Premature ovarian insufficiency: a review on the role of oxidative stress and the application of antioxidants.

The key to counteracting ovarian functional impairment in POI patients caused by OS is to restore the dynamic balance between ROS and the antioxidant system by reducing the production of ROS on the one hand and increasing the activity of the endogenous antioxidant system or supplementing exogenous antioxidant substances to enhance the ability to scavenge ROS on the other hand ( 204 ). The endogenous antioxidant system of the organism consists of two main components, an enzymatic scavenging system consisting of various antioxidant enzymes such as SOD and catalase (CAT), and a non-enzymatic scavenging system comprising antioxidants including GSH and MT ( 205 ). Furthermore, herbal extracts such as ginsenosides, curcumin (CUR), resveratrol and stem cell transplantation also perform important roles in the fight against OS, and we will introduce several antioxidants with outstanding efficacy and good research prospects ( Table 1 ). The application of antioxidants. A randomized controlled trial is a means of testing the effects of a therapy or drug in health care. The basic method of randomized controlled trials is to randomly group study subjects and implement different interventions for different groups to control the differences in effects. It has many advantages such as being able to minimize various possible biases in the design and implementation of clinical trials, balance confounding factors, and improve the validity of statistical tests, and is recognized as the gold standard for evaluating interventions. MT, an amine hormone produced by the pineal gland in mammals and humans, is secreted in tissues such as the ovary and placenta ( 206 ). In the long-term decline of ovarian aging, melatonin treatment is able to regulate ovarian function through anti-oxidative stress and also protect the follicular pool, oocyte quantity and quality ( 207 ). Multiple studies have indicated that MT protects mice’ and hamsters’ GCs and oocytes from ROS damage by reducing OS ( 208 , 209 ), thereby protecting ovarian function and delaying the development of POI. Osatd-Rahimi N et al. ( 210 ) found that melatonin increases the activity of CAT, SOD and thiols, thus it has a protective effect against oxidative stress. Additionally, melatonin plays an effective role in reducing malondialdehyde (MDA) levels, which are a product of lipid peroxidation in cell membranes. It can directly scavenge reactive oxygen radicals, promote the synthesis of the reactive oxygen scavenger GSH in the body, and mitigate OS damage to mitochondrial proteins and DNA ( 211 ). Cyclophosphamide (CTX), an alkylating agent with the highest risk of ovarian damage, is commonly used in the treatment of malignancies such as breast cancer ( 212 ). According to studies, CTX is catalyzed and degraded in vivo by hepatic microsomal enzymes, resulting in the production of phosphoramidite mustard and acrylic acid, which have cytotoxic effects ( 213 ). During the course of treatment, abnormal blood circulation and associated stress damage to the body may result in the release of large amounts of free radicals. It promotes peroxidation and releases large amounts of ROS, which damages the mitochondrial and cellular membranes of ovarian granulosa cells, induces an inflammatory cascade, releases alarm signals (alarms), and initiates cellular responses ( 214 ). Whereas granulosa cells in the ovary provide an important environment for oocyte formation and development ( 215 ). CTX alters follicular growth, oocyte maturation and apoptosis or atresia in the ovary due to the production of ROS and inflammatory substances. Xu et al. ( 216 ) examined that MT improved POI in a cyclophosphamide-induced POI in rat model by maintaining ovarian hormonal status, enhancing ovarian index, promoting follicular development, and inhibiting GCs apoptosis. Animal and human trials showed that short-term use of MT was safe and long-term MT treatment caused only minor adverse effects comparable to placebo ( 217 ). Sphingosine-1-phosphate (S1P) is a biologically active sphingolipid that mediates various biological processes such as apoptosis, immune response and inflammation, and can substantially reduce oocyte apoptosis initiated by various stimuli such as radiation and chemotherapeutic drugs, effectively protecting ovarian tissue to alleviate POI ( 218 ). It was found that OS can induce cellular regulation by altering the balance of Cer/S1P, and with elevated Ceramide (Cer)/sphingosine-1-phosphate (S1P) levels, cells tend to move in a direction favorable to life activity. Conversely, cells tend to die ( 219 ), so S1P is an important antioxidant in reducing apoptosis, and thus treating POI. There are fewer reports on its safety, and one study detected that S1P treatment interferes with the clinical effects of anticancer drugs during chemotherapy, and its anti-apoptotic effects may inhibit normal atresia of DNA-damaged oocytes during follicular development ( 206 ). Coenzyme Q10 (CoQ10) is a fat-soluble quinone, a natural antioxidant found in the mitochondrial respiratory chain. Exogenous supplementation of CoQ10 has been shown to improve ovarian function in mouse ( 220 ). It has a beneficial effect on POI patients by reducing OS, preventing mtDNA mutations caused by peroxidative damage, restoring mitochondrial function, counteracting ovarian physiological aging and improving ovarian reserve function for enhancing ovarian responsiveness ( 221 ). Delkhosh A et al. ( 220 ) discovered that CoQ10 markedly improved ovarian histological morphology and the number of atretic follicles in CTX-treated mouse ovaries by down-regulating ROS levels. It was also shown that CoQ10 is well tolerated, has few adverse effects, and is highly safe ( 222 ). N-Acetyl-L-cysteine (NAC) is a thiol antioxidant whose main role is to disturb the generation of free radicals and to scavenge those already generated ( 223 ). Mantawy EM et al. ( 224 ) revealed that NAC inhibits radiotherapy-induced POI by down-regulating nicotinamide adenine dinucleotide phosphate oxidase (NOX) and p22 proteins and enhancing antioxidant defense to eliminate oxidative damage, and suppresses p53-dependent apoptotic mechanisms by reducing multiple pathways including MAPK signaling pathway, and also reduces follicular atresia and increases ovarian reserve by inhibiting radiation-triggered primordial follicular loss, thereby restraining radiotherapy-induced POI in rats. At the same time, it preserves ovarian function and structure, and plays a positive role in POI patients. Clinical studies have proven that NAC has a good safety and tolerability profile ( 225 ). Vitamin C and E are classic antioxidant molecules. Vitamin C converts oxidized glutathione to reduced glutathione. The α-tocopherol in vitamin E can directly scavenge excess ROS from the body, modulate antioxidant responses, and enhance the expression of antioxidant enzymes like SOD ( 226 ), thus exerting an antioxidant stress effect and favorably affecting POI patients. Resveratrol (RES), a naturally occurring dietary antioxidant flavonoid, is the most recognized antioxidant and the strongest activator of the longevity gene SIRT1. It can effectively improve ovarian function and female germline stem cell (FGSC) regeneration in POI models by alleviating OS, inflammation, and mechanisms involving the Hh signaling pathway ( 159 ), while FGSCs can initiate meiosis in the ovary to produce oocytes and expand follicular reserve to preserve fertility. RES also apparently rescues the activity of antioxidant enzymes including GPX, SOD, and CAT in POI ovaries ( 159 ) and counteracts OS, which plays an essential role in the treatment of POI. The ginseng extract ginsenoside Rg1 has a protective effect on ovarian function and can delay ovarian aging through its antioxidant, anti-aging, anti-inflammatory and estrogenic activities ( 227 ). Rg1, a ginseng extract, not only can inhibit GCs apoptosis and promote the proliferation and growth of GCs by up-regulating the expression of FSHR and activating the FSHR-PI3K-AKT pathway to protect ovarian function in rat model ( 228 ) and thus attenuate POI. D-gal was able to successfully induce POF models by impairing follicular development, decreasing the apoptosis rate of granulosa cells, lowering E2 levels, and increasing the level of FSH in vivo . Studies have shown that D-gal can induce oxidative stress directly in the body, and that increased levels of galactose in women’s blood lead to a decrease in the number of ovarian follicles and damage to oocytes and ovarian granulosa cells, thus leading to POI ( 229 , 230 ). Rg1 can also can postpone D-galactose (D-gal)-induced POI through the PI3K-Akt-mTOR autophagy signaling pathway mouse model of ovarian senescence, effectively facilitating the secretion of estrogen and luteinizing hormone, and increasing the expression of SOD and CAT, thereby alleviating ovarian oxidative damage in mice model ( 231 , 232 ) and delaying the development of POI. Aside from this, Tang’s rat experiments demonstrated that ginsenoside Rb1 has a protective effect on ovarian function by the mechanism that the estrogenic activity of ginsenoside can regulate ovarian function, reverse disorders in sex hormone levels, reduce apoptosis of ovarian granulosa cells, and delay ovarian aging ( 233 ). We have discovered that CUR (the active ingredient of turmeric rhizome) possesses a positive effect on POI patients by potently inhibiting D-gal-induced 0S, apoptosis and ovarian damage through various mechanisms including Nrf2/HO-1 and PI3K/Akt signaling pathways ( 234 ) ( 193 ). Compared with resveratrol and vitamin C, the antioxidant capacity of CUR is superior ( 234 ). Relevant animal and clinical studies have confirmed that both the above natural compounds and herbal extracts have a good safety profile in animal models and human models ( 235 – 239 ). Numerous studies have recently reported promising therapeutic effects of MSC for POI treatment ( 240 ) ( 241 – 243 ). MSCs can secrete a variety of factors, including cytokines and exosomes, that lead to POI recovery through several mechanisms, involving reduction of apoptosis and inflammation and induction of angiogenesis ( 244 ). Stem cell transplantation is a treatment that has the ability to repair damaged ovarian tissues, restore ovarian elasticity, and reduce ovarian tissue fibrosis. In recent years, MSCs for the treatment of POI have become a hot research topic in this field. The mechanisms include MSC homing, promotion of cell proliferation, inhibition of apoptosis, differentiation, anti-inflammatory and immunomodulation, secretion of various cytokines, regulation of genes, and regulation of cell autophagy ( 245 ). The main MSCs used for the treatment of POI are bone marrow mesenchymal stem cells, adipose mesenchymal stem cells (ADMSCs), placental mesenchymal stem cells (PMSCs), amniotic mesenchymal stem cells (AMSCs), umbilical cord mesenchymal stem cells (UCMSCs), amniotic fluid mesenchymal stem cells (AFMSCs), and menstrual stem cells (MenSCs). We have discovered that MSCs can play an anti-OS role by secreting IL-6, VEGF and other related cytokines and exosomes, activating NQO1/MAPK and other related pathways, increasing the production of antioxidant-acting enzymes, and prohibiting ROS production, which in turn improves mice’ ovarian function ( 246 ). For example, fetal mesenchymal stem cells (fMSCs) prominently reduce oxidative damage, increase oxidative protection, improve anti-apoptotic effects, and inhibit apoptotic gene expression in vivo and vitro ( 247 ), protecting ovarian function in POI patients. Exosomes have been recognized to have a vital role in the treatment of POI by MSC, which are carriers of intercellular signaling and drug therapy, and miRNA is a class of single-stranded non-coding RNA in exosomes. Several studies have found that human mesenchymal stem cells derived from bone marrow (BMSC)is at work in the fight against oxidative stress. Proteomic analysis revealed that various antioxidant mediators secreted by BMSCs, such as cyclophilin A, cyclophilin B, thioxapurine, DJ-1, heat shock protein 27, and peroxycin-1, exhibited significant antioxidant stress effects. Proteomic analysis revealed that various antioxidant mediators secreted by BMSCs, such as cyclophilin A, cyclophilin B, thioxapurine, DJ-1, heat shock protein 27, and peroxycin-1, exhibited significant antioxidant stress effects ( 248 ). Insulin-like growth factor 1 (IGF1) released from BMSCs also has potent antioxidant capacity ( 249 ). On top of that, BMSC transplantation affects SOD activity and malondialdehyde content by decreasing cyclin-dependent kinase inhibitor 2A(P16) expression and increasing proliferating cell nuclear antigen, so as to improve the morphology and function of the ovary ( 250 ). Exosomes and miRNAs have the capacity to regulate the expression of downstream target genes to inhibit apoptosis of various cells in the ovary, so as to improve ovarian function ( 251 ) and slow down the progression of POI. For instance, human umbilical cord MSC-derived exosome microRNA-17-5P (miR-17-5p) can reduce ROS accumulation by inhibiting SIRT7 expression to combat OS and thus improve POI ( 252 ). Although MSC has obvious therapeutic effects in animal models of POI, its clinical application still suffers from the issues of insufficient cell source, immunogenicity, passaging culture, ethics, safety and efficacy, which need to be evaluated more comprehensively through more in-depth clinical studies ( 245 , 253 ). Endogenous antioxidant systems exist within the body and cells, including antioxidant enzymes such as SOD, GPX, glutathione sulfhydryltransferase (GST), and CAT, in addition to GSH as a scavenger of ROS ( 205 , 223 ). Superoxide anions in ROS form a spontaneous conversion to H 2 O 2 catalyzed by SOD, and then CAT and GPX further convert H 2 O 2 to O 2 and H 2 O, which exerts antioxidant stress effects in POI patients ( 254 ). In addition, phase II detoxification enzymes have defensive effects against oxidative damage caused by exogenous chemical exposure, mainly including HO-1, HQO1 and γ-glutamylcysteine ligase (γ-GCL), which can moderate POI to some extent by counteracting OS. HO-1 is a classical phase II detoxifying enzyme involved in preventing the generation of more active hydroxyl groups from H 2 O 2 , and also catalyzes the production of biliverdin from heme, of which the latter can form bilirubin, and biliverdin and bilirubin are powerful free radical scavengers in the body. It has been proved in mouse that γ-GCLase catalyzes the formation of GSH and is the most critical rate-limiting enzyme in GSH biosynthesis ( 254 ). Furthermore, OS results in ER stress, and Ufm1-specific ligase 1 (UFL1) alleviates cisplatin-induced apoptosis in ovarian GCs by attenuating ER stress, thereby relieving POI to some extent ( 254 ).The key to POI is the reduction in the quantity and quality of oocytes, which results in symptoms such as hot flashes, excessive sweating, vaginal dryness, and even causes serious consequences such as infertility.

POI is an important medical issue concerning women of reproductive age. It is a clinical syndrome defined by loss of ovarian activity before the age of 40 years. Over the years, several terms have been used to describe this condition, such as ‘primary ovarian insufficiency’, ‘premature menopause’ and ‘premature ovarian failure’. Although the latest guideline from the European Society of Human Reproduction and Embryology (ESHRE) adopted ‘premature ovarian insufficiency’ as the standard nomenclature ( 1 ). The diagnostic criteria are not fully standardized, yet the differences varied little. The latest diagnostic criteria proposed in the latest guidelines are primary or secondary amenorrhea or spaniomenorrhea of>4 months with onset before 40 year of age, and elevated follicle-stimulating hormone (FSH)>25IU/L on 2 assays at>4 weeks’ interval. Estradiol level is low, and anti-Müllerian hormone (AMH) levels have usually collapsed ( 2 ). The prevalence of POI varies among ethnic groups across the world, and studies show a prevalence that ranges from 1% ( 3 ) up to 5.5% ( 4 ). If not treated quickly, women living with POI face the short- and long-term consequences of prolonged hypoestrogenism and can develop symptoms such as lack of libido, vaginal dryness, mood disorders, rise in cardiovascular risk and cognitive impairment ( 5 ). Decreased mineral density promotes osteopenia and osteoporosis, as well as bone fractures. The health provider should be aware of POI signs and symptoms in order to promote prompt diagnosis ( 6 ). The most accepted ovarian aging theory emphasizes that the ability of ovarian GCs to counteract the ROS pathogenic effect is decreased ( 7 ), which ultimately leads to the reduction of the quantity and quality of follicles ( 8 , 9 ). Reactive oxygen species (ROS) are two electron reduction products of oxygen, including superoxide anion, hydrogen peroxide, hydroxyl radical, lipid peroxides, protein peroxides and peroxides formed in nucleic acids ( 10 ). They are maintained in a dynamic balance by a series of reduction-oxidation (redox) reactions in biological systems and act as signaling molecules to drive cellular regulatory pathways ( 11 , 12 ). The production of ROS can be induced by various factors such as heavy metals, tobacco, smoke, drugs, xenobiotics, pollutants and radiation ( 13 ). Excessive oxidative stress derived from ROS accumulation deregulates the antioxidative defense system, which is closely associated with various diseases ( 14 , 15 ). ROS are produced in almost every subcellular organelle in the cell, including the plasma membrane, cytoplasm, mitochondria, core, endoplasmic reticulum, Golgi, and others ( 16 ). The main sources of cellular ROS are inflammation and mitochondrial dysfunction, as ROS are produced by immune cells in response to infection, injury and aerobic metabolism ( 17 ). In ovarian tissue, normal levels of ROS play an important regulatory role in follicular growth, intrathecal angiogenesis and sex hormone synthesis. Nevertheless, when oocytes are exposed to high concentrations of oxygen molecules or chemical derivatives of oxygen, they are susceptible to cellular damage, which may be manifested by impaired follicular oocyte development, POI and decreased reproductive function ( 18 ). The development of POI is associated with excessive accumulation of ROS in the ovary, which accelerates the aging of ovarian cells and reduces ovarian function ( 19 ). Studies have shown that perfluorononanoic acid (PFNA) can induce OS in oocytes, destroy spindle assembly, mitochondrial function and DNA structure, and induce apoptosis and hinder oocyte maturation in vitro ( 20 ). In addition, high levels of OS-mediated ROS were also found to block GCs development and induce apoptosis in the hydrogen peroxide(H 2 O 2 ) – treated POI mouse model, ultimately leading to follicular atresia ( 21 , 22 ). As a product of OS, the pathological effect of ROS has been confirmed in many female reproductive diseases such as endometriosis, polycystic ovary syndrome, spontaneous abortion, hydatidiform mole and pre eclampsia ( 23 ). Therefore, it is of great significance to study the role of ROS, a product of OS, in the pathogenesis of POI for exploring new OS-targeting drugs.

The key to POI is the reduction in the quantity and quality of oocytes, which results in symptoms such as hot flashes, excessive sweating, vaginal dryness. Accordingly, it even causes serious consequences such as infertility. The role of apoptosis, abnormal meiosis, decreased mtDNA numbers, and ovarian microenvironment in POI is well known, but the mechanism of action is unclear. This review proposes that OS can mediate changes in genetic material, signaling pathways, transcription factors and ovarian microenvironment to accelerate oocyte and GCs senescence. Antioxidants are capable of counteracting the adverse effects of OS in the ovary and exhibit good therapeutic effects in the prevention and treatment of POI. We focus on exploring the mechanism of action of antioxidants, mesenchymal stem cells, and bioenzymes against OS in POI. Therefore, studying the relationship between OS and POI will not only help us understand the mechanism of action, but will also provide new insights into the protection of the female reproductive system and infertility.

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Y-QS put forward the article idea and wrote the article. X-TZ wrote articles. S-NZ wrote articles. Y-FM translated and polished the manuscript. Y-HH revised the article. YJ provided technical support for the article. Y-HZ revised the article and provided financial support. All authors contributed to the article and approved the submitted version.