Leukemia cells are sensitized to temozolomide, carmustine and melphalan by the inhibition of O6-methylguanine-DNA methyltransferase

The cytotoxicity of the monofunctional alkylator, temozolomide (TMZ), is known to be mediated by mismatch repair (MMR) triggered by O-6-alkylguanine. By contrast, the cytotoxicity of bifunctional alkylators, including carmustine (BCNU) and melphalan (MEL), depends on interstrand cross-links formed through O-6-alkylguanine, which is repaired by nucleotide excision repair and recombination. O-6-alkylguanine is removed by O-6-methylguanine-DNA methyltransferase (MGMT). The aim of the present study was to evaluate the cytotoxicity of TMZ, BCNU and MEL in two different leukemic cell lines (HL-60 and MOLT-4) in the context of DNA repair. The transcript levels of MGMT, ERCC1, hMLH1 and hMSH2 were determined using reverse transcription-quantitative polymerase chain reaction. In addition, the proliferation was measured using the trypan blue exclusion assay. Drug sensitivity was found to vary between the two cell lines. Treatment of the cells with TMZ, BCNU or MEL in combination with O-6-benzylguanine, an MGMT inhibitor, was demonstrated to sensitize the two cell lines to these agents. However, the extent of sensitization was not found to be correlated with the expression levels of MGMT transcripts. Furthermore, the drug sensitivity was also not associated with the transcript levels of ERCC1, hMLH1 and hMSH2. Thus, leukemic cells were sensitized to alkylating agents by the inhibition of MGMT.