Down-Regulation of Mcl-1 by Small Interference RNA Induces Apoptosis and Sensitizes HL-60 Leukemia Cells to Etoposide

Background: Acute myeloid leukemia (AML) is a fatal hematological malignancy which is resistant to a varietyof chemotherapy drugs. Myeloid cell leukemia-1 (Mcl-1), a death-inhibiting protein that regulates apoptosis,has been shown to be overexpressed in numerous malignancies. In addition, it has been demonstrated that theexpression level of the Mcl-1 gene increases at the time of leukemic relapse following chemotherapy. The aimof this study was to target Mcl-1 by small interference RNA (siRNA) and analyze its effects on survival andchemosensitivity of acute myeloid leukemia cell line HL-60. Materials and
Methods: siRNA transfection wasperformed with a liposome approach. The expression levels of mRNA and protein were measured by real-timequantitative PCR and Western blot analysis, respectively. Trypan blue assays were performed to evaluate tumorcell growth after siRNA transfection. The cytotoxic effects of Mcl-1 siRNA (siMcl-1) and etoposide were determinedusing MTT assay on their own and in combination. Apoptosis was quantified using a DNA-histone ELISA assay.
Results: Transfection with siMcl-1 significantly suppressed the expression of Mcl-1 mRNA and protein in a timedependentmanner, resulting in strong growth inhibition and spontaneous apoptosis. Surprisingly, pretreatmentwith siMcl-1 synergistically enhanced the cytotoxic effect of etoposide. Furthermore, Mcl-1 down-regulationsignificantly increased apoptosis sensitivity to etoposide. No significant biological effects were observed withnegative control siRNA treatment.
Conclusions: Our results suggest that specific suppression of Mcl-1 by siRNAcan effectively induce apoptosis and overcome chemoresistance of leukemic cells. Therefore, siMcl-1 may be apotent adjuvant in leukemia chemotherapy.