ABT-737 accelerates butyrate-induced death of HL-60 cells. Involvement of mitochondrial apoptosis pathway.

The aim of presented study was to determine effect of NaBu in combination with ABT-737 on cell survival of leukemic cell line HL-60. In addition, analysis of molecular mechanism of NaBu action with a focus on mitochondrial apoptosis was performed. Both NaBu and ABT-737 are inducing death of HL-60 cells with different kinetics. ABT-737-induced cell death is fast while NaBu-induced death preceded by cell cycle arrest in G2 phase is rather slow. Cell viability preceded by cell cycle arrest in G2 phase was significantly decreased after 48 hours of incubation with 2 and 5 mmol/l of NaBu while it was significantly decreased after 24 hours of incubation with 1 μmol/l of ABT-737 combined with 2 and 5 mmol/l of NaBu. Incubation of HL-60 cells with NaBu was associated with increased level of pro-apoptotic protein BIMEL and decreased levels of anti-apoptotic proteins of Bcl-2 family as well as GRP78 involved in ER stress signalling. It seems that ABT-737 accelerates NaBu-induced death of HL-60 cells due to mitochondrial apoptosis resulting from ABT-737-mediated inhibition of functions and NaBu-induced decrease of the levels of anti-apoptotic Bcl-2 family proteins as well as due to accelerated decrease of GRP78 observed after the treatment of cells with combination of NaBu and ABT-737. The effect of combination of both drugs on survival of HL-60 cells seems to be synergistic at high concentrations of NaBu (2 and 5 mmol/) while it is rather antagonistic at concentrations of NaBu less than 1 mmol/l. Finally, it might be assumed that NaBu is capable to induce cell death with mechanisms independent from mitochondrial apoptosis.