Identification of a lipolysis-stimulated receptor that is distinct from the LDL receptor and the LDL receptor-related protein

Abstract

This paper provides further characterization of a receptor that, in cells lacking the LDL receptor (FH fibroblasts), mediates lipoprotein binding, uptake, and degradation when incubated with oleate at concentrations not exceeding albumin binding capacity. This oleate-activated receptor is genetically distinct from the LDL receptor and is hereafter referred to as the lipolysis-stimulated receptor (LSR). Its apparent affinity was higher for triglyceride-rich lipoproteins (chylomicrons, VLDL) and for lipid emulsions supplemented with recombinant apoE, than for LDL which contains solely apoB. In contrast, VLDL isolated from a Type III hyperlipidemic patient (apoE2/2 phenotype) failed to bind to the LSR. Five lines of evidence indicated that the LSR is distinct from the LDL receptor-related protein (LRP): (1) the LRP ligand, alpha 2-macroglobulin-methylamine (alpha 2-MG*), did not bind to the oleate-induced LDL binding site; (2) oleate had no effect on the binding of alpha 2-MG* to LRP; (3) the LRP-associated protein, RAP, which inhibits LRP, had no effect on the LSR; (4) binding of lipoproteins to LSR was independent of Ca2+; and (5) LSR activity resolved as two proteins smaller than LRP (apparent molecular masses as determined by ligand blots: 115 and 85 kDa). That LSR provides a new candidate receptor contributing to the clearance of chylomicron remnants (CMR) is supported by the observation that LSR was inhibited by lactoferrin, a milk protein that delays CMR clearance when injected in vivo. Furthermore, in primary cultures of rat hepatocytes, oleate stimulated binding, uptake, and degradation of LDL with kinetic characteristics similar to that of LSR expressed in FH fibroblasts.(ABSTRACT TRUNCATED AT 250 WORDS)

An Extrahepatic Receptor-associated Protein-sensitive Mechanism Is Involved in the Metabolism of Triglyceride-rich Lipoproteins

“…These observations suggest that a novel and hitherto unsuspected RAP-sensitive process is involved in the metabolism of triglyceride-rich lipoproteins. This could involve other RAP-sensitive receptors, such as the hypothetical lipolysis stimulated receptor (18), or a direct or indirect effect of RAP on lipase-mediated conversion of chylomicrons to the smaller remnants.…”

29 Immunoglobulin-Like Domain Containing Receptor Mediates Fat-Stimulated Cholecystokinin Secretion

“…Our finding that DiI-labeled HDL uptake is enhanced in ILDR1-transfected cells indicates that either ILDR1 undergoes internalization or HDL binding is of sufficient duration to allow exchange of DiI from the outside to the inside of the cell. LSR is highly enriched in the endosomal compartment of rat hepatocytes; endosomal membranes showed greater than 10-fold oleate-induced 125 I-LDL binding compared with the plasma membranes (38). A receptor internalization motif (Y-X-X-φ where φ is a hydrophobic amino acid such as F, I, L, M, V) is present in ILDR1 at amino acid 459.…”

Structure and Function of the Lipolysis Stimulated Lipoprotein Receptor

“…This affinity for VLDL was directly related to the size of the particle and therefore TG content of the VLDL fraction, with the highest affinity being for the larger VLDL 1 fraction, as compared to that for VLDL 2 and VLDL 3 . Further studies revealed that chylomicrons or lipid emulsions containing recombinant apoE could compete with 125 I-LDL for binding in the presence of FFA (Yen et al, 1994). Digestion of apoB100 on the LDL with mild pronase treatment renders the LDL unable to bind this oleateinduced binding site, thus clearly demonstrating a direct protein-protein interaction between the lipoprotein ligand and the cell surface binding site.…”

“…While it was necessary to supplement -VLDL with exogenous recombinant apoE before binding by LRP, this was not necessary for -VLDL binding in the presence of oleate in FH fibroblasts (Bihain & Yen, 1992). Experiments using activated 2 -macroglobulin as a known LRP ligand demonstrated that there was no effect of oleate on LRP-mediated internalization and degradation of this ligand (Yen et al, 1994). Nor was the oleate-induced receptor inhibited by the 39 kDa receptor-associated protein (RAP) at concentrations shown to affect LRP activity (Yen et al, 1994).…”

“…Nor was the oleate-induced receptor inhibited by the 39 kDa receptor-associated protein (RAP) at concentrations shown to affect LRP activity (Yen et al, 1994). Furthermore, it had been shown that LRP was able to bind and recognize apoE2/2 (Beisiegel et al, 1989), while this oleate-induced pathway for lipoproteins was unable to bind to VLDL isolated from a type III hypertriglyceridemic patient with the apoE2/2 phenotype (Yen et al, 1994).…”