A Bacteroides fragilis–indole-3-acetic acid–AhR axis aggravates PCOS-like ovarian dysfunction under metabolic stress

Abstract Background Polycystic ovary syndrome (PCOS) is a heterogeneous reproductive and metabolic disorder. Whether gut microbiota-associated metabolites contribute to ovarian dysfunction under metabolically adverse conditions remains unclear. We investigated microbiota-associated metabolic alterations in women with PCOS and overweight/obesity and explored the functional relevance of indole-3-acetic acid (IAA). Methods Women with PCOS and overweight/obesity and non-PCOS infertility controls were enrolled for clinical microbiome and metabolomic profiling. The association between microbial features and IAA metabolism was investigated using integrated multi-omics analysis. The IAA-producing capacity of Bacteroides fragilis was examined in vitro. The effects of B. fragilis and IAA were further evaluated in mice under basal and high-fat diet (HFD)-induced metabolic stress conditions, and IAA-associated AHR signaling responses were examined in KGN granulosa cells with or without palmitic acid (PA)-induced lipotoxic stress. Results Exploratory clinical profiling identified altered gut microbial and indole-related metabolic features in women with PCOS and overweight/obesity, including increased B. fragilis abundance and IAA levels, which were positively correlated. IAA levels were also increased in serum and follicular fluid. The B. fragilis strain used in this study produced IAA in vitro. In mice, B. fragilis or IAA exposure alone did not produce detectable overt ovarian abnormalities under basal dietary conditions, whereas selected ovarian, endocrine, and estrous-cycle abnormalities were observed when either intervention was combined with HFD-induced metabolic stress. In KGN cells, IAA reduced proliferation and altered G1/S cell-cycle progression, accompanied by increased AHR expression and nuclear translocation. AHR inhibition or knockdown partially attenuated PA + IAA-associated cell-cycle alterations. PA-induced lipotoxic stress further enhanced IAA-associated cellular responses. Conclusions These findings support a context-dependent relationship among microbiota-associated IAA signaling, granulosa-cell dysfunction, and selected PCOS-like abnormalities under metabolic stress, with AHR contributing to the cellular response. The clinical microbiota–metabolite associations were identified in women with PCOS and overweight/obesity and require validation in lean PCOS populations and broader PCOS phenotypes.