Regulation of cytochrome P450scc synthesis and activity in the ovine corpus luteum

BDNF promotes mouse follicular development and reverses ovarian aging by promoting cell proliferation

BACKGROUND

Brain-derived neurotrophic factor (BDNF) plays an important role in ovarian function including follicle development and oocyte maturation, and embryonic development. However, whether BDNF treatment can reimpose ovarian aging and impaired fertility remains elusive. In this study, we investigated the reproductive outcomes of BDNF treatment and potential mechanisms in aged mice.

METHOD

"Aged" mice (35-37 weeks old, n = 68) were treated with recombinant human BDNF protein (rhBDNF, 1 µg/200 µL) through daily intraperitoneal (IP) injection for 10 days with/without ovulation induction. Reproductive age mice (8-10 weeks old, n = 28) were treated with ANA 12 (a selective BDNF receptor, TrkB antagonist) through daily IP injection for 5 days with/without ovulation induction. Ovarian function was assessed by ovarian weight, number of follicles, and sex hormone productions. Following induction of ovulation, the number of total oocytes or abnormal oocytes, and blastocyst formation were assessed. Reproductive functions of the mice were evaluated, including pregnancy rate, mating duration for conception, implantation sites, litter size, and weight of offspring. Finally, the molecular mechanism of the effects of BDNF on ovarian cell functions in mice were examined by Western blot and immunofluorescence.

RESULTS

rhBDNF treatment increased the ovarian weight, number of follicles, number and quality of oocytes including increased blastocysts formation, blood estrogen levels, and pregnancy rate in 35-37-week-old mice. Conversely, BDNF receptor antagonist, ANA 12, treatment decreased the ovarian volume and number of antral follicles and increased the proportion of abnormal oocytes in 8-10-week-old mice. We further demonstrated that BDNF treatment promoted ovarian cell proliferation as well as activation of TrkB and cyclinD1-creb signalling.

CONCLUSION

We demonstrated that ten consecutive days of daily IP injection of rhBDNF rescued ovarian function in aged mice. Our results further indicate that TrkB and cyclin D1-creb signaling may underlie the BDNF function in ovaries. Targeting BDNF-TrkB signaling is a potential novel therapeutic strategy to reverse ovarian aging.

History, insights, and future perspectives on studies into luteal function in cattle

The corpus luteum (CL) forms following ovulation from the remnant of the Graafian follicle. This transient tissue produces critical hormones to maintain pregnancy, including the steroid progesterone. In cattle and other ruminants, the presence of an embryo determines if the lifespan of the CL will be prolonged to ensure successful implantation and gestation, or if the tissue will undergo destruction in the process known as luteolysis. Infertility and subfertility in dairy and beef cattle results in substantial economic loss to producers each year. In addition, this has the potential to exacerbate climate change because more animals are needed to produce high-quality protein to feed the growing world population. Successful pregnancies require coordinated regulation of uterine and ovarian function by the developing embryo. These processes are often collectively termed "maternal recognition of pregnancy." Research into the formation, function, and destruction of the bovine CL by the Northeast Multistate Project, one of the oldest continuously funded Hatch projects by the USDA, has produced a large body of evidence increasing our knowledge of the contribution of ovarian processes to fertility in ruminants. This review presents some of the seminal research into the regulation of the ruminant CL, as well as identifying mechanisms that remain to be completely validated in the bovine CL. This review also contains a broad discussion of the roles of prostaglandins, immune cells, as well as mechanisms contributing to steroidogenesis in the ruminant CL. A triadic model of luteolysis is discussed wherein the interactions among immune cells, endothelial cells, and luteal cells dictate the ability of the ruminant CL to respond to a luteolytic stimulus, along with other novel hypotheses for future research.

Impairment of caprine oocyte maturation in vitro and alteration of granulosa cells functions by widely used fungicide mancozeb

Mancozeb is classified as an endocrine disruptor; thus the present study was carried out to investigate the impact of mancozeb on mammalian ovarian functions using in vitro caprine oocyte maturation and granulosa cell culture models. Caprine cumulus oocyte complexes (COCs) and granulosa cells were cultured under standard culture conditions and treated with mancozeb concentrations of 0.3, 3, and 30 μg/ml along with a control for 24 h and assessed. Granulosa cell viability and progesterone concentration in spent culture media after treatments were also assessed. Mancozeb significantly decreased (P < 0.05) the oocytes cumulus expansion and the maturation of caprine oocytes. Marked changes in granulose cell morphology were observed with 30 μg/ml mancozeb and significantly reduced (P < 0.05) cell viability. Interestingly, the same concentrations significantly increased (P < 0.05) the progesterone secretion by the cells. Significant reduction of granulosa cells viability and reduction of cumulus expansion and suppression of metaphase plate formation in oocyte can impair the fertilization ability and developmental potential of the oocytes. High progesterone concentration due to mancozeb treatment may suppress LH surge and suppress ovulation. In conclusion, mancozeb suppresses granulosa cells viability, reduces cumulus expansion, and suppresses metaphase plate formation but induces progesterone secretion from granulosa cells that may inhibit LH surge for ovulation process.

A low protein maternal diet during gestation has negative effects on male fertility markers in rats - A Systematic Review and Meta-analysis

Research indicates that some adult diseases including reproductive pathologies are programmed in utero during foetal development. In particular, maternal low dietary protein, during the most critical developmental periods of male foetal development, may have a detrimental impact on male fertility through direct and epigenetic mechanisms. The aim of our study was to evaluate the impact of a gestational low protein diet on fertility markers in male offspring in rats through a systematic review and meta-analysis. A systematic search using PubMed, and EMBASE databases was performed and two investigators independently screened the 1,703 prospective articles. Eleven articles met the eligibility criteria. Outcome measures were pooled using random-effects models and expressed as mean differences (MDs) at 95% CIs for each study. The results reveal significant reduction in testis weight (MD (mean difference) -0.08 g; -0.12, -0.42; p = .0001), epididymal sperm count (MD -35.34 × 10⁶ cells; -52.15, -18.53; p = .0001), number of Sertoli cells (MD -7.27 × 10⁶ (-13.92, -0.62; p = .03), testosterone (T) concentration (MD -0.29 ng/ml; -0.48, -0.09; p = .004) and luteinising hormone (LH) concentration (MD of -0.24 ng/ml; -0.45, 0.04; p = .02) in comparison with controls. In contrast, follicle-stimulating hormone (FSH) concentration (MD of 0.07 ng/ml; -0.16, 0.29; p = .56) was not significantly different from controls. We conclude that low gestational dietary protein maternal intake potentially negatively impacts fertility in male progeny later in life. The mechanisms of action responsible for these phenomena remain unclear.

Estrogen receptor signaling mechanisms

The primary female sex hormones, estrogens, are responsible for the control of functions of the female reproductive system, as well as the development of secondary sexual characteristics that appear during puberty and sexual maturity. Estrogens exert their actions by binding to specific receptors, the estrogen receptors (ERs), which in turn activate transcriptional processes and/or signaling events that result in the control of gene expression. These actions can be mediated by direct binding of estrogen receptor complexes to specific sequences in gene promoters (genomic effects), or by mechanisms that do not involve direct binding to DNA (non-genomic effects). Whether acting via direct nuclear effects, indirect non-nuclear actions, or a combination of both, the effects of estrogens on gene expression are controlled by highly regulated complex mechanisms. In this chapter, we summarize the knowledge gained in the past 60years since the discovery of the estrogen receptors on the mechanisms governing estrogen-mediated gene expression. We provide an overview of estrogen biosynthesis, and we describe the main mechanisms by which the female sex hormone controls gene transcription in different tissues and cell types. Specifically, we address the molecular events governing regulation of gene expression via the nuclear estrogen receptors (ERα, and ERβ) and the membrane estrogen receptor (GPER1). We also describe mechanisms of cross-talk between signaling cascades activated by both nuclear and membrane estrogen receptors. Finally, we discuss natural compounds that are able to target specific estrogen receptors and their implications for human health and medical therapeutics.

Increased 27-hydroxycholesterol production during luteolysis may mediate the progressive decline in progesterone secretion

STUDY QUESTION

Does 27-hydroxycholesterol (27OH) actively facilitate the progression of luteolysis?

SUMMARY ANSWER

There is increased mRNA expression of the enzyme that produces 27OH during luteolysis in vivo in rhesus macaques and sheep, and 27OH reduces progesterone secretion from human luteinized granulosa cells.

WHAT IS KNOWN ALREADY

There is an increase in mRNA expression of liver x receptor (LXR) and a decrease in sterol regulatory element binding protein 2 (SREBP2) target genes during spontaneous luteolysis in primates, which could result in reduced cholesterol availability for steroidogenesis. Concentrations of 27OH are also increased in primate corpora lutea (CL) during luteolysis, and 27OH is a dual LXR agonist and SREBP2 inhibitor.

STUDY DESIGN SIZE, DURATION

This was an in vitro study using primary human luteinized granulosa cells in a control versus treatment(s) design. Analyses of CL from sheep undergoing induced or spontaneous luteolysis were also performed, along with database mining of microarray data from rhesus macaque CL.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Primary luteinizing granulosa cells were obtained from 37 women aged 24-44 who were undergoing oocyte donation or IVF for male factor or idiopathic infertility, and cells were further luteinized in vitro using human chorionic gonadotropin. Three approaches to test the effect of 27OH produced via CYP27A1 (cytochrome p450, family 27, subfamily A, polypeptide 1) on luteinized granulosa cells were used: (i) direct 27OH supplementation, (ii) induction of endogenous CYP27A1 activity via pharmacologic inhibition of steroidogenesis, and (iii) siRNA-mediated knockdown to directly inhibit CYP27A1 as well as cholesterol transport into the mitochondria via the steroidogenic acute regulatory protein (STAR). Endpoints included: progesterone (P4) secretion into culture media determined by enzyme immunoassay, cholesterol efflux and uptake assays using fluorescent lipid analogs, and mRNA expression determined via semi-quantitative real-time PCR (QPCR). An additional experiment involved QPCR analysis of 40 CL collected from ewes undergoing induced or spontaneous luteolysis, as well as database mining of microarray data generated from 16 rhesus macaque CL collected during spontaneous luteolysis and 13 macaque CL collected during a luteinizing hormone ablation and replacement protocol.

MAIN RESULTS AND THE ROLE OF CHANCE

The mRNA expression of CYP27A1 was significantly increased during luteolysis in rhesus macaques and sheep in vivo, and CYP27A1 transcription was suppressed by luteinizing hormone and hCG. There was a significant decrease in hCG-stimulated P4 secretion from human luteinized granulosa cells caused by 27OH treatment, and a significant increase in basal and hCG-stimulated P4 synthesis when endogenous 27OH production was inhibited via CYP27A1 knockdown, indicating that 27OH inhibits steroidogenesis. Pharmacologic inhibition of steroidogenesis by aminoglutethimide significantly induced LXR and inhibited SREBP2 target gene mRNA expression, indicating that increased oxysterol production occurs when steroidogenesis is suppressed. Inhibiting cholesterol delivery into the mitochondria via knockdown of STAR resulted in reduced SREBP2 target gene mRNA expression, indicating that STAR function is necessary to maintain SREBP2-mediated transcription. The effects of 27OH treatment on markers of LXR and SREBP2 activity were moderate, and knockdown of CYP27A1 did not prevent aminoglutethimide-induced changes in LXR and SREBP2 target gene mRNA expression. These observations indicate that 27OH inhibits P4 secretion partially via mechanisms separate from its role as an LXR agonist and SREBP2 inhibitor, and also demonstrate that other oxysterols are involved in modulating LXR and SREBP2-mediated transcription when steroidogenesis is suppressed.

LARGE SCALE DATA

None.

LIMITATIONS REASONS FOR CAUTION

Luteinized granulosa cells may differ from luteal cells, and the effect on luteal function in vivo was not directly tested. The mechanisms that cause the initial rise in CYP27A1 mRNA expression during luteolysis are also not clear.

WIDER IMPLICATIONS OF THE FINDINGS

The factors causing luteolysis in primates have not yet been determined. This study provides functional evidence of a novel mechanism via increased 27OH synthesis during luteolysis, which subsequently represses progesterone secretion. Increased 27OH may also facilitate the progression of luteolysis in domestic animal species.

STUDY FUNDING AND COMPETING INTEREST(S)

The authors have nothing to disclose. Support was provided by the Eunice Kennedy Shriver National Institute of Child Health & Human Development (NICHD) of the National Institutes of Health (NIH), award number R00HD067678 to R.L.B.

Global gene expression in the bovine corpus luteum is altered after stimulatory and superovulatory treatments

Equine chorionic gonadotrophin (eCG) has been widely used in superovulation and artificial insemination programmes and usually promotes an increase in corpus luteum (CL) volume and stimulates progesterone production. Therefore, to identify eCG-regulated genes in the bovine CL, the transcriptome was evaluated by microarray analysis and the expression of selected genes was validated by qPCR and western blot. Eighteen Nelore crossbred cows were divided into control (n = 5), stimulated (n = 6) and superovulated groups (n = 7). Ovulation was synchronised using a progesterone device-based protocol. Stimulated animals received 400 IU of eCG at device removal and superovulated animals received 2000 IU of eCG 4 days prior. Corpora lutea were collected 7 days after gonadotrophin-releasing hormone administration. Overall, 242 transcripts were upregulated and 111 transcripts were downregulated in stimulated cows (P ≤ 0.05) and 111 were upregulated and 113 downregulated in superovulated cows compared to the control animals (1.5-fold, P ≤ 0.05). Among the differentially expressed genes, many were involved in lipid biosynthesis and progesterone production, such as PPARG, STAR, prolactin receptors and follistatin. In conclusion, eCG modulates gene expression differently depending on the treatment, i.e. stimulatory or superovulatory. Our data contribute to the understanding of the pathways involved in increased progesterone levels observed after eCG treatment.

Stimulatory effect of LH, PGE2 and progesterone on StAR protein, cytochrome P450 cholesterol side chain cleavage and 3beta hydroxysteroid dehydrogenase gene expression in bovine luteal cells

The aim of these studies was to investigate the effect of LH, progesterone (P4), PGE, noradrenaline (NA) and a nitric oxide donor, S-nitroso-N-acetylpenicillamine (S-NAP), on steroid acute regulatory protein (StAR), 3beta-hydroxysteroid dehydrogenase (3beta-HSD) and cytochrome P450 side chain cleavage (P450scc) gene expression and on the synthesis of their protein products. Bovine luteal cells were collected and prepared on days 6-10 of the estrous cycle and preincubated in vitro for 24 h. Thereafter, medium was changed and supplemented with one of six treatments: control medium, LH (100 ng/ml), P4 (10(-5)M), PGE2 (10(-6)M), NA (10(-5)M) or S-NAP (10(-4)M). In Experiment 1, luteal cells (10(6)/well) were incubated for 3, 6, 18 and 24 h. After incubation, total RNA was isolated and P4 concentrations in medium was determined. Semiquantitative RT-PCR was used to measure gene expression. In Experiment 2, luteal cells were preincubated for 24h, then stimulated as in Experiment 1. Total protein was isolated from lysed cells and Western blot analysis was performed using specific antibodies against the StAR, 3beta-HSD and cytochrome P450scc proteins. Bands were analyzed by means of KODAK 1D Image Analysis Software. In Experiment 1, LH and PGE2 stimulated secretion of progesterone from luteal cells. Concentrations of mRNA for StAR, 3beta-HSD, cytochrome P450scc were increased after 6 h in cells stimulated with LH, PGE2 and P4 (P<0.05). Gene expression was not affected by NA. In Experiment 2, LH, P4 and PGE2 induced an increase in the concentration of these three proteins. S-NAP inhibited both concentrations of mRNA and protein for StAR, 3beta-HSD, cytochrome P450scc. Therefore, the increase in secretion of P4 induced by LH and PGE2 is associated with increases in StAR, 3beta-HSD and cytochrome P450scc gene expression. This genomic response may be mediated in part through a positive effect of P4 on the expression of these genes observed in this experiment.

Cholesterol transport and steroidogenesis by the corpus luteum

The synthesis of progesterone by the corpus luteum is essential for the establishment and maintenance of early pregnancy. Regulation of luteal steroidogenesis can be broken down into three major events; luteinization (i.e., conversion of an ovulatory follicle), luteal regression, and pregnancy induced luteal maintenance/rescue. While the factors that control these events and dictate the final steroid end products are widely varied among different species, the composition of the corpus luteum (luteinized thecal and granulosa cells) and the enzymes and proteins involved in the steroidogenic pathway are relatively similar among all species. The key factors involved in luteal steroidogenesis and several new exciting observations regarding regulation of luteal steroidogenic function are discussed in this review.

Luteal expression of cytochrome P450 side-chain cleavage, steroidogenic acute regulatory protein, 3beta-hydroxysteroid dehydrogenase, and 20alpha-hydroxysteroid dehydrogenase genes in late pregnant rats: effect of luteinizing hormone and RU486

A decrease in serum progesterone at the end of pregnancy is essential for the induction of parturition in rats. We have previously demonstrated that LH participates in this process through: 1) inhibiting 3beta-hydroxysteroid dehydrogenase (3beta-HSD) activity and 2) stimulating progesterone catabolism by inducing 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD) activity. The objective of this investigation was to determine the effect of LH and progesterone on the luteal expression of the steroidogenic acute regulatory protein (StAR), cytochrome P450 side-chain cleavage (P450(scc)), 3beta-HSD, and 20alpha-HSD genes. Gene expression was analyzed by Northern blot analysis 24 and 48 h after administration of LH or vehicle on Day 19 of pregnancy. StAR and 3beta-HSD mRNA levels were lower in LH-treated rats than in rats administered with vehicle at both time points studied. P450(scc) mRNA levels were unaffected by LH. The 20alpha-HSD mRNA levels were not different between LH and control rats 24 h after treatment; however, greater expression of 20alpha-HSD, with respect to controls, was observed in LH-treated rats 48 h after treatment. Luteal progesterone content dropped in LH-treated rats at both time points studied, whereas serum progesterone decreased after 48 h only. In a second set of experiments, the anti-progesterone RU486 was injected intrabursally on Day 20 of pregnancy. RU486 had no effect on 3beta-HSD or P450(scc) expression but increased 20alpha-HSD mRNA levels after 8 h treatment. In conclusion, the luteolytic effect of LH is mediated by a drop in StAR and 3beta-HSD expression without effect on P450(scc) expression. We also provide the first in vivo evidence indicating that a decrease in luteal progesterone content may be an essential step toward the induction of 20alpha-HSD expression at the end of pregnancy in rats.

Regulation of steroidogenesis in transgenic mice and zebrafish

Steroid hormones are important physiological regulators in the body. Steroid hormones are mainly synthesized in the adrenal and gonads. Their synthesis is stimulated by pituitary hormones through cAMP as an intracellular mediator. The first and rate-limiting step for steroid biosynthesis is catalyzed by CYP11A1. Important regulatory elements for the control of the CYP11A1 gene expression have been characterized both in vitro and in vivo. The SF-1-binding sites are cis-acting elements controlling the basal and cAMP-stimulated gene expression. Our transgenic mouse studies showed that the 2.3kb promoter contains information controlling developmentally regulated gene expression. Finally, we present our results on the cloning of steroidogenic genes in zebrafish, a new model organism for genetic studies.

Models of luteinization

Luteinization is essential to the success of early gestation. It is the process by which elements of the ovarian follicle, usually including both theca interna and granulosa cells, are provoked by the ovulatory stimulus to develop into the corpus luteum. Although there are significant species differences in luteinization, some elements pervade, including the morphological and functional differentiation to produce and secrete progesterone. There is evidence that luteinization results in granulosa cell exit from the cell cycle. The mechanisms that appear to control luteinization include intracellular signalling pathways, cell adhesion factors, intracellular cholesterol and oxysterols, and perhaps progesterone itself as a paracrine or intracrine regulator. Cell models of luteinization, along with some of the conflicting observations on the luteinization process, are discussed in this review.

Mechanisms controlling the function and life span of the corpus luteum

The primary function of the corpus luteum is secretion of the hormone progesterone, which is required for maintenance of normal pregnancy in mammals. The corpus luteum develops from residual follicular granulosal and thecal cells after ovulation. Luteinizing hormone (LH) from the anterior pituitary is important for normal development and function of the corpus luteum in most mammals, although growth hormone, prolactin, and estradiol also play a role in several species. The mature corpus luteum is composed of at least two steroidogenic cell types based on morphological and biochemical criteria and on the follicular source of origin. Small luteal cells appear to be of thecal cell origin and respond to LH with increased secretion of progesterone. LH directly stimulates the secretion of progesterone from small luteal cells via activation of the protein kinase A second messenger pathway. Large luteal cells are of granulosal cell origin and contain receptors for PGF(2alpha) and appear to mediate the luteolytic actions of this hormone. If pregnancy does not occur, the corpus luteum must regress to allow follicular growth and ovulation and the reproductive cycle begins again. Luteal regression is initiated by PGF(2alpha) of uterine origin in most subprimate species. The role played by PGF(2alpha) in primates remains controversial. In primates, if PGF(2alpha) plays a role in luteolysis, it appears to be of ovarian origin. The antisteroidogenic effects of PGF(2alpha) appear to be mediated by the protein kinase C second messenger pathway, whereas loss of luteal cells appears to follow an influx of calcium, activation of endonucleases, and an apoptotic form of cell death. If the female becomes pregnant, continued secretion of progesterone from the corpus luteum is required to provide an appropriate uterine environment for maintenance of pregnancy. The mechanisms whereby the pregnant uterus signals the corpus luteum that a conceptus is present varies from secretion of a chorionic gonadotropin (primates and equids), to secretion of an antiluteolytic factor (domestic ruminants), and to a neuroendocrine reflex arc that modifies the secretory patterns of hormones from the anterior pituitary (most rodents).

Prostaglandin F(2alpha) induces a rapid decline in progesterone production and steroidogenic acute regulatory protein expression in isolated rat corpus luteum without altering messenger ribonucleic acid expression

With interest in steroidogenic acute regulatory protein (StAR) involvement in the luteolytic process, we studied changes in serum progesterone levels and the concomitant expression of StAR mRNA and protein (37-, 32-, and 30-kDa forms) in postovulatory Day 7 corpora lutea (CL) isolated from rats 1 h after injection with prostaglandin F(2alpha) (PGF(2alpha), n = 6) or saline (n = 6). Serum progesterone levels were determined by RIA, StAR and beta-actin mRNA expression by Northern analysis, and StAR and beta-actin protein expression by Western analysis. Adrenal, brain, and spleen from control animals were used as positive and negative controls for StAR expression. Scanning optical densitometry measurements were standardized by dividing the signal strength from each StAR autoradiogram lane by that from the corresponding beta-actin autoradiogram lane. ANOVA was used for significance testing, with alpha set at 0.05. The 37-, 32-, and 30-kDa forms of StAR protein were expressed in all adrenal samples, whereas only the 37- and 30-kDa forms were found in CL. Serum progesterone levels and expression of the 30-kDa and 37-kDa forms of the StAR protein in CL were all found to be significantly lower in the PGF(2alpha)-treated than the saline-treated group. StAR mRNA expression was not significantly different in the saline- and PGF(2alpha)-treated rats. The rapid decline in StAR protein expression that accompanies PGF(2alpha) induced luteolysis, therefore, does not result from significant decline in mRNA expression.

The mechanism of action of steroidogenic acute regulatory protein (StAR). StAR acts on the outside of mitochondria to stimulate steroidogenesis

Steroidogenic acute regulatory protein (StAR) plays an essential role in steroidogenesis, facilitating delivery of cholesterol to cytochrome P450scc on the inner mitochondrial membrane. StAR is synthesized in the cytoplasm and is subsequently imported by mitochondria and processed to a mature form by cleavage of the NH2-terminal mitochondrial targeting sequence. To explore the mechanism of StAR action, we produced 6-histidine-tagged N-62 StAR (His-tag StAR) constructs lacking the NH2-terminal 62 amino acids that encode the mitochondrial targeting sequence and examined their steroidogenic activity in intact cells and on isolated mitochondria. His-tag StAR proteins stimulated pregnenolone synthesis to the same extent as wild-type StAR when expressed in COS-1 cells transfected with the cholesterol side-chain cleavage system. His-tag StAR was diffusely distributed in the cytoplasm of transfected COS-1 cells whereas wild-type StAR was localized to mitochondria. There was no evidence at the light or electron microscope levels for selective localization of His-tag StAR protein to mitochondrial membranes. In vitro import assays demonstrated that wild-type StAR preprotein was imported and processed to mature protein that was protected from subsequent trypsin treatment. In contrast, His-tag StAR was not imported and protein associated with mitochondria was sensitive to trypsin. Using metabolically labeled COS-1 cells transfected with wild-type or His-tag StAR constructs, we confirmed that wild-type StAR preprotein was imported and processed by mitochondria, whereas His-tag StAR remained largely cytosolic and unprocessed. To determine whether cytosolic factors are required for StAR action, we developed an assay system using washed mitochondria isolated from bovine corpora lutea and purified recombinant His-tag StAR proteins expressed in Escherichia coli. Recombinant His-tag StAR stimulated pregnenolone production in a dose- and time-dependent manner, functioning at nanomolar concentrations. A point mutant of StAR (A218V) that causes lipoid congenital adrenal hyperplasia was incorporated into the His-tag protein. This mutant was steroidogenically inactive in COS-1 cells and on isolated mitochondria. Our observations conclusively document that StAR acts on the outside of mitochondria, independent of mitochondrial import, and in the absence of cytosol. The ability to produce bioactive recombinant StAR protein paves the way for refined structural studies of StAR and StAR mutants.