Arsenic Trioxide Enhances STI571-Induced Apoptosis of K562 Cells through Downregulating Bcl-XL and Bcr-Abl

We aimed to evaluate the effects of arsenic trioxide (ATO, Trisenox) on STI571 (Gleevec, imatinib mesylate)-induced apoptosis of a chronic myelogenous leukemia (CML) cell line, K562 cells. Cell prolifration and colony-forming assays were performed to determine the cytotoxicity of ATO alone and in combination with STI571. Apoptosis was analyzed by morphological changes, apoptosis rate and cell cycles. An Elisa assay was used to detect the levels of cytosolic cytochrome c (cyt c) and caspase-3 in K562 cells exposed to ATO and STI571 at graded concentrations. Semi-quantitative reverse transcriptase polymerase chain reaction (RT-PCR) was used to assay the transcriptional levels of Bcl-XL and Bcr-Abl genes in K562 cells. Results showed that both the colony-forming assay and cell proliferation assay demonstrated additive to synergistic effects of ATO on STI571-induced apoptosis in K562 cells, a Bcr-Abl positive cell line. Caspase-3 was activated during apoptosis and there was an increase in cytosolic accumulation of cytochrome c. Treatment of K562 cells with STI571 alone led to down-regulation of transcriptional levels of Bcl-XL at 12 hours and Bcr-Abl at 96 hours after drug administration. Treatment with ATO alone only led to reduce the mRNA levels of Bcl-XL, but not Bcr-Abl. Combined treatment with ATO and STI571 down regulated the transcripts of Bcl-XL at 12 hours and Bcr-Abl 72 hours after drug administration. We conclude that ATO enhanced cytotoxic and proapoptotic actions of STI571 could be mediated by the down-regulation of Bcr-Abl and Bcl-XL genes in K562 cells. Therefore ATO in combination with STI571 could be a promising therapy for CML.