Hematopoietic Stem and Progenitor Cells Manifest Direct Migration Towards Secreted DEK in a CXCR2- and G Protein Signaling-Dependent Manner

DEK, a nuclear DNA-binding protein implicated in the regulation of transcription, chromatin architecture, and mRNA processing, is secreted by macrophages and acts as a proinflammatory molecule (Mor-Vanknin et al., 2006, Mol. Cell. Bio., 26: 9484). Recombinant (r)DEK functions as a chemotactic factor attracting neutrophils, CD8+ T lymphocytes and natural killer cells. Few cytokines/growth modulating proteins are known to be chemoattractants for hematopoietic stem (HSC) and progenitor (HPC) cells; stromal cell-derived factor-1 (SDF-1/CXCL12) being the most potent known protein with this capability. To test whether rDEK can serve as a chemotactic agent, transwell assays were performed utilizing lineage negative mouse bone marrow (BM) cells with neutrophils (Ly6G+ cells) as a positive control. Both SDF-1 and DEK induced directed migration of Lin-Sca1+cKit+ (LSK) BM cells at a dose of 100ng/mL, as determined by flow cytometry of input and migrated cells, with no significant migration occurring towards 100ng/mL of IL-8 or MIP-2. All four cytokines induced migration of Ly6G+ neutrophils. After examining the ability of LSK cells to migrate towards various doses of rDEK (0-200ng/mL), it was determined that LSK cells can migrate towards rDEK in a dose dependent manner with maximum chemoattraction potential (~20%) occurring at a dose of DEK equal to or greater than 50ng/mL. A checkerboard assay using LSK cells was performed to determine whether rDEK acted more as a chemotactic (directed cell movement) or a chemokinetic (random migration) agent. Checkerboard analysis demonstrated that DEK acted as a chemotactic molecule. Upon our recent discovery and report that the DEK protein has a Glu-Leu-Arg (ELR) motif, similar to that of CXC chemokines such as IL-8 and binds to the chemokine receptor CXCR2 to regulate hematopoiesis (Capitano et al., 2019, J.C.I. 130: 2555-2570), we hypothesized that DEK may manifest its chemotactic actions through CXCR2, known previously to only bind and mediate the actions of the chemokines IL-8 and MIP-2. To examine this, we first confirmed expression of CXCR2 on the surface of HSC and HPC. Next, to determine if LSK migration towards DEK is dependent upon its ability to signal through CXCR2, LSK cells were pretreated with a neutralizing monoclonal antibody for CXCR2 immediately prior to being placed in a transwell chemotaxis assay utilizing 100ng/mL of rDEK in the bottom chamber. Neutralizing anti-CXCR2 antibodies inhibited migration of both LSK and Ly6G+ cells toward DEK; however, if LSK cells were pretreated with an isotype control or a neutralizing antibody towards CXCR4, migration towards DEK still occurred. To confirm that the neutralizing CXCR2 antibody did not inhibit migration in a non-specific manner, transwell assays were performed examining LSK cell migration towards SDF-1, IL-8, and MIP-2. LSK cells were still able to migrate towards SDF-1 except when CXCR4 was neutralized. No migration of LSK cells was observed when IL-8 or MIP-2 was utilized. When Ly6G+ neutrophils were used, CXCR2 neutralizing antibodies blocked migration of the Ly6G+ neutrophils towards DEK, IL-8 and MIP-2. Neutralizing CXCR4 only blocked Ly6G+ neutrophil migration towards SDF-1. CXCR2 is a G protein-coupled receptor and this interaction can be blocked using pertussis toxin which prevents G proteins from interacting with G protein-coupled receptors thus interfering with receptor signaling. Pretreatment of LSK cells with pertussis toxin significantly inhibited the migration of LSK cells towards DEK and SDF-1. To determine if DEK and SDF-1 could inhibit one another, checkboard assays were performed where either different concentrations of DEK was used in the top well and different concentrations of SDF-1 was used on the bottom. Starting at 100ng/mL, DEK in the top well inhibited LSK cell migration towards SDF-1. However, when SDF-1 was in the top well, regardless of the SDF-1 dose, SDF-1 always inhibited LSK cell migration towards DEK. These data suggest that DEK acts as a chemotactic agent for HSC and HPC in vitro but is not as strong of a signaling protein for migration when competing against SDF-1. Thus, DEK may be involved as a compensatory chemotactic agent for HSCs and HPCs, especially under certain inflammatory conditions and when SDF-1 signaling is reduced.