Obesity is a pathological condition resulting from an excessive accumulation of fat. With the prevalence of unhealthy lifestyles such as excessive intake and inadequate physical activity, the incidence of obesity is increasing (Meldrum, 2017). The World Health Organization (WHO) reported in 2016 that 39% of males globally aged 18 years and older are overweight, and approximately 13% were obese (WHO, 2021). Infertility affects 10%–15% of couples of childbearing ages and, statistically, up to 46% of couples of reproductive ages suffer from infertility due to oligoasthenospermia in males (Eisenberg et al., 2014). Obesity is strongly associated with male oligoasthenospermia (Ameratunga et al., 2023). The elevated body mass index (BMI) may be related with oligoasthenospermia, including decreased semen quality, disrupted testicular histomorphology, and impaired spermatogenesis (Kozopas et al., 2020; Ameratunga et al., 2023; Han et al., 2023). An important factor is male hypogonadism due to lipid deposition in the testes, including reduced synthesis of sex hormones, metabolic disorders and, ultimately, high levels of sperm apoptosis and reduced sperm quality (Genchi et al., 2022; Pivonello et al., 2019).

Hormone-sensitive lipase (HSL) is a multifunctional enzyme involved in fatty acid metabolism. It plays a key role in regulating lipid metabolism in the testes. It hydrolyzes triacylglycerols, diacylglycerols, monoacylglycerols, cholesterol esters, retinyl esters and other lipids in multiple tissues (Holm, 2003). Meanwhile, it also plays an important regulatory role in maintaining testicular seminiferous epithelial structure, androgen synthesis and primordial germ cell differentiation. In addition, HSL is the main enzyme in the testis that hydrolyzes the cholesterol esters internalized by the different receptors and stored in the lipid droplets (Nan et al., 2022). Cholesterol esters are hydrolysed by HSL to free cholesterol, which is then used for steroidogenesis and spermatogenesis, an important step in the control of spermatogenesis (Wang et al., 2017a, b). HSL is a critical enzyme involved in spermatogenesis, as revealed by the infertility of male HSL−/− mice (Casado et al., 2021). Studies have revealed that genetically deficient HSL mice show an increase of 2–4 times in the accumulation of cholesterol esters in testes and are infertile (Chung et al., 2001; Haemmerle et al., 2002).

Metformin has been demonstrated to exhibited beneficial effects of on weight loss, cancer, cardiovascular diseases and polycystic ovary syndrome (Wang et al., 2017). Recent research also pinpoints that metformin might have a protective effect on obesity-related reductions in male fertility (Yerevanian and Soukas, 2019; Shpakov, 2021). For example, metformin dramatically increases testosterone levels and sperm quality in HFD-induced obese rats (Leisegang et al., 2021; Yan et al., 2015). Furthermore, studies have shown that HFD can lead to an increase in intratesticular lipid concentration, provoked the deposition of lipids in the seminiferous tubules, stimulated germ cell apoptosis, culminating in a reduction in testosterone synthesis, which could be prevented by metformin treatment (Morgan et al., 2014; Yu et al., 2019; Liu et al., 2020). In obese patients, metformin treatment improves sperm concentration and motility in the same way as observed in obese rats as judged by the decreased number of morphological defects, with higher concentration and motility of sperm (Faure et al., 2018).

Accordingly, in this study, we investigated whether metformin has protective effects against HFD-induced reproductive damage in male mice and whether its effects are mediated via the HSL pathway.