Original contributionPAX8 expression in ovarian surface epithelial cells☆
Introduction
Invasive ovarian carcinoma is a deadly disease characterized by frequent late-stage diagnosis and poor survival rates. It is also a heterogeneous disease encompassing several histological subtypes. High-grade serous ovarian carcinoma (HGSOC) is the most common subtype, accounting for approximately 60% of all ovarian carcinomas. Historically, HGSOCs were thought to arise from ovarian surface epithelial cells (OSECs), the mesothelial-type epithelium covering the ovary, and the epithelial lining of cortical inclusion cysts (CICs), which are derived from invaginations of the ovarian surface. Decades of research have indicated that HGSOCs may arise in these tissues [1], [2], [3], [4]. However, this hypothesis has been criticized because metaplastic ovarian epithelial cells are rarely found on the surface of the ovary, and because ovarian carcinomas exhibited markedly different histological features and marker expression when compared with normal OSECs. More recent evidence indicates that HGSOCs can originate from epithelial cells lining the fallopian tube fimbriae, through a precursor lesion termed serous tubal intraepithelial carcinoma (STIC). STICs are associated with 20% to 60% of sporadic HGSOCs [5], [6], [7], [8], raising the possibility that the remaining 40% to 80% of HGSOCs may arise from an alternative cell of origin.
Several studies have shown that paired box 8 (PAX8) is highly expressed in most HGSOCs [9], [10], [11], [12], [13], [14], suggesting that overexpression of this transcription factor plays a critical role in this tumor type. This is supported by functional evidence showing that PAX8 knockdown in ovarian carcinoma cell lines induces growth arrest and apoptosis, and decreases tumorigenesis [14], [15], [16]. The observation of PAX8 expression in fallopian tube secretory epithelial cells (FTSECs) and in many CICs but not OSECs could suggest that the epithelial lining of CICs is derived from the fallopian tube and that most, if not all HGSOCs, likely originate from FTSECs. Although it is widely considered that OSECs do not express PAX8, there are conflicting reports evaluating PAX8 expression in OSECs [9], [10], [11], [12], [17], [18], [19]. Evidence of PAX8 expression by OSECs has been reported in smaller sample sizes (n > 8 only in 1 study) [4], [10], [12], [17], [19], but this finding has been controversial [20] because it did not agree with the findings of others [9], [11], [18]. The purposes of the current study were to perform a comprehensive analysis of PAX8 protein and messenger RNA (mRNA) expression in both normal ovaries and primary OSECs in culture and to evaluate the functional significance of PAX8 in early stage development of ovarian carcinoma using in vitro models of OSECs.
Section snippets
Tissue culture
Normal OSECs were isolated from histologically normal ovaries of women undergoing surgery for conditions not involving the ovaries (such as endometrial cancer or fibroids). Samples were collected with the approval of the University College London Hospital Ethics Committee and informed patient consent. OSECs were harvested by brushing the surface of the ovary with a cytobrush and cultured in normal OSEC culture media [21]. hTERT (human telomerase reverse transcriptase)–immortalized OSECs
PAX8 protein expression in normal ovaries
PAX8 expression was evaluated in 27 histologically normal ovaries with OSECs present on the surface, 18 of which also contained CICs. From the same patient cohort, 7 fallopian tubes were analyzed as positive controls for PAX8 expression. PAX8 was expressed in the OSECs of 12 ovaries (44%), 9 of which (33% of all ovaries) showed moderate to strong staining (similar to expression levels in FTSECs). OSECs in the remaining 15 ovaries (56%) showed no evidence of PAX8 staining. PAX8 was observed in
Discussion
Ovarian carcinoma is the most lethal gynecological malignancy in Western countries. HGSOC is characterized by frequent late-stage presentation, having no specific symptoms associated with early stage disease. Understanding the molecular mechanisms that underlie cancer initiation and identifying the precursor tissues of the specific subtypes of ovarian carcinoma may lead to the development of novel early stage screening biomarkers for ovarian carcinoma, which would have a substantial impact on
Supplementary data
The following are the supplementary data to this article.
Acknowledgments
We would like to thank Lillian Young and the USC IHC Core for immunohistochemistry services, and Lora Barsky and the USC Flow Cytometry Core Facility for flow cytometry services. We thank Dr Peter Kopp and Aigerim Bizhanova for their kind efforts in generating and providing the pEGFP-hPAX8a construct. We also thank Eva Wozniak and Maria Notaridou for technical assistance in the OSEC collection and culture, and Rod Karevan for technical support in qPCR.
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Funding/Support: K. L. is funded by a K99/R00 award from the National Cancer Institute (Grant No. 1K99CA184415). The project described was supported, in part, by award number P30CA014089 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.
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Both authors contributed equally to this study.