Fig. 6: Proposed model of ROS generation and regulation of survival signals in CLL cells.

Data presented in this study demonstrate that CLL cells contain elevated levels of H₂O₂ with significantly reduced O₂⁻ levels compared to normal B-cells. This is primarily due to the presence of highly active mitochondrial SOD2 enzyme in CLL cells converting O₂⁻ into H₂O₂ more rapidly. Overexpression of SIRT3 in CLL cells deacetylates SOD2 and likely maintains its constitutive enzymatic activity. Moreover, we have also detected reduced expression of catalase in CLL cells causing H₂O₂-accumulation. Although we have identified two distinct CpG-Islands (I and II) for methylation in the human catalase promoter, our results suggest that it is the Island-I, which primarily regulates catalase levels in CLL cells. Indeed, we have detected variable degrees of methylation in CpG-Island-I of the catalase promoter only in CLL cells and not in normal B-cells and that, catalase expression is reduced in CLL cells both at mRNA and protein levels. While released from mitochondria, H₂O₂ may activate the AXL/FGFR/AKT/ERK signaling axis and/or the BCR pathway, independent of ligand, and maintain constitutive activation levels of these cell survival pathways through a positive feedforward loop. For example, activation of AKT/mTOR pathway leads to increase of oxygen consumption and mitochondrial metabolic activities and thus, generation of ROS. Of note, we found that ROS generation or BCR activation induces SIRT3 expression in CLL cells. Therefore, one potential therapeutic intervention can be to increase ROS-scavenging capacity using antioxidants, thereby abrogating ROS signaling and suppressing tumor growth in combination with current CLL therapies.