Wnt's Role in Folliculogenesis

Ovarian steroid production is an important contributor to numerous physiological functions including normal follicle and oocyte maturation, ovulation of a viable oocyte and fertility. Appreciation of the complex mechanisms that affect steroidogenesis is critical to identifying compromising signals that may decrease reproductive efficiency. Follicle maturation and consequent steroidogenesis requires coordinated actions from the pituitary gonadotropins, follicle-stimulating hormone and luteinizing hormone and local ovarian signaling molecules. Beta-catenin (CTNNB1), the lynchpin molecule of the canonical wingless-type mouse mammary tumor virus integration site (WNT) signaling pathway is required for maximal gonadotropin stimulation of steroid production from ovarian granulosa and luteal cells. WNTs are locally secreted glycoproteins known to participate in a diverse range of cellular processes including ovarian development and folliculogenesis. In cultured bovine granulosa cells, WNT2 and AKT mRNAs and CTNNB1 protein increase after FSH stimulation. Likewise, CTNNB1 protein abundance is greater in large antral follicles with high intra-follicular estradiol concentrations suggesting the hormonal milieu responsible for increased estradiol content modulates CTNNB1 accumulation. Additionally, concurrent stimulation of FSH and canonical WNT3A in primary granulosa cells results in marked reduction of key steroidogenic enzymes and ovarian differentiation factors. It is likely that FSH regulation of WNT signaling establishes a negative feedback loop to ensure CTNNB1 remains controlled. To explore the mechanism resulting in the inhibitory effect subsequent studies utilizing AKT pathway activators and inhibitors unveiled a requirement for AKT activity in FSH mediated beta-catenin accumulation. Granulosa cells treated with FSH, IGF-1 and IGF-1+FSH had increased CTNNB1 protein accumulation compared to controls. Similarly, estradiol medium concentrations increased in treated cells compared to non-treated controls. While co-treatment of FSH and IGF-1 with the AKT inhibitor LY294002 reduced CTNNB1 and estradiol production. Subsequent studies evaluated whether FSH regulation of CTNNB1 occurs through a specific phosphorylation event. In bovine granulosa cells phosphorylation of beta-catenin at Ser⁵⁵² was demonstrated in FSH treated cells, while IGF-1 treatment did not phosphorylate CTNNB1 Ser⁵⁵². Therefore, in cattle phosphorylation on CTNNB1 Ser⁵⁵² is a protein kinase A (PKA) dependent, protein kinase B (AKT) independent event. Collectively, these data suggest that beta-catenin regulated by AKT is a fundamental component of FSH-induced estrogen production. However, AKT’s role in estradiol synthesis does not appear to be through phosphorylation of CTNNB1 Ser⁵⁵². The complex interplay between the pituitary gonadotropins and ovarian WNT/CTNNB1 signaling is important for follicle maturation and may be a necessary inhibitory pathway to regulate CTNNB1 target genes.