Is cAMP the obligatory second messenger in the action of lutropin on Leydig cell steroidogenesis

Partial characterization of mitochondrial G proteins in adrenal cells

Four low molecular mass G proteins have been identified in mitochondrial membranes from bovine adrenal cortex. These proteins (referred to as proteins 1 to 4) showed molecular masses of 28, 27, 26 and 24 kDa with isoelectric points (pI) of 8.1, 5.6, and 6.3 respectively for proteins 1, 2 and 4. Protein 3 was shown to be heterogeneous, with isoelectric points of 5.0-6.1. Proteins were identified by binding of [alpha-(32)P]guanosine triphosphate (GTP) after separation by 12% SDS-polyacrylamide gel electrophoresis and transfer to nitrocellulose. Competitive binding by unlabelled competing nucleoside phosphate ligands showed specificity for guanosine triphosphate (GTP) and guanosine diphosphate (GDP) with little binding of guanosine monophosphate and no detectable binding with adenosine nucleoside phosphates. Binding was less than 10% with 100-fold excess GDP and GTP which showed equal intensities of binding. Inhibition of binding by 1000-fold cytidine triphosphate and uridine triphosphate was approx. 10%. Magnesium (Mg(2+)) stimulated binding of GTP by all four proteins. The effect of Mg(2+) was essentially the same for proteins 1, 2 and 3, while protein 4 was less sensitive to Mg(2+) at concentrations <10(-3) M. Centrifugation of sonicated mitochondrial membranes through sucrose density gradients showed the presence of all four proteins in contact points. The presence of lower concentrations (expressed per mg protein) of the proteins in inner and outer membranes suggests that either small amounts of these membranes are part of contact points as presently prepared or that the proteins occur in contact points and to a much smaller extent in inner and outer membranes. It is proposed to examine a possible role for these proteins in transport of cholesterol from outer to inner mitochondrial membranes.

Effect of desialylated human chorionic gonadotropin (hCG) on the bioactivity of rat Leydig cells

Human chorionic gonadotropin is a glycoprotein hormone that, like LH, stimulates steroidogenesis in gonadal cells. Using a desialylation process. 95 per cent of the sialic acid residues from an intact standard hCG molecule were eliminated and then the electrophoretic properties and the bioactivity of the desialylated hCG were determined. Using rat Leydig cells as a biological model, the binding affinity to LH receptors of Leydig cell membranes, steroidogenic activity and second messenger production were studied. The results indicate that the loss of sialic acid from the hCG molecule slightly increases the binding activity to LH receptors and results in steroidogenic activity with an increased ED50. Cyclic AMP production was significantly reduced however and arachidonic acid release was not observed. Several possible mechanisms that could explain these results are discussed.

GTP-binding proteins in adrenocortical mitochondria

We have identified two GTP-binding proteins in mitochondria from bovine adrenal cortex (fasciculata). Sub-mitochondrial particles were fractionated into inner membrane, contact point and outer membrane vesicles on sucrose density gradients. These sub-mitochondrial fractions were identified by the presence of enzyme markers and electron microscopy. Photoaffinity labelling with [gamma-32P]GTP identified a 45 kDa GTP-binding protein in outer mitochondrial membranes and a 19 kDa protein in the contact points. The molecular weight of 45 kDa and requirement for Mg2+ ions raise the possibility that this protein is an alpha subunit of a heterotrimeric GTP-binding protein or a novel GTP-binding protein. The specificity of nucleotide binding, the requirement for low concentrations of Mg2+ (0.1 mM) and molecular weight of 19 kDa suggest that this protein is a typical member of the so-called small GTP-binding protein family. The location of 45 kDa in the outer membrane and that of 19 kDa in the contact points suggest roles for these proteins in the interaction with the extramitochondrial environment and in the regulation of mitochondrial membranes, respectively.

Enzymic remodelling of the N- and O-linked carbohydrate chains of human chorionic gonadotropin. Effects on biological activity and receptor binding

The effects of altered terminal sequences in human chorionic gonadotropin (hCG) N- and O-linked glycans on receptor binding and signal transduction were analyzed using forms of hCG with remodelled carbohydrate chains. hCG derivatives were obtained by enzymic removal of the alpha 3-linked sialic acid residues followed by alpha 6-sialylation, alpha 3-galactosylation or alpha 3-fucosylation of uncovered Gal beta 1-->4GlcNAc (LacNAc) termini, or alpha 3-sialylation of Gal beta 1-->3GalNAc sequences. Also a form that carried GalNAc beta 1-->4-GlcNAc units, which are typical for pituitary hormone oligosaccharides, was derived by enzymic desialylation and degalactosylation followed by beta 4-N-acetylgalactosaminylation. The potency to stimulate testosterone production and the binding to the lutotropin/choriogonadotropin receptor of the preparations were compared with those of native and desialylated hCG (as-hCG). The decrease in bioactivity caused by desialylation of hCG was only restored upon alpha 6-sialylation of the Gal beta 1-->4GlcNAc beta 1-->-2Man alpha 1-->3Man branch of the N-linked glycans. This was without a major effect on receptor binding. Further alpha 6-sialylation, occurring at the Gal beta 1-->4GlcNAc beta 1-->2Man alpha 1-->6Man branch, resulted in a bioactivity below a level found with as-hCG, concomitant with a decreased receptor binding affinity. Similarly alpha 3-galactosylation of the Gal beta 1-->4GlcNAc beta 1-->2-Man alpha 1-->6Man branch yielded a hCG derivative that showed decreased bioactivity and receptor binding. alpha 3-Fucosylation of native as well as as-hCG also led to a decreased activity. Re-alpha 3-sialylation of the O-linked chains on as-hCG had little effect on the bioactivity and receptor binding. Hormone preparations with GalNAc beta 1-->4GlcNAc termini showed lower bioactivity and receptor affinity than as-hCG. It is concluded that the Gal beta 1-->4GlcNAc beta 1-->2Man alpha 1-->3Man- rather than the Gal beta 1-->4GlcNAc beta 1-->2-Man alpha 1-->6Man branch of the N-linked glycans on hCG plays an essential role in signal transduction, whereas the latter branch can potentially interfere with receptor binding. Furthermore attachment of sialic acid, but not of other sugars, to the first branch fulfils the requirement for the full expression of bioactivity, while sialylation of the O-linked chains is of minor importance.

Human chorionic gonadotropin (hCG) increases cytosolic free calcium in adult rat Leydig cells

Regulation of steroidogenesis by luteinizing hormone is mediated by cAMP and calcium. We have investigated changes in cytosolic free calcium ([Ca2+]i) in Leydig cells by using Fura-2 as the fluorescent calcium indicator. Purified Leydig cells were plated on polylysine coated glass coverslips, cultured for 24 h in DMEM/F12 and loaded with Fura-2 at 37 degrees C. [Ca2+]i measurements were made fluorometrically by placing coverslips into 3 ml cuvettes with PBS+calcium. Addition of hCG increased [Ca2+]i gradually after a lag of about 2-3 min and plateaued by 5-6 min. The plateau level was sustained for at least 15 min. Absence of external Ca2+ in the medium or presence of diltiazem or nicardipine or cobalt chloride abolished the rise. Addition of BAY K 8644 or KCl caused a small but significant increase of [Ca2+]i. 8-Br-cAMP, forskolin or cholera toxin produced a gradual rise in [Ca2+]i that plateaued after 5-6 min similar to that observed with hCG. The action of hCG was inhibited by protein kinase A inhibitor (20-residues peptide) but not by protein kinase C inhibitor (staurosporine). We conclude that binding of hCG to its receptors would transmit the signal through G proteins to adenylyl cyclase to increase cAMP which would increase Ca2+ influx into cytosol across plasma membrane Ca2+ channels. Therefore, it appears that the primary action of hCG is to increase cytosolic cAMP which would then regulate [Ca2+]i as well as steroidogenesis.

Vasopressin biosynthesis in rodent Leydig cells

Local biosynthesis of the peptide hormone vasopressin is demonstrated in vitro in Leydig cells derived from rat and mouse testis. Cycloheximide-sensitive production of the nonapeptide was shown for rat Leydig cells in primary culture. A polymerase chain reaction technique demonstrated the presence of functionally constituted vasopressin mRNA in rat and mouse testis, primary mouse Leydig cells and in rat and mouse Leydig tumour cell lines (MA10, R2C). Stimulation of cells with gonadotropins, however, had no effect either on peptide production or on levels of specific mRNA. Similarly, treatment of the MA10 cell line with a phorbol ester, or with rat atriopeptin, which activate other second messenger pathways, had no influence on vasopressin mRNA levels. The results are discussed in terms of an autocrine regulatory system which would provide the cell with information about its microenvironment.

Effects of steroidogenesis inducing protein (SIP) on steroid production in MA-10 mouse Leydig tumor cells: utilization of a non-cAMP second messenger pathway

We have investigated the effects of steroidogenesis inducing protein (SIP) (Endocrinology (1990) 126, 3043-3052) on steroid production in MA-10 mouse Leydig tumor cells. Our results indicate that SIP results in the stimulation of progesterone production in MA-10 cells to the same extent obtained when maximal doses of luteinizing hormone (LH), human chorionic gonadotropin (hCG) and dibutyryl cAMP (dbcAMP) are used. It was also observed that the increased progesterone production in response to SIP was not accompanied by an increase in intracellular cAMP levels as was seen following hCG stimulation. In addition, stimulation of progesterone production using maximal doses of LH, hCG and dbcAMP could be further increased by the addition of SIP to the incubation medium also indicating that this steroidogenic activity was acting through a differential signal transducing system than these hormones. That SIP was not acting through the cAMP second messenger pathway was also demonstrated by its lack of sensitivity to the neutralizing effects of a monoclonal antibody to LH as well as by its insensitivity to the protein kinase A inhibitor HA 1004 while both of these treatments significantly decreased LH and hCG stimulated steroid production. Lastly, SIP was unable to elicit the induction of several mitochondrial proteins which have previously been shown to be synthesized in MA-10 cells in response to LH, hCG and dbcAMP. Our results indicate that SIP stimulates the production of high levels of steroids through a signal transduction pathway which is distinct from that employed by trophic hormone stimulation in Leydig cells.

The role of calcium in luteinizing hormone/human chorionic gonadotrophin stimulation of Leydig cell immunoactive inhibin secretion in vitro

The mechanism by which luteinizing hormone (LH) stimulates Leydig cell immunoactive inhibin (I-inhibin) secretion was investigated using Percoll-purified adult rat Leydig cells. Using a maximally stimulating dose of LH (16 ng/ml). Leydig cell I-inhibin secretion was non-detectable at 1-2 h of incubation, but subsequently increased at all time points during a 25 h incubation period. LH stimulated both Leydig cell content and release of I-inhibin. Increasing concentrations of LH stimulated both inhibin and testosterone immunoactivity in the incubation media over a similar dose-response range, with a 2- to 4-fold rise in I-inhibin secretion at maximal doses of LH. Dibutyryl cAMP stimulated testosterone secretion in a manner similar to that of LH, but I-inhibin secretion was less sensitive than testosterone and a significant stimulation was observed only at the highest doses (200-1000 micrograms/ml). LH-stimulated I-inhibin secretion was significantly decreased when Leydig cells were incubated in calcium-depleted (0.15 mM Ca2+ + 1 mM EGTA) or low [Ca2+] media (0.15 mM) as compared to normal (1.15 mM) or high [Ca2+] (2-5 mM) media. In contrast, LH-stimulated testosterone secretion remained unchanged by altering extracellular [Ca2+], and although decreased in the presence of EGTA, testosterone secretion remained significantly greater than basal levels. Furthermore both diltiazem and verapamil completely blocked the LH and dibutyryl cAMP-stimulated increase in Leydig cell I-inhibin, but did not reduce either LH or dibutyryl cAMP-stimulated testosterone production to basal levels. We conclude that LH stimulates both I-inhibin synthesis and release by adult rat Leydig cells in culture, by mechanisms involving calcium.

Evidence for the involvement of phospholipase A2 in the regulation of luteinizing hormone-stimulated steroidogenesis in rat testis Leydig cells

In this study the effects of modulating the release of arachidonic acid by phospholipase A2 (PLA2) on luteinizing hormone (LH)-stimulated testosterone production in rat testis Leydig cells have been investigated. Exogenously added PLA2 significantly stimulated both basal and LH-stimulated testosterone production. The effects of three structurally unrelated PLA2 inhibitors (dexamethasone, quinacrine and p-bromophenacyl bromide (pBPB)) were determined. Dexamethasone and quinacrine caused a dose-dependent inhibition of LH-induced testosterone production but had no effect on LH-induced cyclic AMP accumulation. Dibutyryl cyclic AMP-, and forskolin-stimulated testosterone production were also inhibited by all three inhibitors used. 22R-OH-cholesterol-stimulated testosterone production was not inhibited by quinacrine or dexamethasone showing that they were not exerting their inhibitory effect on LH-induced testosterone production by decreasing the activity of the steroidogenic enzymes. However, pBPB exerted an inhibitory effect on LH-induced testosterone and cyclic AMP production. Furthermore pBPB also inhibited 22R-OH-cholesterol-induced testosterone production illustrating that apart from its well-documented effect on PLA2, it also exerts a direct inhibitory effect on the steroidogenic enzymes. The finding that PLA2 inhibitors inhibit testosterone production without affecting cyclic AMP accumulation provides further indirect evidence for second messengers in addition to cyclic AMP being involved in the action of LH in Leydig cells. These results indicate that PLA2 is involved in LH-induced testosterone production and that cyclic AMP may exert its actions via this pathway.

Complete dissociation of gonadotropin receptor binding and signal transduction in mouse Leydig tumour cells. Obligatory role of glycosylation in hormone action

Utilizing a clonal cell line of mouse testicular Leydig cells (MA-10 cells) the complete steroidogenic and other hormonal properties of chemically deglycosylated ovine lutropin (DG-LH) and human choriogonadotropin (DG-hCG) were evaluated. In these cells, with the LH receptor-steroidogenic mechanism tightly coupled and in which there are few, if any, spare receptors, both DG-LH and DG-hCG failed to elicit progesterone production, unlike fully glycosylated native LH and hCG. The receptor-binding activity of DG-LH and DG-hCG was 2-3 times that of LH and hCG in competition experiments with radiolabelled hormones. The typical phenomenon of rounding of MA-10 cells induced by LH and hCG was absent when cells were incubated with DG-LH or DG-hCG. This could be directly attributable to their failure to produce cyclic AMP as second messenger. DG-LH and DG-hCG inhibited cell shape changes and steroidogenesis caused by LH and hCG. The deglycosylated hormones were potent antagonists of the action of glycosylated hormones. Delaying DG-hCG (antagonist) addition for up to 1 h after initiation of hCG action was also very effective in preventing further activation of steroidogenesis. Similar effects were produced by addition of affinity-purified anti-hCG antibodies. In affinity cross-linking experiments, both hCG and DG-hCG bound to the same 90 kDa receptor. Studies with MA-10 cells thus provide unequivocal evidence that the presence of antennary sugars in LH and hCG (and perhaps in other similar hormones such as follicle-stimulating hormone and thyroid-stimulating hormone), is essential for signal transduction. Differences observed in the literature in other cellular systems may be attributed to differences in hormone-receptor-effector coupling.

Evidence for the functional coupling of cyclic AMP in MA-10 mouse Leydig tumour cells

A number of studies have indicated that increased production of steroids can be obtained with doses of tropic hormone which do not result in detectable increases in intracellular cAMP. It has been suggested that this may be a result of compartmentalization or functional coupling of cAMP generated by hormone-receptor interactions to specific steroid producing pathways in the cell. In the present study we have stimulated the MA-10 mouse Leydig tumour cell with hCG, dibutyryl cAMP (dbcAMP) and forskolin to determine if functional coupling of cAMP occurs. Treatment with hCG, dbcAMP and forskolin all resulted in significant increases in the production of progesterone, the major steroid produced in these cells. Stimulation with hCG followed by 2D-PAGE analysis of the proteins resulted in the appearance of two proteins in the 30,000 molecular weight range (pI 6.8 and 6.6) and two in the 25,000-27,000 region (pI 5.9-6.0). Stimulation with dbcAMP or forskolin resulted in the appearance of the same proteins seen with hCG, but also in the appearance of two additional proteins, also having molecular weights of approximately 30,000 (pI 6.3 and 6.1). These data indicate that cAMP generated via hCG stimulation, whilst able to generate similar amounts of progesterone, does not stimulate the synthesis of the same proteins as does cAMP added exogenously or generated through indiscriminate activation of adenylate cyclase activity. Thus, it would appear that the gonadotropin activated pathway generates cAMP which remains functionally compartmentalized within the cell.

Physiology of the male reproductive system: endocrine, paracrine and autocrine regulation

This presentation reviews the male reproductive system, concentrating on newer advances in our knowledge of its physiology, biochemistry, and regulation, and introduces the topic of male reproductive toxicology. GnRH is the hypothalamic peptide responsible for the stimulation of LH and FSH release from the pituitary. It is synthesized as a pro-hormone, processed in the hypothalamus and released into the portal system in a pulsatile fashion. The timing of these pulses is critical to the release of LH and FSH into the general circulation. While LH and FSH are the main trophic hormones for the testis, we now realize the importance of not only endocrine control, but also of paracrine and autocrine regulation. Specifically, the local control of Leydig cells, Sertoli cells, and germ cells appears to be modulated by numerous growth factors and local regulators arising from within the testis. This point is emphasized both during a discussion of the interaction of the various cell types in the testis and during a discussion of spermatogenesis, where techniques which show stage-specific secretions are highlighted. Newest advances in the mechanism of action of steroidal and peptide hormones are also emphasized with special reference to the possible interaction between toxicants and endocrine control of the reproductive system. This update of the reproductive system "sets the stage" for an in-depth examination of the site and mechanism of action of reproductive toxicants.

Arginine vasopressin stimulates phosphoinositide turnover in an enriched rat Leydig cell preparation

An enriched rat Leydig cell preparation was preincubated with [14C]arachidonic acid. Stimulation of the cells with arginine vasopressin (AVP) (1 microM) for 2 min caused a significant increase in labelled phosphatidic acid and a significant fall in radioactivity in phosphatidylinositol and phosphatidylinositol 4-monophosphate + phosphatidylinositol 4,5-bisphosphate. Preincubation with dibutyryl cyclic AMP had no effect on the AVP-induced phospholipid turnover. Leydig cells were preincubated with myo-[2-3]inositol for 22 h and then with 10 mM LiCl for 10 min. Exposure to AVP (1 microM) induced a rise in labelled inositol phosphates. The response was inhibited when the cells were preincubated with the phorbol ester, 4 beta-phorbol 12-myristate 13-acetate (0.16 microM) for 10 min. These results provide evidence for an AVP-induced phospholipase C stimulation in rat Leydig cells and suggest a protein kinase C-dependent feedback inhibition of the stimulation. Other agonists that might have a regulatory function in the testis were tested for possible effects on phosphoinositide metabolism. Of prostaglandin E2 (10 microns,) angiotensin II (0.1 microM), and bradykinin (0.9 microM), only the latter induced a significant increase in the labelled inositol phosphates. This suggests that Leydig cells possess a bradykinin receptor which can activate phospholipase C.

Human choriogonadotropin entrapped into liposomes: characterization, biologic effects and interaction with purified mouse Leydig cells in vitro

Liposomes containing human choriogonadotropin (hCG) were prepared from phosphatidylserine by the ether injection method. hCG adsorbed to the outer surface of the liposomes (77% of total liposome-associated hCG) was removed by proteolytic digestion with subtilisin. hCG-containing liposomes digested and not digested with subtilisin stimulated testosterone biosynthesis by Leydig cells in a dose-dependent way; both preparations had identical biologic activities (32% of the activity of free, not liposome-associated hCG) when equal doses of liposome-associated hCG were applied. The onset of stimulation was delayed when compared to the action of free hCG. Liposomes without hCG did not stimulate testosterone biosynthesis. Association of liposomes with Leydig cells was determined by measurement of transfer of radioactive label from liposomes to Leydig cells. The association was not mediated by the hormone receptor. hCG entrapped in liposomes was incorporated by Leydig cells and translated to the cellular surface. This process was impaired by colchicine (10(-5) M). hCG translocated to the external surface of the cell membrane contained a modified alpha-subunit (Mr 16,200 instead of 20,600) which was not detected in unentrapped hCG bound to Leydig cells. We suggest that liposomally entrapped hCG is taken up by Leydig cells and re-exported to the cell membrane by a mechanism resembling retroendocytosis.

Sphingosine and psychosine, suggested inhibitors of protein kinase C, inhibit LH effects in rat luteal cells

The possible involvement of protein kinase C on luteinizing hormone (LH) effects in dispersed rat luteal cells was investigated using two substances that have been reported to be protein kinase C inhibitors, sphingosine and psychosine. Sphingosine efficiently inhibited protein kinase C activity both in brain and luteal cytosol fractions. Both substances inhibited LH-stimulated cyclic adenosine monophosphate (cAMP) accumulation in a dose-dependent fashion with an LD50 at 3-7 microM (sphingosine) and 40 microM (psychosine). LH-stimulated progesterone production was also inhibited with an ID50 at 6-10 microM (sphingosine) and 40-100 microM (psychosine). The inhibition was not due to an increased phosphodiesterase activity since IBMX (3-isobutyl-1-methylxanthine, 0.1 mM) and RO 20-1724 (4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone, 0.1 mM) did not abolish the inhibitory effect of sphingosine. To study the mode of action of sphingosine, forskolin and cAMP analogues were tested. The effect of these substances on steroidogenesis was inhibited, as well as the forskolin-induced cAMP accumulation, by sphingosine. This study demonstrates a clear inhibition of LH-stimulated effects by sphingosine and psychosine. LH action in rat luteal cells is discussed in relation to protein kinase C and the possible mode of sphingosine action.

Stimulation of progesterone production by phorbol-12-myristate-13-acetate in MA-10 Leydig tumor cells

The tumor-promoting phorbol ester, phorbol-12-myristate-13-acetate (PMA) markedly stimulated progesterone production in MA-10 Leydig tumor cells. A slight but significant increase (35%) in the activity of the cholesterol side-chain cleavage (CSCC) enzyme was observed in mitochondria isolated from the PMA-treated MA-10 Leydig cells when compared to mitochondria isolated from non-treated cells. However, this stimulation of CSCC activity appears to be of limited importance when compared to the 240-fold increase observed in progesterone production following PMA stimulation. In contrast, the inactive phorbol ester 4 alpha-phorbol-12,13-didecanoate (alpha-PD) had no effect on either progesterone production or CSCC activity. PMA had no effect on the conversion of 25-hydroxycholesterol and 22R-hydroxycholesterol into progesterone suggesting that one of the mechanism(s) of PMA action may involve the delivery of cholesterol to the mitochondria and/or the affinity of cholesterol with cytochrome P-450scc. Stimulation of steroidogenesis by PMA was also shown to be inhibited by cycloheximide. When PMA was added together with a submaximal dose of hCG, hCG-stimulated steroidogenesis was inhibited. However, at a maximal dose of human chorionic gonadotropin (hCG), PMA inhibited steroid synthesis at 1 and 2 h but had no significant effect at 3 h. Conversely, PMA had an additive effect on cAMP induced steroidogenesis. It was further demonstrated that PMA resulted in a decrease in the hCG-induced accumulation of cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of sterol carrier protein2 (SCP2) levels in the soluble fraction of rat Leydig cells. Kinetics and the possible role of calcium influx

The rate-determining step in steroidogenesis is the conversion of cholesterol to pregnenolone by the cholesterol side-chain cleavage enzyme. The transport of substrate for this reaction may be facilitated by sterol carrier protein2 (SCP2). In rat testis tissue SCP2 is specifically localized in the Leydig cells and tissue levels of SCP2 are regulated by luteinizing hormone (LH). The present study concerns short-term regulation of SCP2 in isolated rat Leydig cells. Levels of SCP2 in the membrane-free supernatant are increased 2-fold already after 2 min incubation with LH and remain elevated for 24 h. The same response occurs with cells preincubated in the presence of cycloheximide for 4 h. SCP2 levels are also 2-fold increased after incubation with dibutyryl cAMP or 4 beta-phorbol 12-myristate 13-acetate (PMA) whereas these compounds stimulate steroid production 5.5- and 2-fold respectively. Luteinizing hormone releasing hormone (LHRH), which can stimulate steroid production more than 3-fold, does not influence SCP2 levels, neither are SCP2 levels altered when LH is added in the presence of the Ca2+-channel blocker diltiazem or in the absence of extracellular Ca2+. A restoration of the LH effect on SCP2 levels was already obtained in the presence of 1 microM extracellular Ca2+. These results suggest that Ca2+ influx through the plasma membrane may play an important role in the control of SCP2 levels. In most of the experiments no correlation between steroid production and SCP2 levels could be observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Concurrent LH and forskolin action on adenylate cyclase activation and progesterone synthesis in corpora lutea from pregnant ewes

The present communication documents LH- and forskolin-induced activation of adenylate cyclase (AC) system and progesterone synthesis in corpora lutea from pregnant ewes. The activation of AC in plasma membranes by LH or forskolin was amplified by Gpp(NH)p. These results suggest that regulatory nucleotide component (Ns) of the AC complex is required for forskolin. Simultaneous addition of maximal concentrations of forskolin (10(-4) M), Gpp(NH)p (10(-4) M) and LH (10(-7) M) led to greater than additive (i.e. synergistic) responses: the experimental value was 4.71 +/- 0.19 nmoles cAMP/mg of membrane protein, whereas the theoretical additive effect was 3.17 +/- 0.10 nmoles/mg of membrane protein (p less than 0.001). These data reveal that more Ns or C component is being activated in these cells when combined treatments with these agents are applied. In intact cells maximum stimulatory concentrations of forskolin or LH caused similar increase in progesterone production with similar time courses. In striking contrast, the exposure of the luteal cells to LH and forskolin simultaneously led to a decrease in progesterone synthesis as early as 1h30 (40%, p less than 0.001). Thus, the synergism observed between LH and forskolin on the stimulation of plasma membranes AC activity did not occur in steroidogenesis. The AC responses in crude plasma membranes form these cells to different stimulants were enhanced (i.e. 15%, p less than 0.2 for Gpp(NH)p, 33%, p less than 0.01 for LH plus Gpp(NH)p and 52%, p less than 0.01 for forskolin). These findings suggest that an early desensitization of the AC system cannot explain the impaired steroidogenic response observed.

Characterization and age dependence of the stimulatory effect of VIP on cyclic AMP accumulation in rat Leydig cells

The neuropeptide vasoactive intestinal peptide (VIP) has been shown to stimulate cyclic AMP accumulation in Leydig cells isolated from rat testis. The effect was dependent on time, temperature and cell concentration. At 15 degrees C, half-maximal and maximal stimulation were observed at about 1 and 100 nM VIP, respectively. The interaction was specific since an order of potencies chicken VIP greater than rat VIP greater than secretin greater than glucagon and no effect of neurotensin and substance P were obtained. The efficiency of VIP was lower in pubertal rats and then increased in young-adult and adult animals. These results together with the known presence of VIP in the testis support the idea that VIP may be involved in the regulation and function of Leydig cells during development.

Arachidonic acid is involved in the regulation of hCG induced steroidogenesis in rat Leydig cells

Phospholipase C (PLC), an enzyme involved in the hydrolysis of membrane phospholipid- phosphatidylinositol-bisphosphate to inositol triphosphate and diacylglycerol, and Phorbol 12, myristate 13, acetate (PMA), a tumor promoting agent, could significantly stimulate testosterone (T) secretion from Leydig cells. Arachidonic acid (AA) stimulated T secretion by about 2 fold. The steroidogenic effect of PLC and AA was biphasic. At low concentrations both PLC and AA (100 mU and 12.5 microM, respectively) augmented hCG induced T secretion, while at higher concentrations (PLC: 500 mU and AA: 200 microM) they inhibited steroid production. AA also had a biphasic effect on hCG induced cyclic AMP secretion. 5, 8, 11, 14 Eicosatetraynoic acid (ETYA), a general inhibitor of AA metabolism, and Nordihydroguaiaretic acid (NDGA), an inhibitor of the lipoxygenase pathway of AA metabolism, inhibited hCG induced T secretion while indomethacin, an inhibitor of cyclo-oxygenase pathway, had no effect on hCG induced T secretion. We conclude from these data that AA plays a role in the regulation of hCG induced steroidogenic responses in rat Leydig cells and that the metabolite(s) of AA that are involved are not cyclooxygenase products.

Stimulatory and inhibitory effects of protein kinase C activation and calcium ionophore on cultured pig Leydig cells

The acute and the long-term (24 h) effects of protein kinase C activators, phorbol 12 myristate 13-acetate (PMA) and 1-oleoyl-2-acetyl-sn-glycerol, and the calcium ionophore A23187 on cultured pig Leydig cell functions were investigated. None of these drugs modified basal cAMP production, but they induced a small (3-4-fold) increase in testosterone secretion. The stimulatory effects of human choriogonadotropin (hCG; 1 nM) on both cAMP and testosterone productions were inhibited by short-term incubation with these drugs. In addition, they suppressed the stimulation of testosterone output by forskolin and 8-bromo-adenosine 3',5'-monophosphate, whereas the forskolin-dependent cAMP production was unaffected. The inhibitory effects of PMA on hCG stimulation of both cAMP and testosterone were due mainly to a decrease of the Vmax without modification of the ED50. Moreover, PMA did not modify the binding of 125I-hCG. Pretreatment of Leydig cells with the three drugs for 24 h induced more pronounced modifications, such as a reduction in the number of hCG binding sites and a decreased responsiveness to hCG and forskolin, the testosterone production being drastically reduced. The effects of PMA were dose- and time-dependent; however, the concentration of PMA required to induce half-maximal effects on hCG receptors (10 nM) was about one order of magnitude higher than those required to reduce cAMP and testosterone productions. Further, the inhibitory effects on cAMP and testosterone secretions appeared within the first 3 h, whereas the hCG receptor number remained constant for at least 8 h. It appears therefore, that the main alteration responsible for the steroidogenic refractoriness of PMA-treated Leydig cells is located beyond cAMP formation. Moreover, since conversion of exogenous pregnenolone to testosterone by control and PMA-treated cells was similar, the alteration was probably located before pregnenolone formation. Kinetic studies with 125I-hCG showed that the rate of internalization of the hormone-receptor complexes was similar in control cells and in PMA-treated cells, suggesting that the decline in receptor number observed in the latter group after an 8-h delay is not due to an increased rate of internalization nor to sequestration of the internalized receptors inside the cells. Since cycloheximide blocked the effects of PMA on hCG down-regulation, it is likely that the phorbol esters and 1-oleoyl-2-acetyl-sn-glycerol induce the synthesis of some proteins which blocked the recycling of internalized receptors. A similar hypothesis has been put forward recently to explain the hCG-induced down regulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple regulation of testicular steroidogenesis

One single injection of ethylene dimethane sulfonate (EDS) to mature rats causes specific degeneration of testicular Leydig cells which is complete after 3 days. At this time no steroidogenic activities can be detected, indicating that Leydig cells are the source of steroids. The mechanism of this cytotoxic effect of EDS has been investigated with isolated cells. Extensive protein alkylation has been shown to occur in Leydig cells, Sertoli cells and hepatocytes. Steroid production by Leydig cells is always inhibited by EDS, but cytotoxic effects of EDS could only be demonstrated in Leydig cells from mature rats or tumour tissue and not in Leydig cells from immature rats. A new population of Leydig cells develops during the next 2-5 weeks after EDS treatment. In hypophysectomized rats this repopulation only occurs when hCG is given daily. FSH has no effects. The proliferative activity in the interstitial tissue increases within 2 days after administration of hCG or EDS and there are indications that LH and locally produced factors are involved in the proliferation of Leydig cells or Leydig cell precursor cells. Inhibition of cAMP production with inhibitors of adenylate cyclase results in an enhancement of the LH-stimulated steroid production similar to that observed with an LHRH agonist and phospholipase C (PLC). Since the effects of LHRH and PLC on protein phosphorylation and steroid production are similar and different from LH or active phorbol esters, it is proposed that LHRH and PLC may stimulate steroid production via liberation of calcium from a specific intracellular pool. Sterol carrier protein2 (SCP2) which is specifically localized in Leydig cells and regulated by LH probably plays a role in the delivery of cholesterol to the mitochondria although the mechanism of this carrier function is not clear. The results indicate that regulation of Leydig cell development and the steroidogenic activities by gonadotrophins and locally produced factors occur via different transducing systems and regulatory pathways.

Inhibition by gossypol of cyclic AMP production in mouse Leydig cells

Gossypol has been shown to inhibit steroidogenesis in leydig cells. This study examined the mechanism of this action by investigating the effect of gossypol on leydig cell hCG receptor binding, adenylate cyclase activity and cyclic AMP production in leydig cells. Gossypol had no effect on hCG binding to cell membranes but inhibited LH-stimulated cyclic AMP production in whole purified leydig cells. It also reduced the stimulation caused by LH, Gpp(NH)p, forskolin and fluoride in membranes from leydig cells and also from liver. It is therefore possible that gossypol affects cyclic AMP production at the level of ATP conversion to cyclic AMP, although effects on the G-protein and catalytic subunit cannot be ruled out.

Effects of seminiferous tubule secreted factor(s) on Leydig cell cyclic AMP production in mature rat

The effects of spent media from seminiferous tubules (STM) on Percoll-purified rat Leydig cells were investigated. Intracellular and extracellular cyclic AMP (cAMP) accumulation and testosterone production were measured. After a 5 h incubation period, STM reduces both the basal and LH-dependent cAMP levels (38 and 20%, respectively for intra- and extracellular cAMP) while, simultaneously, a stimulation of testosterone production is observed (47 to 50%, respectively in the absence or presence of LH). The reduction of cAMP levels observed after 5 h is likely to be due to the potentiating effect of the STM factor on the LH-dependent initial rise of the cAMP level which, in turn, induces a desensitization of the Leydig cell adenylate cyclase. This substance is a thermolabile protein (Mr greater than 50 000) produced by the Sertoli cell, independent of FSH and testosterone controls, and different from the LHRH-like substance.