Lipid transport in the developing bovine follicle: Messenger RNA expression increases for selective uptake receptors and decreases for endocytosis receptors

Differences in rates of steroid production and secretion will, eventually, determine the developmental rates of ovarian follicles. The major supply of cholesterol, the precursor for steroid and androgen biosynthesis, to ovarian cells is from circulating lipoproteins via membrane receptors from the low density lipoprotein receptor (LDL) superfamily. This occurs by either endocytosis, which has been described for very low density lipoprotein receptors (VLDLr), for LDL receptors (LDLr), and by the selective uptake pathway described for the scavenger receptor class B type 1 receptor (SRB1) and the recently described ovarian receptor, lipoprotein receptor-related protein 8 (LRP8). In this study, the mRNA expression of these four cholesterol receptors in bovine ovarian cells was determined at different stages of follicular development. In small antral follicles, mRNA expression of the endocytosis receptors was higher than in large antral follicles. Expression of LRP8 MRNA increased linearly with follicular size together with an increase in LDL, VLDL, and cholesterol concentrations in the follicular fluid. SRB1 mRNA expression tended to increase with follicular diameter. Because different mRNA expression patterns were found for the two types of receptor, this may imply different regulation of cholesterol supply at different stages of follicular development. Accumulation of LDL and VLDL particles in the follicular fluid of large antral follicles may enhance cholesterol availability for the intense steroidogenic activity that is essential at these stages.