Intersection of Polycystic Ovary Syndrome and the Gut Microbiome

Maryan G. Rizk; Varykina G. Thackray


J Endo Soc. 2021;5(2) 

In This Article

Interplay Between Gut Dysbiosis and Hyperandrogenism in PCOS

Gut Dysbiosis as a Driver for HA in PCOS

While the etiology of PCOS remains unknown, there are 2 distinct but related hypotheses that could connect the development of hyperandrogenic PCOS phenotypes to changes in the gut microbiome. As discussed previously, 1 hypothesis proposed by Tremellen and Pearce is that gut dysbiosis, influenced specifically by a high-fat diet (HFD) and a high-carbohydrate diet, leads to inflammation through disruption of the gut barrier, which in turn leads to IR, HA, and ovarian dysfunction.[17] This hypothesis places a strong emphasis on diet and gut dysbiosis as driving factors from which pathogenic features of PCOS, such as HA, emerge. However, it also suggests that obesity and IR are prerequisites for PCOS, although it is well documented that not all women diagnosed with PCOS are obese or insulin resistant.[52,53] In addition, this hypothesis does not take into account that the incidence of PCOS is relatively similar in countries world wide despite differences in diet[2] and that many animal models of PCOS have been recreated independent of diet.[54–58]

In order to begin to mechanistically understand the role of the gut microbiome in the development of PCOS, fecal microbiome transplant (FMT) experiments using stool from women with PCOS are informative. Qi et al performed an FMT from women with PCOS that are normal weight into antibiotic-treated mice and observed reproductive and metabolic changes in the recipient mice.[23] Significant increases in testosterone and luteinizing hormone (LH) levels were observed in mice receiving FMT from women with PCOS (trans-PCOS) compared with mice receiving FMT from healthy women (trans-Control).[23] In addition to HA, trans-PCOS mice had disrupted estrous cyclicity, decreased ovulation as indicated by a decreased number of corpora lutea in the ovaries, the appearance of ovarian cysts, and a decrease in fertility.[23] Additionally, trans-PCOS mice developed IR as assessed by glucose and insulin tolerance tests and the homeostatic model assessment for IR calculated based on fasting glucose and insulin levels.[23] The researchers also transplanted one of the bacteria, Bacteroides vulgatus, that was positively associated with PCOS into antibiotic-treated mice.[23] Similar reproductive and metabolic phenotypes were observed with B. vulgatus compared with trans-PCOS.[23] This potentially ground-breaking study suggests that transplantation of a dysbiotic gut microbiome from women with PCOS or B. vulgatus is sufficient to induce a PCOS-like phenotype in mice and supports the idea that changes in the gut microbiome may play a causal role in this disorder (Figure 2).

Figure 2.

Relationship between the gut microbiome and polycystic ovary syndrome (PCOS). Accumulating evidence, in human studies and rodent models, indicates that there is an association between dysbiosis of the gut microbiome and PCOS. A–B: Gut microbes metabolize substrates that enter the gut, from the diet and the host, and produce metabolites that may act directly on the intestines or enter systemic circulation and influence various host tissues whose function is altered in PCOS, such as ovary, skeletal muscle, liver, and adipose tissue. Gut bacterial metabolites reported to be altered in PCOS include secondary bile acids, SCFAs, and TMA. For instance, bile acids bind to receptors, including FXR, in various tissues and activate intracellular signaling. C: Metabolic tissues, including skeletal muscle, liver, and adipocytes, produce metabolites (such as conjugated primary and secondary bile acids, TMAO, lactate, and glucose) that enter the gut and may alter the composition of gut bacteria by serving as substrates, thus providing selective advantages to certain strains of bacteria over others. D: The host reproductive axis regulates sex steroid hormone production. In PCOS, elevated levels of androgens may alter the composition of the gut microbial community. E: Crosstalk between host metabolic tissues and the reproductive axis also occurs independently of the gut microbiome and may be a driver of the pathology and development of PCOS. Further studies are needed to decipher how the interactions outlined in this figure occur mechanistically. Abbreviations: FSH, follicle-stimulating hormone; FXR, farnesoid X receptor; GnRH, gonadotropin-releasing hormone; IR, insulin resistance; LH, luteinizing hormone; SCFAs, short-chain fatty acids; TMA, trimethylamine; TMAO, trimethylamine N-oxide.

Caveats for this study include the use of antibiotics to deplete the gut microbiome instead of the use of germ-free mice. Antibiotics have been shown to affect metabolism in mice,[59] and no data were provided to demonstrate that the gut microbiome was actually depleted by antibiotic treatment prior to the FMT or that the FMT resulted in the establishment of gut microbes after the FMT. In addition, no data were provided on whether the mice gained weight or not; thus, it is unclear if a dysbiotic gut microbiome from women with PCOS and normal weight could induce obesity in mice. Future studies where the gut microbiome of recipient mice is sampled and sequenced pre- and post-FMT or bacterial transplantation will provide more comprehensive information about the role of the gut microbiome in the emergence of PCOS-like symptoms in mice. Additionally, using germ-free mice as recipients will clarify the direct effects of gut microbiota on the development of PCOS and whether obesity and IR precede HA.[60] Finally, FMT from women with PCOS that are normal weight and obese as well as non-IR versus IR will help parse out the role of different gut microbiota on the development of the different metabolic phenotypes associated with PCOS.

HA as a Driver for Gut Dysbiosis in PCOS

A second hypothesis is that HA leads to gut dysbiosis in association with the development of PCOS (Figure 1). Potential mechanisms through which testosterone could alter the gut microbiome include a direct effect as a substrate for gut microbial enzymes and an indirect effect via activation of host androgen receptors or modulation of the immune system (reviewed previously[61,62]). Although human studies cannot be used to determine causation of gut dysbiosis by HA, it is notable that multiple studies reported correlations between HA and changes in the gut microbiome. Both alpha diversity[22,28,37] and beta diversity[28] were associated with HA, indicating that higher testosterone levels are linked with changes in the overall composition of the gut microbial community. Within the phylum Actinobacteria, the genus Collinsella was positively correlated with testosterone levels in 2 studies[25,26] (Table 5). Within the phylum Bacteroidetes, the genus Bacteroides was positively correlated with testosterone levels in 4 studies.[20,22,24,25] Furthermore, the genus Prevotella was negatively correlated with the levels of testosterone in 1 study[24] and positively correlated with testosterone in 3 studies.[25–27] Within the phylum Proteobacteria, Enterobacteriaceae were positively correlated with testosterone[20–22] (Table 5).

Evidence from rodent models also indicates that HA may drive dysbiosis of the gut microbiome (Figure 1). Two early studies used letrozole, a nonsteroidal inhibitor of aromatase, to induce HA and PCOS-like symptoms in mice and rats. These studies showed that dysbiosis of the gut microbiome occurred as a consequence of treatment with letrozole, most likely due to the resulting HA rather than a direct effect of letrozole.[18,30] In addition to reproductive and metabolic phenotypes similar to women with PCOS,[58,63] pubertal mice treated with letrozole had diet-independent reductions in alpha diversity, changes in beta diversity of the gut microbiome, and changes in the RA of specific bacteria[18] (Table 4). Bifidobacteriaceae was negatively correlated with testosterone while Bacteroides, Streptococcus,[49] and Prevotella were positively correlated with testosterone,[30] consistent with the human studies (Table 5). Two rodent studies showed that a proteobacteria called Desulfovibrio was negatively correlated with testosterone,[32,49] while this genus was not observed to be correlated with testosterone in humans (Table 5). Interestingly, these letrozole-induced changes in the gut microbiome appear to be activational rather than organizational changes; removal of letrozole treatment after the establishment of gut dysbiosis restored gut bacterial diversity.[29] Exogenous treatment of rats with DHT, which cannot be converted to estradiol, led to significant reductions in alpha diversity and changes in beta diversity compared with placebo-treated rats.[32,50] Hyperandrogenism driven by dehydroepiandrosterone (DHEA) treatment altered beta diversity of the gut microbiome in mice when combined with an HFD.[64] Taken together, these results suggest a strong role for testosterone as a modulator of the gut microbiome, although it is unclear whether testosterone exerts an effect on gut microbes directly and/or indirectly through actions in androgen target tissues.

Caveats and Future Directions

The diagnosis of HA is obtained by measuring hirsutism with the Ferriman-Gallway score and/or biochemically by measuring serum testosterone levels. The "gold standard" method of measuring testosterone is liquid chromatography-mass spectrometry, although methods using radioimmunoassays can provide equivalent results, while other methods such as enzyme-linked immunosorbent assay (ELISA) can overestimate the amount of testosterone in the sample.[10,65] In the studies reviewed herein, different methods of measuring testosterone were used, including ELISA-based techniques, radioimmunoassays, and liquid chromatography-mass spectrometry. Given that levels of steroid-hormone binding globulin (SHBG) are decreased in women with PCOS,[66] it may also be relevant to examine the relationship between changes in the gut microbiome and the free androgen index (ratio of total testosterone to SHBG).

While the 2 hypotheses about the interaction between HA and gut dysbiosis were discussed separately, they are likely interconnected (Figure 2). In 2017, vom Steeg and Klein reviewed evidence in support of the crosstalk between sex steroid hormones and the microbiota of the host.[62] However, much remains to be discovered about how sex steroid hormones interact with gut bacteria, especially with regards to how testosterone impacts the gut microbiome in females compared with males. To accomplish this, it will be important to employ metagenomic sequencing to identify gut microbial species/strains and microbial genes that are altered in response to increased levels of testosterone in women and PCOS rodent models. Moreover, transplantation of microbiota from women with PCOS or PCOS-like rodent models into germ-free mice will be crucial to comprehend the temporal changes in testosterone levels relative to other symptoms of PCOS as a result of exposure to PCOS-related microbiota. Finally, pharmacological inhibition of the androgen receptor using antiandrogens such as cyproterone or spironolactone[10] will help elucidate the role of androgen signaling in driving gut dysbiosis in women with PCOS or rodent models. Complementary studies using androgen receptor knockdown within specific host tissues will identify which sites of androgen action are required for gut dysbiosis.