The testis is an important male reproductive organ, which ensures reproductive function via the secretion of testosterone and the generation of spermatozoa. Testis development begins in the embryonic period, continues after birth, and generally reaches functional maturation at puberty. The stress‐activated kinase, p38 mitogen‐activated protein kinase (MAPK), regulates multiple cell processes including proliferation, differentiation, apoptosis, and cellular stress responses. p38 MAPK signalling plays a crucial role in testis development by regulating spermatogenesis, the fate determination of pre‐Sertoli, and primordial germ cells during embryogenesis, the proliferation of testicular cells in the postnatal period, and the functions of mature Sertoli and Leydig cells. In addition, p38 MAPK signalling is involved in decreased male fertility when exposed to various harmful stimuli. This review will describe in detail the biological functions of p38 MAPK signalling in testis development and male reproduction, together with its pathological role in male infertility.

Obesity has increased dramatically worldwide, which is associated with male infertility. Androgen deficiency, impaired spermatogenesis, and erectile dysfunction are characteristics of male infertility. The balance of androgens and estrogens is essential for maintaining normal reproductive function in males. Aromatase, the rate-limiting enzyme in the conversion of androgens into estrogens, is present in various tissues. The expression of aromatase is proportional to body fat mass and causes more fat accumulation, thus forming a vicious cycle. Excessive aromatase activity in adipose tissue leads to increased conversion of androgens into estrogens, eventually results in a reduction of testosterone levels and is the underlying reason for obesity-related infertility. In the male reproductive system, all testicular somatic cells and germ cells express aromatase, except for peritubular myoid cells. The results of studies regarding the effect of aromatase in testicular somatic cells and germ cells have been contradictory. The effect of estrogens in testicular somatic cells is inhibitory, leading to reduced testosterone levels and sperm production; however, it has been observed that aromatase participates in the acquisition of sperm motility. The overall effect of estrogen modulation is an inhibition of spermatogenesis. Aromatase inhibitors are an effective therapy for obesity-associated hypogonadism because they restore normal sex hormone levels and improve semen parameters. This article systematically introduces the basic knowledge of aromatase and provides information of the current advances relating to aromatase in male reproductive function. Increasing our knowledge on the role of aromatase in male obesity could help in proposing new approaches to treat infertile men.

Emerging epidemiological studies indicate that hypercholesterolaemia is a risk factor for testosterone deficiency. However, the underlying mechanism is unclear. Testicular Leydig cells are the primary source of testosterone in males. To identify the effect and mechanism of cholesterol overload on Leydig cell function, rats were fed with a HC (HC) diet to induce hypercholesterolaemia. During the 16‐week feeding period, serum testosterone levels were reduced in a time‐dependent manner in rats fed the HC diet. Accordingly, these steroidogenic enzymes within the Leydig cells, including steroidogenic acute regulatory protein (StAR), cholesterol side‐chain cleavage cytochrome P450 (P450scc) and 3β‐hydroxysteroid dehydrogenase (3β‐HSD), were down‐regulated. Notably, the HC‐fed rats showed evident endoplasmic reticulum (ER) stress in the testis, including a dilated ER as an evident pathological change in the Leydig cell ultrastructure, up‐regulated ER stress biomarker (binding immunoglobulin protein) levels and activation of the activating transcription factor 6 (ATF6)‐related unfolded protein response pathway. Further analysis showed that when 4‐phenyl butyric acid (4‐PBA) was used to block ER stress in HC‐fed rats for 8 weeks, the testosterone deficiency was significantly alleviated. Our findings suggested that high dietary cholesterol intake affected serum testosterone levels by down‐regulating steroidogenic enzymes and that activated ER stress might serve as the underlying mechanism.

Background/Aims. Obesity, which is related to increased oxidative stress in various tissues, is a risk factor for male infertility. Metformin is reported to have an antioxidant effect; however, the precise role of metformin in obesity-induced male infertility remains unknown. The current study is aimed at exploring the effects of metformin and characterizing its underlying mechanism in the fertility of obese males. Methods. An obese male mouse model was generated by feeding mice with a high-fat diet; then, the mice were administered metformin in water for 8 weeks. Reproductive ability, metabolic parameters, and follicle-stimulating hormone (FSH) were assessed by cohabitation, enzymatic methods, and ELISA, respectively. Damage to the integrity of the blood-testis barrier (BTB), which ensures spermatogenesis, was assessed by transmission electron microscopy and immunofluorescence with a biotin tracer. Malondialdehyde (MDA), superoxide dismutase (SOD), and reactive oxygen species (ROS) were employed for the assessments of oxidative stress. BTB-related proteins were measured by immunoblotting. Nuclear factor κB (NF-κB) was assessed by immunofluorescence. Results. High-fat-diet-fed mice presented evident lipid metabolic disturbances, disrupted BTB integrity, and decreased reproductive function. Metformin alleviated the decrease in male fertility, decreased ectopic lipid deposition in the testis, and increased serum FSH levels. A further mechanistic analysis revealed that metformin ameliorated the high-fat-diet-induced injury to the BTB structure and permeability and restored the disordered BTB-related proteins, which might be associated with an improvement in oxidative stress and a recovery of NF-κB activity in Sertoli cells (SCs). Conclusion. Metformin improves obese male fertility by alleviating oxidative stress-induced BTB damage. These findings provide new insights into the effect of metformin on various diseases and suggest future possibilities in the treatment of male infertility.

Introduction: Age-related decline in testosterone is associated with Leydig cell aging with impaired testosterone synthesis in aging. Obesity accelerates the age-related decline in testosterone. However, the mechanisms underlying the Leydig cell aging and the effects of obesity on Leydig cell aging remain unclear.Method: Natural aging mice and diet-induced obese mice were used to assess the process of testicular Leydig cell senescence with age or obesity. Bioinformatic analysis of the young and aged human testes was used to explore key genes related Leydig cell aging. Leydig cell-specific p38 MAPK knockout (p38LCKO) mice were used to further analyze the roles of p38 MAPK in Leydig cell aging. The levels of testosterone and steroidogenic enzymes, activity of p38 MAPK, aging status of Leydig cells, and oxidative stress and inflammation of testes or Leydig cells were detected by ELISA, immunoblotting, immunofluorescence, and senescence-associated β-galactosidase (SA-β-Gal) staining analysis, respectively.Result: The serum testosterone level was significantly reduced in aged mice compared with young mice. In the testis of aged mice, the reduced mRNA and protein levels of LHCGR, SRB1, StAR, CYP11A1, and CYP17A1 and the elevated oxidative stress and inflammation were observed. KEGG analysis showed that MAPK pathway was changed in aged Leydig cells, and immunoblotting displayed that p38 MAPK was activated in aged Leydig cells. The intensity of SA-β-Gal staining on Leydig cells and the number of p21-postive Leydig cells in aged mice were more than those of young mice. Similar to aged mice, the testosterone-related indexes decreased, and the age-related indexes increased in the testicular Leydig cells of high fat diet (HFD) mice. Aged p38LCKO mice had higher levels of testosterone and steroidogenic enzymes than those of age-matched wild-type (WT) littermates, with reduced the intensity of SA-β-Gal staining and the expression of p21 protein.Conclusion: Our study suggested that obesity was an important risk factor for Leydig cell aging. p38 MAPK was involved in Leydig cell aging induced by age and obesity. The inhibition of p38 MAPK could delay Leydig cell aging and alleviate decline in testosterone.