Role of PAX2 in Maintaining the Differentiation of Oviductal Epithelium and Inhibiting the Transition to a Stem Cell State

Several studies have proposed the fallopian tube epithelium as a site of origin of ovarian cancer. The discovery of precursor lesions in the fallopian tube in patients at risk for ovarian cancer supports a probable origin for high-grade serous ovarian carcinoma in this tissue. While the fallopian tube epithelium consists of three distinct cell types, the paired box protein 2 (PAX2) positive cells and potentially the CD44 positive stem-like cells are most relevant to ovarian cancer. Loss of PAX2 expression in the fallopian tube cells is considered to be an early event in epithelial transformation, but the specific role of PAX2 in this transition is unknown. \ud The aim of this study was to define the role of PAX2 in oviductal epithelial cells (OVE) cells and in mouse ovarian surface epithelial cells (MOSE), and to understand its contribution to the formation of serous precursor lesions in the fallopian tubes. Herein, we studied the OVE response to transforming growth factor β (TGFβ, a cytokine found in follicular fluid) and provide evidence of its potential involvement in the regulation of stem cell-like behaviors that may contribute to formation of cancer-initiating cells. Treatment of primary cultures of OVE cells with TGFβ at concentrations found in ovulatory follicular fluid induced an epithelial-mesenchymal transition (EMT) with expected changes in proliferation, cell morphology and expression of SNAIL, Vimentin and E-cadherin. EMT was also associated with decreased expression of PAX2 and an increase in the fraction of cells expressing CD44. Pax2 knockdown in OVE cells and overexpression in ovarian epithelial cells confirmed that PAX2 inhibits CD44 expression and regulates the degree of epithelial differentiation of OVE cells. These results suggest that the loss of PAX2 seen in serous tubal intraepithelial carcinomas (STIC) leads to a shift to a more mesenchymal phenotype associated with stem-like features. Pax2 overexpression in MOSE cells also induced the formation of vascular channels both in vitro and in vivo, which indicate a possible contribution of PAX2 to ovarian cancer progression by increasing the vascular channels to supply nutrients to the tumor cells. \ud Furthermore, since loss of PAX2 in STIC was found associated with P53 and BRCA1 mutations, OVE cells with mutations of the tumor suppressor genes Trp53 and Brca1 were studied. We found that loss of Trp53 with or without loss of Brca1 increased cell proliferation and colony formation in vitro. In addition, loss of Trp53 induced OVE cells to undergo EMT and induced the expression of stem cell–associated genes. We therefore suggest a potential contribution of stem cells in initiating the precursor lesions in the fallopian tubes in combination with tumor suppressor gene mutation