Combination Treatment of p53-Null HL-60 cells with Histone Deacetylase Inhibitors and Chlorambucil Augments Apoptosis and Increases BCL6 and p21 Gene Expression

Background: Treatment of hematological malignancies with conventional DNA-damaging drugs, such as chlorambucil (CLB), commonly results in p53-dependent chemo-resistance. Chromatin modifying agents, such as histone deacetylase inhibitors (HDACIs), sodium butyrate (NaBu) and trichostatin A (TSA), may reverse chemo-resistance by modulating the activity of chromatin remodeling enzymes and/or genes that control cell proliferation, differentiation and survival. Objective: This study examined the potential use of HDACIs and CLB combination therapies in an in vitro chemo-resistant leukemia model. Methods: The p53-null promyelocytic leukemia cell line, HL60, was used as an in vitro model of chemo-resistant leukemia. Drug cytotoxicity was determined by tetrazolium salt-based colorimetric assays and Annexin V/propidium iodide staining (flow cytometry). The level of mRNA expression of the chromatin modifying genes was measured by quantitative real-time PCR. Results: Micromolar concentrations of CLB combined with either NaBu or TSA triggered synergistic cytotoxic effects in HL-60 cells (p < 0.001). The effects of the combination treatments resulted in upregulated p21 gene expression (up to 59-fold; p<0.001) that preceded an increase in BCL6 gene expression (up to 20-fold; p < 0.001). Statistically significant but smaller magnitude changes (≤ 2-fold; p <0.05) were noted in the expression of other genes studied regardless of the treatment type. Conclusion: The combination treatment of p53-null HL-60 cells with DNA-damaging agent CLB and HDACIs NaBu and TSA triggered additive to synergistic effects on apoptosis and upregulated BCL6 and p21 expression. These findings reveal BCL6 and p21 as potential targets of chemo-resistance for the development of anti-leukemic drugs.