Diabetes Mellitus Causes Male Reproductive Dysfunction: A Review of the Evidence and Mechanisms

Diabetes Mellitus Affects Male Reproduction

DM affects male reproduction in four conditions, including erectile dysfunction, ejaculation, structural changes in reproductive organs, and changes in the semen quality.

Studies have found that 59% of diabetic men have erectile dysfunction (ED). The vast majority of patients with diabetic impotence (DMED) have penile nerve thickening or beaded neuropathy. The decrease in serum testosterone (T) caused by DM also affects vascular endothelial function. Nitric oxide (NO) is an important neurotransmitter in penile erection, and the relaxation effect of NO on smooth muscle cells in corpus cavernosum and microvessels of the penis is at the core of penile erection (1). Hyperglycemia leads to increased levels of reactive oxygen species (ROS), increased advanced glycation end products (AGEs), inhibition of endothelial nitric oxide synthase (eNOS) metabolism, and a decrease in endothelial synthesis and the release of NO, that leads to ED. Neuropathy causes a decrease in NO and an increase in vasoconstrictor endothelin (ET) levels, which are related to erectile function. DM causes metabolic changes in sorbitol metabolism bypass, arteriosclerosis, and disturbance in neurotrophic supply (2), resulting in glycogen deposition, thickening of the basal membrane of somatic nerve sheath cells, and disintegration of axons leading to sensory and motor nerve damage, while the demyelination of pelvic sympathetic nerves leads to the inhibition of sympathetic nerves and eventually DMED (3).

The mechanism of ejaculation is coordinated by pelvic sympathetic nerve fibers. Peripheral neuropathy affects the pelvic sympathetic nerve resulting in the weakening of bladder neck sphincter contraction, while abnormal contraction of the external urethral sphincter leads to the obstruction of semen discharge and return to the bladder, which is called retrograde ejaculation. The appearance of retrograde ejaculation symptoms indicates that diabetic neuropathy is very serious. Unexplained retrograde ejaculation should suggest the possibility of diabetes.

As for the diabetes-induced structural and functional changes in reproductive organs, a decrease in testicular blood flow velocity has been detected in diabetic rats by Doppler (4). It is believed that the decrease in vascular endothelial growth factor (VEGF) expression leads to the impairment of vascular endothelial cell (EC) function, and microcirculation disturbance leads to testicular morphological and structural changes. DM can delay gonadal development in immature rats, decrease sexual behavior and testosterone (T) synthesis, and promote gonadal atrophy. Atrophy of seminiferous tubules (STs), thinning of spermatogenic epithelium, and the rate of empty tubules are higher. In some STs, multinucleated cells with two or three nuclei are found, the vascular degeneration and germ cell apoptosis are increased, and the content of cytoplasmic debris in Sertoli cells near the ST cavity is increased. Ultrastructure has shown that Sertoli cells have irregular basement membranes and the numbers of Sertoli cells is significantly reduced. Also, the oval nuclei in Sertoli cells become irregular and the smooth endoplasmic reticulum in these cells is decreased. Likewise, Leydig cells have irregular nuclei, abundant heterochromatin, and lipid droplets. In addition, the structure of Leydig cell is damaged, its mitochondria are swollen, and endoplasmic reticulum is decreased with dilatation. Liu and colleagues (5) have also found that the amount of sperm in epididymis is decreased significantly, the epididymal epithelium shows vacuole like changes, and a part of the cell nucleus is dissolved and ruptured. Further, mitochondria are found to be transformed into spiral and/or doughnut like organelles, and the number of vesicular organelles is reduced.

The main DM-caused changes in the semen quality are the decrease in sperm density and motility and the increase in sperm DNA fragmentation and apoptosis (6-8). In diabetic rats the mitochondria of spermatogonia show vacuolar changes, the transformation of spermatogonia to primary spermatocytes is decreased, the number of inactive spermatogonia is increased, and the number of epididymal sperm is decreased significantly. DM also affects sperm DNA integrity. Compared with healthy controls, 52% of men with diabetes have mitochondrial DNA deletions. The increase in sperm DNA fragmentation, advanced glycation end products (AGEs) and receptor for advanced glycation end products (RAGEs), as well as the changes in spermatogenic genes, all lead to the decrease in sperm quality. NO can inactivate superoxide anion, protect sperm membrane from being damaged by lipid peroxide, stabilize cell and lysosomal membranes, increase cGMP content in sperm cells, and facilitate sperm activation and capacitation. NO regulates both sperm motility and sperm lipid peroxidation and affects spermatogenesis. Endothelin (ET) is a vasoconstrictor peptide, which plays an important role in sperm maturation. ET and NO are closely related to male reproductive hormone secretion and spermatogenesis. NO can inhibit the secretion of ET. Deng and coworkers (9) have shown that ET levels in diabetic patients are increased significantly, whereas NO levels are decreased significantly, and ET/NO imbalance leads to endothelial function damage. Due to dyslipidemia, hypertension, and NO synthesis impairment, VEGF expression is decreased, and vascular basement membrane thickening leads to testicular microcirculation disorder, which affects spermatogenesis and capacitation. NOS plays an important role in regulating androgen levels in Leydig cells. INS signaling regulates the number of Sertoli cells as well as testicular size and sperm production. The regulation of the proliferation of immature Sertoli cells by follicle stimulating hormone (FSH) also requires INS signaling pathway. Furthermore, the lack of INSR in Sertoli cells inhibits the activation of the Akt signaling pathway and decreases the expression of the follicle stimulating hormone receptor (FSHR) gene. Some studies have used lentiviral vectors to achieve stable expression of INS in Sertoli cells for the purpose of developing gene therapy (10).

Effect of Oxidative Stress on Male Reproduction in Diabetes Mellitus

Reactive oxygen species (ROS) is the main cause of cell damage, and one of its main sources is mitochondria. The American Diabetes Association points out that oxidative stress (OS) is a common mechanism of diabetic complications, and OS and microcirculation disorders are the main causes of reproductive system damage. OS can cause insulin resistance (IR) by activating protein kinase C (11), N-terminal kinase pathway (12), and transcription factors (13). Long-term blood sugar stimulation leads to up-regulation of the number of mitochondria in tissue cells, and lipid peroxidation causes damage to the antioxidant defense system. Long-term hyperglycemia in vivo up-regulates the level of mitochondria in tissue cells. The increase in oxygen radicals and AGEs and their interaction with NO induces the secretion of peroxynitrite anion (ONOO -) leading to DNA damage. Hyperglycemia then interacts with NO in vascular endothelial cells to secrete peroxynitrite anion (ONOO -) leading to DNA damage. DNA repair enzymes are then activated in response to DNA damage, and tissue damage is eventually caused by protein kinases and advanced glycation end products (14). Because mitochondrial DNA is almost entirely composed of coding regions, it is easily targeted by ROS because there are no protective mechanisms for mitochondrial DNA. ROS can directly interfere with mitochondrial DNA and RNA replication, resulting in mitochondrial structure damage and dysfunction (15) and decreased glucose oxidation and ATP production, which is a characteristic of IR and hyperglycemia, and mitochondrial damage in turn leads to ROS production. Due to excessive oxidation products, the aldose pathway is activated, and as a result, glucose is converted into sorbitol, nicotinamide adenine dinucleotide phosphate (NADPH) is consumed, and glutathione (GSH) levels are decreased further exacerbating OS. Oxidative damage of islet β cells results in their apoptosis and reduction in their number. Testosterone (T) is positively correlated with mitochondrial function and INS sensitivity. In men, the decrease of T affects mitochondrial function and promotes the occurrence and progress of diabetes (16). T can regulate the levels of glucose metabolism by protecting mitochondrial function (17), while OS can induce dominant lethal mutations in male gametes and lead to embryo death early in pregnancy.

Effect of Abnormal Zinc Metabolism in Males With Diabetes Mellitus

Zinc is a powerful antioxidant which can stabilize and maintain the integrity of sperm cell membrane structure by resisting lipid oxidation of sperm membrane. Semen and testis are rich in zinc. Seminal plasma zinc is positively correlated with free T levels, and zinc regulates T secretion. The concentration of zinc in islet β-cells is high, and the highest zinc concentration is within INS containing secretory granules in islet β-cells. The distribution of zinc in tissues and body fluids of patients with DM is disturbed. Studies have shown that polyuria in type 2 diabetes mellitus (T2DM) may lead to increased excretion of zinc from urine resulting in decrease of plasma zinc concentration. Zinc deficiency may induce SOD mutant conformation and cause endoplasmic reticulum (ER) chronic stress, OS, DNA damage, apoptosis, hypogonadism, spermatogenesis disorder, and sperm morphological defects. Furthermore, the testis and epididymis maintain their function through zinc uptake. Taravati et al. (18) have shown that the concentration of zinc in seminal plasma of asthenospermia is decreased significantly, which suggested the correlation between zinc and sperm motility. During ejaculation, sperm can obtain zinc ions from seminal plasma and improve its motility. Zinc is an important mediator of signal transduction (19) and has been shown to enhance the effect of INS (20). Cellular zinc homeostasis is maintained by a large number of zinc transporters (ZnTs) in cells. ZnTs have been shown to function in reducing cell damage, and zinc homeostasis is precisely regulated by zinc transporters (5). Impaired zinc homeostasis underlies the pathogenesis of many human diseases (1, 2). Altered expression of ZnTs not only interferes with zinc homeostasis in organelles, but also leads to organelle dysfunction. In T2DM, the expression of ZnT genes is significantly reduced. Is there an association between zinc homeostasis and islet β-cells (21)? All members of the SLC30 family (ZnTs) are expressed in human islet β-cells, and all ZnTs can be detected in human and mouse β-cells except ZnT3. ZnT8 is mainly expressed in adult mouse Leydig cells, and the zinc transporters ZnT1-5, 7, 8, and 10 have important roles in immune response (5, 22). ZnT8 is also expressed in islet β-cells. Over-expression of ZnT8 increases the zinc content of β-cells and promotes glucose-stimulated INS secretion, and ZnT8 also plays a role in T production through the protein kinase A (PKA) signaling pathway. ZnT1 is expressed at the plasma membrane of testicular cells, the cytoplasm of spermatogenic cells, and in Leydig cells. In summary, the disturbance of zinc metabolism can diminish male fertility, and the altered ZnT expression in the gonads of DM patients affects zinc homeostasis.

Role of Hypothalamic Pituitary Gonadal Axis in the Reproduction of Diabetic Men

Spermatogenesis is regulated by the hypothalamic-pituitary-gonadal axis (HPGA), and abnormal HPGA can lead to abnormal spermatogenesis. INS stimulates the expression and secretion of gonadotropin-releasing hormone (GnRH) in the hypothalamus and promotes the function of HPGA (23). In vitro INS also promotes the secretion of gonadotropin (GN) induced by luteinizing hormone-releasing hormone (LHRH). Insufficient secretion of INS in DM patients can damage the HPGA. Glucose metabolism in anterior pituitary cells depends on the secretion of INS. Lack of INS causes glucose utilization disorders, which lead to decreased pituitary protein synthesis and decreased GN secretion. Hyperglycemia interferes with the secretion of GnRH, thereby reducing the secretion of GN and prolactin (PRL), which in turn leads to a significant decrease in the secretion of T from Leydig cells and ultimately to spermatogenesis disorders. Because of the DM-mediated damage of Sertoli cells or Leydig cells, the secretion of T is decreased resulting in an increase in the levels of FSH and LH through the negative feedback effect of HPGA. INS receptors and INS signaling proteins are widely distributed in the central nervous system. There is an important relationship between brain INS signal transduction and reproduction. INS is a messenger linking metabolism and HPGA reproductive function. The neuronal expression of INS receptors plays an important role in HPGA through its effect on LH secretion. The most important pathological changes in reproductive function caused by HPGA sexual endocrine disorder are hyperandrogenemia and IR. In the model of INS receptor knockout, the secretion of sex hormone is insufficient, leading to infertility and metabolic syndrome. INS receptor has different expression patterns in hypothalamus and pituitary. Nirko mice (INS receptor gene neuron specific knockout mice) cannot maintain the function of Leydig cells due to hypothalamic dysfunction and insufficient GN secretion. Approximately 20% of seminiferous tubules (STs) lack lumen and have almost no mature spermatogenic cells, indicating that the fertility of Nirko mice is decreased.

The pathogenesis of male infertility and sexual dysfunction caused by abnormal HPGA may be related to the abnormal INS signaling and the dysregulation of kisspeptin expression in hypothalamus (24). G-protein-coupled receptor 54 (GPR54) combined with kisspeptin functions in hypothalamic GnRH neurons to activate the HPGA axis and cause GnRH secretion. Kisspeptin/GPR54 complex is the key to maintain HPGA function and an important factor determining GnRH secretion (25, 26). Kisspeptin is also a “molecular switch” initiating puberty, and its gene mutation leads to hypogonadotropic hypogonadism (HH). The deficiency of INS reduces kisspeptin protein expression in the hypothalamus, resulting in decrease in GnRH secretion leading to HH. Supplementation of INS can correct the abnormality of GnRH (27). Compared with the peripheral gonads, abnormal levels of INS have a greater impact on hypothalamus and pituitary. Lin et al. (28) demonstrated that the upregulation of kisspeptin expression in the preoptic area can be involved in the stimulation of HPGA function in male rats. A study on the Gnv-3 cell line of GnRH-expressing hypothalamic neurons showed that neurons are INS sensitive tissues and express INS receptors (29). The increased expression of kisspeptin in the preoptic area of the hypothalamus indicates that INS affects the expression of kisspeptin in the Gnv-3 cell line, and the target of rapamycin (mTOR) pathway has a regulatory effect on kisspeptin mRNA expression and gene transcription. In the central nervous system, GnRH expression is mediated through the mTOR signaling pathway. Activation of mTOR stimulates the expression of kisspeptin and increases the secretion of LH. Rapamycin, an inhibitor of mTOR, can block the above effects. In conclusion, INS regulates kisspeptin protein through the mTOR pathway and affects HPGA function, and GnRH regulates reproductive function through kisspeptin and kisspeptin neurons.

Effect of Insulin Resistance on Male Reproduction

Insulin resistance (IR) is an important pathogenic factor and a pathological characteristic of T2DM (10). There is a causal relationship between IR and reproductive hormones: reproductive hormones can affect INS sensitivity and androgens can ameliorate IR and slow down the development of DM. Androgens are closely related to DM (30). T levels are significantly lower in DM patients compared to healthy individuals. The lower the T levels, the higher the incidence rates of DM. IR is the key link between T and DM. Decreases in androgen promote the development of IR and DM. Dehydroepiandrosterone (DHEA) can reduce serum triglycerides, interleukin-6, TNF-α, and other inflammatory factors (31). A prospective cohort study of non-diabetic elderly men shows that the prevalence of DM is lower in patients with higher levels of total T and free T, and sex hormone binding protein (SHBG) still exists after the effect is removed (32). Several groups have found that the levels of total T, free T, and SHBG in DM patients are significantly lower than those in non-DM patients, and T and SHBG are negatively correlated with IR (33, 34). T promotes glucose uptake, glycolysis, and oxidative phosphorylation. Additionally, T ameliorates IR by enhancing glucose transport, inhibiting inflammatory response, improving mitochondrial function, and inhibiting adipocyte and tissue proliferation. These indicate that T plays an important role in maintaining INS sensitivity. In ovariectomized rats, the sensitivity to INS is increased, and androgen supplementation ameliorates IR in and out of the liver (35). There is evidence (36, 37) showing that with the increase of homeostatic model assessment of insulin resistance (HOMA-IR) index, T presents a downward trend, and the percentage of sperm forward movement shows a downward trend (36, 37). There is no significant difference between T and sperm concentration or total number (p>0.05). HOMA-IR is negatively correlated with serum T and progesterone (PR) (p<0.05). However, Verit and colleagues (38) observed that HOMA-IR shows no significant correlation with sperm count, motility, and morphology. Zag (zinc alpha glycoprotein) (39) is a new type of adipokines, which plays an important role in alleviating IR. It can enhance INS signal transduction and affect the expression of adipokines and inflammatory factors. Zag can be synthesized in epithelial cells and secreted into the semen to affect physiological processes such as fertilization.

Progression in the Treatment of Diabetes Mellitus-Related Male Reproductive Dysfunction

It is a challenge to treat DM-related male reproductive dysfunction, and blood sugar control is still the key. In the recent years, great efforts have been made to investigate therapeutic approaches for the improvement and treatment of diabetes mellitus erectile dysfunction (DMED). These include stem cell transplantation, gene therapy, vascularization technology, INS treatment, mecobalamin and lipoic therapy, scutellarin (SCU) therapy, acupuncture and moxibustion, androgen therapy, and antioxidant therapy (40-46).

It has been found that stem cells can have a paracrine effect on surrounding tissues, and can also differentiate into a variety of functional cells such as neurons, endothelium, and smooth muscle, thereby repairing damaged tissues caused by DM and improving erectile function (40). Therefore, stem cell transplantation can be a new treatment method for DMED in the clinic.

Gene therapy is also a new direction. The target genes of DMED patients mainly include vascular endothelial growth factor (VEGF) and NOS genes, and the therapy involving these genes can also help improve erectile function. A recent study shows that gene repair therapy activates related cell signal transduction pathways and induces nerve and vascular regeneration (41). In addition, vascularization technology has been used to achieve cavernous smooth muscle angiogenesis and has broad prospects in DMED treatment.

Regarding the treatment of diabetic retrograde ejaculation, blood sugar control is the key, including intensive INS treatment and recovery of pelvic autonomic nerve function. Mecobalamin can repair nerves, and lipoic acid has antioxidant and endothelial functions. Some recent studies have shown that mecobalamin combined with lipoic acid has better effect on the treatment of diabetic neuropathy; however, Ma et al. showed that mecobalamin combined with lipoic acid does not achieve a better therapeutic effect as compared with each agent alone (42). For patients with fertility requirements, in-vitro fertilization, and embryo transfer (IVF-ET) is highly recommended.

Traditional Chinese medicine (TCM)-related therapy is a promising treatment approach for DMED. As the onset of DM is getting younger, reproductive dysfunction caused by DM has gradually attracted attention. Oxidative stress and vascular microcirculation disorders caused by DM can induce cell apoptosis and cause damage to organ function (43), and it is considered to be the main cause of damage to the reproductive system. Therefore, regulating oxidative stress and improving microcirculation are expected to become important means for the prevention and treatment of diabetic complications. Many natural products have significant antioxidant capacity. Breviscapine can be used as a supplement for the prevention or treatment of diabetic complications. Scutellarin (SCU) is another one. Although SCU cannot regulate blood glucose levels, it reduces oxidative stress and has antioxidant effects. SCU improves microcirculation by increasing VEGF expression, reducing blood viscosity, and inhibiting platelet aggregation, thus reducing testicular damage (44, 45). Acupuncture has been widely used in the treatment of T2DM in China. It has been shown that acupuncture improves IR and increases glucose metabolism in T2DM rats. It has also been confirmed that acupuncture and moxibustion have antioxidant effects, improving the levels and activity of SOD, protecting the morphology and structure of mitochondria, and improving ATP synthesis in mitochondria.

Androgen therapy is another treatment of choice for DMED. Androgen inhibits inflammatory response, improves mitochondrial function, enhances glucose transport through glut (glucose transporter), improves glucose metabolism, and reduces IR. It has been shown that T supplementation can slow down the damage of the reproductive system of diabetics.

Lastly, antioxidant therapy has been shown to be effective in the treatment of DMED. Antioxidant therapy reduces OS-induced damage to sperm via stimulating the secretion of FSH and LH as well as T synthesis. Coenzyme Q10 and N-acetylcysteine improve sperm quality by increasing T and inhibin B; while vitamin C, vitamin E, and glutathione can be used as adjuvant therapy for improving sperm quality. The antioxidant L-carnitine improves the levels of antioxidants in seminal plasma and promotes spermatogenesis and maturation (46). Early antioxidant treatment reduces islet cell damage and promotes islet cell proliferation. Vitamin C and antioxidant SOD can delay the process of islet cell apoptosis. SOD also improves autonomic nerve conduction velocity and restores endothelial function. It has been demonstrated that increasing zinc intake reduces the severity of type 1 diabetes in animal models, and Zag also reduces IR through a variety of mechanisms.

In general, the treatment of DM-induced male reproduction-related complications requires comprehensive treatment, and blood sugar control is the key strategy. Only when blood sugar is well controlled, it can be possible to prevent peripheral vascular disease and diabetic neuropathy. When diabetic retrograde ejaculation and DMED occur, and testicular tissue structure and semen quality are changed, it is very important to control blood sugar, together with other treatment strategies, such as improving lipid metabolism, protecting nerves and blood vessels, as well as zinc supplementation and related antioxidants, and all these are helpful to improve semen quality and male reproductive function. Finally, TCM treatment as well as acupuncture and moxibustion can improve microcirculation, protect blood vessels, and eliminate oxygen free radicals. It has been demonstrated that TCM is an effective treatment for DM-caused male reproductive dysfunction.