Progesterone receptor-mediated effects on apoptosis in periovulatory granulosa cells

Abstract

The most common fate of developing ovarian follicles is demise due to a process known as atresia. Regulation of atresia is dependent on the developmental stage of the follicles, resulting in a continuous reduction of the number of follicles as they differentiate and grow towards ovulation. The mechanism behind atresia of growing follicles is apoptosis of the granulosa cells. This thesis focuses on progesterone receptor (PR)-mediated regulation of granulosa cell apoptosis during the final phase of follicular development, the periovulatory interval.

By using two PR antagonists (RU 486 and the more specific Org 31710) we have shown that PR stimulation is important for the survival of periovulatory rat and human granulosa cells in vitro. PR regulated gene expression in rat periovulatory granulosa was characterised by microarray analysis, comparing the expression profiles after incubation in vitro with or without the addition of 10 µM Org 31710. Close to 100 genes were found to be transcriptionally regulated in the presence of Org 31710. This included downregulation of several genes involved in cholesterol synthesis, and a decreased rate of cholesterol synthesis was verified by measuring the incorporation of 14C-acetate into cholesterol, cholesterol ester and progesterone. Based on this we investigated the granulosa cell dependence on cholesterol synthesis and in particular the branch-point reactions supplying cells with prenylation substrates for post-translational lipid modification of proteins. Blocking the cholesterol synthesis with statins increased apoptosis, as did inhibitors of prenyl transferases. The increase in apoptosis after treatment with statins or PR antagonists was partially reversed by the addition of substrates for prenylation.

In conclusion, PR stimulation is important for the survival of periovulatory granulosa cells in both rats and humans. PR stimulation regulates the transcription of several groups of genes including cholesterol synthesis. The cholesterol synthesis also provides the cells with substrates for protein prenylation, which may be one of the factors regulating granulosa cell survival in periovulatory follicles.