Activation of the canonical WNT signaling pathway promotes ovarian surface epithelial proliferation without inducing ß-catenin/Tcf-mediated reporter expression.

BACKGROUND: In response to activation of the canonical WNT signaling pathway, ß-catenin cooperates with Lef/Tcf (lymphoid enhancer factor/T-cell factor) transcription factors to drive expression of Wnt target genes. The canonical WNT signaling pathway is involved in development, wound repair, and tumorigenesis. Studies examining the involvement of the canonical WNT signaling pathway in the development of ovarian surface epithelium (OSE) and ovarian carcinogenesis, however, have recently begun to emerge. In this study, we investigated the modulation of ß-catenin and ß-catenin/Tcf-signaling activity within the OSE using responsive transgenic mice and examined the response of primary OSE cells and ovarian cancer cell lines to activation of the canonical WNT signaling pathway. RESULTS: ß-catenin was localized to the lateral membrane of the ovarian epithelium. Stimulation of primary OSE cells in vitro with LiCl or Wnt3a led to GSK-3ß inhibition and stabilization of ß-catenin but failed to induce ß-catenin/Tcf-mediated lacZ expression. Furthermore, E-cadherin expression was downregulated and the proliferative potency of OSE cells increased. Of four ovarian cancers cell lines screened, only the HEY cell line demonstrated induction of luciferase reporter upon canonical WNT stimulation. CONCLUSIONS: These observations suggest that in ovarian adenocarcinoma, dysregulated WNT signaling may not always be indicative of ß-catenin/Tcf-mediated transcriptional activity.

ß-Catenin/Tcf signaling in murine oocytes identifies nonovulatory follicles.

WNTS are secreted glycoprotein molecules that signal through one of three signaling pathways. The best-characterized pathway involves stabilization of the multifunctional protein ß-catenin, which in concert with members of the T-cell factor (Tcf) family activates specific gene transcription. We have examined putative Wnt/ß-catenin in the murine ovary using transgenic mice harboring a reporter construct that activates ß-galactosidase (lacZ) expression in response to ß-catenin/Tcf binding (TopGal mice). Primordial and primary follicles did not stain for lacZ, and the proportion of ß-catenin/Tcf signaling oocytes was lower than that of nonsignaling oocytes throughout estrous cycle. ß-Catenin/Tcf signaling oocytes were observed in follicles from the secondary stage of development and their proportion increased with follicular maturation (secondary follicles, 20%; early antral and antral follicles, 70%). In contrast, the majority (>90%) of ovulated oocytes did not stain for lacZ. As the oocyte possesses components for WNT signal transduction, our data suggest that ß-catenin/Tcf signaling is involved in the development of follicular ovulatory capability and identifies nonovulatory follicles.

ß-catenin/Tcf-signaling appears to establish the murine ovarian surface epithelium (OSE) and remains active in selected postnatal OSE cells.

BACKGROUND: Wnts are a family of secreted signaling molecules involved in a number of developmental processes including the establishment of cell fate, polarity and proliferation. Recent studies also implicate wnts in epithelial adult stem cell maintenance, renewal and differentiation. Wnts transduce their signal through one of three signaling pathways. The best studied, the wnt/ß-catenin pathway, leads to an increase in intracellular ß-catenin which acts as a co-transcription factor with members of the Tcf/Lef family. A number of wnts are expressed in the ovary, specifically in the membrana granulosa and ovarian surface epithelium (OSE). We investigated the spatio-temporal pattern of ß-catenin/Tcf expression in the OSE using responsive transgenic (TopGal) mice. RESULTS: The generated ß-galactosidase response (lacZ+) identified the cell population that overlies the medio-lateral surface of the indifferent gonad at embryonic day (E) 11.5. From E12.5 onwards, lacZ expression disappeared in cells covering the testis but remained with ovary development. LacZ+ OSE cells were present throughout embryonic and postnatal ovarian development but demonstrated an age-dependent decrease to a small proportion when animals were weaned and remained at this proportion with aging. Flow cytometric (FACS) and ovarian section analyses showed lacZ+ cells constitute approximately 20% of OSE in postnatal (day 1) mice which fell to 8% in 5 day-old animals while in prepubertal and adult mice this accounted for only 0.2% of OSE. Apoptosis was undetected in OSE of neonates and ß-catenin/Tcf-signaling cells were proliferative in neonatal mice indicating that neither cell death nor proliferation failure was responsible for the proportion alteration. It appeared that lacZ+ cells give rise to lacZ- cells and this was confirmed in cell cultures. The DNA-binding dye DyeCycle Violet was used to set up the side population (SP) assay aimed at identifying subpopulations of OSE cells with chemoresistance phenotype associated with ABCG2 transporter activity. FACS analysis revealed lacZ+ cells exhibit cytoprotective mechanisms as indicated by enrichment within the SP. CONCLUSIONS: The study raises the possibility that wnt/ß-catenin-signaling cells constitute a progenitor cell population and could underlie the pronounced histopathology observed for human ovarian cancer.