Erythropoietin (EPO) and thrombopoietin (c-MPL ligand; TPO) are structurally similar cytokines and support respectively, the proliferation and differentiation for erythroid and megakaryocytic lineages, as well as more primitive progenitors. We studied the effect of these cytokines on the induction of adhesion of human growth-factor-dependent hematopoietic cells to immobilized fibronectin, which is a main component of the extracellular matrix in the bone marrow. MO7ER cells that are genetically engineered to express human EPO receptor and MO7e cells that express endogenous c-MPL were used. Stimulation with either TPO or EPO induced rapid increases in adhesion of MO7ER cells to fibronectin without apparent change of expression of integrins. Experiments with inhibitory monoclonal antibodies (mAbs) demonstrated that CD41, which has been reported to be involved in TPO-induced adhesion of megakaryocytic cells, is not responsible for this enhanced adhesion. Anti-β1 integrin mAb inhibited adhesion completely, while inhibition by anti-α4 integrin mAb and anti-α5 integrin mAb was partial. Combination of anti-α4 mAb plus anti-α5 mAb completely abolished adhesion, as did anti-β1 mAb, suggesting that the adhesion is mediated by both α4β1 and α5β1 integrins. Experiments using inhibitors suggested that ligand binding followed by activation of intracellular tyrosine kinases along with PI3-kinase activation is required. After stimulation of MO7ER cells with either TPO or EPO, fibronectin-attached cells, but not cells in suspension, showed tyrosine phosphorylation of focal adhesion kinase, which plays a central role in integrin-mediated signaling. These data suggest that TPO and EPO might be involved in homing/migration to the bone marrow microenvironment by hematopoietic cells that express corresponding receptors.
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