Wnt signaling in the ovary: Identification and compartmentalized expression of wnt-2, wnt-2b, and frizzled-4 mRNAs

Ovarian cadherins, in addition to acting as structural (adhesion) molecules, also function as modulators of gene activity. The dual role of beta-catenin as an intracellular component of the cadherin adhesion complex and as a transcription factor provides a possible explanation for these cadherin effects. Because the transcriptional activity of beta-catenin is dependent on activation by the wnt signaling cascade, we examined whether components of this cascade are expressed in the rat ovary. Using RT-PCR with degenerate primers on RNA from ovaries of hormone-stimulated immature rats, we identified transcripts for wnt-2 and wnt-2b. RT-PCR and in situ hybridization (ISH) demonstrated that granulosa cells express wnt-2 mRNA. Because the sequence for rat wnt-2b has not been reported, we obtained additional sequence by screening a rat ovarian cDNA library. RT-PCR analysis, using primers designed from this wnt-2b cDNA sequence, failed to detect transcripts in the ovarian follicular compartment (granulosa and oocyte). ISH revealed that the ovarian surface epithelium expresses wnt-2b mRNA. Using a similar degenerate RT-PCR approach, we detected expression of a putative wnt receptor, frizzled-4 (fzd-4), and a cytoplasmic component of the wnt signaling cascade, disheveled-2 (dsh-2), in the rat ovary. Further analyses using both RT-PCR and ISH indicated that granulosa cells express fzd-4 mRNA. The expression of wnt-2b transcripts in rat ovarian surface epithelium prompted us to examine whether the homologous gene is expressed in human ovarian cancer cell lines. RT-PCR, using degenerate and specific primers for wnts, on RNA from five ovarian cancer cell lines confirmed the expression of transcripts for wnt-2b. Two additional wnt transcripts (wnt-5a and wnt-11) were detected in the cancer cell lines and in the rat ovary. These results demonstrate that transcripts corresponding to components of the wnt signaling cascade are expressed in the immature rat ovary. The localization of these transcripts in specific ovarian compartments suggests that this signal transduction pathway may be involved in follicular development and ovarian function. Furthermore, because wnts have been implicated in the oncogenic transformation of epithelial cells, our results raise the possibility that aberrant wnt expression may be involved in ovarian tumorigenesis in humans.