Mechanisms of corticotropin action in rat adrenal cells. I. The effects of inhibitors of protein synthesis and of microfilament formation on corticosterone synthesis

STARD1 Functions in Mitochondrial Cholesterol Metabolism and Nascent HDL Formation. Gene Expression and Molecular mRNA Imaging Show Novel Splicing and a 1:1 Mitochondrial Association

STARD1 moves cholesterol (CHOL) from the outer mitochondrial membrane (OMM) to the inner membrane (IMM) in steroidogenic cells. This activity is integrated into CHOL trafficking and synthesis homeostasis, involving uptake through SR-B1 and LDL receptors and distribution through endosomes, ER, and lipid droplets. In adrenal cells, STARD1 is imported into the mitochondrial matrix accompanied by delivery of several hundred CHOL molecules. This transfer limits CYP11A1-mediated generation of pregnenolone. CHOL transfer is coupled to translation of STARD1 mRNA at the OMM. In testis cells, slower CHOL trafficking seems to be limiting. STARD1 also functions in a slower process through ER OMM contacts. The START domain of STARD1 is utilized by a family of genes, which includes additional STARD (forms 3-6) and GRAMD1B proteins that transfer CHOL. STARD forms 2 and 7 deliver phosphatidylcholine. STARD1 and STARD7 target their respective activities to mitochondria, via N-terminal domains (NTD) of over 50 amino acids. The NTD is not essential for steroidogenesis but exerts tissue-selective enhancement (testis>>adrenal). Three conserved sites for cleavage by the mitochondrial processing protease (MPP) generate three forms, each potentially with specific functions, as demonstrated in STARD7. STARD1 is expressed in macrophage and cardiac repair fibroblasts. Additional functions include CHOL metabolism by CYP27A1 that directs activation of LXR and CHOL export processes. STARD1 generates 3.5- and 1.6-kb mRNA from alternative polyadenylation. The 3.5-kb form exclusively binds the PKA-induced regulator, TIS11b, which binds at conserved sites in the extended 3'UTR to control mRNA translation and turnover. STARD1 expression also exhibits a novel, slow splicing that delayed splicing delivery of mRNA to mitochondria. Stimulation of transcription by PKA is directed by suppression of SIK forms that activate a CRTC/CREB/CBP promoter complex. This process is critical to pulsatile hormonal activation in vivo. sm-FISH RNA imaging shows a flow of single STARD1 mRNA particles from asymmetric accumulations of primary transcripts at gene loci to 1:1 complex of 3.5-kb mRNA with peri-nuclear mitochondria. Adrenal cells are similar but distinguished from testis cells by appreciable basal expression prior to hormonal activation. This difference is conserved in culture and in vivo.

Translocator protein/peripheral benzodiazepine receptor is not required for steroid hormone biosynthesis

Molecular events that regulate cellular biosynthesis of steroid hormones have been a topic of intense research for more than half a century. It has been established that transport of cholesterol into the mitochondria forms the rate-limiting step in steroid hormone production. In current models, both the steroidogenic acute regulatory protein (StAR) and the translocator protein (TSPO) have been implicated to have a concerted and indispensable effort in this cholesterol transport. Deletion of StAR in mice resulted in a critical failure of steroid hormone production, but deletion of TSPO in mice was found to be embryonic lethal. As a result, the role of TSPO in cholesterol transport has been established only using pharmacologic and genetic tools in vitro. To allow us to explore in more detail the function of TSPO in cell type-specific experimental manipulations in vivo, we generated mice carrying TSPO floxed alleles (TSPOfl/fl). In this study we made conditional knockout mice (TSPOcΔ/Δ) with TSPO deletion in testicular Leydig cells by crossing with an anti-Mullerian hormone receptor type II cre/+ mouse line. Genetic ablation of TSPO in steroidogenic Leydig cells in mice did not affect testosterone production, gametogenesis, and reproduction. Expression of StAR, cytochrome P450 side chain cleavage enzyme, 3β-hydroxysteroid dehydrogenase/Δ5-Δ4 isomerase type I, and TSPO2 in TSPOcΔ/Δ testis was unaffected. These results challenge the prevailing dogma that claims an essential role for TSPO in steroid hormone biosynthesis and force reexamination of functional interpretations made for this protein. This is the first study examining conditional TSPO gene deletion in mice. The results show that TSPO function is not essential for steroid hormone biosynthesis.

Corticosterone pretreatment suppresses stress-induced hypothalamic-pituitary-adrenal axis activity via multiple actions that vary with time, site of action, and de novo protein synthesis

Glucocorticoid regulation of the hypothalamic-pituitary-adrenal (HPA) axis is believed to depend on multiple actions operative within discrete time domains. However, the underlying cellular and molecular mechanisms for those glucocorticoid actions remain undetermined. Moreover, there is absence of in vivo studies examining whether there are multiple glucocorticoid effects on HPA axis-related function within an intermediate feedback time frame (1-3  h after glucocorticoid elevation), and whether those effects depend on de novo protein synthesis. We examined in rats the effects of protein synthesis inhibition on HPA axis response to restraint (15 min) after 1 and 3  h phasic corticosterone (CORT) pretreatment. We measured HPA axis hormones (ACTH and CORT) and gene expression in the paraventricular nucleus (c-fos and crh genes), as well as gene expression in the anterior and intermediate pituitaries (c-fos and pomc genes). Both CORT pretreatment intervals produced inhibition of stress-induced ACTH secretion, but no inhibition was observed in the presence of protein synthesis inhibition. CORT pretreatment produced inhibitory effects on stress-induced gene expression that varied for each gene depending on the anatomical site, pretreatment time, and protein synthesis dependency. Taken together, the ACTH and gene expression patterns support the presence of multiple independent glucocorticoid actions initiated during the intermediate glucocorticoid negative feedback phase. Moreover, we conclude that those effects are exerted predominantly on the intrinsic anatomical elements of the HPA axis, and some of those effects depend on CORT induction of the expression of one or more regulatory gene products.

cAMP-dependent posttranscriptional regulation of steroidogenic acute regulatory (STAR) protein by the zinc finger protein ZFP36L1/TIS11b

Star is expressed in steroidogenic cells as 3.5- and 1.6-kb transcripts that differ only in their 3'-untranslated regions (3'-UTR). In mouse MA10 testis and Y-1 adrenal lines, Br-cAMP preferentially stimulates 3.5-kb mRNA. ACTH is similarly selective in primary bovine adrenocortical cells. The 3.5-kb form harbors AU-rich elements (AURE) in the extended 3'-UTR, which enhance turnover. After peak stimulation of 3.5-kb mRNA, degradation is seen. Star mRNA turnover is enhanced by the zinc finger protein ZFP36L1/TIS11b, which binds to UAUUUAUU repeats in the extended 3'-UTR. TIS11b is rapidly stimulated in each cell type in parallel with Star mRNA. Cotransfection of TIS11b selectively decreases cytomegalovirus-promoted Star mRNA and luciferase-Star 3'-UTR reporters harboring the extended 3'-UTR. Direct complex formation was demonstrated between TIS11b and the extended 3'-UTR of the 3.5-kb Star. AURE mutations revealed that TIS11b-mediated destabilization required the first two UAUUUAUU motifs. HuR, which also binds AURE, did not affect Star expression. Targeted small interfering RNA knockdown of TIS11b specifically enhanced stimulation of 3.5-kb Star mRNA in bovine adrenocortical cells, MA-10, and Y-1 cells but did not affect the reversals seen after peak stimulation. Direct transfection of Star mRNA demonstrated that Br-cAMP stimulated a selective turnover of 3.5-kb mRNA independent of AURE, which may correspond to these reversal processes. Steroidogenic acute regulatory (STAR) protein induction was halved by TIS11b knockdown, concomitant with decreased cholesterol metabolism. TIS11b suppression of 3.5-kb mRNA is therefore surprisingly coupled to enhanced Star translation leading to increased cholesterol metabolism.

Regulation of steroid hormone biosynthesis by the cytoskeleton

Steroid hormones are synthesized in response to signaling cascades initiated by the trophic peptide hormones derived from the anterior pituitary. The mechanisms by which these peptide hormones regulate steroid hormone production are multifaceted and include controlling the transcription of steroidogenic genes, regulating cholesterol (substrate) uptake and transport, modulating steroidogenic enzyme activity, and controlling electron availability. Cytoskeletal polymers such as microfilaments and microtubules have also been implicated in regulating steroidogenesis. Of note, steroidogenesis is a multi-step process that occurs in two organelles, the endoplasmic reticulum (ER) and the mitochondrion. However, the precise mechanism by which substrates are delivered back and forth between these two organelles is unknown. In this review we will discuss the role of components of the cytoskeleton in conferring optimal steroidogenic potential. Finally, we present data that identifying a novel mechanism by which sphingosine-1-phosphate induces mitochondrial trafficking to promote steroidogenesis.

cAMP increases mitochondrial cholesterol transport through the induction of arachidonic acid release inside this organelle in Leydig cells

We have investigated the direct effect of arachidonic acid on cholesterol transport in intact cells or isolated mitochondria from steroidogenic cells and the effect of cyclic-AMP on the specific release of this fatty acid inside the mitochondria. We show for the first time that cyclic-AMP can regulate the release of arachidonic acid in a specialized compartment of MA-10 Leydig cells, e.g. the mitochondria, and that the fatty acid induces cholesterol transport through a mechanism different from the classical pathway. Arachidonic acid and arachidonoyl-CoA can stimulate cholesterol transport in isolated mitochondria from nonstimulated cells. The effect of arachidonoyl-CoA is inhibited by the reduction in the expression or in the activity of a mitochondrial thioesterase that uses arachidonoyl-CoA as a substrate to release arachidonic acid. cAMP-induced arachidonic acid accumulation into the mitochondria is also reduced when the mitochondrial thioesterase activity or expression is blocked. This new feature in the regulation of cholesterol transport by arachidonic acid and the release of arachidonic acid in specialized compartment of the cells could offer novel means for understanding the regulation of steroid synthesis but also would be important in other situations such as neuropathological disorders or oncology disorders, where cholesterol transport plays an important role.

Rodent StAR mRNA is substantially regulated by control of mRNA stability through sites in the 3'-untranslated region and through coupling to ongoing transcription

The steroidogenic acute regulator (StAR) gene is transcribed to 1.6 kb and 3.5 kb mRNAs that differ only through the length of the 3'-untranslated region (3'-UTR). These forms result from alternative polyadenylation sites in exon 7. These sites are utilized similarly in unstimulated adrenal cells whereas Br-cAMP selectively stimulates 3.5 kb mRNA. After removal of Br-cAMP, 3.5 kb mRNA declines rapidly (t(1/2) = 2 h) while 1.6 kb mRNA responds more slowly. This selective degradation is more evident in testis MA10 cells and is seen even in the presence of Br-cAMP. Transfection of Y-1 cells with CMV promoted StAR vectors confirmed that the 3.5 kb form is less stable and that Br-cAMP slowly increases this instability. Basal instability resides solely in the extended 3'-UTR which contains AU-rich elements. Br-cAMP enhances this degradation of 3.5 kb mRNA but additionally requires translated and 5'-UTR sequences. Degradation of both forms is arrested by inhibitors of transcription or translation, indicating that mRNA stability is coupled to these processes independent of the extended 3'-UTR. Br-cAMP stimulates substantial selective synthesis of 3.5 kb StAR mRNA despite this instability. The preferential generation of the unstable form may facilitate rapid increases and decreases of StAR activity in response to external changes.

Novel signaling stimulated by arsenite increases cholesterol metabolism through increases in unphosphorylated steroidogenic acute regulatory (StAR) protein

Cholesterol metabolism to pregnenolone is dependent on the steroidogenic acute regulatory protein (StAR), which activates mitochondrial transfer of cholesterol to cytochrome CYP450scc. In mouse Y-1 adrenal cells and testis MA10 cells stimulation by 8-Bromo-cAMP (Br-cAMP) is augmented by a novel signaling initiated by low concentrations of arsenite (3-20 microM) and anisomycin (0.2 microM), a more selective stress agent. Each elevated StAR mRNA (three-fold after 6 h treatment) even with simultaneous stimulation by Br-cAMP. Arsenite produced parallel increases in StAR protein expression and cholesterol metabolism, but not for P450scc-mediated metabolism of 20alpha-hydroxycholesterol. Although arsenite and anisomycin each stimulated the phosphorylation of p38, the p38 inhibitor SB203580 (SB) produced additive increases in StAR expression. Cholesterol metabolism increased in parallel but without the increased StAR protein phosphorylation produced by Br-cAMP. Arsenite and anisomycin each elevated StAR mRNA but preferentially increased the 3.5 kb form relative to the 1.6 kb form. Arsenite and anisomycin each enhanced the stability of the more labile 3.5 kb mRNA which contains AU-rich elements that control mRNA stability. Although there were increases in both forms of StAR mRNA, arsenite did not stimulate a StAR promoter-reporter that exhibited a typical three-fold response to Br-cAMP. Arsenite and anisomycin may therefore activate a novel SB-independent MAP kinase which in part increases StAR expression through stabilizing the 3.5 kb mRNA but which may also activate a mechanism that by-passes transcription factors detected by the reporter. SB stimulation, which was completely blocked by a MEK inhibitor, was also selective towards the 3.5 kb StAR mRNA suggesting a second pathway for mRNA stabilization. These activations contrast with inhibition of StAR expression by arsenite at higher concentrations or longer incubation times.

Mitochondrial processing of newly synthesized steroidogenic acute regulatory protein (StAR), but not total StAR, mediates cholesterol transfer to cytochrome P450 side chain cleavage enzyme in adrenal cells

The metabolism of cholesterol by cytochrome P450 side chain cleavage enzyme is hormonally regulated in steroidogenic tissues via intramitochondrial cholesterol transport. The mediating steroidogenic acute regulatory protein (StAR) is synthesized as a 37-kDa (p37) precursor that is phosphorylated by protein kinase A and cleaved within the mitochondria to generate 30-kDa forms (p30, pp30). The effectiveness of modified recombinant StAR forms in COS-1 cells without mitochondrial import has led to a prevailing view that cholesterol transport is mediated by p37 StAR via activity on the outer mitochondrial membrane. The present study of the activation of cholesterol metabolism by bromo-cAMP in adrenal cells in relation to (35)S-StAR turnover indicates that targeting of pp30 to the inner membrane provides the dominant cholesterol transport mechanism. We show that 1) only newly synthesized StAR is functional, 2) phosphorylation and processing of p37 to pp30 occurs rapidly and stoichiometrically, 3) both steps are necessary for optimum transport, and 4) newly synthesized pp30 exhibits very high activity (400 molecules of cholesterol/StAR/min). Segregation of cAMP activation and synthesis of StAR from cholesterol metabolism showed that very low levels of newly synthesized StAR (1 fmol/min/10(6) cells) sustained activated cholesterol metabolism (0.4 pmol/min/10(6) cells, t(1/2) = 70 min) long after complete removal of p37 (t(1/2) = 5 min). This activity was highly sensitive to inhibition of processing by CCCP only until sufficient pp30 was formed. Maximum activation preceded bromo-cAMP-induced StAR expression, indicating other limiting steps in cholesterol metabolism.

Magnolol stimulates steroidogenesis in rat adrenal cells

1. This study investigated the effect of magnolol, a compound purified from Magnolia officinalis, on glucocorticoid production by primary adrenal cell culture. 2. Magnolol increased corticosterone secretion in a dose-dependent manner, this effect being maximal at 40 microM. A similar effect was seen in a minced adrenal gland system. 3. In magnolol-treated cells, the number and total area of cytoplasmic lipid droplets were reduced, suggesting a high utilization rate of cholesterol esters stored in lipid droplets. In control cells, the capsule of the lipid droplet was clearly delineated by immunostaining with antibody A2, whereas capsular staining was discontinuous or undetectable following magnolol treatment. The percentage of decapsulated cells increased significantly from 20% in the control group to 80% in the magnolol-treated group. 4. Magnolol-induced steroidogenesis was not mediated either via the traditional ACTH-cyclic AMP-protein kinase A pathway or by protein kinase C, since the intracellular cyclic AMP level did not change and inhibition of protein kinase A or C did not block the action of magnolol. Furthermore, calcium/calmodulin-dependent protein kinase II was not involved in magnolol-induced steroidogenesis. 5. The stimulatory effect of magnolol on steroidogenesis apparently requires new protein synthesis, since cycloheximide inhibited magnolol-induced corticosterone production by 50%. 6. Although other studies have shown that high concentrations of magnolol inhibit acyl-CoA: cholesterol acyltransferase and 11 beta-hydroxysteroid dehydrogenase in a cell-free system, our data show that, in adrenal cell cultures, low concentrations of magnolol have a stimulatory effect on steroidogenesis, and the glucocorticoid produced may explain the effective control of asthma by Magnolia officinalis.

Relationship of StAR expression to mitochondrial cholesterol transfer and metabolism

Experiments in Y-1 and primary adrenal cells have established that basal StAR mRNA is sufficient for maximum cAMP-stimulated cholesterol metabolism providing that newly synthesized p37 StAR precursor is phosphorylated, transferred to the matrix and proteolytically cleaved to pp30. This form is active at the inner membrane. The majority of mitochondrial StAR redistributes, perhaps with cholesterol, to matrix vesicles but no longer facilitates intermembrane transfer even when appropriately phosphorylated. MA10 cells utilize a similar to Y01 cells mechanism, but sustain a higher rate of cholesterol metabolism at comparable StAR levels and exhibit much higher maximum rates. In Y-1 adrenal cells cholesterol metabolism is fully activated prior to increased StAR expression which then does not affect the rate. Thus factors other than StAR are at least as important in determining overall rates of cholesterol delivery. Following cAMP stimulation StAR is predominantly expressed as the 3.5kb form which arises from alternative polyadenylation following transcription of an extended exon 7. This form contains an AU-rich regulatory element at the 3'-end that potentially mediates the relatively rapid turnover of this form. The 1.6kb form is more stable and reaches a steady state at later time points. Turnover of both forms is coupled tightly to ongoing transcription and translation. In addition to enhanced transcription cAMP appears to direct enhanced turnover of the 3.5kb form. StAR participation in cholesterol metabolism functions at very low levels of mRNA and high efficiency at each step.

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.

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.

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.

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.

Role of the steroidogenic acute regulatory protein (StAR) in steroidogenesis

The rate-limiting, hormone-regulated, enzymatic step in steroidogenesis is the conversion of cholesterol to pregnenolone by the cholesterol side-chain cleavage enzyme system (CSCC), which is located on the matrix side of the inner mitochondrial membrane. However, it has long been observed that hydrophilic cholesterol-like substrates capable of traversing the mitochondrial membranes are cleaved to pregnenolone by the CSCC in the absence of any hormone stimulation. Therefore, the true regulated step in the acute response of steroidogenic cells to hormone stimulation is the delivery of cholesterol to the inner mitochondrial membrane and the CSCC. It has been known for greater than three decades that transfer of cholesterol requires de novo protein synthesis; however, prior to this time the regulatory protein(s) had yet to be identified conclusively. It is the purpose of this commentary to briefly review a number of the candidates that have been proposed as the acute regulatory protein. As such, we have summarized the available information that describes the roles of transcription, translation, and phosphorylation in this regulation, and have also reviewed the supporting cases that have been made for several of the proteins put forth as the acute regulator. We close with a comprehensive description of the Steroidogenic Acute Regulatory protein (StAR) that we and others have identified and characterized as a family of proteins that are synthesized and imported into the mitochondria in response to hormone stimulation, and for which strong evidence exists indicating that it is the long sought acute regulatory protein.

Hormones and the cytoskeleton of animals and plants

It is often overlooked that a cell can exert its specific functions only after it has acquired a specific morphology: function follows form. The cytoskeleton plays an important role in establishing this form, and a variety of hormones can influence it. The cytoskeletal framework has also been shown to function in a variety of cellular processes, such as cell motility (important for behavior), migration (important for the interrelationship between the endocrine and immune systems, e.g., chemotaxis), intracellular transport of particles, mitosis and meiosis, maintenance of cellular morphology, spatial distribution of cell organelles (e.g., nucleus and Golgi system), cellular responses to membrane events (e.g., endocytosis and exocytosis), intracellular communication including conductance of electrical signals, localization of mRNA, protein synthesis, and--more specifically in plants--ordered cell wall deposition, cytoplasmic streaming, and spindle function followed by phragmoplast function. All classes of hormones seem to make use of the cytoskeleton, either during their synthesis, transport, secretion, degradation, or when influencing their target cells. In this review special attention is paid to cytoskeleton-mediated effects of selected hormones related to growth, transepithelial transport, steroidogenesis, thyroid and parathyroid functioning, motility, oocyte maturation, and cell elongation in plants.

Turnover of acyl-CoA-binding protein in four different cell lines measured by using two-dimensional polyacrylamide-gel electrophoresis

Acyl-CoA-binding protein (ACBP), also named diazepam-binding inhibitor or endozepine, is a 10 kDa protein for which a surprisingly large number of biological activities has been suggested. Some of these would seem to require a rapid intracellular turnover of the protein. In this paper we report on the turnover of ACBP in cell lines derived from mouse, rat and man. ACBP was identified in two-dimensional gels by using specific antibodies. Cells were labelled with [35S]methionine and chased for various periods of time. Total protein was extracted, subjected to two-dimensional PAGE, and radioactivity in the spot containing ACBP was determined by liquid-scintillation counting. ACBP half-life was determined, and varied from 25 to 53 h depending on the cell line and the growth conditions. In all cases, radioactivity in ACBP was lost slightly faster than radioactivity in total protein. These results are discussed in relation to the possible function suggested for ACBP.

Heat shock protein induction blocks hormone-sensitive steroidogenesis in rat luteal cells

A variety of agents induce heat shock proteins (HSPs) in addition to heat shock. The heat shock response and its effects on luteal function have not been investigated, but provocatively, many of the agents known to induce HSPs impair progesterone synthesis in luteal cells. We therefore investigated whether HSP induction might influence luteal function. Rat luteal cells exposed to a commonly used heat shock paradigm (45 degrees C; 10 min) were shown to induce HSP of 70 kDa (HSP-70). Heat shock also caused a complete abrogation of LH-sensitive progesterone and 20 alpha-dihydroprogesterone secretion, and blocked steroidogenesis in response to 8-bromo-cAMP and forskolin. In contrast, heat shock had no effect on cAMP accumulation in response to LH or forskolin, or on basal progestin secretion. Heat shock inhibition of steroidogenesis was fully reversed by 22R-hydroxycholesterol (22-OH cholesterol), a cell- and mitochondria-permeant cholesterol analog. Inhibition of transcription with actinomycin D blocked HSP-70 induction and significantly reversed the inhibition of steroidogenesis by heat shock treatment. The antisteroidogenic response of heat shock was coincident with induction of HPSs and both events were transcription dependent. These findings provide strong evidence that HSP induction inhibits steroidogenesis. The mechanism of the antisteroidogenic action of HSP induction appears to be due to interference with translocation of cholesterol to mitochondrial cytochrome P450scc, a conclusion based on reversal of inhibition by 22-OH cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Transfer of cholesterol to cytochrome P450scc is generally the rate-limiting step in steroid synthesis. Depending on the steroidogenic cell, cholesterol is supplied from low or high density lipoproteins (LDL or HDL) or de novo synthesis. ACTH and gonadotropins stimulate this cholesterol transfer prior to activation of gene transcription, both through increasing the availability of cytosolic free cholesterol and through enhanced cholesterol transfer between the outer and inner mitochondrial membranes. Cytosolic free cholesterol from LDL or HDL is primarily increased through enhanced cholesterol ester hydrolysis and suppressed esterification, but increased de novo synthesis can be significant. Elements of the cytoskeleton, probably in conjunction with sterol carrier protein(2) (SCP(2)), mediate cholesterol transfer to the mitochondrial outer membranes. Several factors contribute to the transfer of cholesterol between mitochondrial membranes; steroidogenesis activator peptide acts synergistically with GTP and is supplemented by SCP(2). 5-Hydroperoxyeicosatrienoic acid, endozepine (at peripheral benzodiazepine receptors), and rapid changes in outer membrane phospholipid content may also contribute stimulatory effects at this step. It is suggested that hormonal activation, through these factors, alters membrane structure around mitochondrial intermembrane contact sites, which also function to transfer ADP, phospholipids, and proteins to the inner mitochondria. Cholesterol transfer may occur following a labile fusion of inner and outer membranes, stimulated through involvement of cardiolipin and phosphatidylethanolamine in hexagonal phase membrane domains. Ligand binding to benzodiazepine receptors and the mitochondrial uptake of 37 kDa phosphoproteins that uniquely characterize steroidogenic mitochondria could possibly facilitate these changes. ACTH activation of rat adrenals increases the susceptibility of mitochondrial outer membranes to digitonin solubilization, suggesting increased cholesterol availability. Proteins associated with contact sites were not solubilized, indicating that this part of the outer membrane is resistant to this treatment. Two pools of reactive cholesterol within adrenal mitochondria have been distinguished by different isocitrate- and succinate-supported metabolism. These pools appear to be differentially affected in vitro by the above stimulatory factors.

The role of diazepam binding inhibitor in the regulation of steroidogenesis

It has been well established that one factor influencing the rate of side-chain cleavage of cholesterol is the rate of delivery of the substrate, cholesterol, from depots in the cytoplasm to the inner mitochondrial membrane within the organelle. This process of intracellular transport consists of two steps: transport to the mitochondria and transport from outer to inner membrane. Transport to mitochondria requires the cytoskeleton and Ca(2+)-calmodulin but not newly synthesized protein. Transport within the mitochondria requires newly synthesized protein. Both steps are stimulated by the trophic hormones ACTH and LH, in their respective target organs. Since the steroidogenic responses to these hormones are inhibited by cycloheximide, it is proposed that the new protein(s) required for these responses are synthesized in the cytoplasm. Using an assay based on the production of pregnenolone by mitochondria from bovine adrenal cortex, a protein of mol. wt approximately 8200 (temporarily referred to as 8.2 K) was isolated. The protein was purified to homogeneity and found to possess the following properties: (i) 8.2 K accelerated the synthesis of pregnenolone by isolated mitochondria (ii) it promoted entry of cholesterol from the incubation medium into mitochondria (iii) 8.2 K accelerated the transport of cholesterol from outer to inner membrane (iv) it also promoted loading of P-450scc with cholesterol, when either mitoplasts of the inner mitochondrial membranes were incubated with 8.2 K in vitro. Moreover, (v) the synthesis of 8.2 K was increased by ACTH and (vi) the half-life of the protein was less than 2 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of different steroidogenic stimuli on protein phosphorylation and steroidogenesis in MA-10 mouse Leydig tumor cells

Numerous studies have indicated that treatment of Leydig cells with gonadotropin results in increased levels of intracellular cAMP, binding of cAMP to and activation of protein kinase A, phosphorylation of proteins, synthesis of new proteins and eventually, stimulation of steroidogenesis. In addition, recent studies have indicated that protein phosphorylation is an indispensable event in the production of steroids in response to hormone stimulation in adrenal cells. Because of the important role of phosphorylation in steroidogenic regulation, we investigated the effects of human chorionic gonadotropin (hCG), dibutyryl cyclic AMP (dbcAMP), forskolin and the phorbol ester, phorbol-12-myristate 13-acetate (PMA) on protein phosphorylation in MA-10 mouse Leydig tumor cells. Cells were stimulated with different steroidogenic compounds in the presence of [32P]orthophosphoric acid for 2 h and phosphoproteins analyzed by two-dimensional polyacrylamide gel-electrophoresis (PAGE). Results demonstrated an increase in the phosphorylation of four proteins (22 kDa, pI 5.9; 24 kDa, pI 6.7 and 30 kDa, pI 6.3 and 6.5) in response to 34 ng/ml hCG, 1 mM dbcAMP and 100 microM forskolin. Conversely, treatment of cells with PMA increased the phosphorylation of only one of these proteins (30 kDa, pI 6.3). At least two of these proteins (30 kDa, pI 6.5 and 6.3) appear to be identical to proteins which we and others have shown to be synthesized in response to trophic hormone stimulation in adrenal, luteal and Leydig cells. In addition, they also appear to be identical to adrenal cell mitochondrial proteins demonstrated to be phosphorylated in response to ACTH. These data indicate that proteins similar to those phosphorylated in adrenal cells in response to ACTH are phosphorylated in hormone stimulated testicular Leydig cells and that these proteins may be involved in steroidogenic regulation.

Inhibition of steroidogenesis in rat adrenal cortex cells by a threonine analogue

We have investigated the ability of amino acid analogues of serine and threonine to inhibit the increase in steroidogenesis elicited by addition of ACTH or cAMP in cells isolated from the rat adrenal cortex. We have found that the serine analogues, D, L-isoserine, alpha-methyl-D, L-serine and L-homoserine, are almost totally ineffective in inhibiting this process but that the threonine analogue, D, L-beta-hydroxynorvaline, at a concentration of 300 microM inhibits stimulated steroid hormone biosynthesis by ca 95%, while inhibiting overall protein synthesis by only ca 40%. This inhibition was found to occur in a dose-dependent manner and to be reversible by a stoichiometric concentration of threonine. These studies suggest that beta-hydroxynorvaline is functioning as a threonine analogue in our experimental system. Both the onset of inhibition by analogue and reversal of this inhibition by the natural amino acid occurred rapidly, without detectable lag. Since results obtained using cAMP as stimulant parallel those obtained using ACTH, the inhibitory effect of the analogue seems to occur subsequent to the synthesis of cAMP. Additionally, the analogue does not inhibit the conversion of pregnenolone to corticosterone, suggesting the site of action of analogue occurs prior to the synthesis of pregnenolone from cholesterol. Thus, the analogue may be exerting its effect on a protein that is synthesized subsequent to ACTH addition and is important in the acute phase of stimulated steroid hormone biosynthesis. Further, since ACTH action on adrenal cortex cells causes the activation of protein kinase A, which phosphorylates serine and threonine residues, it is possible that the effect of the analogue is to prevent the phosphorylation of a newly-synthesized protein.

Time-dependent biphasic effect of cytochalasin D on luteal progesterone release in the pregnant rat

Numerous studies have examined the effects of cytoskeletal disruption on steroidogenesis; while some report an inhibition, other studies show a stimulation of steroid hormone production. In the present study, the possibility of a biphasic effect of cytoskeletal inhibitors on steroidogenesis was examined. Luteal tissue from day 12 pregnant rats was incubated for either 3.5 h (short-term) or 12.5 h (long-term) with cytochalasin D or colchicine at 10(-4) M in Medium 199 (medium). Controls were incubated in medium alone. After the incubation, the tissues were separated from the medium, and either processed for electron microscopy, or weighed and snap-frozen for subsequent homogenization and steroid hormone measurements. Progesterone, testosterone, and 17 beta-estradiol levels in the medium were measured by radioimmunoassay. After the short-term incubation, progesterone release decreased with cytochalasin D treatment, while cells became more rounded in shape with a loss of microfilaments. Upon long-term incubation, progesterone release increased and cell contact lessened. Colchicine had no effect at either incubation time, and estradiol and testosterone production remained unchanged throughout the experiments. These results demonstrate that cytochalasin D has a biphasic effect on luteal progesterone release in the rat and provides an explanation for the dichotomy of results thus far reported. In addition, the effects of cytochalasin D on rat luteal progesterone production appear to be the result of changes in cell shape or cell-to-cell contact.

Involvement of translation and transcription in insect steroidogenesis

The involvement of protein and RNA synthesis in insect steroidogenesis was investigated using the prothoracic glands of the tobacco hornworm Manduca sexta. Ecdysone secretion stimulated by prothoracicotropic hormone (PTTH) and by cAMP analogs such as dibutyryl cAMP (dbcAMP), was suppressed by the translation inhibitors cycloheximide and puromycin, and by the transcription inhibitor actinomycin D. Inhibition of protein synthesis did not prevent the activation of glandular kinases, as indicated by continued protein phosphorylation in the presence of cycloheximide. Incorporation of radiolabeled amino acids and uridine increased within 60 min of glandular activation, suggesting that ecdysteroid secretion was accompanied by enhanced protein and RNA synthesis. One-dimensional gel electrophoresis revealed an increase in the translation of glandular proteins within 20 min of activation. The results suggest that the translation of protein from short-lived mRNA is necessary for optimal synthesis of ecdysteroids, and that the requisite proteins act beyond the activation of cAMP-dependent protein kinase.

Distinctive properties of adrenal cortex mitochondria

The mitochondria in cells that synthesize steroid hormones not only have enzymes not present in mitochondria of non-steroidogenic cells but also have unique mechanisms for regulating the steroid substrate availability for certain of these enzymes. We have considered in detail the cytochrome P-450scc system that is located in the inner mitochondrial membrane and that catalyzes the initial and rate-determining step in the steroid hormone biosynthetic pathway. The flux through this pathway is regulated both by the levels of these catalysts themselves and by the availability of the substrate cholesterol for conversion to pregnenolone. These two levels of regulation occur in different time frames but are both controlled externally by the action of tissue-specific peptide hormone. We have used the adrenal cortex fasciculata cells as our paradigmatic cell type. The overall picture seems closely similar for mitochondria in other such steroidogenic cells when analogous data are available. Thus, in adrenal cortex fasciculata cells ACTH triggers several long-term (trophic) and short-term (acute) effects upon and within mitochondria that influence the initial and rate-determining step in the steroid hormone biosynthetic pathway. The only second messenger for both effects characterized thus far is cAMP. An increase in membrane-associated cAMP rapidly activates cAMP-dependent protein kinase, which in turn phosphorylates several cellular proteins, e.g., cholesterol ester hydrolase (vide supra). The trophic action, i.e., that produced by exposure of the cells to increased levels of ACTH or cAMP for a prolonged period (minutes to hours), increases the amounts of the steroid hormone synthesizing proteins in the mitochondria by increasing the transcription of the relevant nuclear genes. This latter process is not needed for the acute increase in the rate of steroid hormone biosynthesis. Whether induction of steroidogenic enzymes requires activation of a kinase has not been determined. However, the postulated SHIP proteins provide a mechanism by which cAMP levels and protein synthesis itself may regulate this induction. Mitochondria in steroidogenic tissues exert control over this process by their ability to recognize, import and process correctly the nuclear encoded precursors of the steroidogenic enzymes. Whether control at this level is ultimately dictated by nuclear or mitochondrial gene products or by an interplay between them is still unknown.(ABSTRACT TRUNCATED AT 400 WORDS)

Cyclic AMP-mediated cytoskeletal effects in adrenal cells are modified by serum, insulin, insulin-like growth factor-I, and an antibody against urokinase plasminogen activator

In adrenocortical cells in culture, increased intracellular cyclic AMP resulting from exposure to agents such as ACTH and cholera toxin causes a change in cell morphology termed 'retraction' or 'rounding'. The breakdown of actin-containing stress fibers in rounding suggested a role for microfilaments in steroidogenesis. Previously, we showed that cultured bovine adrenal cells under standard conditions (medium with 10% fetal bovine serum) do not round in response to intracellular cyclic AMP. Here, we show that these cells do round in defined, serum-free medium. Rounding was maximal within 1 h of addition of 1 nM cholera toxin and after 10 h most cells remained rounded. Cycloheximide at 100 micrograms/ml did not inhibit the response to cholera toxin. The rounding response was abolished when 10% fetal bovine serum, horse serum, or ether-extracted fetal bovine serum was included in the medium. The inhibitory effect of serum was not mimicked by growth factors with the exception that insulin and insulin-like growth factor-I (IGF-I), while not preventing rounding, accelerated the return of cells to a flattened morphology. A monoclonal antibody against urokinase plasminogen activator completely prevented rounding whereas a monoclonal antibody against tissue plasminogen activator had only a slight effect. Fluorescence visualization of F-actin with N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-phallacidin showed that rounding in cultured bovine adrenocortical cells resembles that defined earlier for human and rat adrenocortical cells and includes depolymerization of actin microfilaments. These cytoskeletal changes in adrenal cells are unlikely to play a role in steroidogenesis; however, they may be involved in tissue remodeling occurring as part of the indirect mitogenic effects of ACTH.

Subcellular localization of a protein produced in adrenal cortex cells in response to ACTH

We have reported previously that a protein, ib, is produced in adrenal cortex and other steroidogenic cells with the same tissue-specific peptide hormone or cAMP dose-response and the same kinetics as the increase in steroid hormone biosynthesis. In this study, we have fractionated adrenal cortex cells into subcellular components and used two-dimensional electrophoresis to characterize the proteins in these fractions. We have demonstrated previously that inhibition of cytosolic translation, e.g. by cycloheximide, prevents the production of protein ib. We also report that the production of this protein is not affected by inhibition of mitochondrial translation by chloramphenicol.

Dynamic response to follicle-stimulating hormone of secretion of progesterone by superfused rat ovarian granulosa cells

Granulosa cells from immature female rats, pretreated with pregnant mare's serum gonadotropin, were cultured with microcarrier beads for 24 h, and superfused with culture medium. Progesterone was transiently released following a 10-min pulse of FSH (100 ng/ml), and there was a self-priming effect of FSH. 10-min pulses of 8-bromo-adenosine 3',5'-cyclic monophosphate (8Br-cAMP) (1 mg/ml) mimicked the effects of follicle-stimulating hormone (FSH). Continuous superfusion with FSH induced biphasic secretion of progesterone, which was composed of a parabolic (the first) and a plateau (the second) phase. By contrast, the pattern of secretion induced by continuous superfusion with 8Br-cAMP was monophasic. FSH-stimulated secretion of progesterone was rapidly inhibited by the addition of 10 microM cycloheximide (CX), but secretion recovered upon removal of this inhibitor. In the second phase, the recovery of secretion was accompanied by an overshoot of the plateau value. The present results suggest that: (1) the generation of the time-related biphasic pattern of secretion cannot be interpreted by cAMP alone; (2) FSH stimulates the secretion of progesterone by a mechanism that involves newly synthesized protein.

Steroidogenesis-activator polypeptide isolated from a rat Leydig cell tumor

A cycloheximide-sensitive protein responsive to adenosine 3',5'-monophosphate has been postulated to participate in the regulation of cholesterol side-chain cleavage activity in steroidogenic tissues. Such a steroidogenesis activator polypeptide (SAP) had been isolated from rat adrenocortical tissue and partially characterized. Now a polypeptide with comparable chromatographic behavior and biological activity has been purified from the rat H-540 Leydig cell tumor in quantities sufficient for amino acid sequencing. The activator contains 30 amino acid residues and has a molecular weight of 3215. The synthetic construct based on this sequence is virtually equipotent with native H-540 tumor SAP in an adrenal mitochondrial cholesterol side-chain cleavage assay. Hormonal regulation of the intracellular concentration of this activator may control the rate of cholesterol metabolism in steroidogenic organs.

Characterization of filaments within Leydig cells of the rat testis

Rat Leydig cells were permeabilized and the cytoplasm partially extracted to visualize, describe, and characterize filamentous elements of the cytoskeleton. It was demonstrated by immunofluorescence microscopy that vimentin is abundant within Leydig cells. Ultrastructurally, intermediate filaments in Leydig cells were concentrated at perinuclear sites and comprised bundles that coursed through the cytoplasm. Actin was identified in Leydig cells with the F actin probe, NBD-phallacidin. Fluorescence was strongest at the cortex of the cell. With myosin S-1 subfragments, sparse actin was found positioned almost exclusively in cortical regions of the cell associated with coated pits and in Leydig cell processes.

Steroidogenesis activator polypeptide (SAP) in the rat ovary and testis

A steroidogenesis activator polypeptide (SAP) has previously been identified in the rat adrenal cortex (Pedersen and Brownie, Proc. natn. Acad. Sci. U.S.A. 80 (1983) 1882-1886). This factor apparently facilitates the association of mitochondrial cholesterol with the cholesterol side-chain cleavage cytochrome P-450, a reaction which is generally regarded as rate-controlling in the steroid biosynthetic pathway. The same preparative techniques have now been applied in a search for this material in other rat tissues. Among those investigated, the ovary and testis demonstrate significant concentrations of a factor which is biologically and chromatographically similar to adrenal SAP. In the immature ovary the activator becomes manifest after priming with PMSG and rises dramatically during hCG-stimulated luteinization, an increase which can be blunted with cycloheximide. In the adult rat testis it is increased acutely by treatment with hCG or dibutyryl cAMP and is diminished in response to hypophysectomy or cycloheximide. At approximately equivalent concentrations (10(-7) M), preparations of the activator from the adrenal cortex, the testis, and the superovulated ovary each enhance the activity of cholesterol side-chain cleavage in adrenocortical mitochondria by 5- to 6-fold over basal controls. We conclude that steroidogenic organs share a similar or identical intracellular modulator of cholesterol----pregnenolone conversion which is under pituitary control.

Preparation and characterization of submitochondrial fractions from adrenal cells

A method for preparing submitochondrial fractions from adrenocortical cells was developed by adapting a procedure that has been successful with yeast mitochondria. The method is based upon osmotic swelling, sonication and centrifugation in sucrose. The preparation yields highly purified fractions of outer membrane, intermembrane space, inner membrane and a less purified fraction of matrix. Recoveries are good so that 10(7) cells yield approximately 170 micrograms of inner membrane protein and 12 micrograms of outer membrane protein. Electron microscopy shows that the outer membrane fraction consists of vesicles (0.2-0.6 micron diameter) while inner membrane appears as densely packed sheets of membranous material. Two-dimensional polyacrylamide gels (isoelectric focusing followed by electrophoresis) of all the fractions give highly reproducible patterns of protein spots with Coomassie staining. Steroidogenic proteins were found only in inner membrane fractions which were shown to contain cytochrome P-450 C27 side-chain cleavage and P-450 11 beta-hydroxylase together with adrenodoxin and adrenodoxin reductase. Inner membrane catalyzes side-chain cleavage of cholesterol (conversions to pregnenolone) and 11 beta-hydroxylation (DOC----corticosterone) when substrate and NADPH are added. The preparation yields highly purified submitochondrial fractions from Y-1 mouse adrenal tumor cells and from porcine and bovine adrenocortical mitochondria. The method has the virtue of yielding highly purified intermembrane fluid which is not true of other methods for fractionation of adrenal mitochondria. The procedure also yields cleaner preparations of the two membranes than two other published methods currently used to prepare submitochondrial fractions from adrenocortical cells.

Transduction of ACTH signal from plasma membrane to mitochondria in adrenocortical steroidogenesis. Effects of peptide, phospholipid, and calcium

The conversion of cholesterol to pregnenolone by adrenocortical mitochondria is the rate-limiting step in steroidogenesis. This process is stimulated dramatically by the action of ACTH through the sequential reactions, in which adenyl cyclase, cAMP-dependent protein kinase, cholesterol esterase and ribosomal protein synthesis are all involved. The de novo synthesized protein, the so-called labile protein with a half-life of approx 10 min, is believed to stimulate the cholesterol side chain cleavage reaction by an unknown mechanism. Available evidence indicates that the electron on transfer reaction from NADPH to P-450scc is mediated rapidly by adrenodoxin reductase and p-450 scc. In addition, these redox components are inactivated slowly with a half-life of 3.5 days after hypophysectomy. It is known that the corticoid output from adrenocortical cells starts within 5 min and reaches the maximum after 10-15 min of ACTH administration to animals. One can assume that under normal physiological conditions, both O2 and NADPH are not limiting. Additionally, mitochondrial inner membranes are poor in cholesterol. In this context, the availability of substrate cholesterol to P450scc is the most likely candidate for the regulatory mechanism.

Cyclic AMP-independent stimulation of steroidogenesis in Y-1 adrenal tumor cells by antimitotic agents

The stimulation of steroidogenesis by antimitotic drugs has been studied in wild-type (Y-1) and cAMP-dependent protein kinase-deficient (kin-8) mouse adrenal tumor cell lines. Unlike some other cells, Y-1 cells do not increase their cAMP output upon exposure to antimitotic drugs such as colchicine, vinblastine or podophyllotoxin, which readily increase steroidogenesis. Moreover, no increase in cAMP can be detected over an extended time span. Stabilization of tubulin polymers by taxol or high concentrations of vinblastine blocks ACTH-, cholera toxin- or colchicine-stimulated steroidogenesis without major effects on cAMP levels. Colchicine and podophyllotoxin stimulate steroidogenesis in the cAMP-dependent protein kinase-deficient mutant to the same degree as in the wild-type Y-1 cells, although absolute steroid yields are lower in the mutant cells. We suggest that the antimitotic agents stimulate adrenal steroidogenesis by a cAMP-independent pathway that may involve facilitation of cholesterol access to the mitochondrion.

Effect of vinblastine, a potent antimicrotubular agent on steroid secretion by perifused frog adrenal glands

The role of microtubules in adrenal steroidogenesis was examined in vitro, using frog interrenal tissue. Adrenal dice from Rana ridibunda were perifused with amphibian culture medium and the effect of various antimicrotubular drugs was studied. The amounts of corticosterone and aldosterone released in the effluent perifusate were radioimmunoassayed using specific antisera. Administration of colchicine, nocodazole, and vinblastine (10(-5) M) did not affect spontaneous secretion of corticosterone and aldosterone. These results indicated that, in contrast to microfilaments which play an important role in spontaneous steroidogenesis, the microtubular system is not required for basal corticosteroid secretion. However, vinblastine (10(-5) M) was responsible for a marked decrease in ACTH-induced stimulation of corticosterone and aldosterone production. Conversely, vinblastine did not significantly alter the response of interrenal tissue to dibutyryl cAMP, forskolin and NaF, indicating that the microtubules are involved in an early step of ACTH action, namely at the level of the receptor subunit.

Acute cAMP stimulation in Leydig cells: rapid accumulation of a protein similar to that detected in adrenal cortex and corpus luteum

Addition of cAMP (as the dibutyryl compound) to a primary culture of mouse Leydig cells caused the accumulation of a protein, i(b), with the same dependence on cAMP concentration as the increase in testosterone synthesis. Stimulation of both protein i(b) and testosterone production were inhibited by cycloheximide. Additionally, cAMP caused repression of synthesis of another protein, p(b), with the same approximate molecular weight (28,000 daltons) as i(b), but more basic isoelectric point. This behavior resembles an event which has been documented in the adrenal cortex (Krueger, R.J., and Orme-Johnson, N.R., J. Biol. Chem., 258, 10159-10167, 1983) and corpus luteum (Pon, L.A., and Orme-Johnson, N.R., J. Biol. Chem., 261, 6594-6599, 1986). The discovery of these proteins in a third steroid-producing cell type and the close correlation between conditions causing increased steroid synthesis and increased i(b) production is further indication that protein i(b) may be an intermediary in peptide hormone or cAMP control of steroid hormone biosynthesis.