Thrombopoietin promotes beta1-integrin-mediated adhesion in hematopoietic cells via the small GTPase Rapl.
OBJECTIVE
The interactions between cells and extracellular matrices in the bone marrow microenvironment are critical for normal hematopoiesis, controlling cell survival, proliferation, differentiation, and motility. A number of hematopoietic growth factors and cytokines can mediate these interactions by changing expression and/or activity of specific integrins, or by changing cell shape. Thrombopoietin (TPO) has previously been shown to stimulate adhesion. in certain hematopoietic cell types, although the exact mechanisms by which adhesion is promoted remain elusive.


METHODS
The role of TPO in hematopoietic cell adhesion was determined with fibronectin adhesion and binding assays, flow cytometry, and immunocytochemistry using the hematopoietic cell line UT-7/TPO and bone marrow-derived primary mouse megakaryocytes. The role of Rapl in TPO-mediated adhesion was determined using a RaplGAP overexpressing UT-7/TPO cell line, in which Rapl could not be activated.


RESULTS
We found that TPO promoted hematopoietic cell adhesion by causing cytoskeletal reorganization and not by increasing integrin expression, localization, or affinity, as previously hypothesized. Through studies using the UT-7/TPO-RaplGAP cell line, we found that TPO-mediated cell shape change occurred via activation of Rapl.


CONCLUSIONS
These data demonstrate an important role for TPO in mediating interactions in the bone marrow microenvironment and make a significant contribution to our understanding of how TPO may affect hematopoiesis.
Hitchcock
Ian S
IS
Department of Medicine, University of California San Diego, La Jolla, CA, USA. ihitchcock@ucsd.edu
Kaushansky
Kenneth
K
D000818:Animals; D002448:Cell Adhesion / Q000187:drug effects / Q000502:physiology; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D005353:Fibronectins / Q000378:metabolism; D006412:Hematopoietic Stem Cells / Q000187:drug effects / Q000502:physiology* / Q000648:ultrastructure; D006801:Humans; D019012:Integrin beta1 / Q000187:drug effects / Q000502:physiology*; D008533:Megakaryocytes / Q000187:drug effects / Q000502:physiology / Q000648:ultrastructure; D051379:Mice; D013926:Thrombopoietin / Q000494:pharmacology / Q000502:physiology*; D020827:rap1 GTP-Binding Proteins / Q000096:biosynthesis / Q000187:drug effects / Q000378:metabolism*
D005353:Fibronectins; D019012:Integrin beta1; D013926:Thrombopoietin; D020827:rap1 GTP-Binding Proteins