Mechanisms of thymidine kinase/ganciclovir and cytosine deaminase/5-fluorocytosine suicide gene therapy-induced cell death in glioma cells

Suicide gene transfer using thymidine kinase (TK) and ganciclovir (GCV) treatment or the cytosine deaminase (CD)/5-fluorocytosine (5-FC) system represents the most widely used approach for gene therapy of cancer. However, molecular pathways and resistance mechanisms remain controversial for GCV-mediated cytotoxicity, and are virtually unknown for the CD/5-FC system. Here, we elucidated some of the cellular pathways in glioma cell lines that were transduced to express the TK or CD gene. In wild-type p53-expressing U87 cells, exposure to GCV and 5-FC resulted in a weak p53 response, although apoptosis was efficiently induced. Cell death triggered by GCV and 5-FC was independent of death receptors, but accompanied by mitochondrial alterations. Whereas expression of Bax remained unaffected, in particular, GCV and also 5-FC caused a decline in the level of Bcl-2. Similar findings were obtained in 9L and T98G glioma cells that express mutant p53, and also underwent mitochondrial apoptosis in both the TK/GCV and CD/5-FC system. Upon treatment of 9L cells with 5-FC, Bcl-x(L) expression slowly declined, whereas exposure to GCV resulted in the rapid proapoptotic phosphorylation of Bcl-x(L). These data suggest that TK/GCV- and CD/5-FC-induced apoptosis does neither require p53 nor death receptors, but converges at a mitochondrial pathway triggered by different mechanisms of modulation of Bcl-2 proteins.