Mechanism of action of gonadotropin-releasing hormone stimulated Leydig cell steroidogenesis. III. The role of arachidonic acid and calcium/phospholipid dependent protein kinase

Application of a combined aggregate exposure pathway and adverse outcome pathway (AEP-AOP) approach to inform a cumulative risk assessment: A case study with phthalates

Advancements in measurement and modeling capabilities are providing unprecedented access to estimates of chemical exposure and bioactivity. With this influx of new data, there is a need for frameworks that help organize and disseminate information on chemical hazard and exposure in a manner that is accessible and transparent. A case study approach was used to demonstrate integration of the Adverse Outcome Pathway (AOP) and Aggregate Exposure Pathway (AEP) frameworks to support cumulative risk assessment of co-exposure to two phthalate esters that are ubiquitous in the environment and that are associated with disruption of male sexual development in the rat: di(2-ethylhexyl) phthalate (DEHP) and di-n-butyl phthalate (DnBP). A putative AOP was developed to guide selection of an in vitro assay for derivation of bioactivity values for DEHP and DnBP and their metabolites. AEPs for DEHP and DnBP were used to extract key exposure data as inputs for a physiologically based pharmacokinetic (PBPK) model to predict internal metabolite concentrations. These metabolite concentrations were then combined using in vitro-based relative potency factors for comparison with an internal dose metric, resulting in an estimated margin of safety of ~13,000. This case study provides an adaptable workflow for integrating exposure and toxicity data by coupling AEP and AOP frameworks and using in vitro and in silico methodologies for cumulative risk assessment.

Involvement of PLA2, COX and LOX in Rhinella arenarum oocyte maturation

In Rhinella arenarum, progesterone is the physiological nuclear maturation inducer that interacts with the oocyte surface and starts a cascade of events that leads to germinal vesicle breakdown (GVBD). Polyunsaturated fatty acids and their metabolites produced through cyclooxygenase (COX) and lipoxygenase (LOX) pathways play an important role in reproductive processes. In amphibians, to date, the role of arachidonic acid (AA) metabolites in progesterone (P4)-induced oocyte maturation has not been clarified. In this work we studied the participation of three enzymes involved in AA metabolism - phospholipase A2 (PLA2), COX and LOX in Rhinella arenarum oocyte maturation. PLA2 activation induced maturation in Rhinella arenarum oocytes in a dose-dependent manner. Oocytes when treated with 0.08 μM melittin showed the highest response (78 ± 6% GVBD). In follicles, PLA2 activation did not significantly induce maturation at the assayed doses (12 ± 3% GVBD). PLA2 inhibition with quinacrine prevented melittin-induced GVBD in a dose-dependent manner, however PLA2 inactivation did not affect P4-induced maturation. This finding suggests that PLA2 is not the only phospholipase involved in P4-induced maturation in this species. P4-induced oocyte maturation was inhibited by the COX inhibitors indomethacin and rofecoxib (65 ± 3% and 63 ± 3% GVBD, respectively), although COX activity was never blocked by their addition. Follicles showed a similar response following the addition of these inhibitors. Participation of LOX metabolites in maturation seems to be correlated with seasonal variation in ovarian response to P4. During the February to June period (low P4 response), LOX inhibition by nordihydroguaiaretic acid or lysine clonixinate increased maturation by up to 70%. In contrast, during the July to January period (high P4 response), LOX inhibition had no effect on hormone-induced maturation.

Gonadotropin-releasing hormone positively regulates steroidogenesis via extracellular signal-regulated kinase in rat Leydig cells

Gonadotropin-releasing hormone (GnRH) is secreted from neurons within the hypothalamus and is necessary for reproductive function in all vertebrates. GnRH is also found in organs outside of the brain and plays an important role in Leydig cell steroidogenesis in the testis. However, the signalling pathways mediating this function remain largely unknown. In this study, we investigated whether components of the mitogen-activated protein kinase (MAPK) pathways are involved in GnRH agonist (GnRHa)-induced testis steroidogenesis in rat Leydig cells. Primary cultures of rat Leydig cells were established. The expression of 3β-hydroxysteroid dehydrogenase (3β-HSD) and the production of testosterone in response to GnRHa were examined at different doses and for different durations by RT-PCR, Western blot analysis and radioimmunoassay (RIA). The effects of GnRHa on ERK1/2, JNK and p38 kinase activation were also investigated in the presence or absence of the MAPK inhibitor PD-98059 by Western blot analysis. GnRHa induced testosterone production and upregulated 3β-HSD expression at both the mRNA and protein levels; it also activated ERK1/2, but not JNK and p38 kinase. Although the maximum effects of GnRHa were observed at a concentration of 100 nmnol L⁻¹ after 24 h, activation of ERK1/2 by GnRHa reached peak at 5 min and it returned to the basal level within 60 min. PD-98059 completely blocked the activation of ERK1/2, the upregulation of 3β-HSD and testosterone production. Our data show that GnRH positively regulates steroidogenesis via ERK signalling in rat Leydig cells. ERK1/2 activation by GnRH may be responsible for the induction of 3β-HSD gene expression and enzyme production, which may ultimately modulate steroidogenesis in rat Leydig cells.

CHRONIC PROPRANOLOL TREATMENT CAUSES DESENSITIZATION OF THE STEROIDOGENIC RESPONSE IN TESTICULAR INTERSTITIAL CELLS BUT DOES NOT ALTER PROTEIN KINASE C

We investigated effects of chronic propranolol treatment on the secretory response of rat testicular interstitial cells (testosterone secretion) to subsequent in vitro stimulation with activators of protein kinase-C (PK-C) (L-propranolol, phorbol 12, 13-dibutyrate (PDBu), LHRH) or activators of protein kinase A (PK-A), (hCG or dibutyryl cAMP (dbcAMP)). We determined [3H]PDBu binding and PK-C activity in these cells. Treatment of rats with propranolol (Inderal 500 mg/L of water for 5 weeks) reduced by 48%, 50% and 29% the L-propranolol-, LHRH- or PDBu-induced testosterone secretion, respectively, when compared to cells from controls. This desensitization in testosterone secretion in vitro was also present when the testicular interstitial cells were stimulated with hCG or dbcAMP (secretion decreased by 65%/57%, respectively, when compared to cells from control rats). Challenging the cells originated from rats that received propranolol chronically with the addition in vitro of propranolol resulted in an additional reduction of the hCG/dbcAMP-stimulated testosterone secretion. Chronic propranolol-induced desensitization was not associated with a loss in [3H]PDBu binding or a decrease in PK-C activity. Chronic propranolol-induced desensitization can be uncoupled from down-regulation of protein kinase C. The effector responsible for the desensitization could be distal to the protein kinase C and protein kinase A.

Soluble epoxide hydrolase and epoxyeicosatrienoic acids modulate two distinct analgesic pathways

During inflammation, a large amount of arachidonic acid (AA) is released into the cellular milieu and cyclooxygenase enzymes convert this AA to prostaglandins that in turn sensitize pain pathways. However, AA is also converted to natural epoxyeicosatrienoic acids (EETs) by cytochrome P450 enzymes. EET levels are typically regulated by soluble epoxide hydrolase (sEH), the major enzyme degrading EETs. Here we demonstrate that EETs or inhibition of sEH lead to antihyperalgesia by at least 2 spinal mechanisms, first by repressing the induction of the COX2 gene and second by rapidly up-regulating an acute neurosteroid-producing gene, StARD1, which requires the synchronized presence of elevated cAMP and EET levels. The analgesic activities of neurosteroids are well known; however, here we describe a clear course toward augmenting the levels of these molecules. Redirecting the flow of pronociceptive intracellular cAMP toward up-regulation of StARD1 mRNA by concomitantly elevating EETs is a novel path to accomplish pain relief in both inflammatory and neuropathic pain states.

Annexin 1 (lipocortin 1) mimics inhibitory effects of glucocorticoids on testosterone secretion and enhances effects of interleukin-1beta

Annexin 1 is an important mediator of glucocorticoid action in the hypothalamo-pituitary axis; however, little is known of its role in mediating glucocorticoid actions in the peripheral endocrine organs. Accordingly, we have carried out a preliminary study to investigate the effects of annexin 1 in vitro on the testicular secretion of testosterone, a process inhibited by both glucocorticoids and interleukin-1beta (IL-1beta). Luteinizing hormone (LH) and forskolin stimulated the release of testosterone from dispersed murine testicular cells in vitro. Their effects were reduced in cells from mice pretreated with dexamethasone (DEX). Similarly, preincubation of testicular cells from untreated mice with DEX, corticosterone, or 11-dehydrocorticosterone reduced LH-stimulated testosterone release, as did the 11beta-hydroxysteroid dehydrogenase inhibitors, glycyrrhetinic acid and carbenoxolone. The inhibitory actions of the steroids were mimicked by annexin 1(1-188) (ANXA1(1-188)) (a stable annexin 1 analog). IL-1beta produced a marked decrease in the response to LH, which was blocked by indomethacin, a nonselective cyclooxygenase inhibitor and an additive effect with DEX and ANXA1(1-188). These results confirm reports that glucocorticoids and IL-1beta inhibit LH-stimulated testosterone release from mouse testicular cells. They also show, for the first time, that the effects of the steroids are mimicked by annexin 1 and that, in contrast to their mutually antagonistic effects in the neuroendocrine system, IL-1beta and annexin 1 exert additive actions in the testis.

Effects of polyunsaturated fatty acids and prostaglandins on oocyte maturation in a marine teleost, the European sea bass (Dicentrarchus labrax)

The effects of the polyunsaturated fatty acids (PUFAs), arachidonic acid (AA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and prostaglandins (PGs) on oocyte maturation were investigated in a marine teleost, the sea bass (Dicentrarchus labrax). Follicle-enclosed postvitellogenic, preovulatory oocytes were cultured in vitro and maturation was verified by assessing volume increase, lipid droplet coalescence, yolk clarification, and germinal vesicle migration and breakdown. Human chorionic gonadotropin was administered as the maturation-inducing gonadotropin (GTH) and was capable of inducing maturation in a time- and dose-dependent manner. Free AA induced maturation in a dose- and time-dependent manner and enhanced GTH-induced maturation, while EPA, DHA, and oleic acid were ineffective. Maturation induced by GTH was significantly suppressed by a phospholipase A(2) blocker, suggesting that mobilization of AA was involved in GTH-induced maturation. Moreover, EPA and DHA exhibited a significant, dose-dependent attenuation of GTH-induced maturation. Maturation induced by GTH was inhibited in the presence of a cyclooxygenase inhibitor, indomethacin, and this inhibition was reversed by addition of AA, PGE(2), or PGF(2alpha). PGE(2) and PGF(2alpha) alone were both effective stimulators of maturation, while PGE(1) and PGE(3) were ineffective. The effect of PUFAs on oocyte maturation in vitro were corroborated with studies in vivo. Oocytes were obtained from females fed a commercial, PUFA-enriched diet (RD) and maturational behavior was compared with oocytes from females fed a natural diet (ND) with a higher EPA content and n-3:n-6 ratio. Although no significant difference was observed in the rate of spontaneous oocyte maturation, a higher percentage of GTH-induced maturation and lower percentage of atresia were observed in RD oocytes. Moreover, while basal PGE production from oocytes from both groups was the same, RD oocytes produced significantly higher levels of PGE in the presence of hCG. The results from this study provide evidence for the participation of AA metabolism in GTH-induced oocyte maturation, and suggest that other PUFAs and PGs may play important roles in the induction of maturation in a marine teleost.

Cyclic AMP and arachidonic acid: a tale of two pathways

Increasing evidence in recent years has demonstrated the regulatory effects of arachidonic acid and its metabolites on steroid hormone production in various steroidogenic tissues. In trophic hormone-stimulated steroidogenesis, arachidonic acid is rapidly released from phospholipids. This release is dependent upon hormone-receptor interaction and inhibition of arachidonic acid release results in an inhibition of steroidogenesis. Several of the earlier studies indicated that arachidonic acid acts at the rate-limiting step of steroid biosynthesis, the transfer of substrate cholesterol to the inner mitochondrial membrane, but the manner in which this occurred was not clear. Recently it has been demonstrated that arachidonic acid release can participate in the regulation of gene expression of the steroidogenic acute regulatory (StAR) protein which mediates cholesterol transfer to the inner mitochondrial membrane. These studies suggest that this fatty acid may be instrumental in transducing a signal from trophic hormone/receptor interaction to the nucleus utilizing a pathway different from the reported cyclic AMP pathway. It is possible that these two pathways cooperate and serve to co-regulate transcription factors, resulting in StAR gene expression and subsequent steroid production. This hypothesis may serve to explain and co-ordinate previous observations on the roles of cyclic AMP (cAMP) and arachidonic acid in steroid hormone biosynthesis.

Aspirin inhibits androgen response to chorionic gonadotropin in humans

Eicosanoids play an important role in the regulation of the hypothalamic-pituitary axis; less clear is their role in testicular steroidogenesis. To evaluate the involvement of cyclooxygenase metabolites, such as prostaglandins, in the regulation of human testicular steroidogenesis, we examined the effects of a prostaglandin-blocker, aspirin, on plasma testosterone, pregnenolone, progesterone, 17OH-progesterone, androstenedione, dehydroepiandrosterone, and 17beta-estradiol response to human chorionic gonadotropin (hCG) in normal male volunteers in a placebo-controlled, single-blinded study. To test the efficacy of aspirin, seminal prostaglandin E(2) levels were also determined. hCG stimulation increased peripheral levels of testosterone, 17OH-progesterone, androstenedione, dehydroepiandrosterone, and 17beta-estradiol, without affecting circulating pregnenolone and progesterone values. Aspirin significantly lowered seminal prostaglandin E(2) levels, whereas it did not modify steroid concentrations not exposed to exogenous hCG. Moreover, the drug significantly reduced the response of testosterone, 17OH-progesterone, androstenedione, and dehydroepiandrosterone to hCG, as assessed by the mean integrated area under the curve, whereas it did not influence 17beta-estradiol response. In conclusion, aspirin treatment inhibits androgen response to chorionic gonadotropin stimulation in normal humans. The action of aspirin is probably mediated via an effective arachidonate cyclooxygenase block.

Signal transduction involving cyclic AMP-dependent and cyclic AMP-independent mechanisms in the control of steroidogenesis

The control of steroidogenesis via signal transduction mechanisms involving cAMP-dependent and cAMP-independent mechanisms is reviewed. Several structurally unrelated factors that are potent stimulators of steroidogenesis whose actions do not require cAMP and/or synthesis of proteins have been identified. These include various interleukins, a lipophilic factor from macrophages, a steroidogenic inducing protein from follicular fluid and an imidazole compound, calmidazolium. All of these factors are capable of inducing maximum steroidogenesis. Calcium is required for steroidogenesis in all steroidogenic cells. With the exception of the effects of angiotensin II, there is little evidence for a role of IP3 in the stimulation of the release of calcium from intracellular stores in steroidogenic cells under physiological conditions. There may however, be a cAMP-mediated activation of a plasma membrane calcium channel. Chloride channels that can be regulated by cAMP-dependent and -independent mechanisms, are present in steroidogenic cells. Chloride ions exert a negative effect on steroidogenesis because exclusion of chloride from the extracellular medium markedly enhances cAMP-stimulated steroidogenesis. Arachidonic acid and its lipoxygenase products are involved in the control of steroidogenesis via cAMP mediated processes. An arachidonic acid related thioesterase has been isolated that is activated by ACTH and which may be involved in the release of arachidonic acid. It is concluded that while cAMP is a second messenger for LH/ACTH in the control of steroidogenesis, other signalling systems exist which are potentially equally effective in controlling steroidogenesis. In addition, the action of cAMP requires other signalling pathways involving calcium and chloride ions, as well as arachidonic acid and its lipoxygenase products.

Galanin stimulates steroidogenesis in rat Leydig cells

The present study was designed to evaluate whether galanin could play a role in the regulation of testicular steroidogenesis. To this purpose, using purified rat Leydig cells, we examined the effects of galanin on basal and hCG- or LHRH-induced testosterone production and the interference of a specific galanin receptor antagonist, galantide, on galanin activity. Moreover, since it has been shown that galanin-induced stimulation of LHRH secretion appears to involve the release of prostaglandin E2 (PGE2) as intracellular mediator, we evaluated also the effect of galanin on Leydig cells PGE2 output and the interference of indomethacin, a cycloxygenase blocker, on its activity. Furthermore, the effect of nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, was also examined. Data obtained indicate that galanin amplified testosterone response to hCG or LHRH whilst galantide prevented its potentiating activity. Moreover, galanin stimulated PGE2 output though this fatty acid is not involved in galanin activity on Leydig cells as indomethacin failed to affect its amplification of testosterone production. The possible involvement of leukotrienes should also be excluded as NDGA did not modify galanin action. In summary, the present study indicates that galanin potentiates acute gonadotropin or LHRH steroidogenic action on Leydig cells and that this activity is specific and receptor-mediated as it is prevented by a specific receptor antagonist.

Specific effect of arachidonic acid on 17beta-hydroxysteroid dehydrogenase in rat Leydig cells

It is well known that arachidonic acid (AA) acts as an intratesticular factor regulating luteinizing hormone-mediated testicular steroidogenesis. The present studies were conducted to determine the effect of AA on steroidogenic enzymes in rat Leydig cells. Exogenously added AA significantly inhibited 22(R)-hydroxy-cholesterol-stimulated testosterone production, which is a clear indication that AA is acting at some point after cholesterol transport to the inner mitochondrial membrane. AA failed to block the conversion of 22(R)-hydroxycholesterol to pregnenolone, indicating that the cytochrome P-450 side-chain cleavage enzyme complex is not the site of inhibition. The present results demonstrate that only 17beta-hydroxysteroid dehydrogenase seems to be involved in the AA action, since nearly 60% inhibition of testosterone production was found when the cells were incubated with androstenedione. Furthermore, no effect of AA was found when androstenediol was used as substrate in the testosterone synthesis, which indicates that 3beta-hydroxysteroid dehydrogenase is not affected by AA. The conversion of AA to its metabolites is not required for its action on 17beta-hydroxysteroid dehydrogenase and the activation of protein kinase C is not involved in the inhibitory effect.

GnRH in pregnancy

Human trophoblast produce GnRH and its precursor, immunologically and chemically identical to those of hypothalamic origin. Placental GnRH stimulates human chorionic gonadotropin secretion by the syncytiotrophoblast. It is known that GnRH analogue has negative effect on early rat pregnancy and may cause abortion through its action on the corpus luteum. A significant reduction of progesterone production was found in pregnant rats treated with GnRH agonist. GnRH caused a significant decrease in the maximal contraction intensity of non-pregnant and pregnant uterine muscle strip, following the action of oxytocin and acetylcholine. It was observed that treatment of pregnant rat with pharmacological doses of GnRH was able to delay parturition. Experimentally, GnRH significantly inhibited the release of placental prostaglandins E and F and thromboxane B 2 in a dose dependent fashion. Maternal circulating GnRH levels at 25-35 weeks of gestation were significantly higher in women who later had post-term pregnancies. In an other study maternal circulating GnRH concentration was found to be significantly lower in four patients who developed preterm labor and delivery. Low doses of GnRH in pregnant rats produced inhibition of postpartum lordosis behavior.

Different sites of action of arachidonic acid on steroidogenesis in rat Leydig cells

The present study in purified rat Leydig cells shows that arachidonic acid may act as an intratesticular factor regulating LH-mediated testicular steroidogenesis. Arachidonic acid decreased, in a dose-dependent manner, the LH-stimulated cAMP and testosterone levels, over 2 h incubation. Incubation of Leydig cells with arachidonic acid did not modify 125I-hCG binding to the cells as compared to control, showing that the action of arachidonic acid is not related to a decrease of hCG binding to the cells. Forskolin-stimulated cAMP and testosterone production were inhibited by 51.65 and 70.9%, respectively, in the presence of arachidonic acid (100 microM), although the ED50 for the diterpene was not changed. When isobutyl-methyl-xanthine was added to the incubation medium, the same percentage of inhibition was found indicating that arachidonic acid inhibition of cAMP production is not due to stimulation of Leydig cell phosphodiesterase activity. Pretreatment of the cells with pertussis toxin, to inactivate Gi, was also without effect on arachidonic acid inhibition of LH-stimulated cAMP production, but pertussis toxin abolished the inhibitory effects of arachidonic acid when adenylate cyclase was stimulated with forskolin. However, arachidonic acid addition resulted in inhibition of LH- and forskolin-stimulated testosterone production, even if the cells were pretreated with pertussis toxin. It can be concluded that: (1) The inhibitory effect of arachidonic acid is neither due to a decrease of hCG binding to Leydig cells nor to a stimulation of cell phosphodiesterase activity; (2) arachidonic acid modulates cAMP production at two different levels, either by activation of Gi protein and by inhibition of Gs protein or adenylate cyclase; (3) the effect of arachidonic acid on steroidogenesis is also beyond cAMP formation.

Arachidonic acid inhibits hCG-stimulated progesterone production by corpora lutea of primates: potential mechanism of action

Arachidonic acid (AA) is a precursor of metabolites known to affect the corpus luteum (CL) in many species, including primates. We have shown that some of these products (prostaglandins F2 alpha and E2) inhibit pro-gesterone (P4) production and activate the phosphatidylinositol (PI) pathway in CL of rhesus monkeys. A direct role of AA in luteal function has also been suggested. The current experiments were designed to investigate the effect of AA on P4 synthesis and to examine the ability of AA to activate the PI pathway in CL of rhesus monkeys. Basal and hCG-stimulated P4 production by luteal cells collected during the midluteal phase was measured after treatment with AA (1, 5, and 10 microM) or linoleic acid (1, 5, and 10 microM). Dispersed cells (50,000/tube) were incubated at 37 degrees C for 2 h. AA elicited a dose-dependent decrease in hCG-stimulated, but not in basal, P4 production. hCG-stimulated P4 production was reduced (P < 0.01) at AA doses of 5 microM (12.1 +/- 1.5 ng/mL) and 10 microM (8.6 +/- 1.8 mg/mL) to hCG alone (18 +/- 1.6 ng/mL). There was no significant effect of 1 microM AA (15.2 +/- 1.6). Response to linoleic acid was dissimilar and was not dose-dependent. Viability of cells was not affected by any treatment. Indomethacin, a prostaglandin synthesis inhibitor, and nordihydroguaiaretic acid, an inhibitor of lipoxygenase, did not interfere with the inhibitory effect of AA. Activation of the PI pathway was assessed by monitoring the hydrolysis of phosphatidylinositol-4,5-bisphosphate (PIP2) to inositol phosphates and by monitoring increases in intracellular free calcium concentrations ([Ca2+]i) in individual cells. Moreover, the ability of AA to activate protein kinase C (PKC) in luteal cells was measured using a [3H]phorbol dibutyrate (PDBu) binding assay. AA did not alter PIP2 hydrolysis or [Ca2+]i, however, AA (10 microM) increased specific binding of [3H]PDBu to luteal cells (P < 0.05). We conclude that AA inhibits hCG-stimulated P4 production by primate luteal cells. AA exerts this action without being converted to prostaglandins or leukotrienes. This inhibition may be mediated through the activation of PKC. These results suggest a possible role for AA in the regulation of luteal function in primates, and that PKC-activation by AA may promote its effects.

Arachidonic acid and its metabolites effects on testosterone production by rat Leydig cells

Arachidonic acid (AA) seems to play an important role in testicular steroidogenesis, although controversial data exist in the literature. In the present study AA induced a dose related increase of testosterone (T) formation and, at the highest dose, stimulated the production of prostaglandin E2 (PGE2), leukotrienes B4 (LTB4) and C4 (LTC4) by purified rat Leydig cells. The contemporary addition of the prostaglandin synthesis blocker, indomethacin (IND), and AA further increased T formation, decreased PGE2 levels and did not modify LTB4 and LTC4 concentrations. The addition of a lipoxygenase inhibitor, nordihydroguaiaretic acid (NDGA, 5 microM), did not influence the stimulatory effect of AA on T and PGE2 formation while it decreased the output of LTB4 and LTC4. When 20 microM NDGA was used in addition to AA the expected reduction of leukotrienes release was observed together with a surprising impairment of T and PGE2 secretion. PGE2 and PGF2 alpha did not modify basal T production but reduced HCG-stimulated T secretion at the 10 nM dose. When 5-12- and 15-HETE were tested an enhancement of basal T formation was observed at the 10nM dose. 5-HETE (10nM) stimulated HCG-induced T production. LTA4, LTB4 and LTE4 did not influence basal T output while LTC4 and LTD4 inhibited it. LTC4 (10nM) induced a decrease of HCG-stimulated T production. These findings suggest that: 1) exogenous AA stimulates T secretion; 2) conversion of AA to cycloxygenated and lipoxygenated metabolites is not required for its steroidogenic effect; 3) cycloxygenated and lipoxygenated compounds play a diverse modulatory role on testicular steroidogenesis.

Polyunsaturated fatty acids do not activate protein kinase C in the testis of the goldfish (Carassius auratus)

Earlier studies have established that polyunsaturated fatty acids (PUFA) such as eicosapentaenoic acid and docosahexaenoic acid inhibit steroid production in the goldfish testis. As PUFA inhibit testicular steroidogenesis in the rat through activation of protein kinase C (PKC), the present studies were undertaken to characterize the properties of PKC in the goldfish testis and to test the effects of selected PUFA on PKC activity. PKC activity was quantified in goldfish testis homogenate following partial purification by DEAE-cellulose chromatography by determining the transfer of radiolabelled phosphate from [γ - (32)P]ATP to histone III-S. Testicular PKC activity was defined by the amount of protein phosphorylation in the presence of phosphatidylserine, phasphatidylcholine, Ca(2+) ions and diolein (a 1,2-diacylglycerol analog) above that obtained in response to Ca(2+) ions alone. Western blot analysis of a crude testis homogenate using an antibody specific to the α and β isoforms of mammalian PKC led to the identification a single band of protein (80 kD) that co-migrated with PKC from rabbit brain cytosol. Addition of arachidonic, eicosapentaenoic or docosahexaenoic acids failed to activate PKC. However, PKC activity stimulated by phospholipid, Ca(2+) ions and diolein was inhibited in a dose related fashion by all of these fatty acids. These studies suggest that the inhibitory effects of EPA and DHA on testicular steroidogenesis are not mediated by activation of PKC. The lack of effect of PUFA on PKC activity in the goldfish testis suggests that either the distribution of PKC isoforms differs between the testis of mammals and fish or that PKC is not activated by PUFA in the goldfish.

Arachidonic acid metabolites as intratesticular factors controlling androgen production

The effect of inhibitors and products of arachidonic acid metabolism on rat testicular steroidogenesis has been investigated. In the presence of indomethacin (inhibitor of cyclooxygenase) and nordihydroguaiaretic acid (NDGA) (inhibitor of lipoxygenase), the activity of 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) and 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) were both inhibited. The LH-stimulated increase in secretion of testosterone and progesterone was also inhibited by indomethacin and NDGA. On the other hand, vitamin E (antioxidant and inhibitor of lipoxygenase), stimulated the activity of both 3 beta-HSD and 17 beta-HSD and enhanced LH-stimulated androgen production. The metabolites of lipoxygenase (15-HPETE, 15-HETE, 5-HPETE and 5-HETE) and cyclooxygenase (PGF2 alpha) pathways stimulated 3 beta-HSD and 17 beta-HSD activity and enhanced the secretion of progesterone and testosterone. It is concluded that arachidonic acid metabolites are intratesticular factors which can regulate LH-stimulated testicular steroidogenesis.

Age-related variation of follicle-stimulating hormone-stimulated cAMP production, protein kinase C activity and their interactions in the rat testis

To study further the ontogeny of hormonal regulatory mechanisms in the testis, we measured follicle-stimulating hormone (FSH)- and protein kinase C (PKC)-stimulated cAMP production, PKC activity, and messenger (m)RNA levels of the PKC isoenzymes alpha, beta and gamma in rat testes between day 19 of fetal life and day 90 postpartum. Human FSH (30 mg/l) stimulated slightly but significantly cAMP production of fetal testes (57%; p < 0.05). A higher response (3-fold; p < 0.01) was observed on the day of birth, and the maximum FSH effect on cAMP (23-fold) was observed on day 10 postpartum. Thereafter, a gradual decline of FSH response occurred towards adult age. Concerning testicular PKC, the soluble (inactive) form had its maximum at the age of 1 day and this PKC form declined gradually thereafter. The particulate (active) form was low at birth, increased 6-fold on days 8-11 of age, and declined thereafter. A significant age-dependent variation was also found in the mRNA level of the PKC alpha isoenzyme (maximum on day 10), whereas those of PKC gamma and PKC beta were undetectable at all ages in Northern blots. When the in vitro modulation of basal and FSH-dependent cAMP production by the PKC activator 12-O-tetradecanoylphorbol 13-acetate (TPA, 100 nmol/l) was studied, the substance alone was without effect at all ages studied. The TPA effect on FSH-stimulated cAMP production displayed age-dependent variation: a slight stimulation in fetal testes, no effect at birth, decrease between days 8 and 11, and no effect on day 30.(ABSTRACT TRUNCATED AT 250 WORDS)

Intragonadal signalling mechanisms in the control of steroid hormone production

In the gonads, there are two recognized signal transduction mechanisms which operate in the processing of hormonal stimuli. The gonadotropins, follicle stimulating hormone and luteinizing hormone, act primarily through the generation of cyclic AMP. Several other hormonal regulators in the ovary and the testis, such as gonadotropin releasing hormone and prostaglandin F2 alpha stimulate inositol lipid metabolism following receptor binding. This triggers a cascading mechanism which ultimately results in the generation of increased cytosolic free calcium levels, enhanced protein kinase C activity, and liberation of arachidonic acid. There is also evidence that luteinizing hormone shares in the activation of this pathway. In this review, the significance of these signal transduction pathways is discussed in relation to the effects of various hormones on steroid biosynthesis in the gonads.

Ontogenesis of the in vitro response of rat testis to gonadotropin-releasing hormone

The age-related evolution of the in vitro effects of a gonadotropin releasing hormone agonist ([D-Trp6]-GnRH) on the secretion of testosterone by the testis, cultured during 3 days on a Millipore filter floating on M199 medium, was studied during the perinatal period in the rat. The basal and luteinizing hormone (LH)-stimulated secretions by testes explanted on fetal day 14.5 were unaffected by the agonist. With fetal testes explanted on days 16.5 and 18.5 post-conception, the agonist inhibited, in a concentration-dependent manner, both basal and LH-stimulated secretions from the second or the third day of treatment onwards. With fetal and neonatal testes explanted on days 20.5, 21.5 and 31.5 post-conception, the GnRH agonist also had a long-term inhibitory effect on LH-stimulated secretion, but increased basal secretion. This stimulatory effect was already observed after 4 h of culture, and was maintained for 3 days. These results suggest that, during fetal development, the cellular mechanisms involved in the negative testicular response to GnRH are differentiated 3-5 days before those involved in the positive response. Lastly, after 3 days of preculture in hormone-free medium, fetal testes explanted on day 14.5 displayed long-term GnRH agonist inhibition of in vitro basal secretion of testosterone. This observation points out a spontaneous differentiation of the negative responsiveness to GnRH in the cultured fetal testis.

Phorbol ester and vasopressin activate phospholipase D in Leydig cells

In the present study evidence is provided for the existence of phospholipase D (PLD) activity in rat Leydig cells. Leydig cells were cultured and labelled with [3H]myristic acid. In the presence of ethanol, phorbol 12-myristate 13-acetate (PMA) stimulated the formation of [3H]phosphatidylethanol ([3H]PEt) in a dose-dependent manner at the expense of [3H]phosphatidic acid ([3H]PA). In cells prelabelled with [3H]choline, PMA caused a rapid increase in intracellular free [3H]choline. The time course of [3H]PEt formation was similar to the time course of intracellular [3H]choline formation. The data taken together support the notion that PMA stimulates phosphatidylcholine (PC) hydrolysis by a mechanism, which principally involves PLD. Activation of PLD by PMA was inhibited by long-term pretreatment of cells with PMA to downregulate protein kinase C (PKC) and by pretreatment with staurosporine. These data support the notion that activation of PLD by PMA is dependent on PKC. Arginine vasopressin (AVP) caused a rapid stimulation of PLD activity in the cells. This activation was inhibited after down-regulation of PKC, indicating that the agonist acts by a mechanism similar to that of PMA.

Epidermal growth factor modulates intracellular arachidonic acid levels in MA-10 cultured Leydig tumor cells

Epidermal growth factor (EGF) acts on various cell types, including the mouse Leydig tumor cell line MA-10, where it has been shown to stimulate steroidogenesis, apparently in a cAMP-independent manner. In the process of examining other possible signaling pathways for EGF in these cells, we found rapid changes in the intracellular concentration of arachidonic acid (AA) following addition of EGF. For example, a significant increase in AA was detected 1 min after incubating the cells with EGF, with the maximal effect observed at an EGF concentration of 10 ng/ml. In addition, exogenous AA increased steroidogenesis, and the steroidogenesis enhanced by AA and EGF was reduced by lipoxygenase inhibitors, suggesting a possible role of an AA metabolite(s) in promoting steroidogenesis. Consistent with this hypothesis is our observation that several exogenous lipoxygenase metabolites were capable of enhancing progesterone production. The EGF-stimulated steroidogenesis was also inhibited by two phospholipase A2 inhibitors, again confirming a probable role of AA or a metabolite in this process. Therefore, AA appears to be an important intracellular mediator responsible, at least in part, for some of the acute metabolic effects mediated by EGF in MA-10 cells.

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.

Effects of hormones and intracellular mediators on differentiated functions of cultured Leydig tumor cells

Cellular regulation by hormones that utilize a myriad of intracellular signaling pathways is recognized to be quite complex. To investigate some of these effects in an established cell line, we tested a panel of hormones and modulators for their effects on cyclic AMP (cAMP) and progesterone production, both alone and in combination with human chorionic gonadotropin (hCG), using the MA-10 cultured Leydig tumor cell line. None significantly affected intracellular levels of cAMP, and only epidermal growth factor (EGF) and 12-O-tetradecanoyl-phorbol-13-acetate (TPA) stimulated progesterone production. While EGF, basic fibroblast growth factor, insulin, insulin-like growth factor-1, and transforming growth factor beta all decreased cAMP production only, TPA decreased hCG-stimulated cAMP and progesterone production. Those factors that stimulated progesterone production also induced a characteristic morphological change ("rounding") of these cells. In addition, EGF, insulin, and TPA, like hCG, elevated mRNA levels of competence oncogenes (c-fos and c-myc), albeit to different extents. These data demonstrate the wide range of hormones to which the cultured Leydig tumor cell will respond, as well as the varying degree of responses observed in the intracellular signaling pathways that we examined.

Mechanism of LHRH-stimulated steroidogenesis in rat Leydig cells: lipoxygenase products of arachidonic acid may not be involved

Luteinizing hormone releasing hormone agonist, [(imBzl)-DHis6,Pro9,NEt]-LHRH (LHRH-A), caused a two to threefold increase in in vitro testosterone (T) secretion by rat Leydig cells. This LHRH-A-induced T secretion was completely blocked by quinacrine and chloroquine, inhibitors of phospholipase A2. Addition of phospholipase A2, however, was ineffective in stimulating basal or LHRH-A-induced T secretion. Phospholipase C, on the other hand, significantly stimulated both basal and LHRH-A-induced T secretion. Exogenously added arachidonic acid stimulated basal T secretion in a dose dependent manner, the maximum increase being about 100% over basal at a dose of 100 microM. Higher doses of arachidonic acid had no stimulatory effect. In the presence of LHRH-A, the stimulatory effect of arachidonic acid was additive up to a concentration of 100 microM; but higher concentrations of arachidonic acid (200 microM) were inhibitory. LHRH-A-induced steroidogenesis was inhibited by 5, 8, 11, 14 Eicosatetraynoic acid (ETYA), an inhibitor of all the three known pathways of arachidonic acid metabolism, and by nordihydroguaiaretic acid, and inhibitory of the lipoxygenase pathway of arachidonic acid metabolism. LHRH-A-stimulated T secretion was not inhibited by indomethacin, an inhibitor of the cyclo-oxygenase pathway of arachidonic acid metabolism. ETYA inhibited arachidonic acid-induced T secretion. Nordihydroguaiaretic acid, on the other hand, augmented basal, arachidonic acid-, phospholipase C-, or phorbol 12, myristate 13 acetate-induced testosterone secretion. These results suggest that arachidonic acid, whose release is influenced by phospholipase C, is involved in LHRH-A-induced T secretion by rat Leydig cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential role of protein kinase C in the action of luteinizing hormone-releasing hormone on hormone production in rat ovarian cells

This study was undertaken to determine the involvement of arachidonic acid and protein kinase C in the actions of luteinizing hormone-releasing hormone on steroid and prostaglandin formation in the ovary. In primary culture of rat granulosa cells, treatment with 3 x 10(-7) mol/L melittin stimulates progesterone and prostaglandin E2 accumulation after a 5-hour culture period. Concomitant treatment of the cells with melittin and luteinizing hormone-releasing hormone or 12-0-tetradecanoylphorbol 13-acetate further enhances the stimulatory action of either luteinizing hormone-releasing hormone or 12-0-tetradecanoylphorbol 13-acetate by itself on prostaglandin E2 production. In contrast, no synergistic effects are observed on progesterone production by the same treatments. Treatment with luteinizing hormone-releasing hormone for 24 hours significantly decreases follicle-stimulating hormone-induced progesterone production by approximately 50%. Treatment of the cells with either follicle-stimulating hormone or luteinizing hormone-releasing hormone stimulates prostaglandin E2 production at least tenfold in the same cultures. When follicle-stimulating hormone and luteinizing hormone-releasing hormone are present concomitantly, they synergistically enhance prostaglandin E2 formation (p less than 0.01). Similar effects are observed with the phorbol ester, 12-0-tetradecanoylphorbol 13-acetate, which causes a dose-dependent inhibition of progesterone production by follicle-stimulating hormone whereas follicle-stimulating hormone-stimulated prostaglandin E2 formation is enhanced. Thus luteinizing hormone-releasing hormone-induced activation of protein kinase C may play multiple roles (stimulatory or inhibitory) in hormone production in the ovary.

Testicular GnRH-receptors and direct effects of a GnRH-agonist on human testicular steroidogenesis

GnRH-like substances have been isolated from interstitial fluid from the rat testis and have been found to exert direct, mainly inhibitory effects on steroidogenesis. In rat testis, specific GnRH receptors have been shown, but so far no GnRH-receptors have been isolated from the human testis. In this study testicular tissue from nine elderly men was incubated with either 3H-pregnenolone or 3H-progesterone and the steroid metabolic patterns were analyzed. In parallel incubations a GnRH-agonist was added to the incubate in concentrations of 10(-6) M or 10(-7) M. Furthermore, receptor studies were performed in tissue specimens from four of these untreated patients as well as in tissue specimens from five GnRH-agonist treated patients. No consistent effects were elicited by the addition of GnRH-agonist on the steroid metabolic patterns in vitro after three hours incubation. In the receptor studies the GnRH-agonist was bound to the testicular tissue, although the binding sites were of low affinity and high capacity, indicating a less specific kind of binding than classical receptor binding.

Inhibitory effect of GnRH on isolated rat uterine muscle contractility

The effect of GnRH upon uterine contractions of both non-pregnant and pregnant rats was examined in vitro. In the non-pregnant rat uterus, GnRH inhibited in a concentration-and-time dependent manner the contractions induced by acetylcholine and oxytocin, but not those caused by bradykinin and angiotensin II. GnRH also inhibited the rhythmic contractions induced by oxytocin in uterine strips from late pregnant rats. These findings show that GnRH has a direct inhibitory effect on the rat uterine contractions, suggesting that GnRH-like substances may exert modulatory influences upon rat uterine contractility.

Gonadotropin-releasing hormone agonist activates protein kinase C in rat Leydig cells

Gonadotropin-releasing hormone (GnRH) has specific receptor sites in rat Leydig cells and has direct effects on their steroidogenesis. The purpose of the present study was to examine whether activation of the calcium- and phospholipid-dependent protein kinase C (PK-C) is involved in GnRH effects on rat Leydig cells, as has been shown in pituitary gonadotrophs. Testosterone production of Percoll-purified Leydig cells was similarly stimulated (about 50-100%) by a GnRH agonist (buserelin, maximum effect at concentration of 10(-9) mol/l and above) and a tumor promoting phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA, maximum effect at 10(-8) mol/l), which is known to activate PK-C. In contrast, a GnRH antagonist (10(-5) mol/l) and an inactive phorbol ester, 4 alpha-phorbol-12,13-didecanoate (10(-6) mol/l), were without effect on testosterone. None of these substances had clear effects on cAMP production. The maximum steroidogenic effects of GnRH agonist and TPA were the same whether used separately or together, suggesting that they share a common mechanism of action. TPA translocated the cytosolic proportion of Leydig cell PK-C activity to a membrane-associated form almost instantaneously, within 0.5-1 min. A similar translocation, though less complete, was observed in the presence of buserelin in 1-4 min. Inclusion of a 100-fold excess of a potent GnRH antagonist completely prevented the translocation of PK-C. These results provide evidence that GnRH agonist activates PK-C also in the testis tissue, and this may be the mechanism whereby it affects Leydig cell endocrine function.

Short-term effects of an LHRH-agonist alone or in combination with testosterone propionate or indomethacin on rat testes. Evidence of testosterone independent effects. I

The treatment of adult male Wistar rats with a LHRH-agonist (lutrelin Wyeth/WY 40972) resulted in severe damage of the seminiferous tubules as well as in remarkable changes of the blood vessels within 24 hours. First striking signs of alterations within the blood vessels were already found 6 hours after the injection of lutrelin: the blood vessels were almost totally filled with leucocytes. Neither the effects on the germinal epithelium nor the effects on the blood vessels were prevented by the simultaneous treatment with 3 mg testosterone propionate (TP). The treatment with indomethacin, however, clearly antagonized both events. The complete inefficiency of TP to overcome the inhibitory effects of lutrelin on the testes does argue against an androgen deficiency as the primary cause. The results obtained with indomethacin strengthen the hypothesis, that the early deleterious effects of LHRH-agonists on the germinal epithelium of the rat are primarily caused by circulatory disturbances in the testes and that prostaglandins may act as mediators.

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.

Distribution and activation of protein kinase C in the rat testis tissue

The distribution and role of the calcium-activated, phospholipid-dependent protein kinase C (PK-C) was studied in rat testis. When testis tissue was homogenized in the presence of 2 mmol/l EDTA and EGTA, the majority (greater than 70%) of the PK-C activity was soluble, the rest was released from the particulate fraction by solubilization with 0.3% Triton X-100. Without chelating agents the soluble PK-C activity was undetectable, and only partially recovered from solubilized membranes. Preincubation of the tissue with the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA, 10(-7) mol/l) translocated PK-C to the membranes, and the majority of this activity was recovered by solubilization. Mobility of testicular soluble PK-C activity in HPLC-DEAE cellulose chromatography was similar to that of the brain enzyme. This single step purified testicular PK-C activity 140-fold. The specific activity and subcellular distribution of PK-C was similar in whole testis tissue and separated seminiferous tubules (160-210 pmol 32P X mg protein-1 X min-1 in the soluble and particulate fractions), but 2- to 3-fold higher in purified Leydig cells. However, the majority of total testicular PK-C activity appeared to be of tubular origin. Unilateral cryptorchidism for 1 week reduced PK-C of the abdominal testis by 50%, and the activity of dissected seminiferous tubules varied according to the epithelial wave. Both findings suggest that the bulk of the activity resides in the seminiferous epithelium. Involvement of PK-C in Leydig cell function was demonstrated using the TPA, which at 10(-7) mol/l inhibited basal cAMP production by 50% (P less than 0.01) but increased that of testosterone by 2- to 3-fold (P less than 0.01). On the other hand, when incubated with hCG, TPA inhibited both cAMP and testosterone production; the ED50s of hCG stimulation increased 4- to 10-fold with both parameters. It is concluded that PK-C activity is present in both the seminiferous tubules and Leydig cells, and is involved in the regulation of these testicular compartments. Its total activity and subcellular distribution are at variance according to the functional state and endocrine milieu of the testis.

Fetal Leydig cell culture--an in vitro system for the study of trophic hormone and GnRH receptors and actions

In fetal and neonatal rat Leydig cells cultured in the presence of LH, gonadotropin and GnRH receptors and acute testosterone responses to hCG, were maintained for up to 78 days. Addition of GnRH agonists markedly inhibited steroid production in LH-treated cultures, and abolished the acute testosterone response to hCG. GnRH receptors were not detectable in fetal testes but were present post-natally and increased markedly with age. In cultured fetal testis, GnRH receptors were detected on the third day, and were increased by exposure to GnRH agonists. In LH-treated cultures, GnRH sites were reduced by about 50% and did not increase during incubation with GnRH agonists. LH supported 17 alpha-hydroxylase/17-20 desmolase activities and pregnenolone formation were observed in short (1-4 days) and long-term cultures. Also, LH stimulation of 3 beta-hydroxysteroid dehydrogenase was observed by histochemical studies at 7 days of culture. GnRH agonists inhibited LH dependent steroid production in a dose-dependent fashion and abolished the acute testosterone response to human chorionic gonadotropin. The major component of the steroid inhibitory effect of GnRH agonist occurs beyond cAMP production. A distal lesion of the microsomal enzymes of the androgen pathway is largely responsible for the GnRH-induced decreases in LH-supported androgen production. The expression of functional GnRH receptors during culture and their suppression by LH suggest that pituitary gonadotropins exert a tonic inhibitory effect upon testicular GnRH receptors. The presence of functional GnRH receptors and inhibitory actions in cultured fetal and neonatal Leydig cells indicates that GnRH-related peptides can influence the actions of gonadotropins on the fetal Leydig cell population.