Sustained suppression of Fas ligand expression in cisplatin-resistant human ovarian surface epithelial cancer cells

Although cisplatin derivatives are first line chemotherapeutic agents for t he treatment of ovarian epithelial cancer, chemoresistance is a major thera peutic problem. Although the cytotoxic effect of these agents are believed to be mediated through the induction of apoptosis, the role of the Fas/FasL system in chemoresistance in human ovarian epithelial cancer is not fully understood. In the present study, we have used cultures of established cell lines of cisplatin-sensitive human ovarian epithelial tumours (OV2008 and A2780-s) and their resistant variants (C13* and A2780-cp, respectively) to assess the role ofFas/FasL system in the chemo-responsiveness of ovarian ca ncer cells to cisplatin. Cisplatin was effective in inducing the expression of cell-associated Fas and FasL, soluble FasL and apoptosis in concentrati on and time-dependent fashion in both cisplatin-sensitive cell lines (OV200 8 and A2780-s). In contrast, while cisplatin was effective in increasing ce ll-associated Fas protein content in C13*, it failed to up-regulate FasL (c ell-associated and soluble forms) and induce apoptosis, irrespective of con centration and duration of cisplatin treatment. Concentrated spent media fr om OV2008 cultures after cisplatin treatment were effective in inducing apo ptosis in C13* cells which was partly inhibited by the antagonistic Fas mon oclonal antibody (mAb) suggesting that the soluble FasL present in the spen t media was biologically active. In the resistant A2780-cp cells, neither F as nor FasL up-regulation were evident in the presence of the chemotherapeu tic agent and apoptosis remained low compared to its sensitive counterpart. Activation of the Fas signalling pathway, by addition to the cultures an a gonistic Fas mAb, was equally effective in inducing apoptosis in the cispla tin-sensitive (OV2008) and -resistant variant C13*, although these response s were of lower magnitude compared to that observed with cisplatin in the c hemosensitive cells. A significant interaction between cisplatin and agonis tic Fas mAb was observed in the apoptotic response in OV2008 and C13* when cultured in the presence of both agents. Immunohistochemistry of human ovar ian epithelial carcinomas reveals the presence of Fas in low abundance in p roliferatively active cells but in high levels in quiescent ones. Although the expression pattern of FasL in the tumour was similar to that of Fas, th e protein content was considerably lower. Taken together, these data sugges t that the dysregulation of the Fas/FasL system may be an important determi nant in cisplatin resistance in ovarian epithelial cancer cells. Our result s are also supportive of the notion that combined immuno- and chemo-therapy (i.e., agonistic Fas mAb plus cisplatin) may provide added benefits in the treatment of both chemo-sensitive and -resistant ovarian tumours.