Decreased gremlin 1 expression in women with body mass index ≥35 kg/m2 is mediated by interleukin 10 and interleukin 1β in the follicular fluid

Intracytoplasmic sperm injection (ICSI) is the most common form of assisted reproductive technology (ART) used to help infertile couples conceive a biological child (1). However, suboptimal outcomes in obese women have been recognized. Retrospective studies report lower pregnancy rates and higher miscarriage risks in overweight and obese women compared to women with normal body mass index (BMI) undergoing ART (2–5). Additionally, a recent meta-analysis found the probability of live birth after ART was lower in obese women with BMI ≥30 kg/m2 compared to women with normal BMI (6). The etiology for these outcomes is likely multifactorial; however, follicular impairment due to obesity related chronic inflammation is a mechanism gaining recognition (7, 8). Obesity associated intracellular lipid accumulation in adipose tissue can result in oxidative stress and increased systemic inflammation (7). The oxidative stress is the result of adipocyte hypertrophy causing increase in cytokine production. Similar to adipose tissue, evidence suggests that lipid accumulation can also occur in the ovary (9). Murine studies have demonstrated lipid accumulation and induction of stress markers in the cumulus oocyte complex of mice fed a high-fat diet (10). Additionally, evaluation of inflammatory markers in the follicular fluid of obese women has found increased levels of C-Reactive protein and other pro-inflammatory cytokines, suggesting obesity related follicular inflammation (11, 12). Elevated free fatty acid levels in the follicular fluid results in poor morphology of the cumulus oocyte complex (13). Therefore, derangements to cumulus cell function and oocyte metabolism may arise from the culmination of lipid build up in the follicular fluid, resulting in lipotoxicity and inflammation (7, 8). Such changes to the humoral response can lead to disturbances in folliculogenesis and fertility (14, 15). As the follicular fluid provides the microenvironment for the oocyte, cytokines may negatively influence the developing oocyte and potentially affect embryo development; however, how the inflammation in the follicular fluid of obese women affects signaling within the cumulus oocyte complex has yet to be explored. Oocyte development and maturation occurs in a highly coordinated process involving communication through gap junctions and paracrine signaling between the maturing oocyte and the surrounding cumulus cells (16, 17). Thus, cumulus cells may help identify biomarkers related to oocyte competence (18, 19). Oocyte secreted growth differentiation factor 9 (GDF9) and bone morphogenic protein 15 (BMP15) help regulate cumulus cell differentiation and ovarian folliculogenesis (16, 20, 21). Absence of GDF9 or mutations in BMP15 results in sterility, proving the importance of these signaling molecules for normal folliculogenesis (22, 23). As cumulus cells share an intimate relationship with the oocyte and are discarded following oocyte retrievals, they can potentially serve as an ideal surrogate marker for embryo quality. Cumulus cell expression of genes downstream of GDF9, including gremlin 1 (GREM1), hyaluronan synthase 2 (HAS2), and prostaglandin-endoperoxide synthase 2 (PTGS2 or COX2) were found to be over-expressed in cumulus cells surrounding oocytes that subsequently developed into high grade embryos compared to low grade embryos following ICSI (18). Additionally, versican (VCAN) and PTGS2 expression were higher in cumulus cells surrounding oocytes that resulted in live birth (24). Several other studies have reported similar associations between these markers and positive ICSI outcomes (25–28). With limited information available on obesity related inflammatory changes in women undergoing controlled ovarian stimulation, we sought to explore how BMI affects the cytokine milieu of the follicular fluid. Additionally, GREM1, HAS2, PTGS2, and VCAN, as surrogate markers of positive ICSI outcomes, were measured to identify how obesity induced inflammation impacts cumulus cells.