β-hydroxybutyrate induces endoplasmic reticulum stress in bovine cumulus cells during oocyte maturation in vitro

Ketosis is a significant metabolic disorder caused by negative energy balance (NEB). β-hydroxybutyrate (BHBA) is the most abundant circulating ketone body and is known to accumulate within follicular fluid. Although NEB and metabolic stress conditions have been associated with reduced fertility in cows, the effect of increased BHBA on the viability and functionality of bovine cumulus-oocyte complexes (COC) is poorly understood. The aim of this study was to determine the effects of BHBA on expansion, oxidative status, and endoplasmic reticulum (ER) stress in cumulus cells, oocyte nuclear maturation, and <i>in vitro</i> embryo production (IVP) in cattle. Cumulus-oocyte complexes were treated with 0, 2, or 4 mM of BHBA for 6, 12, 18, or 24 h during oocyte <i>in vitro</i> maturation (IVM.) Cumulus expansion and mRNA levels of genes related to cumulus expansion, oxidative status, ER stress, and autophagy were evaluated. The oocytes were fixed for nuclear maturation analysis at 24 h of IVM. In addition, after IVM with BHBA for 24 hours, cleavage and blastocyst rates were examined. No difference was observed in the rates of oocytes reaching metaphase II, cleavage, and blastocysts. Furthermore, the oxidative status and relative abundance of mRNA for genes associated with antioxidant enzymes and autophagy were not altered by BHBA in cumulus cells. However, BHBA altered the total area of the COC and increased the mRNA levels of genes associated with ER. In conclusion, BHBA affects the expansion and induces ER stress in bovine cumulus cells during IVM without compromising oocyte nuclear maturation, oxidative status, cleavage, and blastocyst rates.