Primary ovarian cancer cultures are resistant to Fas-mediated apoptosis

Objectives. Fas, a primary mediator of cellular apoptosis, is expressed by the normal ovarian epithelium. We analyzed the levels of Fas and soluble Fas (sFas) expression in ovarian cancer tissue and determined the susceptibility of primary ovarian cancer cell (CSOC) cultures to Fas-mediated apoptosis. Methods. Fas mRNA levels were detected by RT-PCR, and Fas protein levels were determined by immunohistochemistry and Western blot analysis. Secreted sFas levels were measured by ELISA. Localization of Fas to the cell surface was demonstrated by flow cytometry. The effect of Fas on cell proliferation was measured by MTT assay. Results. Intense Las staining was detected on the cell surface and in the cytoplasm of ovarian carcinoma specimens. We also found that mean levels of sFas, which can function as a Fas agonist, were significantly increased in 18 sera from cancer patients (0.98 ng/ml) compared to those of 8 healthy individuals (0.61 ng/ml, P = 0.004). Fas mRNA and protein were expressed in all primary ovarian cancer cell cultures. Despite abundant Fas expression, CSOC cultures were significantly less sensitive to Fas-mediated apoptosis (11.3%) than primary cultures of normal ovarian epithelial cells (HOSE) (50.0%) (P = 0.00001). The sFas level in CSOC-conditioned medium was minimal (0.07 ng/ml) and not significantly different from that of HOSE-conditioned medium (0.09 ng/ml). The small amount of sFas secreted by CSOC does not likely account for the observed resistance to Fas-mediated apoptosis. Conclusion. Decreased sensitivity to Fas-mediated apoptosis could contribute to ovarian tumorigenesis through resistance to cytotoxic T lymphocyte-mediated cytotoxicity and may play a role in ovarian tumorigenesis. (C) 1999 Academic Press.