Plastic has become an integral part of modern world, penetrating basically every aspect of our lives. It has been estimated that worldwide plastic production reached 550 million tonnes per year in 2018 (Plastics Europe, 2018). However, a staggering 32% of plastic packaging escapes collection systems owing to low public awareness and mismanagement of plastic waste. It was estimated that poor recycling use has contributed to the accumulation of 15–51 trillion of plastic particles floating on the surface of the oceans (Van Sebille et al., 2015). If improper human behavior and waste mismanagement trends continue, approximately 12,000 tons of plastic waste will eventually enter the natural environment by 2050 (Geyer et al., 2017). Large plastics gradually undergo weathering processes, ultraviolet radiation and microbial breakdown to form the microplastics (particles < 5 mm). Emerging evidence suggests that microplastics are easily transferred and accumulated along the food chain, and eventually enter the human body, posing potential threats to human health (Rahman et al., 2021). Hence, microplastics are recognized as an ecological risk to human health in recent years.
Microplastics are absorbed from the gastrointestinal tract lumen via the endocytosis of M cells, and subsequently transported to the mucosal lymphoid tissues, thereby entering circulatory system (Wright and Kelly, 2017). In recent mammalian experiments demonstrated that microplastics could induce hepatotoxicity, cardiovascular toxicity and behavioral disorders via ingestion or inhalation does occur (Deng et al., 2017; Wright and Kelly, 2017). In addition, rats showed cardiomyocyte apoptosis after being given high doses of microplastics (50 mg/L) for 90 consecutive days, which may be correlated with burst generation of reactive oxygen species (ROS) (Li et al., 2020). It is noted that the reproductive system is one of the most sensitive systems in an organism in microplastics exposure (Yin et al., 2021). Further study reported that polystyrene microplastics accumulates in the testis of mice, causing blood-testis barrier disruption and sperm DNA damage (Jin et al., 2021; Hou et al., 2021a). Balb/c mice exposed to 1 mg/day of polystyrene microplastics showed deterioration of sperm parameters through enhancing ROS production (Deng et al., 2021; Jin et al., 2021; Hou et al., 2021). Coincidentally, male rats treated with 10 mg/kg/day polystyrene nanoparticles (NaPs) showed signs of testicular atrophy and seminiferous tubule degeneration (Amereh et al., 2020). On the other hand, testis accumulation of NaPs could cause alterations in sperm physiology and spermatogenesis disorders (Deng et al., 2021). However, the present knowledge on reproductive toxicity posed by nanoparticles is still incomplete.
Previous study suggest that Excessive ROS is often a hallmark feature in damaged spermatogenesis, which in turn implies its role in the development of impaired reproductive function (Sedha et al., 2015). Uncontrolled ROS production is a driving force for mitochondrial damage in fertility impairment, while nuclear factor erythroid-derived 2-related factor (Nrf2) is responsive to the alternations in cellular redox homeostasis. We have reported that Nrf2 signaling protects adverse effects of microplastics in vivo and in vitro (Li et al., 2021). As a next step in ROS burst event, Nrf2 dissociates from Keap1 and enters the nucleus to trans-activates target gene, such as heme oxygenase-1 (HO-1) and NAD(P)H dehydrogenase quinone 1 (NQO1). Despite the fact that Nrf2 activation is a protective regulator against testicular injury, burgeoning studies have shown the deleterious side of Nrf2 (Zhang et al., 2019; Zhang and Chapman, 2020). Also, it is confirmed that Nrf2 down-regulation is involved in prepubertal testis injury (Zhao et al., 2020). Hence, maintaining the redox balance is important to mitigate microplastics-induced reproductive toxicity.
Pyruvate kinase M2 (PKM2) is a critical rate-limiting enzyme of aerobic glycolysis and catalyzes the reaction of phosphoenolpyruvate to pyruvate with the generation of ATP. In addition to its metabolic functions (tetramer PKM2), dimeric PKM2 exhibits kinase potential beyond glycolysis involved in signal transducer and activator of Nrf2 (Luo et al., 2011; Wei et al., 2020). Previous evidence suggested that nuclear PKM2 promotes transcription of Hif-1α by phosphorylating STAT3 at tyrosine 705 (Dong et al., 2015). Of note, recent research implied that pharmacologic PKM2 inactivation blocked the abnormal cell cycle and apoptosis in vitro (Zheng et al., 2020). Additionally, abnormal expression of PKM2 was observed in many pathological states, including formaldehyde-induced neurotoxicity and arsenic-triggered hepatotoxicity (Wang et al., 2020a; Li et al., 2021). Therefore, we hypothesized that PKM2/Nrf2 signal regulatory oxidative stress and metabolic functions interaction mechanism is involved in microplastics-triggered reproductive toxicity.
Environmental contaminants-induced reproductive toxicity is an area of great concern. Mice spermatocyte cells (GC-2spd cells) have been used in many studies as an in vitro model for reproductive toxicological research (Ren et al., 2019; Jin et al., 2021). According to above studies, we hypothesized that Nrf2-mediated anti-oxidant defence should be an essential target for the molecular mechanism of NaPs-triggered reproductive toxicity. On the other hand, we also investigated the roles of Nrf2-PKM2 signaling in NaPs-induced reproductive toxicity in the GC-2spd cells.