Blood platelets contain and secrete laminin-8 (α4β1γ1) and adhere to laminin-8 via α6β1 integrin

Laminins, a family of heterotrimeric proteins with cell adhesive/signaling properties, are characteristic components of basement membranes of vasculature and tissues. In the present study, permeabilized platelets were found to react with a monoclonal antibody to laminin gamma 1 chain by immunofluorescence. In Western blot analysis of platelet lysates, several monoclonal antibodies to gamma 1 and beta 1 laminin chains recognized 220- to 230-kDa polypeptides, under reducing conditions, and a structure with much slower electrophoretic mobility under nonreducing conditions. Immunoaffinity purification on a laminin beta 1 antibody-Sepharose column yielded polypeptides of 230, 220, 200, and 180 kDa from platelet lysates. In the purified material, mAbs to beta 1 and gamma 1 reacted with the two larger polypeptides, while affinity-purified rabbit antibodies to laminin alpha 4 chain recognized the smallest polypeptide. Identity of the polypeptides was confirmed by microsequencing. One million platelets contained on average 1 ng of laminin (approximately 700 molecules per cell), of which 20-35% was secreted within minutes after stimulation with either thrombin or phorbol ester. Platelets adhered to plastic surfaces coated with the purified platelet laminin, and this process was largely inhibited by antibodies to beta 1 and alpha 6 integrin chains. We conclude that platelets contain and, following activation, secrete laminin-8 (alpha 4 beta 1 gamma 1) and that the cells adhere to the protein by using alpha 6 beta 1 integrin. (C) 1999 Academic Press.