Sustained PU.1 Levels Balance Cell-Cycle Regulators to Prevent Exhaustion of Adult Hematopoietic Stem Cells

Abstract

SUMMARY To provide a lifelong supply of blood cells, hematopoietic stem cells (HSCs) need to carefully balance both self-renewing cell divisions and quiescence. Although several regulators that control this mechanism have been identified, we demonstrate that the transcription factor PU.1 acts upstream of these regulators. So far, attempts to uncover PU.1’s role in HSC biology have failed because of the technical limitations of complete loss-of-function models. With the use of hypomorphic mice with decreased PU.1 levels specifically in phenotypic HSCs, we found reduced HSC long-term repopulation potential that could be rescued completely by restoring PU.1 levels. PU.1 prevented excessive HSC division and exhaustion by controlling the transcription of multiple cell-cycle regulators. Levels of PU.1 were sustained through autoregulatory PU.1 binding to an upstream enhancer that formed an active looped chromosome architecture in HSCs. These results establish that PU.1 mediates chromosome looping and functions as a master regulator of HSC proliferation.

Mechanisms of self-renewal in hematopoietic stem cells

“…These cells totally failed to reconstitute the bone marrow in the third transplantation. Chromatin Immunoprecipitation (ChIP) sequence data suggested that the reduction of PU.1 may stimulate active cycling, possibly resulting in the loss of HSCs [108].…”

The RUNX1–PU.1 axis in the control of hematopoiesis

“…Genome-wide studies have shown that hematopoietic genes such as PU.1 are actually primed in the HE by occupancy of TAL1/SCL and FLI1 to their regulatory regions and that a RUNX1-mediated reorganization of these factors is critical during EHT [44]. PU.1, on the other hand, contributes towards HSC maintenance by balancing the expression of cell-cycle regulators [45]. However, above all, PU.1 is a master regulator of later hematopoiesis and is required for commitment to the myeloid lineage since together with C/EBPα it controls the expression of the receptors for GM-CSF [46], and of CSF-1 which are critical cytokines for myelopoiesis [47,48].…”

“…Both RUNX1 and PU.1 contain distal regulatory elements harboring RUNX and ETS motifs [16,96], indicating that both genes are subject to autoregulation [44,45,89,97]. Using an inducible system, Lichtinger et al [44] showed that the induction of Runx1 leads to binding of RUNX1 to the proximal promoter and to a strong upregulation of its own expression.…”

PU.1 affects proliferation of the human acute myeloid leukemia U937 cell line by directly regulating MEIS1

“…Dysregulation of PU.1 leads to loss of lineage development and leukemia in vitro and in vivo (9,(26)(27)(28). Previous studies, however, have also demonstrated that PU.1 is required for the repopulation or self-renewal capacity of normal hematopoietic stem cells, and sustained PU.1 levels also balance cell cycle-associated regulators to prevent the exhaustion of adult HSC (8,15). These studies indicated that the presence of PU.1 activity may be required to favor the growth of myeloid leukemia stem cells.…”

“…However, PU.1 exerts various functions at distinct hematopoietic stages. Although PU.1 is expressed at low levels in the early stage of hematopoiesis, it has an indispensable function in the maintenance of the HSC pool (6,15). Loss of PU.1 expression in mice leads to a weakening in the self-renewal capacity of long-term HSCs, which are then outcompeted by normal HSCs in bone marrow (8).…”