Advances of the Gut and Vaginal Microbiota in Ovarian Cancer

1. Introduction

Ovarian carcer remains the deadliest gynecologic malignancy worldwide in 2022, with 324,398 incident cases and 206,839 mortality events globally. [1] It’s estimated that the new cases and related deaths in America by 2023 are 19,710 and 13,270, respectively [2]. Notably, more than 60% of women with ovarian cancer are diagnosed at the advanced stage [3], driving the 5-year survival rate of patients in stage IV below 20% [4]. Over the past 30 years, first-line treatment strategies—comprising debulking surgery combined with platinum-based chemotherapy and paclitaxel—have seen limited advancements since the mid-1990s [5]. Though progress in immunotherapies, including poly ADP-ribose polymerase (PARP) inhibitors (e.g. Olaparib, Rucaparib, and Veliparib) [6,7,8]and the anti-angiogenic agent (e.g. bevacizumab) have been deployed clinically, ovarian cancer continues to exhibit a high mortality rate [9]. Diagnostic delay and limited therapy are the leading factors accounting for the high mortality rate [10,11,12].

The risk factors of ovarian cancer remain not yet fully elucidated. Genetic factors [13,14], environmental factors [15], lifestyle factors [16], reproductive factors [17], and microbiome factors have all been implicated in the complex pathogenesis of ovarian cancer. (Figure 1) In recent years, microbiome factors in ovarian cancer have attracted more attention from researchers due to their potential roles in the pathogenesis.

The gut and vaginal microbiomes, as integral parts of the human microbiome, have attracted considerable attention for their relations with human’s health and the crosstalk between them [18,19,20,21,22,23]. Extensive studies have demonstrated a substantial correlation between gut and vaginal microbiomes and ovarian cancer [24,25]. Unique characteristics of gut and vaginal microbiomes of patients with ovarian cancer have been revealed by some clinical trials [25,26]. Further studies have revealed the potential mechanisms have been reported by some research. Gut and vaginal microbiomes affect the development of ovarian cancer mainly through increasing the levels of estrogen [27], regulating inflammatory responses [28], and modulating epigenetic modifications [29]. These results further indict the potential clinical applications of gut and vaginal microbiomes for prevention and early diagnosis.

In this review, we briefly summarize the characteristics of gut and vaginal microbiomes and discuss the correlation between intestinal and vaginal microbiomes and ovarian cancer. We hope to provide a possible solution to prevention and early diagnosis for patients with ovarian cancer from the microbiome point of view.

2. Characteristics of Gut and Vaginal Microbiota in Ovarian Cancer

2.1. Characteristics of Gut and Vaginal Microbiota in Health Women

Gut microbiota: The dominant phyla in the gut of healthy human are Bacteroidetes and Firmicutes, accounting for over 90% of gut microbiota [30]. Phyla Actinobacteria, Proteobacteria, and Verrucomicrobia are common but generally the minor components [31]. The composition of the gut microbiota can be influenced by a variety of factors such as genetic variation, age, diet, drug, etc. and is highly variable between different individuals [32]. Researchers have found obvious differences in the gut microbiota between males and females [33]. Compared with males, a higher α diversity of gut microbiota and a lower abundance of Bacteroidetes were observed in females [34,35]. In addition, one recent research revealed that the ratio of Firmicutes to Bacteroidetes in the gut was significantly higher in female in comparison with male [36]. A variety of metabolically protective bacteria including Akkermansia muciniphila and Alistipesshahii are notably abundant in the intestine of females over males [37]. The levels of estrogen may result in the difference of gut microbiota between male and female [38]. Study has proved that postmenopausal women present with a similarity of the gut microbiota with men [39].

Vaginal Microbiota: The vagina is colonized by a group of microbes. In healthy reproductive-age women, the vaginal microbiota is dominated by Lactobacillus spp. including Lactobacillus crispatus, Lactobacillus gasseri, Lactobacillus iners, and Lactobacillus jensenii [40]. The component of the vaginal microbiota changes dynamically with age, estrogen, living style, etc. [41] To describe its taxonomic composition the term "community state type"(CST) is raised [42]. Vaginal microbiota can be briefly divided into 5 main CSTs according to the subtypes of Lactobacillus, including CST I, II, III, V, and CST IV [43]. The CST I, II, III, and V are dominated by Lactobacillus while CST IV is characterized by a lower proportion of Lactobacillus spp. and a higher proportion of anaerobic bacteria [40,44,45]. CST IV is associated with the development of bacterial vaginosis, which is always accompanied with common gynecological diseases including sexually transmitted diseases, pelvic inflammatory disease, endometriosis, gynecological cancers, etc.[46,47]

2.2. Characteristics of Gut and Vaginal Microbiota in Patients with Ovarian Cancer

The change of components of the gut and vaginal microbiota in women with ovarian cancer has been observed in many studies. Understanding the unique characteristics of the microbiota in these patients can greatly help understand the development of ovarian cancer [48,49].

Patients with ovarian cancer show gut microbiota dysbiosis in comparison with healthy controls. Hu et al. [50] observed decreased Shannon index and α diversity in 20 patients with EOC. The β diversity was observed gradually decreasing with the progression of ovarian cancer in 30 patients [48]. D’ Amico et al. [51] analyzed the gut microbiota in 24 ovarian cancer patients and 24 health controls. They found a decrease in Lachnospiraceae, Bifidobacteriaceae, Clostridiaceae, Rikenellaceae, and Porphyromonadaceae while an increase in Coriobacteriace (Adlercreutzia and Collinsella) Lactococcus, and Lachnobacterium in ovarian cancer patients compared to the healthy control [51]. The critical role of intestinal dysbacteriosis in the development of ovarian cancer was further demonstrated by the studies of epithelium ovarian cancer (EOC) mice. The decreased relative abundance of Actinobacteria phyla, Bifidobacterium, Ruminococcaceae, and Ruminococcus and increased relative abundance of Proteobacteria, Bacteroides, and prevotella were also found in the EOC group. It should be noted that the differences were more significant in advanced stages. More importantly, the growth of tumors was significantly accelerated in the EOC mice with antibiotic deployment, compared with control mice. Chambers et al. [52] found in mice injected with EOC lines that the EOC tumor growth is significantly increased in mice treated with antibiotic-containing water compared to mice in the control group. These findings indicated that gut microenvironment homeostasis might exert inhibitory effects on ovarian cancer. (Table 1)

Obvious changes in the vaginal microbiota are also found in patients with ovarian cancer. In a multicentral case-control study including 360 ovarian cancer, 115 healthy controls, and 69 controls with benign gynecological conditions, Nené et al. [25] divided the vaginal microbiota into two community types, the community type O with less than 50% of Lactobacillus while the community type L on contrary, with at least 50% of Lactobacillus. They claimed that female patients younger than 50 years old had a significantly higher prevalence of community type O microbiota than age-matched controls. This indicates that community type L or Lactobacillus may be a protective factor in ovarian cancer. Jacobson et al. [26] also observed a less proportion of Lactobacillus-dominated vaginal community in ovarian cancer compared to age-matched healthy women. In addition, Asangba et al. [48] observed the difference in α diversity and β-diversity in the lower genital tract of patients with ovarian cancer compared with benign ones and a group of microbes in the lower genital tract that differ in different stages and grades of ovarian cancer. Moreover, vaginal infections including Human Papillomavirus (HPV) [53], Chlamydia trachomatis (CT) [54], and Neisseria gonorrhea [55] have been observed to be connected to the development of ovarian cancer. (Table 1)

These studies indicate the potential association between gut and vaginal microbiomes and ovarian cancer. The characteristics of the microbiomes in patients with ovarian cancer can possibly become the auxiliary diagnostic method of ovarian cancer. Possible mechanisms of how gut and vaginal microbiomes promote the development of ovarian cancer will be described as follow parts.

3. Interaction of Gut and Vaginal Microbiome

3.1. Evidences of the Interaction of Gut and Vaginal Microbiomes

The gut and vagina are anatomically neighbors so it’s possible to assume the existence of a relation between the microbiome in these two organs. In fact, some studies have proved that there is a degree of interaction between the human vaginal microbiota and gut microbiota. In the following, we summarize several evidences of the possible relationship of the gut and vaginal microbiota.

Similar compositions of gut and vaginal microbiota have been presented. A cross-sectional study revealed the correlation of rectal and vaginal microbiota in women who are not pregnant [56]. They found Lactobacilluscrispatus (16%), Lactobacillus jensenii (10%), and Lactobacillus gasseri (10%) were the prevalent lactobacilli colonizing the rectums in most females, which is surprisingly according with the proportion in their vagina. [56]. Another research assessed the rectal and vaginal microbiota in 132 pregnant women at 35-37 weeks of gestation. They found almost half of the bacterial species present in both the rectum and vagina. [57]. A recent study showed similar results. Shin et al. [58] found that from the last trimester of pregnancy to the second month after birth, the composition of the microbiota in these two places became closer. In another study, bacterial vaginosis associated bacteria were detected in both vagina and rectum in women with CST IV (non-Lactobacillus-dominant). This suggested that the rectum stored the bacterial vaginosis associated bacteria of CST IV [59]. The high homogeneity of the microbiota colonized between the vagina and rectum provides strong evidence that the rectal microbiota serves as a reservoir for the vaginal microbiota. Moreover, some evidences have vaguely presented interaction between gut and vaginal microbiomes. the probiotics given orally can change the microbiota in the vagina [60]and cure the Gardnerella vaginalis induced bacterial vaginitis [61]. Oral medication can regulate the vaginal microenvironment, indicating the innate relations between the gut and vagina. Current research also suggested that the vaginal microbiological environment of the mother has a close influence on the gut microbiota of the infant, which is called vaginal seeding [62]. The gut microbial composition of infants born by cesarean section differs from that of neonates born vaginally over a certain period. It may affect neonatal health in terms of obesity [63], and immune-related diseases [64].

Though no definite evidence confirms the interaction between gut and vagina microbiota, the similar composition and the Vaginal Seeding for infants strongly suggest potential association between them.

3.2. Possible Mechanisms of the Interaction of Gut and Vaginal Microbiomes

More researches on the interaction between microbiome located in different parts of the body are still warranted. With the improvement of research techniques and the accumulation of experimental content, we’ll complete the missing parts of the interaction between gut and vaginal microbiomes and get a better understanding of the link between gut and vaginal microbial interaction and ovarian cancer in the future.

Some evidences indicate that gut and vaginal microbiomes may interact with each other via the gut-vaginal axis. The gut-vagina axis is described as two-side interactions between gut and vaginal microbes. In the gut-vagina axis, the gut influences the vaginal microbiome primarily through the estrobolome, but the influence of the vaginal microbiome on the gut microbiome has not yet been revealed. Estrobolome is defined as a part of the gut microbiome with the ability to metabolize estrogen and regulate the levels of estrogen through the estrogen-gut microbiome axis. In other word, estrobolome is an aggregate of genes in gut microbes with the capability of secreting β-glucuronides for estrogen metabolism [65]. Conjugated estrogen can be deconjugated by those enzymes and then return to blood circulation through enterohepatic circulation, resulting in a high level of estrogen in the body [65,66,67]. High levels of circulating estrogen stimulate the production of vaginal glycogen which then promotes the colonization of Lactobacillus in the vagina, thus influencing the vaginal microbiome [68]. Although the interaction mechanisms between vaginal microbes and gut microbiota are largely unknown recently, the link between obesity, gut microbiome dysbiosis, and vaginal microbiota composition suggests the two-side effects of the gut-vagina axis.

More researches on the interaction between microbiome located in different parts of the body are still warranted. With the improvement of research techniques and the accumulation of experimental content, we’ll complete the missing parts of the interaction between gut and vaginal microbiomes and get a better understanding of the link between gut and vaginal microbial interaction and ovarian cancer in the future.

4. Potential Mechanisms of Gut and Vaginal Microbiomes in Ovarian Cancer

Emerging evidence substantiates a significant association between dysbiosis of gut and vaginal microbiota with ovarian cancer. The gut and vaginal microbiota affect the development of ovarian cancer mainly through two routines, the microbe-associated molecular patterns (MAMPs) and metabolites [28,69]. MAMPs represent conserved structural components of microorganisms, including bacterial flagellin and lipopolysaccharide (LPS) [70] which mediate host-microbiome interaction by combining with Pattern Recognition Receptors (PRRs) such as toll-like receptors (TLRs) and nucleotide-binding oligomerization domain-containing protein-like receptors (NLRs) and then activate downstream pathways to regulate the physiological and pathological processes of the host [69]. Metabolites produced by microbiota are also responsible for interacting with the host cells [28]. The gut microbiota can produce metabolites including estrogen, SCFAs, secondary bile acid, indolepropionic acid (IPA), conjugated linoleic acid (CLA), and even genotoxic metabolites. While the vaginal microbiota produces distinct metabolic products including lactic acid, hydrogen peroxide(H₂O₂), SCFAs, peptides, phospholipid, nucleotides, etc. [71] that contribute to regulate host physiological and pathological processes, even cancer genesis [72]. Herein, we conclude the most characterized mechanisms through which gut and vaginal microbiota influence ovarian cancer progression.

4.1. Gut and Vaginal Microbiomes and Estrogen in Ovarian Cancer

In general, estrogen is produced by the ovarian, adrenal gland, and adipose tissue with additional contributions from dietary sources [33]. Hepatic metabolism converts estrogens into conjugated forms via glucuronidation (UDP-glucuronosyltransferase) and sulfation (sulfotransferases), facilitating biliary excretion or enterohepatic recirculation [73]. Circulating estrogen exist in three main forms including estrone, estradiol, and estriol, in either free or protein-bound stats [74]. Estrogen combines with classical nuclear receptors (estrogen receptors α and β, ER-α and ER-β) or non-classical membrane receptors (G-protein-coupled estrogen receptor, GPER) [75,76]. Estrogen and their metabolites can also be processed by the gut microbiome [77], and enrolled in the enterohepatic circulation, reabsorbed into the circulation system to regulate physiological activities. It's widely known that high estrogen levels and ER-positive [78] are risk factors for ovarian cancer. Patients with early menarche, late menopause, and fertility drugs are at a higher risk of developing ovarian cancer [79].

The gut microbiome possibly affects the development of ovarian cancer via the regulation of estrogen levels in two main routines, estrobolome and gut-brain axis. (Figure 2) The gut microbiome can regulate the levels of estrogen through the estrobolome [80]. The estrobolome is mainly composed of Firmicutes and Bacteroidetes [81]. A total of 279 β-glucuronidase proteins encoded by gut microbes have been identified in the Human Microbiome Project database [67,81]. The β-glucuronides produced by estrobolome can uncouple the conjugated estrogen, promote the reabsorption of free estrogen through enterohepatic circulation and increase the total estrogen burden [65]. On the other hand, gut microbiome can also affect the generation of estrogen through gut-brain axis. Gut microbiome can produce metabolites like gamma-aminobutyric acid and norepinephrine acting as neurotransmitters, stimulating local nerve cells or vagus nerves and then affecting brain cells [82]. These effects may lead to stress reactions such as depression and anxiety [83], closely associated with over activation of the hypothalamic-pituitary-adrenal (HPA) axis [84]. The activation of HPA axis in stress circumstances is linked with the generation of gonadal hormones [85], leading to the change of estrogen levels. However, little is known about the impact of vaginal microbes on estrogen.

Available evidence suggested that high unconjugated estradiol levels are positively correlated with a high risk of non-plasmacytoid ovarian cancer [86,87]. Excessive estrogen may promote the proliferation of ovarian epithelial cells [88] and then accelerate the progression of ovarian cancer through receptor-dependent pathways (ER-α and GREP) and receptor-independent pathways (cytochrome P, CYP450 enzymes). ER-α combined with estrogen can activate downstream oncogenesis genes including c-fos, c-myc, and HER2/neu. Moreover, the combined ER-α can active mitogen-activated protein kinase (MAPK) [89] and Wnt/β-catenin signaling pathways [90], which are closely associated with the development of ovarian cancer [91,92]. The GREP induces the second messenger system such as extracellular regulated protein kinases (ERK) and the Phosphatidylinositol-3-kinase (PI3K) [93], promoting the proliferation of cancer cells [27]. In addition, the metabolites of estrogen like quinone intermediates via CYPs can elevate the level of free radicals and induce the mutation of genes [27]. (Figure 2)

In summary, gut and vaginal microbiomes interact with each other via gut-vaginal axis. The disturbance of the gut microbiome changes the estrogen levels via estrobolome and gut-brain axis. The estrogen then combines with the receptors and promotes the proliferation of cancer cells. Meanwhile, the metabolites of estrogen can lead to DNA damage, which results in the development of ovarian cancer [27].

4.2. Gut and Vaginal Microbiomes and Inflammation in Ovarian Cancer

The critical role of inflammation has been well proved in carcinogenesis. Acute inflammation mediates antitumor immunity through dendritic cell activation and antigen presentation while chronic inflammation leads to a cancer-promoting niche [94]. Long-term preserving acute inflammation turns into chronic inflammation, which leads to continuous inflammatory signaling activation and formation of hypoxic and acidic microenvironment. And then the aggregation of immunosuppressive cells and activation of protooncogene lead to damage of DNA, and at last tumor genesis [95]. Notably, the risk factors of ovarian cancer such as psychological factors [15,96,97]and obesity [98] have been proven to promote ovarian cancer through inflammation associated pathways with the help of cytokines including interleukins and interferons [99]. The inflammatory response is mainly regulated by microbe associated molecular patterns (MAMPs) and the metabolites [69] [100,101]. (Figure 3)

The MAMPs of the gut microbiota interact with PRRs and then cause the host inflammatory reaction. TLR-4 is one of the most reported PRRs in ovarian cancer. Gut microbiota-derived LPS activates TLR4/NF-κB signaling in ovarian carcinoma cells via myeloid differentiation factor 88 (MYD88) adaptor recruitment. [102] Then increase the levels of proinflammatory cytokines including tumor necrosis factor α (TNF-α), interleukin 6 (IL-6), interleukin 8 (IL-8) and monocyte chemotactic protein 1 (MCP-1) combining with anti-apoptotic family activation. This may promote the development of ovarian cancer by stimulating pro-angiogenic cytokines and anti-apoptotic responses [103,104].

Emerging evidence further implicates TLR4 crosstalk with Hedgehog (Hh) pathway. Hu et al. [50] demonstrated the pathway in murine EOC model. The gut microbiota from mice with EOC could promote the proliferation of tumors, while Hh inhibition (GANT61) significantly restrained the tumor progression [50]. Gut microbiota promotes EOC via Sonic Hedgehog (Shh) /Gli-1 axis. NF-κB regulates the Hh signaling pathway by increasing the production of TNF-α and IL-1β, enhancing the activity of Sonic Hedgehog (Shh) promoter, and promoting the expression of Gli-1[50,105]. As a result, it increases migration and invasion of cancer cells. [106] A retrospectively including 102 patients revealed association between TLR4 and tumor associated macrophage (TAM). The over expression of TLR4 on macrophage and the increasing levels of TAM biomarkers are linked to reduced progression-free survival [107]. This correlation was further confirmed by Xu et al. with murine model [108]. Gut microbiota dysbiosis induced by high-dose antibiotics was associated with ovarian cancer progression. Gut microbiota dysbiosis enhanced the secretion of TAM-derived TNF-α and IL-6, and then promoted the epithelial-mesenchymal transition (EMT) procedure. As a result, the development of ovarian cancer was accelerated. (Figure 3)

TLR5 is reported to be associated with ovarian cancer via activating MyD88/ TNF receptor associated factor 6 (TRAF6) signaling. Rutkowski et al. [24] suggested that TLR5 in microbiota increased the levels of IL-6 and activating the adaptor molecules MyD88 and TRAF6. Then the recruitment of Myeloid-derived suppressor cells and γδ lymphoid cells promoting the secretion of Galectin-1, which accelerated the growth of tumor cells. This result was further proved in EOC patients. Patients with TLR5 deficient were claimed to have a higher long-term survival rate when compared with those without TLR5 deficient [24]. Moreover, IL-6 was also demonstrated to be able to activate the Janus Kinase-signal transducer and activator of transcription 3 pathway (JAK-STAT3) and MAPK pathways, which are closely associated with the aggressive disease course and poor prognosis of high-grade ovarian cancer [109].(Figure 3)

Beyond MAMPs, gut microbiota-derived metabolites, including secondary bile acids, SCFAs, IPA, genotoxic metabolites, etc. pose critical influence on ovarian cancer development. [28] These metabolites act locally or at distal sites through circulation system [110] and affect ovarian carcinogenesis through promoting cell apoptosis, scavenging free-radicals and suppressing inflammation [100,101].

Gut microbiota can uncouple conjugated bile acids via Bacterial bile salt hydrolase (BSH) enzymes and further process and modify them to generate secondary bile acids, including deoxycholic acid (DCA) and lithocholic acid [111]. Secondary bile acids can inhibit the proliferation of ovarian cancer. DCA and ursodeoxycholic acid (UDCA) can induce cell apoptosis through a PKC-independent pathway in the ovarian cell line (A2780) and cisplatin-resistant cell line(A2780-CP) [112]. This result further proves the anti-tumor effect of secondary bile acid on ovarian cancer. SCFAs, generally including acetate, propionate, and butyrate, are a group of compounds fermented from undigested carbohydrates by intestinal flora such as Bacteroides, Roseburia, Bifidobacterium, Fecalibacterium, and Enterobacteria [113]. SCFAs were claimed to be implicated in the promotion of macrophage differentiation and inhibition of the NF-κB pathway in cancer patients, which can inhibit carcinogenesis [114]. Tryptophan can be metabolized by the gut microbiota, and then produce indole compounds including IPA [115]. IPA has been proven with anti-tumor effects in ovarian cancer. A study found a decreased level of IPA in patients with ovarian cancer. And it further suggested that IPA can scavenge free radicals and was responsible for anti-tumor function. [116] In addition, intestinal dysbacteriosis can also promote carcinogenesis by producing genotoxic metabolites (e.g. Colibactin) and promoting a pro-inflammatory state that contributes to cancer progression [117]. The research is still limited in the field of gut microbiota metabolisms and ovarian cancer. (Figure 3)

Moreover, the metabolism including secondary bile acids, CLAs, and SCFAs can suppress inflammation via anti-obesity mechanisms. Evidence shows those microbiota metabolites can suppress obesity and the adipocytokines such as TNF-α and IL-6. It has been shown that Bile acids may affect obesity via altering energy, lipid, and glucose metabolism [118]. Available evidence indicated that levels of secondary bile acids are abnormal in obese patients and may be associated with pathological changes. Secondary bile acids can reduce inflammation and lipoprotein uptake, and decrease the development of atherosclerosis [119]. CLAs are a family of fatty acids with weight-loss properties, widely found in beef and dairy products and produced by some bacteria in the Firmicutes and Actinobacteria phyla. Recent studies reported that CLAs can promote weight loss by reducing the synthesis of anti-inflammatory eicosanoids, activating PPARs involved in lipid metabolism and immune function, promoting the browning of white adipose tissue [120], and reducing adiponectin secreted by adipose cells [121]. There is a contradiction between the anti-obesity and pro-obesity effects of SCFAs. On the one hand, SCFAs can increase intestinal energy accumulation and promote the development of obesity. On the other hand, SCFAs increase energy expenditure and anorexia hormone production, reduce appetite, and maintain the intestinal mucosal barrier integrity to inhibit obesity. In conclusion, SCFAs contribute to the improvement of insulin sensitivity and play a beneficial role in maintaining normal metabolism and function of multiple organ systems [122,123].

The vaginal microbiota is also essential in the prevention of infection by external pathogens and the maintenance of an immune tolerant environment. Several studies have linked reproductive tract infections with an increased risk of ovarian cancer [124,125]. Commensal vaginal microbiota is dominated by lactobacillus, which can protect the host from infection by producing lactic acid and H2O2[56]. Rahbar et al. [126] found that Lactobacillus, isolated from the vagina, enabled to induce the apoptosis of CAOV-4 cells and suppress the development of ovarian cancer by down-regulation of miR-21, miR-200b, and TLR-4. Available evidence indicated that vaginal dysbacteriosis is associated with an increased risk of infection and ovarian cancer. Patients with ovarian cancer are observed with less Lactobacillus and more pathogens including Chlamydia trachomatis and HPV in the vagina [25]. Chlamydia trachomatis can block caspase 3 and cytochrome C release and inhibit apoptosis, which can promote the development of ovarian cancer [125]. In addition, Chlamydia trachomatis can also active the TLRs in the epithelial cells and promote the ovarian cancer carcinogenesis [127]. Other cancerogenic mechanisms associated with pathogen infection include regulation of DNA damage repairing, degeneration of p53, and stimulation of the MAPK signaling pathway [125].

4.3. Gut and Vaginal Microbiomes and Inflammation in Ovarian Cancer

The genetic factor is an essential risk factor for ovarian cancer. Although identified susceptibility genes have offered insights into the complex genetic basis of ovarian cancer, the identified risk loci only explain a small part of overall disease variance. The epigenetic modifications, mainly including methylation and histone modifications, mediating the interaction between genetics and environment, have been widely observed in the development of ovarian cancer. Some studies reported that the methylation of 59 cytosine phosphate guanine (CpG) rich regions was increased in many kinds of tumors, including ovarian cancer. methylated CpG islands are implicated in the regulation of cell cycle, apoptosis, and drug-sensitive gene silencing. Methylation of some risk genes such as BRCA1, RASSF1A, and PTEN has been demonstrated in ovarian cancer [29,128,129]. Histone modifications, such as acetylation and methylation are implicated in the pathogenesis of ovarian cancer [130]. The loss of acetylation of histone H4 lysine 16 (H4K16) in ovarian cancer tissue has been reported associated with the development of ovarian cancer [131]. Meanwhile, the histone methyltransferases, responsible for histone methylation, such as enhancers of zeste homologue 2 (EZH2), disruptors of telomeric silencing-1-like (DOT1L), and protein arginine methyltransferase (PRMT) are reported in correlation with the development of ovarian cancer [130]. Microbiota are closely related to host cell epigenetics. Helicobacter pylori can cause host gene methylation and suppress damaged DNA repairing in gastric cancer and colorectal malignant tissues [132,133,134]. Fusobacterium is also reported to accelerate the methylation of colorectal cancer in patients with UC, which is closely associated with the development of colorectal cancer [135]. In addition, some metabolites such as SCFAs can also influence the epigenetics in host cells [136,137]. Among SCFAs, butyrate has been reported with the activity of histone deacetylase (HDAC) inhibitor in the nucleus to prevent cell multiplication and trigger apoptosis [137]. Moreover, the folate and some B vitamins produced by gut microbiota can participate in the methylation of DNA and histone by providing methyl groups [138].

Although recent research has revealed epigenetic modifications in ovarian cancer, the role of microbiota remains unknown in this procedure [29,128]. Some studies indirectly suggested a link between microbe-induced alterations in host epigenetics and ovarian cancer development. In patients with vaginal Chlamydia trachomatis, Chlamydia trachomatis infection induces methylation of the calreticulin promoter, which in turn leads to a down-regulation of calreticulin expression and Epithelial–mesenchymal transition (EMT) [139]. Chlamydia trachomatis infection is the leading cause of pelvic inflammatory disease (PID). Chlamydia trachomatis infected women have been shown to have a higher risk of ovarian cancer than non-infected women [124]. The difference may be related to its ability to induce EMT in ovarian epithelial cells. In humans with Firmicutes dominated gut microbiota, a total of 803 genes with differentially methylated promoters are claimed to be associated with lipometabolism, obesity, and the inflammatory response, while obesity and inflammation have been verified as the risk factors of ovarian cancer [140]. More efforts are needed to clarify how specific microbiota or metabolites can regulate epigenetic modifications and then affect the development of ovarian cancer. Key metabolites and critical signaling pathways among them may become novel targets for epigenetic-based therapies for ovarian cancer.

5. Discussion

Ovarian cancer is a complex disease with an unclear pathogenesis and complex aetiologic factors. The disease prediction and early diagnosis of ovarian cancer are far from satisfactory. The unclear pathogenesis and delayed diagnosis in ovarian cancer are urgent to be solved. Recent advances in microbiomes suggest the potential of microbiota in the diagnosis and management of human disease [141,142,143] and offer us an entirely new way to better understand the mechanism of ovarian cancer and the opportunity to optimize the prevention and early diagnosis of ovarian cancer.

As we discussed above, ovarian cancer has been reported mainly associated with factors including environmental factors [144,145], genetic factors [13,146,147,148], demographic and lifestyle factors [96,149,150], reproductive factors [13,151,152], microbiome factors, and other factors [16,87]. (Figure 1) Though recent research revealed the separate effects of these risk factors, their cooperation in individuals is almost ignored, which may be a blockage of exploring the development of ovarian cancer. The risk factors exert different effects in specific patients with ovarian cancer. The Wheel of Causation and Web of Causation [153], models that focus on the etiology of chronic diseases, may help us understand the interaction of risk factors in ovarian cancer. The multi-causal models will help to further elucidate the effect of risk factors on a specific individual and the interactions of each factor, which are essential for individual prevention [154]. These will greatly help us make the strategy of ovarian cancer prevention.

Moreover, women with ovarian cancer are generally diagnosed at advanced stages (stage III/IV) [3], which is closely related to the poor prognosis. The main reason for the difficulties in early diagnosis should be the lack of effective screening methods. The present screening methods mainly include transvaginal ultrasonography (TVS) and serum tests for cancer antigen 125 (CA125) [155]. However, none of them shows satisfactory effects in decreasing the morbidity of ovarian cancer in asymptomatic, average-risk women [156]. Updating of ovarian cancer screening methods is in urgency. Microbiomes in the gut and vagina provide us with a potential way to screen for ovarian cancer. Combining gut and vaginal microbiomes and other associated factors, we may improve the diagnostic model of ovarian cancer, shortening the diagnostic delay and reducing mortality. To ensure the accuracy of the early diagnosis model, we need to make full use of modern molecular biology technology to screen and identify ovarian cancer biomarkers from the aspects of microbiology, metabolomics, genomics, transcriptomics, key signaling pathways, and tissue target cells. At the same time, combining with the traditional factors related to ovarian cancer (including symptoms, serum index, genetic index, etc.) is also essential to enhance the sensitivity and specificity of the early diagnosis prediction model.

6. Conclusions

The gut and vaginal microbiomes show extremely complex interactions in different kinds of diseases including ovarian cancer. Some important roles of microbiomes in the development and treatment of ovarian cancer have been demonstrated in several studies. The gut and vaginal microbiomes may affect the progression of ovarian cancer via the regulation of estrogen, modulation of inflammatory, and immune responses, and modulation of epigenetic modifications. There are great differences in gut and vaginal microbiomes between patients with ovarian cancer and those with benign ones, providing a possibility of using microbiomes for differential diagnosis. Moreover, incorporating gut and vaginal microbiomes into multi-aetiologic models will help to fully elucidate the mechanisms of ovarian cancer development. Combining multi-omics research for novel ovarian cancer-related markers with traditional ovarian cancer-associated factors is crucial for the prevention and early diagnosis of ovarian cancer, which reduces the morbidity and mortality rate of ovarian cancer.