Duocarmycin A (Duo), which is one of well-known antitumor
antibiotics, efficiently alkylates
adenine N3 at the 3' end of AT-rich sequences in the
DNA. The addition of a minor groove binder,
distamycin A (Dist), not only accerelates the reactivity of Duo with
oligonucleotide duplex but also switches the
DNA-alkylation site to
guanine in GC-rich sequences. Here we examined cytotoxic effect of Duo in the coexistence of Dist using human lung
carcinoma (HLC-2) cells. The cytotoxicity of Duo to HLC-2 cells was enhanced 10 times by the addition of 0.5microg/ml Dist, which was much lower than the IC(50) value of 16microg/ml. Addition of Duo alone to HLC-2 cells resulted in typically apoptotic changes, including
chromatin condensation, sub-G1 accumulation in
DNA histogram pattern, and decrease in
procaspase-3 and 9 levels. Interestingly, these apoptotic characteristics in Duo-treated cells were suppressed by the addition of 0.5microg/ml Dist, and the G2/M population in the cell cycle progression of HLC-2 cells was largely unchanged in the coexistence of Dist along with the extremely low accumulation of p53 and higher induction of p21. In contrast, the treatment of HLC-2 cells with Dist (16microg/ml) alone was observed to induce the accumulation of p53 and cell cycle arrest at the G1 phase. These results indicate that Dist suppresses apoptosis induced by Duo as well as enhances Duo-induced cytotoxicity in living cells, and may contribute to
chemotherapy for
tumors resistant to inducing apoptotic cell death.