Further Evidence That HO-1 Regulates SDF-1 Expression in the Bone Marrow Microenvironment and That HO-1-Deficient Mice Show a Defect in the SDF-1–CXCR4 Retention Axis of Hematopoietic Stem/Progenitor Cells in Bone Marrow and Thus Are Easy Mobilizers - Studies in HO-1 Mutant Mice, Irradiation Chimeras, and the Effect of in Vivo Pharmacological Inhibition of HO-1
Authors: Wysoczynski Marcin , Ratajczak Janina , Rokosh Gregg , Bolli Roberto , Ratajczak Mariusz Z 
Published: October 2019

Abstract
Abstract 344

Background:
Stromal derived factor-1 (SDF-1), which binds to the CXCR4 receptor expressed on the surface of hematopoietic stem/progenitor cells (HSPCs), plays an important role in the retention of HSPCs in BM niches. Heme oxygenase (HO-1) is a stress-responsive enzyme that catalyzes the degradation of heme and plays an important function in various physiological and pathophysiological states associated with cellular stress, such as ischemic/reperfusion injury, atherosclerosis, and cancer. Interestingly, it has also been reported that HO-1 regulates the expression of SDF-1 in myocardium (J Mol Cell Cardiol. 2008;45:44–55).

Aim of study:
Since SDF-1 plays a crucial role in retention and survival of HSPCs in BM, we become interested in whether HO-1 is expressed by BM stromal cells and whether deficiency of HO-1 affects normal hematopoiesis and retention of HSPCs in BM.

Experimental approach:
To address this issue, we employed several complementary strategies to investigate HO-1–/–, HO-1+/–, and wild type (wt) mouse littermates for i) the expression level of SDF-1 in BM, ii) the number of clonogenic progenitors from major hematopoietic lineages in BM, iii) peripheral blood (PB) cell counts, iv) the chemotactic responsiveness of HSPCs to an SDF-1 gradient as well as to other chemoattractants, including sphingosine-1-phosphate (S1P), ceramide-1-phosphate (C1P), and extracellular nucleotiodes (ATP, UTP), iv) the adhesiveness of clonogenic progenitors to immobilized SDF-1 and stroma, v) the number of circulating HSPCs in PB, and vi) the degree of mobilization in response to granulocyte-colony stimulating factor (G-CSF) or AMD3100, assessed by enumerating the number of CD34–SKL cells and clonogeneic progenitors (CFU-GM) circulating in PB. We also exposed mice to the small HO-1 molecular inhibitor tin protoporphyrin IX (SnPP) and studied the effect of this treatment on G-CSF- or AMD3100-induced mobilization of HSPCs. Finally, to prove an environmental HSPC retention defect in HO-1-deficient mice, we created radiation chimeras, wild type mice transplanted with HO-1-deficient BM cells, and, vice versa, HO-1-deficient mice reconstituted with wild type BM cells.

Results:
Our data indicate that under normal, steady-state conditions, HO-1–/– and HO+/– mice have normal PB cell counts and numbers of circulating CFU-GM, while a lack of HO-1 leads to an increase in the number of erythroid (BFU-E) and megakaryocytic (CFU-GM) progenitors in BM. However, while BMMNCs from HO-1–/– have normal expression of the SDF-1-binding receptor, CXCR4, we observed that the mRNA level for SDF-1 in BM-derived fibroblasts was ∼4 times lower. This corresponded with the observation in vitro that HSPCs from HO-1–/– animals respond more robustly to an SDF-1 gradient, and HO-1–/– animals mobilized a higher number of CD34–SKL cells and CFU-GM progenitors into PB in response to G-CSF and AMD3100. Both G-CSF and AMD3100 mobilization were also significantly enhanced in normal wild type mice after in vivo administration of HO-1 inhibitor. Finally, mobilization studies in irradiation chimeras confirmed the crucial role of the microenvironmental SDF-1-based retention mechanism of HSPCs in BM niches.

Conclusions:
Our data demonstrate for the first time that HO-1 plays an important and underappreciated role in modulating the SDF-1 level in the BM microenvironment and thus plays a role in retention of HSPCs in BM niches. Furthermore, our recent data showing a mobilization effect by a small non-toxic molecular inhibitor of HO-1 (SnPP), suggest that blockage of HO-1 could be a promising strategy to facilitate mobilization of HSPCs. Further studies are also needed to evaluate the role of HO-1 in homing of HSPCs after transplantation to BM stem cell niches.

Disclosures:
No relevant conflicts of interest to declare.