Overview of Nrf2 as a target in ovary and ovarian dysfunctions focusing on its antioxidant properties

Abstract

Female infertility is a common issue caused by various factors, such as hormonal imbalances, age-related decline in oocyte quality, and lifestyle choices. Ovarian dysfunction is a prevalent cause, impacting fertility by damaging cells and impairing functions. Oxidative stress (OS) is a condition resulting from an imbalance between natural antioxidants and the generation of oxidants. This phenomenon acts as a double-edged sword, playing a crucial role as a signaling mechanism in both physiological and pathological processes related to the female reproductive system. OS is linked to ovarian dysfunction, leading to cell damage and reduced fertility. Nrf2 is a key regulator in oxidative homeostasis, helping to defend against OS and improve ovarian function in women of reproductive age. Therefore, this review aims to highlight the role of Nrf2 in the female reproductive system, focusing on its antioxidant properties.

Introduction

Female infertility is a complex and often challenging issue that affects millions of women worldwide [1]. This condition can be attributed to a variety of factors, including hormonal imbalances, reproductive disorders, age-related declines in egg quality, and lifestyle choices [2]. Among the various causes of infertility, ovarian dysfunction represents a prevalent and significant challenge for women of reproductive age [3]. This condition can significantly hinder the ability to conceive, making it a critical concern in the field of reproductive health.

Oxidative stress (OS) is characterized by an imbalance between pro-oxidants, including reactive oxygen species (ROS) and reactive nitrogen species (RNS), and the body's antioxidant defenses. This condition arises when the production of these harmful species exceeds the capacity of antioxidants to neutralize them [4].

OS is a significant factor influencing various physiological processes such as implantation, fertilization, pregnancy, and the normal course of parturition. Additionally, it is involved in the onset of preterm labor and contributes to the age-related decline in fertility [5, 6]. Numerous external factors, alongside physiological alterations, can contribute to OS in the ovaries. These factors include smoking, significant psychological stress, the natural aging process, superovulation associated with assisted reproductive technology (ART), diets high in sugar, chemotherapy medications, and exposure to industrial environmental pollutants [7].

The optimal concentration of OS is recognized as a significant signal for triggering apoptosis in the granulosa cells, thereby facilitating the onset of ovulation. However, research has demonstrated that the OS present in the ovarian microenvironment can lead to significant pathological consequences. These include apoptosis of granulosa cells, meiotic arrest in oocytes, and dysfunction of the corpus luteum, all of which contribute to the acceleration of ovarian aging. The implications of these findings are profound, as they highlight the critical role of OS in ovarian health. Understanding these mechanisms may provide insights into potential therapeutic strategies aimed at mitigating the effects of oxidative damage and preserving ovarian function over time [7].

Various studies have indicated that OS plays a critical role in the development of preeclampsia. It appears that the excessive generation of ROS, which occurs as a consequence of placental ischemia–reperfusion injury, is a key factor in the pathophysiology of this condition [6].

Recent findings indicate that OS is implicated in the development of ovarian dysfunction and contributes to the emergence of various related etiologies [7,8,9]. This encompasses a range of issues, including the shortening of telomeres, impairment of biomacromolecules, dysfunction of mitochondria, apoptosis, and the activation of inflammatory responses [6, 10]. OS has the potential to impact various cell types involved in fertility, including germ cells, somatic cells, oocytes, cumulus cells, and granulosa cells [11, 12]. This disruption can lead to reduced oocyte quality and significant damage to the metabolic and endocrine functions of the ovaries, ultimately resulting in a decline in female fertility [6].

A key strategy for enhancing the female fertility rate involves reducing or normalizing OS to physiologically optimal levels. This can be achieved by fostering a reproductive-friendly microenvironment through healthy lifestyle choices and antioxidant supplements [13, 14]. Antioxidants can eliminate, neutralize, or inhibit the production of ROS, thereby mitigating OS. This protective function is essential for preserving cellular integrity and preventing various conditions related to oxidative damage [15].

As a crucial regulator, nuclear factor erythroid 2-related factor 2 (Nrf2) plays a pivotal role in antioxidant defense mechanisms against OS. This transcription factor is instrumental in modulating a wide array of antioxidant enzymes that facilitate the detoxification and removal of OS and has been extensively studied, particularly concerning infertility disorders [16].

The Nrf2 signaling pathway is essential for sustaining ovarian proliferation and facilitating normal follicular development, while also contributing to the enhancement of ovarian function [17]. It is hypothesized that a relationship exists between the expression of the Nrf2 protein and ovarian reproductive function, suggesting that Nrf2 may play a significant role in safeguarding ovarian reproductive capabilities. Throughout the stages of youth, growth, and aging, as ovarian reproductive function initially rises, the levels of Nrf2 protein within the ovarian tissue demonstrate a pattern of increase followed by a subsequent decline [18].

The objective of the present study is to guide researchers regarding the role of the Nrf2 signaling pathway in regulating OS and influencing ovarian. This review aims to highlight the role of Nrf2 in the female reproductive system by emphasizing its crucial contribution to the enhancement of ovarian function in women of reproductive age.

Structure and mechanisms of nrf2 action

Nrf2 is a Cap 'n' Collar (CNC)-structured basic leucine zipper (bZIP) transcription factor comprising 605 amino acids. These amino acids are organized into seven highly conserved domains known as Nrf2-ECH homology (Neh) domains, each of which is assigned specific functional roles [19,20,21]. The Neh1 domain functions as a CNC-bZIP domain, facilitating the dimerization of Nrf2 with small muscle aponeurosis fibromatous (sMAF) proteins, specifically MAFK, MAFG, and MAFF, thus enabling DNA binding [22]. Furthermore, the Neh1 domain interacts with UbcM2, an E2 ubiquitin-conjugating enzyme, thereby regulating the stability of the Nrf2 protein [23]. Nrf2 associates with its cytosolic repressor, Kelch-like ECH-associated protein 1 (Keap1), through the Neh2 domain [24]. The transcriptional activation of genes containing an antioxidant response element (ARE) in their promoter regions is mediated by the Neh3, Neh4, and Neh5 domains [25]. The Neh3 domain interacts with components of the transcriptional machinery, serves as a transactivation domain, and contributes to the stability of the Nrf2 protein [26]. The Neh4 and Neh5 domains bind to the cAMP response element binding protein, which possesses intrinsic histone acetyltransferase activity, rather than to other proteins that also act as transactivation domains [27]. The Neh6 domain contains a serine-rich region involved in a redox-insensitive mechanism that regulates Nrf2 independently of Keap1 [28]. Lastly, the Neh7 domain plays a role in the suppression of Nrf2/ARE signaling pathway by interacting with the retinoid X receptor alpha (RXRα) [29] (Fig. 1).

Fig. 1

Diagram of Nrf2 structure with Nrf2-ECH homology domains (Neh1-Neh7)

NRF2 activity is primarily regulated by its interaction with Keap1. In homeostatic states, Nrf2 is inactive in the cytosol due to binding with Keap1. However, the Keap1-Nrf2 complex is disrupted under OS conditions, allowing Nrf2 to be transferred into the nucleus. There, Nrf2 initially interacts with small MAF proteins to form a heterodimer, which then binds to ARE and triggers the expression of phase II antioxidant genes, including CAT, PRDX1, SOD, and HO-1, to neutralize the effects of OS and promote survival under suboptimal environmental conditions [30,31,32,33,34].

Previous reports have indicated that the MAPK and AMPK signaling pathways play a role in the upstream signaling of Nrf2 [35]. During OS, AMPK becomes active through phosphorylation at Thr172 and helps to increase antioxidants by activating the expression of genes such as Nrf2, DAF-16, FOXO factors, and SIRT [36]. Recently, it was demonstrated that over 85 miRNAs may target the Nrf2 gene. This suggests that the Nrf2 signaling pathway can be regulated through DNA methylation, histone modifications, interactions of microRNAs, and RNA-binding proteins [36, 37] (Fig. 2).

Fig. 2

The Keap1-Nrf2 antioxidant signaling pathway. Under basal conditions, Nrf2 is degraded by Keap1. However, the Keap1–Nrf2 complex is disrupted under OS, allowing Nrf2 to enter the nucleus. There, Nrf2 binds to ARE and triggers the expression of CAT, PRDX1, SOD, and HO-1 to neutralize the effects of OS and promote survival under suboptimal environmental conditions

Nrf2 and ovary

The health of women's oocytes and ovaries plays the most important role in their reproductive capability. Various environmental and physiological factors are capable of eliciting detrimental effects on ovarian tissue. Extensive research has been conducted to recognize these factors and their underlying mechanisms.

The probable risks inquiry associated with environmental exposure to cadmium [38], the herbicide atrazine [39], carbon tetrachloride [40], and mercury chloride [41] in different species indicated the induction of OS and inflammatory responses in the ovary. This phenomenon is attributed to convoluted regulation occurring at multiple molecular levels and the activity of the Nrf2 signaling pathway and its associated genes. The activation of the apoptosis pathway and the overexpression of Keap1 and Nrf2 genes were observed following high-dose zinc oxide (≥ 100) application in ovarian tissue. Zinc oxide toxicity can be mitigated by administering OS inhibitors, such as N-acetylcysteine (NAC) and salubrinal [42].

Oxidative damage to ovarian tissue has manifested as a consequence of contact with sodium fluoride (NaF) and dietary vanadium. Trying Vitamin C as a countermeasure proved ineffective in reversing the oxidative effects caused by NaF while analyzing the Keap1-Nrf2-sMaf antioxidant pathway, the researchers elucidated how epigallocatechin-3-gallate supplementation can alleviate the detrimental effects of vanadium and enhance overall antioxidant capacity [43, 44]. Raising environmental temperature and the induction of heat stress are considered suppressors of the Nrf2 signaling pathway and its associated proteins; however, capsaicinoid supplements reduce OS and boost antioxidant indicators [45]. A considerable portion of the research concentrates on protective antioxidants and dietary interventions aimed at mitigating ovarian dysfunction. Hydrogen-rich saline (HRS) and the inhibition of glycogen synthase kinase-3 (GSK-3) in countering chemotherapy-induced ovarian damage [46, 47], diselenonicotinamide (DSNA) against radiation-created damage [48], melatonin during cryopreservation [49, 50], and resveratrol [51] by targeting the OS pathway have shown promise for enhanced antioxidant production and diminished injuries originating from OS in the ovaries.

Age-related infertility represents a global concern, especially impacted by OS within ovarian tissue. Aging results in unusual upregulation of proteins involved in iron transport and storage: transferrin, ferritin, and IRP2-mediated transferrin receptor 1 (TfR1), which leads to OS and inflammatory responses in ovarian tissue. Intermediation in iron metabolism is a therapeutic approach that could alleviate aging-associated ovarian disorder [52]. Administration of dimethylfumarate and Tanshinone IIA as anti-aging interventions elevates Nrf2 and antioxidant markers, telomere length, and anti-Müllerian hormone levels in aged female mice [53, 54]. An experimental investigation elucidated that a deficiency in the transcription factor Nrf2 accelerates ovarian aging, particularly in response to exposure to toxic compounds [55]. According to recent findings, proteasome expression and activity decline meaningfully in aged somatic tissues, resulting in the aggregation of ubiquitinated and oxidized proteins. By contrast, proteasomes in gonadal tissues possess greater activity that is independent of age, enhancing these tissues' ability to withstand OS. Moreover, Nrf2 activation causes restoration of proteasome expression in aged somatic tissues, suggesting that age-related Nrf2 dysfunction is correlated with the diminished levels of proteasomes in these tissues [56]. Targeting pigment epithelium-derived factor (PEDF) may assist in the management of diseases related to ovarian oxidative damage. The factor is a multifunctional protein with anti-tumorigenic and neurotrophic properties. Mice lacking PEDF exhibit oxidative damage in the ovaries, evidenced by ROS accumulation and activation of the Nrf2 pathway [57]. Hypoxia-inducible factor-1 (HIF-1) mediates the activation of the HO-1 gene under hypoxic conditions. Over the past decades, Gong et al. explained that HO-1 gene expression is regulated by hypoxia and cobalt chloride (CoCl2) exposure through HIF-dependent and HIF-independent pathways. CoCl2 activates HO-1 gene expression via OS and the involvement of Nrf2 and MafG, whereas hypoxia employs a distinct pathway without involving these proteins in mutant Chinese hamster ovary (CHO) cells (Ka13), which lack HIF activity.

Overall, the review underscores the critical role of OS and its regulators in maintaining ovarian health and presents potential therapeutic pathways to mitigate environmental and age-related damage (Table 1 and Fig. 3).

Table 1 The role of Nrf2 in ovary

Fig. 3

Signaling pathways involved in Nrf2 inhibitors and simulators in ovary

Ovarian steroidogenesis is the biochemical process through which ovarian cells synthesize hormones essential for maintaining reproductive tissues, regulating ovarian activities, facilitating ovulation, and establishing pregnancy. GCs are essential components of the ovaries, contributing significantly to steroid hormone production and oocyte maturation [72, 73]. The results indicate that Nrf2 enhances steroidogenesis through its influence on granulosa cells. Furthermore, the upregulation of Nrf2 led to an increase in the expression of genes associated with steroidogenesis, such as steroidogenic acute regulatory protein (StAR), cytochrome P450, family 19, subfamily A, polypeptide 1 (CYP19A1), 3 beta-hydroxysteroid dehydrogenase/isomerase (HSD3b1), all of which play a crucial role in promoting luteinization [74, 75].

Nrf2 and oocyte

In vitro maturation (IVM) is the technique of oocyte maturation to produce embryos in vitro. Additionally, it is an experimental in vitro method to study the effect of external intervention on oocyte cells. One problem with this technique is the rising OS. Researchers have added some components with antioxidative features to IVM culture to target signaling pathways related to Nrf2 to ameliorate the OS in IVM oocytes.

In this regard, astaxanthin was used to target the Nrf2 pathway and found the upregulation of Nrf2 and its downstream antioxidants like SOD1, SOD2, GPX4, and CAT, and reduced OS in IVM oocytes [76]. Similar to this study, [59] enriching the maturation medium of oocytes during IVM with epigallocatechin-3-gallate significantly enhanced the transcriptional level of Nrf2 and its downstream antioxidants in oocytes during IVM. The mechanism action of some interventions may be related to the upstream or downstream signaling pathway. For example, N-acetylcysteine as a potent antioxidant acts through protein kinase C (PKC) as an upregulated factor which disconnects Nrf2 from Keap-1 [58]. Another mechanism of Nrf2 function in oocyte maturation is described by Kim et al. [64]. They used melatonin as a Nrf2 activator and brutal as a Nrf2 inhibitor. They demonstrated that melatonin, by binding to the melatonin receptor 2, activates a series of cascades, leading to the detachment of Nrf2 from Keap1. Nrf2 is then translocated to the nucleus and targets ARE to express catalase, which prevents ROS accumulation and translocates to peroxisomes. In peroxisomes, catalase also prevents H2O2, which is produced in peroxisomes in porcine oocytes. Brutal reduced the detachment of Nrf2 from Keap1, thus reversing this signaling pathway and reducing oocyte quality. In contrast, it was shown that melatonin attenuated Nrf2 overexpression induced by heat stress in IVM of porcine oocytes [65]. This reverse effect of melatonin on the expression of Nrf2 may have originated from the neutralizing effect of heat stress. The mammalian target of rapamycin (mTOR) is another pathway that has been reported as a trigger for Nrf2 in oocyte cells. It was reported that targeting mTOR by rapamycin upregulated Nrf2 and its downstream antioxidant in IVM oocytes [66]. p38α MAPK is another regulator of Nrf2 that separates Nrf2 from Keap1 by Nrf2 phosphorylation. Myostatin acts through its upregulating factor [60].

Another intervention in IVM is cell therapy. One of the problems with the IVM process is the separation of the COC complex from follicular mural somatic cells. Mural cells have an essential function in protecting oocytes, such as myostatin production. To solve this miscommunication, the researchers added cumulus-derived somatic cells to IVM and observed modulation of ROS and enhancement of Nrf2 [63]. Furthermore, using equine amniotic fluid mesenchymal stem cell conditioned medium passages 7 to increase Nrf2 in cumulus cell-mediated oocyte maturation in IVM was to solve this phenomenon [67].

In vivo studies also confirm the essential effect of Nrf2 in oocytes. In this regard, it was reported that lead exposure increases ROS and MDA by reducing antioxidant activity in oocyte cells, and its effect was related to the Nrf2/Keap1 pathway [68]. Similarly, oral administration of isoniazid, (a synthetic first-line tuberculosis antibiotic) increased the level of ROS and activated the Keap1-Nrf2 pathway as an OS response [69]. This also led to apoptosis of oocytes and mitochondrial dysfunction.

Another function of Nrf2 is involvement in chromosome condensation. This role of Nrf2 is mediated through the Nrf2-Cyclin B1 signaling pathway. Cyclin B1 synthesis and degradation regulate maturation-promoting factor activity, which increases chromosome condensation and microtubule polymerization in prophase I oocytes. In support of this notion, it was reported that using brusatol as an inhibitor of the Nrf2-mediated signaling pathway disrupted oocyte maturation through the reduction of Nrf2. Reduced Nrf2 caused a reduction in Cyclin B1, thus disturbing proper spindle assembly and chromosome condensation in meiotic oocytes [61].

As a result, Nrf2 has a critical effect on oocyte maturation, especially in IVM. Its function is clarified by studies that used an activator or inhibitor intervention of the Nrf2 pathway (Table 2 and Fig. 4).

Table 2 The role of Nrf2 in oocyte cell

Fig. 4

Signaling pathways involved in Nrf2 inhibitors and simulators in oocyte

Nrf2 and granulosa cells

The follicular growth phase of mammalian folliculogenesis is characterized by the proliferation and differentiation of granulosa cells (GCs). These cells, which encase the oocyte, are crucial for the protection, support, and nourishment of maturing oocytes. GCs facilitate the transfer of essential nutrients and small molecules to oocytes via gap junctions and are integral to ovarian function due to their roles in steroidogenesis and oocyte development [33, 72]. Consequently, the quantity and morphology of GCs serve as important indicators for assessing the potential for embryo development and the success of pregnancy outcomes. Additionally, OS, which encompasses endoplasmic reticulum stress, significantly contributes to the apoptosis of GCs and oocytes, ultimately resulting in follicular atresia and infertility in women. Therefore, the management of OS in granulosa cells may enhance the outcomes of infertility treatments [33].

There are some studies designed in this context. In one of them, researchers isolated GCs and cultured them with H2O2, dimethylfumarate, and a Nrf2 activator. The objective was to investigate the interaction between Nrf2 and antioxidant mechanisms. The study concluded that Nrf2 activation contributes to the reduction of OS in GCs, highlighting its potential role as a protective factor against cellular damage caused by OS [74]. miR-28, miR-153, and miR-708 are endogenous regulators of the Nrf2 pathway in GCs. The findings indicated that OS triggers the activation of Nrf2 expression as a protective response while concurrently decreasing the levels of these specific miRNAs. Additionally, elevated levels of these miRNAs were associated with the downregulation of Nrf2 expression, resulting in increased ROS accumulation, decreased mitochondrial activity, and reduced cellular proliferation [37]. It was also shown that exosomal miR-27 could elevate ROS and promote apoptosis by downregulating the p-ERK/Nrf2 signaling pathway, which is negatively influenced by the regulation of SPRY2 [77].

Furthermore, recent studies have revealed that OS-suppressing compounds such as sulforaphane [30, 78,79,80], morroniside [81], astaxanthin [73], quercetin [82, 83], vitamin E [33, 75], and selenium [33] can alleviate the OS condition of GCs via the Nrf2 pathway.

By using sulforaphane, the elevation of the protein level of Nrf2 was demonstrated, which was followed by an increase in Nrf2 downstream factors like SOD, CAT, PRDX1, and TXN1. These factors improved GCs' defense mechanisms against OS. By ameliorating OS, sulforaphane improved the viability, apoptosis, and mitochondrial function in GCs exposed to H2O2 [30, 80]. This antioxidative effect of sulforaphane is achieved by stimulating the AMPK, AKT, and Nrf2 pathways, thereby providing protective effects against OS in GCs [78].

Another antioxidant is morroniside. It was reported that pretreatment of GCs with morroniside significantly reduced ROS, MDA, and 8-hydroxydeoxyguanosine (8-OHdG) in GCs. This effect of morroniside was achieved through upregulating the phosphorylation of Nrf2 and facilitating its translocation into the nucleus, leading to the transcriptional activation of antioxidant enzymes such as SOD and NQO1 [81].

Quercetin is another component with antioxidative properties. It was reported that quercetin upregulates the messenger RNA expressions for Nrf2 and its associated downstream genes, which were also correlated with enhanced cellular viability. In addition, quercetin alleviated H2O2-induced steroidogenic dysfunction in goat GCs, a process that may be facilitated through the Nrf2 signaling pathway [83]. Also, its upregulating function on Nrf2 may occur through Thrx gene expression [82].

Various studies have indicated that administering vitamin E, both independently and alongside selenium, can substantially decrease intracellular ROS levels. These studies revealed that vitamin E influences GC dynamics, specifically their proliferation and apoptosis through a defense mechanism mediated by Nrf2 and its downstream antioxidants. This process is facilitated by the activation of the PI3K/AKT and ERK1/2 signaling cascades [33, 75]. Similarly, astaxanthin influenced GCs. Astaxanthin enhanced both gene and protein expression, as well as the nuclear localization of Nrf2, while simultaneously exerting an inhibitory influence on the protein levels of Keap1 [73].

In conclusion, the Nrf2 signaling pathway has a potential effect on the modulation of redox in GCs, and any downregulatory or upregulatory effect of this factor could have a potential impact on the GCs (Table 3 and Fig. 5).

Table 3 The role of Nrf2 in granulosa cells

Fig. 5

Signaling pathways involved in Nrf2 inhibitors and simulators in granulosa cell

Throughout the processes of follicular development and ovulation, GCs activate an intrinsic defense mechanism that encompasses the NRF2-ARE pathway, which serves to protect against a range of stressors [80]. Recent research indicates that Nrf2 may serve as a significant factor in promoting the survival and maintenance of follicles. This finding highlights the potential role of Nrf2 in supporting the health and longevity of follicular structures [73, 74]. A significant presence of NRF2 protein has been identified within the follicular cells of the preovulatory follicle as well as in the newly developed corpus luteum. This localization plays a crucial role in initiating the activation of antioxidant mechanisms, which serve to safeguard the cells against apoptosis [86].

Nrf2 and polycystic ovarian syndrome

Systemic and ovarian imbalances of OS are critical characteristics of polycystic ovarian syndrome (PCOS). This evidence positions the Nrf2 pathway as a viable target for therapeutic intervention in the disease. Numerous comprehensive studies have been conducted to investigate beneficial strategies for modulating the Nrf2 pathway in order to alleviate symptoms associated with PCOS.

The combination administration of MitoQ10 and Vitamin D3 in induced PCOS reduced the expression of genes involved in OS, including Keap1, HO-1, and Nrf2 [87]. Another combination comprising linagliptin and I3C has revealed a positive impact on PCOS metrics by upregulating the Nrf2/HO-1 pathway and enhancing SOD and CAT antioxidant enzymes in ovarian tissue [88]. The utilization of diacerein, an IL-1β inhibitor, in the experimental PCOS model exhibited a pronounced protective effect against OS by effectively reducing elevated levels of MDA and total nitrite, while concurrently enhancing the production of SOD and CAT. The Keap-1/Nrf2/OH-1 signaling pathway is introduced for diacerein's ameliorating effect [89]. Luteolin (a naturally occurring flavonoid present in a diverse range of herbs) improved ovarian function by activating the PI3K/AKT signaling pathway. Also, luteolin enhanced antioxidant defenses by restoring the Nrf2 pathway and its downstream genes, specifically Nqo1 and Hmox1 in PCOS [90]. Astaxanthin and sulforaphane are commonly employed as advantageous dietary supplements. Both of them have been proposed as favorable candidates for mitigating OS in PCOS patients, which could respectively lead to increased levels of total antioxidant capacity (TAC) in the patient's serum and promote cell survival and growth in granulosa cells through activating the Nrf2 pathway and its relevant genes, AMPK/AKT [91, 92]. The traditional herbal formula known as Qi Gong Wan (QGW) has long been used in Ancient Chinese medicine as a remedy for obesity-induced infertility in women. The efficacy of QGW has been considered in PCOS, specifically in individuals with insulin resistance. QGW's therapeutic advantages are uncovered with improvements in ovarian health, insulin levels, and insulin resistance markers such as the HOMA-IR index. These beneficial effects are attributed to the activation of the Nrf2/HO-1/Cyp1b1 pathway, which plays an important role in protecting adipose tissue in cases of PCOS [93]. Humanin, a peptide biomarker derived from mitochondria, is well-known for its antioxidant properties. Humanin has yielded promising results in reducing OS and preventing PCOS by activating the Keap1/Nrf2 signaling pathway [32].

Some scientists have scrutinized the role of the unusual expression of miRNAs and unveiled an upregulation of miR-873-5p in patients with PCOS. Silencing of miR-873-5p activates the p38/Nrf2/HO-1 signaling pathway, thereby diminishing the generation of OS factors, including ROS and MDA. Histone deacetylase is an epigenetic regulator believed to contribute to ovarian dysfunction in PCOS. The hypothesis that sodium acetate, an inhibitor of histone deacetylase, could protect against ovarian dysfunction in PCOS, was examined recently. Acetate treatment illustrated positive outcomes, including reductions in body and ovarian weight, improvements in insulin sensitivity, and reconditioning of hormonal balance. Antioxidant response was determined by increasing ovarian levels of glutathione peroxidase and Nrf2 [94].

This extensive overview discusses recent advances in understanding and treating PCOS through targeted intervention in OS pathways, specifically focusing on the Nrf2 signaling pathway (Table 4).

Table 4 the role of Nrf2 on polycystic ovarian syndrome

Nrf2 and primary ovarian insufficiency

Primary ovarian insufficiency (POI) is a condition in which follicle development and sex hormone production in the ovaries are disrupted before the age of 40. One of the signaling pathways involved in POI is the Keap1-Nrf2-ARE pathway. In POI, the increase in OS and ROS inactivates this pathway, causing accumulated Nrf2 to be targeted by PKC, JNK, PIK3, and ERK and translocated to the nucleus. In the nucleus, Nrf2 induces the transcription of HO-1, NQO1, and SOD through ARE. These proteins are then transported to the cytoplasm to protect cells from OS damage [9, 95].

However, there are other interventions that can activate upstream proteins. For example, PI3K enhances the disconnection of the Keap1-Nrf2 complex by activating AKT [96]. Epicatechin [97], curcumin [98], and EGF [99] have been shown to alleviate POI by upregulating PI3K. Additionally, it has been suggested that EGF may directly activate Nrf2 or inhibit PTEN, which is an upregulator of PI3K. SESN2 is another protein that disrupts the Keap1-Nrf2 complex by promoting the autophagic degradation of Keap1 in POI [100]. Some components with antioxidative activity, such as chitooligosaccharide-zinc, have a therapeutic effect on POI by activating SESN2 [101]. PKC and HDAC-2 act as upregulators and downregulators, respectively, of the Nrf2 signaling pathway [102]. A study by Liu et al. found that squid ink polysaccharide positively affects POI by reducing Keap1 and HDAC-2 while increasing PKC, Nrf2, and downstream factors like SOD, HO-1, and NQO-1 [103]. SIRT1 is another upregulator of Nrf2 [104]. It was indicated that icariin influences POI by activating the Nrf2/HO-1/Sirt1 axis in an animal model [105]. Foxo3a is a downstream factor in the Nrf2 pathway. It was reported that 4-Vinyl Cyclohexene Diepoxide reduces OS damage in an animal model of POI by upregulating Nrf2, Foxo3a, and HO-1 [106]. Another study identified the Nrf2/SOD2 pathway as a target for the antioxidative effect of puerarin in POI [107]. Additionally, daphnetin was introduced as another promoter of Nrf2 in D-galactose-induced POI [108]. Some antioxidative interventions, such as preventive electroacupuncture, alleviate POI by directly interfering with the Keap1-Nrf2 complex [109].

Furthermore, Nrf2 has been shown to regulate angiogenesis through the HO-1-mediated HIF-1α/VEGF signaling axis [110]. Supporting this idea, one study demonstrated that Si-Wu-Tang (a traditional Chinese medicine) promoted angiogenesis by targeting the Nrf2/HO-1 pathway in POI [111] (Table 5).

Table 5 The role of Nrf2 on primary ovarian insufficiency

Nrf2 and ovarian hyperstimulation syndrome

Ovarian hyperstimulation syndrome (OHSS) is a significant complication that can occur following gonadotropin-driven ovarian stimulation, particularly when human chorionic gonadotropin (hCG) is administered to promote the development of multiple ovarian follicles. One of the critical issues associated with OHSS is OS, which can adversely affect oocyte quality and overall reproductive health. Nrf2 is crucial for regulating the expression of antioxidant enzymes, which help protect cells from oxidative damage.

In a study by Fan et al., the effects of N-acetyl-cysteine (NAC) on OS induced by repeated controlled ovarian hyperstimulation were examined in mice. Their findings demonstrated that NAC enhances Nrf2 activity and PKC, a regulator that activates Nrf2, while simultaneously suppressing Keap1, a negative regulator of Nrf2. The authors concluded that NAC alleviates OS in oocytes during controlled ovarian hyperstimulation and improves oocyte quality by upregulating the Nrf2 signaling pathway, thereby increasing the activity of various antioxidant enzymes [58]. Similarly, Guan et al. investigated Kunling Wan, a traditional Chinese herbal formulation comprising 31 components with antioxidant properties. Their study found that Kunling Wan activates Nrf2, leading to the upregulation of antioxidant enzymes such as SOD and GSH-Px. This activation helps counteract OS and prevents mitochondrial damage in oocytes and ovaries during controlled ovarian hyperstimulation. The formulation enhances oocyte quality by modulating the PKC/Keap1/Nrf2 pathway, effectively inhibiting oxidative damage associated with repeated stimulation [112].

In contrast, Liu et al. identified a different mechanism involving microRNA-27 (miR-27) that may contribute to reduced Nrf2 function in OHSS. Their research indicated that miR-27 mimics increase ROS levels by downregulating the phosphorylated ERK/Nrf2 pathway in granulosa cells. They noted that SPRY2 acts as an upregulator for the ERK/Nrf2 signaling pathway, and miR-27 increases OS by downregulating SPRY2, subsequently leading to decreased p-ERK/Nrf2 signaling in an OHSS model [77].

Overall, these studies underscore the complex interplay between OS and the Nrf2 signaling pathway in the context of OHSS, highlighting potential therapeutic strategies to mitigate oxidative damage and improve oocyte quality.

Nrf2 and ovarian cancer

There is an overview discussing Nrf2 and ovarian cancer [113]. Briefly, Nrf2 controls how cells respond to DNA damage [113, 114]. Recent studies have shown that Nrf2 levels are significantly increased in ovarian cancer, demonstrating a positive correlation with ovarian lesions. Additionally, the Keap1/Nrf2/ARE pathway interacts with essential components of the endogenous antioxidant and detoxifying systems in ovarian cancer. By activating the Keap1/Nrf2/ARE signaling pathway, Nrf2 can prevent ROS-induced apoptosis. By regulating proteins such as ERα, CD99, EGFR, ErbB2, AKR1C1, AKR1C2, and ABCF2, Nrf2 can influence the apoptotic process in ovarian cancer cells. It can also regulate the cell cycle and the proliferation of cancer cells [113, 115,116,117].

About the role of Nrf2 in ovarian health, it is worth noting that most of the ovarian tissue protectors during chemotherapy, including GSK-3 inhibitors, epigallocatechin gallate, resveratrol, melatonin, and theaflavins, exert their positive effects by activating Nrf2 signaling to preserve female fertility [113]. Comprehensive studies have confirmed that miRNAs, such as miR-141 and miR-181d, can influence the expression of Nrf2 in breast and ovarian cancer, which, in turn, affects chemoresistance. According to the data, Nrf2 activation increases chemoresistance to chemotherapy by inhibiting drug-mediated OS (such as platinum drugs), which usually leads to cancer cell death [19, 115].

Conclusion

In conclusion, Nrf2 plays a significant role in the pathogenesis of ovarian dysfunction. The studies indicate that interventions affecting the Nrf2 signaling pathway may influence OS and redox balance within cells. The evidence reviewed suggests that the primary signaling pathway involved in mitigating OS is the Keap/Nrf2 signaling pathway. Various antioxidants that provide protective effects against ovarian disorders target this signaling pathway or its upstream regulatory factors.