Novel link between plasma bilirubin and anti-inflammatory miRNA profiles in follicular fluid of IVF patients

Maternal metabolic factors are increasingly recognized as critical preconceptional determinants of fertility outcomes. To investigate how metabolic health influences female fertility, we investigated the molecular composition of follicular fluid (FF), with a focus on microRNA (miRNA) expression. Blood and FF samples from 15 women undergoing controlled ovarian stimulation were examined in a pilot study. Clinical traits (glucometabolic markers, lipid profiles, liver function markers, inflammatory cytokines, and hormonal parameters) and oocyte outcomes were measured and recorded. Elevated plasma bilirubin levels were associated with a distinct miRNA profile in FF, characterized by an enrichment of anti-inflammatory miRNAs, including miR-146a-5p, miR-146b-5p, miR-487b-3p, and miR-21-5p. Bioinformatic analysis revealed that these miRNAs directly target key inflammatory mediators, including interleukin-6, cyclooxygenase 2, Toll-like receptor 4, interleukin 1 receptor-associated kinase 1, and nuclear factor kappa β1, suggesting a regulatory role in intrafollicular inflammation. Furthermore, patients with a fertilization rate of ≤50% exhibited higher transcript levels of miRNAs associated with elevated plasma bilirubin. Our findings provide a novel perspective on the growing body of evidence supporting bilirubin's regulatory properties, including antioxidative and anti-inflammatory effects, and highlight the relationship between plasma bilirubin and FF miRNA expression. The observed associations between bilirubin levels, follicular fluid miRNA composition, and oocyte quality underscore the critical influence of metabolic factors on reproductive outcomes. This exploratory work provides a foundation for further studies to investigate the functional role of plasma bilirubin in follicular physiology and its potential as a biomarker to optimize fertility treatments.<b>NEW & NOTEWORTHY</b> Maternal plasma bilirubin levels are associated with a distinct anti-inflammatory miRNA signature in follicular fluid that targets key inflammatory mediators. This signature is diminished in patients with higher fertilization rates, suggesting a novel role for plasma bilirubin in potentially modulating the ovarian follicular environment.