Stromal-derived factor 1-induced megakaryocyte migration and platelet production is dependent on matrix metalloproteinases

Despite the discovery of thrombopoietin (TPO) and its contribution to megak aryocytopoiesis, the exact mechanisms and sites of platelet production are unknown. It has been shown that mature megakaryocytes (MKs) functionally ex press the stromal-derived factor 1 (SDF-1) receptor, CXCR4. SDF-1-induced m igration of mature MKs through endothelial cell layers results in increased platelet production. Because the migration of polyploid MKs from the bone marrow microenvironment requires remodeling of the perivascular extracellul ar matrix, it was hypothesized that mature polyploid MKs may express matrix metalloproteinases (MMPs), facilitating their exit into the bone marrow ex travascular space. In this report, it is demonstrated that SDF-1 induces th e expression and release of gelatinase B (MMP-9) by purified mature polyplo id human MKs and an adeno-CXCR4-infected megakaryocytic cell line. Neutrali zing antibody to MMP-9, but not MMP-2, blocked SDF-1-induced migration of M Ks through reconstituted basement membrane, suggesting that expression of M MP-9 is critical for MK migration. Incubation of mature MKs with a syntheti c MMP inhibitor, 5-phenyl-1,10-phenanthrolene, resulted in the inhibition o f platelet formation, suggesting that the expression of MMPs is not only cr itical for megakaryocyte migration but also for subsequent platelet release . Confirming these results, adeno-SDF-1 injection into normal mice resulted in increased platelet counts, a process that could be blocked by a synthet ic MMP inhibitor. These results suggest mobilization of MKs involves sequen tial expression and activation of chemokine receptors such as CXCR4, MMP-9, followed by transendothelial migration, MMP inhibitors may have potential use in the treatment of thrombotic and myeloproliferative disorders. (C) 20 00 by The American Society of Hematology.