Effects of p-selectin on hematopoietic cell proliferation, colony formation and adhesion to fibronectin

Abstract

P-selectin, inducibly expressed on platelets and endothelial cells, interacts with ligands on leukocytes and hematopoietic cells to mediate adhesion. Occupancy of the P-selectin ligand CD162 (PSGL-1) has recently been reported to activate β2 integrins in human neutrophils, and to reduce proliferation and induce apoptosis in a subset of human bone marrow CD34+ hematopoietic progenitor cells. Our laboratory has further characterized the effects of P-selectin on a variety of hematopoietic cells. Addition of soluble P-selectin to clonogenic assays in methylcellulose increased colony formation of HL-60 cells but not CD34+ cells from human bone marrow (mBM). In suspension culture, P-selectin suppressed proliferation of hBM and hCB cells, but not mBM or HL-60 cells. Of interest, P-selectin induced a rapid and specific increase in attachment of HL-60 and mBM cells but not hBM or hCB cells to immobilized fibronectin (FN). The contradictory effects of P-selectin on HL-60 and mBM versus hBM and hCB suggest that this molecule is capable of triggering both positive and negative effects, depending on its target cell. Further studies with HL-60 cells demonstrated that adhesion to FN was both time- and P-selectin-dose-dependent. P-selectin-induced adhesion of HL-60 to FN was blocked with specific mAbs against α5 or β1 integrin chains. Addition of P-selectin did not increase cell surface integrin expression or induce cell-cell aggregation, suggesting that occupancy of P-selectin ligand on HL-60 increases the binding affinity of α5/β1 integrin. These data illustrate that P-selectin binding to its ligand can activate α5/β1 integrin and imply that mBM hematopoietic cell responses to exogenous stimuli may not directly extrapolate to hBM cells.