151. LIPOTOXICITY MEDIATED ENDOPLASMIC RETICULUM STRESS, MITOCHONDRIAL DYSFUNCTION AND APOPTOSIS CONTRIBUTE IMPAIRED OOCYTE QUALITY IN RESPONSE TO OBESITY

Abstract

In obesity, accumulation of lipid in non-adipose tissues, a process termed lipotoxicity, is associated with endoplasmic reticulum (ER) stress, mitochondrial dysfunction and ultimately apoptosis . We have previously shown that diet-induced obesity in mice causes impaired oocyte developmental competence, but whether this is due to activation of lipotoxicity pathways in the ovary is not known. The present study examined the hypothesis that diet-induced lipid accumulation in the cumulus oocyte complex (COC) disrupts ER homeostasis and mitochondrial membrane potential which leads to apoptosis. COCs and mural granulosa cells were collected from ovaries of adult mice fed a high fat (HFD) or control diet for 4 weeks. ER homeostasis was assessed by measuring expression of known ER stress marker genes, GRP78, ATF4 and CHOP. COCs from mice fed HFD showed significantly increased expression of GRP78 and ATF4. There was a similar trend towards increased expression in granulosa cells. Mitochondrial function was assessed by measuring membrane potential using the dual emission probe JC-1. In COCs from mice fed HFD there were reduced numbers of active mitochondria but instead large aggregated clusters of inactive mitochondria. Apoptosis in granulosa cells was determined by DNA laddering assay which showed significantly increased DNA fragmentation in cells from mice fed HFD. Apoptosis was also assessed by TUNEL staining of paraffin embedded ovaries from identical treatment groups. Ovaries from HFD mice appeared to have increased TUNEL positivity in both granulosa and cumulus cells. Our results demonstrate that the ER stress, mitochondrial dysfunction and apoptosis are markedly increased in granulosa cells and COCs from mice fed HFD, suggesting that lipotoxicity contributes to the impaired oocyte quality and reduced fertility observed in response to obesity.