Immunolocalization of beta 1 integrins in platelets and platelet- derived microvesicles

Abstract

Human platelets contain several adhesion receptors belonging to the integrin superfamily. At least three beta 1 integrins are present on platelets and have been shown to mediate platelet adhesion to collagen, fibronectin, and laminin. To study the cellular localization of the beta 1 integrins in platelets, we produced a polyclonal antibody by immunization of goat 172 with purified beta 1 subunit from HPB-ALL cells. Antibody 172 (Ab172) specifically immunoblotted a 135-Kd protein in a lysate of whole platelets. The reactivity of Ab172 with platelet membrane proteins was further determined by immunoprecipitation of lysates of surface-radioiodinated platelets. Ab172 immunoprecipitates, resolved by nonreducing/reducing two-dimensional sodium dodecyl sulfate- polyacrylamide gel electrophoresis consisted of three labeled proteins with migrational properties of platelet glycoprotein (GP)Ia, GPIc and GPIIa. Neither GPIIb/IIIa nor the vitronectin receptor were immunoprecipitated by Ab172, confirming a lack of cross-reactivity with the beta 3 integrins in platelets. Immunofluorescence studies using Ab172 were performed to investigate the cellular distribution of beta 1 integrins in platelets. Fluorescent labeling of intact cells demonstrated the presence of beta 1 antigen on the surface of resting cells. Permeabilization of platelets with Triton X-100 showed the presence of an intracellular pool of beta 1 antigen. Double-label experiments using Ab172 and AP-2 (anti-GPIIb/IIIa) showed identical labeling patterns, suggesting a similar subcellular distribution for these integrins. Following thrombin stimulation, permeabilized cells showed a centralized clearing of both beta 1 antigen and GPIIb/IIIa as well as an intensification of surface labeling for beta 1 antigen. These findings suggest the translocation of intracellular beta 1 antigen to the platelet surface as a result of thrombin stimulation. Because platelet-derived microvesicles have been reported to contain GPIIb/IIIa, we investigated the possible distribution of beta 1 integrins in these structures. Microvesicles, produced as a result of platelet activation, were labeled with Ab172, suggesting the distribution of beta 1 integrins in these structures as well as in intact cells.