A brief exposure to cadmium impairs Leydig cell regeneration in the adult rat testis

Fibroblast growth factor homologous factor 1 stimulates Leydig cell regeneration from stem cells in male rats

Fibroblast growth factor homologous factor 1 (FHF1) is an intracellular protein that does not bind to cell surface fibroblast growth factor receptor. Here, we report that FHF1 is abundantly present in Leydig cells with up‐regulation during its development. Adult male Sprague Dawley rats were intraperitoneally injected with 75 mg/kg ethane dimethane sulphonate (EDS) to ablate Leydig cells to initiate their regeneration. Then, rats daily received intratesticular injection of FHF1 (0, 10 and 100 ng/testis) from post‐EDS day 14 for 14 days. FHF1 increased serum testosterone levels without affecting the levels of luteinizing hormone and follicle‐stimulating hormone. FHF1 increased the cell number staining with HSD11B1, a biomarker for Leydig cells at the advanced stage, without affecting the cell number staining with CYP11A1, a biomarker for all Leydig cells. FHF1 did not affect PCNA‐labelling index in Leydig cells. FHF1 increased Leydig cell mRNA (Lhcgr, Scarb1, Star, Cyp11a1, Hsd3b1, Cyp17a1, Hsd17b3, Insl3, Nr5a1 and Hsd11b1) and their protein levels in vivo. FHF1 increased preadipocyte biomarker Dlk1 mRNA level and decreased fully differentiated adipocyte biomarker (Fabp4 and Lpl) mRNA and their protein levels. In conclusion, FHF1 promotes Leydig cell regeneration from stem cells while inhibiting the differentiation of preadipocyte/stem cells into adipocytes in EDS‐treated testis.

In utero exposure to bisphenol A disrupts fetal testis development in rats

Bisphenol A (BPA) is widely used in consumer products and is a potential endocrine disruptor linked with abnormal development of male reproductive tract. However, its action and its effects on the pathways in the development of male gonad are still unclear. Here we report that effects of BPA exposure during gestation on male gonad development. Sprague-Dawley rats were gavaged daily with BPA (0, 4, 40, and 400 mg/kg body weight) from gestational day 12 to day 21. BPA dose-dependently decreased serum testosterone levels (0.45 ± 0.08 ng/ml and 0.32 ± 0.08 ng/ml for 40 and 400 mg/kg BPA, respectively) versus the control level (1.11 ± 0.22 ng/ml, Mean ± SE). BPA lowered Leydig cell Insl3 and Hsd17b3 mRNA and their protein levels at doses of 40 and 400 mg/kg. BPA also lowered Leydig cell (Lhcgr, Cyp11a1, and Cyp17a1) and Sertoli cell (Amh) mRNA and their protein levels at 400 mg/kg. BPA decreased fetal Leydig cell number via inhibiting their proliferation, but it did not affect fetal Sertoli cell number. In conclusion, the current study shows that in utero exposure to BPA inhibits fetal Leydig and Sertoli cell differentiation, possibly disrupting the development of male reproductive tract.

Fibroblast growth factor 16 stimulates proliferation but blocks differentiation of rat stem Leydig cells during regeneration

Objectives

We aim to investigate the effects of fibroblast growth factor 16 (FGF16) on Leydig cell regeneration in ethane dimethane sulphonate (EDS)‐treated rat testis.

Methods

We intraperitoneally inject 75 mg/kg EDS to adult male Sprague Dawley rats and then intratesticularly inject FGF16 (0, 10 and 100 ng/testis/day) from post‐EDS day 14 for 14 days. We investigate serum hormone levels, Leydig cell number, gene and protein expression in vivo. We also explore the effects of FGF16 treatment on stem Leydig cell proliferation in vitro.

Results

FGF16 lowers serum testosterone levels (21.6% of the control at a dose of 100 ng/testis) without affecting the levels of luteinizing hormone (LH) and follicle‐stimulating hormone (FSH) on post‐EDS day 28 in vivo. FGF16 increases Leydig cell number at doses of 10 and 100 ng/mg without affecting Sertoli cell number, increases the percentage of PCNA‐positive Leydig cells, and down‐regulates the expression of Leydig cell genes (Lhcgr, Scarb1, Star, Cyp11a1, Cyp17a1 and Hsd17b3) and Sertoli cell genes (Fshr, Dhh and Sox9) and their proteins in vivo. FGF16 increases phosphorylation of AKT1 and AKT2 as well as EKR1/2 in vivo, indicating that it possibly acts via AKT1/ATK2 and ERK1/2 pathways. FGF16 also lowers medium testosterone levels and down‐regulates the expression of Leydig cell genes (Lhcgr, Scarb1, Star, Cyp11a1, Cyp17a1 and Hsd17b3) but increases EdU incorporation into stem Leydig cells in vitro.

Conclusions

These data suggest that FGF16 stimulates stem and progenitor Leydig cell proliferation but blocks their differentiation, thus lowering testosterone biosynthesis.

Dehydroepiandrosterone and Its CYP7B1 Metabolite 7α-Hydroxydehydroepiandrosterone Regulates 11β-Hydroxysteroid Dehydrogenase 1 Directions in Rat Leydig Cells

Background: The purpose of this study was to investigate cytochrome P450-7B1 (CYP7B1) in the human and rat testes to regulate 11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) activity. We hypothesized that dehydroepiandrosterone (DHEA) and its product 7α-hydroxydehydroepiandrosterone (7αOHD) after catalysis of CYP7B1 played a critical role in driving the direction of 11β-HSD1, because 7αOHD is an alternative substrate for 11β-HSD1. Methods: We examined the influence of DHEA and 7αOHD on 11β-HSD1 activities in both intact Leydig cells and microsomes using radioactive substrates and identified the location of CYP7B1 in Leydig cells using immunohistochemical staining, Western blot, and qPCR. Results: We found that DHEA stimulated 11β-HSD1 oxidase activity in intact cells (EC₅₀ = 0.97 ± 0.11 μM) and inhibited its reductase activity (IC₅₀ = 1.04 ± 0.06 μM). In microsomes, DHEA was a competitive inhibitor of the reductase activity. The 11β-HSD1 oxidase activity in intact cells was inhibited by 7αOHD (IC₅₀ = 1.18 ± 0.12 μM), and the reductase activity was enhanced (EC₅₀ = 0.7 ± 0.04 μM). 7αOHD was a competitive inhibitor of 11β-HSD1 oxidase. CYP7B1 was present in rat Leydig cells, as shown by immunohistochemistry, Western blotting, and qPCR analysis. Conclusion: Our results are consistent with a conclusion that DHEA in the circulation driving 11β-HSD1 toward an oxidase in Leydig cells mainly through inhibiting the reductase of the enzyme, while 7αOHD (CYP7B1 catalytic product of DHEA) drives the enzyme toward the opposite direction.

Fibroblast Growth Factor 1 Promotes Rat Stem Leydig Cell Development

Fibroblast growth factor 1 (FGF1) is reported to be expressed in the testis. How FGF1 affects stem Leydig cell development remains unclear. Here, we report the effects of FGF1 on rat stem Leydig cell development in an ethane dimethane sulfonate (EDS)-treated model. FGF1 (100 ng/testis) significantly increased serum testosterone level, increased PCNA-positive Leydig cell percentage and Leydig cell number, but down-regulated the expression of Lhcgr, Star, Cyp11a1, Hsd3b1, Cyp17a1, and Hsd11b1 in Leydig cells per se, after its daily intratesticular injection from post-EDS day 14 for 14 days. Primary culture of the seminiferous tubules showed that FGF1 stimulated EdU incorporation to stem Leydig cells but blocked the differentiation into the Leydig cell lineage, possibly via FGFR1-mediated mechanism. In conclusion, FGF1 promotes stem Leydig cell proliferation but blocks its differentiation.

Delayed Puberty by Ziram Is Associated with Down Regulation of Testicular Phosphorylated AKT1 and SIRT1/PGC-1α Signaling

Ziram is a dimethyldithiocarbamate fungicide, which may influence the male reproductive system as a potential endocrine disruptor. We interrogated the disruption of ziram on rat progenitor Leydig cell development. Prepubertal male Sprague-Dawley rats were orally treated with 0, 2, 4, or 8 mg/kg ziram for 2 weeks. We investigated the effects of ziram on serum testosterone levels, Leydig cell number, and Leydig and Sertoli cell gene and protein expression, SIRT1/PGC-1α levels, and phosphorylation of AKT1, ERK1/2, and AMPK in vivo. We also interrogated the effects of ziram on reactive oxidative species (ROS) level, apoptosis rate, and mitochondrial membrane potential of progenitor Leydig cells in vitro. Ziram decreased serum testosterone and follicle-stimulating hormone levels, the down-regulated Leydig cell-specific gene ( Lhcgr, Scarb1, Star, Cyp17a1, and Hsd17b3), and their protein expression. However, ziram stimulated anti-Müllerian hormone production. Ziram lowered SIRT1/PGC-1α and phosphorylated protein levels of AKT1. Ziram induced ROS and apoptosis and lowered the mitochondrial membrane potential of progenitor Leydig cells in vitro. In conclusion, ziram disrupts Leydig cell development during the prepubertal period potentially through the SIRT1/PGC-1α and phosphorylated AKT1 signaling.

4-Bromodiphenyl ether delays pubertal Leydig cell development in rats

Polybrominated diphenyl ethers are a class of brominated flame retardants that are potential endocrine disruptors. 4-Bromodiphenyl ether (BDE-3) is the most abundant photodegradation product of higher polybrominated diphenyl ethers. However, whether BDE-3 affects Leydig cell development during puberty is still unknown. The objective of this study was to explore effects of BDE-3 on the pubertal development of rat Leydig cells. Male Sprague Dawley rats (35 days of age) were gavaged daily with BDE-3 (0, 50, 100, and 200 mg/kg body weight/day) for 21 days. BDE-3 decreased serum testosterone levels (1.099 ± 0.412 ng/ml at a dose of 200 mg/kg BDE-3 when compared to the control level (2.402 ± 0.184 ng/ml, mean ± S.E.). BDE-3 decreased Leydig cell size and cytoplasmic size at a dose of 200 mg/kg, decreased Lhcgr, Star, Dhh, and Sox9 mRNA levels at ≥ 100 mg/kg and Scarb1, Cyp11a1, Hsd17b3, and Fshr at 200 mg/kg. BED-3 also decreased the phosphorylation of AKT1, AKT2, ERK1/2, and AMPK at 100 or 200 mg/kg. BDE-3 in vitro induced ROS generation, inhibited androgen production, down-regulated Lhcgr, Scarb1, Star, Cyp11a1, Hsd3b1, Srd5a1, and Akr1c14 expression in immature Leydig cells after 24-h treatment. In conclusion, the current study indicates that BDE-3 disrupts Leydig cell development via suppressing AKT, ERK1/2, and AMPK phosphorylation and inducing ROS generation.

Dicyclohexyl phthalate blocks Leydig cell regeneration in adult rat testis

Dicyclohexyl phthalate (DCHP) is a phthalate plasticizer with a ring structure in the alcohol moiety. The objective to the current study was to determine the effects of DCHP on Leydig cell regeneration in the adult rat-testis. Adult male Sprague Dawley rats received intraperitoneally an injection of ethane dimethane sulfone (EDS) to eliminate all Leydig cells in the testis and then were divided into 4 groups of 0 (control), 10, 100, and 1000 mg/kg/day DCHP. Rats were gavaged either vehicle (corn oil, control) or DCHP from post-EDS day 7 to day 21 and 28. On post-EDS day 21 and day 28, rats were euthanized and serum testosterone, luteinizing hormone (LH), follicle-stimulating hormone (FSH) levels were measured, and Leydig cell number, cell size, gene, and protein expression were evaluated. During the course of exposure, DCHP did not cause the general toxicity to rats. On post-EDS day 21, DCHP significantly increased serum testosterone level at 10 and 100 mg/kg and increased Leydig cell number at 10 mg/kg via stimulating their mitosis. On post-EDS day 28, DCHP lowered serum testosterone levels and Leydig cell number at 1000 mg/kg. DCHP dose-dependently down-regulated the expression of many Leydig cell genes (Lhcgr, Scarb1, Star, Cyp11a1, Hsd3b1, Cyp17a1, Hsd17b3, Hsd11b1, and Insl3) and their proteins, especially at 1000 mg/kg. DCHP also lowered the pAKT1/AKT1 and pERK1/2/ERK1/2 ratios. In conclusion, DCHP at low doses (10 and 100 mg/kg) increased Leydig cell number during the initial regeneration and inhibited Leydig cell regeneration during the course of its exposure.

In utero exposure to triphenyltin disrupts rat fetal testis development

Triphenyltin is an organotin that is widely used as an anti-fouling agent and may have endocrine-disrupting effects. The objective of the current study was to investigate effects of triphenyltin on the development of rat fetal testis. Female pregnant Sprague Dawley dams were gavaged daily with triphenyltin (0, 0.5, 1, and 2 mg/kg body weight/day) from gestational day 12 to day 21. Triphenyltin dose-dependently decreased serum testosterone levels (0.971 ± 0.072 and 0.972 ± 0.231 ng/ml at 1 and 2 mg/kg, respectively) from control level (2.099 ± 0.351 ng/ml). Triphenyltin at 1 and 2 mg/kg doses also induced fetal Leydig cell aggregation, decreased fetal Leydig cell size and cytoplasmic size. Triphenyltin decreased the expression levels of Lhcgr, Scarb1, Star, Cyp11a1, Cyp17a1, Insl3, Fshr, Pdgfa, and Sox9 by 0.5 mg/kg dose and above. However, triphenyltin did not affect Leydig and Sertoli cell numbers. In conclusion, the current study indicated that in utero exposure of triphenyltin disrupted fetal Leydig and Sertoli cell development.

Lambda-cyhalothrin delays pubertal Leydig cell development in rats

Lambda-cyhalothrin (LCT) is a widely used broad-spectrum pyrethroid insecticide and is expected to cause deleterious effects on the male reproductive system. However, the effects of LCT on Leydig cell development during puberty are unclear. The current study addressed these effects. Twenty-eight-day-old male Sprague Dawley rats orally received LCT (0, 0.25, 0.5 or 1 mg/kg body weight/day) for 30 days. The levels of serum testosterone, luteinizing hormone, and follicle-stimulating hormone, Leydig cell number, and its specific gene and protein expression were determined. LCT exposure lowered serum testosterone levels at doses of 0.5 and 1 mg/kg and luteinizing hormone levels at a dose of 1 mg/kg, but increased follicle-stimulating hormone levels at doses of 0.5 and 1 mg/kg. LCT lowered Star and Hsd3b1 mRNA or their protein levels at a dose of 1 mg/kg. Immature Leydig cells were purified from pubertal rats and treated with different concentrations of LCT for 24 h and medium androgen levels, Leydig cell mRNA and protein levels, the mitochondrial membrane potential (△Ψm), and the apoptotic rate of immature Leydig cells were investigated. LCT inhibited androgen production at 5 μM and downregulated Scarb1 at 0.05 μM, Hsd3b1 and Hsd11b1 at 0.5 μM, and Cyp11a1 at 5 μM. LCT also decreased △Ψm at 0.5 and 50 μM. In conclusion, LCT can influence the function of Leydig cells.

Oncostatin M inhibits differentiation of rat stem Leydig cells in vivo and in vitro

Oncostatin M (OSM) is a pleiotropic cytokine within the interleukin six family of cytokines, which regulate cell growth and differentiation in a wide variety of biological systems. However, its action and underlying mechanisms on stem Leydig cell development are unclear. The objective of the present study was to investigate whether OSM affects the proliferation and differentiation of rat stem Leydig cells. We used a Leydig cell regeneration model in rat testis and a unique seminiferous tubule culture system after ethane dimethane sulfonate (EDS) treatment to assess the ability of OSM in the regulation of proliferation and differentiation of rat stem Leydig cells. Intratesticular injection of OSM (10 and 100 ng/testis) from post-EDS day 14 to 28 blocked the regeneration of Leydig cells by reducing serum testosterone levels without affecting serum luteinizing hormone and follicle-stimulating hormone levels. It also decreased the levels of Leydig cell-specific mRNAs (Lhcgr, Star, Cyp11a1, Hsd3b1, Cyp17a1 and Hsd11b1) and their proteins by the RNA-Seq and Western blotting analysis. OSM had no effect on the proliferative capacity of Leydig cells in vivo. In the seminiferous tubule culture system, OSM (0.1, 1, 10 and 100 ng/mL) inhibited the differentiation of stem Leydig cells by reducing medium testosterone levels and downregulating the expression of Leydig cell-specific genes (Lhcgr, Star, Cyp11a1, Hsd3b1, Cyp17a1 and Hsd11b1) and their proteins. OSM-mediated action was reversed by S3I-201 (a STAT3 antagonist) or filgotinib (a JAK1 inhibitor). These data suggest that OSM is an inhibitory factor of rat stem Leydig cell development.

In utero exposure to hexavalent chromium disrupts rat fetal testis development

Hexavalent chromium (Cr⁶⁺) acts as an endocrine disruptor. Herein, we investigated effects of Cr⁶⁺ on the development of rat fetal Leydig and Sertoli cells, which support differentiation of the male reproductive tract in late gestation. Female pregnant Sprague Dawley rats were gavaged with potassium dichromate (0, 3, 6, and 12 mg/kg) from gestational days (GD) 12 to GD 21. Leydig and Sertoli cell function was evaluated by investigating serum testosterone levels, cell number and distribution, and the expression levels of Leydig and Sertoli cell genes and proteins. Cr⁶⁺ increased serum testosterone level at dose of 3 mg/kg (1.170 ± 0.121 ng/ml vs. 0.720 ± 0.082 ng/ml in the control), while lowered it at dose of 12 mg/kg (0.400 ± 0.098 ng/ml). In addition, it showed that Cr⁶⁺ dose-dependently reduced Leydig cell size and cytoplasmic size and decreased the percentage of medium fetal Leydig cell cluster at dose of 12 mg/kg. Further study demonstrated that the expression of Leydig cell (Lhcgr, Scarb1, and Hsd3b1) and Sertoli cell (Fshr, Pdgfa, and Lif) genes in the testis was upregulated at dose of 3 mg/kg while the expression of Lhcgr, Hsd17b3 and Igf1 was downregulated by Cr⁶⁺ at dose of 12 mg/kg. In conclusion, Cr⁶⁺ had biphasic effects on fetal Leydig cell development with low dose to stimulate testosterone production and high dose to inhibit it, possibly via biphasically regulating growth factor gene expression in fetal Sertoli cells.

Interleukin 6 inhibits the differentiation of rat stem Leydig cells

Inflammation causes male hypogonadism. Several inflammatory cytokines, including interleukin 6 (IL-6), are released into the blood and may suppress Leydig cell development. The objective of the present study was to investigate whether IL-6 affected the proliferation and differentiation of rat stem Leydig cells. Leydig cell-depleted rat testis (in vivo) and seminiferous tubules (in vitro) with ethane dimethane sulfonate (EDS) were used to explore the effects of IL-6 on stem Leydig cell development. Intratesticular injection of IL-6 (10 and 100 ng/testis) from post-EDS day 14 to 28 blocked the regeneration of Leydig cells, as shown by the lower serum testosterone levels (21.6% of the control at 100 ng/testis dose), the down-regulated Leydig cell gene (Lhcgr, Star, Cyp11a1, Cyp17a1, and Hsd17b3) expressions, and the reduced Leydig cell number. Stem Leydig cells on the surface of the seminiferous tubules were induced to enter the Leydig cell lineage in vitro in the medium containing luteinizing hormone and lithium. IL-6 (1, 10, and 100 ng/ml) concentration-dependently decreased testosterone production and Lhcgr, Cyp11a1, Cyp17a1, Hsd17b3 and Insl3 mRNA levels. The IL-6 mediated effects were antagonized by Janus kinase 1 (JAK) inhibitor (filgotinib) and Signal Transducers and Activators of Transcription 3 (STAT3) inhibitor (S3I-201), indicating that a JAK-STAT3 signaling pathway is involved. In conclusion, our results demonstrated that IL-6 was an inhibitory factor of stem Leydig cell development.

Triphenyltin Chloride Delays Leydig Cell Maturation During Puberty in Rats

Triphenyltin chloride (TPT) is present in a wide range of human foods. TPT could disrupt testis function as a potential endocrine disruptor of Leydig cells. However, the effect of TPT on pubertal Leydig cell development is still unclear. The objective of the current study was to explore whether exposure to TPT affected Leydig cell developmental process and to clarify the underlying mechanisms. Male Sprague-Dawley rats at 35 days of age were randomly divided into four groups and received normal corn oil (control), 0.5, 1, or 2 mg/kg/day TPT for 18 days. Immature Leydig cells isolated from 35-day-old rat testes were treated with TPT (10 and 100 nM) for 24 h in vitro. In vivo exposure to ≥0.5 mg/kg TPT lowered serum testosterone levels and lowered Star mRNA. TPT at 2 mg/kg also lowered Lhcgr, Cyp11a1, Hsd3b1, Hsd17b3 as well as pAKT1/AKT1, pAKT2/AKT2, and pERK1/2/ERK1/2 ratios. In vitro exposure to TPT (100 nM) increased ROS production and induced cell apoptosis rate in rat immature Leydig cells. In conclusion, TPT exposure disrupts Leydig cell development possibly via interfering with the phosphorylation of AKT1, AKT2, and ERK1/2 kinases.

Diverged Effects of Piperine on Testicular Development: Stimulating Leydig Cell Development but Inhibiting Spermatogenesis in Rats

Background: Piperine is the primary pungent alkaloid isolated from the fruit of black peppercorns. Piperine is used frequently in dietary supplements and traditional medicines. The objective of the present study was to investigate the effects of piperine on the testis development in the pubertal rat.

Methods: Piperine (0 or 5 or 10 mg/kg) was gavaged to 35-day-old male Sprague-Dawley rats for 30 days. Serum levels of testosterone (T), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were measured. The development of adult Leydig cell population was also analyzed 65 days postpartum. For in vitro studies, immature Leydig cells were isolated from 35-day-old male rats and treated with 50 μM piperine in the presence of different steroidogenic stimulators/substrates for 24 h.

Results: Thirty-day treatment of rats with piperine significantly increased serum T levels without affecting LH concentrations. However, piperine treatment reduced serum FSH levels. Consistent with increase in serum T, piperine increased Leydig cell number, cell size, and multiple steroidogenic pathway proteins, including steroidogenic acute regulatory protein, cholesterol side-chain cleavage enzyme, 3β-hydroxysteroid dehydrogenase 1, 17α-hydroxylase/20-lyase, and steroidogenic factor 1 expression levels. Piperine significantly increased the ratio of phospho-AKT1 (pAKT1)/AKT1, phosphos-AKT2 (pAKT2)/AKT2, and phospho-ERK1/2 (pERK1/2)/ERK1/2 in the testis. Interestingly, piperine inhibited spermatogenesis. Piperine in vitro also increased androgen production and stimulated cholesterol side-chain cleavage enzyme and 17α-hydroxylase/20-lyase activities in immature Leydig cells.

Conclusion: Piperine stimulates pubertal Leydig cell development by increasing Leydig cell number and promoting its maturation while it inhibits spermatogenesis in the rat. ERK1/2 and AKT pathways may involve in the piperine-mediated stimulation of Leydig cell development.

In utero single low-dose exposure of cadmium induces rat fetal Leydig cell dysfunction

Cadmium chloride (Cd) is a potent endocrine disruptor and may cause the malformation in the male reproductive tract. However, the effects of a single in utero exposure to low doses of Cd on fetal Leydig cell development are still unknown. The objective of this study is to investigate the effects of a single in utero exposure to low doses of Cd on rat fetal Leydig cell development. Adult 64-day-old Sprague-Dawley dams received a single intraperitoneal injection of 0, 0.25, 0.5, and 1.0 mg/kg Cd on gestational day 12. Cd dose-dependently reduced testosterone production of fetal testis, lowered fetal Leydig cell numbers, downregulated protein expression levels of Leydig (LHCGR, SCARB1, STAR, CYP11A1, HSD3B1, and CYP17A1), and Sertoli cells (HSD17B3, DHH, and FSHR). In conclusion, our results demonstrated that a single in utero exposure to low doses of Cd blocked fetal Leydig cell development.

Parathyroid Hormone-Related Protein Promotes Rat Stem Leydig Cell Differentiation

The regulatory factors for stem Leydig cell development are largely unknown. Herein, we reported that parathyroid hormone-related protein (PTHrP) may be a factor to regulate this process. The effects of PTHrP on rat stem Leydig cell proliferation and differentiation were investigated using a stem Leydig cell culture system and an ethane dimethane sulfonate (EDS)-treated in vivo Leydig cell regeneration model. PTHrP (1,000 pg/ml) significantly increased medium testosterone level and up-regulated STAR, CYP17A1, and 17β-HSD3 expressions. Co-treatment with PKA inhibitor H-89 or PKC inhibitor U73122 reversed PTHrP-mediated increase of testosterone production in vitro. Intratesticular injection of PTHrP (100 ng/testis) into the Leydig cell-depleted testis from post-EDS day 7 to 21 significantly increased serum testosterone level, up-regulated LHCGR, SCARB1, CYP11A1, 11β-HSD1, and CYP17A1 expressions. It also enlarged Leydig cell size without affecting PCNA-labeled Leydig cell number. This indicates that PTHrP promotes stem Leydig cell differentiation. PTHrP in vivo increased CREB and p-CREB levels, suggesting that PTHrP acts via a PKA-CREB signaling pathway. In conclusion, PTHrP stimulates stem Leydig cell differentiation without affecting its proliferation, showing its novel action and mechanism on rat stem Leydig cell development.