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Review
. 2013 Aug;169(8):1652-71.
doi: 10.1111/bph.12253.

To breathe or not to breathe: the haematopoietic stem/progenitor cells dilemma

Affiliations
Review

To breathe or not to breathe: the haematopoietic stem/progenitor cells dilemma

C Piccoli et al. Br J Pharmacol. 2013 Aug.

Abstract

Adult haematopoietic stem/progenitor cells (HSPCs) constitute the lifespan reserve for the generation of all the cellular lineages in the blood. Although massive progress in identifying the cluster of master genes controlling self-renewal and multipotency has been achieved in the past decade, some aspects of the physiology of HSPCs still need to be clarified. In particular, there is growing interest in the metabolic profile of HSPCs in view of their emerging role as determinants of cell fate. Indeed, stem cells and progenitors have distinct metabolic profiles, and the transition from stem to progenitor cell corresponds to a critical metabolic change, from glycolysis to oxidative phosphorylation. In this review, we summarize evidence, reported in the literature and provided by our group, highlighting the peculiar ability of HSPCs to adapt their mitochondrial oxidative/bioenergetic metabolism to survive in the hypoxic microenvironment of the endoblastic niche and to exploit redox signalling in controlling the balance between quiescence versus active cycling and differentiation. Especial prominence is given to the interplay between hypoxia inducible factor-1, globins and NADPH oxidases in managing the mitochondrial dioxygen-related metabolism and biogenesis in HSPCs under different ambient conditions. A mechanistic model is proposed whereby 'mitochondrial differentiation' is a prerequisite in uncommitted stem cells, paving the way for growth/differentiation factor-dependent processes. Advancing the understanding of stem cell metabolism will, hopefully, help to (i) improve efforts to maintain, expand and manipulate HSPCs ex vivo and realize their potential therapeutic benefits in regenerative medicine; (ii) reprogramme somatic cells to generate stem cells; and (iii) eliminate, selectively, malignant stem cells.

Linked articles: This article is part of a themed section on Emerging Therapeutic Aspects in Oncology. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.169.issue-8.

Keywords: NADPH oxidases; globins; haematopoietic stem/progenitor cells; hypoxia inducible factor-1; mitochondria; oxidative phosphorylation; redox signalling.

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Figures

Figure 1
Figure 1
Redox signalling governing haematopoietic stem cell fate. Bone marrow stem cell endoblastic and vascular niches are schematically drawn on the left and bottom sides of the diagram respectively. The oxygen gradient, from the endoblastic niche to the sinusoid vessel, is also shown with the approximate values of concentration given in μM and % pO2 units (for conversion in other units, consider that 1.0 μM O2 = 0.77 mmHg = 5.77 kPa at 37°C). A quiescent long-term haematopoietic stem cell (LT-HSC) is elicited by niche-related factors to undergo asymmetric division. One of the two daughter cells remains in the niche, whereas the other short-term (ST)-HSC moves away from the hypoxic area and experiences a progressive increase in O2 concentration. The ST-HSC proliferate and convert to multipotent progenitor (MPP) cells that finally are committed to one or other of the fully differentiated blood cells (DBC). The progressive loss of the stemness profile is accompanied by a metabolic shift from glycolysis to oxidative phosphorylation. The two pictures on the right-hand side are confocal images of cells co-stained for the stemness marker CD34 and the mitochondrial compartment showing an increase in mitochondrial mass in the CD34low-HSPC. The increased number of functional mitochondria in the pre-commitment stage of the ST-HSC/MPP provides the cell with the energy-generating systems needed to cope with a proliferating/differentiating metabolism. On the left-hand side of the picture: schematic depictions of the main signalling pathways reported to control the quiescence/maintenance of the HSCs or to induce their proliferation/differentiation (the prime factors of the pathways are highlighted in blue and red respectively). All the pathways are linked directly or indirectly to the regulation of the cellular redox state with the low and high reactive oxygen species (ROSLOW and ROSHIGH) level priming the HSC maintenance and commitment respectively. See text for definitions of the abbreviations.
Figure 2
Figure 2
Schematic view of the suggested functional role of globins under hypoxic and normoxic conditions in quiescent and early committed HSPCs. The pictures on the left are artificial colour images of CD34++ (A) and CD34+ (B) in human HSPCs immunostained with a myoglobin (Mb)-Ab. The Mb-related fluorescence signal is shown in red and the nuclear compartment outlined with a dashed black line. The schemes on the right illustrate the suggested functions of Mb and neuroglobin (Ngb) in the context of hypoxic (A) and normoxic (B) conditions in early and late HSPCs respectively. The prevailing de-oxygenated and oxygenated state of the globins is highlighted with a different colour tone in (A) and (B). It is shown that under hypoxia (A) the de-oxygenated form of Mb/Ngb function as a nitrite reductase generating NO. This inhibits the mitochondrial respiratory chain (RC) and the oxidative phosphorylation promoting electron leak to O2 with formation of ROS, which, along with NO, inhibit the prolyl-hydroxylases (PHDs) thereby stabilizing the hypoxia inducible factors (HIF-1/2α). The transcriptional activity of HIF induces the expression of a set of pro-survival genes. The nuclear localization of Mb under this condition prevents ROS from scavenging the redox-linked activation of NFs/genes involved in the proliferation/differentiation of HSPCs or oxidative damage of DNA. Under normoxic conditions (B) the oxygenated form of the globins converts NO in nitrate unleashing the NO inhibition of the RC. This restores mitochondrial O2 consumption and ATP production by oxidative phosphorylation. Under these conditions the globins might favour the delivery of O2 and/or fatty acids (FA) to the mitochondria. It is further shown that the normoxic condition promotes the activity of the NADPH oxidases (NOXs) with the formation of ROS. The ROS level is suggested to be eventually increased by means of a ROS-induced ROS release (RIRR) mechanism involving the mitochondrial RC. The consequent enhanced pro-oxidative state of the cell functions as a signal inducing the HSPC to differentiate. The protective role of the nuclear Mb under this condition is removed by its cytoplasmic re-localization. Depending on the prevailing conditions, set by extrinsic signals, ROS can also activate under normoxic conditions HIF thereby delaying differentiation of HSPCs if these are mobilized to restore exhausted niches or act at hypoxia-injured tissues. See text for further considerations.

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