Cell-Stage Dependent Outcomes of the NAMPT/SIRT2-Mediated Activation of LMO2 By Deacetylation in Normal Hematopoiesis and in T-ALL

NAMPT (nicotinamide phosphoribosyltransferase) regulates cellular functions through the protein deacetylation activity of NAD⁺-dependent sirtuins (SIRT). Here we demonstrated that NAMPT is indispensable for hematopoiesis and leukemogenesis through SIRT2-mediated deacetylation and activation of the hematopoietic transcription factor, LMO2 (LIM domain only 2). LMO2 is essential for the blood cell formation through the interaction with LIM Domain Binding 1 (LDB1) protein and activation of the TAL1 transcriptional complex. We found that NAMPT/SIRT2-mediated deacetylation of LMO2 on two evolutionary highly conserved lysines (K74 and K78) enables its interaction with LDB1 protein, activation of the TAL1 complex target genes (e.g. GATA1, KDR, KLF1) and hematopoietic differentiation. NAMPT- or SIRT2 inhibition abrogated LMO2 deacetylation and early hematopoietic differentiation of the induced pluripotent stem cells (iPSCs) in vitro as well as blood cell formation in zebrafish embryos in vivo on 24 hours post fertilization. Intriguingly, transfection of NAMPT- compromised zebrafish embryos with the deacetylation-mimic LMO2 mutant restore defective hematopoiesis, demonstrating the essential role of LMO2 deacetylation on K74 and K78 in NAMPT-mediated early hematopoiesis. Intriguingly, LMO2 is also known as an oncogene, which hyper-activation transforms lymphoid progenitor cells into T-cell acute lymphoblastic leukemia (T-ALL) blasts. We found, that NAMPT- or SIRT2 inhibition markedly suppressed proliferation of LMO2-expressing T-ALL cells in vitro and in zebrafish xenograft model in vivo . It was in line with impaired LMO2:LDB1 interaction and downregulation of T-ALL-specific LMO2 target genes such as HHEX.

Taken together, we identified a new molecular mechanism of LMO2 activation by NAMPT/SIRT2-mediated deacetylation, which may explain the pathogenesis of hematopoietic diseases, involving disturbed TAL1 complex functions. Moreover, we found, that targeting of LMO2 deacetylation by NAMPT/SIRT2 inhibition may be a potential novel therapy for LMO2-related T-ALL.