The conformation-dependent interaction of alpha(2)-macroglobulin with vascular endothelial growth factor - A novel mechanism of alpha(2)-macroglobulin/growth factor binding

alpha(2)-Macroglobulin (alpha(2)M) is a highly conserved proteinase inhibit or present in human plasma at high concentration (2-4 mg/ml), alpha(2)M exi sts in two conformations, a native form and an activated, receptor-recogniz ed form. While alpha(2)M binds to numerous cytokines and growth factors, in most cases, the nature of the alpha(2)M interaction with these factors is poorly understood. We examined in detail the interaction between alpha(2)M and vascular endothelial growth factor (VEGF) and found a novel and unexpec ted mechanism of interaction as demonstrated by the following observations: 1) the binding of VEGF to alpha(2)M occurs at a site distinct from the rec ently characterized growth factor binding site; 2) VEGF binds different for ms of alpha(2)M with distinct spatial arrangement, namely to the interior o f methylamine or ammonia-treated alpha(2)M and to the exterior of native an d proteinase-converted alpha(2)M; and 3) VEGF (molecular mass similar to 40 kDa) can access the interior of receptor-recognized alpha(2)M in the absen ce of proteinase trapped within the molecule. VEGF bound to receptor-recogn ized forms of alpha(2)M is internalized VEGF bound to receptor-recognized f orms of alpha(2)M is internalize and degraded by macrophages via the alpha( 2)M receptor, the low density lipoprotein receptor-related protein, Oxidati on of both native and receptor-recognized alpha(2)M results in significant inhibition of VEGF binding. We also examined the biological significance of this interaction by studying the effect of alpha(2)M On VEGF-induced cell proliferation and VEGF-induced up-regulation of intracellular Ca2+ levels. We demonstrate that under physiological conditions, alpha(2)M does not impa ct the ability of VEGF to induce cell proliferation or up-regulate Ca2+.