LDL-receptor-related protein regulates beta2-integrin-mediated leukocyte adhesion

Beta2-integrin clustering on activation is a key event in leukocyte adhesion to the endothelium during the inflammatory response. In the search for molecular mechanisms leading to this clustering, we have identified low-density lipoprotein (LDL) receptor-related protein (LRP) as a new partner for beta2-integrins at the leukocyte surface. Immobilized recombinant LRP fragments served as an adhesive surface for blood-derived leukocytes and the U937 cell line. This adhesion was decreased up to 95% in the presence of antibodies against beta2-integrins, pointing to these integrins as potential partners for LRP. Using purified proteins, LRP indeed associated with the alphaMbeta2 complex and the alphaM and alphaL I-domains (K(d, app) approximately 0.5 microM). Immunoprecipitation experiments and confocal microscopy revealed that endogenously expressed LRP and alphaLbeta2 colocalized in monocytes and U937 cells. Furthermore, activation of U937 cells resulted in clustering of alphaLbeta2 and LRP to similar regions at the cell surface, indicating potential cooperation between both proteins. This was confirmed by the lack of alphaLbeta2 clustering in U937 cells treated by antisense oligonucleotides to down-regulate LRP. In addition, the absence of LRP resulted in complete abrogation of beta2-integrin-dependent adhesion to endothelial cells in a perfusion system, demonstrating the presence of a previously unrecognized link between LRP and leukocyte function.