SIRT3 overexpression and epigenetic silencing of catalase regulate ROS accumulation in CLL cells activating AXL signaling axis.

Mitochondrial metabolism is the key source for abundant ROS in chronic lymphocytic leukemia (CLL) cells. Here, we detected significantly lower superoxide anion (O2-) levels with increased accumulation of hydrogen peroxide (H2O2) in CLL cells vs. normal B-cells. Further analysis indicated that mitochondrial superoxide dismutase (SOD)2, which converts O2- into H2O2 remained deacetylated in CLL cells due to SIRT3 overexpression resulting its constitutive activation. In addition, catalase expression was also reduced in CLL cells suggesting impairment of H2O2-conversion into water and O2 which may cause H2O2-accumulation. Importantly, we identified two CpG-islands in the catalase promoter and discovered that while the distal CpG-island (-3619 to -3765) remained methylated in both normal B-cells and CLL cells, variable degrees of methylation were discernible in the proximal CpG-island (-174 to -332) only in CLL cells. Finally, treatment of CLL cells with a demethylating agent increased catalase mRNA levels. Functionally, ROS accumulation in CLL cells activated the AXL survival axis while upregulated SIRT3, suggesting that CLL cells rapidly remove highly reactive O2- to avoid its cytotoxic effect but maintain increased H2O2-level to promote cell survival. Therefore, abrogation of aberrantly activated cell survival pathways using antioxidants can be an effective intervention in CLL therapy in combination with conventional agents.