Evidence for the involvement of lutropin-independent RNA synthesis in Leydig cell steroidogenesis

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.

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.

Inhibition of transcription affects synthesis of steroidogenic acute regulatory protein and steroidogenesis in MA-10 mouse Leydig tumor cells

Hormonal induction of steroidogenesis in the adrenal and gonads is dependent on the synthesis and function of the steroidogenic acute regulatory protein (StAR). As a first approach to investigate the role of translation in the control of StAR expression, we examined StAR protein synthesis and steroid production in MA-10 mouse Leydig tumor cells in the presence of the transcriptional inhibitor, actinomycin D. We show that human CG (hCG)-induced StAR synthesis, as determined by radiolabeling MA-10 cells with [35S]methionine and immunoprecipitation of StAR, is blocked by actinomycin D. The rate of hCG-stimulated progesterone production is also decreased, but not completely blocked, suggesting a possible StAR-independent mechanism that may contribute approximately 10-20% of the acute steroidogenic potential of the cells. When MA-10 cells were pretreated with hCG to increase StAR messenger RNA levels and then the proteins radiolabeled in the presence of hCG or hCG plus actinomycin D, no difference was observed in the amount of the 30-kDa StAR protein synthesized. However, a 50% increase in the precursor form of StAR protein was detected with hCG treatment alone. These data suggest that ongoing StAR protein synthesis is not inhibited by actinomycin D, but that continued synthesis requires transcriptional activity. Progesterone production was inhibited by actinomycin D in the hCG-pretreated cells, supporting the proposal that maintaining StAR protein synthesis is required for optimal steroid production in MA-10 mouse Leydig tumor cells.

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.

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.

Lutropin stimulates de novo synthesis of short-lived proteins required for lutropin-dependent steroid production in tumour Leydig cells

Continuous protein synthesis is required for the hormonal regulation of cholesterol side chain cleavage activity. A protein with a short half-life (t1/2 = 2-13 min) is believed to play an important role, but the regulation of the synthesis of this putative rapidly-turning-over protein is largely unknown. The steroid production rate in tumour Leydig cells can be increased more than 4-fold after addition of lutropin. However, steroid production by cells preincubated for 60 min with medium containing cycloheximide (89 microM) could not be stimulated when lutropin was added to the medium. Thus, the putative protein with the short half-life is apparently not derived from a stable precursor protein. Moreover, in tumour Leydig cells incubated with low concentrations of cycloheximide (0.2-0.8 microM), inhibition of steroid production was significantly greater in lutropin-stimulated cells than in control cells. These results support the hypothesis that lutropin regulates the de novo synthesis of rapidly-turning-over proteins by increasing the rate of initiation of the translation step of protein synthesis.

Polypeptide activators of cholesterol side-chain cleavage

A rate-determining step in the cAMP-dependent action of ACTH on adrenal steroid biosynthesis is the interaction of cholesterol substrate with the cholesterol side-chain cleavage cytochrome P-450 in the mitochondrion. This interaction is rapidly and reversibly sensitive to inhibitors of protein synthesis. For this reason a hormone-dependent, labile protein activator of cholesterol side-chain cleavage has long been postulated as an obligatory intermediate in the tropic regulation of this reaction. Applying recent advances in liquid chromatography, two-dimensional gel electrophoresis, and enzyme reconstitution into liposomes, several laboratories have now reported the isolation and partial characterization of polypeptide candidates for the status of "labile protein."

Primary cultures of Leydig cells from rat, mouse and pig: advantages of porcine cells for the study of gonadotropin regulation of Leydig cell function

Primary cultures of interstitial cells were prepared from the testis of mice, rats, and pigs. The cells were grown in a defined medium supplemented with low (0.1%) serum and insulin, transferrin and epidermal growth factor. Comparisons of the interstitial cell cultures from the three species were made for plating efficiency, cell survival, maintenance of hCG receptors and maintenance of steroidogenic responsiveness to hCG. The porcine cultures had a higher plating efficiency and higher hCG receptor levels per cell than Leydig cells from either rodent. Additionally, the porcine cells showed an increase in testosterone (T) production with hCG stimulation throughout their lifespan in culture while the rodent cultures showed a decrease in T stimulation with time with no stimulation by day 6 in culture. These data indicate that species differences exist in hCG receptor concentrations per cell, the maintenance of hCG receptors and steroidogenic response in culture. The initial high survival, purity and continued functional response of porcine interstitial cell cultures make them a superior system for the study of gonadotropin regulation of Leydig cell function.

Effect of lutropin and cycloheximide on lutropin receptors and cyclic AMP production in Leydig tumour cells in vitro

A system to study lutropin-induced desensitization of tumour Leydig cells in vitro has been investigated. Tumour Leydig cells were purified on a Percoll gradient and then incubated for 30 min with lutropin (0-1000ng/ml). The cells were then washed and incubated in suspension media at 32 degrees C. 125I-labelled human choriogonadotropin binding and basal and lutropin-stimulated cyclic AMP production were determined at various times. Initially the cells showed a dose-dependent decrease in human choriogonadotropin binding (1.18 and 0.13fmol/10(6) cells respectively) followed by an increase at 1 h (2.32 and 0.87fmol/10(6) cells respectively). Human choriogonadotropin binding remained elevated in the cells pre-incubated without lutropin, whereas the cells pre-incubated with lutropin showed a dose-dependent decrease over the next 10 h (2.20-0.18fmol/10(6) cells respectively). Basal production of cyclic AMP initially reflected the pre-incubation conditions (1.17-21.19ng/10(6) cells per h for 0-1000ng of lutropin/ml respectively). However, by 1 h there was a marked rise in basal cyclic AMP production which returned to the initial lower values by 4 h. At all time intervals studied, lutropin-induced cyclic AMP production showed a decrease that was proportional to lutropin concentration in the pre-incubated media. The decreases in human choriogonadotropin binding produced by pre-incubations with lutropin (100ng/ml) was partially inhibited by the presence of cycloheximide in the pre-incubation media and totally prevented by the continuous presence of cycloheximide. These results demonstrate that desensitization of tumour Leydig cells occurs after exposure to lutropin in vitro. This desensitization involves both a loss of plasma membrane receptors for lutropin and lutropin-stimulated adenylate cyclase. These events can be prevented by cycloheximide and are therefore probably dependent on protein synthesis.

Modulation of steroidogenic activities in testis Leydig cells

Recent developments in the regulation of testicular Leydig-cell function support the following conclusions. (1) Enzymic activities involved in steroid production in the testis are mainly localized in Leydig cells. The aromatase enzyme complex for oestrogen production appears to be localized in Leydig cells as well as in Sertoli cells. (2) LH- (or hCG-) induced alterations of Leydig cells depend on dose and duration of exposure of the cell to the hormone. Locally produced oestradiol is probably involved in the inhibition of steroidogenesis. (3) The stimulatory action of LH on Leydig cells involves different proteins in concert with the activation of the cleavage activity of mitochondrial cholesterol side-chain. However, most of the functional properties of these proteins are yet unknown. (4) Different populations of Leydig cells are present in the testis. These different cell populations can be characterized by quantitatively and qualitatively different responses to hormones.