Endoplasmic Reticulum Stress: A New Research Direction for Polycystic Ovary Syndrome?

Abstract

Polycystic ovary syndrome (PCOS) is one of the most common gynecological endocrine disorders, with sporadic ovulation, excessive androgens, and polycystic ovarian changes as the main clinical manifestations. Due to the high heterogeneity of its clinical manifestations, the discussion on its pathogenesis has not been unified. Current research has found that genetic factors, hyperandrogenism, chronic inflammation and oxidative stress, insulin resistance, and obesity are strongly associated with PCOS. Recently, when studying the specific mechanisms of the abovementioned factors in PCOS, the biological response process of endoplasmic reticulum stress (ERS) has gradually come to researchers' attention, and several studies have confirmed the involvement of ERS in the pathogenesis of PCOS and the improvement of a series of pathological manifestations of PCOS after the application of ERS inhibitors, which may be a new entry point for the treatment of PCOS. In this article, we review the relationship between ERS and various pathogenic factors of PCOS.

Introduction

Polycystic ovary syndrome (PCOS) is a common chronic systemic endocrine disorder in women of reproductive age, with a prevalence of ∼6–15%. PCOS is thought to be associated with insulin resistance (IR), hyperandrogenism, and chronic inflammation and oxidative stress (OS), but the exact pathogenesis is not yet clear (Aversa et al., 2020). PCOS not only affects a woman's fertility but also seriously affects her physical and psychological state. Due to the hyperandrogenic state of the body, patients with PCOS develop skin disorders such as hirsutism and acne, and minor degrees of androgenetic alopecia are more frequent.

In addition, the risks of developing type 2 diabetes, metabolic syndrome, nonalcoholic fatty liver, and cardiovascular disease increase (Escobar-Morreale, 2018) and may even be accompanied by some malignant lesions, such as endometrial cancer (Barry et al., 2014). Given the risks that PCOS poses to women, it is imperative to identify the causes of PCOS and to find safer and more effective treatment options for PCOS.

The endoplasmic reticulum (ER) is the largest membrane-like organelle in eukaryotic cells, consisting of continuous lamellar or tubular structures that span the cytoplasm and are involved in important life processes, including protein synthesis, modification, and transport, lipid and steroid synthesis, Ca²⁺ homeostasis, and regulation of secretion (Schwarz, 2015). The ER is exceptionally sensitive to changes in the peripheral environment (Gong et al., 2017). Under the stimulation of exogenous stimuli such as hypoxia, nutritional deficiency, and virus infection and endogenous factors such as cell differentiation and metabolic abnormalities, the phenomenon of steady-state imbalance can easily occur.

At this time, to maintain the stability of the intracellular environment and the normal life activities within the cell, the endoplasmic reticulum will carry out a series of stress responses, known as ER stress (ERS) (Guzel et al., 2017). ERS can be broadly classified into three types: unfolded protein response (UPR), endoplasmic reticulum overload response, and sterol regulatory element-binding protein (Ni et al., 2021).

Taking the UPR cell signaling pathway as an example, this pathway mainly includes the phosphorylated endoplasmic reticulum kinase (PERK) pathway, the inositol-requiring enzyme 1 (IRE1) pathway, and the activating transcription factor 6 (ATF6) pathway. The three pathways attempt to restore cellular homeostasis by expanding the size of the ER, increasing protein folding capacity, and degrading unfolded/misfolded proteins (Walter and Ron, 2011). The proteostasis mechanisms mediated by early UPR signaling are involved in a wide range of physiological events, including cell proliferation, differentiation, migration, invasion, angiogenesis, and/or growth factor/cytokine release (Guzel et al., 2017), with the intention of maintaining normal cellular function and restoring homeostasis in the body.

In addition to being a self-protective mechanism for cells, ERS is also an important apoptotic pathway independent of the classical mitochondrial damage pathway and the death receptor (DR) signaling pathway, which can cause apoptosis if endoplasmic reticulum function is persistently disrupted (Hu et al., 2019).

Mitochondria are the place of aerobic respiration in cells and are closely related to energy metabolism in cells. In addition to supplying energy to cells, mitochondria are also involved in processes such as intercellular information transmission and apoptosis and have the ability to regulate cell growth and the cell cycle. Mitochondria are also abundant in ovarian granulosa cells (GCs) and participate in the regulation of the cell cycle, metabolism, and signal transduction in GCs (Sirard, 2019).

Through the study of mitochondrial DNA variation and mitochondrial epigenetics in PCOS model mice (Ernst and Lykke-Hartmann, 2018), it was found that abnormal mitochondrial function can not only affect follicular development but also induce cell death (Li et al., 2021), which shows that mitochondria also play an important role in the pathogenesis of PCOS. The endoplasmic reticulum is closely related to mitochondria through mitochondria-associated membranes (MAMs).

MAMs, also known as the mitochondria-associated endoplasmic reticulum membrane, are a region between the mitochondria and the endoplasmic reticulum that is in close proximity, but not fused, allowing the exchange of information and materials between the ER and mitochondria, while maintaining its independence, and this structure plays an important role in the regulation of mitochondrial dynamics, Ca²⁺ signaling, lipid processing and transport, mitochondrial autophagy, and ERS (Gao et al., 2018).

One of the most critical roles of MAMs is to enable calcium signaling between two organelles. Ca²⁺ is a widespread intracellular signaling molecule that is involved in intracellular signaling and plays an important role in a variety of cellular physiological processes, including oocyte maturation. Recent studies have found that some Ca²⁺ channel inhibitors can inhibit oocyte activation, which further supports the important role of Ca²⁺ in follicular development (Wang et al., 2021).

The ER is an important place for intracellular Ca²⁺ storage. The accumulation of misfolded proteins in the ER lumen can lead to Ca²⁺ leakage from the ER, which can then enter mitochondria through MAMs, affecting the function of mitochondria. During the UPR adjustment phase, the ER regulated by the MAM proteome undergoes Ca²⁺ transfer to the mitochondria, maintaining mitochondrial metabolism and ATP production, thus sustaining normal cellular function. However, severe and sustained ERS can induce Ca²⁺ overload, reactive oxygen species (ROS) accumulation, and ATP depletion in mitochondria, which activates mitochondria-dependent apoptosis (Chaudhari et al., 2014). In addition to MAMs, MFN2 is also a key regulatory protein in the interaction between mitochondria and the ER, with a wide range of contact sites between mitochondria and the ER.

It was found that the deletion of MFN2 in mouse embryonic fibroblasts leads to ER morphological changes, induces ER binding to mitochondria, and ultimately leads to abnormal Ca²⁺ signal transduction, thereby causing amplification of apoptotic signals (Filadi et al., 2018). Therefore, it is not difficult to find that the ER connects with mitochondria through specific tissue structures, such as MAMs and Mfn2, participates in the process of ovarian function and follicular development, and thus affects the development of oocytes and GCs, which may be one of the reasons for PCOS.

In addition to participating in the development of PCOS through correlation with mitochondria, many other studies have shown that ERS is involved in hyperandrogenism (Azhary et al., 2019; Jin et al., 2020), inflammation and OS (Hang et al., 2020; Li et al., 2020), IR (Yaribeygi et al., 2019), cancer (Urra et al., 2016; Xu et al., 2017), cardiovascular disease (Hamczyk et al., 2019; Yang et al., 2020), metabolic syndrome (Yaribeygi et al., 2019), nonalcoholic fatty liver (Lebeaupin et al., 2018), and other physiological or pathological processes, including causative factors and distant complications of PCOS. The aim of this review is to investigate whether there is an association between ERS and the development of PCOS, with the aim of exploring a new and feasible treatment for PCOS.

ERS and Hyperandrogenism

GC apoptosis and follicular atresia

Apoptosis is an important pathway of cell death in organisms and an important part of follicular development in the female life cycle. In women of reproductive age, ∼3–11 follicles are recruited each month, and usually only one dominant follicle develops to maturity and is expelled, while the rest of the follicles develop to a certain extent and degenerate on their own through apoptotic mechanisms, that is, follicular atresia (Matsuda et al., 2012). Ovarian GCs can form receptors for various hormones, such as follicle-stimulating hormone (FSH) and estrogen (E), which are closely related to the processes of follicle development, selection of the dominant follicle, ovulation, and formation of the corpus luteum (Juengel et al., 2006; Regan et al., 2018).

There is a cell gap connection between GCs and oocytes, through which GCs can transmit various information to oocytes, including apoptotic information. Atresia of that follicle occurs when GC apoptosis reaches 10% or more (Wang et al., 2014); thus, apoptosis of GCs is an important part of the dominant process of follicular development (Markstrom et al., 2002) and a direct cause of follicular atresia in all stages of follicular cyclic development (Perez et al., 2000). One of the prominent manifestations of PCOS is polycystic ovarian changes, which manifest as an excess of immature small antral follicles that stop growing at 2–9 mm and fail to form dominant follicles (American College of Obstetricians and Gynecologists, 2018). Therefore, it is hypothesized that abnormal apoptosis of ovarian GCs may be an important pathogenic factor in PCOS.

Hyperandrogenism and GC apoptosis

High androgen can lead to clinical manifestations such as acne, hirsutism, ovulation disorder, and menstrual disorders (Ye et al., 2021). Studies have shown that androgens can induce apoptosis by activating the intrinsic apoptotic pathway and reducing follicular growth factor production (Lim et al., 2017a, 2017b). A significant increase in the number of apoptotic GCs compared to normal ovaries can be found in the ovaries of PCOS patients and animal models of PCOS (Mikaeili et al., 2016; Lima et al., 2018), and the rate of apoptosis of ovarian GCs can be reduced by androgen reduction therapy in PCOS patients (Xia et al., 2015), suggesting that high levels of androgens may induce apoptosis of ovarian GCs through some pathways, which leads to impaired follicular development.

ERS and GC apoptosis

C/EBP homologous protein (CHOP) is a transcription factor in the UPR signaling pathway and is involved in the ERS-induced apoptosis pathway (Urra et al., 2013). DR 5, a transcriptional target of CHOP, plays a key role in ERS-induced apoptosis by increasing the production of autocrine death ligand signals. Azhary et al. (2019) observed in antral follicles of PCOS patients and dehydroepiandrosterone (DHEA)-induced PCOS model mice that increased GC apoptosis was accompanied by upregulated expression of multiple UPR pathway transcription factors, including CHOP and DR5. The application of ERS inhibitors to PCOS mice resulted in a decrease in the number of apoptotic GCs and a concomitant decrease in the expression of DR5 and CHOP.

These findings suggest that androgens alter the pathological state of PCOS by inducing ERS within GCs, which in turn causes GC apoptosis (Fig. 1).

FIG. 1.

High levels of androgens may induce apoptosis of ovarian granulosa cells through some UPR pathways. The application of ERS inhibitors to PCOS mice resulted in a decrease in the number of apoptotic granulosa cells and a concomitant decrease in the expression of DR5 and CHOP. PCOS, polycystic ovary syndrome; ERS, endoplasmic reticulum stress; CHOP, C/EBP homologous protein; DR5, death receptor 5; DHEA, dehydroepiandrosterone; UPR, unfolded protein response.

Advanced glycation end products (AGEs) are compounds produced by the spontaneous reaction of macromolecules such as proteins, lipids, or nucleic acids with glucose or other reducing monosaccharides, without the involvement of enzymes (Merhi et al., 2018). Diabetes, IR, aging, and OS can stimulate the production of AGEs directly or increase their levels by decreasing renal clearance (Unoki and Yamagishi, 2008; Tatone and Amicarelli, 2013).

AGEs can stimulate the production of the proinflammatory cytokines IL-6 and IL-8 in GLCs (Takahashi et al., 2019), which may disrupt the process of oocyte development, growth, and maturation in the ovary through the ERK1/MAPK signaling pathway (Diamanti-Kandarakis et al., 2013). In addition to the body's own formation, long-term intake of high-calorie food will also lead to an increase in AGE content in the body (Uribarri et al., 2005).

In animals, a high AGE diet triggers IR, resulting in increased serum testosterone, increased ovarian weight, and overexpression of the receptor for advanced glycation end products (RAGEs) in the ovary (Diamanti-Kandarakis et al., 2007). The AGE-RAGE system can directly or indirectly induce ERS and play an important role in the pathogenesis of many metabolic diseases, such as diabetes, obesity, inflammation, and PCOS (Diamanti-Kandarakis et al., 2016).

The interaction between the two interferes with the intracellular signal transduction of insulin and glucose transport in human GCs and aggravates OS in the body (Papalou et al., 2016), which may affect ovarian function and follicular growth, resulting in a decline in fertility (Diamanti-Kandarakis et al., 2016). The effect is especially pronounced under conditions such as hyperglycemia, hypoxia, and OS (Piperi et al., 2012). Previous studies found that AGEs were significantly increased in ovarian GCs of PCOS patients (Diamanti-Kandarakis et al., 2007), and further studies based on this phenomenon by Azhary et al. found that overexpression of AGEs and RAGEs in ovarian GCs of PCOS patients and PCOS model mice was caused by excess androgens in vivo, which in turn was mediated by the transcription factor UPR—CHOP.

Subsequently, researchers treated PCOS mice with ERS inhibitors and found decreases in the expression levels of AGEs and RAGEs, an improvement in estrous cycles, and a consequent decrease in the number of sinus follicular atresia (Azhary et al., 2020). From these observations, the investigators speculated that the hyperandrogenic state in PCOS patients could trigger the occurrence of ERS in ovarian GCs, resulting in the accumulation of AGEs, which in turn could lead to abnormal apoptosis of GC and ultimately trigger the metabolic and reproductive consequences of PCOS (Fig. 2).

FIG. 2.

Excess androgens can cause the overexpression of AGEs and RAGEs in ovarian granulosa cells of PCOS patients and PCOS model mice. After treating PCOS mice with ERS inhibitors, the expression levels of AGEs and RAGEs decreased, the estrous cycles improved, and the number of sinus follicular atresia decreased. AGEs, advanced glycation end products; RAGEs, receptor for advanced glycation end products.

The above findings suggest that chronic androgen overload may disrupt the balance between the body's intracellular protein demand and the ER protein supply capacity, leading to the onset of ERS, and this unadjustable imbalance can lead to abnormal apoptosis of ovarian GCs, affecting normal follicle development and ultimately leading to the development of PCOS.

IR and ERS

IR and PCOS

IR manifests as a state in which cells fail to respond normally to a certain concentration of insulin, resulting in abnormal glucose transfer and utilization functions (Petersen and Shulman, 2018). Studies have shown that insulin signaling pathways are significantly abnormal in ovarian tissues of PCOS patients (Zhang et al., 2016), and insulin can be involved in the development of PCOS by mediating two molecular signaling pathways, the phosphatidylinositol 3-kinase (PI-3K)/Akt pathway (Munir et al., 2004) and the mitogen-activated protein kinase (MAPK) pathway, which are involved in metabolic actions and promote cell growth, proliferation, and differentiation (Zhang et al., 2016; Arkun and Yasemi, 2018).

IR plays an important role in metabolic disorders and anovulation in PCOS and is strongly associated with distant metabolic complications such as obesity, dyslipidemia, metabolic syndrome, hypertension and atherosclerosis, nonalcoholic fatty liver disease (NAFLD), and type 2 diabetes mellitus (T2DM) (Yaribeygi et al., 2019), and IR is present in ∼44–85% of PCOS patients (Jeanes and Reeves, 2017).

IR and ERS

The body maintains adequate insulin levels in the body by increasing insulin transcription and translation to maintain normal blood glucose. Corresponding to the powerful secretory function of islets, islet β-cells are rich in ER. Prolonged persistence of IR forces islet β-cells to secrete more insulin, resulting in compensatory hyperinsulinemia (Gutiérrez-Rodelo et al., 2017). To maintain the abnormally high demand for insulin in vivo, the ER usually works at the limit of its capacity, which can easily lead to the accumulation of misfolded insulinogen in the ER of β-cells, which in turn activates the ERS (Ozcan et al., 2008).

ERS in pancreatic β-cells disrupts insulin receptor synthesis and glucose transporter 4 (GLUT-4) expression and reduces autophagy, leading to impaired insulin signaling (Yaribeygi et al., 2019). The UPR, an acute mechanism for re-establishing cellular homeostasis in ERS, often has both physiological and pathological roles in the regulation of insulin signaling and causes apoptosis of islet cells if cellular homeostasis is not restored within its capacity (Zhu et al., 2021).

IR, ERS, and PCOS

A series of research results shows that IR and hyperandrogenemia also affect each other. IR and hyperinsulinemia in PCOS patients can lead to hyperandrogenemia in many ways. In thecal cells, IR can directly increase the activity of CYP17A1, which enhances the production of androstenedione and testosterone, thereby inducing hyperandrogenism (Cadagan et al., 2016). Due to the persistence of the IR state in vivo, more insulin is secreted in a compensatory manner in the organism, which can stimulate the ovary to secrete more androgens directly or by increasing the secretion of LH. Therefore, IR is also an important reason for hyperandrogenism in PCOS (Bremer, 2008). In animal experiments, the use of DHEA induced systemic IR in PCOS mice (Song et al., 2018).

Since ERS is involved in the pathological process of hyperandrogenism and IR in PCOS patients at the same time, Zhu's group investigated the molecular regulatory mechanisms between hyperandrogenism, ERS, and β-cell function in PCOS. Testosterone caused insulin overexpression in pancreatic β-cells in mice, which in turn induced an ERS state and islet cell apoptosis in mice. Subsequent treatment of mice with an androgen receptor antagonist (Flut) and an ERS inhibitor (TUDAC) revealed that ERS and β-cell apoptosis were significantly reduced in the mice and gradually restored homeostasis in the mice. These findings suggest that ERS and the corresponding apoptotic mechanisms are important causes of islet β-cell dysfunction and IR in PCOS mice and patients (Rocha et al., 2016; Zhu et al., 2021) (Fig. 3).

FIG. 3.

Testosterone caused insulin overexpression in pancreatic β-cells in mice, which in turn induced an ERS state and islet cell apoptosis in mice. Subsequent treatment of mice with Flut and TUDAC revealed that ERS and β-cell apoptosis were significantly reduced in mice and gradually restored homeostasis in the mice. AR, androgen receptor; TUDCA, tauroursodeoxycholic acid.

Inflammation, OS, and ERS

Inflammation and OS

Chronic low-grade inflammation, also known as metabolic inflammation, is below the level of infectious and autoimmune inflammation, without local and systemic symptoms such as redness, swelling, and pain, and is mainly characterized by associated inflammatory response products such as C-reactive protein (CRP), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) (Rostamtabar et al., 2021).

Low-grade systemic inflammation is a key feature of the pathophysiology of PCOS. Toll-like receptor (TLR), CRP, TNF-α, interleukin-1beta (IL-1β), and other inflammatory response factors can be found in the serum, ovary, and GCs of patients with PCOS (Popovic et al., 2019; Kuliczkowska-Płaksej et al., 2021; Rostamtabar et al., 2021; Zhang et al., 2021), which will lead to premature apoptosis of GCs (Popovic et al., 2019) and have adverse effects on follicular maturation and ovulation.

ROS are byproducts of mitochondrial respiration and can be involved in the regulation of a variety of cellular functions (Qiao et al., 2020). Under physiological conditions, the production and neutralization of ROS are in a relatively balanced state.

However, when the body is exposed to various harmful stimuli, its own oxidation and antioxidant systems are unbalanced, which can lead to excessive ROS accumulation and cause peroxidation damage to major biological molecules such as DNA, proteins, and lipids in cells, further affecting enzyme activity, ion channel transport, and receptor signaling, causing dysregulation of gene expression and impairment of cell function, that is, OS (Dan Dunn et al., 2015). The level of ROS, a marker of OS in PCOS patients, was also significantly higher than that in normal subjects (Newsholme et al., 2012; Zuo et al., 2016; Ilie, 2018; Masjedi et al., 2019; Zhang et al., 2019).

OS and inflammation often interact with each other, and it is difficult to distinguish them completely. In the process of inflammation, phagocytes such as neutrophils produce ROS under the action of proinflammatory factors. If the inflammatory state is continuously stimulated, excessive ROS will spread out from phagocytes, which can cause OS in local tissues, further induce inflammation, and finally cause damage to multiple organs of the whole body (Siti et al., 2015; Papalou et al., 2016). In an experimental study, it was found that the expression of inflammatory factors and peroxidation metabolites increased in the PCOS rat model, indicating that chronic inflammation and OS exist simultaneously in PCOS rats (Furat Rencber et al., 2018).

Inflammation and OS are also mutually induced with ERS. The OS state in vivo can lead to the accumulation of excessive ROS and destroy intracellular homeostasis, thus inducing the occurrence of ERS (Wan et al., 2021). All three branches included in the UPR have been proven to mediate proinflammatory transcriptional programs through transcription factors such as NF-κB and activator protein 1 (AP-1) (Brenjian et al., 2020). These results show that inflammation, OS, and ERS exist simultaneously and closely in patients with PCOS (Garg et al., 2012).

Inflammation and OS seem to be closely related to hyperandrogenemia and IR. Studies have found that OS and inflammatory markers are positively correlated with androgen levels in patients with PCOS (Yang et al., 2011). IR can aggravate the inflammatory state in patients with PCOS (Victor et al., 2016). Similarly, inflammation can induce IR by interfering with the postinsulin receptor signaling pathway and the insulin receptor substrate 1-phosphatidylinositol 3-kinase-protein kinase B (IRS1-PI3K-PKB/Akt) pathway (Keane et al., 2015). In the IR state, hyperglycemia and high levels of free fatty acids in the body lead to the production of ROS, which can aggravate OS in the body (Lee et al., 2010).

Resveratrol, a natural polyphenol found in grapes, nuts, and berries, inhibits the production of proinflammatory gene products such as IL-1β, IL-6, and cyclooxygenase-2 (COX-2) and has anti-inflammatory and antioxidant effects (Yang et al., 2018). A recent study showed increased expression of inflammatory factors such as IL-1β, IL-6, TNF-α, IL-18, NF-κB, and CRP, as well as UPR pathway-related transcription factors such as CHOP and glucose-regulated protein 78 (GRP78), in ovarian GCs isolated from PCOS patients.

The levels of all these factors were decreased in GCs of the experimental group after treatment with resveratrol (Brenjian et al., 2020). The above study further supports that ERS, inflammation, and OS coexist in PCOS patients, and the application of resveratrol alleviates some clinical symptoms in PCOS patients. Therefore, we boldly suspect that the chronic inflammatory state in ovarian GCs can induce ERS, which leads to abnormal apoptosis of GCs and consequently to PCOS, and whether resveratrol controls inflammation through the ERS state in GCs remains to be further investigated.

Obesity

Obesity negatively affects the ovarian function of the body, leading to a decline in oocyte function (Snider, 2019), which is an important risk factor for PCOS. Patients with PCOS appear to have a higher risk of obesity, with ∼38–66% of PCOS patients having obesity problems, mostly in the form of increased visceral fat (Jena et al., 2018).

Adipocytes are important endocrine organs of the body and can secrete a series of bioactive factors, such as adiponectin, leptin, resistin, and interleukin (Coelho et al., 2013), which can also induce the development of inflammatory responses in other tissues, including the ovaries. Therefore, obesity itself can be considered a systemic chronic inflammatory state that increases the inflammatory response load in patients with PCOS (Snider, 2019). The hyperandrogenic environment in PCOS patients increases the number of abdominal subcutaneous adipocytes and the preferential accumulation of abdominal visceral fat and aggravates obesity (Dumesic et al., 2016).

As visceral adiposity increases, adipose-derived proinflammatory factors also increase, which can lead to a low-grade inflammatory cascade in the whole body (Spritzer et al., 2015). Foreign studies have found (Liu and Zhang, 2012) that obesity is also directly related to OS in PCOS patients, and the two are mutually causal. This vicious cycle is particularly obvious in obese patients with PCOS. Abdominal obesity can aggravate the oxidative damage of adipose tissue in PCOS patients, which in turn causes cellular damage, leading to dysfunctional secretion of cytokines by adipocytes and fat accumulation, thus triggering and aggravating obesity (Chen et al., 2014). Based on the interdependent relationship between inflammation and OS, obesity is also a chronic OS state of the body.

It has been shown that the expression levels of three branches of the UPR pathway—ATF6, IRE1α, and PERK—and the transcription factor GRP78—are elevated in obese people (Khadir et al., 2016), while the use of ERS inhibitors can reduce the expression levels of the above transcription factors and suppress the ERS status in obese patients to achieve lipid lowering. This finding suggests that ERS is involved in the process of lipid metabolism (Khadir et al., 2016; Sun et al., 2018).

Further exploration of this result in animal experiments revealed that follicular fluid from obese PCOS patients and PCOS mouse models can induce lipid hoarding in the eggs, leading to a large accumulation of unfolded or misfolded proteins in the ER and a disruption of intracellular and extracellular Ca²⁺ homeostasis, which induces ERS. The persistence of a long-term stress state induces the generation of apoptotic signals in eggs, which in the long run can reduce the developmental potential of obese mouse eggs (Wu et al., 2021). This suggests that obesity may induce ERS in ovarian cells through some mechanisms that may lead to PCOS.

Obesity is a chronic inflammatory state caused by the infiltration of proinflammatory M1 macrophages into white adipose tissue, which also leads to the occurrence of IR (Suzuki et al., 2017).

The effect of obesity on IR in patients with PCOS has been previously studied, and compared with healthy subjects, PCOS patients have obvious IR. The IR state of obese PCOS patients is more obvious compared with nonobese PCOS patients, suggesting that although obesity may not be a necessary factor for the generation of IR, obesity in PCOS patients can indeed exacerbate the degree of IR (Behboudi-Gandevani et al., 2016). Visceral adipose tissue in obese patients can induce ER stress and increase the expression levels of cytokines such as IL-6 and TNF-α through the UPR signaling pathway, thereby inhibiting the effect of insulin, causing IR, and resulting in the development of compensatory hyperinsulinemia (Jiao et al., 2011).

Weight loss in obese individuals, such as through surgery or weight loss, can lead to significant reductions in markers of ERS in adipose tissue to improve the status of ERS and IR in vivo (Khadir et al., 2016). Another study found that a mouse model fed a high-fat diet (HFD) showed hyperlipidemia and an IR state, and the level of the UPR signaling transcription factor CHOP in mouse adipocytes was also upregulated, thus inducing ERS. The use of chemical chaperones such as phenylbutyric acid and tauroursodeoxycholic acid can reduce CHOP expression, effectively relieve ERS in liver and adipose tissue, increase insulin sensitivity, and reverse systemic IR (Sun et al., 2018; Wu et al., 2021).

These results show that obesity, as a chronic inflammatory or OS state in the body, can aggravate the pathological state of PCOS. As an important mechanism of PCOS, IR is also closely related to obesity through ERS. We found that there seems to be a causal relationship between obesity, inflammation and OS, and IR, which can induce ERS in ovarian cells and thus lead to the development of PCOS. In contrast, the pathological state of PCOS can also be alleviated by suppressing ERS to alleviate the corresponding clinical symptoms (Fig. 4).

FIG. 4.

It seems to be a causal relationship between obesity, inflammation, OS, and IR, which can induce ERS in ovarian cells and thus lead to the development of PCOS. In contrast, the pathological state of PCOS can also be alleviated by suppressing ERS to alleviate the corresponding clinical symptoms. GRP78, glucose-regulated protein 78; IL-6, interleukin-6; TNF-α, tumor necrosis factor-α; IL-1β, interleukin-1beta; MDA, 3,4-methylenedioxyamphetamine; SOD, superoxide dismutase; GPx, glutathione peroxidase.

Ovarian Fibrosis and ERS

One of the classic pathological changes of PCOS is thickening, fibrosis, and interstitial hyperplasia of the ovarian cortex due to the deposition of collagen and an increase in fibrous tissue (Ghafurniyan et al., 2015). Transforming growth factor (TGF)-β1, a member of the TGF-β superfamily, is involved in regulating cell growth and differentiation, and its moderate expression is important for follicular development and expulsion and luteal formation. Some studies have suggested a close relationship between TGF-β1 and ovarian fibrosis (Chang et al., 2016a, 2016b; Zhou et al., 2021).

ERS is also considered to be an important factor in the process of tissue fibrosis. At present, the important role of ERS has been found in the process of tissue fibrosis and in the promotion of fibrosis remodeling of the liver, lung, kidney, and other organs (Cybulsky, 2017; Burman et al., 2018; Xia et al., 2020). Takahashi et al. found that the expression of fibrogenic growth factors in GCs of patients with PCOS increased. The use of ERS inducers can also induce the expression of several profibrotic growth factors, including TGF-β1, in healthy human GCs.

Then, PCOS mice were treated with ERS inhibitors. Takahashi found reductions in interstitial fibrosis and collagen deposition in the ovaries, as well as a reduction in TGF-β1 expression in GCs (Fig. 5). These findings suggest that the ERS state in ovarian GCs of PCOS patients can induce the expression of profibrotic growth factors during ovarian fibrosis, leading to fibrotic proliferation of ovarian tissue and promoting anatomical changes in PCOS. This undoubtedly provides a new idea for the clinical treatment of PCOS (Takahashi et al., 2017).

FIG. 5.

It was found that the expression of fibrogenic growth factors in granulosa cells of patients with PCOS increased. The use of ERS inducers can also induce the expression of several profibrotic growth factors, including TGF-β1, in healthy human granulosa cells. Treating PCOS mice with ERS inhibitors can reduce interstitial fibrosis and collagen deposition in the ovaries, as well as a reduction in TGF-β1 expression in granulosa cells. TGF-β1, transforming growth factor-β1.

Aryl Hydrocarbon Receptor and ERS

Other views on the exploration of the pathogenesis of PCOS are that it is the result of a combination of genetic and environmental factors, which synergistically contribute to the clinical phenotype of PCOS (Palioura and Diamanti-Kandarakis, 2015). Studies have shown that some environmental pollutants are related to reproductive and endocrine disorders and can affect the function and development of ovaries and follicles (Gregoraszczuk and Ptak, 2013). For example, exposure to endocrine disruptors (EDCs) will lead to ovarian physiological damage and changes in the morphology and function of the female reproductive system (Costa et al., 2014).

Aryl hydrocarbon receptor (AHR) is a mature endocrine-disrupting chemical receptor that controls the expression of a variety of genes and plays an important role in various metabolic, developmental, and pathological processes (Esser et al., 2018). Even if it does not depend on the identity of its EDC receptor, AHR can also play a role in a variety of metabolic and pathological processes, such as obesity, NAFLD, and cancer, by mediating its related pathways (Esser et al., 2018; Bock, 2020).

AHR also exists in ovarian tissue and plays an important role in the regulation of ovarian follicle growth and steroidogenesis (Hernandez-Ochoa et al., 2010). High levels of LH and relatively low levels of FSH in PCOS patients may lead to abnormal expression of AHR (Teino et al., 2014) and reduce its ability to metabolize pollutants, resulting in elevated serum levels in PCOS patients. It has been found that AHR can regulate ERS in a variety of tissues, including lung tissue, through some mechanism (Yang et al., 2015; Guerrina et al., 2021). This result suggests that there may be a link between AHR and ERS in the ovarian tissue of PCOS patients that contributes to the development of PCOS.

After activation in the body, AHR often binds to the AHR nuclear translocator (ARNT) and regulates the expression of CYP1A1. Cytochrome P450 1B1 (CYP1A1) is a member of the cytochrome enzyme P450 family and plays an important role in the metabolic pathways of several substances (Esser et al., 2018). Kunitomi et al. (2021) found that the expression levels of AHR, ARNT, and CYP1B1 increased in ovarian GCs of PCOS patients and PCOS model mice. Subsequently, the study group also found high expression of AHR, ARNT, and CYP1B1 mRNA in human GLCs treated with an ERS inducer.

After PCOS mice were treated with an AHR antagonist, it was found that the expression of the above cytokines in mouse ovarian GCs decreased, and the estrous cycle and ovarian morphology of mice returned to normal (Kunitomi et al., 2021). The study has shown that AHR plays an important role in the development of the pathological state of PCOS: the ERS state in ovarian cells induces the expression of AHR, and the overexpression of AHR activates some downstream signaling pathway, promoting the development of the body to the pathological state of PCOS (Fig. 6).

FIG. 6.

The expressions levels of AHR, ARNT, and CYP1B1 increased in ovarian GCs of PCOS patients and PCOS model mice. The same results could be seen in the treatment of healthy human GLCs with ERS inducers. After treating PCOS mice with AHR antagonists, the expression of these cytokines in ovarian GCs decreased, while the estrous cycle and ovarian morphology of mice returned to normal. AHR, aryl hydrocarbon receptor; ARNT, AHR nuclear translocator; CYP1B1, cytochrome P450 1B1; GLC, granulosa lutein cell; GC, granulosa cell.

Environmental factors are also an important inducer of PCOS. It was found that testosterone can stimulate the AHR system in GCs, induce the expression of AHR and ARNT at the mRNA and protein levels, and activate the downstream signaling pathway of AHR (Wu et al., 2013). The hyperandrogen status in PCOS patients can induce ERS (Azhary et al., 2019), which suggests that hyperandrogenism in ovarian GCs in PCOS and ERS may jointly participate in the induction of AHR (Kunitomi et al., 2021). In addition to participating in the activation of inflammatory pathways (Guarnieri et al., 2020), AHR also plays a key role in liver steatosis induced by an HFD in obese mice (Rojas et al., 2020).

Inflammation and obesity are both characteristic clinical manifestations and risk factors for PCOS, and further experiments are needed to investigate whether AHR affects PCOS by participating in these pathological processes.

Conclusion

An appropriate follicular microenvironment is essential for follicular health, and changes in the microenvironment, such as excessive androgens, inflammation, OS, and insulin, present systemically or locally in the follicle, may negatively affect oocyte quality and thus reduce the quality of oocytes (Da Broi et al., 2018). By analyzing the results of previous studies on the relationship between ERS and multiple causes of PCOS, we found that the mechanism of PCOS is complex and does not result from the action of a single factor, but from the interaction of multiple pathogenic factors in the body, which often form a vicious cycle that together promote the development of PCOS.

The ER is a necessary structure responsible for the production of proteins, lipids, and other important life substances in the body. The abnormal function of ER caused by various reasons will more or less affect the life process. Initially, the presence of ERS is intended to restore cellular homeostasis, but if ERS persists, it can cause apoptosis, leading to the appearance of various pathological states. Several animal and cellular experiments support the role of ERS in the pathogenesis and pathological progression of PCOS, but the reality is far from simple. There are three branches of the ERS pathway, and different branches are involved in different physiological or pathological processes. How ERS acts in organisms and its specific mechanism of action still need to be further investigated.

Moreover, since any kind of PCOS animal model cannot fully simulate the pathological changes caused by PCOS in the human body (McNeilly, 2013), future studies on ERS and PCOS should be carried out in PCOS patients or cells from PCOS patients to enhance the strength of the research results. In conclusion, the results of previous studies suggest that ERS is a viable treatment for PCOS, and understanding the role of ERS in the pathogenesis of PCOS may open up new avenues for the development of therapeutic approaches for PCOS.

Footnotes

Acknowledgment

The authors thank Professor Zhang for her guidance on the professionalism and rigor of the article.

Authors' Contribution

C.W. conceptualized and prepared the article. Y.Z. edited and revised critically for important intellectual content in the article.

Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received.

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