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

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): the LRP ligand, alpha 2-macroglobulin-methylamine (alpha 2-MG*), did not bind to the oleate-induced LDL binding site; oleate had no effect on the binding of alpha 2-MG* to LRP; the LRP-associated protein, RAP, which inhibits LRP, had no effect on the LSR; binding of lipoproteins to LSR was independent of Ca2+; and 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. LDL binding to LSR was also demonstrated in isolated liver membranes. Membranes from liver endocytic organelles showed marked (10-15-fold) enrichment in LSR activity when compared to total liver membranes; virtually no LSR activity was detected in liver mitochondrial membranes.