Effects of Decitabine and Homoharringtonine in Combination in Leukemia Cell Lines

Background: The methylation inhibitor decitabine (DAC) has great therapeutic value for myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). However, DAC monotherapy is associated with relatively low rates for overall response and complete remission. We aimed to investigate the effects of the combination of DAC and homoharringtonine (HHT) in the SKM-1 and Kg-1a cell lines and explore their associated mechanisms.

Methods: Cell viability was estimated by MTS assay. Cell apoptosis were detected by flow cytometry analysis and PI/Annexin V staining. Western blot and quantitative reverse transcription-polymerase chain reaction assays were performed to detect the expression of apoptosis-related genes including caspase-3, caspase-9, BCL-2 and BCL-XL and DNA methyltransferases(DNMT) including DNMT1, DNMT3A and DNMT3B. Changes in LINE-1 gene methylation were assessed by quantitative polymerase chain reaction after bisulfite conversion. The effects of the combinations were estimated using CalcuSyn software.

Results: The combination of DAC and HHT showed synergistic effects for inhibiting cell viability in SKM-1 and KG-1a cell lines. This combination can enhance inhibition of colony formation and apoptosis induction of DAC alone in SKM-1 cell line. However, in Kg-1a cells, this combination only enhanced the apoptosis induction of DAC alone. For SKM-1 cells, further study found that different doses of DAC and HHT alone have different effects on the expression of the apoptosis-related genes caspase-3, caspase-9, BCL-XL and BCL-2. The combination of 0.4 μM DAC and 5 nM HHT treatment significantly increased the mRNA expression of caspase-3 (P=0.0005) and caspase-9 (P=0.0075) and decreased the expression of BCL-2 (P=0.0331), but has no significantly effect on the BCL-XL (P=0.3436) compared with 0.4 μM DAC alone. However, 4 μM DAC plus 50 nM HHT had no significant effects on the mRNA and protein expression of the apoptosis-related genes examined compared with 4 μM DAC alone (P>0.05). Low-dose DAC induced greater hypomethylation than higher doses of the drug. Whereas HHT had no demethylation effects, and also had no enhancement effects with DAC on the hypomethylation and mRNA and protein expression of DNMT1, DNMT3A and DNMT3B in SKM-1 cells.

Conclusion: The combination of DAC and HHT has synergistic effects on cell viability inhibition and enhancement effects on cell apoptosis in SKM-1 and KG-1a cell lines. But this combination did not enhance the hypomethylation effect of DAC.These data suggest that DAC used in combination with HHT has clinical potential in the treatment of MDS and AML.