Heme Oxygenase 1 (HO-1) Is a Novel Negative Regulator of Normal and Malignant Hematopoietic Cell Trafficking

Background. Cell migration is a crucial process regulating the homing and mobilization of hematopoietic stem/progenitor cells (HSPCs), the trafficking of immune cells, and the metastatic spread of leukemic cells. Several factors have been described that promote cell migration, but very little is known about how this process is negatively controlled. Since inflammation triggers the release of several factors (including chemokines, bioactive lipids, extracellular nucleotides, and complement cascade cleavage fragments) that enhance chemotaxis or chemokinesis of normal and malignant hematopoietic cells, we became interested in the physiological mechanisms that limit these pro-migratory effects. Heme oxygenase 1 (HO-1) is an inducible enzyme that is upregulated in response to inflammation and tissue injury. It degrades extracellular as well as intracellular heme and is a known negative regulator of inflammation.

Hypothesis. We hypothesized that one of the anti-inflammatory effects of HO-1 is negative regulation of cell trafficking, and HO-1 could negatively affect both mobilization and homing of normal hematopoietic stem/progenitor cells (HSPCs) and the spread of malignant cells.

Materials and Methods. To address this question, we performed several complementary experiments to evaluate the role of HO-1 in hematopoietic cell trafficking. First, we evaluated the mobilization of normal HSPCs in HO-1-deficient (HO-1^-/-) mice and studied the responsiveness of hematopoietic cells from these mice to major HSPC chemoattractants (SDF-1, S1P, C1P, and ATP). Next, we downregulated or upregulated expression of HO-1 in established human hematopoietic cell lines (K-562, Raji, Jurkat, Nalm6) and studied the effect of changes in HO-1 expression on the migration of these cells. Finally, in in vitro and in vivo experimental models, we employed small-molecule activators and inhibitors of HO-1 and modulated the expression of p38 MAPK, which inhibits HO-1 expression in normal and leukemic cells.

Results. In all experimental strategies employed, genetic deficiency or downregulation of HO-1 activity by shRNA or small-molecule inhibitors correlated with enhanced motility of hematopoietic cells. HSPCs after exposure to small-molecule HO-1 inhibitors homed and engrafted better after transplantation into normal animals and HO-1-deficient mice were found to be easy mobilizers. By contrast, upregulation of HO-1 in hematopoietic cell lines by HO-1-overexpressing vectors or small-molecule activators or inhibiting p38 MAPK activity resulted in decreased cell migration. Accordingly, overexpression of HO-1 in leukemic cells before injection into immune-deficient mice decreased their seeding efficiency and spread into BM and other organs.

Conclusions. Our studies demonstrate for the first time the pivotal role of HO-1 in regulating the trafficking of hematopoietic cells. These results are significant for developing more efficient mobilization and homing strategies for normal HSPCs and for controlling the migration and spread of leukemic cells. Since small-molecule modifiers of HO-1 activity are available to be employed in patients, these observations open up new therapeutic possibilities.