The role of Forkhead transcription factors in hematopoiesis and leukemogenesis

Hematopoietic development and long-term homeostasis are tightly regulated by a dynamic balance of stem cell self-renewal and differentiation. The Forkhead O (FoxO) family of transcription factors plays an important role in diverse physiologic processes, including induction of cell cycle arrest, stress resistance, differentiation and apoptosis, but their role in normal hematopoietic development and leukemia is unknown. To advance our understanding of FoxO in leukemia and lymphoma, we examined the role of FoxO3 in survival and proliferative signaling by the fusion protein nucleophosmin-anaplastic lymphoma kinase (NPM-ALK) and demonstrated its important contribution to transformation in vitro. To better understand the role of the FoxO family in vivo, we studied the impact of somatic deletion of all FoxO genes in the adult hematopoietic system of mice engineered with the interferon-inducible Mx-Cre transgene and various conditional FoxO alleles. Cre-mediated excision of FoxO1, FoxO3 and FoxO4 resulted in myeloid lineage expansion and lymphoid developmental abnormalities, which were associated with development of a subtle myeloproliferative phenotype and thymic lymphomas. In addition, we noted a marked decrease in the lineage-negative, Sca-1 + , c-Kit + (LSK) compartment that contains the short and long-term hematopoietic stem cell (HSC) populations, but normal numbers of myeloid progenitors. Furthermore, FoxO -deficient bone marrow had defective long-term repopulating ability that correlated with increased exit out of quiescence, cell cycling and apoptosis of HSC cells. Notably, there was a marked context-dependent increase in reactive oxygen species (ROS) in FoxO -deficient HSC compared with wildtype HSC that correlated with changes in expression of genes that regulate ROS, and that was not observed in committed progenitors. In addition, gene set enrichment analysis supported the existence of a developmentally regulated program that is engaged with the transition from HSC to myeloid progenitors and independent of FoxO. Lastly, in vivo treatment with the anti-oxidative agent N-acetyl-L-cysteine resulted in reversion of the FoxO -deficient HSC phenotype. Taken together, these results demonstrate that in the hematopoietic compartment, FoxO act as both tumor suppressors and key regulators of HSC response to physiologic oxidative stress, and thereby mediate quiescence and enhanced survival of the HSC compartment, a function that is required for its long-term regenerative potential.