Fas ligand is constitutively secreted by prostate cancer cells in vitro

LNCaP, DU145, and PC3 prostate carcinoma cells secrete the 27-kDa soluble Fas ligand (sFasL) into their local environment. sFasL arises from the 40-kDa membrane-bound form (mFasL), which can be found on the cell surface in the LNCaP line, as demonstrated by monoclonal antibody staining. mFasL was also found in extracts of all three cell lines, as demonstrated by Western blotting. FasL mRNA was detected not only in the cell lines, but in the normal prostate as well. sFasL protein could also be detected immunohistochemically in prostate secretions and in human semen. Cleavage of mFasL to sFasL could be inhibited by several matrix metalloprotease inhibitors without a change in the cellular levels of FasL. Prostate-derived sFasL is biologically active, as demonstrated by its induction of apoptosis in Fas-positive Ramos cells, which was detected by terminal deoxynucleotidyl transferase-mediated nick end labeling assay. Mitoxantrone induces cellular apoptosis in all three prostate cancer cell lines. Mitoxantrone treatment and doxorubicin treatment also cause up-regulation of Fas, the cell surface receptor for FasL, in LNCaP cells, but not in DU145 or PC3 cells. Furthermore, the up-regulation of Fas expression by mitoxantrone at a high concentration was potentiated by hydrocortisone. When FasL interacts with its Fas, the Fas-bearing cell undergoes apoptosis. When LNCaP cells were treated with mitoxantrone and incubated with an anti-FasL monoclonal antibody, apoptosis was partially blocked. This not only further suggests that the sFasL is biologically active, but that the up-regulation of Fas in the presence of sFasL accounts, in part, for the cytotoxicity of mitoxantrone.