beta-lapachone induces cell cycle arrest and apoptosis in human colon cancer cells

Background: Human colon cancers have a high frequency of p53 mutations, and cancer cells expressing mutant p53 tend to be resistant to current chemo- and radiation therapy. It is thus important to find therapeutic agents that can inhibit colon cancer cells with altered p53 status. beta-Lapachone, a novel topoisomerase inhibitor, has been shown to induce cell death in human promyelocytic leukemia and prostate cancer cells through a p53-independent pathway. Here we examined the effects of beta-lapachone on human colon can cer cells. Materials and Methods: Several human colon cancer cell Lines, SW480, SW620, and DLD1, with mutant or defective p53, were used. The antiproliferative e ffects of beta-lapachone were assessed by colony formation assays, cell cyc le analysis, and apoptosis analysis, including annexin V staining and DNA l addering analysis. The effects on cell cycle and apoptosis regulatory prote ins were examined by immunoblotting. Results: All three cell lines, SW480, SW620, and DLD1, were sensitive to be ta-lapachone, with an IC50 of 2 to 3 mu M in colony formation assays, a fin ding similar to that previously reported for prostate cancer cells. However , these cells were arrested in different stages of S phase. At 24 hr post-t reatment, beta-lapachone induced S-, late S/G2-, and early S-phase arrest i n SW480, SW620, and DLD1 cells, respectively. The cell cycle alterations in duced by beta-lapachone were congruous with changes in cell cycle regulator y proteins such as cyclin A, cyclin B1, cdc2, and cyclin D1. Moreover, beta -lapachone induced apoptosis, as demonstrated by annexin V staining, now cy tometric analysis of DNA content, and DNA laddering analysis. Furthermore, down-regulation of mutant p53 and induction of p27 in SW480 cells, and indu ction of pro-apoptotic protein Bar in DLD1 cells may be pertinent to the an ti-proliferative and apoptotic effects of beta-lapachone on these cells. Conclusions: beta-Lapachone induced cell cycle arrest and apoptosis in huma n colon cancer cells through a p53-independent pathway. For human colon can cers, which often contain p53 mutations, beta-lapachone may prove to be a p romising anticancer agent that can target cancer cells, especially those wi th mutant p53.