As a crucial arm of innate immunity, the complement cascade (ComC) is involved both in mobilization of normal hematopoietic stem/progenitor cells (HSPCs) from bone marrow (BM) into peripheral blood (PB) and in their homing to BM. Despite the fact that ComC cleavage fragments alone do not chemoattract normal HSPCs, we found that leukemia cell lines as well as clonogenic blasts from CML and AML patients respond robustly to C3 and C5 cleavage fragments by chemotaxis and increased adhesion. This finding was supported by the detection of C3a and C5a receptors in cells from human malignant hematopoietic cell lines and patient blasts at the mRNA (RT-PCR) and protein level (FACS) and by the demonstration that these receptors respond to stimulation by C3a and C5a by phosphorylation of p42/44 and p38 MAPK and AKT. We also found that inducible heme oxygenase 1 (HO-1) is a negative regulator of ComC-mediated trafficking of leukemic cells and that stimulation of leukemic cells by C3 or C5 cleavage fragments activates p38 MAPK, which downregulates HO-1 expression, rendering cells more mobile. We conclude that activation of the ComC in leukemia/lymphoma patients (e.g., as a result of accompanying infections) enhances the motility of malignant cells and contributes to their spread in a p38 MAPK–HO-1-dependent manner. Therefore, inhibition of p38 MAPK or upregulation of HO-1 by small-molecule modulators would have a beneficial effect on ameliorating expansion of leukemia/lymphoma cells when the ComC becomes activated.

Bioactive phospholipids, including sphingosine-1-phosphate (S1P), ceramide-1-phosphate (C1P), lysophosphatidylcholine (LPC), and its derivative lysophosphatidic acid (LPA), have emerged as important mediators regulating the trafficking of normal and cancer cells. While the role of S1P in regulating migration of hematopoietic cells is well established, in this work we compared its biological effects to the effects of C1P, LPC, and LPA. We employed ten human myeloid and lymphoid cell lines as well as blasts from AML patients. We observed that human leukemic cells express functional receptors for phospholipids and respond to stimulation by phosphorylation of p42/44 MAPK and AKT. We also found that bioactive phospholipids enhanced cell migration and adhesion of leukemic cells by downregulating expression of HO-1 and iNOS in a p38 MAPK-dependent manner but did not affect cell proliferation. By contrast, downregulation of p38 MAPK by SB203580 enhanced expression of HO-1 and iNOS and decreased migration of leukemic cells in vitro and their seeding efficiency to vital organs in vivo after injection into immunodeficient mice. Based on these findings, we demonstrate that, besides S1P, human leukemic cells also respond to C1P, LPC, and LPA. Since the prometastatic effects of bioactive phospholipids in vivo were mediated, at least in part, by downregulating HO-1 and iNOS expression in a p38 MAPK-dependent manner, we propose that inhibitors of p38 MAPK or stimulators of HO-1 activity will find application in inhibiting the spread of leukemic cells in response to bioactive phospholipids.