Improving screening and treatment options for patients with epithelial ovarian cancer has been a major challenge in cancer research. Development of novel diagnostic and therapeutic approaches, particularly for the most common subtype, high-grade serous ovarian cancer (HGSC), has been hampered by controversies over the origin of the disease and a lack of spontaneous HGSC models to resolve this controversy. Over long-term culture in our laboratory, an ovarian surface epithelial (OSE) cell line spontaneously transformed OSE (STOSE). The objective of this study was to determine if the STOSE cell line is a good model of HGSC. STOSE cells grow faster than early passage parental M0505 cells with a doubling time of 13 and 48 h, respectively. STOSE cells form colonies in soft agar, an activity for which M0505 cells have negligible capacity. Microarray analysis identified 1755 down-regulated genes and 1203 up-regulated genes in STOSE compared to M0505 cells, many associated with aberrant Wnt/β-catenin and Nf-κB signaling. Upregulation of Ccnd1 and loss of Cdkn2a in STOSE tumors is consistent with changes identified in human ovarian cancers by The Cancer Genome Atlas. Intraperitoneal injection of STOSE cells into severe combined immunodeficient and syngeneic FVB/N mice produced cytokeratin+, WT1+, inhibin−, and PAX8+ tumors, a histotype resembling human HGSC. Based on evidence that a SCA1+ stem cell-like population exists in M0505 cells, we examined a subpopulation of SCA1+ cells that is present in STOSE cells. Compared to SCA1− cells, SCA1+ STOSE cells have increased colony-forming capacity and form palpable tumors 8 days faster after intrabursal injection into FVB/N mice. This study has identified the STOSE cells as the first spontaneous murine model of HGSC and provides evidence for the OSE as a possible origin of HGSC. Furthermore, this model provides a novel opportunity to study how normal stem-like OSE cells may transform into tumor-initiating cells.

Epithelial ovarian cancer (EOC) is thought to arise in part from the ovarian surface epithelium (OSE); however, the molecular events underlying this transformation are poorly understood. Germline mutations in the BRCA1 tumor suppressor gene result in a significantly increased risk of developing EOC and a large proportion of sporadic EOCs display some sort of BRCA1 dysfunction. To generate a model in which Brca1-mediated transformation can be studied, we previously inactivated Brca1 alone in murine OSE, which resulted in an increased accumulation of premalignant changes, but no tumor formation. In this study, we examined tumor formation in mice with conditionally expressed alleles of Brca1, p53 and Rb, alone or in combination. Intrabursal injection of adenovirus expressing Cre recombinase to inactivate p53 resulted in tumors in 100% of mice. Tumor progression was accelerated in mice with concomitant inactivation of Brca1 and p53, but not Rb and p53. Immunohistologic analyses classified the tumors as leiomyosarcomas that may be arising from the ovarian bursa. Brca1 inactivation in primary cultures of murine OSE cells led to a suppression of proliferation that could be rescued by concomitant inactivation of p53 and/or Rb. Brca1-deficient OSE cells displayed an increased sensitivity to the DNA damaging agent cisplatin, and this effect could be modulated by inactivation of p53 and/or Rb. These results indicate that Brca1 deficiency can accelerate tumor development and alter the sensitivity of OSE cells to chemotherapeutic agents. Intrabursal delivery of adenovirus intended to alter gene expression in the ovarian surface epithelium may, in some strains of mice, result in more rapid transformation of adjacent cells, resulting in leiomyosarcomas.

The ovarian surface epithelium, a single layer of poorly differentiated epithelial cells, covers the surface of the ovary and is ruptured during ovulation. Little is known about the changes that occur in this layer before or during ovulation, and even less is known about the regenerative processes that occur after the surface is ruptured to release a mature oocyte. Recently, a population of mouse ovarian surface epithelial (MOSE) cells that exhibit progenitor/stem cell characteristics has been identified, though neither a genetic marker nor how these cells are regulated has been determined. We have identified a defined population of MOSE cells with progenitor cell characteristics that express the stem cell marker lymphocyte antigen 6 complex, locus A (LY6A; also known as stem cell antigen-1 [SCA-1]). By testing the effect of factors found in the follicular fluid at ovulation on proliferation, sphere formation, and LY6A expression, we have determined that the size of the LY6A-expressing (LY6A+) progenitor cell population is regulated by at least two ovulation-associated factors present in the follicular fluid: transforming growth factor beta 1 and leukemia-inhibitory factor. Our work has identified a population of LY6A+ MOSE progenitor cells on the surface of the ovary that may play a role in ovulatory wound healing.