title: Engagement of integrin alpha4beta1 enhances thrombopoietin-induced megakaryopoiesis.
abstract: OBJECTIVE
  Studies in numerous adherent cell systems have indicated that engagement of integrins is required for cell survival and proliferation. Although not classically thought of as an adherent cell type, megakaryocytes in the marrow develop in juxtaposition to endothelial cells which display a number of integrin counter-receptors. Moreover, a number of other hematopoietic cell types, including stem cells and erythroid progenitors, have been shown to engage and be affected by integrin ligands.
  
  
  METHODS
  The role of beta1 integrins in thrombopoietin-mediated megakaryopoiesis was studied using both gain-of-function and loss-of-function strategies.
  
  
  RESULTS
  We found that pan-blockade of integrins with a relatively nonspecific disintegrin blocked TPO-induced MK growth, but that an alpha5beta1 disintegrin, and a function-blocking monoclonal antibody, failed to affect megakaryopoiesis in vitro. In contrast, a neutralizing alpha4beta1 monoclonal antibody blocked TPO-induced MK growth, and an integrin alpha4beta1 ligand, the H296 fragment of fibronectin, enhanced MK growth at all concentrations of TPO.
  
  
  CONCLUSIONS
  These findings have important implications for thrombopoiesis in general, and potentially for the enhanced platelet production found in states of systemic inflammation and following the use of therapeutic strategies designed to block alpha4beta1 integrin engagement in states of chronic inflammation and autoimmunity.
authors.affiliation: Department of Medicine, University of California San Diego, 92103-8811, USA.
authors.forename: Norma E
authors.initials: NE
authors.lastname: Fox
authors.forename: Kenneth
authors.initials: K
authors.lastname: Kaushansky
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000378:metabolism / Q000494:pharmacology; D001854:Bone Marrow Cells / Q000166:cytology; D049109:Cell Proliferation / Q000187:drug effects; D019483:Disintegrins / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D042783:Endothelial Cells / Q000166:cytology; D005353:Fibronectins / Q000378:metabolism / Q000494:pharmacology; D039041:Integrin alpha4beta1 / Q000037:antagonists & inhibitors / Q000378:metabolism / Q000502:physiology*; D008533:Megakaryocytes / Q000166:cytology; D051379:Mice; D034061:Thrombopoiesis / Q000187:drug effects; D013926:Thrombopoietin / Q000494:pharmacology / Q000502:physiology*
chemical_list: D000911:Antibodies, Monoclonal; D019483:Disintegrins; D005353:Fibronectins; D039041:Integrin alpha4beta1; D013926:Thrombopoietin