Conversion of pregnenolone to DHEA by human 17alpha-hydroxylase/17, 20-lyase (P450c17). Evidence that DHEA is produced from the released intermediate, 17alpha-hydroxypregnenolone

RNA-Seq reveals ACTH-induced steroid hormone pathway participating in goat adrenal gland response to castration

The content of androgen from adrenal is elevated under castration, and the mechanisms of compensatory secretion of adrenal androgen remain unknown. This study was designed to compare the transcript profiles between adrenals from noncastrated, orchiectomized and immunocastrated Yiling goats. Fifteen goats were randomly divided into three groups: pVAX-asd injection (control) group, pVAX-B2L-(G4S)3-kisspeptin-54-asd immunization (PBK-asd) group, and surgical castration (SC) group. Subsequently, serum was collected every two weeks after the initial immunization for hormone assays. At week 14 after immunization, adrenal glands were collected for transcriptome sequencing and qPCR. Serum testosterone concentration was significantly reduced in PBK-asd and SC group, demonstrating the effectiveness of castration. Both surgical and immunized castration resulted in adrenal hyperplasia, and thickness of adrenal cortex elevated. The specific genes involving castration were enriched in many pathways, including Steroid hormone biosynthesis pathway. The adrenocorticotropic hormone (ACTH), which promotes the production of adrenal steroids, and dehydroepiandrosterone (DHEA), a steroid hormone secreted by adrenal glands, both increased after castration. Further construction of co-expression network for transcription genes and traits (including adrenal weight and cortex thickness, ACTH and DHEA concentration) showed that the trait-related genes were enriched in multiple steroid-related pathways. These results showed that adrenal compensatory hyperplasia and androgen secretion caused by castration may involve in ACTH-induced steroid hormone synthesis.

Steroid hormones of the octopus self-destruct system

Among all invertebrates, soft-bodied cephalopods have the largest central nervous systems and the greatest brain-to-body mass ratios, yet unlike other big-brained animals, cephalopods are unusually short lived.^1-5 Primates and corvids survive for many decades, but shallow-water octopuses, such as the California two-spot octopus (Octopus bimaculoides), typically live for only 1 year.^6,7 Lifespan and reproduction are controlled by the principal neuroendocrine center of the octopus: the optic glands, which are functional analogs to the vertebrate pituitary gland.^8-10 After mating, females steadfastly brood their eggs, begin fasting, and undergo rapid physiological decline, featuring repeated self-injury and leading to death.¹¹ Removal of the optic glands completely reverses this life history trajectory,¹⁰ but the signaling factors underlying this major life transition are unknown. Here, we characterize the major secretions and steroidogenic pathways of the female optic gland using mass spectrometry techniques. We find that at least three pathways are mobilized to increase synthesis of select sterol hormones after reproduction. One pathway generates pregnane steroids, known in other animals to support reproduction.^12-16 Two other pathways produce 7-dehydrocholesterol and bile acid intermediates, neither of which were previously known to be involved in semelparity. Our results provide insight into invertebrate cholesterol pathways and confirm a remarkable unity of steroid hormone biology in life history processes across Bilateria.

The Utilization of Dehydroepiandrosterone as a Sexual Hormone Precursor in Premenopausal and Postmenopausal Women: An Overview

Dehydroepiandrosterone (DHEA), and its metabolite, dehydroepiandrosterone sulfate ester (DHEAS), are the most abundant circulating steroid hormones, and are synthesized in the zona reticularis of the adrenal cortex, in the gonads, and in the brain. The precise physiological role of DHEA and DHEAS is not yet fully understood, but these steroid hormones can act as androgens, estrogens, and neurosteroids, and perform many roles in the human body. Since both levels decline with age, use of DHEA supplements have gained more attention due to being advertised as an antidote to aging in postmenopausal women, who may have concerns on age-related diseases and overall well-being. However, current research has not reached an overall consensus on the effects of DHEA on postmenopausal women. This overview is a summary of the current literature, addressing the metabolic pathway for DHEA synthesis and utilization, as well as the effects of DHEA on premenopausal and postmenopausal women with disease states and other factors. As for the therapeutic effects on menopausal syndrome and other age-related diseases, several studies have found that DHEA supplementations can alleviate vasomotor symptoms, preserve the integrity of the immune system, reduce bone loss, and increase muscle mass. Intravaginal DHEA has shown significant beneficial effects in menopausal women with severe vulvovaginal symptoms. On the other hand, DHEA supplements have not shown definitive effects in cardiovascular disease, adrenal insufficiency, insulin sensitivity, and cognition. Due to inadequate sample sizes and treatment durations of current studies, it is difficult to assess the safety and efficacy of DHEA and draw reliable conclusions for the physiological role, the optimal dosage, and the effects on premenopausal and postmenopausal women; therefore, the study of DHEA warrants future investigation. Further research into the roles of these steroid hormones may bring us closer to a therapeutic option in the future.

Association of Endogenous Pregnenolone, Progesterone, and Related Metabolites with Risk of Endometrial and Ovarian Cancers in Postmenopausal Women: The BFIT Cohort

BACKGROUND

Postmenopausal pregnenolone and/or progesterone levels in relation to endometrial and ovarian cancer risks have been infrequently evaluated. To address this, we utilized a sensitive and reliable assay to quantify prediagnostic levels of seven markers related to endogenous hormone metabolism.

METHODS

Hormones were quantified in baseline serum collected from postmenopausal women in a cohort study nested within the Breast and Bone Follow-up to the Fracture Intervention Trial (B∼FIT). Women using exogenous hormones at baseline (1992-1993) were excluded. Incident endometrial (n = 65) and ovarian (n = 67) cancers were diagnosed during 12 follow-up years and compared with a subcohort of 345 women (no hysterectomy) and 413 women (no oophorectomy), respectively. Cox models with robust variance were used to estimate cancer risk.

RESULTS

Circulating progesterone levels were not associated with endometrial [tertile (T)3 vs. T1 HR (95% confidence interval): 1.87 (0.85-4.11); P _trend = 0.17] or ovarian cancer risk [1.16 (0.58-2.33); 0.73]. Increasing levels of the progesterone-to-estradiol ratio were inversely associated with endometrial cancer risk [T3 vs. T1: 0.29 (0.09-0.95); 0.03]. Increasing levels of 17-hydroxypregnenolone were inversely associated with endometrial cancer risk [0.40 (0.18-0.91); 0.03] and positively associated with ovarian cancer risk [3.11 (1.39-6.93); 0.01].

CONCLUSIONS

Using sensitive and reliable assays, this study provides novel data that endogenous progesterone levels are not strongly associated with incident endometrial or ovarian cancer risks. 17-hydroxypregnenolone was positively associated with ovarian cancer and inversely associated with endometrial cancer.

IMPACT

While our results require replication in large studies, they provide further support of the hormonal etiology of endometrial and ovarian cancers.

Mechanism of the Dual Activities of Human CYP17A1 and Binding to Anti-Prostate Cancer Drug Abiraterone Revealed by a Novel V366M Mutation Causing 17,20 Lyase Deficiency

The CYP17A1 gene regulates sex steroid biosynthesis in humans through 17α-hydroxylase/17,20 lyase activities and is a target of anti-prostate cancer drug abiraterone. In a 46, XY patient with female external genitalia, together with a loss of function mutation S441P, we identified a novel missense mutation V366M at the catalytic center of CYP17A1 which preferentially impaired 17,20 lyase activity. Kinetic experiments with bacterially expressed proteins revealed that V366M mutant enzyme can bind and metabolize pregnenolone to 17OH-pregnenolone, but 17OH-pregnenolone binding and conversion to dehydroepiandrosterone (DHEA) was impaired, explaining the patient’s steroid profile. Abiraterone could not bind and inhibit the 17α-hydroxylase activity of the CYP17A1-V366M mutant. Molecular dynamics (MD) simulations showed that V366M creates a “one-way valve” and suggests a mechanism for dual activities of human CYP17A1 where, after the conversion of pregnenolone to 17OH-pregnenolone, the product exits the active site and re-enters for conversion to dehydroepiandrosterone. The V366M mutant also explained the effectiveness of the anti-prostate cancer drug abiraterone as a potent inhibitor of CYP17A1 by binding tightly at the active site in the WT enzyme. The V366M is the first human mutation to be described at the active site of CYP17A1 that causes isolated 17,20 lyase deficiency. Knowledge about the specificity of CYP17A1 activities is of importance for the development of treatments for polycystic ovary syndrome and inhibitors for prostate cancer therapy.

Kinetic processivity of the two-step oxidations of progesterone and pregnenolone to androgens by human cytochrome P450 17A1

Cytochrome P450 (P450, CYP) 17A1 plays a critical role in steroid metabolism, catalyzing both the 17α-hydroxylation of pregnenolone and progesterone and the subsequent 17α,20-lyase reactions to form dehydroepiandrosterone (DHEA) and androstenedione (Andro), respectively, critical for generating glucocorticoids and androgens. Human P450 17A1 reaction rates examined are enhanced by the accessory protein cytochrome b₅ (b₅), but the exact role of b₅ in P450 17A1-catalyzed reactions is unclear as are several details of these reactions. Here, we examined in detail the processivity of the 17α-hydroxylation and lyase steps. b₅ did not enhance reaction rates by decreasing the k_off rates of any of the steroids. Steroid binding to P450 17A1 was more complex than a simple two-state system. Pre-steady-state experiments indicated lag phases for Andro production from progesterone and for DHEA from pregnenolone, indicating a distributive character of the enzyme. However, we observed processivity in pregnenolone/DHEA pulse-chase experiments. (S)-Orteronel was three times more inhibitory toward the conversion of 17α-hydroxypregnenolone to DHEA than toward the 17α-hydroxylation of pregnenolone. IC₅₀ values for (S)-orteronel were identical for blocking DHEA formation from pregnenolone and for 17α-hydroxylation, suggestive of processivity. Global kinetic modeling helped assign sets of rate constants for individual or groups of reactions, indicating that human P450 17A1 is an inherently distributive enzyme but that some processivity is present, i.e. some of the 17α-OH pregnenolone formed from pregnenolone did not dissociate from P450 17A1 before conversion to DHEA. Our results also suggest multiple conformations of P450 17A1, as previously proposed on the basis of NMR spectroscopy and X-ray crystallography.

Role of cytochrome b5 in the modulation of the enzymatic activities of cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1)

Cytochrome b5 (cyt b5) is a small hemoprotein that plays a significant role in the modulation of activities of an important steroidogenic enzyme, cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1, CYP17A1). Located in the zona fasciculata and zona reticularis of the adrenal cortex and in the gonads, P450 17A1 catalyzes two different reactions in the steroidogenic pathway; the 17α-hydroxylation and 17,20-lyase, in the endoplasmic reticulum of these respective tissues. The activities of P450 17A1 are regulated by cyt b5 that enhances the 17,20-lyase reaction by promoting the coupling of P450 17A1 and cytochrome P450 reductase (CPR), allosterically. Cyt b5 can also act as an electron donor to enhance the 16-ene-synthase activity of human P450 17A1. In this review, we discuss the many roles of cyt b5 and focus on the modulation of CYP17A1 activities by cyt b5 and the mechanisms involved.

Structural and kinetic basis of steroid 17α,20-lyase activity in teleost fish cytochrome P450 17A1 and its absence in cytochrome P450 17A2

Cytochrome P450 (P450) 17A enzymes play a critical role in the oxidation of the steroids progesterone (Prog) and pregnenolone (Preg) to glucocorticoids and androgens. In mammals, a single enzyme, P450 17A1, catalyzes both 17α-hydroxylation and a subsequent 17α,20-lyase reaction with both Prog and Preg. Teleost fish contain two 17A P450s; zebrafish P450 17A1 catalyzes both 17α-hydroxylation and lyase reactions with Prog and Preg, and P450 17A2 is more efficient in pregnenolone 17α-hydroxylation but does not catalyze the lyase reaction, even in the presence of cytochrome b5. P450 17A2 binds all substrates and products, although more loosely than P450 17A1. Pulse-chase and kinetic spectral experiments and modeling established that the two-step P450 17A1 Prog oxidation is more distributive than the Preg reaction, i.e. 17α-OH product dissociates more prior to the lyase step. The drug orteronel selectively blocked the lyase reaction of P450 17A1 but only in the case of Prog. X-ray crystal structures of zebrafish P450 17A1 and 17A2 were obtained with the ligand abiraterone and with Prog for P450 17A2. Comparison of the two fish P450 17A-abiraterone structures with human P450 17A1 (DeVore, N. M., and Scott, E. E. (2013) Nature 482, 116-119) showed only a few differences near the active site, despite only ∼50% identity among the three proteins. The P450 17A2 structure differed in four residues near the heme periphery. These residues may allow the proposed alternative ferric peroxide mechanism for the lyase reaction, or residues removed from the active site may allow conformations that lead to the lyase activity.

Steroid profiling in pregnancy: a focus on the human fetus

In this review we focused on steroid metabolomics in human fetuses and newborns and its role in the physiology and pathophysiology of human pregnancy and subsequent stages of human life, and on the physiological relevance of steroids influencing the nervous systems with regards to their concentrations in the fetus. Steroid profiling provides valuable data for the diagnostics of diseases related to altered steroidogenesis in the fetal and maternal compartments and placenta. We outlined a potential use of steroid metabolomics for the prediction of reproductive disorders, misbalance of hypothalamic-pituitary-adrenal axis, and impaired insulin sensitivity in subsequent stages of human life. A possible role of steroids exhibiting a non-genomic effect in the development of gestational diabetes and in the neuroprotection via negative modulation of AMPA/kainate receptors was also indicated. Increasing progesterone synthesis and catabolism, declining production of tocolytic 5β-pregnane steroids, and rising activities of steroid sulfotransferases with the approaching term may be of importance in sustaining pregnancy. An increasing trend was demonstrated with advancing gestation toward the production of ketones (and 3β-hydroxyl groups in the case of 3α-hydroxy-steroids) was demonstrated in the fetus on the expense of 3α-hydroxy-, 17β-hydroxy-, and 20α-hydroxy-groups weakening in the sequence C17, C3, and C20. There was higher production of active progestogen but lower production of active estrogen and GABAergic steroids with the approaching term. Rising activities of placental CYP19A1 and oxidative isoforms of HSD17B, and of fetal CYP3A7 with advancing gestation may protect the fetus from hyperestrogenization. This article is part of a Special Issue entitled 'Pregnancy and Steroids'.

Cytochrome b5: novel roles in steroidogenesis

Cytochrome b(5) (cyt-b(5)) is essential for the regulation of steroidogenesis and as such has been implicated in a number of clinical conditions. It is well documented that this small hemoprotein augments the 17,20-lyase activity of cytochrome P450 17α-hydroxylase/17,20-lyase (CYP17A1). Studies have revealed that this augmentation is accomplished by cyt-b(5) enhancing the interaction between cytochrome P450 reductase (POR) and CYP17A1. In this paper we present evidence that cyt-b(5) induces a conformational change in CYP17A1, in addition to facilitating the interaction between CYP17A1 and POR. We also review the recently published finding that cyt-b(5) allosterically augments the activity of 3β-hydroxysteroid dehydrogenase/Δ(5)-Δ(4) isomerase (3βHSD), a non cytochrome P450 enzyme, by increasing the enzymes affinity for its cofactor, NAD(+). The physiological importance of this finding, in terms of understanding adrenal androstenedione production, is examined. Finally, evidence that cyt-b(5) is able to form homomeric complexes in living cells is presented and discussed.

Adrenarche in nonhuman primates: the evidence for it and the need to redefine it

Adrenarche is most commonly defined as a prepubertal increase in circulating adrenal androgens, dehydroepiandrosterone (DHEA) and its sulfo-conjugate (DHEAS). This event is thought to have evolved in humans and some great apes but not in Old World monkeys, perhaps to promote brain development. Whether adrenarche represents a shared, derived developmental event in humans and our closest relatives, adrenal androgen secretion (and its regulation) is of considerable clinical interest. Specifically, adrenal androgens play a significant role in the pathophysiology of polycystic ovarian disease and breast and prostate cancers. Understanding the development of androgen secretion by the human adrenal cortex and identifying a suitable model for its study are therefore of central importance for clinical and evolutionary concerns. This review will examine the evidence for adrenarche in nonhuman primates (NHP) and suggest that a broader definition of this developmental event is needed, including morphological, biochemical, and endocrine criteria. Using such a definition, evidence from recent studies suggests that adrenarche evolved in Old World primates but spans a relatively brief period early in development compared with humans and some great apes. This emphasizes the need for frequent longitudinal sampling in evaluating developmental changes in adrenal androgen secretion as well as the tenuous nature of existing evidence of adrenarche in some species among the great apes. Central to an understanding of the regulation of adrenal androgen production in humans is the recognition of the complex nature of adrenarche and the need for more carefully conducted comparative studies and a broader definition in order to promote investigation among NHP in particular.

Variation in 3β-hydroxysteroid dehydrogenase activity and in pregnenolone supply rate can paradoxically alter androstenedione synthesis

The 3β-hydroxysteroid dehydrogenase/Δ(5)-Δ(4) isomerase (3β-HSD) and 17α-hydroxylase/17,20-lyase cytochrome P450 (P450c17) enzymes are important in determining the balance of the synthesis of different steroids such as progesterone (P4), glucocorticoids, androgens, and estrogens. How this is achieved is not a simple matter because each of the two enzymes utilizes more than one substrate and some substrates are shared in common between the two enzymes. The two synthetic pathways, Δ(4) and Δ(5), are interlinked such that it is difficult to predict how the synthesis of each steroid changes when any of the enzyme activities is varied. In addition, the P450c17 enzyme exhibits different substrate specificities among species, particularly with respect to the 17,20-lyase activity. The mathematical model developed in this study simulates the network of reactions catalyzed by 3β-HSD and P450c17 that characterizes steroid synthesis in human, non-human primate, ovine, and bovine species. In these species, P450c17 has negligible 17,20-lyase activity with the Δ(4)-steroid 17α-hydroxy-progesterone (17OH-P4); therefore androstenedione (A4) is synthesized efficiently only from dehydroepiandrosterone (DHEA) through the Δ(5) pathway. The model helps to understand the interplay between fluxes through the Δ(4) and Δ(5) pathways in this network, and how this determines the response of steroid synthesis to the variation in 3β-HSD activity or in the supply of the precursor substrate, pregnenolone (P5). The model simulations show that A4 synthesis can change paradoxically when 3β-HSD activity is varied. A decrease in 3β-HSD activity to a certain point can increase A4 synthesis by favouring metabolism through the Δ(5) pathway, though further decrease in 3β-HSD activity beyond that point eventually limits A4 synthesis. The model also showed that due to the competitive inhibition of the enzymes' activities by substrates and products, increasing the rate of P5 supply above a certain point can suppress the synthesis of A4, DHEA, and 17OH-P4, and consequently drive more P5 towards P4 synthesis.

Phase III data for abiraterone in an evolving landscape for castration-resistant prostate cancer

At the 2010 meeting of the European Society for Medical Oncology (ESMO), a landmark development in prostate cancer therapy was unveiled. In a phase III study, the CYP17 inhibitor abiraterone yielded a survival advantage over placebo in patients with metastatic castration-resistant prostate cancer (mCRPC) who had progressed despite prior docetaxel therapy. The data for abiraterone follow the publication of successful phase III studies earlier this year supporting two mechanistically distinct agents-namely, the novel taxane cabazitaxel and the autologous dendritic cell vaccine sipuleucel-T. A challenge that lies ahead for the scientific community is to discern the appropriate positioning of abiraterone in an increasingly crowded therapeutic landscape. Several ongoing trials are examining the agent in earlier settings (i.e., a phase III in mCRPC pre-docetaxel, and smaller studies in combination with radiation therapy or as neoadjuvant pre-surgery for localized disease). Herein, several potential strategies for abiraterone are presented to clarify the clinical utilization of this agent in the future.

Multi-step oxidations catalyzed by cytochrome P450 enzymes: Processive vs. distributive kinetics and the issue of carbonyl oxidation in chemical mechanisms

Catalysis of sequential oxidation reactions is not unusual in cytochrome P450 (P450) reactions, not only in steroid metabolism but also with many xenobiotics. One issue is how processive/distributive these reactions are, i.e., how much do the "intermediate" products dissociate. Our work with human P450s 2E1, 2A6, and 19A1 on this subject has revealed a mixture of systems, surprisingly with a more distributive mechanism with an endogenous substrate (P450 19A1) than for some xenobiotics (P450s 2E1, 2A6). One aspect of this research involves carbonyl intermediates, and the choice of catalytic mechanism is linked to the hydration state of the aldehyde. The non-enzymatic rates of hydration and dehydration of carbonyls are not rapid and whether P450s catalyze the reversible hydration is unknown. If carbonyl hydration and dehydration are slow, the mechanism may be set by the carbonyl hydration status.

Oxidation of N-Nitrosoalkylamines by human cytochrome P450 2A6: sequential oxidation to aldehydes and carboxylic acids and analysis of reaction steps

Cytochrome P450 (P450) 2A6 activates nitrosamines, including N,N-dimethylnitrosamine (DMN) and N,N-diethylnitrosamine (DEN), to alkyl diazohydroxides (which are DNA-alkylating agents) and also aldehydes (HCHO from DMN and CH(3)CHO from DEN). The N-dealkylation of DMN had a high intrinsic kinetic deuterium isotope effect ((D)k(app) approximately 10), which was highly expressed in a variety of competitive and non-competitive experiments. The (D)k(app) for DEN was approximately 3 and not expressed in non-competitive experiments. DMN and DEN were also oxidized to HCO(2)H and CH(3)CO(2)H, respectively. In neither case was a lag observed, which was unexpected considering the k(cat) and K(m) parameters measured for oxidation of DMN and DEN to the aldehydes and for oxidation of the aldehydes to the carboxylic acids. Spectral analysis did not indicate strong affinity of the aldehydes for P450 2A6, but pulse-chase experiments showed only limited exchange with added (unlabeled) aldehydes in the oxidations of DMN and DEN to carboxylic acids. Substoichiometric kinetic bursts were observed in the pre-steady-state oxidations of DMN and DEN to aldehydes. A minimal kinetic model was developed that was consistent with all of the observed phenomena and involves a conformational change of P450 2A6 following substrate binding, equilibrium of the P450-substrate complex with a non-productive form, and oxidation of the aldehydes to carboxylic acids in a process that avoids relaxation of the conformation following the first oxidation (i.e. of DMN or DEN to an aldehyde).

Day-to-day differences in cortisol levels and molar cortisol-to-DHEA ratios among working individuals

PURPOSE

The present study was carried out to determine day-to-day differences in cortisol levels and the molar cortisol-to-dehydroepiandrosterone (DHEA) ratio (molar C/D ratio) in working subjects.

MATERIALS AND METHODS

The cortisol and DHEA levels were measured from saliva samples collected 30 minutes after awakening for 7 consecutive days in full-time working subjects that worked Monday through Saturday. To determine the day-to-day differences within subjects, the collected data was analyzed using variance (ANOVA) for a randomized complete block design (RCBD).

RESULTS

The cortisol levels from samples collected 30 minutes after awakening on workdays were similar to each other, but were significantly different from the cortisol levels on Sunday. The DHEA levels were not significantly different between the days of week. The DHEA levels on Monday and Tuesday were relatively lower than the levels on the other weekdays. The DHEA levels on Thursday and Friday were relatively higher than the other days. The molar C/D ratios on Sunday were significantly lower than those on workdays. The molar C/D ratios on Monday and Tuesday were significantly higher than those on Wednesday or other workdays.

CONCLUSION

The cortisol levels and the molar C/D ratios demonstrate differences in adrenocortical activities between workdays and non-workdays, but the molar C/D ratio additionally represents differences in adrenocortical status between the first two workdays and other workdays. Thus, it is possible that the day-to-day differences in the cortisol levels and the molar C/D ratio represent the adrenal response to upcoming work-related stress.

The developmental increase in adrenocortical 17,20-lyase activity (biochemical adrenarche) is driven primarily by increasing cytochrome b5 in neonatal rhesus macaques

Adrenarche is thought to be experienced only by humans and some Old World primates despite observed regression of an adrenal fetal zone and establishment of a functional zona reticularis (ZR) in other species like rhesus macaques. Adrenal differentiation remains poorly defined biochemically in nonhuman primates. The present studies defined ZR development in the neonatal rhesus by examining androgen synthetic capacity and factors affecting it in rhesus and marmoset adrenals. Western immunoblots examined expression of 17alpha-hydroxylase/17,20-lyase cytochrome P450 (P450c17), cytochrome b5 (b5), and 3beta-hydroxysteroid dehydrogenase (3betaHSD), among other key enzymes. 17,20-lyase activity was quantified in adrenal microsomes, as was the contribution of b5 to 17,20-lyase activity in microsomes and cell transfection experiments with rhesus and marmoset P450c17. Expression of b5 increased from birth to 3 months, and was positively correlated with age and 17,20-lyase activity in the rhesus. Recombinant b5 addition stimulated 17,20-lyase activity to an extent inversely proportional to endogenous levels in adrenal microsomes. Although 3betaHSD expression also increased with age, P450c17, 21-hydroxylase cytochrome P450, and the redox partner, reduced nicotinamide adenine dinucleotide phosphate-cytochrome P450 oxidoreductase, did not; nor did recombinant cytochrome P450 oxidoreductase augment 17,20-lyase activity. Cotransfection with b5 induced a dose-dependent increase in dehydroepiandrosterone synthesis by both nonhuman primate P450c17 enzymes. We conclude that the increase in 17,20-lyase activity characteristic of an adrenarche in rhesus macaques is driven primarily by increased b5 expression, without the need for a decrease in 3betaHSD, as suggested from human studies. The rhesus macaque is a relevant and accessible model for human ZR development and adrenal function.

The role of porcine cytochrome b5A and cytochrome b5B in the regulation of cytochrome P45017A1 activities

Male pigs are routinely castrated to prevent the accumulation of testicular 16-androstene steroids, in particular 5alpha-androst-16-en-3-one (5alpha-androstenone), which contribute to an off-odour and off-flavour known as boar taint. Cytochrome P450C17 (CYP17A1) catalyses the key regulatory step in the formation of the 16-androstene steroids from pregnenolone by the andien-beta synthase reaction or the synthesis of the glucocorticoid and sex steroids via 17alpha-hydroxylase and C17,20 lyase pathways respectively. We have expressed CYP17A1, along with cytochrome P450 reductase (POR), cytochrome b5 reductase (CYB5R3) and cytochrome b5 (CYB5) in HEK-293FT cells to investigate the importance of the two forms of porcine CYB5, CYB5A and CYB5B, in both the andien-beta synthase as well as the 17alpha-hydroxylase and C17,20 lyase reactions. Increasing the ratio of CYB5A to CYP17A1 caused a decrease in 17alpha-hydroxylase (p<0.013), a transient increase in C17,20 lyase, and an increase in andien-beta synthase activity (p<0.0001). Increasing the ratio of CYB5B to CYP17A1 also decreased 17alpha-hydroxylase, but did not affect the andien-beta synthase activity; however, the C17,20 lyase, was significantly increased. These results demonstrate the differential effects of two forms of CYB5 on the three activities of porcine CYP17A1 and show that CYB5B does not stimulate the andien-beta synthase activity of CYP17A1.

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.

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.

Assessment of porcine and human 16-ene-synthase, a third activity of P450c17, in the formation of an androstenol precursor. Role of recombinant cytochrome b5 and P450 reductase

Recently, we have shown that the biosynthesis of androstenol, a potential endogenous ligand for the orphan receptors constitutive androstane receptor and pregnane-X-receptor, requires the presence of enzymes of the steroidogenic pathway, such as 3 beta-hydroxysteroid dehydrogenase, 5 alpha-reductase and 3 alpha-hydroxysteroid dehydrogenase. In this report, we examine at the molecular level whether the enzyme 17 alpha-hydroxylase/17,20-lyase (P450c17), which possesses dual 17 alpha-hydroxylase and 17,20-lyase activities and catalyzes the production of precursors for glucocorticoids and sex steroids, is also able to catalyze the formation of a third class of active steroids, 16-ene steroids (including androstenol). The role of components of the P450 complex is also assessed. We transfected human embryonic kidney (HEK-293) cells with various amounts of vectors expressing P450c17, NADPH-cytochrome P450 reductase, and cytochrome b5. Our results showed that P450c17 possesses a 16-ene-synthase activity able to transform pregnenolone into 5,16-androstadien-3 beta-ol, without the formation of the precursor 17-hydroxypregnenolone. Cytochrome b5 has a much stronger effect on the 16-ene-synthase activity than on the 17 alpha-hydroxylase/17,20-lyase activities. On the other hand, P450reductase has a drastic effect on the latter, but a negligible one on 5,16-androstadien-3 beta-ol synthesis. Our results therefore demonstrate that human P450c17, as other enzymes of the classical steroidogenic pathway, is involved in the biosynthetic pathway leading to the formation of androstenol.

Neuroactive steroids, their precursors and polar conjugates during parturition and postpartum in maternal and umbilical blood: 3.3beta-hydroxy-5-ene steroids

Five 3beta-hydroxy-5-ene steroids involved in the metabolic route from pregnenolone sulfate to dehydroepiandrosterone and its sulfate, of which three are known allosteric modulators of neurotransmitter receptors, were monitored in the serum of 20 women around parturition. In addition, their levels in maternal and umbilical serum were compared at delivery. On the basis of these data, a scheme of steroid biosynthesis in maternal organism during the critical stages around parturition is proposed. In maternal serum, all the steroids except dehydroepiandrosterone sulfate decreased during labor and even first day after delivery, although their changes were less distinct the more distant from pregnenolone sulfate (PregS) in the metabolic pathway. Calculation of product/immediate precursor ratios in maternal serum over all stages around parturition enabled identification of the respective changes in the activities of the relevant enzymes. The ratio of 17-hydroxypregnenolone/pregnenolone did not change significantly, while that of dehydroepiandrosterone/17-hydroxypregnenolone grew, indicating increased C17,20 side chain cleavage on the account of C17-hydroxylation both catalyzed by C17-hydroxylase-C17,20-lyase. As was shown by factor analysis, the changes in the maternal steroids were associated with a single common factor, which strongly correlated with all the steroids except dehydroepiandrosterone sulfate. The lack of change in the pregnenolone sulfate/pregnenolone ratio and a marked increase of the ratio dehydroepiandrosterone sulfate to unconjugated dehydroepiandrosterone indicate a different means of formation of both steroid sulfates. On the basis of these data, a scheme of steroid biosynthesis in maternal organism during the critical stages around parturition is proposed.

Molecular differences caused by differentiation of 3T3-L1 preadipocytes in the presence of either dehydroepiandrosterone (DHEA) or 7-oxo-DHEA

The effects of dehydroepiandrosterone (DHEA) and 7-oxo-DHEA on the cell size, adiposity, and fatty acid composition of differentiating 3T3-L1 preadipocyte cells are correlated with stearoyl-CoA desaturase (SCD) expression (mRNA and protein levels) and enzyme activity. Fluorescence-activated cell sorting shows that preadipocyte cells treated with methylisobutylxanthine, dexamethasone, and insulin (MDI) plus DHEA comprise a population distribution of predominantly large cells with reduced adiposity. In contrast, cells treated with MDI plus 7-oxo-DHEA comprise a population distribution of almost equal proportions of small and large cells that have an adiposity equivalent to cells differentiated with MDI alone. The cells treated with MDI plus DHEA have significantly reduced levels of total fatty acid, mainly due to a dramatic reduction in the level of palmitoleic (Delta(9)-16:1) acid. The cells treated with MDI plus 7-oxo-DHEA have a significantly increased level of total fat, primarily due to increased levels of Delta(9)-16:1 and palmitic (16:0) acids. At the molecular level, the DHEA-treated cells contain lowered amounts of SCD1 mRNA and antibody-detectable desaturase protein, while 7-oxo-DHEA-treated cells contained elevated levels of SCD1 mRNA and protein. Inhibition of differentiation in DHEA-treated cells was also suggested by a reduction in the mRNA level of the adipogenic gene aP2. At the level of microsomal enzymatic activity, SCD activity was decreased in DHEA-treated cells while the SCD activity was increased in 7-oxo-DHEA-treated cells. The changes in mRNA levels and enzyme activity were concentration-dependent and appeared as early as day 3 of the differentiation protocol. The results show that DHEA and 7-oxo-DHEA have distinct modes of action with respect to the complex transcriptional cascade required for differentiation. Furthermore, differences in the insulin-stimulated uptake of 2-deoxyglucose and in the activity of carnitine palmitoyl transferase observed from either DHEA- or 7-oxo-DHEA-treated cells support the ability of DHEA to produce a thermogenic effect in differentiating preadipocytes, while 7-oxo-DHEA promotes differentiation without other changes typical of thermogenesis.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Assessment of the ability of type 2 cytochrome b5 to modulate 17,20-lyase activity of human P450c17

The 17 alpha-hydroxylase and 17,20-lyase activities of P450c17 lead to the production of 17 alpha-hydroxypregnenolone (17 alpha-OH-Preg) and dehydroepiandrosterone (DHEA), respectively, in different tissues. The mechanisms of differential regulation of these two activities are not yet fully elucidated. It has been previously shown that cytochrome b5 (cyt-b5) could facilitate the 17,20-lyase activity of human P450c17. Recently, a cDNA (type 2 cyt-b5) sharing 45.8% homology with type 1 cyt-b5 has been isolated from human testis. Since high 17,20-lyase activity is required for the production of androgens in the testis, we wanted to determine the importance of this second cDNA in the modulation of P450c17 17,20-lyase activity and hence, its role in the formation of active androgens. We therefore isolated type 2 cyt-b5 from human testis by RT-PCR and analyzed, by transient transfection in transformed human embryonic kidney cells (HEK-293) of various amounts of vectors expressing cyt-b5, P450-reductase and P450c17, its ability to modulate the 17,20-lyase activity of human P450c17. Results show that, in the presence of NADPH cytochrome P450 reductase (P450-red), type 2 cyt-b5 increases 17,20-lyase activity to a level comparable to that of type 1. These results support the idea that types 1 and 2 cyt-b5 could be involved in the differential modulation of 17 alpha-hydroxylase and 17,20-lyase activities of P450c17. Furthermore, the analysis of mRNA expression of types 1 and 2 cyt-b5 by RT-PCR using primers specific to each type showed that both types are present in the liver but also in the adrenal and testis.