Unraveling the links between estrogen and gut microbiota in sex-hormone driven cancers

Abstract

Estrogens are a group of steroid hormones produced by ovary, placenta, and other organs. They have historically been associated with female reproduction, but according to current evidence estrogens regulate also male reproductive and nonreproductive organs. Estrogens play a crucial role in female reproductive development and maintenance either directly by increasing glycogen levels, epithelial thickness and mucus secretion or indirectly, by decreasing vaginal pH through the maintenance of lactobacilli dominance and lactic acid production. Several studies demonstrated that dysbiosis and/or specific bacteria could have impact on the development of sex-hormone driven cancers such as endometrial, cervical, ovarian, breast and prostate cancers, through mechanisms involving modulation of estrogen metabolism. This modulation is realized through secretion of β-glucuronidase which deconjugates estrogens into their active forms. When gut dysbiosis occurs, microbial diversity decreases and so the deconjugation diminishes leading to a decrease of circulating estrogens. Low levels of circulating estrogen may adversely affect a wide range of physiological factors, with clinical implications especially for gut health. In this review, we discuss the different aspects of the critical interplay between gut microbiome and estrogens in sex-hormone driven cancers and the potential outcomes on their clinical management.

Key Words: Diet; Prostate; Breast; Estrobolome; Cancer; Gut microbiome; Estrogen

Core Tip: The effect of the gut microbiome (GM) expands upon the intestine in altering the metabolism and inducing inflammation. Mutually, the host microenvironment impacts the GM through the variation of estrogen levels. The circulating estrogen itself is regulated through the β-glucuronidase enzyme which deconjugates estrogen to its biologically active form, allowing it to link to estrogen receptors and thus contributing to its physiological and pathological effects (e.g. cancers and specially sex-hormone driven cancers). In this review, the interaction between estrogen and microbiota in sex-hormone-mediated cancers as well as the potential health outcomes on their clinical management are emphasized.

INTRODUCTION

The gut microbiome (GM) is defined as the whole of the microbiota genetic material that forms the intestinal epithelial barrier[1] and comprises primarily 4 phyla: Firmicutes, Bacteriodetes, Actinobacteria and Proteobacteria[2]. Dietary compounds generate essential nutrients for the body as well as substrates for the commensal flora of the gastrointestinal tract. The GM microorganisms metabolize undigested food components into different metabolites that are the major mediators in the relationship between host and GM[3]. Thereby, food modulates the composition, structure and function of the GM, which, by the connections with gut epithelium and mucosal immune system, controls intestinal homeostasis regulating the host functions[3]. A balanced bacterial composition and metabolic profile are crucial to maintain microbiota and immunological homeostasis. When the Firmicutes/Bacteriodetes ratio is increased and the bacterial diversity decreased, dysbiosis occurs[2]. Overall, dysbiosis is basically characterized by an increase of anaerobes and/or pathobionts[4]. A high-fat diet supports pathobiont expansion through impacting the bile acid metabolism, which can favor intestinal inflammation in genetically predisposed subjects[3]. The gut pathogenic bacteria may indirectly drive carcinogenesis through estrobolome[4] that is defined as the entire genes collection of the GM, in charge of metabolizing estrogens[1]. It is worthy of note that the estrobolome influence is not exclusively dedicated to women[2]. Mutually, estrogens can also modify the gut epithelial barrier integrity[2]. These female steroid hormones play with their transcription factors [estrogen receptor (ERs)] a crucial role not only in maintaining the female reproductive organs and secondary sexual characteristics but also in regulating several physiological processes of cell growth, reproduction, development, and differentiation[5]. Many tissues such as intestine, adipose tissue and brain express ERs[6]. As a consequence of this global expression, estrogen has been revealed to impact various physiological responses including cancers and specially sex-hormone driven cancers[2]. Currently, many anticancer therapies can be applied through counteracting dysbiosis[4]. Microbial therapy is obtained through diverse approaches such as diet, probiotics, prebiotics and postbiotics as well as antibiotics, microbiota transplantation and engineering bacteria. Since some bacterial species possessing β-glucuronidase enzyme that deconjugate estrogen to its conjugated form, modulating the microbiome composition and function, we modulate the estrogens’ levels[7]. The GM modulation and subsequently metabolic profile is an emerging way of treating sex-hormone driven cancers[2]. In this review, we highlighted the links between estrogen and microbiome in sex-hormone-mediated cancers and provided the potential health outcomes in resolving these cancers.

INTERPLAY BETWEEN ESTROGEN AND MICROBIOME

Metabolism of estrogens

Although their classical association with female reproduction, estrogens and their main nuclear receptors (ESR1 and ESR2) and G protein-coupled ERs also control male genital and non-genital organs[8]. Indeed, 17 β-Estradiol is assessable in males’ blood and rete testis fluids where it can attain concentrations usually found only in females[8]. In addition, nanomolar concentrations of estrone sulfate were recorded in the semen of some animal species[8]. Moreover, aromatase, which transform androgens into estrogens, is expressed in several male organs like Leydig cells and seminiferous epithelium[8].

Through optimal concentrations, estrogens have a deep involvement in health and disease[9]. They undergo extensive metabolism by microbial secreted β-glucuronidase that transforms them from their conjugate forms to their deconjugated or unbound forms which are the "active" versions able to enter the bloodstream through enterohepatic circulation[10] and act on the two isoforms: ERs alpha (ERα) and ERs beta (ERβ)[2]. ERα and ERβ are ERs for the estrogen and play crucial roles in human ERs tissues regulating various physiological processes[5].

The biologically active endogenous estrogens are estrone (E1), estradiol (E2), estriol (E3) and estetrol (E4) which is exclusively produced in pregnancy. These estrogens are also applied exogenously for their high therapeutic potential[9].

E1, E2, and E3 are metabolized through phase I including hydroxylation, oxidation and reduction reactions and phase II comprising primarily conjugation reactions, while E4undergoes exclusively phase II reactions[9].

Estrogen metabolites are variable in terms of nature and quantity depending on their endogenous formation, chemical structure and route of administration of exogenous estrogens. Before being excreted in urine and/or feces, some hydroxylated metabolites may interact with ERs (ERα and ERβ) and generate significant hormonal responses in target tissues while others can elicit unique biological effects unrelated to ER activation[9]. In fact, catechol and 16α-hydroxylated estrogens are estrogen metabolites recognized for their carcinogenic potential[9]. Studies performed in preclinical models showed that microbial metabolites modify phenotypes of tumor somatic mutations and regulate immune checkpoint inhibitor efficiency[11].

Furthermore, epidemiological studies have documented that the metabolic syndrome, referring to a disorder in the metabolism of carbohydrates, fats, proteins and other substances in the human body[5], can increase the cancer risk[12]. Sex hormone related cancers like endometrial and prostate cancers (PC) have been found in relationships with metabolic syndrome. Most components of metabolic syndrome are in some way linked to the cancer development[5]. Indeed, after menopause, women are more likely to develop metabolic syndrome since the decreased: (1) Estrogen levels; and (2) ERs-sensitivity[5,12].

Estrogen and oncobiome in females

Estrogens are a main GM regulator and reciprocally, the GM is acting in the control of estrogen levels[13]. Dysbiosis minimizes β-glucuronidase activity and consequently reduces deconjugation of estrogens into their circulating active forms. The reduction in circulating estrogens impairs ERs activation and may lead to hypestrogenic pathologies like metabolic syndrome[13]. On the other hand, an increase in the abundance of bacteria producing β-glucuronidase can result in heightened levels of circulating estrogens and lead to diseases like cancer[13], and especially the sex hormone-driven cancers, such as ovarian, endometrial, cervical and breast cancers (BC)[13].

Despite the fact that microbial diversity is multifactorial, many studies found an association between estrogen with its subsequent metabolites and microbial taxa characterized by specific diversity and GM composition in both healthy and disease conditions[11,14].

Finally, the GM is suggested to modify host immunity through affecting various immunologic pathways, therefore influencing cancer risk and treatment outcomes in many malignancies[15].

Ovarian cancer: Ovarian cancer (OC) is characterized by significantly increased levels of Proteobacteria and Firmicutes phylum bacteria and the presence of Chlamydia trachomatis, Lactobacillus and Mycobacterium[4]. In OC tissues, Proteobacteria phylum is the most abundant[4,16]. Proteobacteriahas a lipopolysaccharide layer that allows direct interaction with intestinal mucosal cells through bacterial secretion systems[16,17]. Proteobacteria may get involved, in combination with host genes, in the metabolic by-products of fatty acid metabolism[18]. The second most prevalent phylum in the OC tissues are Firmicutes bacteria that were significantly enhanced in intestinal lumen[16]. Zhou et al[16] revealed that dysbiosis of Proteobacteria and Firmicutes ratios can impact ovarian carcinogenesis if they lead to persisted infection. In addition, Acinetobacter_lwoffii species are within Acinetobacters genera and normally colonize theoropharynx, perineum, human skin, and have tropism for urinary tract mucosa[19]. It has been documented that they were significantly heightened in OC tissues[16]. On the contrary, Lactococcus_piscium species within Lactococcus genera, were generally considered as gut commensal with probiotic features and they were significantly decreased in OC tissues[16] (Table1).

Endometrial cancer: Elevated levels of estrogen can stimulate the proliferation of epithelial cells in female reproductive tract and subsequently cause endometrial cancer (EC)[20,21]. Chadchan et al[22] reported that estrogen levels are decreased after antibiotic therapy in mice confirming the crucial role played by intestinal bacteria in estrogen metabolism. As previously reported in the estrogen-GM axis, microbes comprising Bacteroidetes, Firmicutes, and Bifidobacterium possess genes coding for glucuronidase activity and acting as an indicator of an impaired estrobolome and disrupted estrogen metabolism in mice affected by endometriosis[2,23]. This dysregulation rises circulating estrogen levels and induces ectopic endometrial invasion and growth as well as cyclic bleeding and ache[21]. In the gut of endometriosis patients, an expand in β-glucuronidase-producing bacteria resulted in increased circulating estrogen levels and alteration in estrogen metabolism. Indeed, analysis of endometriosis patients revealed the presence of significant differences in the expression of 16-keto-17β-estradiol, 17β-estradiol,2-hydroxyestradiol and 2-hydroxyestrone in comparison with healthy individuals, and their GM was positively correlated with urinary estrogen[24]. The imbalance in estrogen metabolism is a risk factor for the EC development and it is intimately related to GM changes[4]. In addition, the GM can indirectly induce endometrial carcinogenesis by changing genital microbial communities[4]. EC is characterized by the presence of Porphyromonas and Atopobium vaginae species, which favor hyperplasia and lead to carcinoma[4]. In fact, sequencing analysis of these species detected in the endometrium of EC patients revealed a high similarity with bacteria of the same species identified in the vagina[4,25] (Table 1).

Table 1 Gut oncobiome in the different types of cancer.

Gender	Cancer type	Gut oncobiome	Ref.
Female	Ovarian cancer	Proteobacteria	[4,16]
		Firmicutes	[4,16]
		Acinetobacter_lwoffii	[16]
		Lactococcus_piscium	[16]
		Chlamydia trachomatis	[4]
		Lactobacillus	[4]
		Mycobacterium	[4]
	Endometrial cancer	Bacteroidetes	[2]
		Firmicutes	[2]
		Bifidobacterium	[2]
		Porphyromonas	[4]
		Atopobium vaginae species	[4]
	Cervical cancer	Proteobacteria	[31]
		Porphyromonas	[15]
		Dialister	[15]
		Prevotella	[15]
	Breast cancer	Firmicutes	[41]
		Pediococcus (Firmicutes)	[39]
		Desulfovibrio (Proteobacteria)	[39]
		Feacalibacterium (Firmicutes)	[40]
		Clostridiacea (Firmicutes)	[40]
		Ruminococcaceae (Firmicutes)	[40]
		Lachnospiracea (Firmicutes)	[40]
		Doreae (Firmicutes)	[40]
		Blautia (Lachnospiraceae)	[44]
		C. coccoides (Clostridiaceae)	[44]
Male	Prostate cancer	Streptococcus	[55,57]
		Bacteroides spp.	[55,57]
		Bacteroides massiliensis	[48]
		Alistipes	[58]
		Rikenellaceae	[58]
		Lachnospira	[58]
	Breast cancer	No information available

Cervical cancer: Cervical cancer (CC) is the most widespread gynecologic cancer. This tumor, characterized by the development of cervical dysplasia, is the result of a persistent human papillomavirus infection causing immune evasion[15,26]. Previous data suggested that diversity, abundance and specific GM composition may lead to enhanced anti-tumoral immune response[27]. For instance, in neoplasm patients receiving immunotherapy, Gopalakrishnan et al[28] disclosed that favorable baseline GM identified by high diversity and abundance of Faecalibacterium and Ruminococcaceae showed improved systemic and antitumor immune responses.

Several studies established that the fecal microbiome diversity in CC patients is different between young and older women[15,29,30]. In addition, they also noticed that α and β diversity are significantly different between CC patients and cancer-free controls, proposing differences in the GM composition[15,29,30]. The analysis of the GM in CC patients and healthy controls showed higher α-diversity particularly in older patients and β-diversity differences, correlated with healthy controls[15,31]. Furthermore, for the differences in the GM composition, Wang et al[31] observed several genera differentiating CC patients and healthy controls with higher abundance of the members of the phylum Proteobacteria in CC patients, while Sims et al[15] found in similar patients that the relative abundance of specific taxa was mainly regarding Porphyromonas, Dialister and Prevotella. These differences between these two studies are probably due to geography, ethnicity or subsistence-specific variations in human GM composition[15,32,33].

In menopause, the drastic decrease in estrogen levels is accompanied with a decline in Lactobacilli composition. This rises the alkalinity of the cervico-vaginal environment, promoting the affluence of other anaerobic bacteria including Porphyromonas, Gardnerella vaginalis, Sneathia, Leptotrihia, Prevotellabivia and Fusobacterium, making the cervical cells vulnerable to oncogenesis[4] (Table 1).

BC: Circulating estrogen is considered as a chief biomarker of BC[34]. A growing body of evidence studying pre- or postmenopausal status revealed a relationship between circulating estrogen levels, GM and BC risk[35]. In case of dysbiosis, an increase in β-glucuronidase-possessing bacteria occurs resulting in an elevation in circulating estrogen levels and potentially promoting the BC development[36,37]. Based on menopausal status, the findings seem to be divergent. Indeed, some studies did not detect significant differences regarding α-diversity between microbiota of premenopausal patients and healthy women[38,39]. A different investigation found changes in the composition of intestinal flora in premenopausal BC patients with respect to the controls and revealed that Pediococcus and Desulfovibrio affiliated with Firmicutes and Proteobacteria respectively, might differentiate premenopausal BC patients from normal premenopausal women[39]. For postmenopausal patients, a significant increase in α- and β-diversity in addition to a significant difference in the relative abundance of 45 species were recorded in comparison to healthy controls[38]. Moreover, Goedert et al[40] documented that the postmenopausal fecal microbiota of BC patients presents a significant decrease in α-diversity in addition to some differences within the Firmicutes phyla manifested by increased levels of Feacalibacterium, Clostridiacea and Ruminococcaceae and decreased levels of Lachnospiracea and Doreae in comparison to healthy controls. Moreover, Bobin-Dubigeon et al[41] highlighted a decrease in microbial diversity, a decline in Bacteroidetes along with a relative enrichment in Firmicutes at the expense of Coprococcussp., Butyricimonas sp., and Odoribacter sp. in early BC patients compared to healthy women; While, Aarnoutse et al[42] documented no difference regarding the intestinal microbiota diversity, richness and composition between postmenopausal BC patients and controls. Overall, no significant α-diversity or phyla-level differences were recorded in the GM of patients regarding a specific tumor subtype, ERs or cancer stages[43-45]. Nonetheless, Wu et al[43] identified specific taxa for each stage and receptor status as well as a reduced intra-sample diversity and Firmicutes level in HER2+ BC compared to HER2- ones. In line with these results, Luu et al[44] reported that Blautia and C. coccoides genus within the Lachnospiraceae and Clostridiaceae families respectively were more abundant in stage II/III than in earlier BC stages, supporting a potential relationship between microbial composition and tumor progression. Interestingly, several investigations noticed an increase in bacteria associated with estrogen metabolism in fecal samples of BC patients[38,41,44,46] (Table 1).

Estrogen and oncobiome in males

PC: In PC, the carcinogenesis process is associated with chronic inflammation[4]. The breakdown of epithelial barrier and disruption of hormonal balance seem to be among the main reasons of inflammation[47].

The microbiota plays a crucial role in the stimulation of chronic inflammation condition and PC promotion[4]. In fact, the GM can impact on the pathogenesis process and subsequently the PC risk[48]. The composition of gut microbial community modulates the metabolism of dietary compounds with potentially high PC risk such as red meat, high fat and dairy products[49]. Chronic inflammation can also be maintained by microbial dysbiosis due in part to the use of antibiotics[50]. Indeed, Plottel et al[51] suggested that the estrobolome seems to be an actor in PC development. It was reported that estrogen can trigger polycyclic hydrocarbons, resulting in the generation of carcinogenic metabolites like radical cations that elicit DNA damage and subsequently carcinogenesis[52]. The analysis performed by Sfanos et al[53] showed that the fecal microbiota profiles of patients with different clinical PC states had lower α diversity than those of healthy controls. Overall, these findings support the hypothesis of a potential link between specific GM species and the risk of PC occurrence and progress[4]. In the gut, the microbiota participates in sex hormone synthesis and metabolism[2,21,54]. Indeed, the genus Bacteroides express 17 β-dehydrogenase, which can degrade testosterone into androstenedione[2,21,54].

Multiple investigations have suggested a potential relationship between GM and PC risk and development[55-57]. For instance, GM analysis on 133 United States men showed that Streptococcus and Bacteroides spp. were more important and GM folate and arginine pathways were significantly modulated in PC patients[55,57]. Golombos et al[48] assessed the GM of a group of 20 men comprising 12 with PC high-risk of PC and 8 with benign prostate hypertrophy. Findings showed that Bacteroides massiliensis are more prevalent in PC patients[48], while Matsushita et al[58] documented that Alistipes, Rikenellaceae, and Lachnospira were highly increased in men with elevated Gleason PC score in the GM of a group of 152 Japanese men. In addition, they found that the GM profile contains 18 gut bacteria that were not affected by metastasis and that may serve to predict PC with a higher Gleason score in comparison with the prostate-specific antigen test. Finally, they noticed that modulations in the GM generates PC and are not its consequence[57,58] (Table 1).

BC: Representing about 1% of all BC, male BC seems to be the rarest[59]. Actually, its incidence is still unknown among the public. Indeed, an investigation conducted into above 400 men and women showed that 34.3% of participants believed that BC is only associated with women and 61.1% were unconscious that BC can happen in men[60]. Given that several studies clearly ignore males with BC, the treatment recommendations for male patients are generally extrapolated from results of clinical trials performed on female BC patients[59,61].

In males, an excess of estrogen in the body called “Hyper-Estrogenism” can promote a higher risk of BC through stimulating the proliferation of breast epithelium[59,62]. For instance, an excess of estrogen accompanied by a reduced testosterone level can lead to gynecomastia making BC susceptible to occur[59]. In addition, the use of exogenous estrogen can be involved in excess BC risk and development[59,63]. In fact, transgender females who utilize exogenous estrogen present an elevated risk of BC occurrence in comparison with cisgender males but relatively reduced in comparison with cisgender females[64,65]. On the other hand, transgender males present lower risk of BC in comparison with cisgender females, however, higher in comparison with cisgender males[59].

The study of Ervin et al[35], sought to assess the reactivation of estrogen glucuronides by intestine microbial glucuronidase enzymes, was performed on fecal samples of 6 Female and 5 male BALB/c mice. After in vitro estrogen-processing coupled assay, the authors concluded that the active components of the estrobolome in all samples, including males, are gut microbial glucuronidase enzymes[35].

POTENTIAL OUTCOMES ON SEX-HORMONE DRIVEN CANCERSTREATMENT

The impact of bacterial therapy on the estrobolome is an emerging area of research[2]. Obviously, the modulation of the GM and metabolic profile results in an impact on estrogen levels[2]. In consequence, estrogen regulation seems to be feasible through several promising approaches, such as diet, probiotics and antibiotics, that have more benefits and afford valuable support to current estrogen-driven cancers treatments[2]. This action on the estrobolome is effective not only in women but also in men[2].

Diet

Theoretically, a dietary regimen can be pro-oncogenic or anti-oncogenic. For example, a high-fat dietary regimen like lard modified the GM in mice, by rising the levels of Lachnospiraceae, thus promoting cancer progression[46,66]. Many epidemiological associations confirmed that dietary interventions to modulate the GM are promising in cancer prevention[67].

Enteric microbes are not only the main actor in metabolizing estrogens but also the key player in synthesis of estrogen mimics or estrogen-like compounds called “Phytoestrogens”, starting from dietary sources[68]. The phytoestrogens are a kind of plant compounds analogous to 17-estradiol in structure and molecular weight[69]. In addition to structural similarities, phytoestrogens are able to bind to ERs (ERα and ERβ), but only a few of them have really estrogenic or anti-estrogenic activities[68]. These compounds include flavonoids, isoflavanoids, coumestans, lignans, stilbenes, genistein, ellagitannins, daidzein and its metabolite S-equol, and coumestrol[68,70]. A significant impact of phytoestrogens can be reached at circulating concentrations in the range of micromolar levels which require 10 to 100 mg per day of precursors given that the affinity of ERs for phytoestrogens is around 4 folds further down than 17β-estradiol[71]. Phytoestrogens possess more affinity for ERβ in comparison to ERα which could be greatly significant considering that estrogen-dependent BCs are generally associated with ERα while other physiological effects are associated with ERβ[68]. In addition, some phytoestrogens interfere with production and bioavailability of steroid hormones including E2 and testosterone[71]. In humans, estrogenic compounds can be activated through intestinal bacteria like Eubacterium limosum[71] and their biotransformation is managed by Eggerthellaspp., Adlercreutzia equolifaciens and Slackia isoflavoniconvertens[68,71]. For instance, several studies have assessed the anti-tumor effects of dietary lignan, secoisolariciresinol and secoisolariciresinol diglucoside consumption on BC development[72]. Moreover, some phytoestrogens are able to modulate inflammation, suppress the nuclear factor κB, act as androgen receptor antagonists and thus stop the effects of androgens (namely testosterone or dihydrotestosterone)[73]. Notably, the expression of androgen receptors is mostly controlled by the nuclear factor κB pathway and increased nuclear factor κB expression is observed in the majority of PC-patients[72,74].

In addition to phytoestrogens, dietary fiber is suggested to have a relationship with the GM that, as previously explained, raises β -glucuronidase activity and circulating 17β-estradiol in female postmenopausal BC patients[75]. Indeed, Zengul et al[75] found that elevated levels of soluble and total dietary fibers are correlated with decreased levels of two bacteria that increases β-glucuronidase activity namely Clostridium belonging to Erysipelotrichaceae family and Clostridium hathewayi sp. As well, they detected a positive and significant link between insoluble fiber and Bacteroides uniformis which also boost β -glucuronidase activity. This finding may be connected to the impact of Bacteroides uniformis on glycolysis pathways. Increased glycolysis is associated with elevated glucose uptake, and it is considered as a BC indicator[75].

Various investigations showed that certain dietary antioxidants may reduce the risk of a specific cancer while others suggested that they can increase it[76]. For example, high vitamin E consumption may increase the PC risk, impacting on the expression of different cytochrome P450 enzymes[76,77].

Probiotics, prebiotics and postbiotics

Probiotics, prebiotics and postbiotics interventions are auspicious to modulate both gut and tumor microbiomes. Probiotics are defined as live microorganisms that have beneficial impact on host health when consumed in adequate amounts[1,78]. A minority of commercialized probiotic formulations have been assessed for their effects on antitumor activity and some of them are even promoting tumorigenesis[79]. Besides, in intensive care units ‘patients, the use of commercial probiotics can induce bacteremia[80]. Therefore, non-selective consumption of commercially available probiotics in cancer is not advisable[11]. On the other hand, there are many epidemiological evidences that promote a protective role of probiotics in cancer and especially in in sex-hormone driven cancers[81]. Indeed, specific probiotics (e.g. lactic acid bacteria) have been recommended to repress the activity of estrogen metabolizing-microbiota and subsequently to decrease the estrogens’ deconjugation in the gut[81]. Notwithstanding, so far there is no evidence of their preventive or therapeutic effect in BC[45]. In spite of the efficiency of probiotic products, researchers have established the notion of postbiotic to refine their beneficial effects and enhance their safety[11]. Postbiotic compounds are described as microbial-derived molecules that can be relevant alternative agents to live probiotic cells[11]. About the impact on cancer, the experimental evidence is restricted, but postbiotics are expected to give advantages through specific composition and manufacturing reproducibility[11,82].

Prebiotics are defined as a non-digestible food component that improves growth and activity of beneficial microbes[1,11]. Prebiotics such as mucin, inulin and resistant starch are bright in preclinical trials and prebiotics like NCT03950635 and NCT03870607 in clinical trials[11].

The findings of Muccee et al[34] regarding the β-glucuronidase enzyme called also “estrogen reactivating protein” discovered in bacteria of normal and BC patients could allow an easier modification of this enzyme and/or its active sites at the level of probiotics in comparison to human genes. This modification could help in decreasing BC risk via impairing reactivation of this estrobolome-associated enzyme[34].

Antibiotics

Historically, exogenous estrogens were utilized as PC treatment of cis men because of their capacity to inhibit testosterone via the pituitary axis and to decrease the incidence of PC[62]. On the other hand, high estrogens’ levels may stimulate carcinogenesis. Thus, their role was decreased due to their adverse reactions and the emergence of gonadotrophin releasing hormone analogues[62]. For instance, the role of exogenous estrogens in PC is more complex since it depends on which ER isoform is activated[62]. Indeed, the activation of ERα has pro-oncogenic effects, while activation of ERβ has a tumor inhibitive role in the PC[62,83].

The interaction between GM, sex-hormone-mediated cancers’ outcomes and drug metabolism is worthy of investigation[45,84]. In fact, in two clinical settings, data supported that antibiotics impinge on the anticancer activity of cyclophosphamide and cisplatin[85,86]. In another study, β-lactams or quinolones administration, within 30 days from anthracyclines, taxanes and cyclophosphamide therapy, might be associated with decreased efficacy of neoadjuvant therapy and poor chance of recovery in BC patients[45,87]. Furthermore, mice treated with vancomycin or streptomycin, showed a worse response to neoadjuvant therapy namely trastuzumab[88]. Indeed, the administration of these antibiotics decreased the relative abundance of Clostridiales, Turicibacteraceae, Actinobacteria and Firmicutes phyla, however vancomycin also diminished Bacteroidetes and increased Verrucomicrobia and Proteobacteria in BC models[88]. Additionally, some studies demonstrated that a regular aspirin uptake decreases BC incidence[46,89].

Tamoxifen is the essential treatment for estrogen+-BC via its active metabolite, endoxifen, for over half a century[90]. This selective ERs modulator therapy presents the preferred choice even for male BC since this cancer type is almost always ER+[91]. Recent investigational studies suggested potential interactions between tamoxifen and GM. Hillege et al[90] found that after tamoxifen therapy, microbial community structure remains invariable, however, an increase in gut microbial diversity was recorded in postmenopausal ER+/HER2- BC patients[90]. In addition, Alam et al[92] suggested that GM plays a key role in tamoxifen pharmacokinetics and reported that functional differences in the GM, especially the amount of β-glucuronidase enzyme, results in an impact on drug efficacy manifested by inter-individual variability in systemic exposure to tamoxifen[92,93]. Li et al[94] studied BC in mouse model and found that tamoxifen treatment modified GM composition, promoting taxa positively related with inflammation like Prevotellaceae and Akkermansia[93,94]. Thus, these reports describing drug-microbiota interactions, or pharmacomicrobiomics, may enhance drug responses in sex-hormone-mediated cancers[93].

Transplantation of microbiota

In cancer disease, GM may be modified via fecal microbiota transplantation (FMT). This process implies the transfer of active fecal microbiota from a healthy individual into a patient’s intestinal tract to restore balance in the gut flora and improve intestinal and extraintestinal diseases[78]. Current studies have manifested a great interest in FMT for its ability to enhance outcomes in cancer therapy[78,95,96]. However, its stability and long-term effectiveness are still undetermined[97]. Clinically, some reasons, including administration route, antibiotic pre-conditioning, dietary recommendations and frequency of modulation, make this method relatively tricky[97]. Clinical trials supported that FMT treatment may provide beneficial changes in antitumor immune responses immune in both of the tumor microenvironment and the gut lamina propria[11,98]. Studies have investigated FMT as a method to regulate GM dysbiosis[21] and to promote the effectiveness of antitumor immunotherapy[78]. However, no study investigated the link between FMT and estrobolome in sex-hormone mediated cancers.

Engineering bacteria

Engineered microbes are regarded as innovative cancer drugs that have an affinity for tumor tissues creating a natural bridge between synthetic biology and cancer therapies. This tumor tropism is implemented by natural bacterial mechanisms via intravenous, intratumoral or oral delivery routes resulting in a high bacteria accumulation in the tumor tissue[11,99,100].

In preclinical models, Listeria, Escherichia, Lactococcus, Bifidobacterium, Shigella, Vibrio, Clostridium and Salmonella species were genetically modified to attenuated, inducible and auxotrophic versions and manifested antitumor efficacy with the different delivery routes[101]. Some studies are advancing towards intracellular delivery of drugs via phagocytic ingestion of bacteria and others are modulating bacteria to proceed like “intratumoral bioreactors” that continually generate and liberate payloads extracellularly[11]. Another prevalent approach was developed in order to decrease bacterial colony size and avoid systemic toxicities. Indeed, it provides engineered bacterial lysis that promotes antitumor protein generation or liberation once a predefined population density of bacteria is reached[102-105]. For instance, it was reported that, at a threshold population density, non-pathogenic E. coli and Salmonella might be programmed to lyse, releasing pro-apoptotic protein, hemolysin or chemokine, or all three, into the tumor microenvironment at desirable periodic intervals[103]. In addition, this lysis circuit design was applied to produce and liberate an antibody-fragment nanobody against CD47 that tumors are able to overexpress to inhibit dendritic cells phagocytosis[104]. This engendered a tumor-antigen-specific CD8+ T cell response that precluded metastasis and moderated the abscopal effect that degenerated regressed distal non-injected tumors[11]. Lysis circuit designs can also offer a possibility to engineer tumor-specific commensal strains, patient-specific strains, or many strains in response to each other to modulate payload release[11]. In fact, the immunomodulatory effects of host microbiota can be considered as a form of immunotherapy.

Despite extensive preclinical evidence through murine immunotherapy models, engineered bacteria as cancer therapies have not yet been translated into humans as therapeutic interventions[11].

CONCLUSION

Many human sex-hormone driven cancers were correlated with alterations on GM composition and function[106]. For instance, BC patients were characterized by the abundance of Clostridiales while CC and OC patients by heightened levels of Prevotella[106]. Hence, studying the compositional deviations of a such metabolite-secreting gland might be of great relevance. Whether specific GM is causative or curative, discerning the microbiota’s involvement in the different sex-hormone driven cancers is still largely unknown. GM is involved in the estrogen cycle, thus establishing the estrogen-GM axis[11]. The therapeutic approaches such as the use of probiotics, prebiotics, engineered bacteria and/or antimicrobial agents can be mainly conceptualized to modify GM populations as well as the β-glucuronidase enzyme activity in order to decrease the risk of estrogen-related cancers. In addition, specific gut bacterial genera are capable of metabolizing phytoestrogens transforming them into active metabolites that can protect from cancer development. The GM modulation approaches could be considered as complementary treatments. Generally, amicrobial frame of reference of sex-hormone driven cancers can enhance patient outcomes and promote a better understanding of the complex cancer-estrogen-microbial relationship.