroidogenic enzymes. The molecular aspects of steroid regulation are poorly understood in fishes, so as a first step we determined the pattern of expression of four key steroidogenic genes throughout the ovarian cycle in an annually spawning teleost, the channel catfish (Ictalurus punctatus). The abundance of transcripts encoding 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) and cholesterol side chain cleavage (P450(scc)), 17 alpha-hydroxylase/lyase (P450(c17)), and aromatase (P450(arom)) were determined by rtqRT-PCR or ribonuclease protection assay and correlated to ovarian growth and plasma titers of estradiol (E(2)) and testosterone (T) in two populations of catfish. Elevations in transcript abundance for P450(c17), P450(scc), and P450(arom) were observed at the onset of ovarian recrudescence and during early vitellogenic growth of the oocytes; however, all three decreased precipitously with the completion of vitellogenesis. Changes in the expression of these genes strongly suggest a direct correlation to E(2) and T titers. Alternatively 3 beta-HSD transcript abundance was relatively stable throughout the year. This study suggests that the genes encoding the three steroidogenic cytochrome P450s have a similar regulatory mechanism.

Characterization of gonadal and extra-gonadal forms of the cDNA encoding the Atlantic stingray (Dasyatis sabina) cytochrome P450 aromatase (CYP19)

Cytochrome P450 aromatase (P450arom; CYP19) mediates the conversion of androgens to estrogens and its activity has been found in all vertebrates studied to date. This study describes the full-length cDNA encoding the ovarian form of P450arom and the differences in the 5'-untranslated region (5'-UTR) of the extra-gonadal P450arom transcript expressed by the Atlantic stingray (Dasyatis sabina). Elasmobranchs (cartilaginous fishes such as sharks, rays and skates) diverged from the other vertebrates more than 350 million years ago, therefore the stingray P450arom cDNA may represent an ancient form of this gene. Northern blot analysis showed that the ovarian follicle expressed transcripts of 3.1 and 1.7 kb in size which correspond to the clones isolated from a stingray ovarian follicle cDNA library. Both transcripts consisted of an identical 1.5 kb coding region and a 41 bp 5'-UTR, however the 3'-UTRs differed in the use of the most proximate and the most distal of four polyadenylation signals. COS cells transfected with the 1.7 kb cDNA had twice the aromatase activity as cells transfected with the 3.1 kb cDNA. The coding region of the cDNA predicted a 58.5 kDa protein which consisted of 511 residues. Alignment of the stingray protein indicates that the P450arom is equally identical (53-59%) to all other vertebrate forms of P450arom characterized to date, thus indicating a common ancestry. The evolutionary relationship of the stingray form of P450arom clearly predates the other forms and belongs to a unique lineage. Transcripts of P450arom were expressed in ovarian follicles (of all sizes), the testis, the pituitary, in all sections of the brain, and in the kidney. The extra-gonadal transcripts appear to encode a protein identical to the ovarian form, however, the 5'-UTR was 657 bp longer presumably due to the transcription of an untranslated 'first exon' as seen in the mammalian form of this gene.

Molecular cloning and characterization of Japanese eel ovarian P450c17 (CYP17) cDNA

As a first step in investigating the mechanism underlying the steroidogenic shift from the production of ovarian androgens (vitellogenic stage) to that of 17alpha-hydroxylated progestins (maturational stage) in Japanese eel during induced oogenesis, a cDNA encoding Japanese eel (Anguilla japonica) ovarian P450c17 (CYP17: steroid 17alpha-hydroxylase/C(17-20) lyase) was cloned and sequenced. This cDNA contained the complete coding region representing 510 amino acid residues, which showed high sequence homology to those of rainbow trout (74%) and mammals (45-55%). The protein encoded by this cDNA possessed high enzymatic activities of 17alpha-hydroxylase and C(17-20) lyase, thus quickly converting pregnenolone and progesterone to their respective delta(4) and delta(5) C19 products. P450c17 produced a single transcript of 2.4 kb in length, as assessed by Northern blot. Transcript levels of this enzyme significantly increased throughout artificially induced ovarian development. Considering this together with the previous data showing that C(17-20) lyase activity decreased from the vitellogenic to the maturational stage, whereas 17alpha-hydroxylase activity increased, the present data suggest that changes in C(17-20) lyase activity (the production of androgens) do not depend on transcriptional changes of the P450c17 gene.

Role of cytochrome P450c17 in polycystic ovary syndrome

The hyperandrogenism of polycystic ovary syndrome (PCOS) appears to be due to dysregulation of steroidogenesis within the ovaries and adrenal glands. P450c17 is the key enzyme that regulates androgen synthesis. It is the only enzyme known to have the capacity to convert C21-precursors to the androgen pre-hormones, the 17-ketosteroids. It is a single enzyme with two activities, 17-hydroxylase and 17,20-lyase. Thus, its regulation is a significant factor in the expression of hyperandrogenism. Androgen secretion is LH-dependent in the ovary and ACTH-dependent in the adrenal glands. The androgenic response to each of these tropic hormones seems to be modulated by intra-ovarian or intra-adrenal autocrine and paracrine mechanisms. This modulation serves to regulate steroid hormone secretion in tissue-specific ways. Insulin, IGFs and inhibin are among the many growth factors capable of augmenting the response to LH and ACTH. The insulin/IGF system stimulates P450c17 mRNA expression and activities in the ovaries and adrenal glands. An integrating link between insulin resistance and hyperandrogenemia may be serine phosphorylation, which inhibits activity of the insulin receptor and promotes the 17,20-lyase activity of P450c17. However, it must be kept in mind that there is some evidence for the existence of P450c17-independent pathways of androgen biosynthesis.

Modulation of 17alpha-hydroxylase/17,20-lyase activity of guinea pig cytochrome P450c17 by site-directed mutagenesis

Microsomal cytochrome P450c17 (17a-hydroxylase/17,20-Lyase) catalyzes two reactions in the delta5 and delta4 pathways leading to the production of C19 steroids. Transient expression of human, bovine, porcine, rat, and mouse P450c17 cDNAs showed that the protein has 17alpha-hydroxylase and 17,20-Lyase activities, converting pregnenolone and progesterone into delta5- and delta4-Cl9 steroids, respectively, although the rat and mouse proteins have a preferential pathway toward the delta4 steroids. The guinea pig (gp) P450c17 shares 46% to 70% amino acid identity with the corresponding proteins of other species, and further characterization indicated that the guinea pig enzyme only converts progesterone to androstenedione. In this study, we have tried to identify amino acid(s) in the gpP450c17 that governs such a steroid specificity. Among the various mutants that we have created, change of the arginine (R) residue at position 200 to an asparagine (N) (R200N) in the gpP450c17 protein increased reactivity toward pregnenolone compared with the wild-type enzyme. Pregnenolone was converted into 17alpha-hydroxypregnenolone and dehydroepiandrosterone. However, this gain occurred at the expense of the 17,20-lyase activity toward 17alpha-hydroxyprogesterone. The R200N mutation in the gpP450c17 protein introduced a potential N-linked glycosylation site (200Asn-X-Thr202); however, substitution of the Thr202 residue by an asparagine (R200N/T202N), which abolishes the site, did not change the preference of the gpP450c17 mutant for pregnenolone. Furthermore, introduction of a putative glycosylation site at amino acid 185 in the gpP450c17 enzyme did not alter substrate specificity. The properties of the amino acid were also investigated, and neither the charge nor the size of the sidechain elicited change in the substrate specificity of gpP450c17. Thus, our results demonstrate that the mutation of arginine to asparagine at position 200 changes the substrate specificity of the gpP450c17 enzyme.

Isolation of the putative cDNA encoding cholesterol side chain cleavage cytochrome P450 (CYP11A) of the southern stingray (Dasyatis americana)

Cholesterol side chain cleavage cytochrome P450 (P450scc; CYP11A) catalyzes the first step in the production of steroid hormones. By utilizing degenerate oligonucleotide primers in a reverse transcriptase-coupled polymerase chain reaction (RT-PCR), a specific 252 bp fragment of the putative P450scc was amplified from RNA of interrenal tissue (the adrenal cortex homolog) from the southern stingray (Dasyatis americana), blacktip shark (Carcharhinus limbatus), and the spiny dogfish shark (Squalus acanthias). The amino-acid sequences predicted by these PCR products were 73-90% identical to each other. Using the homologous PCR-generated probe, five positive clones were isolated from a cDNA library constructed from interrenal mRNA of the southern stingray. The longest clone (4619 bp) contained the 3'-untranslated region, including four putative polyadenylation signals. Northern blot analysis of stingray interrenal RNA revealed a single transcript of 4.2 kb in length. The incomplete amino-acid sequence predicted by the open reading frame of the cDNA (514 residues in length) is 48% homologous to the trout form and 39-40% homologous to mammalian forms. Even though the stingray P450scc contains an amino terminus longer than the other forms of P450scc, no translation initiation signal (ATG) was evident within the open reading frame. This report presents the first sequence of cytochrome P450scc from this evolutionary unique taxon of vertebrates.

Characterization of the adrenal cytochrome P450C17 in the hamster, a small animal model for the study of adrenal dehydroepiandrosterone biosynthesis

The hamster, like the human produces cortisol as its major glucocorticoid, rather than corticosterone, typical of most enzyme rodents. It is not known, however, if the hamster cytochrome P450C17 (P450C17), a key enzyme for cortisol formation, also exhibits 17,20-lyase activity and if it catalyzes the formation of dehydroepiandrosterone (DHEA) at the adrenal level. To study this, we isolated the cDNA of P450C17 from a hamster adrenal library. This cDNA was sequenced and was found to have an open reading frame for a protein of 511 amino acids, as compared to the human P450C17, which contains 508 amino acids. The hamster P450C17 cDNA, in the coding region, is 76% homologous with the human P450C17 cDNA. The cDNA was then cloned in the expression vector pSV-SPORT 1, which was transiently transfected into COS 1 cells. The transfected cells were used for temporal studies on the transformation of radiolabeled C21-delta5- and C21-delta4-precursors. When transfected cells were incubated with [14C]pregnenolone, rapid formation of [14C]DHEA occurred. The intermediate 17alpha-hydroxypregnenolone accumulated initially with subsequent metabolism to DHEA. Likewise, when incubated with C21-delta4-steroids, [14C]progesterone and [3H]17alpha-hydroxyprogesterone, the 17,20-lyase product androstenedione was produced efficiently. In these studies, with respect to the delta5 pathway, the expressed hamster P450C17 gave similar results to bovine P450C17 cDNA inserted in the same expression vector. However, in contrast to the bovine enzyme, which converted low amounts of progesterone to androstenedione, the expressed hamster P450C17 enzyme showed an active metabolism via the delta4 pathway. Northern blot analysis, using the complete alpha-32P labeled hamster P450C17 cDNA as the probe, demonstrated a strong presence of P450C17 mRNA in hamster adrenals, a weaker presence in testes and ovaries, and no detectable species in brain, mesentery, and kidney. Immunoblotting analysis using an anti-rat P450C17 antibody demonstrated the presence of P450C17 protein in hamster adrenals, testes, and ovaries. Hamster adrenal cell suspensions and microsomal preparations were used to demonstrate the biosynthesis of [14C]17alpha-hydroxypregnenolone and [14C]DHEA from [14C]pregnenolone; both metabolites were formed during incubations. However, the ratio of [14C]DHEA/[14C]17alpha-hydroxypregnenolone was much lower in adrenal cells than in transfected COS 1 cells, indicating the presence of putative factors in hamster adrenal cells, favoring the 17alpha-hydroxylase activity rather than that of the 17,20-lyase. In conclusion, these studies demonstrate that the hamster adrenal is both a DHEA and a cortisol producer, and, therefore, this animal could be a suitable small animal model for the study of the role of DHEA in relation to human biochemistry and physiology.