Unique effects of Stat3 on the early phase of hematopoietic stem cell regeneration

Abstract

IntroductionHematopoietic stem cells (HSCs) constitute a rare subpopulation in hematopoietic tissues and are uniquely defined by their ability to divide many times with full retention of their pluripotentiality. 1 This self-renewal function underlies the ability of HSCs to produce new blood cells throughout the life of the individual. It also allows them to regenerate the entire blood-forming system in vivo, including the HSC compartment, following their transplantation into irradiated recipients. 2,3 In mice, the ability of individual HSCs to permanently reconstitute all blood-cell lineages provides a functional end point for the specific quantification of HSCs in limiting-dilution transplantation assays. 4 The cells thus defined are operationally referred to as competitive repopulating units (CRUs), since these cells must compete with other HSCs whose presence is required for the survival of the host mice.Expansion of the HSC (CRU) population normally occurs in vivo during fetal and early adulthood. 5 Thereafter, the number of these cells remains stable with self-renewal divisions exactly balancing HSC losses by differentiation or death. However, a rapid and extensive net expansion of HSCs can be reinitiated in vivo following damage to the hematopoietic system. 6 Accumulating evidence points to the involvement of both extrinsic cues (eg, Steel factor, factors that activate gp130, 7 notch ligands, 8 and Wnt 9 ) as well as intrinsic regulators (eg, p21 cip1/waf1 , 10 Bmi-1, 11,12 and HoxB4 13-15 ) in controlling HSC self-renewal responses following their mitogenic activation in vitro or in vivo. Relatively little, however, is known about the signaling intermediates that direct or can modulate changes in HSC self-renewal when these cells are stimulated to proliferate in vivo.Signal transducer and activator of transcription 3 (STAT3) is an intermediate signaling molecule that allows ligand-induced signals from gp130 and the granulocyte colony-stimulating factor (G-CSF) receptor, and from various endogenous nonreceptor kinases such as Src, 16,17 to transactivate specific target genes. A key event in this process is the induced phosphorylation of the Tyr 705 residue in the Src-homology 2 (SH2) domain of STAT3, which then leads to the formation of STAT3 dimers and their translocation into the nucleus. Distinct biologic roles of STAT3 in various types of tissues including hematopoietic cells have been described in several studies. 18 In hematopoietic tissue, we showed that the forced expression of a dominant-negative (dn) form of Stat3 in fetal liver cells markedly reduced the competitive repopulating activity of the transduced HSCs in irradiated recipients of cotransplanted normal adult bone marrow cells. However, neither the proliferation nor the differentiation of slightly more mature progenitors in the original suspension of dnSTAT3-transduced fetal liver cells (detected in vivo as colony-forming units-spleen, CFU-Ss, or in vitro as colony-forming cells, CFCs) was affected. 19 Subsequently, it was found that induc...

Disruption of Bis Leads to the Deterioration of the Vascular Niche for Hematopoietic Stem Cells

“…Repopulation of transplanted cells in the recipients was assessed by flow cytometry to measure the proportion of leukocytes expressing the donor-origin (Ly5.2) surface antigen in their blood or bone marrow. The lineages of repopulated hematopoietic cells were analyzed by staining with anti-Mac-1 (BD Pharmingen, San Diego), anti-Gr-1 antibody (BD Pharmingen) for myeloid cells, and anti-B220 antibodies (BD Pharmingen) for lymphoid engraftment, as described previously [26]. For quantitative measurements of HSC numbers, competitive repopulating unit (CRU) assays were performed as previously described [27].…”

Steel factor coordinately regulates the molecular signature and biologic function of hematopoietic stem cells

“…These include regulators of the cell cycle, such as Cdkn1a (p21) 13 and Cdkn2c (p18) 14 and members of the transcriptional control machinery, such as HoxA9, 15 HoxB4, 16 Phc1 (Rae28), 17 Bmi1, 18 Ezh2, 19 Gfi1, 20 Gata2 21 and Gata3, 22 Etv6, 23 Pcgf2 (Mel18), 24 Mcl1, 25 Meis1, 15 and Runx1. 21 In addition, growth factor receptors, such as GP130 26 and KIT, 27 and signaling factors, such as STAT3, 28 STAT5, 29 PTEN, 30 and LNK, 11 have been shown to affect the ability of HSCs to execute self-renewal divisions. These findings reinforce data suggesting that multiple external cues, including Steel factor (SF), 31 thrombopoietin, 32,33 transforming growth factor-␤, 34 flt3-ligand, 35 and factors, such as interleukin-6 (IL-6) and IL-11, 36,37 which signal through GP130, can modulate HSC self-renewal, proliferation, and viability independently.…”

Hematopoietic stem cells: generation and self-renewal

“…Gain and loss of function studies have shown that both Stat5 and Stat3 are positive regulators of HSC self-renewal. 63,64 In addition to transcription factors, other proteins involved in the modulation of gene expression have been found to regulate self-renewal of HSCs. Bmi-1 is a polycomb group (PcG) protein that forms the Polycomb repression complex 1 (PRC1)with other members of the PcG family, such as 65 The mechanisms whereby Bmi-1 and its protein complex maintains HSC self-renewal are unclear, but it may be attributed to the repression of p16 and p19, and/or repression of differentiation-related gene expression programs.…”