The alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein binds lipoprotein lipase and beta-migrating very low density lipoprotein associated with the lipase

Abstract

Lipoprotein lipase (LPL) causes a marked increase in the cellular binding of beta-migrating very low density lipoprotein (beta-VLDL) to a large receptor compatible with the alpha 2-macroglobulin receptor (alpha 2MR)/low density lipoprotein receptor-related protein (LRP) (Beisiegel, U., Weber, W., and Bengtsson-Olivecrona, G. (1991) Proc. Natl. Acad. Sci. U. S. A. 88, 8342-8346). Here we demonstrate that LPL binds to the alpha-chain of purified alpha 2MR/LRP immobilized on microtiter plates. The binding, apparently to multiple sites, was blocked by heparin and inhibited by the alpha 2MR-associated protein (alpha 2MRAP) and by EDTA. Immobilized LPL bound alpha 2MR/LRP in solution as well as beta-VLDL prepared from cholesterol-fed rabbits. Both binding reactions were dependent on an intact carboxyl-terminal folding domain of LPL, but were independent of its dimeric structure and intact catalytical function. Dimeric LPL could mediate binding of beta-VLDL to immobilized alpha 2MR/LRP and to cells, e.g. monocytes. In contrast, LPL monomers were not able to mediate binding to immobilized alpha 2MR/LRP, presumably because of cross-inhibition due to close relation between the binding regions for the lipoprotein and for the receptor in the carboxyl-terminal domain of the LPL monomer. Heparin, but not alpha 2MRAP, inhibited cellular binding of 125I-LPL or 125I-beta-VLDL supplemented with LPL. However, alpha 2MRAP inhibited degradation of the two ligands by about 90% and 40-50%, respectively. The results show that LPL is a ligand for alpha 2MR/LRP and, because of its affinity for lipoprotein particles, dimeric LPL can mediate or strengthen binding of beta-VLDL to this receptor. It is proposed that LPL binds primarily to cell surface heparan sulfate in monocytes and is presented for endocytosis and degradation by alpha 2MR/LRP. Moreover, beta-VLDL may be further supplemented with LPL at the cell surface and achieve affinity for alpha 2MR/LRP.