Antidiabetic agents and male fertility: unraveling the impact of glucose-lowering therapies on sperm quality in diabetic males—a narrative review

Abstract

Background and Objective

Diabetes mellitus imposes a substantial burden on male fertility, adversely affecting sperm quality through structural damage in the reproductive tract, endocrine disruption, and dysregulation of cellular metabolism. While glucose-lowering agents help achieve glycemic control, they may also ameliorate diabetes-induced impairment of sperm quality. However, these drugs exert heterogeneous effects on male reproduction via divergent mechanisms, with outcomes often varying between in vivo and in vitro settings. This review aims to systematically evaluate the impacts of different antihyperglycemic therapies on semen quality and elucidate their mechanistic bases, to guide therapy selection for improving fertility in diabetic men.

Methods

A comprehensive search of the PubMed and Embase databases was conducted, covering the period from their inception to 2025.

Key Content and Findings

Insulin enhances sperm DNA integrity via Hsp70-2/TP-1/TP-2 upregulation and acts as a cryoprotectant. Metformin activates AMPK-mediated autophagy and synergizes with zinc to preserve fertility. SGLT-2 inhibitors suppress testicular apoptosis via Nrf2/HO-1 antioxidant pathways. Critically, sulfonylureas improve in vivo sperm quality but exert spermicidal effects in vitro by disrupting calcium homeostasis.

Conclusions

Current evidence supports the potential of certain antihyperglycemic agents in mitigating diabetes-related male infertility, though drug-specific and context-dependent effects must be considered. Further phenotype-targeted studies are needed to optimize clinical treatment strategies.

Introduction

Infertility is increasingly recognized as a critical public-health challenge that substantially impedes demographic expansion; its global prevalence currently approximates 10–15%. Concurrently, a recent modelling study forecasts that the age-standardized worldwide prevalence of male infertility will escalate at a markedly accelerated rate compared with that of female infertility during the period 2022–2040 (1). Among these, the population of infertile men concomitantly afflicted with diabetes mellitus is exhibiting an escalating trend (2), and a substantial body of investigations has already delineated the deleterious impact of diabetes on human spermatozoa (3-6). The blood-testis barrier (BTB), one of the most impermeable vascular barriers, exerts stringent control entry of molecules into the seminiferous epithelium. Diabetic insults compromise male fertility via oxidative stress-mediated structural disruption of the BTB, thereby precipitating a perturbed testicular immune milieu. Supporting cells not only constitute the physical framework of the BTB but also deliver indispensable nutritional support for spermatogenic progression; consequently, any diabetes-induced metabolic derangement within these BTB-forming cells can disturb spermatogenesis and ultimately impair sperm quality (7,8). In addition, diabetes mellitus also affects spermatogenesis and development by altering the secretory activity of the hypothalamic-pituitary-testicular (HPT) axis (9,10). In the testicular stage, this impairment is mediated through reduced testosterone secretion (11); in the hypothalamic stage, low insulin levels lead to reduced levels of leptin, and consequently, gonadotropin-releasing hormone (GnRH) secretion, and subsequently, reduced luteinizing hormone (LH) and follicle-stimulating hormone (FSH) signaling to the testis, which in turn causes secondary hypogonadism (12-14). There are particular effects on sperm viability, DNA fragmentation, and chromatin quality (15,16).

In a clinical investigation, both lean and obese diabetic individuals exhibited markedly attenuated conventional semen parameters alongside significantly reduced circulating total testosterone and sex hormone-binding globulin concentrations relative to normoglycemic, normal-weight controls; concurrently, serum leptin concentrations were elevated, whereas serum insulin levels displayed inverse correlations with sperm concentration, viability and morphology (17). In a subsequent case-control study, patients with type 1 diabetes mellitus demonstrated a higher prevalence of infertility and erectile dysfunction, accompanied by impaired conventional semen indices—most notably diminished semen volume, total sperm count and progressive sperm motility. Additionally, type 1 diabetic men exhibited an increased incidence of positive anti-sperm antibodies in seminal plasma (18).

Antihyperglycemic agents, while executing both direct and indirect glycemic control at the systemic level, simultaneously exert precise regulatory effects on sperm viability, morphological integrity, and DNA fragmentation indices. These pharmacotherapeutic entities not only enhance intrinsic semen quality in vivo, but also serve as exogenous “sperm agonists” under in vitro conditions, thereby imparting measurable protective advantages to cryopreserved-thawed spermatozoa. The mechanistic repertoire underpinning these actions encompasses the modulation of testicular redox equilibrium, the preservation of acrosomal membrane structural integrity, and the fine-tuning of mitochondrial bioenergetics within spermatozoa; these phenomena have been comprehensively delineated across both experimental animal paradigms and human clinical studies. As shown in Figure 1, there are mechanisms of the effect of diabetes mellitus on sperm quality in males. We present this article in accordance with the Narrative Review reporting checklist (available at https://tau.amegroups.com/article/view/10.21037/tau-2025-440/rc).

Figure 1.

Mechanisms of the effect of diabetes mellitus on sperm quality in males. BTB, blood-testis barrier; FSH, follicle-stimulating hormone; GnRH, gonadotropin-releasing hormone; HPT, hypothalamic-pituitary-testicular; LH, luteinizing hormone; NO, nitric oxide.

Methods

We conducted a literature review of the most relevant articles on the overview topic within the PubMed and Embase databases, limiting the search scope to the period from database inception to May 2025 to ensure capture of the latest relevant research. Search keywords included “diabetes”, “sperm”, “antidiabetic drugs”, “biguanides”, “sulfonylureas”, “glucagon-like peptide-1 (GLP-1) receptor agonists”, “SGLT-2 inhibitors”, “thiazolidinediones”, and “insulin”. To ensure comprehensive coverage, we manually reviewed reference lists of articles to include additional papers relevant to the topic. Our screening process comprised two stages: title/abstract screening and full-text screening. During the title/abstract screening phase, three independent researchers conducted an initial review of each document’s title and abstract to exclude studies irrelevant to the research topic. In the full-text screening phase, three researchers independently assessed the full text of each article and screened them according to predetermined inclusion and exclusion criteria. The summary of the search strategy is shown in Table 1 (all included studies are compiled in Table S2).

Table 1. The search strategy summary.

Items	Specification
Date of search	June 5, 2025
Databases	PubMed and Embase
Search terms used	“diabetes”, “sperm”, “antidiabetic drugs”, “biguanides”, “sulfonylureas”, “glucagon-like peptide-1 (GLP-1) receptor agonists”, “SGLT-2 inhibitors”, “thiazolidinediones”, and “insulin” (for details on the PubMed database search strategy, please refer to Table S1)
Timeframe	Database inception–May 2025
Inclusion and exclusion criteria	(I) Including randomized controlled trials, clinical trials, animal studies, case reports, and systematic reviews concerning antidiabetic drugs and male sperm quality. (II) Duplicate studies and studies without English versions were excluded
Selection process	During the title/abstract screening phase, Z.L., L.S., Lina Wang conducted an initial review of each document’s title and abstract to exclude studies irrelevant to the research topic. In the full-text screening phase, J.L., L.S., Lina Wang independently assessed the full text of each article and screened them according to predetermined inclusion and exclusion criteria

Insulin

Insulin is a protein hormone secreted by pancreatic β-cells in the pancreatic organ of the body, which dominates the blood glucose regulation of the body. In type I diabetes mellitus, autoimmune- and genetically-mediated destruction of pancreatic β-cells results in an absolute deficiency of insulin secretion that is insufficient to maintain euglycemia (19); conversely, in type II diabetes mellitus, the dysregulation of blood glucose arises from a state of relative insulin secretory insufficiency coupled with peripheral insulin resistance (20). Persistent hyperglycemia constitutes a detrimental prognostic indicator for male seminal quality, whereas exogenous insulin replacement therapy has been demonstrated to elicit favorable and clinically meaningful improvements in sperm parameters.

In an in vivo study, Guo et al. found that insulin can up-regulate the expression of lipocalin (an insulin sensitizing hormone, which can be secreted locally by the testis to regulate testicular function through the autocrine/paracrine pathway) and its receptor and down-regulate oxidative stress and inflammatory cytokine levels, and that lipocalin binds to its receptor and activates and phosphorylates the AMPK, thus regulating the activity of steroidogenic enzymes (6). This regulates the activity of steroidogenic enzymes, which controls the synthesis of steroid hormones and regulates sperm production and reproductive system function, as well as inhibiting the inflammatory response and decreasing inflammatory cytokines in order to increase testosterone secretion by Leydig cells. In addition, it was found that insulin may promote NO release and regulate testicular microcirculation by modulating the Akt/eNOS/NO signaling pathway, exerting a protective effect on the testes of diabetic rats, improving the diabetes-induced low levels of sperm counts, viability, serum and testicular testosterone, as well as lipocalin and its receptor, and decreasing the level of reactive oxygen species (ROS) in the testes of rats (21,22). Meanwhile, it has been shown that insulin intervention in combination with the micronutrients zinc, Omega-3 and moderate-intensity exercise showed better synergistic effects on sperm quality than insulin alone, mainly in terms of sperm morphology, counts, viability, DNA integrity and testosterone production (23-25). When utilizing insulin in combination with other therapies to modulate sperm quality, careful consideration must be given to exposure dose and duration. For instance, low zinc levels are associated with impaired spermatogenesis and male infertility, whereas elevated zinc levels can induce oxidative stress and other deleterious changes contributing to infertility (26). Similarly, moderate-intensity exercise training protects testicular Leydig cells from diabetes-induced apoptosis, thereby maintaining testicular cellular integrity and promoting spermatogenesis. However, when combining exercise with insulin therapy, the antioxidant effects of exercise on sperm quality must be accounted for. The above study indicates that while this exercise-mediated mechanism independently improves spermatogenesis, primarily through enhanced testosterone production, it more significantly potentiates the beneficial effects of insulin on sperm parameters (24).

In in vitro studies, the addition of insulin and leptin to washed human sperm increased sperm total motility, forward motility, and NO production, thereby increasing in vitro sperm fertilization; However, it is worth exploring the fact that, although leptin is involved in the regulation of reproduction-related hormones, its direct effects on sperm motility and sperm morphology are controversial. Lampiao et al. demonstrates that the synergistic effect of leptin and insulin on enhancing sperm fertilization potential primarily manifests through modulation of sperm signaling and function during capacitation (27), but it is not clear whether this positive effect is an effect on the process of capacitation or a stimulation of the acrosome reaction itself (28). Insulin can also be used as a sperm agonist to improve the quality of frozen and thawed spermatozoa. Diogo et al. found that spermatozoa from diabetic males exhibited a significant reduction in sperm viability, plasma membrane activity, and DNA integrity (15). After cryopreservation of spermatozoa, it exhibits no elongation of telomere length; nevertheless, the procedure triggers oxidative stress via both physical and chemical pathways, resulting in an elevated proportion of oxidized DNA fragmentation, concomitant with reductions in sperm viability and the percentage of morphologically normal cells (29,30). Shokri et al. found that the addition of different doses of insulin to frozen spermatozoa reduced the level of ROS in sperm mitochondria, and in particular, the 1,000 ng/mL dose of insulin significantly reduced DNA fragmentation rate and increased sperm viability, implying that insulin needs in high levels to induce protection against sperm DNA fragmentation. However, the above studies are, after all, for animal experiments, in human spermatozoa has not been explored, and the lack of mechanistic studies on the effect of insulin on spermatozoa in vitro, it remains to be further clarified the function of insulin on human semen in vitro (31).

The molecular protective mechanism of insulin in hyperglycemic sperm DNA is mainly realized by enhancing the antioxidant capacity of spermatozoa, and it has been demonstrated that insulin treatment improves the expression of Hsp70-2a, TP-1 and TP-2, and enhances the role of Hsp90 in the DNA repair process, and TP-1 and TP-2 are mainly involved in the chromatin composition and precision changes of haploid spermatozoa, whereas the Hsp70-2a and Hsp90 of the heat shock protein family are involved in epigenetic processes related to nuclear chromatin condensation, and all of the above pathways contribute to the maintenance of sperm DNA integrity (32-34). At the gene expression level, it has been suggested that type I diabetes inhibits B-cell lymphoma-2 (Bcl-2) and increases BAX expression, initiating intrinsic apoptosis signaling through caspase-3 overexpression, and that insulin inhibits type I-triggered intrinsic apoptosis in the testis by maintaining preexisting Bcl-2 protein and inhibiting BAX overexpression and thus caspase-3 expression; The Physiological levels of cell cycle protein D1 interact with its kinase Cdk-4 to help accelerate the cell cycle transition from G1 to S phase, whereas its overexpression or under-expression promotes apoptosis, and insulin improves testicular cell proliferation by restoring diabetes-induced deficiencies in cell cycle protein D1; Moreover, diabetic conditions upregulate p53—a DNA damage checkpoint protein in the G1 phase that promotes apoptosis upon DNA damage. Conversely, insulin therapy suppresses p53-dependent pathways, inhibiting G1 cell cycle arrest and/or apoptotic onset (35).

Biguanides

Biguanide antihyperglycemic agents represent the first-line pharmacotherapy for type II diabetes mellitus, and their principal glucose-lowering mechanisms encompass: (I) enhancement of anaerobic glycolysis within skeletal muscle; (II) inhibition of hepatic gluconeogenesis; (III) augmentation of peripheral glucose uptake and glycolytic flux (36); and (IV) amplification of systemic insulin sensitivity (37). Type II diabetes mellitus constitutes the predominant form of diabetes among infertile males, and biguanides have accordingly been extensively prescribed in this population. Over recent years, a growing body of clinical investigations and animal studies has begun to examine the impact of biguanide-based hypoglycemic therapy on human and experimental spermatozoa. However, the majority of these works have concentrated on seminal quality in obesity-related male infertility, whereas investigations specifically targeting sperm parameters within diabetic males remain comparatively scarce (38,39).

In in vivo studies, Naghibi et al. found metformin significantly improves sperm quality, increases testosterone levels, and reduces levels of ROS, glucose, and BAX to Bcl-2 ratios in blood and testicular tissues in diabetic rats. The levels of BAX to Bcl-2 ratios correlate with apoptosis, and a decrease in their levels inhibited apoptosis in testicular cells, which promoted the preservation of male fertility. The combination of metformin and mauve forskolin especially had significant effects on testosterone levels and antioxidant capacity, but for other routine sperm parameters there was little change compared to the two alone (40). Metformin augments testicular antioxidant capacity primarily via AMPK activation, whereby oxidative stress within the testis is attenuated through both AMPK-dependent and AMPK-independent signaling cascades. In parallel, accumulating evidence demonstrates that metformin further mitigates testicular oxidative injury by reducing the levels of membrane lipid-peroxidation product malondialdehyde (MDA) and apoptotic marker caspase-3 (41), concomitant with a decrease in the BAX/Bcl-2 ratio, thereby inhibiting germ-cell apoptosis and preserving testicular cytoarchitecture. Forskolin likewise attenuates oxidative stress while promoting testosterone biosynthesis and spermatogenesis; nevertheless, it does not confer a significant synergistic advantage when combined with metformin, possibly attributable to disparities in exposure duration or dosage, or to an as-yet-unidentified antagonistic interaction that warrants further investigation (40). A study by Zhao et al. found a significant increase in sperm motility, sperm membrane functional integrity and acrosome integrity with the addition of 150 mg/kg dose of metformin to the diet of male goats. In contrast, the addition of metformin at a dose of 300 mg/kg to the diet of male goats revealed a significant increase in catalase (CAT), glutathione peroxidase (GSH-px) and superoxide dismutase (SOD) activities, as well as AMPK and p-AMPK protein expression. It is through the activation of AMPK, Belicin-1 and LC3II/I protein expression that metformin maintains sperm energetic homeostasis and regulates autophagy and oxidative stress, decreasing ROS levels and improving sperm quality and plasma membrane integrity. Furthermore, the present study revealed that serum testosterone concentrations exhibited a progressive decline as the metformin dose escalated, implying that an optimal therapeutic window, rather than simple dose escalation, must be identified to maximize patient benefit during in vivo metformin therapy. Concomitantly, the protein expression levels of key autophagy markers beclin-1 and LC3-II/I were up-regulated in a dose-dependent manner, indicating that metformin alleviates oxidative stress by facilitating the autophagic clearance of senescent or aberrant cells, thereby attenuating oxidative injury within the testicular microenvironment (42). The combination of metformin and the trace element zinc promotes a significant increase in the expression of blood-epididymis barrier (BEB)-related molecules (including ZO-1, β-catenin, and N-cadherin) as well as water channel proteins (AQP3, AQP9, and AQP11) in epididymal tissues, which ameliorates diabetes-induced imbalance of epididymal zinc homeostasis, BEB, and uptake dysfunction, and consequently increases spermatozoa. There are studies found that metformin combined with zinc could correct diabetes-induced zinc homeostasis imbalance and improve sperm quality by activating the PI3K/AKT/mTOR pathway and improving the expression of related proteins, such as PI3K, p-AKT/AKT and p-mTOR/mTOR (43,44). Meanwhile, it was found that PI3K, p-AKT/AKT, and p-mTOR/mTOR are key signaling pathway proteins in the regulation of autophagy, and their increased expression contributes to the elimination of testicular senescent and abnormal cells and the reduction of oxidative stress induced by cellular metabolites (45).

In in vitro study, Rindone et al. found that metformin interfered with the metabolism and tight composition of testicular supporting cells during BTB formation, did not impair BTB permeability, and did not affect spermatogenesis (46). Metformin also plays a positive role in the improvement of sperm quality; the viability of freeze-thawed spermatozoa mainly depends on the amount of sperm adenosine triphosphate (ATP) stored before activation. During sperm freezing and thawing, the mitochondria will be impaired due to a significant decrease in high membrane potential affecting the production of ATP. Thus, maintaining sufficient ATP content during storage will be beneficial to thawed spermatozoa viability enhancement, similar to the in vivo mechanism of action. Metformin can improve sperm viability and fertilization ability while prolonging the storage capacity of spermatozoa in vivo through activation of the AMPK pathway. Moreover, similar to its in vivo mechanism, metformin maintains sperm ATP content by activating AMPK expression and enhancing glucose uptake, thereby increasing sperm viability and fertilization potential and further extending the storage capacity of spermatozoa in vitro (47). Low concentrations of metformin exert no deleterious effects on fresh spermatozoa; however, exposure to high concentrations significantly compromises fresh sperm viability. Importantly, supplementation of cryopreservation medium with metformin not only enhances post-thaw sperm motility and fertilization competence, but also diminishes the incidence of abnormal zygotes and reduces lipid peroxidation following in vitro fertilization. Lipid peroxidation of the sperm plasma membrane precipitates impaired motility and viability, and concurrently disrupts the spermatozoon’s ability to bind to the zona pellucida. The observed improvement in embryonic quality conferred by metformin may be attributable to enhanced genomic stability of the male gamete. Certainly, the concentration of metformin tolerated by spermatozoa varies between species, and the optimal concentration for human spermatozoa needs to be further investigated (48,49). Meanwhile, metformin has been found to limit mitochondrial respiration and reduce oxidative stress in spermatozoa in vitro; 50 μM metformin improves sperm viability and increases spermatozoa mitochondrial activity and nicotinamide adenine dinucleotide+ (NAD+) content, which is positively correlated with the functional state of the mitochondria, and the NAD+ content decreases in case of mitochondrial damage or dysfunction (50).

However, the use of metformin in diabetic men is controversial, with Wensink et al. noting in a cohort study that metformin takers were at increased risk of birth defects in their offspring, with a particular increased risk of developing genital defects (51). Controversially, Rotem et al. in a cohort study noted the safety of its use and found that metformin monotherapy did not increase the risk of birth defects in newborns (52). Obesity and diabetes often go hand in hand, McPherson et al. found that short-term metformin treatment in men who are obese could be a potential intervention for the treatment of subfertility and increase fetal weights and lengths (53). Further validation of the safety of metformin on birth defects in offspring is still needed from a large number of clinical studies and evidence-based medicine. As shown in Figure 2, there are mechanisms by which metformin and insulin improve sperm quality in diabetes males.

Figure 2.

Mechanisms by which metformin and insulin improve sperm quality in diabetic males. BEB, blood-epididymal barrier.

Sulfonylureas

Sulfonylurea hypoglycemic agents belong to insulinotropic agents, which stimulate insulin secretion to lower blood glucose by blocking ATP-sensitive K+ channels and cAMP-dependent signaling proteins on β-cell membranes (54,55), and common medications include glibenclamide, glipizide, gliclazide, gliquidone, and glimepiride.

In an in vivo study, Shokri et al. demonstrated that glipalamide significantly enhanced fundamental sperm parameters, reduced testicular MDA concentrations, and elevated systemic antioxidant capacity in diabetic rats. The drug also mitigated diabetes-induced pathological alterations, as evidenced by decreased testicular varicocele diameter, restoration of spermatogenic epithelial height, and overall improvement in testicular histological architecture. However, the mechanistic relationship between glipalamide-mediated suppression of ROS and the reversal of testicular histological lesions was not investigated in the study (56). Animal experiments revealed that the sulfonylurea antidiabetic drugs glimepiride and glibenclamide could increase the normal sperm morphology rate and enhance the antioxidant capacity of the testis, and glibenclamide could not only reduce the lipid peroxidation of sperm plasma membrane but also increase the levels of the antioxidant enzymes SOD, CAT, and glutathione-s-transferase (GST). Rabbani et al. suggested that glibenclamide might reduce xanthine oxidase reaction by inhibiting the ATP-sensitive potassium channel, and then decrease the production of ROS, which in turn reduces the abnormal oxidative stress caused by the abnormal sperm morphology. ATP-sensitive potassium channels reduce the xanthine oxidase reaction and the production of ROS, thereby reducing the oxidative stress that causes spermatozoa morphological abnormalities (57,58).

Glibenclamide has strong systemic antioxidant activity outside of the reproductive system and can reverse the diabetic-induced decrease in CAT and SOD activity in the liver, thereby reducing systemic ROS production (59,60). However, it is noteworthy that the combination of pioglitazone and glimepiride did not improve the diabetes-induced sperm morphology abnormalities and oxidative stress state, probably due to the lack of free radical clarity, which might be related to the dose and duration of exposure to the combination of the two (61). Öztaş et al. reported that glipizide elevated sperm count and the proportion of morphologically normal spermatozoa while ameliorating testicular architecture in diabetic rats, yet exerted no discernible influence on the HPT axis. Notably, testosterone concentrations remained persistently low in glipalamide-treated diabetic animals, implying that glipalamide may confer protection to the GnRH-LH signaling cascade of the HPT axis against diabetes-induced damage, thereby preserving physiological testosterone levels. Consequently, the observed improvements in spermatozoa quality were primarily ascribed to glipalamide-mediated enlargement of testicular seminiferous tubule diameter and restoration of inter-Sertoli-cell tight-junction integrity (62).

Glibenclamide can increase GSH, CAT and SOD activities and reduce ROS levels, and carrying glibenclamide through nanocarriers can increase the drug dissolution rate, which can significantly reduce ROS levels in type 2 diabetes compared with glibenclamide alone. In addition, nanocarriers carrying glibenclamide can increase the transcriptional levels of Nrf2, HO-1, and GPx4 through Nrf2/HO-1 and GPx4 signaling pathways, and protect sperm cell membranes from oxidative stress damage through adenylate-rich uridine, which is important for spermatogenesis and motility. GPx4 transcript levels, resist oxidative stress and protect sperm cell membranes from oxidative stress damage, Nrf2 plays an important role in spermatogenesis and motility, and stimulates antioxidant enzymes to reduce ROS levels through adenylate-uridylic acid-rich elements (63,64). Glibenclamide nano-carrying shows significant clinical benefits and provides direction for new drug development and fertility protection in diabetic men.

However, the in vitro effects of sulfonylurea hypoglycemic agents on spermatozoa diverge markedly from those of insulin and bisphosphonates. Spermatozoa function within the female reproductive tract is critically dependent on calcium-channel modulation. When sulfonylureas such as glipalamide or gliclazide are added to semen in vitro, they exert a dual, opposing action: first, they block ATP-sensitive K⁺ channels, provoking membrane depolarization and subsequent opening of voltage-gated calcium channels, which promotes calcium influx; second, they simultaneously inhibit the Na⁺-Ca²⁺ exchanger, thereby preventing Ca²⁺ efflux from spermatozoa. The net consequence is a sustained elevation of intracellular Ca²⁺ concentration and disruption of calcium homeostasis, ultimately compromising sperm viability and, in extreme cases, inducing spermatozoa death (65,66). The in vitro properties of this class of drugs can mostly be utilized in male contraception and in vitro spermicide.

GLP-1 receptor agonists

GLP-1 is a multifunctional peptide hormone that regulates blood glucose, and GLP-1 receptor agonists commonly used in clinical practice, including somatostatin, liraglutide, dulaglutide, etc. The hypoglycemic mechanisms of GLP-1 receptor agonists include promotion of insulin secretion and inhibition of glucagon secretion, and they can inhibit gastrointestinal motility and reduce appetite (67,68).

Pourheydar et al. demonstrated that, under physiological conditions, liraglutide may exert deleterious effects on the normal metabolic machinery of germ cells, thereby eliciting excessive generation of ROS and compromising sperm DNA integrity. This adverse outcome is presumably attributable to the route-dependent administration of liraglutide, which inhibits the endogenous redox system and subsequently impairs the scavenging of ROS derived from aberrant cellular metabolism. In diabetic mice, neither 1.20 nor 1.8 mg/kg liraglutide significantly increased sperm survival or viability; nevertheless, both doses effectively ameliorated diabetes-induced impairments in sperm DNA integrity and chromatin condensation. These findings imply that, despite its insulin-mimetic properties, liraglutide fails to attenuate diabetes-evoked, mitochondria-dependent apoptosis, thereby conferring no substantial improvement on overall spermatogenesis or seminal quality (69). However, a recent animal study found that liraglutide failed to affect sperm concentration and viability, both in vivo and in vitro, and that sperm concentration was instead increased in mice if GLP-1 receptor was absent, further validating the negative effects of liraglutide on germ cell metabolism under physiological conditions (70). The improvement of male sperm quality by liraglutide was mostly found in animal studies in obesity models (71,72). Although in a randomized open clinical study, somatostatin was found to significantly improve sperm quality in patients with obesity combined with type 2 diabetes mellitus, which not only increased the normal sperm morphology, concentration and number of subjects, but also increased total testosterone and libido (73). However, this study does not directly prove whether the improvement in sperm quality acts on obesity-induced sperm damage mechanisms, diabetes-induced sperm damage mechanisms, or both interventions, and further studies are needed.

A meta-analysis showed that GLP-1 receptor agonists do not act directly to intervene in testicular function (74); meanwhile, in a randomized, double-blind, placebo-controlled crossover study, it was found that the GLP-1 receptor agonist, dulaglutide, did not significantly intervene in the secretion of hormones of the Hypothalamic-Pituitary-Ovarian (HPO) axis (75). It has been hypothesized that the effect of GLP-1 receptor agonists on sperm quality may be either direct by triggering sperm GLP-1 receptor expression or indirect through paracrine bursts of mesenchymal cells acting on the GLP-1 receptor present in supporting cells (76,77). However, evidence from relevant studies is still lacking to support that the mechanism of action of GLP-1 receptor agonists on spermatozoa needs to be further explored.

SGLT-2 inhibitors

Sodium-glucose cotransporter proteins are active transport proteins that carry out isotropic transport of glucose relying on the difference in sodium ion concentration gradient on both sides of the cell membrane (78). SGLT-2 inhibitors lower blood glucose mainly by blocking glucose reabsorption in the kidney. It has also been demonstrated that SGLT-2 inhibitors have a positive effect on male spermatozoa as well. The common clinical drugs include dapagliflozin and empagliflozin.

In an animal study, it was found that empagliflozin improved oxidative stress, sperm quality and pathological changes in testicular tissues, and indirectly modulated serum sex hormone, insulin, leptin levels and expression of kisspeptin (a hypothalamic neuropeptide hormone involved in the regulation of energy metabolism, GnRH, and reproductive function) in testicular tissues in diabetic rats, which enhances fertility (79). Kiani et al. found that empagliflozin improved the antioxidant capacity of spermatozoa by increasing the viability of SOD, as well as significantly lowering the values of single-stranded DNA (AO+), immature nucleated spermatozoa (AB+), and the BAX/Bcl-2 ratio to improve sperm quality and reduce apoptosis. The improvement of sperm viability by Empagliflozin is mainly due to the reduction of oxidative stress, the increase of ATP content required by spermatozoa, and the modulation of calcium levels. It is worth noting that empagliflozin, while improving the morphological abnormalities of the head of the spermatozoa, increases the morphological abnormalities of the tail of spermatozoa, which may be related to the lowering of testicular cholesterol levels, and a high proportion of the spermatozoa tails are more susceptible to damage after lowering of cholesterol (80). In addition, dapagliflozin inhibited the testicular apoptotic process by up-regulating the expression of the anti-apoptotic proteins Bcl-2 and X-linked inhibitor of apoptosis (XIAP) and suppressed testicular apoptotic process by enhancing the total antioxidant capacity, total SOD activity, and GPx activity as well as by decreasing the level of 4-hydroxynonenal (4-HNE) oxidative stress (81).

In addition to the mechanism of action at the level of oxidative stress, at the chromosomal and genetic levels, diabetes increases the incidence of diploid nondisjunction or chromosomal defects in male spermatozoa, as well as disruption of genes such as Ogg1, Parp1, and P53, and dapagliflozin repairs diabetes-induced DNA damage thereby decreasing the oxidative stress response and reducing the rate of diabetes-induced spermatozoa aneuploidy (82). At the level of intestinal flora metabolism, dapagliflozin modulates microbial composition and cascade regulates the metabolism of adenosine, cAMP, and 2'-deoxyinosine and 2'-deoxyhypoxanthine in serum and testis, and decreases apoptosis and ROS production in spermatogenic cells. High levels of adenosine in the cecum, cAMP in plasma, and 2'-deoxyinosine in testis induces ROS production and apoptosis, in which cAMP is involved in oxidative stress and apoptosis through the cAMP-PKA signaling pathway (83). Based on the above studies, it can be found that SGLT-2 inhibitors protect male fertility and improve sperm quality mainly by responding to antioxidant stress, decreasing apoptosis and promoting DNA repair.

Meanwhile, the combination of SGLT-2 inhibitors and other hypoglycemic agents also showed positive improvements in sperm quality in diabetic males. Ribeiro et al. found that the combination of the GLP-1 agonist, exenatide, and dapagliflozin had a cumulative effect on the production of lactic acid and alanine in the testicular supportive cells and on glucose depletion, which was favorable for improving spermatogenesis and development. Alshamrani et al. found that the combination of dipeptidyl peptidase-4 (DPP-4) inhibitor saxagliptin and dapagliflozin reduced testicular ROS levels and increased glutathione levels to restore the gonadal redox imbalance in diabetic mice, but unlike dapagliflozin, saxagliptin exacerbated the decline in spermatozoa viability and number in diabetic mice. in diabetic mice, so caution should be exercised when combining the two (84,85). A randomized controlled trial exploring the effects of dapagliflozin on sperm quality in pre-diabetic individuals is currently underway, and further validation of the effects of SGLT-2 inhibitors on human fertility is awaited (86).

Thiazolidinediones

Thiazolidinediones hypoglycemic agents are insulin sensitizers that lower blood glucose by increasing insulin sensitivity and do not directly promote insulin secretion (87,88). Common clinical thiazolidinediones hypoglycemic agents include rosiglitazone and pioglitazone, and also act as peroxisome proliferator-activated receptor γ (PPARγ) agonists, which can affect spermatozoa quality by directly acting on spermatozoa PPARγ receptors (89).

In in vivo studies, Rabbani et al. found that higher dose (10 mg/kg) of pioglitazone significantly ameliorated the oxidative stress status of male spermatozoa and increased the normal morphology ratio of spermatozoa as well as sperm counts by decreasing the levels of serum lipid peroxide (LPO), GSH, and GPx in diabetic rats (90). Some studies have shown that pioglitazone can also reduce early and late sperm cell apoptosis by improving sperm mitochondrial activity and increase sperm viability and linear velocity, and pioglitazone has a time- and concentration-dependent effect in vivo, with lower concentrations of pioglitazone instead showing higher mitochondrial activity. At the same time, pioglitazone can act directly on damaged mitochondria, altering glycolytic metabolism and fuel Substrate-specific changes, increasing mitochondrial activity and reducing apoptosis (91,92). It is noteworthy that, in leptin-receptor-deficient obese mice, the PPARγ agonist pioglitazone failed to reverse diabetes-induced degeneration of the spermatogenic epithelium or to restore testicular architecture; instead, it diminished epididymal and testicular weight. Moreover, pioglitazone did not attenuate testicular lipid peroxidation or oxidative DNA damage in diabetic mice and further suppressed the base excision repair (BER) machinery within diabetic testes, as evidenced by down-regulation of the BER proteins 8-oxoguanine DNA glycosylase (OGG1/2), X-ray repair cross-complementary protein-1 (XRCC1), and DNA polymerase δ. Elevated levels of OGG1/2 and DNA polymerase δ suggest that the BER mechanism attempts to remove damaged bases and generate depurination/pyrimidineless sites and initiate single-strand breaks, exhibiting a negative effect on diabetic male spermatozoa. However, PPAR-γ stimulation in leptin receptor-intact lean mice reduced testicular ROS levels, suggesting that the reduction of testicular ROS by pioglitazone may be mediated through the leptin and its receptor-associated pathway, and that when leptin and its receptor-associated pathway are deficient, PPAR-γ stimulation instead exacerbates the production of ROS and exacerbates apoptosis (93,94). Thus, the in vivo effects of pioglitazone on diabetic male spermatozoa are controversial, and the effects on human male fertility require further clinical validation.

In in vitro studies, thiazolidinediones showed mostly positive effects on spermatozoa in vitro. The addition of rosiglitazone at a concentration of 60 µM to the frozen-thawed spermatozoa of rams formed a protective layer on the outside of the sperm membrane, protected the sperm membrane from lipid peroxidation caused by oxidative stress during freezing and thawing, and significantly improved sperm viability, sperm acrosomes and membrane integrity. It was mainly due to the regulation of the energy metabolism of spermatozoa and the protection of the mitochondrial membrane potential, while rosiglitazone reduced the level of ROS, mainly due to the activation of the AMPK-dependent mechanism to inhibit NAD(P)H oxidase. The reduction of ROS levels by rosiglitazone is mainly due to its activation of AMPK, an AMPK-dependent mechanism that inhibits NAD(P)H oxidase, and the hydroxylation of the phenyl and pyridine rings in the chemical structure of rosiglitazone, which may be able to remove hydroxyl radicals (95,96). Ortiz-Rodriguez et al. found that rosiglitazone reduces oxidative stress induced by the freezing process and regulates sperm mitochondrial function by increasing Akt phosphorylation and decreasing caspase 3 activation (97). Meanwhile, Rabbani et al. found that pioglitazone in combination with metformin improved the state of oxidative stress and increased the percentage of normal morphology due to diabetes (61). Further evidence of synergistic effects between hypoglycemic agents provides options for individualized diagnosis and treatment.

It is important to note that advanced age is a significant confounding factor in this context, as type II diabetes predominantly affects older men who are already experiencing age-related decline in fertility (98-100). This factor has been accounted for in Table S2.

Conclusions

To date, animal experiments, clinical studies, and meta-analyses focusing on the relationship between glucose-lowering drugs and male sperm quality have yielded different conclusions. Firstly, in in vivo studies, insulin, biguanide hypoglycemic agents, sulfonylureas, and SGLT-2 inhibitors have shown positive effects on improving sperm quality. However, the effects of GLP-1 receptor agonists and thiazolidinediones on sperm quality in vivo are controversial and further meta-analyses are needed to clarify their significance. Secondly, in in vitro studies, insulin, biguanide hypoglycemic agents and thiazolidinediones showed positive effects on in vitro sperm quality, however, sulfonylurea hypoglycemic agents showed negative effects on in vitro sperm quality.

Glucose-lowering drugs have different mechanisms of action in vivo and in vitro, and antidiabetic drugs play a dual role in male fertility. Insulin improves sperm quality by enhancing antioxidant capacity, upregulating DNA repair proteins (e.g., Hsp70-2, TP-1/2), and acting as a “sperm booster” in cryopreservation. Metformin maintains fertility through AMPK-mediated autophagy, reduction of oxidative stress, and synergistic effects with zinc through PI3K/AKT/mTOR signaling, and SGLT-2 inhibitors (e.g., empagliflozin, dapagliflozin) show promise in combating testicular apoptosis and ROS through metabolic reprogramming and antioxidant upregulation. In contrast, sulfonylureas (e.g., glibenclamide) exhibit spermicidal effects in vitro due to disruption of calcium homeostasis, despite their antioxidant effects in vivo. GLP-1 receptor agonists show contradictory results, potentially improving DNA integrity in diabetic models, but exhibit neutral or unfavorable effects on sperm metabolism in a physiological setting thiazolidinedione (e.g., pioglitazone) enhances sperm mitochondrial function in vitro but exhibits inconsistent efficacy in vivo.

The literature on this line of research is mostly animal experiments, rarely supported by clinical evidence, and the only relevant clinical studies are still in progress. The mechanisms and results explored in the current animal studies may provide support and direction for clinical studies, but the effects on sperm quality in human males need to be further explored in clinical studies.

In conclusion, antidiabetic drugs have great potential as adjuncts to fertility management in diabetic male. Currently, there are few or several studies aimed at exploring the effects of synergistic use of glucose-lowering drugs on sperm quality, such as the combination of metformin with zinc and insulin omega-3, which provides a new opportunity for exploring nanocarrier-delivered sulphonylureas to improve efficacy while mitigating off-target spermicidal effects. Also, the tissue-specific expression of GLP-1 receptor in testis and spermatozoa provides new directions to study the epigenetic modifications induced by antidiabetic drugs in germ cells. Clarifying the effects of different types of hypoglycemic agents on sperm quality in diabetic males will not only help to rationally avoid reproductive toxicity drugs, but also help to provide individualized treatment for infertile men.

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