Cytoplasmic Expression of p21 in High Grade Serous Ovarian Cancer Confers a Pro-proliferative Role and Correlates with Poor Prognostic Markers

Introduction

High grade serous carcinoma (HGSC) is the most common subtype of epithelial ovarian cancer (EOC). p21^waf1/cip1 is a well-known tumor suppressor that exerts cellular growth arrest via binding of nuclear cell-cycle proteins. The role of p21 in HGSC and EOC is largely unknown. Herein we study the function, subcellular localization, and clinical correlates of p21 in HGSC and other EOC subtypes.

Materials and methods

In order to test the role of p21 in HGSC we knocked down or overexpressed p21 in HGSC lines and performed an XTT assay to measure cell viability. Next, we studied the subcellular localization of p21 by performing an immune-fluorescent stain of p21 in HGSC lines, and immunohistochemical stains of p21 in human HGSC samples. Since it is now widely accepted that HGSC often originates from the fallopian tube epithelium, we tested the evolution of p21 subcellular localization during the process of normal fallopian tube epithelium transformation to HGSC. This was accomplished by staining human samples of normal fallopian tube epithelium, early non-invasive lesions and HGSC for p21. Lastly, we performed a p21 stain on a tissue microarray (TMA) containing human EOC samples and correlated p21 subcellular localization with clinical parameters.

Results and discussion

Knocking down p21 in two different HGSC lines resulted in significantly reduced viability of HGSC lines and accordingly p21 overexpression resulted in enhanced viability. This suggests that p21 plays a pro-proliferative role in HGSC, contrary to its canonical growth suppressive role. Previous reports in breast cancer and monocyte cells have shown that a pro-proliferative role of p21 is mediated by a cytoplasmic p21. Therefore, we tested the subcellular localization of p21. Immunofluorescent stains of p21 in HGSC lines shows cytoplasmic localization of p21, and IHC of p21 in HGSC samples shows cytoplasmic p21 localization in human EOC tumors. We show that while in normal human FTSEC p21 is localized to the nucleus, during the process of transformation p21 shuttles to the cytoplasmic compartment. By staining an EOC TMA for p21 we show that the extent of cytoplasmic p21 correlates with poor prognostic factors like type II histology and higher cancer stage.

Conclusion

We show for the first time cytoplasmic expression of p21 in EOC, which plays a non-canonical pro-proliferative role and correlates with poor prognosis. Therefore, cytoplasmic p21 can potentially serve as an additional marker to differentiate ovarian cancer from other cancer types and serve as a poor prognostic marker.