Bisphenol A and Its Analogues Deteriorate the Hormones Physiological Function of the Male Reproductive System: A Mini-Review

BPA is identified as an endocrine-disrupting chemical that deteriorates the physiological function of the hormones of the male reproductive system. Bisphenol F (BPF), bisphenol S (BPS), and bisphenol AF (BPAF) are actively explored as substitutes for BPA and are known as BPA analogues in most manufacturing industries. These analogues may demonstrate the same adverse effects as BPA on the male reproductive system; however, toxicological data explaining the male reproductive hormones’ physiological functions are still limited. Hence, this mini-review discusses the effects of BPA and its analogues on the physiological functions of hormones in the male reproductive system, focusing on the hypothalamus-pituitary-gonad (HPG) axis, steroidogenesis, and spermatogenesis outcomes. The BPA analogues mainly show a similar negative effect on the hormones’ physiological functions, proven by alterations in the HPG axis and steroidogenesis via activation of the aromatase activity and reduction of spermatogenesis outcomes when compared to BPA in in vitro and in vivo studies. Human biomonitoring studies also provide significant adverse effects on the physiological functions of hormones in the male reproductive system. In conclusion, BPA and its analogues deteriorate the physiological functions of hormones in the male reproductive system as per in vitro, in vivo, and human biomonitoring studies.


Introduction
Bisphenol A (BPA) (4,4 -dihydroxy-2,2-diphenyl propane) is an organic synthetic monomer, reported to be one of the most produced chemicals in the world. About 7 million tonnes of BPA was produced in 2013, and this figure is estimated to be growing annually [1,2]. BPA was initially synthesized by the scientist Aleksandr Pavlovich Dianin in 1891. BPA's resistance to high temperatures, shatterproof nature, and electrical insulation capability has led to this chemical being predominantly used in most epoxy resin and polycarbonate plastic industries [3][4][5]. Epoxy resin is a precursor chemical of any product that requires resilience and high durability, such as a dental sealant, amalgams, tanks, sports equipment, roads, structural adhesives, and coating materials in cans [6][7][8][9]. Polycarbonate plastic is also used as packaging or as containers for many daily consumable products, such as vegetables, fruits, milk bottles, plastic bags, and food and beverage containers. Furthermore, these polycarbonate plastics can also be involved in the packaging of personal care products such as shampoo, body wash, and cosmetics [7][8][9][10][11][12][13].
pituitary glands, either in the anterior or posterior gland. One of the neuroendocrine activities regulated by the anterior pituitary gland is the HPG axis [3]. The HPG axis encounters three levels of hormone production: the hypothalamus releases the gonadotropin-releasing hormone (GnRH), the anterior pituitary gland secretes the follicle-stimulating hormone (FSH) and luteinizing hormone (LH), and the testis, specifically the LC, synthesises testosterone. GnRH, which is released from the hypothalamus, stimulates the anterior pituitary gland to release FSH and LH. Both hormones act on the testes to release target hormones, such as testosterone, oestrogen, progesterone, and inhibin. The primary purpose of this mechanism is to achieve homeostasis balance and modulate the positive and negative feedback of hormone regulation [4,5]. GnRH, LH, and FSH secretion are controlled by the neuropeptide kisspeptin (KiSS1), which is regulated by gene kiss1 [6][7][8]. Generally, the kiSS1 and G protein-coupled receptor 54 (GPR54) complex are involved in HPG axis feedback regulations [86]. KiSS1 binds to GPR54, also known as the kiss1 receptor, to form a Kiss1/GPR54 complex. This complex regulates the neuroendocrine reproductive axis by targeting the GnRH neuron to stimulate GnRH release [87]. Subsequently, GnRH stimulates neuron transmission at the anterior pituitary gland to induce the secretion of gonadotropic hormones, specifically LH and FSH. Furthermore, ERα also plays a significant role in regulating reproductive and sexual behaviour [88,89]. Once E2 binds to ERα in the hypothalamus, it suppresses GnRH secretion [90].
Several experimental studies revealed that BPA and its analogues, such as BPF, BPS, and BPAF, disturbed the HPG axis via KiSS1 and ERα by targeting mRNA gene expression. However, BPA and its analogues have either direct or indirect effects and are still controversial. Previous studies have reported that laboratory animals, such as rats and zebrafish, exposed to BPA showed an increase in Kiss1 mRNA expression in the brain [62,[71][72][73]. Exposure to BPA at a dose of 50 µg/kg/bw via drinking water caused an increase in Kiss 1 mRNA expression [71]. The same findings were also recorded in the offspring and pups of rats and the transgenic embryo of zebrafish [62,72,73]. Exposure of zebrafish embryos to 1000 µg/L of BPA and BPS at 120 h post-fertilization (hpf) revealed an increased expression of the Kiss1 gene. Moreover, the Kiss 1 receptor was highly expressed, leading to an increased number of GnRH3 neurons in the hypothalamus [62]. GnRH3 neuron is a neuromodulator that indirectly controls the pituitary gonadal axis of the reproductive system in zebrafish [91,92]. Furthermore, Yang et al. [56] found that male zebrafish in an aquarium containing 0.1 and 1 mg/L BPF showed an increase in the expression of GnRH receptors (GnRHR1 and GnRHR2), which influences the increase of GnRH neurons. Another BPA analogue, BPAF, has also been reported to disturb the HPG axis in the offspring of male zebrafish by increasing the mRNA expression of gnrh2, fshβ, and lhβ [74]. In zebrafish, gnrh2, fshβ, and lhβ are orthologous to human GNRH2, FSHβ, and LHβ, respectively.
BPA and its analogues also showed the ability to interfere with ERα in in vivo and in vitro studies. The BPA and its analogues, BPF and BPAF, increased the binding affinity towards ERα, while BPS has no effect on this receptor in zebrafish embryos [59]. This observation is supported by in vivo studies where subcutaneous exposure to BPA at a dose of 50 mg/kg/bw for two days increased the expression of ERα in the hypothalamus [73]. The same finding was also shown in the transgenic zebrafish embryo exposed to BPA and BPS at doses of 1000 µg/L and 100 µg/L, respectively. In contrast, perinatal exposure to BPA at a dose of 50 µg/kg/bw via drinking water caused a decrease in the expression of both ERα and β in the hypothalamus of male Wistar rats during adulthood [71]. The increase of Kiss1 expression in the brain increases GnRH secretion, stimulating LH and FSH secretion. A study by Bai et al. [72] found that perinatal exposure to BPA at a dose of 2 µg/kg/bw increased the GnRH neuron in the brain, leading to an increase in LH levels in the blood of male rat offspring.
In contrast, several previous studies have reported a decrease in the FSH and LH levels in the blood of adult male rats when exposed to various dosages of BPA ranging from 25 mg/kg/bw to 200 mg/kg/bw either via oral gavage or intraperitoneal injection [64][65][66][67][75][76][77]80]. A similar finding was also shown in rats who were exposed to BPA analogues. A study by Ullah et al. [68] showed that BPF at a dose of 1 mg/kg/bw via oral gavage significantly reduced the LH and FSH levels in the plasma of male rats. Therefore, from the previous findings, we may assume that BPA and its analogues at a lower dosage may increase LH and FSH levels via Kiss1 expression. However, a contrasting finding was noted when BPA was exposed at a higher dosage. LH in the blood binds to the LH receptor on the LC membrane to stimulate testosterone synthesis [93]. A previous study reported a decrease in the testosterone levels in the blood of adult male rats when exposed to BPA either during adulthood or exposure of offspring via oral gavage, subcutaneously, or intraperitoneal injection [64][65][66][75][76][77]80].
Furthermore, exposure to BPF and BPS for 28 days in male rats also decreased testicular and plasma testosterone [52,[68][69][70]. BPA and its analogues, BPF and BPAF, also caused a decrease in testosterone levels in the plasma or testis of male rats and zebrafish [42,72,[74][75][76]. In contrast, a study by Stoker et al. [71] reported that perinatal exposure to BPA at a dose of 50 µg/kg/bw via drinking water caused an increase in the testosterone level of male Wistar rats during their adulthood. The contradictory results might be due to the dose, route, and length of BPA exposure. The effects of BPA and its analogues on the correlation between Kiss1, ERα, and GnRH in the brain with the levels of FSH, LH, and testosterone in the plasma is still doubtful. The differences may be due to the different doses, way of exposure, types of animals, and study duration. There might also be other mechanisms involving the correlation; for instance, the bisphenols can disrupt the mechanism of negative/positive feedback system in regulating the reproductive hormones or may do so via competitive inhibition at the receptor level either in the brain or the testis. Table 1 shows a summary of the effect of BPA and its analogues on the HPG axis involving the male reproductive system. ↑ gnrh2, fshβ, lhβ, and cyp19b in 125 µg/L ↑ estradiol ↓ testosterone [74] Abbreviations: ↑ Increase; ↓ Decrease.

Bisphenols and Steroidogenesis
Testicular steroidogenesis is another important process in regulating the normal physiology of the male reproductive system. Steroidogenesis products such as testosterone, oestrogen, inhibin B, and progesterone play an essential role in maintaining the homeostasis of hormones in blood circulation. LC is a well-known site for steroidogenesis, particularly in the male reproductive system [94]. Steroidogenesis occurs in two different locations in the LC: the mitochondria and endoplasmic reticulum [93,94]. LH in the circulation binds to the LH receptor (LHR) at the LC membrane, thus activating the G protein groups to form the LHR/G protein complex. This complex activates two pathways by increasing cyclic adenosine monophosphate (cAMP) production and allowing the entry of arachidonic acid (AA) into the LC [95]. Next, cAMP activates protein kinase A (PKA) and mitogenactivated protein kinase (MAPK) for the stimulation of the steroidogenic acute regulatory (StAR) protein. This StAR protein is responsible for the transportation and movement of cholesterol from the outer membrane to the inner membrane of mitochondria [96,97].
Meanwhile, the presence of AA in the LC helps control testosterone production by inhibiting cholesterol movement to the mitochondria [98,99]. AA produces prostaglandin-E2 (PGE-2) via the activation of cyclooxygenase-2 (COX-2) and inhibits StAR functions [100][101][102]. Cholesterol is the primary substrate acting as a precursor in testicular steroidogenesis [94,96]. In mitochondria, cholesterol is converted to pregnenolone through the action of CYP11A1 [103]. The pregnenolone then moves into the endoplasmic reticulum, and the steroidogenic cascade of enzyme reaction that takes place involves the CYP450 enzyme (CYP17) and hydroxy steroid dehydrogenase (HSD) enzymes (3β-HSD and 17β-HSD). The conversion of pregnenolone into testosterone can be divided into two pathways (∆4 and ∆5). These pathways can be alternated depending on the binding affinity of the CYP17 towards the substrates, 17α-hydroxy pregnenolone and 17α-hydroxyprogesterone, which activate the ∆5 and ∆4 pathways, respectively [103]. Humans mainly undertake this activity through the ∆5 pathway, while rats and mice mostly take the ∆4 pathway. In normal physiology, in testicular steroidogenesis, the ∆5 pathway is less prone to be converted to the alternative pathway synthesizing the E2 than the ∆4 pathway.
Nowadays, growing evidence has demonstrated the ability of bisphenols to disturb the steroidogenesis pathway. StAR, a protein responsible for transporting and moving cholesterol into the mitochondria, is among the proteins affected by exposure to BPA and its analogues. The gene and protein expression of StAR was decreased in adult male rats exposed to BPA for 28 days and 42 days, respectively, at a dose of 200 mg/kg by oral gavage [65,67]. Furthermore, BPA analogues such as BPF, BPAF, and BPS also caused a decrease in the expression of StAR mRNA. The expression of StAR mRNA was decreased in BPF and BPAF in the adults and offspring of male zebrafish, respectively [56,74]. Meanwhile, Eladak et al. [78] found that BPF and BPS also significantly decreased the expression of StAR mRNA in mouse foetal testicular cells (mFeTA) at a concentration of 10,000 nmol/L. The disturbance of the StAR mRNA expression may lead to the deterioration of testicular steroidogenesis due to the disturbance in cholesterol transportation and movement into the mitochondria in the LC.
Previous findings have also shown disturbance in the gene and protein expression of cytochrome P450 and HSD enzymes either in the mitochondrial or reticulum endoplasmic of LC, such as CYP11A1, CYP 17A1, 3β-HSD, and 17β-HSD. Exposure to BPA at a dose of 200 mg/kg for 28 days reduced the gene expression of CYP11A1 in the testicular mitochondria of male Sprague-Dawley rats [67]. Furthermore, exposure to BPA at the same dose for 42 days also decreased the protein expression of CYP11A1 [65]. In contrast, the expression of CYP11A1 was found to increase in adults and embryos of male zebrafish exposed to BPF and BPAF, respectively [56,74]. The disruption of CYP11A1 either involving the gene or protein expression decreases the conversion of cholesterol to pregnenolone in the mitochondria of LC. The CYP17A1 and 3β-HSD gene expression involved in the steroidogenic enzyme cascade in the endoplasmic reticulum also decreased in BPA-intoxicated rats [65]. The 17β-HSD, 3β-HSD, and CYP17A1 protein expression also decreased in the testis of male Sprague-Dawley rats exposed to BPA [80]. The same findings were also noted with exposure to BPA analogues in either in vivo or in vitro studies. BPF and BPAF exposure decreased CYP17 expression in the testis of zebrafish, while 17βHSD was found to be decreased in the testis of adult male zebrafish after 21 days of exposure to BPF [56,74]. An in vitro study conducted by Eladak et al. [78] found that BPF and BPS at the highest dose (10,000 nmol/L) caused a decrease in the expression of HSD3β1 and CYP17A1 in mFeTA after three days of exposure.
Furthermore, BPF and BPAF also decreased the gene expression of HSD3β2 and CYP17A1 in the human adrenocortical carcinoma cell line [79]. However, only BPS exposure has been reported to decrease the gene expression of CYP17A1 in the same cell line [79]. Hence, the interference of the steroidogenic enzymes, demonstrated by the changes in the gene and protein expression of 17β-HSD, 3β-HSD, and CYP17A1, may be the reason for the abnormal testosterone level in BPA, BPF, BPS, and BPAF exposure [56,65,74,78,79].
Most exposure to BPA and its analogues causes decreased testosterone levels due to the low level of LH and the disturbance of the steroidogenic enzyme cascade in the LC [52,56,[64][65][66][67][68][69]72,[74][75][76][77][78][79]. In contrast, Roelofs et al. [55] reported that exposure to BPF and BPS for 48 h caused an increase in the level of testosterone in the MA-10 LC culture. Moreover, Stoker et al. [71] also reported an increase in the testosterone level in male Wistar rats during their adulthood when exposed to BPA in their perinatal stage [71]. Decreasing testosterone synthesis activates the backdoor pathway, whereby dehydroepiandrosterone (DHEA) is converted into androstenedione (AD). AD is responsible for the ∆4 pathway, and this pathway poses a high risk of stimulating the overproduction of E2. This backdoor pathway is also known as the alternative pathway in testicular steroidogenesis involving the activation of p450 aromatase. The activation of aromatase is exhibited by the increased gene and protein expression of CYP19A1, which leads to the activation of cAMP. This mechanism causes the overproduction of E2 in the testis, which also presents as an effect of BPA and its analogues [56,59,72,[74][75][76]79]. However, studies conducted by Stoker et al. [71] and Alboghobeish et al. [64] found a decrease in E2 levels in the blood circulation of adult male rats exposed to BPA. The disruption in this steroidogenesis pathway leads to a disturbance in sperm synthesis known as spermatogenesis. Table 2 shows the effects of BPA and its analogues on steroidogenesis in the male reproductive system.

Bisphenols and Spermatogenesis
Spermatogenesis occurs within the seminiferous tubules in the testis. The germ cells, such as spermatogonia, spermatocyte, and spermatid, undergo various stages of spermatogenesis to form sperm. Spermatogenesis occurs via specific processes, such as proliferation, differentiation, mitosis, meiosis, and spermiogenesis, to develop mature spermatozoa. Among these specific processes, proliferation, differentiation, and mitosis occur in the basement membrane, while the remaining processes occur in the adluminal compartment [104]. The blood-testis barrier (BTB) is formed after the basal membrane to protect the microenvironment of the adluminal compartment for the processes relevant to that area. Therefore, the germ cells found in the basement membrane are more vulnerable to any toxicants than the germ cells found in the adluminal compartment [93]. The integrity of the BTB is also crucial because changes in its structure may affect the production and morphological structure of the sperm [95]. Spermatogenesis involves not only different stages of germ cells but also SCs. These cells secrete pyruvate and lactate to nourish germ cells during their development and are responsible for the organization of the germ cells [103]. Therefore, any disturbance in the SC causes degeneration and disorganization of the germ cells. Testosterone plays a critical role in spermatogenesis owing to its ability for BTB maintenance, meiosis, Sertoli-spermatid adhesion, and the release of mature spermatozoa [104]. Testosterone maintains the remodelling of the BTB by binding with AR to form the protein involved in the integrity of tight junctions. Testosterone is needed in the completion of meiosis during the development of spermatocytes. Moreover, testosterone also plays an essential role in preventing the elongated spermatid from being released earlier. However, the testosterone hormone helps release the mature spermatozoa into the lumen of the seminiferous tubule, thus preventing spermatozoa from being engulfed by the SC [104].
BPA and its analogues were reported to disturb spermatogenesis by diminishing the BTB integrity, changing testicular histopathology, and causing sperm defects [52,[64][65][66][67][68][69][70][75][76][77]80,81]. A study carried out by Li et al. [105] found a disturbance in the BTB of male Wistar rats, which was proven by the reduction of occludin and nectin-3 when exposed to BPA in a dose-dependent manner. The reduction of occludin was also found in the SCs, which were exposed to BPA in in vitro studies [82,83]. Furthermore, both studies also showed a decrease in the ZO-1 protein level [82,83]. The disturbances of these proteins lowered the integrity of BTB, which was proven by the reduction in cell viability and androgen receptor (AR) after 6 h of BPA exposure [82]. Moreover, Feng et al. [83] also found that the reduction of occludin and ZO-1 in SCs significantly perturb the tight junction barrier, lowering the integrity of the BTB. The disruption of BTB integrity may allow germ cells in the adluminal compartment to be exposed to toxicants, leading to the disturbance of spermatogenesis in the seminiferous tubules.
Spermatogenesis disruption after exposure to BPA and its analogues can be shown by histological observations, such as reduction in the diameter and epithelial height of germ cells, atrophy and separation of germinal epithelium, and irregular seminiferous tubule structure [66,67,80]. Previous studies reported that BPA exposure causes histopathological changes, proven by the vacuolation, degeneration, and disorganization of germ cells [66,67,80]. The vacuolation and degeneration of germ cells were reported after BPA exposure either via oral gavage for 52 days or intraperitoneal injection on alternate days for 30 days in adult male Sprague-Dawley rats [66,80]. The spermatogenesis process was found to be weak, arrested in the seminiferous tubule of adult male Wistar rats exposed to BPA at a dose of 50 mg/kg via oral gavage for 14 days [77]. Furthermore, the same study also found that spermatocytes are among the most affected germ cells in BPA-intoxicated rats [77]. Wang et al. [67] found that BPA at a dose of 200 mg/kg via oral gavage caused disorganization of germ cells [67]. However, these changes were not observed in BPA analogue-intoxicated rats. Moreover, BPA analogues such as BPF and BPS cause spermatids to become longer, and the absence of mature spermatozoa in the lumen of seminiferous tubules disrupts spermatogenesis in adult male Sprague-Dawley rats [65,68,69]. According to Liang et al. [81], BPA and its analogues (BPS and BPAF) decrease cell viability and increase the DNA damage of the spermatogonia cell line (C18-4). Among these bisphenols, BPAF causes significant outcomes at the lowest concentrations within 24 h of exposure [81].
BPA and its analogues disrupt spermatogenesis, leading to the deterioration of its outcome, which is proven by a reduction in sperm quality. Low sperm production induced by toxicants is usually associated with oxidative stress and the reduction of testosterone in the blood circulation [49,106]. Furthermore, the reduction in sperm development may also be due to abnormal SC causing insufficient nutrition, which is necessary for spermato-genesis [105]. According to previous studies, adult male rats' exposure to BPA causes a decrease in sperm quality, proven by a reduction in sperm production, count, motility, viability, and the integrity of sperm acrosome and plasma membrane mitochondrial activity [64][65][66][67][75][76][77]. BPA at a dose of 50 mg/kg/bw for 14 days caused mild oedema in the LC, leading to a reduction in testosterone, thus lowering the sperm quality of adult male Wistar rats [77]. There is also an association reported between mitochondrial activity and motility in sperm because the mitochondria is the only source of ATP that enables the energy production necessary for sperm movement [76]. The sperm-specific ion calcium (Ca 2+ ) channel (CatSper) is also crucial for sperm motility, hyperactivation, and acrosome reaction. This pH-sensitive channel is responsible for providing enough Ca 2+ for sperm function [107]. Progesterone is a factor that influences the activation of the CatSper channel for sperm hyperactivation and acrosomal reaction to penetrate the oocyte [107]. Previous findings reported that the expression of the CatSper channel and charges were significantly downregulated and decreased after exposure to 10, 50, and 250 µg/kg/kg doses of BPA to the sperm mice orally [84]. These reductions parallel the finding where the motility and acrosome reaction in the presence of progesterone were significantly decreased as well. Exposure of healthy human sperm to 10 µM BPA analogues showed a similar effect on the CatSper channel's ability. In a study, the scholars found that BPG, BPAF, BPBP, BPC, and BPB are potent chemicals that inhibit progesterone-induced Ca 2+ [85]. These BPA analogues are shown to affect Ca 2+ signaling, which can interfere with normal CatSper signaling and result in infertility [85]. Table 3 shows the effects of BPA and its analogues on spermatogenesis in the male reproductive system.  Abbreviations: ↑ Increase; ↓ Decrease.

The Effects of BPA and Its Analogues on Male Reproductive Hormones: Human Biological Studies Evidence
Human epidemiology findings show a strengthened impact of BPA on the male reproductive system, involving the sexual hormones and sperm characteristics, through in vivo and in vitro studies. However, to the best of our knowledge, the number of human studies on the effects of BPA analogues on the male reproductive system focusing on the HPG axis is still limited. Most human studies have shown an association between the presence of BPA and its analogues in biological samples, such as urine and serum, with male sexual hormones and its effects on spermatogenesis outcomes, such as sperm characteristics parameters. A cross-sectional study was carried out between male children and adolescents in the United States of America. This study found a significant association between a high concentration of BPA in the urine and a low level of total testosterone in the serum of male adolescents only [108]. Researchers have estimated that 1 unit of BPA might lower the total testosterone levels by approximately 50% [108]. The finding prevailed for associations between BPA and other male reproductive hormones, such as AD and FSH, in men who work in the epoxy resin industry in Shanghai, China [109]. Liu et al. [109] found that increased BPA concentration in the urine indicated a significant decrease in AD and FSH levels in the worker's serum [109]. Another study among men in the same industry in Guangdong, China, also presented the same findings [110]. The researcher found that BPA concentration and the duration of BPA exposure also influence hormone levels. Higher concentration and longer duration of BPA exposure decreased AD levels increasingly. Furthermore, Zhuang et al. [110] also found that a high concentration and a longer duration of BPA exposure also increased the sex hormone binding globulin (SHBG) in the serum of workers in the industry [110]. SHBG is a globulin protein that carries testosterone, dihydrotestosterone (DHT), and E2. Among these hormones, testosterone is carried the most by this protein. Therefore, an increase in SHBG level suggests that the testosterone level has decreased in the serum of the workers exposed to BPA. However, when comparing worker and non-worker groups, no association was found on the levels of SHBG, total testosterone, inhibin B, and AD [110].
Meanwhile, a study by Lassen et al. [111] found that increased BPA concentration in the urine significantly increased the total and free testosterone, LH, and E2 levels in the serum of the population of young men in Denmark [111]. Furthermore, a cross-sectional study conducted by Adoamnei et al. [112] found that increased concentrations of BPA in the urine also present a significant increase in the LH level among the population of young men in Spain. However, no association was found between BPA and FSH, free testosterone, SHBG, inhibin B, or E2 levels in the same population [112]. BPA acts as an anti-androgen agent by its ability to competitively bind to the AR, causing an increase in testosterone levels in circulation. However, even though BPA can bind to AR, it does not have any effects on testosterone, as a massive amount of BPA is required in serum to exhibit any effect as an antagonist towards AR. Therefore, the HPG axis is being induced to secrete LH for steroidogenesis activation in the LC, leading to increasing testosterone formation. This mechanism explains the increased levels of LH and free testosterone reported by Lessen et al. [111] and Adoamnei et al. [112].
BPA is known to raise oestrogenic activity either by stimulating or inhibiting the ER [113]. BPA has been reported to have a higher affinity towards ERβ than ERα in an in vitro study [112]. The action of BPA towards these receptors appears to be very complex, but the response depends on the presence of the ER subtypes (ERα and ERβ) and the coregulatory protein, which either acts as a co-activator protein (stimulate) or the co-repressor protein (inhibit) [113]. The stimulation or inhibition of ER causes an increase or decrease of E2 in circulation, respectively [112]. Several studies have reported increasing E2 levels in the blood after exposure to BPA [108,111]. BPA can strongly bind to oestrogen-related receptors, such as ERR-γ, which interfere with the steroid synthetase functions and gene expression involved in steroidogenesis [110]. BPA has also been identified to bind with the ER at the anterior and posterior pituitary gland, leading to HPG axis disturbance and activation of prolactin secretion. The increased prolactin level in men who were occupationally exposed to BPA was evidence for this [109]. Furthermore, BPA can also increase the E2 level by its ability to activate an alternative steroidogenesis pathway. In humans, the ∆5 pathway is dominant; however, BPA may activate the alternative pathway where DHEA is converted into AD, which has a high incidence of stimulating the overproduction of E2 [109,110].
A retrospective study known as the ELEMENT project was carried out by Ferguson et al. [114], who reported no association between BPA levels found in mothers' urine during pregnancy with SHBG, inhibin B, or free and total testosterone levels in the serum of male children aged between 8 and 14 years in Mexico [114]. Furthermore, the same study also found no association between BPA in the urine of the male children with SHBG, inhibin B, and the free and total testosterone levels in the serum [114]. However, a retrospective study done by Hart et al. [115] found a significant weak positive correlation between BPA in the serum of pregnant women and sperm motility and concentration in the semen of young men aged between 20 and 22 years old in Australia [115]. BPA was found in almost 90% of the maternal serum sample [115]. Furthermore, other epidemiological studies showed that a high concentration of BPA decreased sperm quality, proven by decreasing sperm concentrations, count, and motility [111,112,116,117]. A study by Lessen et al. [111] found that a high concentration of BPA present in the urine caused a decrease in the percentage of progressively motile sperm in the semen of young men in Denmark [111]. The same finding was reported between BPA and sperm count and concentration in young men aged 18-23 years in Spain. The high BPA concentration found in the urine of young men decreased the sperm count and concentration. However, the sperm motility and morphology in the same study population did not show any alterations [112]. Furthermore, there was no association between BPA and semen analysis, such as sperm morphology, sperm concentration, total sperm count, and semen volume, among fertile men aged over 18 years in Michigan and Texas [118]. Interestingly, BPA has also been identified to affect semen quality in IVF patients in Slovenia, where sperm count, concentration, vitality and motility decreased with the increasing concentration of BPA in the urine [116].
BPA analogues, such as BPS, also presented adverse effects in semen analysis among the population of young men involved in the FEPOS project in Denmark. The semen volume was lowered in subjects who had high BPA in their urine [119]. In contrast, Abou Ghayda et al. [117] found that infertile patients in Boston, MA, USA, with a high BPS concentration showed a high semen volume; however, this semen has a lower sperm concentration [117]. Low sperm concentration was found in infertile patients who were exposed to BPS. The researcher also reported that an increased level of BPS causes a significant decrease in sperm quality among obese or overweight patients who have a BMI >25 kg/m 2 . Therefore, BPS can increase the severity of sperm defects observed in obese or overweight infertile patients. Table 4 summarises human epidemiological studies of BPA and its analogues towards the male reproductive system.
across the quartiles. [111] Sperm characteristics: Increased level of BPA caused a significant decreased in percentage of progressive motile spermatozoa across the quartiles.   Abbreviations: ↑: Increase; ↓ Decrease

Discussion
There is significant evidence that endocrine disrupting chemicals such as BPA and its analogues are an additional risk factor to be considered. Understanding the link between them and hormonal physiological function in the male reproductive system would aid in ensuring that suitable efforts are made to enhance public awareness and prevent their detrimental impacts on health.
BPA and its analogues have been the topic of extensive research and controversy in recent decades, and the amount of information available about them today is astounding. There are numerous papers that dealing with BPA and its analogues on human health, and the trend is for more to be published in the future. It is considered that the effects of BPA and its analogues on human health are a relatively new field with many unresolved problems. As previously stated, BPA and its analogues are commonly used substances in developing and underdeveloped countries. This means that scientists cannot dismiss the possibility that one of the functions of these compounds is to disturb the endocrine system. Numerous studies, both epidemiological and experimental, published in recent years have contributed to our understanding of some of the features and how they influence the hormonal physiological functions in the male reproductive system.
BPA and its analogues have a negative impact on hormonal physiological functions in the male reproductive system, which have been proven in vitro [55,78,79], in animals [56,[64][65][66][67][68][69][70]72,[74][75][76][77]80] and in human studies [108][109][110][111]116,118]. These hormonal physiological dysfunctions affect spermatogenesis outcomes. Some studies have shown decreased LH, FSH and testosterone levels when exposed to certain concentrations of BPA and its analogues as occurs in the HPG axis, specifically when this exposure occurs during adulthood [64][65][66][67][68][75][76][77]. The result of this review suggests that BPA and its analogues are capable of competitively binding to AR and ER and stimulating KiSS1 expression in the hypothalamus of the brain, causing the HPG axis feedback mechanism disturbance. These effects will alter GnRH secretion, resulting in variations in pituitary secretion of LH and FSH, with most studies reporting lower levels of these hormones. It is, therefore, logical that experimental animal studies showed low levels of testosterone as the negative feedback mechanism for these phenomena. Furthermore, BPA and 17-beta-oestrogen molecules are structurally similar; therefore, the former can adhere to the ER and mimic the oestrogenic effects in a human study [110,111], which is supported by the animal data [60,72,73]. Epidemiology studies have also shown that BPA and its analogues found in human urine are associated with the disturbance of male reproductive hormones via its effects as anti-androgenic and anti-oestrogenic agents [110][111][112]116].
Not only involving the HPG axis mechanism, BPA and its analogues could interfere with the steroidogenesis pathway via upregulation and downregulation of genes and proteins such as CYP and HSD, causing steroidogenesis disturbance [110,112]. BPA and its analogues can activate an alternative pathway known as the aromatase pathway, which alternates from testosterone synthesis to oestrogen synthesis [110,112]. This aromatase activity happens due to the upregulation of genes and protein expression of CYP19A1, which has been proven in in vitro and animal studies [56,59,74]. One of the primary results of this analysis is that BPA and its analogues have the potential to alter the steroidogenesis pathway, resulting in outcomes that are analogous to those of prostate cancer, such as high oestrogen levels in blood circulation. Therefore, BPA and its analogues exposure could be listed as one of the risk factors for prostate cancer.
In the Section 3 of the findings of this literature review, we examined articles that investigated the effects of BPA and its analogues on spermatogenesis outcomes. The disturbance of male reproductive hormones reduces spermatogenesis outcomes proven by sperm and semen quality defects. Testosterone and FSH play important roles in spermatogenesis; therefore, a decrease in these hormones will result in sperm quality defects, which were reported in animals and cell cultures exposed to BPA and its analogues [64][65][66][67][68][69][70][75][76][77][80][81][82]. The human findings also reported a decrease in sperm and semen quality when exposed to BPA and its analogues [111,112,116]. CatSper channel is another factor that influences sperm motility, hyperactivation and acrosomal reaction to penetrate the oocyte. As discussed previously, CatSper is one of the channels found in the sperm responsible for the influx of Ca 2+ for sperm function. Therefore, downregulation and alteration in this channel after BPA and its analogues exposure could lead to immotile sperm and reduction in oocyte penetration ability even with the presence of progesterone. All these events in spermatogenesis disturbances lead to sperm quality defects resulting in infertility [85].
The significant drawback of this work is that most publications examine animal studies rather than human studies, owing to the logical and ethical challenges of doing this type of study in humans. Some findings are parallel between humans and animals; however, some findings showed differences, such as the doses used in animal and human studies. The low observed adverse effects level (LOAEL) dose for BPA in a mammalian animal is 50 mg/kg/bw. Even though the dose of BPA used in the previous studies was below the LOAEL, it reportedly caused changes in the reproductive hormone, sperm characteristic, and histological of the testis in laboratory animals [66,[70][71][72]76]. Rats were more susceptible to BPA when compared to humans. Therefore, the adverse effect is more severe in rats, specifically the male reproductive system, proven by the disturbance of reproductive hormone, gene, and protein expression in steroidogenesis and reduced sperm quality.
Even though the BPA concentration found in human urine is lower than the tolerable daily intake (TDI), which is below than 4 µg/kg, significant negative effects were found in our literature findings proven by alterations in the reproductive hormones, sperm characteristics, and semen quality. However, the data from human biomonitoring studies are uncertain, which might be due to different metabolism rates in individuals influencing their ability to excrete the BPA [9]. Furthermore, the concentration of BPA and the duration of exposure might also be significant factors that must be considered in assessing the effect of BPA on the hormones' physiological function in the male reproductive system. There are a few possible challenges from the previous literature findings which derive interpretation concern: subject population (fertile versus infertile, young adult versus children; BPA and its analogues exposure levels (due to non-linear effects of BPA); sampling frequency limitations (sample may not be representative of exposure such as retrospective study); and BPA exposure window which may not be related to the spermatogenesis outcomes because the spermatogenesis process in the human takes about three months to be complete [9]. Another problem is that the effects of BPA and its analogues on physiological hormonal functions were explored without taking into account probable interactions with other environmental factors. Figure 1 shows the possible mechanism of BPA and its analogues on the hormonal physiology pathway of the male reproductive system. BPA and its analogues negatively impact the physiological hormonal functions in the male reproductive system and its outcome, which were proven by sperm and semen quality defects. The BPA and its analogues can alter the HPG axis by increasing the expression of KiSS1 and competitively bind with ERα in the brain's hypothalamus. These effects will influence GnRH secretion, leading to changes in the pituitary secretion of LH and FSH, which most of the studies reported decreased levels of these hormones. Decreased LH levels in the plasma could disturb the steroidogenesis process in Leydig cells, resulting in decreased testosterone levels. Furthermore, these endocrine disrupting chemicals were also reported to alter the StAR protein, HSD and CYP450 genes and enzymes in the mitochondrial and ER of Leydig cells involved in the steroidogenesis pathway. These alterations also lead to the decrease of testosterone synthesis. Therefore, once testosterone synthesis is inhibited, the alternative pathway will be activated. BPA and its analogues can activate this alternative pathway of steroidogenesis by activating aromatase activity, thus increasing the formation of E2. As the consequences of HPG axis disturbance and activation of the alternative steroidogenesis pathway specifically proven by the FSH and testosterone levels decreased, respectively, these phenomena lead to the disturbance of spermatogenesis reported in most studies. Therefore, the spermatogenesis outcome, such as sperm quality, will have deteriorated.

Conclusions
In conclusion, BPA and its analogues may be an additional risk factor to consider because they disrupt the hormonal physiological functions of the male reproductive system. The results of the experimental studies mostly point to BPA and its analogues having the ability to disrupt the endocrine system, resulting in HPG axis disturbances and activation of the steroidogenesis alternative pathway, which is why more experimental and epidemiological research will be required to establish the scale of the effects caused by these chemicals in large populations and its molecular mechanism to a better understanding of the connection between HPG axis, steroidogenesis, and spermatogenesis outcomes. Despite the fact that many nations have adopted policies to limit exposure to BPA and its analogues in their populations, epidemiological research on humans imply that the same abnormalities seen in experimental studies on animals may be detected. Understanding the association between BPA and its analogues and physiological hormonal activities can aid in raising public awareness and implementing public health campaigns to prevent exposure to these compounds, particularly among those attempting to conceive and elderly populations.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.