Deficient activation of CD95 (APO-1/Fas)-mediated apoptosis: a potential factor of multidrug resistance in human renal cell carcinoma

The pronounced resistance of human renal cell carcinoma (RCC) to anticancer -induced apoptosis has primarily been related to the expression of P-glycop rotein and effective drug detoxification mechanisms. Because the CD95 syste m has recently been identified as a key mediator of anticancer drug-induced apoptosis, we analysed the contribution of the CD95 system to chemotherapy -induced apoptosis in four newly established RCC cell lines. Here, we demon strate that all RCC cell lines expressed CD95-receptor and -ligand. Exposur e to agonistic anti-CD95 antibodies resulted in induction of apoptosis and significant (P < 0.05) reduction of cell number in three out of four cell l ines, indicating that the essential components for CD95-mediated apoptosis were present and functionally intact in the majority of these RCC cell line s. Moreover, treatment of cultures with bleomycin or topotecan, a novel top oisomerase I inhibitor with little substrate affinity for P-glycoprotein, l ed to induction of apoptosis and significant (P < 0.05) dose-dependent redu ction of cell number in ail RCC cell lines. Both anticancer drugs also indu ced upregulation of CD95 ligand expression in all cell lines. Additionally, augmentation of CD95 receptor expression was found in three RCC cell lines , including one p53-mutated cell line, whereas another p53-mutated cell lin e showed no or only a weak CD95 receptor upregulation after exposure to top otecan or bleomycin, respectively. Despite this upregulation of CD95 recept or and ligand, antagonistic antibodies directed against CD95 receptors or l igands could not inhibit induction of apoptosis by topotecan and bleomycin in any cell line. Thus, although a functionally intact CD95 signalling casc ade is present in most RCC cell lines, the anticancer drugs topotecan and b leomycin that induce upregulation of CD95 receptor and ligand fail to effec tively activate CD95-mediated apoptosis. This deficient activation of CD95- mediated apoptosis might be an important additional factor for the multidru g resistance phenotype of human RCCs. (C) 2000 Cancer Research Campaign.