Apolipoprotein E-binding proteins isolated from dog and human liver.

Abstract

Chylomlcron remnant catabollsm appears to be mediated by apolipoprotein (apo) E binding to hepatic llpoprotein receptors. Previously, the apo B.E(LDL) receptor and a unique apo E-blndlng protein (referred to as the apo E receptor) were Isolated from solublllzed canine and human livers. In the present study, the apo E-blndlng fraction was further characterized and found to contain at least three proteins, all of which bind apo E-contalnlng llpoprotelns with high affinity. The 56-kDa band was found to contain the a-and B-subunlts of F,-ATPase, presumably derived from mltochondrlal membranes. In addition, an apo E-blndlng protein with an apparent M r =-59000 was identified. The 59-kDa protein displays calcium-Independent binding on llgand blots, but displays both calcium-dependent and -Independent binding In assays performed with detergent-solubilized protein. The 59-kDa protein recognized llpld-free as well as llpld-bound apo E In llgand blots, and also bound apo E-2, apo E-3, and apo E-4 in a comparable way. Monoclonal antibodies produced against the 59-kDa protein did not react with the 56-kDa proteins. Normal human liver, as well as the liver of a patient lacking the apo B,E(LDL) receptor, possessed the 56-kDa and 59-kDa proteins. These data Indicate that liver cells possess at least three proteins, in addition to the apo B.E(LDL) receptor, that bind apo E-contalnlng llpoprotelns with high affinity. The physiological role of these proteins In apo E metabolism remains to be determined. (LDL) at a much reduced rate. In addition, Watanabe Heritable Hyperlipidemic (WHHL) rabbits, which express less than 5% of the normal number of apo B,E(LDL) receptors, 6 are still able to catabolize chylomicron remnants at normal rates.7 Furthermore, down-regulation of hepatic apo B.E(LDL) receptors by cholesterol feeding results in retarded clearance of LDL but has no effect on the hepatic uptake of remnant lipoproteins. 8In vivo observations have shown that apo E on the chylomicron remnant surface is required for hepatic clearance of these lipoproteins. 49 -13 Competitive binding studies in the perfused rat liver have shown that both chylomicron remnants and apo E HDL C [the cholesterol-induced high density lipoproteins (HDL) that contain apo E] are taken up by the same process. 4 Furthermore, chylomicron remnants without apo E are removed by the perfused rat liver at a slow rate. 11 The importance of apo E in chylomicron remnant metabolism is also evident from studies of type III hyperiipoproteinemic (dysbetalipoproteinemic) subjects. The apo E-2 isolated from these patients is defective, in comparison with the normal apo E-3, in promoting hepatic uptake of model lipoproteins. 13 In addition, chylomicron remnants accumulate in the plasma of patients who have a marked deficiency in plasma apo E.

Scavenger receptor BI facilitates the metabolism of VLDL lipoproteins in vivo

“…Martinez et al identified the b chain of ATP synthase, a principal protein complex of the mitochondrial inner membrane, as a high-affinity receptor for HDL that is expressed on the plasma membrane of hepatocytes (69). Interestingly, Beisiegel et al (70) and Mahley et al (71) described almost two decades ago that the b chain of ATP synthase binds apoE-containing lipoproteins with high affinity. It is highly interesting to speculate that in addition to SR-BI, as we demonstrate in this study, the b chain of ATP synthase is the recognition site responsible for displacement of b-VLDL association to hepatocytes by HDL.…”

Lipoprotein lipase enhances the binding of chylomicrons to low density lipoprotein receptor-related protein.

“…One unit corresponds to 1 ,mol of fatty acid released per min. Protein concentrations were calculated by using the absorbance at 280 nm and the absorption coefficient (A1%) 16.8 cm 1. Human LPL was purified from postheparin plasma by adsorption to heparin/agarose.…”

“…Several earlier attempts to characterize the potential apoE receptor protein (15,16) were not successful. Recently, Herz et al (17) described the LDL receptor-related protein (LRP) with the structural potential to be a lipoprotein receptor.…”

The Molecular Mechanisms of HDL and Associated Vesicular Trafficking Mechanisms to Mediate Cellular Lipid Homeostasis

“…It has also been reported that both the ␤-chain and ␣-chain of ATP synthase are receptors for apoE-enriched HDL in the plasma membrane. 23 The possible involvement of the F0/F1-ATPase in the lipid influx/efflux rheostat together with ABCA1 is shown in Figure 1. A fraction of ABCA1 is located in internal endocytic compartments where it may facilitate cholesterol efflux from late endosomes.…”