DOI 10.1515/hmbci-2014-0003 Horm Mol Biol Clin Invest 2014; 17(3): 137–144
Eleni Palioura, Eleni Kandaraki and Evanthia Diamanti-Kandarakis*
Endocrine disruptors and polycystic ovary syndrome: a focus on Bisphenol A and its potential pathophysiological aspects Abstract: Polycystic ovary syndrome (PCOS) is a heterogeneous disorder of unknown etiology that may arise from a combination of a number of underlying genetic interactions and predispositions with environmental factors. Endocrine disruptors and, in particular, Bisphenol A may represent one of the many underlying causes of the syndrome as they are experimentally linked to metabolic and reproductive derangements resembling PCOS-related disorders. Exposure to endocrine-disrupting chemicals may act as an environmental modifier to worsen symptoms of PCOS in affected females or to contribute to the final phenotype of the syndrome in genetically predisposed individuals. Keywords: Bisphenol A; endocrine disruptors; metabolism; PCOS; reproduction. *Corresponding author: Evanthia Diamanti-Kandarakis, Third Department of Internal Medicine, University of Athens Medical School, Sotiria General Hospital, Mesogion Avenue 152, Athens 11527, Greece, Phone/Fax: +003210 7778838, E-mail: [email protected] Eleni Palioura and Eleni Kandaraki: Third Department of Internal Medicine- Endocrine Section-, University of Athens Medical School, Sotiria Hospital, Greece
Introduction Polycystic ovary syndrome (PCOS) represents a common endocrine disorder among women of reproductive age [1]. With a lack of a clear etiology for its generative roots, PCOS is now considered to enclose genetic and environmental components [2] leading to phenotypic variability. Traditionally, women with PCOS manifest both reproductive (chronic anovulation, hyperandrogenism, polycystic ovarian morphology) and metabolic (insulin resistance, metabolic syndrome, obesity) derangements. However, there is much heterogeneity of clinical and biochemical features raising the possibility that a cluster of etiological factors synergistically contribute to the final PCOS phenotype.
One hypothesis proposes that androgen excess early in life (prenatal/prepubertal) may lead to manifestation of PCOS in adulthood [3, 4], and furthermore, the heterogeneity of phenotypes can be explained on the basis of the interaction of this disorder with other genes and with the environment [5]. The role of the environment is currently emerging especially in view of the modern epidemics of obesity and type 2 diabetes sweeping developed countries. According to the recent endocrine society scientific statement, there is accumulative evidence pointing to a role of endocrine disruptors in the etiology of complex diseases such as obesity, diabetes mellitus, and cardiovascular disease [6]. For over 30 years, scientists have focused on the impact of endocrine disrupting chemicals on human health. Endocrine disruptors (EDs) are defined as “exogenous agents that interfere with synthesis, secretion, transport, metabolism, binding action, or elimination of natural blood-borne hormones that are present in the body and are responsible for homeostasis, reproduction, and developmental process” [7]. They are found abundantly in the environment, and people are constantly exposed to them via a variety of sources including contaminated water, oil, air, and food. Furthermore, there is no endocrine system that is immune to these substances [6]. As endocrine-disrupting chemicals are involved in many aspects of female reproductive disorders, it is reasonable to assume that similar factors may have also factored in the PCOS epidemic targeting both the metabolic and the reproductive characteristics of the syndrome. This article will review evidence linking exposure to EDs, and more specifically to Bisphenol A, with metabolic and reproductive disorders resembling PCOS traits providing a pathophysiologic background for such interplay.
PCOS pathophysiology Metabolic abnormalities in PCOS Hyperinsulinemia and peripheral insulin resistance (IR) are the central features of the metabolic disorder, which
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138 Palioura et al.: Endocrine disruptors and PCOS: impact on metabolic and reproductive phenotypes is now recognized as a significant characteristic of PCOS. Insulin resistance occurs in approximately 60%–80% of women with PCOS partly independently of body weight [8]. At a molecular level, there is a postbinding defect in receptor signaling likely due to increased receptor and insulin receptor substrate-1 serine phosphorylation [9]. Furthermore, increased basal insulin β-cell secretion with decreased hepatic insulin clearance is an additive factor contributing to hyperinsulinemia of PCOS women [9]. The defects in insulin action and secretion increase the risk for development of metabolic disorders. Indeed, 30%–40% of women with the polycystic ovary syndrome have metabolic disturbances manifested as impaired glucose tolerance and 10% have type 2 diabetes by their fourth decade [10]. Obesity is another common trait of PCOS encountered in 30%–70% [10] of affected females and has an aggravating impact on the syndrome’s symptoms. Obesity has an aggravating impact on the ovulatory process through a yet, not totally understood, mechanism in which insulin resistance and compensatory hyperinsulinemia seem to be major pathophysiologic mechanisms of reproductive disorders through a direct effect on the insulin-sensitive PCOS ovary.
Reproductive abnormalities in PCOS The ovulatory dysfunction in PCOS is characterized by disturbed follicular development with excessive early follicular growth and abnormal later stages of arrested follicle growth well before expected maturation [11]. This disturbed follicle growth results to the classic PCO morphology with accumulation of small antral follicles (typically arrested at a diameter of 4–8 mm) without selection of a dominant, preovulatory follicle [5]. Clinically, women experience irregular ovulation with menstrual irregularities of oligo-/amenorrhea that may cause infertility due to difficulties in conceiving. Apart from impaired folliculogenesis, the PCO ovary is also characterized by impaired steroidogenesis with theca cells remaining the primary source of hyperandrogenism. There is clear evidence for intrinsic abnormality of constitutive hypersecretion of androgen by ovarian theca cells [12]. Though specific steroidogenic enzyme defects – namely, CYP17 (coding for P450c17 and the associated P450 reductase) and CYP11a (P450scc) have been incriminated in the genesis of ovarian hyperandrogenemia, a causal interplay is not currently proven [13]. Apart from theca cells, granulosa cell deregulation may also play a role in PCOS pathogenesis, via secretion of intra-ovarian regulatory factors, such as anti- Müllerian hormone
(AMH), that are believed to modulate follicular steroidogenesis [14].
Neuroendocrine dysfunction in PCOS Despite the various PCOS recognized phenotypes and the unclear etiology, derangement of the neuroendocrine features is almost a unanimous finding of the syndrome and undoubtedly correlated with its pathogenesis. Women with PCOS show an acceleration of the gonadotropinreleasing hormone (GnRH) pulse generator activity, which causes inappropriate gonadotropin synthesis and release, with elevated luteinizing hormone (LH) and low folliclestimulating hormone (FSH) levels [15]. Normally, during luteal phase, increased levels of progesterone in the presence of estradiol, inhibit and, as a result, decrease the pulsatility of GnRH and LH [16]. The loss of this physiological pattern and the decreased sensitivity of GnRH pulse generator to progesterone suppression in the presence of hyperandrogenemia, favor LH production over FSH and impairs follicular development [17]. Furthermore, the increased LH frequency pulsatility promotes the increased androgen production from theca cells, whereas the compensatory aromatization to estrogens expected by the action of FSH on the granulosa cells is impaired, due to the reduced FSH levels. As a result a ‘vicious circle’ promoting hyperandrogenemia and anovulation is developed [15]. In fact, the use of antiandrogens, such as flutamide, was shown to restore the sensitivity of GnRH pulse generator in women with PCOS in 4 weeks [18].
Endocrine disruptors and PCOS As previously analyzed, PCOS pathophysiology combines metabolic and reproductive disorders both of which could be targeted by exposure to endocrine-disrupting chemicals. In the following section, experimental evidence will be presented to show how EDs can disrupt hormonal balance and result in abnormalities of metabolism and reproduction resembling those observed in PCOS-affected females. This interaction may represent a plausible pathophysiologic link between EDs and the clinical PCOS.
Metabolic actions of endocrine disruptors The role of environmental chemicals in the etiology of complex metabolic diseases such as obesity, insulin resistance, metabolic syndrome, type 2 diabetes is a new area of
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research. Accumulating data are pointing to the potential involvement of these chemicals either directly or indirectly in the pathogenesis of obesity and diabetes [6], both of which are metabolic characteristics of PCOS women. The idea that chemicals in the environment could be contributing to the obesity epidemic was first introduced in an article by Paula Baillie-Hamilton published in 2002. In this paper, the author presented evidence from earlier toxicologic studies in which low-dose chemical exposures were associated with weight gain in experimental animals [19]. In the following years, the term “obesogens” was applied to describe “molecules that inappropriately regulate lipid metabolism and adipogenesis to promote obesity” [20]. Most known or suspected obesogens are endocrine disruptors. The list of compounds suspected to have “obesogenic” properties is heterogeneous and includes a variety of molecules; industrial chemicals [Bisphenol A (BPA), organotins, perfluorooctanoic acid, phthalates, polybrominated diphenyl ethers, polychlorinated biphenyl ethers], pharmaceutical agents such as diethylstilbestrol, organophosphate pesticides, dietary factors such as genistein, and other environmental pollutants [21]. The complexity of obesogen effects is reflected by their diverse modes of action; some obesogenic effects are believed to be mediated by sex steroid dysregulation (ex. DES, genistein, and BPA mainly act upon estrogen receptors), others include effects on metabolic sensors (ex. organotins and phthalates activate peroxisome proliferator-activated receptor gamma (PPARγ), the master regulator of adipogenesis), while others could potentially target the central integration of energy balance [22]. At the level of the adipocyte, environmental chemicals have been shown to promote adipocyte differentiation and to affect mature adipose tissue metabolism by targeting lipogenesis, lipolysis, and adipokine secretion [23]. Epidemiology studies support the findings in experimental animals and show a link between exposure to environmental chemicals and the development of obesity [24, 25]. Similarly, there are findings to connect endocrine disruptors with metabolic disturbances including diabetes and cardiovascular diseases [26]. Widespread EDs, such as dioxins, pesticides, and bisphenol A, cause insulin resistance and alter β-cell function in animal models [27] conditions that are both known as underlying causes of diabetes.
Reproductive actions of endocrine disruptors Female reproductive system seems particularly vulnerable to exposure to endocrine disrupting chemicals, which
have been linked with a variety of reproductive disorders from anatomical abnormalities of the reproductive tract to disturbed puberty onset and adult life disorders such as endometriosis and subfertility [6]. The female gonad is also targeted either through a direct effect on the level of the ovarian function [28] or as an indirect effect on its neuroendocrine control. At the level of the ovary, it seems that both follicular growth and steroid hormone production [29] can be disturbed by exposure to EDs. Several environmental chemicals have been shown to exert direct inhibitory and/or stimulatory effects on the expression and/or activity of key ovarian steroidogenic enzymes [28, 29], and therefore, as the hormone is no longer available, normal follicular development and maturation are disrupted. Furthermore, there are studies to show that environmental contaminants also target ovarian hormone receptors including gonadotropin receptors, which bind LH and FSH [29] probably leading to inappropriate ovary response to gonadotropin stimulation. Among the chemicals incriminated as ovarian disruptors are pesticides (e.g., dichlorodiphenyltrichloroethane and methoxychlor), plasticizers (e.g., bisphenol A and phthalates), dioxins, polychlorinated biphenyls, pharmaceutical agents (diethylstilbestrol), and phytoestrogens (genistein) [30].
Neuroendocrine actions of endocrine disruptors Αn indirect effect on female reproduction could be accomplished by targeting reproductively relevant neuroendocrine function at the hypothalamus-pituitary level. Indeed, accumulative evidence illustrate the interference of endocrine-disrupting chemicals (Bisphenol A, genistein, methoxychlor, polychlorinated biphenyls, estradiol benzoate) at several points of the axis leading to alteration of GnRH signaling in the pituitary [31]. The action of individual chemicals at different levels, via different mechanisms, adds to the level of complexity in determining the adverse effects on neuroendocrine physiology. The pathways involved in the disruption of GnRH signaling include a direct affect on the expression of estrogen-sensitive neuropeptides associated with the regulation of GnRH such as kisspeptin and galanin as well as a direct effect on GnRH neuron expression in the hypothalamus [31]. Furthermore, impaired steroid feedback on GnRH neurons has also been described as another potential pathway [31]. Differences have also been observed in behavior and mating success of laboratory animals as a result of endocrine-disrupting effects on sexually dimorphic brain regions [6].
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140 Palioura et al.: Endocrine disruptors and PCOS: impact on metabolic and reproductive phenotypes
Endocrine disruptors and PCOS: focus on Bisphenol A (BPA) BPA is one of the world’s highest production – volume synthetic chemicals with a large number of applications including use in certain food and drink packaging, in many plastic consumer products as well as in dental materials [32]. Human exposure is widespread and constant given that measurable levels of BPA have been detected in the serum and in many biological fluids [32]. Laboratory rodent studies incriminate BPA as a disruptor of multiple endocrine systems including neuroendocrine function, ovarian function and fertility, as well as metabolism [33]. Human studies indicate that BPA exposure in adults may be associated with both male (reduced male sexual function, reduced sperm quality) and female (reduced ovarian response and IVF success, reduced fertilization success and embryo quality, implantation failure, miscarriage, premature delivery endometrial disorders) reproductive disorders as well as metabolic diseases (type-2 diabetes, cardiovascular disease, hypertension, obesity, altered liver function) [34]. Furthermore, other potential health effects include altered thyroid hormone concentrations and immune system function, while exposure during critical stages of development is associated with disorders during gestation such as increased spontaneous abortion, abnormal gestation time, reduced birth weight or early in life such as altered behavior and disrupted neurodevelopment in children, as well as increased probability of childhood wheeze and asthma [34]. Neonatal BPA exposure has been linked to the development of PCOS reproductive phenotype later in life [35], and women with PCOS have increased BPA levels compared to controls [36]. These interactions will be subsequently analyzed providing the pathophysiological background of a potential interplay between this endocrine disruptor and PCOS.
Evidences for the relation between exposure to BPA and obesity and diabetes induction Bisphenol A is identified as a potential “obesogen” molecule. In vitro studies using 3T3-L1 cells (mouse fibroblasts that can differentiate into adipocytes) show a link between this environmental chemical with obesity by a promoting effect on adipocyte differentiation [37–39] and lipogenesis [40]. In vivo studies with rodents associate pre- [41] and perinatal [42, 43] exposure to low doses of BPA with increased adipose storage later in adult life. Similar concentrations are also able in vitro to affect human adipocyte cells’ endocrine function. In these cells, BPA was shown to inhibit
adiponectin release [44], a critical adipokine that increases insulin sensitivity and reduces tissue inflammation and stimulate the release of inflammatory adipokines such as interleukin-6 (IL-6) and tumor necrosis factor α (TNF-α) [45]. Both actions could lead to insulin resistance and increased susceptibility to development of metabolic syndrome. Epidemiological studies show a positive association between urinary BPA and obesity in general adult population of the US [25] as well as in children and adolescents [46]. Apart from obesity, higher BPA exposure, reflected in higher urinary concentrations of BPA in the general adult population of the US was shown to be associated with diabetes and cardiovascular diseases [26]. Furthermore, higher urinary BPA levels were found to be associated with prediabetes in 3516 subjects, free of diabetes risk factors [47]. The connection between BPA and metabolic disturbances including diabetes is plausible given that recent experimental studies have suggested that BPA causes pancreatic β-cell dysfunction. BPA has been demonstrated to enhance insulin biosynthesis and secretion in β-cells in adult male mice, leading to β-cell overstimulation and dysfunction and subsequent peripheral insulin resistance [48]. Pancreatic α-cells are also affected. Low doses of BPA are shown to impair the molecular signaling that leads to secretion of glucagon by suppressing intracellular calcium ion oscillations in α-cells in response to low blood glucose levels [49].
Evidences for the relation between exposure to BPA and reproductive disorders Detectable concentrations of BPA have been measured in human follicular fluid [50]. Many experimental studies show reproductive effects in exposed animal models at the level of ovarian follicle itself targeting both follicle growth and ovarian steroid production, processes affected in PCOS individuals. Culture of antral follicles isolated from 32-day-old mice with BPA inhibits growth of antral follicles and reduces production of progesterone, dehydroepiandrosterone, androstenedione, estrone, testosterone, and estradiol. Furthermore, BPA-induced inhibition of steroidogenesis is accompanied by decreased expression of Star andP450scc mRNA [51]. Impaired ovarian follicular development following developmental exposure to BPA has been reported in rodents manifested as an excess of antral follicles with decreased corpora lutea formation [52] in a dose-related manner [53]. Interestingly, these effects that are suggestive of a reduced number of ovulated oocytes were observed at low, environmental equivalent doses of BPA. Similarly, in
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a diverse reptile animal model, the blocking interference of BPA at the level of oocyte maturation has also been demonstrated but only at the highest dose [54]. In another rat model, prepubertal exposure to BPA disrupts normal ovarian follicle development by decreasing the expression of follicle development-promoting genes and increasing the expression of AMH gene that has a follicular development inhibitory effect [55]. Another study evaluating the effects of BPA on cultured rat thecainterstitial and granulosa cells showed dose-dependent alterations in sex steroid levels and mRNA for steroido genic enzymes [56]. In this study, Zhou et al. demonstrated increased testosterone synthesis and mRNA expression of 17-β hydroxylase (P450c17) and cholesterol side chain cleavage enzyme (P450scc) in theca-interstitial cells [56], key enzymes believed to be involved in PCOS pathogenesis [13]. In granulosa cells, BPA increased progesterone, but decreased E2 and P450arom mRNA levels [56].
Evidences for the relation between exposure to BPA and neuroendocrine disorders Developmental exposure to BPA has been shown to affect the hypothalamo-pituitary axis as indicated by significant impairments in gonadotropin secretion observed in laboratory animals. Collet et al. [57] presented data, obtained from prepubertal ovariectomized lambs exposed intravenously for 54 h to BPA, concerning the impact of the chemical on the hypothalamus-pituitary axis. Researchers observed an inhibitory effect of BPA on LH secretion [57]. Analogous findings are reported in rodents. Perinatal rat exposure to BPA is accompanied with interruption of gonadotropin secretion indicated by the low circulating LH levels [42]. In another experimental study, neonatal exposure to BPA decreased basal and GnRH-stimulated serum LH and also increased GnRH pulse frequency in infantile rats, an effect that persisted throughout life. Interestingly, GnRH signaling in the adult pituitary was severely affected as shown by impaired GnRH-stimulated inositol 1,4,5-triphosphate (IP3) formation [58].
Experimental and clinical evidences for the relation between exposure to BPA and PCOS It is evident from the above that BPA exposure is experimentally related to the induction of both metabolic and reproductive abnormalities resembling PCOS disorders. A pathophysiologic interference is biologically plausible in all aspects of PCOS pathogenesis from ovarian
follicle level to adipose tissue function and neuroendocrine system regulation. Such interplay is proven in an animal rat model of neonatal exposure to increasing doses of Bisphenol A that led to a PCOS-resembling phenotype in adult life [35]. Rats given subcutaneous injections daily of 50 or 500 μg/day from postnatal days 1–10 had higher levels of circulating testosterone and estrogen and lower levels of progesterone as adults. This sex steroid status resembles the abnormally elevated androgen levels in PCOS women. Furthermore, ovarian follicle growth and ovarian morphology were also affected; ΒPA-exposed rats (either 50 or 500 μg/day) had lower numbers of antral follicles, while animals treated with 500 μg/day BPA as neonates also had ovarian cysts. Regarding fertility outcomes, the highest BPA dose resulted in anovulation in affected rodents, and consequently, no pups were delivered from this group. Animals treated with 50 μg/day BPA were also affected as they delivered ~30% fewer pups without changes in the number of oocytes. Finally, exposure to 500 and 50 μg/day BPA had a neuroendocrine effect with accelerated GnRH pulse frequency in hypothalamic explants from adult rats [35]. In this animal model, the reproductive disorders found in PCOS are present in the rodents even with the dose of 50 μg/day BPA (a dose lower than the lowest observed adverse effect level). On the whole, in this rat model of BPA exposure, the coexistence of biochemical hyperandrogenemia, unovulation, infertility, polycystic ovarian morphology, and increased GnRH pulse interestingly points to a link with PCOS characteristics. Unfortunately, the metabolic effects, commonly seen in PCOS, were not investigated. Concerning humans, a recent study by E. Kandaraki et al. [36] showed that BPA levels are significantly higher in women with polycystic ovarian syndrome compared to normal women independently of body weight and could possibly be one of many underlying causes of this disorder. Confirmative of this hypothesis was the strong association observed between BPA levels with androgens and insulin resistance indices, implying a potential role of this endocrine disruptor to the two major components of PCOS pathophysiology [36]. Furthermore, it is known that women with ovulatory dysfunction compared to regularly ovulating women have higher serum BPA levels [59], and both groups display lower BPA levels compared to males [60]. A connection between androgens and BPA is obvious; however, it is not clear whether this is the result of an androgen-induced inhibition of BPA catabolism in the liver [61], or it is the consequence of BPA-induced alteration in androgens metabolism. BPA has been reported to significantly inhibit the activity of two different testosterone hydroxylases leading to decreased testosterone catabolism
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142 Palioura et al.: Endocrine disruptors and PCOS: impact on metabolic and reproductive phenotypes [62]. Additionally, BPA has the ability to bind potently with sex hormone-binding globulin (SHBG), thus, competing natural androgens whose circulating levels increase [63].
Conclusion Experimental studies converge to the involvement of EDs in all aspects of metabolic and reproductive regulation leading to disorders similar to PCOS aberrations. Among the wide variety of chemicals, Bisphenol A provides a clear example of development of a PCOS reproductive phenotype in laboratory animals. Furthermore, the increased level of this estrogenic substance in PCOS females and its association with androgens and insulin resistance, the cardinal features of the syndrome, may interestingly point to a link between BPA and PCOS pathophysiology. Whether this interplay is related to androgen-induced modifications of BPA clearance or BPA per se affects androgen metabolism, and production remains to be explored. New animal models combining both metabolic and reproductive PCOS phenotypes are necessary to comprehend the generative roots of this, yet not fully understood, endocrine disorder.
Highlights –– Experimental evidence link exposure to EDs with PCOS-related metabolic abnormalities; obesity, insulin resistance, diabetes.
–– Experimental evidence link exposure to EDs with PCOS-related reproductive abnormalities; impaired follicle development and steroidogenesis, neuroendocrine dysfunction. –– In vivo and in vitro studies associate BPA exposure to the induction of both metabolic and reproductive disorders. –– Neonatal BPA exposure leads to a PCOS reproductive phenotype later in life. –– BPA levels are elevated in PCOS women. –– Association between BPA and androgens – a cause or an effect?
Outlook Future research should focus on the development of an animal model of exposure to endocrine-disrupting chemicals combining both the metabolic and reproductive malfunctions occurring in the syndrome. This will provide insights into the environmental component of PCOS pathogenesis especially in early life exposures. Furthermore, the association between Bisphenol A and androgens should be elucidating as a potential aggravating factor that may help to comprehend the etiology and the ideal management/treatment of the syndrome. Received January 11, 2014; accepted February 12, 2014; previously published online March 8, 2014
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29. Craig ZR, Wang W, Flaws JA. Endocrine-disrupting chemicals in ovarian function: effects on steroidogenesis, metabolism and nuclear receptor signaling. Reproduction 2011;142:633–46. 30. Uzumcu M, Zama AM, Oruc E. Epigenetic mechanisms in the actions of endocrine-disrupting chemicals: gonadal effects and role in female reproduction. Reprod Domest Anim 2012;47: 338–47. 31. Fowler PA, Bellingham M, Sinclair KD, Evans NP, Pocar P, Fischer B, Schaedlich K, Schmidt JS, Amezaga MR, Bhattacharya S, Rhind SM, O′Shaughnessy PJ. Impact of endocrine disrupting compounds (EDCs) on female reproductive health. Mol Cell Endocrinol 2012;355:231–9. 32. Vandenberg LN, Hauser R, Marcus M, Olea N, Welshons WV. Human exposure to Bisphenol A (BPA). Reprod Toxicol 2007;24:139–77. 33. Richter CA, Birnbaum LS, Farabollini F, Newbold RR, Rubin BS, Talsness CE, Vandenbergh JG, Walser-Kuntz DR, vom Saal FS. In vivo effects of bisphenol A in laboratory rodent studies. Reprod Toxicol 2007;24:199–224. 34. Rochester JR. Bisphenol A and human health: a review of the literature. Reprod Toxicol 2013;42:132–55. 35. Fernandez M, Bourguignon N, Lux-Lantos V, Libertun C. Neonatal exposure to bisphenol a and reproductive and endocrine alterations resembling the polycystic ovarian syndrome in adult rats. Environ Health Perspect 2010;118:1217–22. 36. Kandaraki E, Chatzigeorgiou A, Livadas S, Palioura E, Economou F, Koutsilieris M, Palimeri S, Panidis D, Diamanti-Kandarakis E. Endocrine disruptors and polycystic ovary syndrome (PCOS): elevated serum levels of bisphenol A in women with PCOS. J Clin Endocrinol Metab 2011;96:480–4. 37. Masuno H, Kidani T, Sekiya K, Sakayama K, Shiosaka T, Yamamoto H, Honda K. Bisphenol A in combination with insulin can accelerate the conversion of 3T3-L1 fibroblasts to adipocytes. J Lipid Res 2002;43:676–84. 38. Masuno H, Iwanami J, Kidani T, Sakayama K, Honda K. Bisphenol a accelerates terminal differentiation of 3T3-L1 cells into adipocytes through the phosphatidylinositol 3-kinase pathway. Toxicol Sci 2005;84:319–27. 39. Sakurai K, Kawazuma M, Adachi T, Harigaya T, Saito Y, Hashimoto N, Mori C. Bisphenol A affects glucose transport in mouse 3T3-F442A adipocytes. Br J Pharmacol 2004;141:209–14. 40. Wada K, Sakamoto H, Nishikawa K, Sakuma S, Nakajima A, Fujimoto Y, Kamisaki Y. Life style-related diseases of the digestive system: endocrine disruptors stimulate lipid accumulation in target cells related to metabolic syndrome. J Pharmacol Sci 2007;105:133–7. 41. Honma S, Suzuki A, Buchanan DL, Katsu Y, Watanabe H, Iguchi T. Low dose effect of in utero exposure to bisphenol A and diethylstilbestrol on female mouse reproduction. Reprod Toxicol 2002;16:117–22. 42. Rubin BS, Murray MK, Damassa DA, King JC, Soto AM. Perinatal exposure to low doses of bisphenol A affects body weight, patterns of estrous cyclicity, and plasma LH levels. Environ Health Perspect 2001;109:675–80. 43. Somm E, Schwitzgebel VM, Toulotte A, Cederroth CR, Combescure C, Nef S, Aubert ML, Hüppi PS. Perinatal exposure to bisphenol A alters early adipogenesis in the rat. Environ Health Perspect 2009;117:1549–55. 44. Hugo ER, Brandebourg TD, Woo JG, Loftus J, Alexander JW, Ben-Jonathan N. Bisphenol A at environmentally relevant
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144 Palioura et al.: Endocrine disruptors and PCOS: impact on metabolic and reproductive phenotypes doses inhibits adiponectin release from human adipose tissue explants and adipocytes. Environ Health Perspect 2008;116:1642–7. 45. Ben-Jonathan N, Hugo ER, Brandebourg TD. Effects of bisphenol A on adipokine release from human adipose tissue: implications for the metabolic syndrome. Mol Cell Endocrinol 2009;304: 49–54. 46. Brent RL. Bisphenol A and obesity in children and adolescents. J Am Med Assoc 2013;309:134. 47. Sabanayagam C, Teppala S, Shankar A. Relationship between urinary bisphenol A levels and prediabetes among subjects free of diabetes. Acta Diabetol 2013;50:625–31. 48. Alonso-Magdalena P, Morimoto S, Ripoll C, Fuentes E, Nadal A. The estrogenic effect of bisphenol A disrupts pancreatic beta-cell function in vivo and induces insulin resistance. Environ Health Perspect 2006;114:106–12. 49. Alonso-Magdalena P, Laribi O, Ropero AB, Fuentes E, Ripoll C, Soria B, Nadal A. Low doses of bisphenol A and diethylstilbestrol impair Ca2+ signals in pancreatic alpha-cells through a nonclassical membrane estrogen receptor within intact islets of Langerhans. Environ Health Perspect 2005;113:969–77. 50. Ikezuki Y, Tsutsumi O, Takai Y, Kamei Y, Taketani Y. Determination of bisphenol A concentrations in human biological fluids reveals significant early prenatal exposure. Hum Reprod 2002;17:2839–41. 51. Peretz J, Gupta RK, Singh J, Hernández-Ochoa I, Flaws JA. Bisphenol A impairs follicle growth, inhibits steroidogenesis, and downregulates rate-limiting enzymes in the estradiol biosynthesis pathway. Toxicol Sci 2011;119:209–17. 52. Kato H, Ota T, Furuhashi T, Ohta Y, Iguchi T. Changes in reproductive organs of female rats treated with bisphenol A during the neonatal period. Reprod Toxicol 2003;17:283–8. 53. Adewale HB, Jefferson WN, Newbold RR, Patisaul HB. Neonatal bisphenol-a exposure alters rat reproductive development and ovarian morphology without impairing activation of gonadotropin-releasing hormone neurons. Biol Reprod 2009;81:690–9.
54. Stoker C, Beldoménico PM, Bosquiazzo VL, Zayas MA, Rey F, Rodríguez H, Muñoz-de-Toro M, Luque EH. Developmental exposure to endocrine disruptor chemicals alters follicular dynamics and steroid levels in Caiman latirostris. Gen Comp Endocrinol 2008;156:603–12. 55. Li Y, Zhang W, Liu J, Wang W, Li H, Zhu J, Weng S, Xiao S, Wu T. Prepubertal bisphenol A exposure interferes with ovarian follicle development and its relevant gene expression. Reprod Toxicol 2013 [Epub ahead of print]. 56. Zhou W, Liu J, Liao L, Han S, Liu J. Effect of bisphenol A on steroid hormone production in rat ovarian theca-interstitial and granulosa cells. Mol Cell Endocrinol 2008;283:12–8. 57. Collet SH, Picard-Hagen N, Viguié C, Lacroix MZ, Toutain PL, Gayrard V. Estrogenicity of bisphenol A: a concentration-effect relationship on luteinizing hormone secretion in a sensitive model of prepubertal lamb. Toxicol Sci 2010;117:54–62. 58. Fernandez M, Bianchi M, Lux-Lantos V, Libertun C. Neonatal exposure to bisphenol a alters reproductive parameters and gonadotropin releasing hormone signaling in female rats. Environ Health Perspect 2009;117:757–62. 59. Takeuchi T, Tsutsumi O, Ikezuki Y, Takai Y, Taketani Y. Positive relationship between androgen and the endocrine disruptor, bisphenol A, in normal women and women with ovarian dysfunction. Endocr J 2004;51:165–9. 60. Takeuchi T, Tsutsumi O. Serum bisphenol A concentrations showed gender differences, possibly linked to androgen levels. Bioch Bioph Res Com 2002;291:76–8. 61. Takeuchi T, Tsutsumi O, Ikezuki Y, Kamei Y, Osuga Y, Fujiwara T, Takai Y, Momoeda M, Yano T, Taketani Y. Elevated serum bisphenol A levels under hyperandrogenic conditions may be caused by decreased UDP-glucuronosyltransferase activity. Endocr J 2006;53:485–91. 62. Hanioka N, Jinno H, Nishimura T, Ando M. Suppression of malespecific cytochrome P450 isoforms by bisphenol A in rat liver. Arch Toxicol 1998;72:387–94. 63. Dechaud H, Ravard C, Claustrat F, de la Perriere AB, Pugeat M. Xenoestrogen interaction with human sex hormone-binding globulin (hSHBG). Steroids 1999;64:328–34.
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Eleni Palioura, Eleni Kandaraki and Evanthia Diamanti-Kandarakis*
Endocrine disruptors and polycystic ovary syndrome: a focus on Bisphenol A and its potential pathophysiological aspects Abstract: Polycystic ovary syndrome (PCOS) is a heterogeneous disorder of unknown etiology that may arise from a combination of a number of underlying genetic interactions and predispositions with environmental factors. Endocrine disruptors and, in particular, Bisphenol A may represent one of the many underlying causes of the syndrome as they are experimentally linked to metabolic and reproductive derangements resembling PCOS-related disorders. Exposure to endocrine-disrupting chemicals may act as an environmental modifier to worsen symptoms of PCOS in affected females or to contribute to the final phenotype of the syndrome in genetically predisposed individuals. Keywords: Bisphenol A; endocrine disruptors; metabolism; PCOS; reproduction. *Corresponding author: Evanthia Diamanti-Kandarakis, Third Department of Internal Medicine, University of Athens Medical School, Sotiria General Hospital, Mesogion Avenue 152, Athens 11527, Greece, Phone/Fax: +003210 7778838, E-mail: [email protected] Eleni Palioura and Eleni Kandaraki: Third Department of Internal Medicine- Endocrine Section-, University of Athens Medical School, Sotiria Hospital, Greece
Introduction Polycystic ovary syndrome (PCOS) represents a common endocrine disorder among women of reproductive age [1]. With a lack of a clear etiology for its generative roots, PCOS is now considered to enclose genetic and environmental components [2] leading to phenotypic variability. Traditionally, women with PCOS manifest both reproductive (chronic anovulation, hyperandrogenism, polycystic ovarian morphology) and metabolic (insulin resistance, metabolic syndrome, obesity) derangements. However, there is much heterogeneity of clinical and biochemical features raising the possibility that a cluster of etiological factors synergistically contribute to the final PCOS phenotype.
One hypothesis proposes that androgen excess early in life (prenatal/prepubertal) may lead to manifestation of PCOS in adulthood [3, 4], and furthermore, the heterogeneity of phenotypes can be explained on the basis of the interaction of this disorder with other genes and with the environment [5]. The role of the environment is currently emerging especially in view of the modern epidemics of obesity and type 2 diabetes sweeping developed countries. According to the recent endocrine society scientific statement, there is accumulative evidence pointing to a role of endocrine disruptors in the etiology of complex diseases such as obesity, diabetes mellitus, and cardiovascular disease [6]. For over 30 years, scientists have focused on the impact of endocrine disrupting chemicals on human health. Endocrine disruptors (EDs) are defined as “exogenous agents that interfere with synthesis, secretion, transport, metabolism, binding action, or elimination of natural blood-borne hormones that are present in the body and are responsible for homeostasis, reproduction, and developmental process” [7]. They are found abundantly in the environment, and people are constantly exposed to them via a variety of sources including contaminated water, oil, air, and food. Furthermore, there is no endocrine system that is immune to these substances [6]. As endocrine-disrupting chemicals are involved in many aspects of female reproductive disorders, it is reasonable to assume that similar factors may have also factored in the PCOS epidemic targeting both the metabolic and the reproductive characteristics of the syndrome. This article will review evidence linking exposure to EDs, and more specifically to Bisphenol A, with metabolic and reproductive disorders resembling PCOS traits providing a pathophysiologic background for such interplay.
PCOS pathophysiology Metabolic abnormalities in PCOS Hyperinsulinemia and peripheral insulin resistance (IR) are the central features of the metabolic disorder, which
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138 Palioura et al.: Endocrine disruptors and PCOS: impact on metabolic and reproductive phenotypes is now recognized as a significant characteristic of PCOS. Insulin resistance occurs in approximately 60%–80% of women with PCOS partly independently of body weight [8]. At a molecular level, there is a postbinding defect in receptor signaling likely due to increased receptor and insulin receptor substrate-1 serine phosphorylation [9]. Furthermore, increased basal insulin β-cell secretion with decreased hepatic insulin clearance is an additive factor contributing to hyperinsulinemia of PCOS women [9]. The defects in insulin action and secretion increase the risk for development of metabolic disorders. Indeed, 30%–40% of women with the polycystic ovary syndrome have metabolic disturbances manifested as impaired glucose tolerance and 10% have type 2 diabetes by their fourth decade [10]. Obesity is another common trait of PCOS encountered in 30%–70% [10] of affected females and has an aggravating impact on the syndrome’s symptoms. Obesity has an aggravating impact on the ovulatory process through a yet, not totally understood, mechanism in which insulin resistance and compensatory hyperinsulinemia seem to be major pathophysiologic mechanisms of reproductive disorders through a direct effect on the insulin-sensitive PCOS ovary.
Reproductive abnormalities in PCOS The ovulatory dysfunction in PCOS is characterized by disturbed follicular development with excessive early follicular growth and abnormal later stages of arrested follicle growth well before expected maturation [11]. This disturbed follicle growth results to the classic PCO morphology with accumulation of small antral follicles (typically arrested at a diameter of 4–8 mm) without selection of a dominant, preovulatory follicle [5]. Clinically, women experience irregular ovulation with menstrual irregularities of oligo-/amenorrhea that may cause infertility due to difficulties in conceiving. Apart from impaired folliculogenesis, the PCO ovary is also characterized by impaired steroidogenesis with theca cells remaining the primary source of hyperandrogenism. There is clear evidence for intrinsic abnormality of constitutive hypersecretion of androgen by ovarian theca cells [12]. Though specific steroidogenic enzyme defects – namely, CYP17 (coding for P450c17 and the associated P450 reductase) and CYP11a (P450scc) have been incriminated in the genesis of ovarian hyperandrogenemia, a causal interplay is not currently proven [13]. Apart from theca cells, granulosa cell deregulation may also play a role in PCOS pathogenesis, via secretion of intra-ovarian regulatory factors, such as anti- Müllerian hormone
(AMH), that are believed to modulate follicular steroidogenesis [14].
Neuroendocrine dysfunction in PCOS Despite the various PCOS recognized phenotypes and the unclear etiology, derangement of the neuroendocrine features is almost a unanimous finding of the syndrome and undoubtedly correlated with its pathogenesis. Women with PCOS show an acceleration of the gonadotropinreleasing hormone (GnRH) pulse generator activity, which causes inappropriate gonadotropin synthesis and release, with elevated luteinizing hormone (LH) and low folliclestimulating hormone (FSH) levels [15]. Normally, during luteal phase, increased levels of progesterone in the presence of estradiol, inhibit and, as a result, decrease the pulsatility of GnRH and LH [16]. The loss of this physiological pattern and the decreased sensitivity of GnRH pulse generator to progesterone suppression in the presence of hyperandrogenemia, favor LH production over FSH and impairs follicular development [17]. Furthermore, the increased LH frequency pulsatility promotes the increased androgen production from theca cells, whereas the compensatory aromatization to estrogens expected by the action of FSH on the granulosa cells is impaired, due to the reduced FSH levels. As a result a ‘vicious circle’ promoting hyperandrogenemia and anovulation is developed [15]. In fact, the use of antiandrogens, such as flutamide, was shown to restore the sensitivity of GnRH pulse generator in women with PCOS in 4 weeks [18].
Endocrine disruptors and PCOS As previously analyzed, PCOS pathophysiology combines metabolic and reproductive disorders both of which could be targeted by exposure to endocrine-disrupting chemicals. In the following section, experimental evidence will be presented to show how EDs can disrupt hormonal balance and result in abnormalities of metabolism and reproduction resembling those observed in PCOS-affected females. This interaction may represent a plausible pathophysiologic link between EDs and the clinical PCOS.
Metabolic actions of endocrine disruptors The role of environmental chemicals in the etiology of complex metabolic diseases such as obesity, insulin resistance, metabolic syndrome, type 2 diabetes is a new area of
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research. Accumulating data are pointing to the potential involvement of these chemicals either directly or indirectly in the pathogenesis of obesity and diabetes [6], both of which are metabolic characteristics of PCOS women. The idea that chemicals in the environment could be contributing to the obesity epidemic was first introduced in an article by Paula Baillie-Hamilton published in 2002. In this paper, the author presented evidence from earlier toxicologic studies in which low-dose chemical exposures were associated with weight gain in experimental animals [19]. In the following years, the term “obesogens” was applied to describe “molecules that inappropriately regulate lipid metabolism and adipogenesis to promote obesity” [20]. Most known or suspected obesogens are endocrine disruptors. The list of compounds suspected to have “obesogenic” properties is heterogeneous and includes a variety of molecules; industrial chemicals [Bisphenol A (BPA), organotins, perfluorooctanoic acid, phthalates, polybrominated diphenyl ethers, polychlorinated biphenyl ethers], pharmaceutical agents such as diethylstilbestrol, organophosphate pesticides, dietary factors such as genistein, and other environmental pollutants [21]. The complexity of obesogen effects is reflected by their diverse modes of action; some obesogenic effects are believed to be mediated by sex steroid dysregulation (ex. DES, genistein, and BPA mainly act upon estrogen receptors), others include effects on metabolic sensors (ex. organotins and phthalates activate peroxisome proliferator-activated receptor gamma (PPARγ), the master regulator of adipogenesis), while others could potentially target the central integration of energy balance [22]. At the level of the adipocyte, environmental chemicals have been shown to promote adipocyte differentiation and to affect mature adipose tissue metabolism by targeting lipogenesis, lipolysis, and adipokine secretion [23]. Epidemiology studies support the findings in experimental animals and show a link between exposure to environmental chemicals and the development of obesity [24, 25]. Similarly, there are findings to connect endocrine disruptors with metabolic disturbances including diabetes and cardiovascular diseases [26]. Widespread EDs, such as dioxins, pesticides, and bisphenol A, cause insulin resistance and alter β-cell function in animal models [27] conditions that are both known as underlying causes of diabetes.
Reproductive actions of endocrine disruptors Female reproductive system seems particularly vulnerable to exposure to endocrine disrupting chemicals, which
have been linked with a variety of reproductive disorders from anatomical abnormalities of the reproductive tract to disturbed puberty onset and adult life disorders such as endometriosis and subfertility [6]. The female gonad is also targeted either through a direct effect on the level of the ovarian function [28] or as an indirect effect on its neuroendocrine control. At the level of the ovary, it seems that both follicular growth and steroid hormone production [29] can be disturbed by exposure to EDs. Several environmental chemicals have been shown to exert direct inhibitory and/or stimulatory effects on the expression and/or activity of key ovarian steroidogenic enzymes [28, 29], and therefore, as the hormone is no longer available, normal follicular development and maturation are disrupted. Furthermore, there are studies to show that environmental contaminants also target ovarian hormone receptors including gonadotropin receptors, which bind LH and FSH [29] probably leading to inappropriate ovary response to gonadotropin stimulation. Among the chemicals incriminated as ovarian disruptors are pesticides (e.g., dichlorodiphenyltrichloroethane and methoxychlor), plasticizers (e.g., bisphenol A and phthalates), dioxins, polychlorinated biphenyls, pharmaceutical agents (diethylstilbestrol), and phytoestrogens (genistein) [30].
Neuroendocrine actions of endocrine disruptors Αn indirect effect on female reproduction could be accomplished by targeting reproductively relevant neuroendocrine function at the hypothalamus-pituitary level. Indeed, accumulative evidence illustrate the interference of endocrine-disrupting chemicals (Bisphenol A, genistein, methoxychlor, polychlorinated biphenyls, estradiol benzoate) at several points of the axis leading to alteration of GnRH signaling in the pituitary [31]. The action of individual chemicals at different levels, via different mechanisms, adds to the level of complexity in determining the adverse effects on neuroendocrine physiology. The pathways involved in the disruption of GnRH signaling include a direct affect on the expression of estrogen-sensitive neuropeptides associated with the regulation of GnRH such as kisspeptin and galanin as well as a direct effect on GnRH neuron expression in the hypothalamus [31]. Furthermore, impaired steroid feedback on GnRH neurons has also been described as another potential pathway [31]. Differences have also been observed in behavior and mating success of laboratory animals as a result of endocrine-disrupting effects on sexually dimorphic brain regions [6].
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140 Palioura et al.: Endocrine disruptors and PCOS: impact on metabolic and reproductive phenotypes
Endocrine disruptors and PCOS: focus on Bisphenol A (BPA) BPA is one of the world’s highest production – volume synthetic chemicals with a large number of applications including use in certain food and drink packaging, in many plastic consumer products as well as in dental materials [32]. Human exposure is widespread and constant given that measurable levels of BPA have been detected in the serum and in many biological fluids [32]. Laboratory rodent studies incriminate BPA as a disruptor of multiple endocrine systems including neuroendocrine function, ovarian function and fertility, as well as metabolism [33]. Human studies indicate that BPA exposure in adults may be associated with both male (reduced male sexual function, reduced sperm quality) and female (reduced ovarian response and IVF success, reduced fertilization success and embryo quality, implantation failure, miscarriage, premature delivery endometrial disorders) reproductive disorders as well as metabolic diseases (type-2 diabetes, cardiovascular disease, hypertension, obesity, altered liver function) [34]. Furthermore, other potential health effects include altered thyroid hormone concentrations and immune system function, while exposure during critical stages of development is associated with disorders during gestation such as increased spontaneous abortion, abnormal gestation time, reduced birth weight or early in life such as altered behavior and disrupted neurodevelopment in children, as well as increased probability of childhood wheeze and asthma [34]. Neonatal BPA exposure has been linked to the development of PCOS reproductive phenotype later in life [35], and women with PCOS have increased BPA levels compared to controls [36]. These interactions will be subsequently analyzed providing the pathophysiological background of a potential interplay between this endocrine disruptor and PCOS.
Evidences for the relation between exposure to BPA and obesity and diabetes induction Bisphenol A is identified as a potential “obesogen” molecule. In vitro studies using 3T3-L1 cells (mouse fibroblasts that can differentiate into adipocytes) show a link between this environmental chemical with obesity by a promoting effect on adipocyte differentiation [37–39] and lipogenesis [40]. In vivo studies with rodents associate pre- [41] and perinatal [42, 43] exposure to low doses of BPA with increased adipose storage later in adult life. Similar concentrations are also able in vitro to affect human adipocyte cells’ endocrine function. In these cells, BPA was shown to inhibit
adiponectin release [44], a critical adipokine that increases insulin sensitivity and reduces tissue inflammation and stimulate the release of inflammatory adipokines such as interleukin-6 (IL-6) and tumor necrosis factor α (TNF-α) [45]. Both actions could lead to insulin resistance and increased susceptibility to development of metabolic syndrome. Epidemiological studies show a positive association between urinary BPA and obesity in general adult population of the US [25] as well as in children and adolescents [46]. Apart from obesity, higher BPA exposure, reflected in higher urinary concentrations of BPA in the general adult population of the US was shown to be associated with diabetes and cardiovascular diseases [26]. Furthermore, higher urinary BPA levels were found to be associated with prediabetes in 3516 subjects, free of diabetes risk factors [47]. The connection between BPA and metabolic disturbances including diabetes is plausible given that recent experimental studies have suggested that BPA causes pancreatic β-cell dysfunction. BPA has been demonstrated to enhance insulin biosynthesis and secretion in β-cells in adult male mice, leading to β-cell overstimulation and dysfunction and subsequent peripheral insulin resistance [48]. Pancreatic α-cells are also affected. Low doses of BPA are shown to impair the molecular signaling that leads to secretion of glucagon by suppressing intracellular calcium ion oscillations in α-cells in response to low blood glucose levels [49].
Evidences for the relation between exposure to BPA and reproductive disorders Detectable concentrations of BPA have been measured in human follicular fluid [50]. Many experimental studies show reproductive effects in exposed animal models at the level of ovarian follicle itself targeting both follicle growth and ovarian steroid production, processes affected in PCOS individuals. Culture of antral follicles isolated from 32-day-old mice with BPA inhibits growth of antral follicles and reduces production of progesterone, dehydroepiandrosterone, androstenedione, estrone, testosterone, and estradiol. Furthermore, BPA-induced inhibition of steroidogenesis is accompanied by decreased expression of Star andP450scc mRNA [51]. Impaired ovarian follicular development following developmental exposure to BPA has been reported in rodents manifested as an excess of antral follicles with decreased corpora lutea formation [52] in a dose-related manner [53]. Interestingly, these effects that are suggestive of a reduced number of ovulated oocytes were observed at low, environmental equivalent doses of BPA. Similarly, in
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a diverse reptile animal model, the blocking interference of BPA at the level of oocyte maturation has also been demonstrated but only at the highest dose [54]. In another rat model, prepubertal exposure to BPA disrupts normal ovarian follicle development by decreasing the expression of follicle development-promoting genes and increasing the expression of AMH gene that has a follicular development inhibitory effect [55]. Another study evaluating the effects of BPA on cultured rat thecainterstitial and granulosa cells showed dose-dependent alterations in sex steroid levels and mRNA for steroido genic enzymes [56]. In this study, Zhou et al. demonstrated increased testosterone synthesis and mRNA expression of 17-β hydroxylase (P450c17) and cholesterol side chain cleavage enzyme (P450scc) in theca-interstitial cells [56], key enzymes believed to be involved in PCOS pathogenesis [13]. In granulosa cells, BPA increased progesterone, but decreased E2 and P450arom mRNA levels [56].
Evidences for the relation between exposure to BPA and neuroendocrine disorders Developmental exposure to BPA has been shown to affect the hypothalamo-pituitary axis as indicated by significant impairments in gonadotropin secretion observed in laboratory animals. Collet et al. [57] presented data, obtained from prepubertal ovariectomized lambs exposed intravenously for 54 h to BPA, concerning the impact of the chemical on the hypothalamus-pituitary axis. Researchers observed an inhibitory effect of BPA on LH secretion [57]. Analogous findings are reported in rodents. Perinatal rat exposure to BPA is accompanied with interruption of gonadotropin secretion indicated by the low circulating LH levels [42]. In another experimental study, neonatal exposure to BPA decreased basal and GnRH-stimulated serum LH and also increased GnRH pulse frequency in infantile rats, an effect that persisted throughout life. Interestingly, GnRH signaling in the adult pituitary was severely affected as shown by impaired GnRH-stimulated inositol 1,4,5-triphosphate (IP3) formation [58].
Experimental and clinical evidences for the relation between exposure to BPA and PCOS It is evident from the above that BPA exposure is experimentally related to the induction of both metabolic and reproductive abnormalities resembling PCOS disorders. A pathophysiologic interference is biologically plausible in all aspects of PCOS pathogenesis from ovarian
follicle level to adipose tissue function and neuroendocrine system regulation. Such interplay is proven in an animal rat model of neonatal exposure to increasing doses of Bisphenol A that led to a PCOS-resembling phenotype in adult life [35]. Rats given subcutaneous injections daily of 50 or 500 μg/day from postnatal days 1–10 had higher levels of circulating testosterone and estrogen and lower levels of progesterone as adults. This sex steroid status resembles the abnormally elevated androgen levels in PCOS women. Furthermore, ovarian follicle growth and ovarian morphology were also affected; ΒPA-exposed rats (either 50 or 500 μg/day) had lower numbers of antral follicles, while animals treated with 500 μg/day BPA as neonates also had ovarian cysts. Regarding fertility outcomes, the highest BPA dose resulted in anovulation in affected rodents, and consequently, no pups were delivered from this group. Animals treated with 50 μg/day BPA were also affected as they delivered ~30% fewer pups without changes in the number of oocytes. Finally, exposure to 500 and 50 μg/day BPA had a neuroendocrine effect with accelerated GnRH pulse frequency in hypothalamic explants from adult rats [35]. In this animal model, the reproductive disorders found in PCOS are present in the rodents even with the dose of 50 μg/day BPA (a dose lower than the lowest observed adverse effect level). On the whole, in this rat model of BPA exposure, the coexistence of biochemical hyperandrogenemia, unovulation, infertility, polycystic ovarian morphology, and increased GnRH pulse interestingly points to a link with PCOS characteristics. Unfortunately, the metabolic effects, commonly seen in PCOS, were not investigated. Concerning humans, a recent study by E. Kandaraki et al. [36] showed that BPA levels are significantly higher in women with polycystic ovarian syndrome compared to normal women independently of body weight and could possibly be one of many underlying causes of this disorder. Confirmative of this hypothesis was the strong association observed between BPA levels with androgens and insulin resistance indices, implying a potential role of this endocrine disruptor to the two major components of PCOS pathophysiology [36]. Furthermore, it is known that women with ovulatory dysfunction compared to regularly ovulating women have higher serum BPA levels [59], and both groups display lower BPA levels compared to males [60]. A connection between androgens and BPA is obvious; however, it is not clear whether this is the result of an androgen-induced inhibition of BPA catabolism in the liver [61], or it is the consequence of BPA-induced alteration in androgens metabolism. BPA has been reported to significantly inhibit the activity of two different testosterone hydroxylases leading to decreased testosterone catabolism
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142 Palioura et al.: Endocrine disruptors and PCOS: impact on metabolic and reproductive phenotypes [62]. Additionally, BPA has the ability to bind potently with sex hormone-binding globulin (SHBG), thus, competing natural androgens whose circulating levels increase [63].
Conclusion Experimental studies converge to the involvement of EDs in all aspects of metabolic and reproductive regulation leading to disorders similar to PCOS aberrations. Among the wide variety of chemicals, Bisphenol A provides a clear example of development of a PCOS reproductive phenotype in laboratory animals. Furthermore, the increased level of this estrogenic substance in PCOS females and its association with androgens and insulin resistance, the cardinal features of the syndrome, may interestingly point to a link between BPA and PCOS pathophysiology. Whether this interplay is related to androgen-induced modifications of BPA clearance or BPA per se affects androgen metabolism, and production remains to be explored. New animal models combining both metabolic and reproductive PCOS phenotypes are necessary to comprehend the generative roots of this, yet not fully understood, endocrine disorder.
Highlights –– Experimental evidence link exposure to EDs with PCOS-related metabolic abnormalities; obesity, insulin resistance, diabetes.
–– Experimental evidence link exposure to EDs with PCOS-related reproductive abnormalities; impaired follicle development and steroidogenesis, neuroendocrine dysfunction. –– In vivo and in vitro studies associate BPA exposure to the induction of both metabolic and reproductive disorders. –– Neonatal BPA exposure leads to a PCOS reproductive phenotype later in life. –– BPA levels are elevated in PCOS women. –– Association between BPA and androgens – a cause or an effect?
Outlook Future research should focus on the development of an animal model of exposure to endocrine-disrupting chemicals combining both the metabolic and reproductive malfunctions occurring in the syndrome. This will provide insights into the environmental component of PCOS pathogenesis especially in early life exposures. Furthermore, the association between Bisphenol A and androgens should be elucidating as a potential aggravating factor that may help to comprehend the etiology and the ideal management/treatment of the syndrome. Received January 11, 2014; accepted February 12, 2014; previously published online March 8, 2014
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