SARS-CoV-2 Enters Human Leydig Cells and Affects Testosterone Production In Vitro

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a SARS-like coronavirus, continues to produce mounting infections and fatalities all over the world. Recent data point to SARS-CoV-2 viral infections in the human testis. As low testosterone levels are associated with SARS-CoV-2 viral infections in males and human Leydig cells are the main source of testosterone, we hypothesized that SARS-CoV-2 could infect human Leydig cells and impair their function. We successfully detected SARS-CoV-2 nucleocapsid in testicular Leydig cells of SARS-CoV-2-infected hamsters, providing evidence that Leydig cells can be infected with SARS-CoV-2. We then employed human Leydig-like cells (hLLCs) to show that the SARS-CoV-2 receptor angiotensin-converting enzyme 2 is highly expressed in hLLCs. Using a cell binding assay and a SARS-CoV-2 spike-pseudotyped viral vector (SARS-CoV-2 spike pseudovector), we showed that SARS-CoV-2 could enter hLLCs and increase testosterone production by hLLCs. We further combined the SARS-CoV-2 spike pseudovector system with pseudovector-based inhibition assays to show that SARS-CoV-2 enters hLLCs through pathways distinct from those of monkey kidney Vero E6 cells, a typical model used to study SARS-CoV-2 entry mechanisms. We finally revealed that neuropilin-1 and cathepsin B/L are expressed in hLLCs and human testes, raising the possibility that SARS-CoV-2 may enter hLLCs through these receptors or proteases. In conclusion, our study shows that SARS-CoV-2 can enter hLLCs through a distinct pathway and alter testosterone production.

Alterations of gene profiles in Leydig-cell-regenerating adult rat testis after ethane dimethane sulfonate-treatment

Only occupying about 1%–5% of total testicular cells, the adult Leydig cell (ALC) is a unique endocrine cell that produces androgens. Rat Leydig cells regenerate after these cells in the testis are eliminated with ethane dimethane sulfonate (EDS). In this study, we have characterized Leydig cell regeneration and messenger ribonucleic acids (mRNA) profiles of EDS treated rat testes. Serum testosterone, testicular gene profiling and some steroidogenesis-related proteins were analyzed at 7, 21, 35 and 90 days after EDS treatment. Testicular testosterone levels declined to undetectable levels until 7 days after treatment and then started to recover. Seven days after treatment, 81 mRNAs were down-regulated greater than or equal to two-fold, with 48 becoming undetectable. These genes increased their expression 21 days and completely returned to normal levels 90 days after treatment. The undetectable genes include steroidogenic pathway proteins: steroidogenic acute regulatory protein, Scarb1, Cyp11a1, Cyp17a1, Hsd3b1, Cyp1b1 and Cyp2a1. Seven days after treatment, there were 89 mRNAs up-regulated two-fold or more including Pkib. These up-regulated mRNAs returned to normal 90 days after treatment. Cyp2a1 did not start to recover until 35 days after treatment, indicating that this gene is only expressed in ALCs not in the precursor cells. Quantitative polymerase chain reaction, western blotting and semi-quantitative immunohistochemical staining using tissue array confirmed the changes of several randomly picked genes and their proteins.

Identification, proliferation, and differentiation of adult Leydig stem cells

Leydig cells, the testosterone-producing cells of the adult testis, rarely turn over. However, their elimination with ethane dimethanesulfonate (EDS) is followed by the appearance of new, fully functional adult Leydig cells. The cells that give rise to the new Leydig cells have not been well characterized, and little is known about the mechanism by which they are regulated. We isolated cells expressing platelet-derived growth factor receptor-α, but not 3β-hydroxysteroid dehydrogenase (3β-HSD(neg)) from the testes of EDS-treated adult rats. Depending on conditions, these cells proliferated indefinitely or differentiated and produced testosterone. To localize these cells and to determine the effect of the testicular environment on their function, the seminiferous tubules and testicular interstitium were physically separated and cultured. During the first 72 h in culture, 3β-HSD(neg) cells on the tubule surfaces underwent divisions. Some of these cells later expressed 3β-HSD and produced testosterone. Removal of the newly formed 3β-HSD(pos) cells from the tubule surfaces with EDS, followed by further culture of the stripped tubules, resulted in the reappearance of testosterone-producing cells. These results, taken together, suggest that the precursors for newly formed Leydig cells are stem cells, with many if not all situated on the surfaces of the seminiferous tubules. Although normally quiescent, the stem cells are capable of self-renewal and differentiation. The development of the tubule culture system should provide a valuable in vitro approach to assess the role(s) of niche components on the function of adult Leydig stem cells despite their residing in a complex mammalian tissue.

Effect of brominated flame retardant BDE-47 on androgen production of adult rat Leydig cells

As one of the most abundant polybrominated diphenylethers (PBDEs) detected in adipose tissue and breast milk of humans, 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) is considered as a potential endocrine disruptor. The objective of this study is to explore whether environment-related level of BDE-47 could affect the androgen production in rat Leydig cells. Rat adult Leydig cells (ALCs) were treated with 10(-8) to 10(-4)M BDE-47 in vitro, the production of testosterone (T) and steroidogenic acute regulatory (StAR) protein level were determined. BDE-47 significantly increased basal T production and steroidogenic acute regulatory protein (StAR) level of ALCs after treatment with 10(-4)M BED-47. Overall, LH (0.1ng/ml) stimulated T production in ALCs by 6 folds, however it did not increase T production in BDE-47-treated ALCs when compared to untreated ALC. Both 8-Br-cAMP (for cAMP signaling) and 22R-hydroxycholesterol (22-diol, for P450 cholesterol side chain cleavage enzyme P450scc activity) significantly increased T production in ALCs treated with BDE-47 from 10(-7) to 10(-5)M. The results of this study indicate that environment-related level of BDE-47 in vitro increased T production in a dose-dependent manner. The stimulated effects of BDE-47 on StAR and P450scc might play key roles in BDE-47-mediated stimulation of T production.

Role of 11β-OH-C(19) and C(21) steroids in the coupling of 11β-HSD1 and 17β-HSD3 in regulation of testosterone biosynthesis in rat Leydig cells

Here we describe further experiments to support our hypothesis that bidirectional 11β-HSD1-dehydrogenase in Leydig cells is a NADP(H) regenerating system. In the absence of androstenedione (AD), substrate for 17β-HSD3, incubation of Leydig cells with corticosterone (B) or several C(19)- and C(21)-11β-OH-steroids, in the presence of [(3)H]-11-dehydro-corticosterone (A), stimulated 11β-HSD1-reductase activity. However, in presence of 30 μM AD, testosterone (Teso) synthesis is stimulated from 4 to 197 picomole/25,000 cells/30 min and concomitantly inhibited 11β-HSD1-reductase activity, due to competition for the common cofactor NADPH needed for both reactions. Testo production was further significantly increased (p<0.05) to 224-267 picomole/25,000 cells/30 min when 10 μM 11β-OH-steroids (in addition to 30 μM AD) were also included. Similar results were obtained in experiments conducted with lower concentrations of AD (5 μM), and B or A (500 nM). Incubations of 0.3-6.0 μM of corticosterone (plus or minus 30 μM AD) were then performed to test the effectiveness of 17β-HSD3 as a possible NADP(+) regenerating system. In the absence of AD, increasing amounts (3-44 pmol/25,000 cells/30 min) of 11-dehydro-corticosterone were produced with increasing concentrations of corticosterone in the medium. When 30 μM AD was included, the rate of 11-dehydro-corticosterone formation dramatically increased 1.3-5-fold producing 4-210 pmol/25,000 cells/30 min of 11-dehydro-corticosterone. We conclude that 11β-HSD1 is enzymatically coupled to 17β-HSD3, utilizing NADPH and NADP in intermeshed regeneration systems.

Normoxic expression of hypoxia-inducible factor 1 in rat Leydig cells in vivo and in vitro

Hypoxia-inducible factors (HIF) are transcription factors that serve essential regulatory roles in cellular and molecular responses to oxygen debt. HIFs are composed of hypoxia-dependent α subunits (1α, 2α, 3α) and an oxygen-independent β subunit. Previously we demonstrated that HIF-1, the master regulator of hypoxic responses, is expressed in the adult rat testis. We hypothesized that HIF-1 is involved in regulating responses to oxygen tension in the testis. Goals of this study were to determine if HIF-2α and HIF-3α are expressed in rat testis, identify testis cell types that express HIF-1α, and examine patterns of testicular HIF-1α protein expression under conditions of ischemia and hypoxia in vivo and in vitro. Reverse transcriptase polymerase chain reaction revealed that mRNA for Hif-1α, Hif-2α, and Hif-3α is expressed in the testis. The HIF-1α protein is the predominant subunit in testis. HIF-1α protein was abundant in normoxic testis, and its levels remained unchanged following ischemia created by surgically induced testicular torsion and reperfusion. Immunoblot and immunocytochemical experiments demonstrated that Leydig cells are the major source of HIF-1α in normoxic and hypoxic testes. To examine potential mechanisms of testicular HIF-1 stabilization, nuclear proteins from Leydig cells cultured in 5% or 21% oxygen, or cells cultured with H₂O₂, were analyzed by immunoblotting. Levels of HIF-1α were significantly diminished in 5% or 21% oxygen cultures compared with freshly isolated cells. Treating Leydig cells with H₂O₂ as a source of reactive oxygen species did not affect HIF-1α levels. High levels of constitutively expressed HIF-1α in normoxic Leydig cells suggest potentially unique roles for HIF-1 in Leydig cell responsiveness to oxygen.

Inhibition of LH-stimulated androgen production in rat immature Leydig cells: Effects on nuclear receptor steroidogenic factor 1 by FGF2

Both fibroblast growth factor 2 (FGF2) and luteinizing hormone (LH) have been reported to regulate androgen production in Leydig cells in progenitor Leydig cells. The objective of the present study is to examine the regulation of androgen production in rat immature Leydig cells (ILCs). ILCs were isolated from 35-day-old rat testes and cultured in DMEM/F12 medium with LH (1 ng/ml) or FGF2 (10 ng/ml). 5alpha-Androstane-3alpha, 17beta-diol (3alpha-DIOL), the primary androgen in ILCs, and testosterone (T) were measured by Radioimmuno assay. The results showed the LH stimulated androgen production in ILCs, and FGF2 did not. However, FGF2 decreased the LH-stimulated androgen production. Real-time PCR and enzyme assay showed that FGF2 decreased levels of several steroidogenic enzymes, inhibited the expressions of steroidogenic acute regulatory (StAR) protein and steroidogenic factor 1 (Nr5a1) in LH-stimulated ILCs. FGF2-mediated inhibition of Nr5a1gene expression may be the mechanism through which FGF2 inhibits LH-stimulated androgen production.

Normal responses to restraint stress in mice lacking the gene for neuronal nitric oxide synthase

The hormonal changes associated with immobilization stress (IMO) include a swift increase in corticosterone (CORT) concentration and a decrease in circulating testosterone (T) levels. There is evidence that the production of the short-lived neuromodulator nitric oxide (NO) is increased during stress in various tissues, including the brain. NO also suppresses the biosynthesis of T. Both the inducible and the neuronal isoforms of NO synthase (iNOS and nNOS, respectively) have been implicated in this suppression, but the evidence has not been conclusive. We used adult wild-type (WT) and nNOS knockout male mice (nNOS-/-) to assess the respective roles of CORT and nNOS-derived NO in stress mediated inhibition of T production. Animals were assigned to either basal control or 3-hour IMO groups. No difference in basal plasma and testicular T levels were observed between WT and nNOS-/-, although testicular weights of mutant mice were slightly lower compared to WT animals. The plasma contents of luteinizing hormone (LH) and CORT in unstressed mice of both genotypes were similar. Exposure to 3 hours of IMO increased plasma CORT and decreased T concentrations in mice of both genotypes. However, comparable levels of plasma LH and testicular nitrite and nitrate (NOx), NO stable metabolites, were detected in control and stressed WT and nNOS-/- mice. Adrenal concentrations of NOx declined after IMO, but the reduction was not statistically significant. These findings implicate CORT rather than NO generated by nNOS in the rapid stress-induced suppression of circulating T.

Increased proliferation but decreased steroidogenic capacity in Leydig cells from mice lacking cyclin-dependent kinase inhibitor 1B

Proliferating cells express cyclins, cell cycle regulatory proteins that regulate the activity of cyclin-dependent kinases (CDKs). The actions of CDKs are regulated by specific inhibitors, the CDK inhibitors (CDKIs), which are comprised of the Cip/Kip and INK4 families. Expression of the Cip/Kip CDKI 1B (Cdkn1b, encoding protein CDKN1B, also called p27(kip1)) in developing Leydig cells (LCs) has been reported, but the function of CDKN1B in LCs is unclear. The goal of the present study was to determine the effects of CDKN1B on LC proliferation and steroidogenesis by examining these parameters in Cdkn1b knockout (Cdkn1b(-/-)) mice. LC proliferation was measured by bromodeoxyuridine incorporation. Testicular testosterone levels, mRNA levels, and enzyme activities of steroidogenic enzymes were compared in Cdkn1b(-/-) and Cdkn1b(+/+) mice. The labeling index of LCs in Cdkn1b(-/-) mice was 1.5% +/- 0.2%, almost 7-fold higher than 0.2% +/- 0.08% (P < 0.001) in the Cdkn1b(+/+) control mice. LC number per testis in Cdkn1b(-/-) mice was 2-fold that seen in the Cdkn1b(+/+) control mice. However, testicular testosterone levels, mRNA levels of steroidogenic acute regulatory protein (Star), cholesterol side-chain cleavage enzyme (Cyp11a1), and 3beta-hydroxtsteroid dehydrogenase 6 (Hsd3b6), and their respective proteins, were significantly lower in Cdkn1b(-/-) mice. We conclude that deficiency of CDKN1B increased LC proliferation, but decreased steroidogenesis. Thus, CDKN1B is an important regulator of LC development and function.

In utero and lactational exposures to diethylhexyl-phthalate affect two populations of Leydig cells in male Long-Evans rats

Diethylhexylphthalate (DEHP) has been classified as an antiandrogen. However, whether in utero and lactational exposures of DEHP affect Leydig cells has not been well established. In the present study, the effects of DEHP exposures on fetal Leydig cells (FLCs) and adult Leydig cells (ALCs) were assessed. Pregnant dams of Long-Evans rats were treated with 0, 10, and 750 mg/kg body weight DEHP from Gestational Day 12.5 to Postnatal Day (PND) 21.5. Fetal Leydig cell clustering and FLC-specific gene expression were examined. Anogenital distances (AGDs) of male pups were assessed at PND 2. Serum testosterone levels of male pups and mRNA levels of ALC-specific genes were measured at PNDs 21 and 49. The AGDs of male pups were significantly shorter in the group treated with 750 mg/kg DEHP (mean +/- SEM, 3.68 +/- 0.16 mm) compared with control (4.62 +/- 0.13 mm). The FLCs were aggregated after 10 and 750 mg/kg DEHP exposures. Several FLC-specific genes, including luteinizing hormone receptor (Lhcgr) and steroidogenic enzyme genes, were downregulated at both doses. Serum testosterone levels were significantly lower compared with control at PND 21 after treatment of 10 or 750 mg/kg DEHP, and continued to be lower even up to 49 days postpartum at the higher dose. The mRNA levels for Lhcgr and steroidogenic enzyme genes were significantly lower at both doses of DEHP at PND 21, whereas there were no significant differences for these genes at PND 49. In conclusion, in utero and continued lactational exposures to DEHP exert long-term disruption of steroidogenesis of ALCs.

Gene expression during development of fetal and adult Leydig cells

In rats and mice, Leydig cells are formed as two morphologically and functionally different generations. The first generation develops in utero, from undifferentiated stem Leydig cells (SLCs) that differentiate into fetal Leydig cells (FLCs). After birth, SLCs that may differ from the fetal SLCs undergo lineage-specific commitment and give rise to adult Leydig cells (ALCs). The intermediates of ALCs first become apparent by day 11 postpartum. These first-appearing intermediates, progenitor Leydig cells (PLCs), are spindle shaped and identifiable as steroidogenic because they express luteinizing hormone receptor (LHR) and 3beta-hydroxysteroid dehydrogenase (3betaHSD). The next step in the transition of PLCs to ALCs is the appearance of the immature Leydig cells (ILCs), most commonly seen in the testis during days 28 to 56 postpartum. ILCs have a more abundant smooth endoplasm reticulum (SER), the network of membranes providing a scaffold for steroidogenic enzyme localization, compared to PLCs, but are considered immature because they secrete higher levels of 5alpha-reduced androgen than testosterone. ILCs undergo a final division before ALC steroidogenic function matures by postnatal day 56. ALCs mark the point of maximum differentiation, and at this stage, the Leydig cell secretes testosterone at the highest rate. In this review, trends of gene expression during development of the two Leydig-cell generations, and recent information from gene profiling by microarray, are evaluated. The expression profiles are distinct, indicating that FLCs and ALCs may originate from separate pools of stem cells.

Development of a cryopreservation protocol for Leydig cells

BACKGROUND

In the present study, we describe a procedure to cryopreserve the postnatal members of the Leydig cell lineage, including progenitor (PLC), immature (ILC) and adult (ALC) Leydig cells from, respectively 21-, 35- and 90-day-old rats.

METHODS

The cells were resuspended in a culture medium supplemented with 1% bovine serum albumin (Dulbecco's Modified Eagle's Medium [DMEM]/F12) to a final concentration of 2 x 10(6)cells/ml and the effects of varying concentrations of dimethylsulfoxide (DMSO) (5, 10, 15 or 20%) were assessed after freezing at -70 degrees C and then storing in liquid nitrogen. After 12 months of frozen storage, these cells were thawed rapidly at 37 degrees C and Trypan Blue exclusion staining and attachment to culture dishes were assessed as measures of viability.

RESULTS

The trypan blue exclusion and attachment rates for Leydig cell stages were around 85% in the presence of 15% DMSO. After frozen storage, Leydig cell steroidogenic capacity in response to a range of LH doses, (0.01-100 ng/ml) was unchanged compared with freshly isolated control cells. Furthermore, the steady-state mRNA levels for Leydig cell specific transcripts were maintained.

CONCLUSIONS

This study demonstrates that purified rat Leydig cells at a range of developmental stages can be frozen and that the cryopreserved cells retain normal function.

Biphasic Effects of Postnatal Exposure to Diethylhexylphthalate on the Timing of Puberty in Male Rats

Phthalate esters such as di(2-ethylhexyl)phthalate (DEHP), which are commonly found in cosmetics and in flexible plastics distributed by the food, construction, and medical products industries, have been classified as anti-androgens. High-dose DEHP exposure in utero is associated with decreased androgen levels. However, when administered after birth, low doses of DEHP (eg, 10 mg/kg body weight) may stimulate androgen production. In the present study, the potential of phthalate exposure to advance or delay the timing of puberty was assessed. Male Long-Evans rat pups were chronically subjected to low or high doses of DEHP, with the androgen-driven process of preputial separation serving as an index of pubertal timing. Rats were treated with 0, 10, 500, or 750 mg/kg body weight DEHP for 28 days starting at day 21 postpartum. The average age at which the animals completed preputial separation was measured in each group. The age of preputial separation was 41.5 +/- 0.1 days postpartum in controls (vehicle). The 10 mg/kg DEHP dose advanced pubertal onset significantly to 39.7 +/- 0.1 days postpartum, whereas the 750 mg/kg DEHP dose delayed pubertal onset to 46.3 +/- 0.1 days postpartum. The 10 mg/kg DEHP dose also significantly increased serum testosterone (T) levels (3.13 +/- 0.37 ng/mL) and seminal vesicle weights (0.33 +/- 0.02 g) compared with control serum T (1.98 +/- 0.20 ng/mL) and seminal vesicle weight (0.26 +/- 0.02 g), while the 750 mg/kg dose decreased serum T (1.18 +/- 0.18 ng/mL) as well as testes and body weights. Direct action of the DEHP metabolite, monoethylhexylphthalate (MEHP), on Leydig cell steroidogenic capacity was investigated in vitro. MEHP treatment at a low concentration (100 microM) increased luteinizing hormone-stimulated T production, whereas 10 mM concentrations were inhibitory. In conclusion, data from the present study indicate that DEHP has a biphasic effect on Leydig cell function, with low-dose exposure advancing the onset of puberty. High doses of DEHP, which are anti-androgenic, may also be outside the range of real environmental exposure levels.

Testosterone production in mice lacking inducible nitric oxide synthase expression is sensitive to restraint stress

Immobilization stress (IMO) induces a rapid increase in glucocorticoid secretion [in rodents, corticosterone CORT)] and this is associated with decreased circulating testosterone (T) levels. Nitric oxide (NO), a reactive free radical and neurotransmitter, has been reported to be produced at higher rates in tissues such as brain during stress. The biosynthesis of T is also known to be dramatically suppressed by NO. Specifically, the inducible isoform of nitric oxide synthase (iNOS) was directly implicated in this suppression. To assess the respective roles of CORT and NO in stress-mediated inhibition of T production, adult wild-type (WT) and inducible nitric oxide synthase knockout (iNOS(-/-)) male mice were evaluated. Animals of each genotype were assigned to either basal control or 3-h IMO groups. Basal plasma and testicular T levels were equivalent in both genotypes, whereas testicular weights of mutant mice were significantly higher compared with WT animals. Exposure to 3-h IMO increased plasma CORT and decreased T concentrations in mice of both genotypes. Testicular T levels were also affected by stress in WT and mutant males, being sharply reduced in both genotypes. However, the concentrations of nitrite and nitrate, the stable metabolites of NO measured in testicular extracts, did not differ between control and stressed WT and iNOS(-/-) mice. These results support the hypothesis that CORT, but not NO, is a plausible candidate to mediate rapid stress-induced suppression of Leydig cell steroidogenesis.

In search of rat stem Leydig cells: identification, isolation, and lineage-specific development

Leydig cells (LCs) are thought to differentiate from spindle-shaped precursor cells that exhibit some aspects of differentiated function, including 3beta-hydroxysteroid dehydrogenase (3betaHSD) activity. The precursor cells ultimately derive from undifferentiated stem LCs (SLCs), which are postulated to be present in testes before the onset of precursor cell differentiation. We searched for cells in the neonatal rat testis with the abilities to: (i) proliferate and expand indefinitely in vitro (self renew); (ii) differentiate (i.e., 3betaHSD and ultimately synthesize testosterone); and (iii) when transplanted into host rat testes, colonize the interstitium and subsequently differentiate in vivo. At 1 week postpartum, spindle-shaped cells were seen in the testicular interstitium that differed from the precursor cells in that they were 3betaHSD-negative, luteinizing hormone (LH) receptor (LHR)-negative, and platelet-derived growth factor receptor alpha (PDGFR alpha)-positive. These cells were purified from the testes of 1-week-old rats. The cells contained proteins known to be involved in LC development, including GATA4, c-kit receptor, and leukemia inhibitory factor receptor. The putative SLCs expanded over the course of 6 months while remaining undifferentiated. When treated in media that contained thyroid hormone, insulin-like growth factor I, and LH, 40% of the putative SLCs came to express 3betaHSD and to synthesize testosterone. When transplanted into host rat testes from which LCs had been eliminated, the putative SLCs colonized the interstitium and subsequently expressed 3betaHSD, demonstrating their ability to differentiate in vivo. We conclude that these cells are likely to be the sought-after SLCs.

Stimulation of testosterone production in rat Leydig cells by aldosterone is mineralocorticoid receptor mediated

The testis is known to be a site of corticosterone action, and testosterone production in Leydig cells is directly inhibited by glucocorticoids. Glucocorticoids bind to both glucocorticoid receptors (GRs) and to mineralocorticoid receptors (MRs). In Leydig cells, selective mineralocorticoid binding could result from oxidative inactivation of glucocorticoid by type 1 and/or 2 11beta-hydroxysteroid dehydrogenase (11betaHSD), as both isoforms are expressed. However, it remains unclear whether Leydig cells express MRs and respond directly to mineralocorticoid action. Therefore, the aims of the present study were to ascertain: (1) whether MR mRNA, protein and receptor binding are present in Leydig cells; and (2) if the mineralocorticoid modulates testosterone production. The mRNA encoding MR, as well as protein, and binding activity were each observed in adult rat Leydig cells. MR-ligand binding specificity within isolated Leydig cells was evaluated further by measuring displacement of MR binding to aldosterone by corticosterone in the presence and absence of carbenoxolone, an inhibitor of 11betaHSD1 and 2 that decreases conversion to biologically inert 11-dehydrocorticosterone. Carbenoxolone inhibited 11betaHSD oxidative activity, and reduced corticosterone-binding by 50%. Mineralocorticoid effects on steroidogenesis were assessed in the presence of aldosterone (0.01-10 nM) with or without the MR antagonist, RU28318. Aldosterone induced dose-dependent increases in both basal and luteinizing hormone-stimulated testosterone production. RU28318 eliminated the increase, indicating that these effects of aldosterone were mediated by the MR. The effects of aldosterone and luteinizing hormone (0.1 ng/ml) on testosterone production were synergistic, suggesting that the two hormones increased steroidogenesis through separate pathways. We conclude that Leydig cells express MRs and that testosterone production is subject to regulation by aldosterone.

Paracrine modulation of androgen synthesis in rat leydig cells by nitric oxide

The free radical nitric oxide (NO), generated through the oxidation of L-arginine to L-citrulline by NO synthases (NOSs), has been shown to inhibit steroidogenic pathways. NOS isoforms are known to be present in rat and human testes. Our study examined the sensitivity of Leydig cells to NO and determined whether NOS activity resides in Leydig cells or in another cell type such as the testicular macrophage. The results showed a low level of L-[14C]arginine conversion in purified rat Leydig cell homogenates. Administration of the NOS inhibitor L-N(G)-nitro-arginine methyl ester (L-NAME), or the calcium chelator ethylenebis (oxyethylenenitrilo)tetraacetic acid (EGTA), had no effect on L-[14C]citrulline accumulation. Increased intracellular Ca2+ concentrations that were induced by a calcium ionophore, or the addition of luteinizing hormone (LH), failed to affect NO formation in intact cells that were cultured in vitro. Introduction of a high concentration of the NO precursor L-arginine did not decrease testosterone (T) production, and NOS inhibitors did not increase T biosynthesis. However, exposing Leydig cells to low concentrations of the NO donor S-nitrosoglutathione (GSNO) induced a dramatic blockade of T production under basal and LH-stimulated conditions. DNA array assays showed a low level of expression of endothelial NOS (eNOS), while the neuronal and inducible isoforms of NOS (nNOS and iNOS) were below detection levels. Reverse transcriptase-polymerase chain reaction (RT-PCR) analyses confirmed these findings and demonstrated the presence of high iNOS messenger RNA (mRNA) levels in activated testicular macrophages that produced large amounts of NO. These data suggest that, while T production in rat Leydig cells is highly sensitive to NO and an endogenous NO-generating system is not present in these cells, NOS activity is more likely to reside in activated testicular macrophages.

11{beta}-Hydroxysteroid dehydrogenase 2 in rat leydig cells: its role in blunting glucocorticoid action at physiological levels of substrate

Corticosterone (CORT) suppresses Leydig cell steroidogenesis by inhibiting the expression of proteins involved in testosterone biosynthesis including steroidogenic acute regulatory protein and steroidogenic enzymes. In most cells, intracellular glucocorticoid levels are controlled by either or both of the two known isoforms of 11beta-hydroxysteroid dehydrogenase (11beta HSD): the nicotinamide adenine dinucleotide phosphate reduced-dependent low-affinity type I 11beta HSD (11beta HSD1) oxidoreductase and the nicotinamide adenine dinucleotide-dependent 11beta HSD2 high-affinity unidirectional oxidase. In Leydig cells, 11beta HSD1 alone may not be sufficient to prevent glucocorticoid-mediated suppression due to its low affinity for CORT at basal concentrations. The high-affinity unidirectional 11beta HSD2, if also present, may be critical for lowering intracellular CORT levels. In the present study, we showed that 11beta HSD2 is present in rat Leydig cells by PCR amplification, immunohistochemical staining, enzyme histochemistry, immunoprecipitation, and Western blotting. Real-time PCR showed a 6-fold enrichment of 11beta HSD2 mRNA in these cells, compared with whole testis and that the amount of 11beta HSD2 message was about 1000-fold lower, compared with 11beta HSD1. Diffuse immunofluorescent staining of 11beta HSD2 protein in the Leydig cell cytoplasm was consistent with its localization in the smooth endoplasm reticulum. 11beta HSD1 or 11beta HSD2 activities were selectively inhibited using antisense methodology: inhibition of 11beta HSD1 lowered reductase activity by 60% and oxidation by 25%, whereas inhibition of 11beta HSD2 alone suppressed oxidase activity by 50%. This shows that the high-affinity, low-capacity 11beta HSD2 isoform, present at only one thousandth the level of the low-affinity isoform may significantly affect the level of CORT. The inhibition of either 11beta HSD1 or 11beta HSD2 significantly lowered testosterone production in the presence of CORT. These data suggest that both types I and II 11beta HSD in Leydig cells play a protective role, opposing the adverse effects of excessive CORT on testosterone production.

Rapid glucocorticoid mediation of suppressed testosterone biosynthesis in male mice subjected to immobilization stress

Physical and psychosocial stress challenge homeostasis, increasing glucocorticoid secretion (in rodents, corticosterone [CORT]) while decreasing testosterone (T) levels. The dynamics of stress-induced changes in T, CORT, and luteinizing hormone (LH) concentrations in mice have not been investigated previously. In particular, it remains to be established whether there is a rapid effect of CORT that is directly mediated by glucocorticoid receptors (GRs) in the testis. Therefore, serum and intratesticular T, serum CORT, and LH levels were measured during acute immobilization (IMO) stress, using the C57BL/6 strain of mice. The effects of testicular GR blockade were investigated by administration of the GR antagonist, RU486, via intratesticular (IT) or intraperitoneal (IP) injection. CORT levels increased in stressed males starting at 15 minutes, reaching a fivefold higher plateau by 1 hour compared with controls (P < .01). Conversely, starting from 30 minutes on, both serum and intratesticular T levels decreased in stressed males to 30% and 8% of control values, respectively, by 6 hours (P < .01). In contrast, LH was unchanged by IMO stress for up to 6 hours. Intratesticular treatment with RU486 partially prevented the IMO-induced decline in T levels. CORT treatment reduced intracellular cyclic adenosine monophosphate (cAMP) content in Leydig cells by 15 minutes and T production by 30 minutes in vitro. We conclude that 1) the rapid changes in T suggest a suppression of T biosynthesis by glucocorticoid through a nongenomic mechanism, lowering the production of cytoplasmic cAMP; 2) changes in gonadotropic stimulation of Leydig cells are unlikely to explain the suppression of T levels during acute stress; and 3) the results are consistent with a direct inhibitory action of CORT on Leydig cells.

Gene expression in rat leydig cells during development from the progenitor to adult stage: a cluster analysis

The postnatal development of Leydig cells can be divided into three distinct stages: initially they exist as fibroblast-like progenitor Leydig cells (PLCs) appearing in the testis by Days 14-21; subsequently, by Day 35, they become immature Leydig cells (ILCs) acquiring steroidogenic organelle structure and enzyme activities but metabolizing most of the testosterone they produce; finally, as adult Leydig cells (ALCs) by Day 90, they actively produce testosterone. The factors controlling proliferation and differentiation of Leydig cells remain largely unknown, and the aim of the present study was to identify changes in gene expression during development through cDNA array analysis of PLCs, ILCs, and ALCs. By cluster analysis, it was determined that the transitions from PLC to ILC to ALC were associated with downregulation of mRNAs corresponding to 107 genes. The downregulated genes included cell-cycle regulators, e.g., cyclin D1 (Ccnd1); growth factors, e.g., basic fibroblast growth factor (Fgf2); growth-factor-related receptors, e.g., platelet-derived growth factor alpha receptor (Pdgfra); oncogenes, e.g., kit oncogene (Kit); and transcription factors, e.g., early growth response 1 (Egr1). Conversely, expression levels of 264 genes were increased by at least twofold. Most of these were related to differentiated function and included steroidogenic enzymes, e.g., 11beta-hydroxysteroid dehydrogenase 2 (Hsd11b2); neurotransmitter receptors, e.g., acetylcholine receptor nicotinic alpha 4 (Chrna4); stress response factors, e.g., glutathione transferase 8 (Gsta4); and protein turnover enzymes, e.g., tissue inhibitor of metalloproteinase 2 (Timp2). The detection of Hsd11b2 mRNA in the array was the first indication that this gene is expressed in Leydig cells, and parallel increases in Hsd11b2 mRNA and enzyme activity were recorded. Thus, gene profiling demonstrates that postnatal development is associated with changes in the expression levels of several different clusters of genes consistent with the processes of Leydig cell growth and differentiation.

Inhibition of testicular steroidogenesis by the xenoestrogen bisphenol A is associated with reduced pituitary luteinizing hormone secretion and decreased steroidogenic enzyme gene expression in rat Leydig cells

Exposure of humans to bisphenol A (BPA), a monomer in polycarbonate plastics and a constituent of resins used in food packaging and dentistry, is significant. In this report exposure of rats to 2.4 microg/kg.d (a dose that approximates BPA levels in the environment) from postnatal d 21-35 suppressed serum LH (0.21 +/- 0.05 ng/ml; vs. control, 0.52 +/- 0.04; P < 0.01) and testosterone (T) levels (1.62 +/- 0.16 ng/ml; vs. control, 2.52 +/- 0.21; P < 0.05), in association with decreased LHbeta and increased estrogen receptor beta pituitary mRNA levels as measured by RT-PCR. Treatment of adult Leydig cells with 0.01 nm BPA decreased T biosynthesis by 25% as a result of decreased expression of the steroidogenic enzyme 17alpha-hydroxylase/17-20 lyase. BPA decreased serum 17beta-estradiol levels from 0.31 +/- 0.02 ng/ml (control) to 0.22 +/- 0.02, 0.19 +/- 0.02, and 0.23 +/- 0.03 ng/ml in rats exposed to 2.4 microg, 10 microg, or 100 mg/kg.d BPA, respectively, from 21-35 d of age (P < 0.05) due to its ability to inhibit Leydig cell aromatase activity. Exposures of pregnant and nursing dams, i.e. from gestation d 12 to postnatal d 21, decreased T levels in the testicular interstitial fluid from 420 +/- 34 (control) to 261 +/- 22 (P < 0.05) ng/ml in adulthood, implying that the perinatal period is a sensitive window of exposure to BPA. As BPA has been measured in several human populations, further studies are warranted to assess the effects of BPA on male fertility.

Müllerian-inhibiting substance inhibits rat Leydig cell regeneration after ethylene dimethanesulphonate ablation

The postnatal development of Leydig cell precursors is postulated to be controlled by Sertoli cell secreted factors, which may have a determinative influence on Leydig cell number and function in sexually mature animals. One such hormone, Mullerian inhibiting substance (MIS), has been shown to inhibit DNA synthesis and steroidogenesis in primary Leydig cells and Leydig cell tumor lines. To further delineate the effects of MIS on Leydig cell proliferation and steroidogenesis, we employed the established ethylene dimethanesulphonate (EDS) model of Leydig cell regeneration. Following EDS ablation of differentiated Leydig cells in young adult rats, recombinant MIS or vehicle was delivered by intratesticular injection for 4 days (Days 11-14 after EDS). On Days 15 and 35 after EDS (1 and 21 days post-MIS injections), endocrine function was assessed and testes were collected for stereology, immunohistochemistry, and assessment of proliferation and steroidogenesis. Although serum testosterone and luteinizing hormone (LH) were no different, intratesticular testosterone was higher on Day 35 in MIS-treated animals. At both time points, intratesticular 5alpha-androstan-3alpha,17beta-diol concentrations were much higher than that of testosterone. MIS-treated animals had fewer mesenchymal precursors on Day 15 and fewer differentiated Leydig cells on Day 35 with decreased numbers of BrdU+ nuclei. Apoptotic interstitial cells were observed only in the MIS-treated testes, not in the vehicle-treated group on Day 15. These data suggest that MIS inhibits regeneration of Leydig cells in EDS-treated rats by enhancing apoptotic cell death as well as by decreasing proliferative capacity.

A role for kit receptor signaling in Leydig cell steroidogenesis

Kit and its ligand, Kitl, function in hematopoiesis, melanogenesis, and gametogenesis. In the testis, Kitl is expressed by Sertoli cells and Kit is expressed by spermatogonia and Leydig cells. Kit functions are mediated by receptor autophosphorylation and subsequent association with signaling molecules, including phosphoinositide (PI) 3-kinase. We previously characterized the reproductive consequences of blocking Kit-mediated PI 3-kinase activation in KitY(719F)/Kit(Y719F) knockin mutant male mice. Only gametogenesis was affected in these mice, and males are sterile because of a block in spermatogenesis during the spermatogonial stages. In the present study, we investigated effects of the Kit(Y719F) mutation on Leydig cell development and steroidogenic function. Although the seminiferous tubules in testes of mutant animals are depleted of germ cells, the testes contain normal numbers of Leydig cells and the Leydig cells in these animals appear to have undergone normal differentiation. Evaluation of steroidogenesis in mutant animals indicates that testosterone levels are not significantly reduced in the periphery but that LH levels are increased 5-fold, implying an impairment of steroidogenesis in the mutant animals. Therefore, a role for Kit signaling in steroidogenesis in Leydig cells was sought in vitro. Purified Leydig cells from C57Bl6/J male mice were incubated with Kitl, and testosterone production was measured. Kitl-stimulated testosterone production was 2-fold higher than that in untreated controls. The Kitl-mediated testosterone biosynthesis in Leydig cells is PI 3-kinase dependent. In vitro, Leydig cells from mutant mice were steroidogenically more competent in response to LH than were normal Leydig cells. In contrast, Kitl-mediated testosterone production in these cells was comparable to that in normal cells. Because LH levels in mutant males are elevated and LH is known to stimulate testosterone biosynthesis, we proposed a model in which serum testosterone levels are controlled by elevated LH secretion. Leydig cells of mutant males, unable to respond effectively to Kitl stimulation, initially produce lower levels of testosterone, reducing testosterone negative feedback on the hypothalamic-pituitary axis. The consequent secretion of additional LH, under this hypothesis, causes a restoration of normal levels of serum testosterone. Kitl, acting via PI 3-kinase, is a paracrine regulator of Leydig cell steroidogenic function in vivo.

Trends of reproductive hormones in male rats during psychosocial stress: role of glucocorticoid metabolism in behavioral dominance

Stress in socially subordinate male rats, associated with aggressive attacks by dominant males, was studied in a group-housing context called the visible burrow system (VBS). It has been established that subordinate males have reduced serum testosterone (T) and higher corticosterone (CORT) relative to dominant and singly housed control males. The relationship of the decreased circulating T levels in subordinate males to changes in serum LH concentrations has not been evaluated previously. Since decreases in LH during stress may cause reductions in Leydig cell steroidogenic activity, the present study defined the temporal profiles of serum LH, T, and CORT in dominant and subordinate males on Days 4, 7, and 14 of a 14-day housing period in the VBS. The same parameters were followed in serum samples from single-housed control males. Leydig cells express glucocorticoid receptors and may also be targeted for direct inhibition of steroidogenesis by glucocorticoid. We hypothesize that Leydig cells are protected from inhibition by CORT at basal concentrations through oxidative inactivation of glucocorticoid by 11beta-hydroxysteroid dehydrogenase (11betaHSD). However, Leydig cell steroidogenesis is inhibited when 11betaHSD metabolizing capacity is exceeded. Therefore, 11betaHSD enzyme activity levels were measured in Leydig cells of VBS-housed males at the same time points. Significant increases in LH and T relative to control were observed in the dominant animals on Day 4, which were associated with the overt establishment of behavioral dominance as evidenced by victorious agonistic encounters. Serum LH and T were lower in subordinate males on Day 7, but T alone was lower on Day 14, suggesting that lowered LH secretion in subordinates may gradually be reversed by declines in androgen-negative feedback. Serum CORT levels were higher in subordinate males compared to control at all three time points. In contrast, oxidative 11betaHSD activity in Leydig cells of dominant males was higher relative to control and unchanged in subordinates. These results suggest the following: 1) failure of Leydig cells of subordinate males to compensate for increased glucocorticoid action during stress, by increasing 11betaHSD oxidative activity, potentiates stress-mediated reductions in T secretion; and 2) an inhibition of the reproductive axis in subordinate males at the level of the pituitary.

Prenatal exposure to dexamethasone alters Leydig cell steroidogenic capacity in immature and adult rats

This study examines the effects of prenatal exposure to dexamethasone (DEX) on postnatal testosterone production in male rats. Pregnant female rats were treated on gestation days 14-19 with DEX (100 microg/kg body weight per day; n = 9) or vehicle (n = 9). Results show that 35-day-old male offspring from DEX-treated pregnant females (n = 42) had decreased levels of serum testosterone (45.6% lower, P < .05) compared with control offspring (n = 43), although serum luteinizing hormone (LH) levels were not significantly altered. These findings suggest that a direct programming of developing gonadal cells occurs in response to high levels of maternal glucocorticoid. Indeed, testosterone production was significantly reduced in Leydig cells isolated from immature offspring of DEX-treated pregnant females compared with controls (48.3%, P < .001), and LH stimulation of these cells did not compensate for the lowered steroidogenic capacity. The hypothalamic-pituitary-adrenal axis was also affected, because significant reductions in both serum adrenocorticotropic hormone (ACTH; 26.2%, P < .001) and corticosterone (CORT; 32.3%, P < .001) were measured in DEX-exposed immature male offspring. In contrast, adult male offspring from DEX-treated dams had significantly higher levels of serum ACTH (39.2%, P <. 001) and CORT (37.8%, P < .001). These same animals had higher serum testosterone (31.6%, P < or = .05) and a significant reduction in serum LH (30.8%, P < .001). Moreover, Leydig cells isolated from these adult offspring exhibited an increased capacity for testosterone biosynthesis under basal (38.6%, P < .001) and LH-stimulated conditions (33.5%, P < .001). In summary, sustained changes in steroidogenic capacity were observed in male rats exposed to high levels of glucocorticoid during prenatal development. More specifically, DEX exposure in utero perturbed Leydig cell testosterone production in both pubertal and adult rats.

Modulation of rat Leydig cell steroidogenic function by di(2-ethylhexyl)phthalate

Exposure of rodents to phthalates is associated with developmental and reproductive anomalies, and there is concern that these compounds may be causing adverse effects on human reproductive health. Testosterone (T), secreted almost exclusively by Leydig cells in the testis, is the primary steroid hormone that maintains male fertility. Leydig cell T biosynthesis is regulated by the pituitary gonadotropin LH. Herein, experiments were conducted to investigate the ability of di(2-ethylhexyl)phthalate (DEHP) to affect Leydig cell androgen biosynthesis. Pregnant dams were gavaged with 100 mg(-1) kg(-1) day(-1) DEHP from Gestation Days 12 to 21. Serum T and LH levels were significantly reduced in male offspring, compared to control, at 21 and 35 days of age. However, these inhibitory effects were no longer apparent at 90 days. In a second set of experiments, prepubertal rats, from 21 or 35 days of age, were gavaged with 0, 1, 10, 100, or 200 mg(-1) kg(-1) day(-1) DEHP for 14 days. This exposure paradigm affected Leydig cell steroidogenesis. For example, exposure of rats to 200 mg(-1) kg(-1) day(-1) DEHP caused a 77% decrease in the activity of the steroidogenic enzyme 17beta-hydroxysteroid dehydrogenase, and reduced Leydig cell T production to 50% of control. Paradoxically, extending the period of DEHP exposure to 28 days (Postnatal Days 21-48) resulted in significant increases in Leydig cell T production capacity and in serum LH levels. The no-observed-effect-level and lowest-observed-effect-level were determined to be 1 mg(-1) kg(-1) day(-1) and 10 mg(-1) kg(-1) day(-1), respectively. In contrast to observations in prepubertal rats, exposure of young adult rats by gavage to 0, 1, 10, 100, or 200 mg(-1) kg(-1) day(-1) DEHP for 28 days (Postnatal Days 62-89) induced no detectable changes in androgen biosynthesis. In conclusion, data from this study show that DEHP effects on Leydig cell steroidogenesis are influenced by the stage of development at exposure and may occur through modulation of T-biosynthetic enzyme activity and serum LH levels.

Müllerian-inhibiting substance type II receptor expression and function in purified rat Leydig cells

Müllerian-inhibiting substance (MIS), a gonadal hormone in the transforming growth factor-beta superfamily, induces Müllerian duct involution during male sexual differentiation. Mice with null mutations of the MIS ligand or receptor develop Leydig cell hyperplasia and neoplasia in addition to retained Müllerian ducts, whereas MIS-overexpressing transgenic mice have decreased testosterone concentrations and Leydig cell numbers. We hypothesized that MIS directly modulates Leydig cell proliferation and differentiated function in the maturing testis. Therefore, highly purified rat Leydig and Sertoli cells were isolated to examine cell-specific expression, binding, and function of the MIS type II receptor. These studies revealed that this receptor is expressed abundantly in progenitor (21-day) and immature (35-day) Leydig cells as well as in Sertoli cells. Prepubertal progenitor Leydig cells exhibit high affinity (Kd = 15 nM), saturable binding of MIS. No binding, however, is detected with either peripubertal immature Leydig cells or Sertoli cells at either age. Moreover, progenitor, but not immature Leydig cells, respond to MIS by decreasing DNA synthesis. These data demonstrate that functional MIS type II receptors are expressed in progenitor Leydig cells and support the hypothesis that MIS has a direct role in the regulation of postnatal testicular development.

Increased proliferation of Leydig cells induced by neonatal hypothyroidism in the rat

Treatment of male rat pups with the reversible goitrogen 6-n-propyl-2-thiouracil (PTU), administered by adding 0.1% PTU to the mother's drinking water from birth through weaning, increases adult testis size, number of Sertoli cells, and daily sperm production. Adult Leydig cell numbers are also increased by 70%, despite permanent suppression of serum luteinizing hormone (LH). The present study was designed to test whether this effect results from an increase in proliferation of Leydig cells or the mesenchymal precursors. The labeling indices (LI) of six interstitial cell types were measured by tritiated thymidine autoradiography. With the exception of Leydig cells, LI declined after birth for all interstitial cell types in both control and PTU-treated rats, but mesenchymal cell proliferation was not significantly different in control and treated rats. In contrast, the LI of Leydig cells from PTU-treated rats rose by day 10, remained elevated through day 45 (highest on day 35 at 3.6-fold higher than control, P < 0.05), and declined to control values by day 50. The LI of Leydig cells in control rats was unchanged during the experiment. Proliferating Leydig cells were immature, as shown by their cytoplasmic lipid droplets. Adult Leydig cells, which lack lipid droplets, did not proliferate. Mesenchymal and immature Leydig cells contained thyroid hormone receptor mRNA; levels in adult Leydig cells were reduced but detectable. In conclusion, proliferation of Leydig cells, rather than increased proliferation of their mesenchymal precursors, is the principal mechanism responsible for the increase in Leydig cell number after neonatal hypothyroidism. Increased Leydig cell proliferation could result from direct effects of hypothyroidism on these cells. Alternatively, the increased Sertoli cell population that results from PTU treatment could also stimulate increased Leydig cell proliferation, or both of these mechanisms could contribute to the Leydig cell increase.