Regulation of cholesterol movement to mitochondrial cytochrome P450scc in steroid hormone synthesis

Differential effect of chronic inhibition of calcium channel and angiotensin II type 1-receptor on aldosterone synthesis in spontaneously hypertensive rats

We have investigated the in vivo effect of chronic blockade of Ca(2+)-channels and angiotensin II type 1 (AT(1))-receptors on aldosterone (Aldo)-synthesis in the adrenal glands of spontaneously hypertensive rats (SHR). Male SHR were administered Ca(2+)-antagonist, amlodipine (10 mg/kg per day) or AT(1)-receptor-antagonist, TCV-116 (1 mg/kg per day) from 7 until 11 weeks of age. Systolic blood pressure (SBP) and heart rate (HR) were significantly higher in SHR than Wistar-Kyoto (WKY) rats. Both treatments resulted in equivalent and significant reduction in SBP in SHR. Aldo-secretion in SHR, which was significantly higher than in WKY rats, was profoundly suppressed by TCV-116 compared with amlodipine. Both treatments resulted in thickening of the zona glomerulosa, which immunohistochemically contains Aldo, at the end of therapy. Competitive reverse transcription-polymerase chain reaction (RT-PCR) showed that CYP11A (P450scc) mRNA regulating the first step of Aldo-synthesis was significantly reduced from week 9 of age by amlodipine, and that CYP11B2 (P450aldo) mRNA regulating the last step of Aldo-synthesis was potently suppressed from 9 weeks of age by TCV-116. Our results indicate that chronic treatment with different antihypertensive agents directly modulates adrenocortical aldosterone synthesis in SHR in vivo via different mechanisms.

A trypsin-sensitive protein is required for utilization of exogenous cholesterol for pregnenolone synthesis by placental mitochondria

The utilization of cholesterol for steroid hormone synthesis by human placental mitochondria is poorly understood. The human placenta does not express the steroidogenic acute regulator protein, which is critical for cholesterol delivery to the cholesterol side chain cleavage system in adrenal and gonadal mitochondria. We explored the mechanism underlying cholesterol transport in human placental mitochondria by measuring its transformation into pregnenolone. Mitochondria of syncytiotrophoblast from human term placenta were isolated by centrifugation through a sucrose gradient. The synthesis of pregnenolone in the presence of exogenous cholesterol was increased two-fold in syncytiotrophoblast mitochondria. Treatment of mitochondria with trypsin prevented the increase in the synthesis of pregnenolone in the presence of exogenous cholesterol. However, when 22-OH cholesterol, a substrate that readily crosses membranes, was added, the trypsin-treated mitochondria synthesized increased amounts of pregnenolone. The trypsin-treated mitochondria were intact, since oxygen consumption, succinate dehydrogenase and the adenine nucleotide translocase activities were not significantly different from in untreated mitochondria. However, activity of NADH cytochrome c oxidoreductase, an outer mitochondrial membrane enzyme, was reduced in the trypsin-treated mitochondria, reflecting the selective degradation of proteins. In addition, SDS-PAGE analysis revealed the loss of a prominent 34 kDa band which proved to be a novel porin-like protein that binds to cholesterol. These results support our previous assumption that human placental mitochondria employ a novel protein(s)-mediated the mechanism to take up cholesterol for steroidogenesis.

The peroxisome proliferator perfluorodecanoic acid inhibits the peripheral-type benzodiazepine receptor (PBR) expression and hormone-stimulated mitochondrial cholesterol transport and steroid formation in Leydig cells

The peroxisome proliferator perfluordecanoic acid (PFDA) has been shown to exert an antiandrogenic effect in vivo by acting directly on the interstitial Leydig cells of the testis. The objective of this study was to examine the in vitro effects of PFDA and identify its site of action in steroidogenesis using as model systems the mouse tumor MA-10 and isolated rat Leydig cells. PFDA inhibited in a time- and dose-dependent manner the hCG-stimulated Leydig cell steroidogenesis. This effect was localized at the level of cholesterol transport into the mitochondria. PFDA did not affect either the total cell protein synthesis or the mitochondrial integrity. Moreover, it did not induce any DNA damage. Morphological studies indicated that PFDA induced lipid accumulation in the cells, probably due to the fact that cholesterol mobilized by hCG did not enter the mitochondria to be used for steroidogenesis. In search of the target of PFDA, we examined its effect on key regulatory mechanisms of steroidogenesis. PFDA did not affect the hCG-induced steroidogenic acute regulatory protein (StAR) levels. However, it was found to inhibit the mitochondrial peripheral-type benzodiazepine receptor (PBR) ligand binding capacity, 18-kDa protein, and messenger RNA (mRNA) levels. Further studies indicated that PFDA did not affect PBR transcription, but it rather accelerated PBR mRNA decay. Taken together, these data suggest that PFDA inhibits the Leydig cell steroidogenesis by affecting PBR mRNA stability, thus inhibiting PBR expression, cholesterol transport into the mitochondria, and the subsequent steroid formation. Moreover, this action of PFDA on PBR mRNA stability indicates a new mechanism of action of peroxisome proliferators distinct from the classic transcription-mediated regulation of target genes.

Intramitochondrial cholesterol transfer

Cholesterol serves as the initial substrate for all steroid hormones synthesized in the body regardless of the steroidogenic tissue or final steroid produced. The first steroid formed in the steroidogenic pathway is pregnenolone which is formed by the excision of a six carbon unit from cholesterol by the cytochrome P450 side chain cleavage enzyme system which is located in the inner mitochondrial membrane. It has long been known that the regulated biosynthesis of steroids is controlled by a cycloheximide sensitive factor whose function is to transfer cholesterol from the outer to the inner mitochondrial membrane, thus, the identity of this factor is of great importance. A candidate for the regulatory factor is the mitochondrial protein, the steroidogenic acute regulatory (StAR) protein. Cloning and sequencing of the StAR cDNA indicated that it was a novel protein, and transient transfections with the cDNA for the StAR protein resulted in increased steroid production in the absence of stimulation. Mutations in the StAR gene cause the potentially lethal disease congenital lipoid adrenal hyperplasia, a condition in which cholesterol transfer to the cytochrome P450 side chain cleavage enzyme, P450scc, is blocked, filling the cell with cholesterol and cholesterol esters. StAR knockout mice have a phenotype which is essentially identical to the human condition. The cholesterol transferring activity of StAR has been shown to reside in the C-terminal part of the molecule and a protein sharing homology with a region in the C-terminus of StAR has been shown to display cholesterol transferring capacity. Recent evidence has indicated that StAR can act as a sterol transfer protein and it is perhaps this characteristic which allows it to mobilize cholesterol to the inner mitochondrial membrane. However, while it appears that StAR is the acute regulator of steroid biosynthesis via its cholesterol transferring activity, its mechanism of action remains unknown.

Human neuroblastoma SH-SY5Y cell line: neurosteroid-producing cell line relying on cytoskeletal organization

Pregnenolone, the precursor of all steroids, is synthesized by CNS structures. The synthesis requires an obligatory step involving cholesterol transport to mitochondrial cytochrome P450-cholesterol side chain cleavage (cytP450scc), although the underlying mechanism(s) are still mostly unknown. We used the human neuroblastoma SH-SY5Y cell line to investigate cytP450scc expression and activity and to establish a role of cytoskeleton in pregnenolone synthesis. Immunocytochemical and biochemical approaches revealed that undifferentiated as well as differentiated cells either by retinoic acid (RA) or phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), possess cytP450scc and rapidly synthesize pregnenolone in the presence of a NADPH-generating system. The newly neurosteroid formation by SH-SY5Y cells was increased by 22R-hydroxycholesterol and blocked by the cytP450scc inhibitor, aminoglutethimide. When trilostane was used to inhibit 3beta-hydroxysteroid dehydrogenase catalyzing pregnenolone conversion into progesterone, a higher pregnenolone accumulation occurred in TPA-differentiated cells than in RA-differentiated ones. Although SU 10603, a blocker of 17alpha-hydroxylase/c17,20-lyase enzyme involved in DHEA formation from pregnenolone, gave rise to an elevated neurosteroid content only in RA-differentiated cells. No difference in pregnenolone levels was found in undifferentiated cells treated with each inhibitor. Thus, differentiation seems to promote pregnenolone-metabolizing enzyme activities that may vary upon phenotypic changes induced by RA or TPA. Treatments of differentiated cells with the microtubule-depolymerizing drug colchicine and the actin microfilament-altering agent cytochalasin D decreased pregnenolone synthesis without affecting cell viability or cytP450scc amount. Addition of the cell-permeant cholesterol analogue 22R-hydroxycholesterol known to elude cholesterol transport systems induced pregnenolone synthesis, however, indicating that perturbations in cytoskeleton likely affect endogenous cholesterol transport. The relevance of this finding may rest on the observed involvement of cytoskeletal organization in such events as neuronal plasticity, cognitive function and also neurodegenerative disorders in which neurosteroids have been shown to have a part.

Steroidogenic acute regulatory (StAR) protein: what's new?

In response to trophic hormone stimulation of steroidogenic adrenal and gonadal cells, the acute biosynthesis of steroid hormones occurs in the order of minutes to tens of minutes and can be contrasted to chronic regulation, which occurs on the order of hours. The steroidogenic acute regulatory (StAR) protein is an indispensable component in the acute regulatory phase and functions by rapidly mediating the transfer of the substrate for all steroid hormones, cholesterol, from the outer to the inner mitochondrial membrane where it is cleaved to pregnenolone, the first steroid formed. This transfer of cholesterol constitutes the rate-limiting step in steroidogenesis. To underscore its importance, mutations in the StAR gene have been shown to be the only cause of the potentially fatal disease lipoid congenital adrenal hyperplasia, in which affected individuals synthesize virtually no steroids. Since the cloning of the murine cDNA in 1994, many observations have substantiated the critical role of StAR in regulated steroidogenesis. The purpose of this review will be to summarize briefly some background material on StAR and then attempt to update several recent and interesting findings on the StAR protein.

Stimulation of aldosterone secretion by benzodiazepines in bovine adrenocortical cells

Previous studies have indicated that peripheral benzodiazepine receptor (PBR) ligands inhibit aldosterone secretion in isolated adrenal zona glomerulosa cells although positive responses have been demonstrated in other steroidogenic tissues. In the present study, aldosterone secretion was measured in bovine cells after 6 days of primary culture. At this time, basal aldosterone secretion was very low and cells appeared less sensitive to the steroidogenic effects of extracellular [K+] (maximal response required K+ concentration > 32 mmol/L) but were sensitised to angiotensin II (maximal response achieved with 3 nM) when compared with previous studies with freshly isolated cells. Diazepam concentration in the range 0.1 nM to 1 microM increased basal aldosterone secretion, an effect which was not enhanced by pre-treatment with diazepam. The effects were small compared with those of angiotensin II or K+. Over the same concentration range, diazepam also potentiated the stimulatory effects of sub-maximally effective concentrations of angiotensin II. When cells were treated with high-density lipoprotein (HDL-3) as a source of cholesterol, diazepam and the PBR ligands Ro5-4864 and PK11195 also stimulated aldosterone secretion at nanomolar concentrations. In addition, the conversion of added 11-deoxycorticosterone (DOC) to aldosterone was increased by nanomolar concentrations of diazepam and Ro5-4864 but inhibited by high micromolar concentrations of these drugs (100 microM). We conclude that adrenocortical responses to PBR ligands are complex. At high concentrations, inhibitory effects involving competition for steroidogenic enzymes and calcium channel blockage predominate. At low concentrations, an enhancement of basal, angiotensin-II and cholesterol-dependent aldosterone synthesis is revealed which may involve a PBR-mediated mitochondrial uptake of cholesterol and DOC.

Plasma aldosterone, plasma lipoproteins, obesity and insulin resistance in humans

Aldosterone production in vitro can be affected by many hormones, autacoids, ions, and lipids, but regulation in humans is incompletely understood. We measured plasma aldosterone in adult subjects with a wide range of obesity and insulin resistance. Aldosterone levels correlated with measures of visceral obesity in one predominantly male cohort and in the women of a second cohort. In the same subjects, aldosterone correlated with insulin resistance. Aldosterone also correlated with plasma cortisol in men and women, and with DHEA-S in women. The data suggested that visceral fat stimulates adrenal steroidogenesis. We found that certain fatty acids stimulated aldosterone production in vitro by rat adrenal cells incubated with rat hepatocytes, but not adrenal cells alone. The results suggested that fatty acids from visceral adipocytes induce hepatic formation of an adrenal secretagogue. This may explain the correlation of plasma steroids with visceral obesity. Aldosterone may contribute to vascular diseases that complicate obesity.

Functional assessment of the calcium messenger system in cultured mouse Leydig tumor cells: regulation of human chorionic gonadotropin-induced expression of the steroidogenic acute regulatory protein

The steroidogenic acute regulatory (StAR) protein, a 30-kDa mitochondrial factor, is a key regulator of steroid hormone biosynthesis, facilitating the transfer of cholesterol from the outer to the inner mitochondrial membrane. StAR protein expression is restricted to steroidogenic tissues, and it responds to hormonal stimulation through different second messenger pathways. The present study was designed to explore the mechanisms of extracellular calcium (Ca2+) involved in the hCG-stimulated expression of StAR protein and steroidogenesis in a mouse Leydig tumor cell line (mLTC-1). Extracellular Ca2+ (1.5 mmol/liter) enhanced the hCG (50 microg/liter)-induced increases in StAR messenger RNA (mRNA) and protein levels (1.7 +/- 0.3-fold; 4 h), as monitored by quantitative RT-PCR and immunoblotting. The potentiating effect of Ca2+ on the hCG-stimulated StAR response correlated with the acute progesterone (P) response. In accordance, omission of Ca2+ from the extracellular medium by specific Ca2+ chelators, EDTA or EGTA (4 mmol/liter each), markedly diminished the hCG-stimulated P production. The Ca2+ effect on hCG-induced StAR mRNA expression was dramatically suppressed by 10 micromol/liter verapamil, a Ca2+ channel blocker. The Ca2+-mobilizing agonist, potassium (K+; 4 mmol/liter), greatly increased the hCG responses of StAR expression and P production, which conversely were attenuated by Ca2+ antagonists, further supporting the involvement of intracellular free Ca2+ ([Ca2+]i) in these responses. The interaction of Ca2+ or K+ with hCG accounted for a clear increase in the StAR protein level (1.4-1.8-fold; 4 h) compared with that after hCG stimulation. Inhibition of protein synthesis by cycloheximide (CHX) drastically diminished the hCG-induced StAR protein content, indicating the requirement for on-going protein synthesis for hCG action. The transmembrane uptake of 45Ca2+ was increased by 26% with hCG and was strongly inhibited by verapamil. [Ca2+]i moderately augmented the response to hCG in fura-2/AM-loaded mLTC-1 cells within 30-40 sec, reaching a plateau within 1-3 min. Interestingly, the calcium ionophore (A23187) clearly increased (P < 0.01) StAR mRNA expression, in additive fashion with hCG. Northern hybridization analysis revealed four StAR transcripts at 3.4, 2.7, 1.6, and 1.4 kb, with the 1.6-kb band corresponding to the functional StAR protein; all of them were up-regulated 3- to 5-fold upon hCG stimulation, with a further increase in the presence of Ca2+. The mechanism of the Ca2+ effect on hCG-stimulated StAR expression and P production was evaluated by assessing the involvement of the nuclear orphan receptor, steroidogenic factor 1 (SF-1). Stimulation of hCG significantly elevated (2.1 +/- 0.3-fold) the SF-1 mRNA level, which was further augmented in the presence of Ca2+, whereas EGTA and verapamil completely abolished the increase caused by Ca2+. Cells expressing SF-1 marginally increased StAR expression, but coordinately elevated StAR mRNA levels in response to hCG and hCG plus Ca2+ compared with those in mock-transfected cells. On the other hand, overexpression of the nuclear receptor DAX-1 remarkably diminished (P < 0.0001) the endogenous SF-1 mRNA level as well as hCG-induced StAR mRNA expression. In summary, our results provide evidence that extracellular Ca2+ rapidly increases [Ca2+]i after hCG stimulation, presumably through opening of the transmembrane Ca2+ channel. Neither extracellular Ca2+ nor K+ alone has a noticeable effect on StAR expression and steroidogenesis, whereas they clearly potentiate hCG induction. The Ca2+-mediated increase in hCG involved in StAR expression and P production is well correlated to the levels of SF-1 expression. The stimulatory effect of hCG that rapidly increases [Ca2+]i is responsible at least in part for the regulation of SF-1-mediated StAR expression that consequently regulates steroidogenesis in mouse Leydig tumor cells.

In vitro studies on the role of the peripheral-type benzodiazepine receptor in steroidogenesis

In vitro studies using isolated cells, mitochondria and submitochondrial fractions demonstrated that in steroid synthesizing cells, the peripheral-type benzodiazepine receptor (PBR) is an outer mitochondrial membrane protein, preferentially located in the outer/inner membrane contact sites, involved in the regulation of cholesterol transport from the outer to the inner mitochondrial membrane, the rate-determining step in steroid biosynthesis. Mitochondrial PBR ligand binding characteristics and topography are sensitive to hormone treatment suggesting a role of PBR in the regulation of hormone-mediated steroidogenesis. Targeted disruption of the PBR gene in Leydig cells in vitro resulted in the arrest of cholesterol transport into mitochondria and steroid formation; transfection of the mutant cells with a PBR cDNA rescued steroidogenesis demonstrating an obligatory role for PBR in cholesterol transport. Molecular modeling of PBR suggested that it might function as a channel for cholesterol. This hypothesis was tested in a bacterial system devoid of PBR and cholesterol. Cholesterol uptake and transport by these cells was induced upon PBR expression. Amino acid deletion followed by site-directed mutagenesis studies and expression of mutant PBRs demonstrated the presence in the cytoplasmic carboxy-terminus of the receptor of a cholesterol recognition/interaction amino acid consensus sequence. This amino acid sequence may help for recruiting the cholesterol coming from intracellular sites to the mitochondria.

Effects of adrenocorticotropic hormone and dexamethasone on adrenal and hepatic alpha-tocopherol concentrations

Studies were done to determine the effects of ACTH treatment on adrenal alpha-tocopherol (alpha-T) concentrations in female rats. Administration of dexamethasone (DEX) to inhibit endogenous ACTH secretion increased whole adrenal alpha-T levels as well as the fractional amount in adrenal cytosol. Adrenal ascorbic acid (AA) concentrations were unaffected by DEX. DEX treatment also had no effect on hepatic AA content but decreased alpha-T concentrations in the liver. The subcellular distribution of alpha-T in the liver was not altered by DEX. Administration of ACTH to DEX-treated animals decreased adrenal alpha-T content and restored the pattern of subcellular distribution to that seen in controls. ACTH had no effect on hepatic alpha-T concentrations or subcellular distribution. ACTH treatment also had no effect on AA concentrations in adrenals or livers. The results demonstrate that ACTH has a role in the regulation of adrenal alpha-T but the mechanism(s) involved remain to be determined. The data also indicate that glucocorticoids such as DEX directly influence hepatic alpha-T levels independent of their effects on ACTH secretion.

Human chorionic gonadotropin induces an inverse regulation of steroidogenic acute regulatory protein messenger ribonucleic acid in theca interna and granulosa cells of equine preovulatory follicles

The time- and gonadotropin-dependent regulation of steroidogenic acute regulatory protein (StAR) has not been characterized in vivo in preovulatory follicles of large monoovulatory species or sexually mature animals. The objectives of this study were to clone equine StAR and describe the regulation of its messenger RNA (mRNA) in equine follicles after the administration of an ovulatory dose of hCG. The screening of an equine follicle complementary DNA (cDNA) library with a mouse StAR cDNA probe revealed two forms of equine StAR that differ only in the length of their 3'-untranslated region (3'-UTR); a long form of 2918 bp and a short form of 1599 bp. The StAR long form cDNA contains a 5'-UTR of 117 bp, an open reading frame (ORF) of 855 bp, and a 3'-UTR of 1946 bp. Primer extension analysis showed that the cDNA clone lacked the first 10 bp of the primary transcript, giving a total of 127 bp for the complete StAR 5'-UTR. The ORF encodes a 285-amino acid protein that is 86-90% identical to StAR of other species characterized to date. The regulation of StAR mRNA in vivo was studied in equine preovulatory follicles isolated during estrus at 0, 12, 24, 30, 33, 36, and 39 h (n = 4-5 follicles/time point) after an ovulatory dose of hCG. Results from Northern blots showed no significant changes in StAR mRNA levels after hCG treatment when analyses were performed on intact follicle wall (theca interna with attached granulosa cells). However, Northern blots performed on isolated follicle cells revealed an unexpected regulation of StAR mRNA. In granulosa cells, StAR transcripts were undetectable at 0 h but were significantly increased at 30 h post-hCG, and this induction was associated with a rise in follicular fluid concentrations of progesterone (P < 0.05). In contrast, StAR mRNA levels were high in theca interna at 0 h, remained unchanged until 33 h post-hCG, and dropped dramatically thereafter (P < 0.05). Thus, this study describes the primary structure of equine StAR, documents the regulation of StAR mRNA in vivo in preovulatory follicles of a large monoovulatory species, and identifies a novel inverse regulation of StAR transcripts in theca interna and granulosa cells of equine follicles before ovulation.

Ontogeny of diazepam binding inhibitor/acyl-CoA binding protein mRNA and peripheral benzodiazepine receptor mRNA expression in the rat

The Diazepam Binding Inhibitor/Acyl-CoA Binding Protein (DBI/ACBP) has been implicated in different functions, as acyl-CoA transporter and as an endogenous ligand at the GABA(A) receptor and the peripheral benzodiazepine receptor (PBR). The latter is thought to be involved in control of steroidogenesis. We studied the ontogeny of DBI/ACBP and PBR mRNA expression in embryos and offspring of time-pregnant Long Evans rats by in-situ hybridization with 33P-endlabelled oligonucleotides. Both mRNAs were present in embryo and placenta at gestational day (G)11, the earliest stage studied. DBI/ACBP mRNA was strongly expressed from embryonic through mid-foetal stages in central nervous system (maximum in neuroepithelium), cranial and sympathetic ganglia, anterior pituitary, adrenal cortex, thyroid, thymus, liver and (late foetal) brown adipose tissue, moderately in testis, heart, lung and kidney. In brain, a late foetal decrease of DBI/ACBP mRNA was followed by an increase at postnatal day 6. Peripheral benzodiazepine receptor mRNA expression started very low and increased to moderate levels in adrenal cortex and medulla, testis, thyroid, brown adipose tissue, liver, heart, lung, salivary gland at mid- to late-foetal stages. Data suggest a significant role of DBI/ACBP at early developmental stages. Both proteins may be involved in the control of foetal steroidogenesis. However, differences in developmental patterns indicate that additional functions may be equally important during ontogeny, such as the involvement in lipid metabolism in the case of DBI/ACBP.

Calcium ion as a second messenger for o-nitrophenylsulfenyl-adrenocorticotropin (NPS-ACTH) and ACTH in bovine adrenal steroidogenesis

o-Nitrophenyl sulfenyl-modified ACTH (NPS-ACTH) stimulated steroidogenesis acutely in bovine fasciculata-reticularis cells without increase in cellular cAMP synthesis. Application of NPS-ACTH to the cultured cells induced Ca2+ signals in individual cells as detected by video-enhanced microscopic fluorescence measurements. The percentage of Ca2+ signaling cells corresponded well with the increase of steroidogenesis induced by NPS-ACTH below 1 nM. Treatment of the cells with nicardipine, a Ca2+ channel blocker, suppressed the Ca2+ signals except for the transient increase just after the addition of NPS-ACTH and also blocked completely the stimulative effect on the steroidogenesis of NPS-ACTH below 1 nM. At a dosage of NPS-ACTH higher than 10 nM, the stimulative effect of steroidogenesis was partly suppressed by nicardipine and also by AA-861, a lipoxygenase inhibitor. The action of NPS-ACTH might be mediated by both Ca2+ and lipoxygenase metabolite(s) of arachidonic acid as dual second messengers. The effect of ACTH in pM range on the steroidogenesis was suppressed completely by the treatment with nicardipine and AA-861 at the same time, indicating that the action was mediated by both Ca2+ and the lipoxygenase metabolite(s) but not by cAMP. cAMP plays a significant role as a second messenger for ACTH action only at ACTH concentrations greater than 10 pM.

The acute regulation of mineralocorticoid biosynthesis: scenarios for the StAR system

The zona glomerulosa cell of the adrenal cortex produces mineralocorticoids in response to physiological stimuli (angiotensin II and extracellular K(+)) activating the Ca(2+) messenger system. The mechanisms underlying the generation of the Ca(2+) signal have been analyzed extensively and recent developments have contributed to bridging the gap between intracellular signals and activation of the biological function. This article summarizes the current knowledge on the intracellular targets of the Ca(2+) messenger, obtained mainly in bovine glomerulosa cells. Ca(2+) appears to exert a dual effect, both at the intramitochondrial level and at the nuclear level, where it activates steroidogenic acute regulatory protein (StAR) gene transcription.

Effect of truncated forms of the steroidogenic acute regulatory protein on intramitochondrial cholesterol transfer

It has been proposed that the steroidogenic acute regulatory (StAR) protein controls hormone-stimulated steroid production by mediating cholesterol transfer to the mitochondrial inner membrane. This study was conducted to determine the effect of wild-type StAR and several modified forms of StAR on intramitochondrial cholesterol transfer. Forty-seven N-terminal or 28 C-terminal amino acids of the StAR protein were removed, and COS-1 cells were transfected with pCMV vector only, wild-type StAR, N-47, or the C-28 constructs. Lysates from the transfected COS-1 cells were then incubated with mitochondria from MA-10 mouse Leydig tumor cells that were preloaded with [3H]cholesterol. After incubation, mitochondria were collected and fractionated on sucrose gradients into outer membranes, inner membranes, and membrane contact sites, and [3H]cholesterol content was determined in each membrane fraction. Incubation of MA-10 mitochondria with wild-type StAR containing cell lysate resulted in a significant 34.9% increase in [3H]cholesterol content in contact sites and a significant 32.8% increase in inner mitochondrial membranes. Incubations with cell lysate containing N-47 StAR protein also resulted in a 16.4% increase in [3H]cholesterol in contact sites and a significant 26.1% increase in the inner membrane fraction. In contrast, incubation with the C-28 StAR protein had no effect on cholesterol transfer. The cholesterol-transferring activity of the N-47 truncation, in contrast to that of the C-28 mutant, was corroborated when COS-1 cells were cotransfected with F2 vector (containing cytochrome P450 side-chain cleavage enzyme, ferridoxin, and ferridoxin reductase) and either pCMV empty vector or the complementary DNAs of wild-type StAR, N-47 StAR, or C-28 StAR. Pregnenolone production was significantly increased in both wild-type and N-47-transfected cells, whereas that in C-28-transfected cells was similar to the control value. Finally, immunolocalization studies with confocal image and electron microscopy were performed to determine the cellular location of StAR and its truncated forms in transfected COS-1 cells. The results showed that wild-type and most of the C-28 StAR protein were imported into the mitochondria, whereas most of N-47 protein remained in the cytosol. These studies demonstrate a direct effect of StAR protein on cholesterol transfer to the inner mitochondrial membrane, that StAR need not enter the mitochondria to produce this transfer, and the importance of the C-terminus of StAR in this process.

Atrial natriuretic peptide inhibits calcium-induced steroidogenic acute regulatory protein gene transcription in adrenal glomerulosa cells

Atrial natriuretic peptide (ANP) is a potent inhibitor of mineralocorticoid synthesis induced in adrenal glomerulosa cells by physiological agonists activating the calcium messenger system, such as angiotensin II (Ang II) and potassium ion (K+). While the role of calcium in mediating Ang II- and K(+)-induced aldosterone production is clearly established, the mechanisms leading to blockade of this steroidogenic response by ANP remain obscure. We have used bovine adrenal zona glomerulosa cells in primary culture, in which an activation of the calcium messenger system was mimicked by a 2-h exposure to an intracellular high-calcium clamp. The effect of ANP was studied on the following parameters of the steroidogenic pathway: 1) pregnenolone and aldosterone production; 2) changes in cytosolic ([Ca2+]c) and mitochondrial ([Ca2+]m) Ca2+ concentrations, as assessed with targeted recombinant aequorin; 3) cholesterol content in outer mitochondrial membranes (OM), contact sites (CS), and inner membranes (IM); 4) steroidogenic acute regulatory (StAR) protein import into mitochondria by Western blot analysis; 5) StAR protein synthesis, as determined by [35S]methionine incorporation, immunoprecipitation, and SDS-PAGE; 6) StAR mRNA levels by Northern blot analysis with a StAR cDNA; 7) StAR gene transcription by nuclear run-on analysis. While clamping Ca2+ at 950 nM raised pregnenolone output 3.5-fold and aldosterone output 3-fold, ANP prevented these responses with an IC50 of 1 nM and a maximal effect of 90% inhibition at 10 nM. In contrast, ANP did not affect the [Ca2+]c or [Ca2+]m changes occurring under Ca2+ clamp or Ang II stimulation in glomerulosa cells. The accumulation of cholesterol content in CS (139.7 +/- 10.7% of control) observed under high-Ca2+ clamp was prevented by 10 nM ANP (92.4 +/- 4% of control). Similarly, while Ca2+ induced a marked accumulation of StAR protein in mitochondria of glomerulosa cells to 218 +/- 44% (n = 3) of controls, the presence of ANP led to a blockade of StAR protein mitochondrial import (113.3 +/- 15.0%). This effect was due to a complete suppression of the increased [35S]methionine incorporation into StAR protein that occurred under Ca2+ clamp (94.5 +/- 12.8% vs. 167.5 +/- 17.3%, n = 3). Furthermore, while the high-Ca2+ clamp significantly increased StAR mRNA levels to 188.5 +/- 8.4 of controls (n = 4), ANP completely prevented this response. Nuclear run-on analysis showed that increases in intracellular Ca2+ resulted in transcriptional induction of the StAR gene and that ANP inhibited this process. These results demonstrate that Ca2+ exerts a transcriptional control on StAR protein expression and that ANP appears to elicit its inhibitory effect on aldosterone biosynthesis by acting as a negative physiological regulator of StAR gene expression.

Characterization of the rat Star gene that encodes the predominant 3.5-kilobase pair mRNA. ACTH stimulation of adrenal steroids in vivo precedes elevation of Star mRNA and protein

The steroidogenic acute regulatory protein (STAR) participates in steroidogenesis through the mitochondrial transfer of cholesterol to cytochrome P450scc. The rat adrenal Star gene is transcribed as a 3. 5-kilobase pair (kb) and 1.6-kb mRNA with the larger mRNA predominating ( approximately 85% of total) in vivo. Hypophysectomy (HPX) produced a 3-5-fold decrease in Star mRNA along with a loss of adrenal steroids, whereas P450scc mRNA decreased by less than 2-fold. Adrenocorticotropic hormone (ACTH) treatment of HPX rats maximally stimulated steroidogenesis rates within 5 min with over 10-fold elevation of steady state blood levels occurring within 10 min. For intact rats there was a 5-10-fold larger increase, paralleling previously observed elevations of cholesterol-cytochrome P450scc association and metabolism in subsequently isolated adrenal mitochondria. ACTH did not increase either total STAR protein or a group of modified forms until at least 30 min after completion of acute stimulation, indicating that elevated translation of STAR protein cannot alone mediate this acute stimulation. Parallel slow changes in STAR protein and corticosterone formation after ACTH treatment are consistent with participation of STAR forms as co-regulators of these hormonal responses. ACTH stimulation of HPX rats increased Star mRNA by 2.5-fold within 20 min and by 4.5-fold after 1 h, thus preceding the rise in the STAR protein. A 3.5-kb Star cDNA clone isolated from a rat adrenal cDNA library exhibited a 0.9-kb open reading frame and a 2.5-kb 3'-untranslated region (3'-UTR). The open reading frame sequence differed at only 12 amino acids from that of the mouse Star. The rat Star gene seven exons with exon 7 encoding the entire 2.5 kb of 3'-UTR of the 3.5-kb mRNA. The 3'-UTR sequence suggests that 1.6- and 3.5-kb mRNA are formed by an alternative usage of different polyadenylation signals. Multiple UUAUUUA(U/A)(U/A) motifs also suggest additional regulation through this extended 3'-UTR. Although elevation of STAR protein by ACTH does not cause the acute increase in adrenal cholesterol metabolism, changes in the turnover or distribution of an active STAR subfraction cannot be excluded.

Targeted disruption of the peripheral-type benzodiazepine receptor gene inhibits steroidogenesis in the R2C Leydig tumor cell line

To evaluate the role of the mitochondrial peripheral-type benzodiazepine receptor (PBR) in steroidogenesis, we developed a molecular approach based on the disruption of the PBR gene, by homologous recombination, in the constitutive steroid producing R2C rat Leydig tumor cell line. Inactivation of one allele of the PBR gene resulted in the suppression of PBR mRNA and ligand binding expression. Immunoblot and electron microscopic immunogold labeling analyses confirmed the absence of the 18-kDa PBR protein in the selected clone. Although mitochondria from the PBR-negative cells contained high levels of the constitutively expressed 30-kDa steroidogenic activity regulator protein, these cells produced minimal amounts of steroids compared with normal cells (5%). Moreover, mitochondria from PBR-negative cells failed to produce pregnenolone when supplied with exogenous cholesterol. Addition of the hydrosoluble cholesterol derivative, 22R-hydroxycholesterol, increased steroid production by the PBR-negative R2C cells, indicating that the cholesterol transport mechanism was impaired. Stable transfection of the PBR-negative R2C Leydig cells with a vector containing the PBR cDNA resulted in the recovery of the steroidogenic function of the cells. These data demonstrate that PBR is an indispensable element of the steroidogenic machinery, where it mediates the delivery of the substrate cholesterol to the inner mitochondrial side chain cleavage cytochrome P-450.

Ex vivo regulation of adrenal cortical cell steroid and protein synthesis, in response to adrenocorticotropic hormone stimulation, by the Ginkgo biloba extract EGb 761 and isolated ginkgolide B

We previously demonstrated that repeated treatment of rats with the standardized extract of Ginkgo biloba leaves, EGb 761, and its bioactive component ginkgolide B (GKB), specifically reduces the ligand binding, and protein and messenger RNA expression of the adrenal mitochondrial peripheral benzodiazepine receptor (PBR), a key element in the regulation of cholesterol transport, resulting in decreased circulating corticosterone levels. Adrenocortical cells were isolated from rats treated with EGb 761 or GKB and cultured for 2 and 12 days. The effect of ACTH on normal and metabolically labeled cells was examined. Corticosterone levels were measured by RIA, and protein synthesis was analyzed by two-dimensional gel electrophoresis. Ex vivo treatment with EGb 761 and GKB resulted, respectively, in 50% and 80% reductions of ACTH-stimulated corticosterone production by adrenocortical cells cultured for 2 days compared with that by cells isolated from saline-treated rats. Two-dimensional gel electrophoresis analysis revealed that in cells from both control and drug-treated animals, ACTH induced the synthesis, at the same level, of a 29-kDa and pI 6.4-6.7 protein identified as the adrenal steroidogenic acute regulatory protein (StAR). In addition, treatment with EGb 761 and GKB specifically altered the synthesis of seven proteins, including inhibition of synthesis of a 17-kDa, identified as PBR. After 12 days in culture, ACTH-stimulated adrenocortical cell steroid synthesis was maintained, and it was identical among the cells isolated from animals treated with GKB or saline. Under the same conditions, the expression of PBR was recovered, whereas no effect of ACTH on the 29-kDa and 6.4-6.7 pI protein (StAR) or other protein synthesis could be seen. A comparative analysis of the effects of GKB and EGb 761 on adrenocortical steroidogenesis and protein synthesis identified, in addition to the 17-kDa PBR, target proteins of 32 kDa (pI 6.7) and 40 kDa (pI 5.7-6.0) as potential mediators of the effect of EGb 761 and GKB on ACTH-stimulated glucocorticoid synthesis. In conclusion, these results 1) validate and extend our previous in vivo findings on the effect of EGb 761 and GKB on ACTH-stimulated adrenocortical steroidogenesis, 2) demonstrate the specificity and reversibility of EGb 761 and GKB treatment, 3) question the role of the 29-kDa, 6.4-6.7 pI protein (mature StAR) as the sole mediator of ACTH-stimulated steroid production, and 4) demonstrate the obligatory role of PBR in hormone-regulated steroidogenesis.

Peripheral-type benzodiazepine receptors

1. The pharmacological effects of benzodiazepines are mediated through a class of recognition sites associated with the gamma-aminobutyric acid A receptor. A second class of benzodiazepine binding sites is found in virtually all mammalian peripheral tissues and is therefore called the peripheral type benzodiazepine receptor (PBR). 2. The first section of this review describes the tissue and subcellular distribution of the PBR in mammalian tissues and analyzes its many putative endogenous ligands. 3. The next section deals with the pharmacological, structural and molecular characterization of the PBR that has taken place in the past few years. 4. The final section describes the possible physiological role(s) of the PBR and identifies future work that would help deepen our understanding of the PBR and its function.

Submitochondrial distribution of three key steroidogenic proteins (steroidogenic acute regulatory protein and cytochrome p450scc and 3beta-hydroxysteroid dehydrogenase isomerase enzymes) upon stimulation by intracellular calcium in adrenal glomerulosa cells

In adrenal glomerulosa cells, angiotensin II (Ang II) and potassium stimulate aldosterone synthesis through activation of the calcium messenger system. The rate-limiting step in steroidogenesis is the transfer of cholesterol to the inner mitochondrial membrane. This transfer is believed to depend upon the presence of the steroidogenic acute regulatory (StAR) protein. The aim of this study was 1) to examine the effect of changes in cytosolic free calcium concentration and of Ang II on intramitochondrial cholesterol and 2) to study the distribution of StAR protein in submitochondrial fractions during activation by Ca2+ and Ang II. To this end, freshly prepared bovine zona glomerulosa cells were submitted to a high cytosolic Ca2+ clamp (600 nM) or stimulated with Ang II (10 nM) for 2 h. Mitochondria were isolated and subfractionated into outer membranes, inner membranes (IM), and contact sites (CS). Stimulation of intact cells with Ca2+ or Ang II led to a marked, cycloheximide-sensitive increase in cholesterol in CS (to 143 +/- 3. 2 and 151.1 +/- 18.1% of controls, respectively) and in IM (to 119 +/- 5.1 and 124.5 +/- 6.5% of controls, respectively). Western blot analysis revealed a cycloheximide-sensitive increase in StAR protein in mitochondrial extracts of Ca2+-clamped glomerulosa cells (to 159 +/- 23% of controls). In submitochondrial fractions, there was a selective accumulation of StAR protein in IM following stimulation with Ca2+ (228 +/- 50%). Similarly, Ang II increased StAR protein in IM, and this effect was prevented by cycloheximide. In contrast, neither Ca2+ nor Ang II had any effect on the submitochondrial distribution of cytochrome P450scc and 3beta-hydroxysteroid dehydrogenase isomerase. The intramitochondrial presence of the latter enzyme was further confirmed by immunogold staining in rat adrenal fasciculata cells and by immunoblot analysis in MA-10 mouse testicular Leydig cells. These findings demonstrate that under acute stimulation with Ca2+-mobilizing agents, newly synthesized StAR protein accumulates in IM after transiting through CS. Moreover, our results suggest that the import of StAR protein into IM may be associated with cholesterol transfer, thus promoting precursor supply to the two first enzymes of the steroidogenic cascade within the mitochondria and thereby activating mineralocorticoid synthesis.

Peripheral benzodiazepine receptor in cholesterol transport and steroidogenesis

Steroidogenesis begins with the metabolism of cholesterol to pregnenolone by the inner mitochondrial membrane cytochrome P450 side-chain cleavage (P450scc) enzyme. The rate of steroid formation, however, depends on the rate of cholesterol transport from intracellular stores to the inner mitochondrial membrane and loading of P450scc with cholesterol. In previous in vitro studies, we demonstrated that a key element in the regulation of cholesterol transport is the mitochondrial peripheral-type benzodiazepine receptor (PBR). We also showed that the polypeptide diazepam binding inhibitor (DBI), an endogenous PBR ligand, stimulates cholesterol transport and promotes loading of cholesterol to P450scc in vitro, and that its presence is vital for hCG-induced steroidogenesis by Leydig cells. Based on these data and the observations that i) the mitochondrial PBR binding and topography are regulated by hormones; ii) the 18-kDa PBR protein is functionally coupled to the mitochondrial contact site voltage-dependent anion channel protein; iii) the 18-kDa PBR protein is a channel for cholesterol, as shown by molecular modeling and in vitro reconstitution studies; iv) targeted disruption of the PBR gene in steroidogenic cells dramatically reduces the ability of the cells to transport cholesterol in the mitochondria and produce steroids; v) endocrine disruptors, with known anisteroidogenic effect, inhibit PBR ligand binding; and vi) in vivo reduction of adrenal PBR expression results in reduced circulating glucocorticoid levels, we conclude that PBR is an indispensable element of the steroidogenic machinery.

Control of cholesterol access to cytochrome P450scc in rat adrenal cells mediated by regulation of the steroidogenic acute regulatory protein

Cholesterol conversion to pregnenolone by cytochrome P450scc in steroidogenic cells, including those of the adrenal cortex, is determined by hormonal control of cholesterol availability. Intramitochondrial cholesterol movement to P450scc, which retains hormonal activation in isolated mitochondria, is apparently dependent on peripheral benzodiazepine receptor and the recently cloned steroidogenic acute regulatory (StAR) protein. In rat adrenal cells, StAR is formed as a 37-kDa precursor that is transferred to the mitochondrial inner membrane following phosphorylation by hormonally activated protein kinase A, and processed to multiple forms, some of which turn over very rapidly. In bovine cells, StAR undergoes three modifications forming a set of eight proteins seen in both glomerulosa and fasciculata cells. In the former, cyclic AMP and angiotensin II each decrease two forms and elevate six forms. Significantly, the major change seen after activation may not involve phosphorylation of StAR. Cholesterol transfer across mitochondrial membranes is also activated in isolated mitochondria by GTP and low concentrations of Ca2+, apparently prior to activation by StAR. Depletion of StAR by cycloheximide inhibits cholesterol transfer but is overcome by uptake of Ca2+ into the matrix. This activation of cellular cholesterol transport is sustained in adrenal cells permeabilized by Streptolysin O. In rat adrenal cells cAMP elevates 3.5- and 1.6-kb mRNA, hybridized by a 1.0-kb StAR cDNA. A 3.5-kb rat adrenal cDNA that encodes all except the 5' end of the longest StAR mRNA has been characterized. The corresponding gene sequence is distributed across seven exons. The shorter mRNA may arise from polyadenylation signals early in exon 7. However, the 3.5-kb mRNA comprises 80-90% of untreated rat adrenal StAR mRNA and may therefore provide the prime source for in vivo translation of StAR protein.

Steroidogenic acute regulatory protein (StAR) retains activity in the absence of its mitochondrial import sequence: implications for the mechanism of StAR action

Steroidogenic acute regulatory protein (StAR) plays a critical role in steroid hormone biosynthesis, presumably by facilitating the delivery of cholesterol to P450scc in the inner mitochondrial membranes. StAR is synthesized as a 37-kDa preprotein that is processed to a 30-kDa mature form by cleavage of an N-terminal mitochondrial import sequence. To identify structural features required for StAR biological activity, we mutated the human StAR cDNA, including the deletion of N- and C-terminal sequences, and examined the ability of the mutants to promote steroidogenesis and enter the mitochondria of transfected COS-1 cells. Deletion of up to 62 residues from the N terminus (N-62) did not significantly affect steroidogenesis-enhancing activity. The N-terminal deletion mutants were associated with mitochondria-enriched fractions, but import and processing were progressively impaired with increasing length of the deletion. Immunogold electron microscopy and in vitro import assays showed that the active N-62 mutant was not imported into the mitochondria. Removal of the 28 C-terminal amino acids (C-28) inactivated StAR. Deletion of the C-terminal 10 amino acids (C-10) reduced steroidogenic activity by 53%, while truncation of the last 4 amino acids had no effect. The C-28 mutant StAR was not efficiently imported into mitochondria or processed, whereas some of the C-10 mutant was processed, indicating that import had occurred. We conclude that in the COS-1 cell system used, StAR does not need to enter into mitochondria to stimulate steroidogenesis and that residues in the C terminus are essential for steroidogenesis-enhancing activity. These findings imply that StAR acts via C-terminal domains on the outside of the mitochondria.

Calcium stimulates intramitochondrial cholesterol transfer in bovine adrenal glomerulosa cells

In adrenal glomerulosa cells, angiotensin II (Ang II) stimulates aldosterone synthesis through rises of cytosolic calcium ([Ca2+]c). The rate-limiting step in this process is the transfer of cholesterol to the inner mitochondrial membrane, where it is converted to pregnenolone by the P450 side chain cleavage enzyme. The aim of the present study was to examine the effect of changes in [Ca2+]c and of Ang II on intramitochondrial cholesterol distribution. Freshly prepared bovine zona glomerulosa cells were submitted to a cytosolic Ca2+ clamp (600 nM) or stimulated with Ang II (10 nM). Mitochondria were isolated and subfractionated into outer membranes (OM), inner membranes (IM), and contact sites (CS). Cholesterol content was determined by the cholesterol oxidase assay. Stimulation of intact cells with Ca2+ led to a marked decrease in cholesterol content of OM (to 54 +/- 24% of controls, n = 5) and to a concomitant increase of cholesterol in CS and IM (to 145 +/- 14%, n = 5). When glomerulosa cells were exposed to Ang II, a marked increase of cholesterol in CS occurred (to 172 +/- 16% of controls, n = 5). No significant changes were detected in OM cholesterol, suggesting a stimulation of cholesterol supply to the mitochondria in response to Ang II. Cycloheximide specifically and significantly reduced Ca2+-activated cholesterol transfer to CS and IM. In conclusion, our data indicate that one of the main functions of the Ca2+ messenger is to increase cholesterol supply to the P450 side chain cleavage enzyme by enhancing endogenous intermembrane cholesterol transfer to a mitochondrial site containing the enzymes responsible for the initial steps of the steroidogenic cascade.

StAR-A tissue specific acute mediator of steroidogenesis

The rate-limiting and acutely regulated step in steroid hormone biosynthesis is the translocation of cholesterol, the precursor of all steroid hormones, from the mitochondrial outer membrane to the inner membrane, where it is converted to pregnenolone by the cytochrome P450 cholesterol side-chain cleavage enzyme (P450scc). This step has long been known to be dependent upon the de novo synthesis of a labile protein factor, which is required for the intramitochondrial translocation of cholesterol. Recently, the Steroidogenic Acute Regulatory (StAR) protein has been shown to have an indispensable role in acute steroid production and is proposed to be this labile protein factor. Given the fundamental importance of StAR as a key regulator of steroid hormone biosynthesis, the next frontier for researchers is elucidating the molecular mechanisms that control StAR expression and function.

Role of calmodulin-dependent protein kinase II in the acute stimulation of aldosterone production

Acute aldosterone production in adrenocortical cells is highly dependent on calcium (Ca2+) and calmodulin (CaM) activation. To determine the role of calmodulin-dependent protein kinase II (CaM kinase II) in human adrenal aldosterone production, the action of KN93 (a specific CaM kinase II inhibitor) on human adrenocortical H295R cells was examined. The stimulation of aldosterone, production by angiotensin II (Ang II) and potassium (K+) were inhibited by KN93 in a concentration-dependent manner with an IC50 of approximately 0.9 and approximately 0.5 microM, respectively. Aldosterone production was also stimulated by treatment with the calcium channel activator Bay K 8644 (Bay K) (1 microM). This production was inhibited in a concentration-dependent manner by KN93 with an IC50 of between 1 and 3 microM. No inhibition by KN93 (0.3-3 microM) or by the calmodulin inhibitor calmidazolium (0.03-0.3 microM) was observed for 22R-hydroxycholesterol (22R-OHChol) stimulation of aldosterone production. Because 22R-OHChol is a substrate for the cytochrome P450 cholesterol side-chain cleavage enzyme (P450scc) and does not require active transport to the mitochondria, these results indicate that KN93 does not directly inhibit P450scc or later steps leading to aldosterone synthesis. To investigate the site of KN93 action further we examined its effect on agonists induction of steroidogenic acute regulatory (StAR) protein, which was recently shown to regulate the movement of cholesterol from the outer to the inner mitochondrial membranes. Induction of StAR protein in H295R cells by Ang II, or Bay K was not affected by co-treatment with KN93 at concentration which blocked steroidogenesis by 60-80%. These results indicate a direct role of CaM kinase II in Ang II and K+ simulation of aldosterone production and support the hypothesis that CaM kinase II may be involved in the process of cholesterol mobilization to the mitochondria.

The possible involvement of LH/hCG induced mitochondrial proteins in the regulation of steroidogenesis in bovine luteal cells

In previous studies we described the synthesis of three mitochondrial proteins (A, B and C) in response to acute in vitro stimulation by lutropin of small bovine luteal cells. Protein A had a molecular weight of 28 kDa and an isoelectric point (pI) of 6.7. Proteins B and C had a molecular mass of 27 kDa and pI of 6.2 and 6.4, respectively. The appearance of these proteins was prevented by 100 microM cycloheximide. In the present study, we have shown that the time course of synthesis of protein A and its hCG dose-response closely parallel the increase in progesterone production. The induction by hCG of protein A was already observed after a 5 min incubation. Pulse chase experiments by addition of excess unlabelled methionine after prelabelling with [35S]methionine indicated that its half-life was approximately 15-20 min. Study of 32P labelled phosphate incorporation into individual proteins and treatment by alkaline phosphatase of [35S]methionine-labelled proteins demonstrated that none of the three proteins A, B or C was a phosphoprotein. Localization of protein A in mitochondria, at the site of the rate limiting step in steroidogenesis, and the high degree of correlation between its 35S labelling and progesterone production argue in favour of its involvement in the acute regulation of steroidogenesis.

Organization of 3 beta-hydroxysteroid dehydrogenase/isomerase and cytochrome P450scc into a catalytically active molecular complex in bovine adrenocortical mitochondria

We have previously reported the co-localization [Cherradi et al., Endocrinology 134 (1994) 1358-1364] of 3 beta-hydroxysteroid dehydrogenase/isomerase (3 beta-HSD) and cytochrome P450scc (cyt. P450scc) in the inner membrane and in the intermembrane contact sites of adrenocortical mitochondria. This observation raises the question of a possible functional association between the two proteins. Isolated bovine adrenocortical mitochondria are able to convert cholesterol to progesterone without the need of exogenous cofactors. An association of 3 beta-HSD and cyt. P450scc is observed during the purification of 3 beta-HSD from mitochondria. The behaviour of 3 beta-HSD on a column of Heparin-Sepharose is modified by the presence of cyt. P450scc. Immunoprecipitations from mitochondria with either anti-cyt. P450scc or anti 3 beta-HSD antibodies result in a co-precipitation of the two proteins. Both proteins engaged in these immunocomplexes are catalytically active. The interaction was further demonstrated by the surface plasmon resonance method using purified components. An affinity demonstrated by the surface plasmon resonance method using purified components. An affinity constant of 0.12 microM between 3 beta-HSD and P450scc was obtained. These observations suggest that P450scc and 3 beta-HSD may associate into a molecular complex in the mitochondrial compartment and may constitute a functional steroidogenic unit, thus opening new possibilities in the regulation of the production of progesterone and its flow in the adrenocortical cell.

Glucocorticoids enhance the cholesterol side-chain cleavage activity of ovine adrenocortical mitochondria

We have shown previously that a chronic treatment with glucocorticoids enhances cAMP- or ACTH-induced steroidogenesis of cultured ovine adrenocortical cells. This effect appears to involve a greater amount of cholesterol in mitochondria. Hence, the present study aimed to define the role of glucocorticoids in cholesterol metabolism by these cells. 2-day-old cultures were exposed to different hormones or inhibitors (10(-6) M ACTH, 10(-5) M metyrapone) for 28-48 h. At the end of the treatment period, the cells were stimulated for 2 h with 10(-3) M 8Br-cAMP, in the presence of 10(-3) M aminoglutethimide (in order to load mitochondria with cholesterol). Mitochondria were then isolated and incubated without or with 100 microM cholesterol either in the presence or absence of 10(-3) M CaCl2, or with 25 microM 22R-hydroxycholesterol. Mitochondria isolated from dexamethasone-treated cells produced consistently more pregnenolone than mitochondria from control cells, suggesting that at least part of the additional cholesterol present in these mitochondria was available for steroidogenesis. However, similar differences were obtained when mitochondria were incubated in the presence of exogenous cholesterol, both with or without calcium, or in the presence of 22R-hydroxycholesterol. Pregnenolone production under these latter conditions was much higher than when endogenous cholesterol was the only substrate. Conversely, metyrapone treatment of the cells resulted in lower production of pregnenolone from 22R-hydroxycholesterol by their mitochondria. Likewise ACTH treatment enhanced pregnenolone production by isolated mitochondria irrespective of the incubation conditions. These effects of dexamethasone and ACTH were not related to higher amounts of adrenodoxin, adrenodoxin reductase or cytochrome P450scc. These results indicate that exposure of ovine adrenocortical cells to glucocorticoids or ACTH enhances their steroidogenic potency not only by increasing the amount of cholesterol available for steroidogenesis but also by enhancing some step(s) involved in the transformation of cholesterol into pregnenolone.

Routes and regulation of NADPH production in steroidogenic mitochondria

The first and rate-limiting step of steroidogenesis is catalyzed by the mitochondrial cholesterol side chain cleavage system that is dependent on NADPH. The pathways of NADPH generation in steroidogenic mitochondria include three major routes catalyzed by: 1. NADP-linked malic enzyme, 2. NADP-linked isocitrate dehydrogenase, and 3. nicotinamide nucleotide transhydrogenase. The main route may differ among cell types and across species. Generally operation of alternative routes, with different substrates is not excluded. The oxidation of NADPH by the mitochondrial P450 systems is not tightly coupled with substrate metabolism, as these systems can reduce O2 by a single electron to produce harmful superoxide radical. To minimize such futile NADPH oxidation, NADPH generation may be regulated by two types of mechanisms: 1. Feedback mechanisms that maintain the ratio of NADPH/NADP+ at a steady-state level by enhancing the rate of NADPH production to keep up with its rate of oxidation, e.g., allosteric regulation of enzymes involved in NADPH production. 2. Hormonal signals that enhance the level of NADPH production in coordination with steroidogenesis. One major hypothesis with experimental evidence is that stimulation of mitochondrial NAD(P)H synthesis is mediated by Ca++ as a second messenger of tropic factors. Tropic stimulation of cells increases the levels of Ca++ in the cytosol and then in the mitochondrial matrix, and the rise in Ca++ activates enzymes involved in NAD(P)H synthesis. These regulatory mechanisms most probably operate in concert adjusted to the steroidogenic activity of the cell.

Metabolism of exogenous cholesterol by rat adrenal mitochondria is stimulated equally by physiological levels of free Ca2+ and by GTP

Adrenal mitochondria metabolize cholesterol at inner membrane (IM) cytochrome P450scc. Exogenous and outer membrane (OM) cholesterol are metabolized more slowly due to a limiting transfer of cholesterol from OM to IM. This process is stimulated by in vivo ACTH treatment and inhibited by cycloheximide (CX)-induced depletion of labile regulatory proteins. In isolated rat adrenal mitochondria, GTP enhances the metabolism of exogenous cholesterol, consistent with enhanced intermembrane cholesterol transfer (Xu et al. (1989) J. Biol Chem. 264, 17674), but metabolism of 20 alpha-hydroxycholesterol, which readily traverses mitochondrial membranes, is not affected. The non-hydrolyzable analog, GTP gamma S, completely inhibits the activation of cholesterol metabolism by GTP, suggesting a requirement for GTP hydrolysis. Low concentrations of Ca2+ (0.4-4 microM) stimulate two independent cholesterol transport processes. For exogenous cholesterol, a Ca(2+)-mediated process can replace GTP since each produces comparable stimulation and the combination produces little additional activity. This Ca2+ stimulation is insensitive to GTP gamma S and also to Ruthenium Red (RR), which prevents Ca2+ entry into the matrix. Ca2+ also enhances availability to P450 scc of endogenous OM cholesterol, which accumulates during in vivo CX-inhibition. This stimulation is, however, distinguished by insensitivity to GTP and complete inhibition by RR. Ca2+, therefore, enhances intermembrane transfer of exogenous cholesterol from OM without entry into the matrix through a process which is independently stimulated by GTP. Ca2+ induces transfer of endogenous OM cholesterol through a completely different mechanism involving RR-inhibited matrix changes.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning of ACTH-regulated genes in the adrenal cortex

To search for genes that are induced by ACTH in adrenocortical cells, we screened adrenal cortex cDNA libraries by a differential hybridization method using cDNA probes representing mRNAs from cells with or without ACTH stimulation. Forty clones were identified as ACTH induced (yielding a frequency of about 1/2500 plaques screened), and two clones as ACTH repressed. The cDNAs isolated and sequenced include nuclear genes for microsomal steroidogenic enzymes and novel proteins of yet unidentified functions, and mitochondrial genes encoding subunits of oxidative phosphorylation enzymes. Northern blot analysis of RNA from cells stimulated with ACTH confirmed the induction of these genes by ACTH, yet revealed important differences in the relative responses of the respective mRNAs. The time courses showed the major increase in the initial 6 h; and a decline after 24-36 h. The enhancement of the levels of the mRNAs could be ascribed to transcriptional activation. Since the mitochondrial genome is transcribed as a single polycistronic unit, to account for the > 20-fold differences in the levels of the mitochondrial mRNAs it is necessary to invoke differential stabilities of these mRNAs. The synchronous increase in the expression of both the steroidogenic enzymes and the mitochondrial oxidative phosphorylation system subunits, provides evidence for coregulation of steroidogenic and energy producing capacities of adrenal cells to meet the metabolic needs of steroid hormone production. Suppression of beta-actin gene expression may be related to changes in actin polymerization during ACTH-dependent cytoskeletal reorganization.

Effects of cyclosporine-A on steroid secretion of dispersed rat adrenocortical cells

The acute effect of cyclosporine-A (CSA), a potent immunosuppressive agent, on the secretory activity of dispersed rat adrenocortical cells was investigated. The production of the following steroid hormones was assayed by high performance liquid chromatography: pregnenolone (PREG), progesterone (PROG), 11-deoxycorticosterone (DOC), corticosterone (B), 18-hydroxy-11-deoxycorticosterone (18OH-DOC), 18-hydroxycorticosterone (18OH-B) and aldosterone (ALDO); B and ALDO outputs were also measured by radioimmunoassay. Low concentrations of CSA (0.1-0.2 mg/ml) enhanced basal, but not ACTH- or angiotensin-II (ANG-II) 10(-8) M-stimulated, secretions of PREG, non-18-hydroxylated steroids (PROG, DOC and B) and 18-hydroxylated steroids (18OH-DOC, 18OH-B and ALDO) of both zona glomerulosa (ZG) and zonae fasciculata and reticularis (ZF/ZR) cells. Middle concentrations of CSA (from 0.3 to 0.5 mg/ml) did not affect PREG yield, nor did they alter basal and ACTH-stimulated post-PREG output of both ZG and ZF/ZR cells; however, they elicited a marked decrease in ANG-II-enhanced production of 18-hydroxylated steroid by AG cells. Concentrations of CSA higher than 0.5 mg/ml strikingly reduced either basal and agonist-stimulated over-all steroidogenesis of both ZG and ZF/ZR cells. These findings suggest that CSA at low concentrations strongly stimulates the conversion of cholesterol to PREG (i.e. the rate-limiting step of steroidogenesis), while at middle concentrations it did not affect this early step, but specifically interferes with the intracellular events which transduce the stimulatory signal of ANG-II on the late steps of mineralocorticoid production (i.e. the conversion of B to ALDO). At higher concentrations, CSA probably exerts a cytotoxic effect.

Selective chemical modification of Cys264 with diiodofluorescein iodacetamide as a tool to study the membrane topology of cytochrome P450scc (CYP11A1)

Cys264 of cytochrome P450scc (CYP11A1) was selectively labelled with diiodofluorescein iodacetamide in solution and in proteoliposomes. The labelling affected the interaction of P450scc with adrenodoxin and significantly inhibited the side-chain cleavage activity of the soluble and membrane-bound hemeprotein in the reconstituted system. In proteoliposomes both the labelled and unlabelled hemeproteins were susceptible to trypsin and split into F1 and F2, two fragments corresponding to the two main domains of P450scc. These results suggest that the hinge connecting the two domains in the region Arg250-Asn257 is exposed to the surface of the membrane and involved in the interaction of P450scc with adrenodoxin.

cDNA cloning and sequence analysis of the bovine adrenocorticotropic hormone (ACTH) receptor

We isolated five independent cDNAs of nearly 3000 bp for the bovine ACTH receptor by screening adrenal cortex cDNA libraries with a PCR cloned cDNA fragment. The deduced receptor sequence includes 297 residues (M(r) = 33,258) with 81% identity with the human ACTH receptor, and shows seven hydrophobic transmembrane domains. The calculated M(r) of the receptor is smaller than the 40-45 kDa observed in crosslinking studies with labeled ACTH. Since the bovine and human receptors have two glycosylation motifs in the N-terminus, the difference may result from glycosylation of the receptor. Analysis of the sequences of both bovine and human receptors revealed a single protein kinase. A phosphorylation motif located in the third intracellular loop (Ser-209) juxtaposed to a protein kinase C phosphorylation motif (Thr-204). Thus, the involvement of protein kinase A and C pathways in ACTH action may be mediated in part by phosphorylation of the ACTH receptor at these motifs. The 3'-untranslated region of the bovine cDNA is > 2000 bp and includes two inverse repeats giving an extensive and strong secondary structure to the ACTH receptor RNA.

Plasticity of cell organization during differentiation of normal and oncogene transformed granulosa cells

Granulosa cells, which nurse the oocyte and serve as a major source for estradiol and progesterone production, undergo major morphological changes which correlate very well with modulation of their steroidogenic capacity. These include changes in intercellular contacts and communication, in cell membrane receptors, and in the development and organization of organelles associated with steroidogenesis (i.e., mitochondria, smooth endoplasmic reticulum, lipid droplets, and lysosomes). These biochemical and morphological changes can also be obtained in primary cultures as well as in oncogene transformed granulosa cell lines established recently in our laboratory. A growing body of evidence suggests that plasticity of the cytoskeleton plays a major role in the biochemical and morphological differentiation of granulosa cells as well as in other steroidogenic cells.

Rapid modulation of ovarian 3 beta-hydroxysteroid dehydrogenase/delta 5-delta 4 isomerase gene expression by prolactin and human chorionic gonadotropin in the hypophysectomized rat

In order to better understand the role of prolactin (PRL) and luteinizing hormone (LH) on progesterone biosynthesis in the ovary, we have investigated the time course (1-9 days) of the effect of PRL and human chorionic gonadotropin (hCG) on ovarian 3 beta-hydroxysteroid dehydrogenase/delta 5-delta 4 isomerase (3 beta-HSD) expression in the hypophysectomized rat. As evaluated by quantitative in situ hybridization using a 35S labelled type I 3 beta-HSD cDNA probe, the administration of hCG for 2, 3 and 9 days induced increases of 63%, 145% and 146% above control, respectively, in 3 beta-HSD mRNA levels in ovarian interstitial cells. The absence of apparent effect of the gonadotropin in other ovarian cell types could explain the small modulation of ovarian 3 beta-HSD protein content and enzymatic activity observed in total ovarian tissue. On the other hand, treatment with PRL caused a rapid decrease in 3 beta-HSD mRNA levels in corpus luteum by 23%, 63%, 76% and 78% (P < 0.01) following 1, 2, 5 and 9 days of treatment, respectively. The short-term inhibitory effect of PRL was also observed on ovarian immunoreactive 3 beta-HSD protein, as measured by Western blot analysis, and on 3 beta-HSD activity measured by the conversion of [14C]dehydroepiandrosterone into [14C]androstenedione.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of C6 glioma cell steroidogenesis by adenosine 3',5'-cyclic monophosphate

Incubation of C6 glioma cells in the presence of aminoglutethimide, an inhibitor of cholesterol metabolism, together with either adenosine 3',5'-cyclic monophosphate (cAMP) analogues or agents that increase cAMP synthesis, such as cholera toxin, forskolin, and isoproterenol, stimulated the rate of pregnenolone formation by their isolated mitochondria. This effect of cAMP was blocked by the antagonist (Rp)-cAMPS. The incorporation rate of mevalonolactone into pregnenolone was also increased by the stimulation of adenylyl cyclase activity in intact C6 cells. It is concluded that cAMP stimulates glial cell steroidogenesis by increasing the movement of the substrate, cholesterol, to the mitochondria, where it will be metabolized to pregnenolone by the side chain cleavage cytochrome P450 enzyme.