Stromal cell–derived factor 1α increases polyploidization of megakaryocytes generated by human hematopoietic progenitor cells

The alpha chemokine receptor CXCR4 has been shown to be expressed on human hematopoietic progenitor cells and during the megakaryocytic differentiation pathway. Stromal cell–derived factor 1 (SDF-1) is the ligand for CXCR4. In this study, the role of SDF-1α in megakaryocytopoiesis was investigated. CD34+ progenitors purified from peripheral blood were grown in serum-free liquid suspension culture supplemented with thrombopoietin to obtain a virtually pure megakaryocytic progeny. In this condition, the addition of SDF-1α gives rise to megakaryocytes (MKs) showing an increased DNA content and a rise of lobated nuclei, as compared with untreated cells: at day 5, approximately 20% of the cells already showed the presence of more than one nuclear lobe versus fewer than 5% in the control cells; at day 12, approximately 85% of the cells were of large size and markedly polyploid, whereas approximately 60% of the control cells were polyploid, showed fewer lobes, and were a smaller size. This effect was dose-dependent and did not affect the megakaryocytic proliferation. Experiments with the mitogen-activated protein kinase (MAPK) inhibitor PD98059 suggested a role for MAPK pathway on SDF-1α–induced endomitosis. Furthermore, SDF-1α induced a significant increase in the number of proplatelet-bearing MKs and promoted the migration of megakaryocytic cells. Treatment with SDF-1α caused reduction in CXCR4 abundance on the plasma membrane, seemingly owing to receptor internalization. Furthermore, the presence of SDF-1α did not affect the expression of megakaryocytic markers, indicating that differentiation and polyploidization are independently regulated events.