Inhibitory effects of C apolipoproteins from rats and humans on the uptake of triglyceride-rich lipoproteins and their remnants by the perfused rat liver

Like rat C apolipoproteins, each of the C apolipoproteins from human blood plasma (C-I, C-11, C-111-1, and C111-2) bound to small chylomicrons from mesenteric lymph of estradiol-treated rats and inhibited their uptake by the isolated perfused rat liver. This inhibitory effect of the C apolipoproteins was independent of apolipoprotein E, which is present only in trace amounts in these chylomicrons. Addition of rat apolipoprotein E to small chylomicrons from mesenteric lymph of normal rats did not displace C apolipoproteins and had no effect on the uptake of these particles by the perfused liver, indicating that an increased ratio of E apolipoproteins to C apolipoproteins on chylomicron particles, unaccompanied by depletion of the latter, may not promote recognition by the chylomicron remnant receptor. The hepatic uptake of remnants of rat hepatic very low density lipoproteins (VLDL) and small chylomicrons, which had been produced in functionally eviscerated rats, was al o inhibited by addition of C apolipoproteins.lThese observations are consistent with the hypothesis that the addition of all of the C apolipoproteins to newly secreted chylomicrons and VLDL inhibits premature uptake of these particles by the liver and that depletion of all of these apolipoproteins from remnant particles facilitates their hepatic uptake. Remnants of chylomicrons and VLDL incubated with rat C apolipoproteins efficiently took up C-I11 apolipoproteins, but not apolipoprotein C-I1 (the activator protein for lipoprotein lipase). Preferential loss of apolipoprotein C-I1 during remnant formation may regulate the termination of triglyceride hydrolysis prior to complete removal of triglycerides from chylomicrons and VLDL. “Winder, E., and R. J. Havel. Inhibitory effects of C apolipoproteins from rats and humans on the uptake of triglyceride-rich lipoproteins and their remnants by the perfused rat liver. J. Lipid &J. 1985. 26: 556-565. Supplementary key words apoE chylomicrons * VLDL apoB The liver, as the major site from which cholesterol and its degradation products, the bile acids, are excreted from the body, is of central importance in the regulation of plasma cholesterol metabolism. The hepatic uptake of cholesterol in plasma lipoproteins has been studied extensively and it is now recognized that all of the lipoproteins that contain apolipoprotein (apo) B, apoE, or both 556 Journal of Lipid Research Volume 26, 1985 of these proteins are taken up into hepatic parenchymal cells by saturable processes of high affinity (1). Preparations of membranes from liver cells have been found to exhibit saturable binding of these lipoprotein species, but the binding affinity is substantially greater for those species that contain apoE, as compared with low density lipoproteins (LDL), which contain solely apoB (2). The affinity of binding of triglyceride-rich lipoproteins to liver membranes is reduced by addition of C apolipoproteins to these lipoproteins. These findings are consistent with our observations of lipoprotein uptake by the isolated, perfused rat liver (3). We found that small chylomicrons from mesenteric lymph and very low density lipoproteins (VLDL) from liver perfusates were readily taken up by perfused livers. When these lipoproteins were incubated with rat serum, their content of C apoproteins increased, as occurs normally when these lipoproteins enter the blood. Hepatic uptake of the modified chylomicrons and VLDL was substantially reduced. Remnant particles, produced when the chylomicrons or VLDL were injected into functionally eviscerated rats, were depleted of the C apoproteins, but not of apoE and apoB. These remnants were removed with high efficiency from perfusates of isolated livers. These observations, which suggested that the C apoproteins and apoE have opposing effects upon the hepatic uptake of triglyceride-rich lipoproteins, were confirmed by experiments in which small chylomicrons from mesenteric lymph of rats treated with pharmacological amounts of 17-a-ethinyl estradiol were used (4). These chylomicrons contain apolipoprotein B-48 but little C apoproteins and only trace amounts of apoE. The apoAbbreviations: apo, apolipoprotein; VLDL, very low density lipoprotein(s); TG, triglycerides; CE, cholesteryl esters. ‘Present address: Kemklinik fir Innere Medizin, Universitits-Krankenrankenhaus Eppendorf, Martinstr. 52, D-2000, Hamburg 20, West Germany. by gest, on N ovem er 6, 2017 w w w .j.org D ow nladed fom protein composition of these chylomicrons was modified by incubating them with purified rat apoproteins without alteration of their size or the composition of core lipids. Addition of apoE resulted in rapid hepatic uptake of the particles, whereas addition of individual C apoproteins (C-11, C-111-0, and (3-111-3) all caused similar retardation of hepatic uptake (4). These results are consistent with the hypothesis that the changes in the apoprotein composition of triglyceride-rich lipoproteins that occur as they are secreted from the intestine or liver and as they are subsequently metabolized can explain both the low rate of initial hepatic uptake and the rapid uptake of their remnants. In the current mearch we have examined the effect of adding individual human C apoproteins ((3-1, C-11, C111-1, and (2-111-2) to chylomicrons from estradiol-treated rats on the uptake of these particles by pehsed rat livers. We have also evaluated further the opposing effects of apoE and the C apoproteins on hepatic uptake by enriching chylomicrons from normal rats (which contain appreciable C apoproteins) with apoE. Finally, we have evaluated the effects of enriching remnants of both chylomicrons and hepatic VLDL (which contain substantial amounts of apoE) with C apoproteins. A preliminary report of this work has appeared (5). EXPERIMENTAL PROCEDURES