Ovarian follicular steroidogenic acute regulatory protein, low-density lipoprotein receptor, and cytochrome P450 side-chain cleavage messenger ribonucleic acids in cattle undergoing superovulation

Abstract

Ovarian hyperstimulation with eCG in cattle results in increased plasma estradiol and progesterone concentrations, whereas hyperstimulation with FSH increases only estradiol concentrations. This study tested the hypothesis that eCG, compared to FSH, increases mRNA abundance for steroidogenic acute regulatory protein (STAR), low-density lipoprotein receptor (LDL-R), and/or cytochrome P450 cholesterol side-chain cleavage (P450(scc)), the main elements of the progesterone biosynthetic pathway. Heifers were stimulated with eCG (n = 10) or commercial FSH (n = 10), and ovaries were removed by colpotomy the day before and at 12 and 24 h after luteolysis was induced with prostaglandin (PG) F(2α). RNA was extracted from individual follicles, and relative abundance of STAR, LDL-R, and P450(scc) mRNA was assessed by slot blots. In ovaries of abattoir origin, StAR mRNA was detected in all follicles and was present in the theca but not the granulosa cell layer, as shown by Northern blotting. Levels of StAR mRNA increased 2-fold (p < 0.05) after PGF(2α) injection in small (< 6 mm) follicles from eCG-treated but not from FSH-treated animals. After PGF(2α) injection, StAR mRNA levels were 2- to 3- fold higher (p < 0.01) in large (> 9 mm) and medium (6-9 mm) follicles in eCG- compared with FSH-treated heifers. In contrast, P450(scc) and LDL-R mRNA levels did not consistently differ according to treatments. We show that StAR is expressed in the theca cells of bovine follicles and that stimulation with eCG increases follicular accumulation of StAR mRNA in comparison to stimulation with FSH.