Topoisomerase I inhibitors have been shown to have clinical activity against human
colorectal cancer. Previous studies showed that the cytotoxicity of
camptothecin, a
topoisomerase I inhibitor, occurs mainly in the S -phase of the cell cycle and is protectable by
aphidicolin, an inhibitor of replicative
DNA polymerase in some
camptothecin-sensitive colorectal cells.
Transcription factor E2F-1 regulates the G1/S transition, and recent studies have shown that E2F-1 potentiated the cytotoxicity of some cell-cycle-related drugs. Therefore, the present study was designed to investigate the effect of adenovirus-mediated E2F-1 gene transfer on chemosensitivity of
colorectal cancer to
camptothecin, in vitro and in vivo. Two human
colorectal cancer cells, SW620 (mutant p53) and RKO (wild-type p53), were treated with
camptothecin, alone or in combination with adenoviral vectors expressing
beta-galactosidase (Ad-LacZ), or E2F-1 (Ad-E2F-1). E2F-1 overexpression was confirmed by Western blot analysis. Ad-E2F-1 gene transfer at low doses (less than the LD(20) dose) markedly increased the sensitivity of human
colorectal cancer cells to
camptothecin in vitro, which is because of induction of apoptosis.
Aphidicolin did not have any protective effect on the Ad-E2F-1/
camptothecin-mediated cytotoxicity. The level of
topoisomerase I expression was not affected by combination treatment as well, suggesting that DNA replication and
topoisomerase I activity may not account for the molecular mechanism of cell killing in response to Ad-E2F-1/
camptothecin treatment. Fas and
Fas ligand expression were not altered by treatment with
camptothecin and/or Ad-E2F-1. Moreover, combination of
camptothecin and Ad-E2F-1 has an additive antitumor effect in an in vivo nude mouse xenograft model. When combined with
camptothecin, E2F-1 adenovirus
therapy resulted in a 95.7% decrease in
tumor size compared to control groups (P<.05). These results suggest a chemosensitization strategy that may have clinical utility in human
colorectal cancer.