The Role of NADPH Oxidase 2 in Normal and Malignant Hematopoiesis

NADPH dependent oxidase 2 (NOX2) is the founding member of a family of multimeric, oxido-reductase enzymes that catalyze the production of superoxides by transferring a single electron from the cofactor NADPH to molecular oxygen. It is primarily utilized in neutrophils and macrophages to generate copious amount of reactive oxygen species (ROS) to facilitate the neutralization of engulfed particulates during phagocytosis. In sharp contrast to this specialized function however, recent evidence implies a non-phagocytic role for NADPH oxidases in which physiologic levels of ROS generated by these enzymes modulate key signaling proteins and transcription factors to exert profound biological effects. Based on this information we decided to investigate the potential role of NOX2 in normal and leukemic stem cells. Using transgenic NOX2 knock out mice, genetically defined murine models of myeloid leukemia and primary human acute myeloid leukemia (AML) specimens, we show that NOX2 is critical for the proper function of normal and malignant hematopoietic stem cells.

In silico analysis using published transcriptional profiles of hematopoietic populations revealed that multiple subunits of the NOX2 complex are expressed at low levels in hematopoietic stem cells (HSCs) and at relatively higher levels in multipotent progenitors (MPPs). Next, we characterized the different hematopoietic compartments from age and sex matched wild type (WT) and transgenic NOX2 knock out (KO) mice. Our studies revealed that in the bone marrow of KO mice, a subset of multipotent progenitor populations (MPP2 & MPP3), which often have biased myelo-erythroid output are markedly expanded relative to their wild type counterparts. Consistently, we found increased levels of granulocytes and monocytes in the peripheral circulation of NOX2 KO mice. To test whether NOX2 has a functional, biological role in the self-renewal of HSCs, we performed competitive transplantation assays using equal numbers of whole BM cells from WT and KO mice to co-repopulate lethally irradiated hosts. Analysis of engrafted mice showed that the contribution from NOX2 KO HSCs was severely compromised in all lineages and developmental stages of hematopoiesis examined. Collectively, these results suggest a critical biological role for NOX2 in maintaining the quiescence and long term self-renewal of HSCs.

Similar to normal hematopoiesis, we found out that NOX2 is also widely expressed by functionally defined leukemic stem cells in a murine model of myeloid leukemia generated by expressing the oncogenic translocations BCR-ABL and NUP98-HOXA9. To evaluate the role of NOX2 in leukemogenesis, we established the BCR-ABL/NUP98-HOXA9 model using primitive cells derived from either WT or KO. Intriguingly, NOX2 KO leukemic cells generated a much less aggressive disease upon transplantation into primary and subsequently into secondary recipients. Furthermore, leukemic cells in which NOX2 is suppressed displayed aberrant mitotic activity and altered developmental potential marked by loss of quiescence, enhanced entry into cycle and terminal differentiation. To gain mechanistic insight into the observed phenotype, we isolated leukemic stem cells and performed whole genome expression analysis. The data showed that deficiency of NOX2 leads to downregulation of the cell cycle inhibitor CDKN2C (p18) and robust activation of the granulocyte fate determining transcription factor CEBPε. Thus we conclude that loss of NOX2 impacts leukemogenesis through rewiring of the cell cycle machinery and developmental programs in leukemic stem cells.

Finally, we found that in CD34+ primary human AML cells, NOX2 and the other subunits of the complex are abundantly expressed. Furthermore, pharmacologic inhibition of NOX2 with VAS2870, a selective NADPH oxidase inhibitor, reduced the level of ROS and limited the in vitro proliferation and survival of leukemic cells. Next we genetically suppressed the expression of NOX2 in primary human AML cells using sh-RNAs and transplanted these cells into immune compromised mice. Consistent with the murine leukemia, NOX2 knocked down AML cells failed to engraft and expand in vivo. Taken together, our results firmly establish a hitherto unrecognized, prominent regulatory role for NOX2 in the biology of normal and malignant hematopoietic stem cells and imply a potential therapeutic opportunity that can get exploited to treat AML.