The NFkB System Regulates Flt3-Mediated Hematopoiesis

FMS-like tyrosine kinase 3 (Flt3) plays pivotal roles in the survival, proliferation and differentiation of hematopoietic stem/progenitor cells (HSPCs). However, how proliferation and differentiation via Flt3 is regulated is poorly understood. In this study, our bioinformatic meta-analysis of existing cancer microarray datasets in Oncomine correlated reduced nfkb1 expression with acute myeloid leukemia (AML). The nfkb1 gene products p105 and p50 are not transcriptional activators but regulators of NFκB activators. Examining bone marrow HSPCs, we found that nfkb1^-/- mice produce much higher (~5-8X) total cell yields compared to wild type (WT) when cultured with Flt3 Ligand (Flt3L) ex vivo. Cell cycle analysis and apoptosis assay revealed that nfkb1^-/- cells have profoundly increased proliferation and enhanced survival compared to WT cells. Nfkb1^-/- cells show skewed differentiation towards plasmacytoid DCs and delayed differentiation of conventional DCs. To examine the molecular mechanism, we investigated how the NFκB system is assembled as HSPCs, which have little NFκB, progress through hematopoiesis to become differentiated immune cells with robust NFκB systems. In nfkb1^-/- cells, the distribution of NFκB signaling complexes was altered and resulted in overall elevated RelA expression. Reducing RelA expression in compound rela^+/- nfkb1^-/- HSPCs partially rescued the myelopoiesis phenotype, suggesting that hyper NFκB activity is the cause. Further in vivo studies are underway to further test NFκB's role in a Flt3L injection model and a Flt3-ITD-mediated mouse leukemia model. In conclusion, our work reveals that precisely timed control of NFκB system assembly is a hallmark of hematopoiesis and that its misregulation perturbs normal Flt3-mediated hematopoiesis and can be a cause of myeloid cancer.