The Combination of PEITC (Phenehyl Isothiocyanate) with a Histone Deacetylase Inhibitor (HDACi) Has Synergistic Antileukemia Activity by Overcoming a Redox-Mediated Resistance Pathway.

Histone deacetylase inhibitors (HDACI) have limited but well established clinical activity in human leukemia. Results of a phase 1 trial of vorinostat in AML indicate that a gene signature composed mainly of antioxidants was associated with clinical resistance to vorinostat (Blood 2008;111:1060-60). This study suggested that generation of reactive oxygen species (ROS) appears to be a mechanism of action of vorinostat whereas increase of antioxidants may correlate with vorinostat resistance. The aims of this study were to further investigate the underlying molecular mechanisms and test the combination effect of vorinostat and redox modulation agents.

The parental HL60 and the pan-HDACI resistant HL60/LR were used to compare the redox parameters in this pair of cell lines. Real time PCR and western blot analysis demonstrated that a variety of glutathione related antioxidant defense enzymes were substantially increased in HL60/LR compared to its parental HL60, which is consistent with the clinical findings cited above. Most importantly, Nrf2, a master transcription factor that activates the transcription of cellular defense and antioxidant genes was also upregulated in HL60/LR. Confocal microscopy study showed that vorinostat treatment of HL60 cells caused translocation of Nrf2 from cytosol to nucleus. Furthermore, its downstream antioxidant genes including GST (glutathione S- Transferase), GSR (glutathione reductase), GCLC (glutathione synthase) and SOD (superoxide dismutase) were upregulated, demonstrating that the cellular defense against oxidative stress was induced by vorinostat. Overexpression of Nrf2 in HEK293 cells prevented ROS generation induced by vorinostat. Knock-down of Nrf2 by siRNA in colon cancer cell HCT116 caused increase of ROS production and cytotoxicity induced by vorinostat. These findings further demonstrated the role of Nrf2 in protecting cells from oxidative stress caused by vorinostat. We also observed that vorinostat substantially activated a ROS generating enzyme NADPH Oxidase (NOX) in various AML cell lines including HL60, U937 and ML1. Vorinostat induced ROS in both HL60 and the mitochondrial deficient cell line HL60-C6F. This indicates that NOX is a major source of ROS generation induced by vorinostat. As a result, modulation of antioxidant response may potentiate the cytotoxic activity of vorinostat. In order to modulate cellular redox balance and overcome the resistance to vorinostat, PEITC, a compound known to deplete cellular glutathione was used to test its combination effect with vorinostat. We found that a subtoxic concentration of PEITC (1-2.5 uM) substantially potentiated cytotoxicity of vorinostat in a dose-dependent manner in various AML cell lines, as demonstrated by Annexin-PI assay after 48 hrs and MTT assay after 72 hrs. Treatment with subtoxic concentrations of vorinostat (1.5 uM) and PEITC (1-2.5 uM) for 48 hrs also resulted in synergistic cytotoxicity in primary leukemia cells obtained from AML patient samples as demonstrated by Annexin-PI assay. Parallel results were also obtained with other HDACI such as MGCD0103.

Our study indicates that NADPH Oxidase is likely a major source of ROS generation induced by vorinostat. Nrf2, a master transcription factor and its downstream antioxidant genes, which protect cells from oxidative stress, contributes to leukemia cellular resistance to vorinostat. Modulation of cellular redox balance such as depletion of glutathione by PEITC significantly potentiates the anti-leukemia activity of vorinostat. Our study provides important information for further development of a mechanism-based combination strategy to maximize the potential of vorinostat and other HDACI and provides an alternative mechanism of the anti-leukemia activity of HDACI.

No relevant conflicts of interest to declare.