Involvement of the mismatch repair system in temozolomide-induced apoptosis

Postreplicative mismatch repair plays a major role in mediating the cytotoxicity of agents generating O⁶-methylguanine in DNA. We previously showed that a methylating antitumor triazene compound, temozolomide, induces apoptosis and that the persistence of O⁶-methylguanine in DNA is required to trigger the process. We wanted to test whether the latter apoptotic signal is dependent on a functional mismatch repair system. To this end, we used two human lymphoblastoid cell lines (i.e., the mismatch repair-proficient TK6 line and its mismatch repair-deficient subline MT1) that are both deficient in O⁶-methylguanine repair. Temozolomide treatment of TK6 cells brought about efficient cell growth inhibition, G₂/M arrest, and apoptosis, as indicated by the results of cytofluorimetric analysis of 5-bromo-2'- deoxyuridine incorporation and DNA content and evaluation of DNA fragmentation. The drug treatment resulted also in the induction of p53 and p21/waf-1 protein expression. In contrast, MT1 cells were highly resistant to the drug and no p53 and p21/waf-1 induction was observed. Importantly, we could show that MT1 cells are not deficient in the p53-dependent apoptosis pathway; treatment with etoposide, a topoisomerase II inhibitor, resulted in p53 and p21/waf-1 protein expression and apoptosis in both cell lines. In conclusion, we demonstrate the existence of a link between a functional mismatch repair system and the trigger of apoptosis in cells exposed to clinically relevant concentrations of temozolomide. The results also suggest that p53 induction in response to O⁶-guanine methylation involves the mismatch repair system.