A Novel Observation That Heme Oxygenase-1 (HO-1) Deficient Mice Are Easy Mobilizers and That HO-1 Plays An Important Role in Maintaining Expression of SDF-1 in Bone Marrow (BM) Stroma and Promotes Retention of Hematopoietic Stem/Progenitor Cells (HSPCs) in the Bone Marrow Microenvironment

Abstract 315

Background. Heme oxygenase (HO) is an enzyme that catalyzes the degradation of heme. Two distinct HO isoforms have been identified: HO-2, which is constitutively expressed, and HO-1, which is stress-responsive and plays an important function in various physiological and pathophysiological states associated with cellular stress. HO-1 plays a role in ischemic/reperfusion injury, atherosclerosis, and cancer. It has also been reported that HO-1 regulates expression of a-chemokine stromal derived factor-1 (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 hematopoietic stem cell/progenitor cells (HSPCs) in BM, we become interested in 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^+/–, 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) chemotactic responsiveness of HSPCs to an SDF-1 gradient, iv) adhesiveness of clonogenic progenitors, 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. Results: Our data indicate that under normal, steady-state conditions, HO-1^−/− and HO^+/– mice have normal peripheral blood cell counts and numbers of circulating CFU-GM. Interestingly, lack of HO-1 leads to an increase in the number of erythroid (BFU-E) and megakaryocytic (CFU-GM) progenitors in BM. Next, BMMNCs from HO-1^−/−have normal expression of the SDF-1-binding receptor, CXCR4, but a 5-times lower level of CXCR7, which is another SDF-1-binding receptor. Of note, 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 responded more robustly to an SDF-1 gradient, and HO-1^−/−animals mobilized a higher number of CD34^–SKL cells and CFU-GM progenitors into peripheral blood in response to G-CSF and AMD3100. Conclusions: Our data demonstrate for the first time that heme oxygenase plays an important and underappreciated role in BM retention of HSPCs and may affect their trafficking. Since small non-toxic molecular inhibitors of HO-1 have been developed for clinical use (e.g., metaloporhirins), blockage of HO-1 could be a novel strategy for mobilizing HSPCs. Our recent in vivo mobilization studies lend support to this hypothesis.