Down-regulation of genes encoding steroidogenic enzymes and hormone receptors in late preovulatory follicles of the cow coincides with an accumulation of intrafollicular steroids

The transformation of the dominant follicle into a functional corpus luteum is accompanied by a profound molecular and morphological reorganization of somatic cell layers. Several studies have focused on gene expression during early processes of follicular differentiation as it relates to recruitment and selection of dominant follicles. However, little information exists on changes of gene expression profiles in late preovulatory follicles. This lack of information is addressed here to elucidate molecular mechanisms behind the LH-induced transition from the large dominant estrogen-active to the preovulatory follicle, an intermediate stage toward full luteinization. Transcripts encoding key molecules for the biosynthesis of steroid hormones and prostaglandins, as well as receptors for gonadotropic and growth hormones (Star, Cyp11a1, Hsd3b, Cyp17, Cyp19, Ptgs2, Fshr, Lhr, and Ghr), were quantified by real-time polymerase chain reaction (PCR) in the granulosa and theca of large dominant and late preovulatory follicles. The steroid hormones progesterone (P4) and estradiol-17β (E2) were monitored to distinguish estrogen-active and estrogen-inactive follicles. We found that (1) independent of the follicular stage, the gene expression profile was very different in granulosa and theca; (2) the abundance of several key transcripts was lower in estrogen-inactive, compared with estrogen-active, dominant follicles; (3) in the granulosa of late preovulatory follicles, transcripts encoding steroidogenic enzymes and hormone receptors were largely down-regulated, whereas (4) progesterone and E2 were found at high concentrations in the follicular fluid. Collectively, our data show that late preovulatory follicles have a transient and unique gene expression profile and are clearly different from both the preceding and subsequent (follicular and luteal, respectively) stages.