Abstract
The role of the low density lipoprotein (LDL) receptor in the binding of chylomicron remnants to liver membranes and in their uptake by hepatocytes was assessed using a monospecific polyclonal antibody to the LDL receptor of the rat liver. The anti-LDL receptor antibody inhibited the binding and uptake of chylomicron remnants and LDL by the poorly differentiated rat hepatoma cell HTC 7288C as completely as did unlabeled lipoproteins. The antireceptor antibody, however, decreased binding of chylomicron remnants to liver membranes from normal rats by only about 10%. This was true for intact membranes and for solubilized reconstituted membranes and with both a crude membrane fraction as well as with purified sinusoidal membranes. Further, complete removal of the LDL receptor from solubilized membranes by immunoprecipitation with antireceptor antibody only decreased remnant binding to the reconstituted supernatant by 10% compared to solubilized, nonimmunoprecipitated membranes. Treatment of rats with ethinyl estradiol induced an increase in remnant binding by liver membranes. All of the increased binding could be inhibited by the antireceptor antibody. The LDL receptor-independent remnant binding site was not EDTA sensitive and was not affected by ethinyl estradiol treatment. LDL receptor-independent remnant binding was competed for by beta-VLDL = HDLc greater than rat LDL greater than human LDL (where VLDL is very low density lipoprotein, and HDL is high density lipoprotein). There was weak and incomplete competition by apoE-free HDL, probably due to removal of apoE from the remnant. The LDL receptor-independent remnant-binding site was also present in membranes prepared from isolated hepatocytes and had the same characteristics as the site on membranes prepared from whole liver. In contrast, when chylomicron remnants were incubated with a primary culture of rat hepatocytes, the anti-LDL receptor antibody prevented specific cell association by 84% and degradation of chylomicron remnants completely. Based on these studies, we conclude that although binding of chylomicron remnants to liver cell membranes is not dependent on the LDL receptor, their intact uptake by hepatocytes is.
Highlights
10%.This was true for intact membranes and fsoorlubilized reconstitutedmembranesandwithboth a crude membrane fraction as well as with purified sinusoidal membranes.Further, complete removal of the low density lipoprotein (LDL) receptor from solubilized membranes by immunoprecipitationwithantireceptorantibodyonly de
A number of studies have demonstrated that the liver is the principle site of LDL removal in vivo and that the bulk of creased remnant binding to the reconstituted superthis removal is dependent on the LDL receptor [4, 5]
LDL receptor-independent remnant binding site was [6, 7], and the liver remains the principle site of removal (5, not EDTA sensitive and was not affected by ethinyl 6).there musbte mechanisms other than LDL receptors estradiol treatment
Summary
Effect of Anti-LDLReceptor Antibody on '251-Chylomicron vanen et al [37] from the livers of normal rats. The ability of Remnant and '251-LDL Uptakbey Rat Hepatoma (HTC7288C) the antireceptor antibody to compete for '251-chylomicron. Cells-The polyclonal antireceptor antibody used in these remnant binding was studied. We have previously reported that rat hepatoma cells have a marked diminution in their ability to bind and internalize chylomicron remnants as compared to normal liver [31]. The effect of the anti-LDL receptor antibody on lipoprotein binding by these cells was examined. The antireceptor antibody inhibited the binding and internalization of both '251-chylomicronremnants (Fig. l a ) and '251-LDL (Fig. l b ). In three experiments, a 100250-fold protein excess of antireceptor antibody decreased for as much binding and internalization acsould the unlabeled lipoprotein.
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