Regulation of FSH target genes in ovarian granulosa cells requires input from the WNT signaling pathway

Abstract Text: Ovarian follicles develop in response to endocrine regulation by the hypothalamic-pituitary-gonadal axis, and several intraovarian factors.  Pituitary derived follicle-stimulating hormone (FSH) and luteinizing hormone up-regulate a specific subset of genes in ovarian follicles necessary for follicle maturation and steroid production.  Follicle-stimulating hormone regulation of aromatase and consequent production of estradiol relies on input from the transcriptional co-factor, beta-catenin.  Beta-catenin, which was initially identified as a cell adhesion molecule, is also a key effector of canonical wingless-type mouse mammary tumor virus integration site family (WNT) signaling by interacting with TCF transcription factors to activate gene expression.   Contributions of WNT signaling to ovarian development were first demonstrated in Wnt4 null mice which presented with sex reversed ovaries and a paucity of oocytes at birth. Subsequent studies in postnatal rodent ovaries identified several WNT ligands and WNT downstream signaling components expressed at distinct stages of folliculogenesis and luteinization.   Among the WNT family of signaling molecules found in the adult rodent ovary, Wnt4, Fz-1, and Fz-4 appear to be hormonally regulated.   Similarly, data from our lab has shown WNT2 mRNA expression to be increased in large estrogen active bovine follicles which are likely under influence of elevated endogenous FSH. In primary cultures of granulosa cells simultaneous stimulation of WNT and FSH pathways results in an unexpected reduction in the ability of FSH to stimulate aromatase, P450 side chain cleavage, steroidogenic acute regulatory protein and ovarian differentiation factor transcripts.  The stimulatory effect of 100 ng/mL of FSH on media concentrations of estradiol and progesterone was also reduced in a dose dependent fashion with increasing doses of WNT. These data suggest a mechanism whereby FSH regulates WNT ligands to set up a negative feedback loop through Axin2 in an effort to ensure beta-catenin remains controlled so that TCF responsive genes are not overexpressed. Additionally, in cultured primary bovine granulosa cells, beta-catenin and protein kinase B (AKT) are shown to be directly up-regulated by FSH. Utilization of AKT pathway activators and inhibitors unveiled a requirement for AKT activity in FSH mediated beta-catenin accumulation.   Collectively, recent data indicate that beta-catenin regulated by AKT is a fundamental component of FSH-induced estrogen production.

Keywords: follicle, estradiol, beta-catenin