Hepatic uptake of 0-VLDL in cholesterol-fed rabbits

The hepatic uptake of intravenously injected P-very low density lipoprotein (8-VLDL) in rabbits fed 2% (w/w) cholesterol for 3 weeks was investigated. In vitro studies were also conducted to examine the specificity and the capacity of the uptake in isolated liver parenchymal cells. The hepatic uptake of P-VLDL was 15.8 * 6.7% (n = 6) in the cholesterol-fed rabbits as compared to 26.6 * 7.5% (n = 6) of the injected dose in control rabbits (P < 0.05). Although this is a fractional reduction, it represents a more than 10-fold increase in absolute hepatic uptake of lipoproteins in the cholesterol-fed rabbits. In these animals the liver parenchymal, endothelial, and Kupffer cells took up 10.2 * 2.7%, 3.0 * 0.9%, and 1.8 * 0.4% of the injected dose, respectively, compared to 25.9 * 6.1%, 3.6 * 1.6%, and 1.5 * 0.8% of the injected dose in chow-fed controls. However, taking into account the high plasma lipoprotein levels in the cholesterol-fed rabbits, the absolute cellular uptake was 10-fold increased in the parenchymal liver cells and more than 20-fold increased in the nonparenchymal cells. In vitro results indicated a 40% down-regulation of the specific receptor for 8VLDL in the parenchymal cells, and this, together with an increased competition for binding sites in the hypercholesterolemic rabbits, probably explains the reduced uptake of 0-VLDL in terms of % of injected dose observed in vivo. In vitro data suggested that the receptor involved in both hypercholesterolemic and normolipemic rabbits was the apolipoprotein (apo) B,E receptor. On a per cell basis, parenchymal cells from chowfed control animals took up 2.4 * 0.8% of the injected dose per 109 cells; this uptake was reduced to 1.1 f 0.5% in hypercholesterolemic animals. No differences in uptake of P-VLDL in nonparenchymal liver cells were observed on a per cell basis between the two feeding groups, indicating that binding sites involved in this uptake are not down-regulated by cholesterol feeding. On the contrary, the absolute uptake in the nonparenchymal liver cells is greatly increased in hypercholesterolemic rabbits as compared to controls. In cholesterolfed rabbits the three different liver cell types took up approximately the same amount of P-VLDL per cell. The liver nonparenchymal cells, therefore, assume a prominent role in uptake of 0-VLDL in hypercholesterolemic rabbits, accounting for more than 30% of the total hepatic uptake as compared to 16% in control animals. Electron microscopic studies showed a significant accumulation of lipids in all cell types investigated from cholesterol-fed rabbits, while no such lipid accretion could be observed in control rabbits. The in vivo data show that the nonparenchymal cells, notably the Kupffer cells, play an important role in catabolizing P-VLDL in hypercholesterolemic rabbits.-Gudmundsen, O., T. Berg, N. Roos, and M. S. Nenseter. Hepatic uptake of P-VLDL in cholesterol-fed rabbits. J. Lipid Res. 1993. 34: 589-600. Supplementary key words liver parenchymal cells Kupffer cells liver endothelial cells plasma clearance hypercholesterolemic rabbits Feeding rabbits diets high in cholesterol causes marked alterations in the plasma lipoproteins. O n e of these changes is the appearance of 0-VLDL. P-VLDL are cholesterol-enriched lipoproteins that contain apoB and apoE as their major protein constituents. 0-VLDL is also produced in patients with type I11 hyperlipidemia (1, 2). Cholesterol-rich P-VLDL has been shown to cause cholesteryl ester accumulation in macrophages (3), similar to that seen after uptake of chemically modified low density lipoprotein (LDL) mediated by the scavenger receptor. It might also induce monocyte adhesion to endothelial cells (4). 0-VLDL may accumulate in aorta and atherosclerotic lesions (5, S) , and this lipoprotein has therefore been considered to have a potential role in development of atherosclerosis. Although macrophages take u p P-VLDL avidly, the liver is the main organ of 0-VLDL catabolism (7-10). Yet, the role of the different liver cell types in the uptake of /3-VLDL in hypercholesterolemic animals has not been investigated. T h e functional units of the mammalian liver are hexagonally shaped lobuli consisting of many layers of platelets of parenchymal cells. The sinusoids, which are specialized capillaries, bring both venous blood from the digestive system and arterial blood through the lobuli. T h e sinusoids are lined with the endothelial cells that separate the sinusoidal lumen from the space of Disse and the underlying parenchymal cells. The endothelial cells often cover the whole circumference of a sinusoid. They display 100-nm fenestrations that allow free passage of macromolecules and lipoproteins (except chylomicrons) Abbreviations: apo, apolipoprotein; P-VLDL, 0-very low density lipoprotein; LDL, low density lipoprotein; LDL(cr), cholesterol-rich low density lipoprotein; 'IC, tyramine cellobiose; PBS, phosphate-buffered saline; FCR, fractional catabolic rate; ACR, absolute catabolic rate. 'To whom correspondence should be addressed. Journal of Lipid Research Volume 34, 1993 589 by gest, on O cber 8, 2017 w w w .j.org D ow nladed fom to the space of Disse (11). Within the sheets of parenchymal cells are bile canaliculi that transport bile to the bile ducts. The Kupffer cells, the liver macrophages, are usually observed in the lumen of the sinusoid. In normolipidemic rats, P-VLDL is taken up mainly by parenchymal liver cells; however, contrary to expectations, rat Kupffer cells do not take up significant amounts of 0-VLDL (8). In rat liver a relatively high proportion of the apoB,E receptor is located in the Kupffer cells. The rat liver parenchymal cells express relatively low levels of the apoB,E receptor. Thus, in this respect the liver macrophages behave differently from other types of macrophages which avidly take up 0-VLDL via an apoB,E receptor (12). The finding that the P-VLDL is taken up exclusively by the parenchymal liver cells, therefore, suggests that P-VLDL uptake is mediated via an apoB,E receptor-independent mechanism in rats (13). In the present investigation rabbits were used, because lipoprotein metabolism in rabbits resembles that of humans, in that the plasma concentrations of LDL are relatively high and the liver expresses high levels of the apoB,E receptor (14, 15). P-VLDL has high affinity for the apoB,E receptor (16) and mRNA for this receptor has been identified in liver parenchymal, endothelial, and Kupffer cells in cholesterol-fed rabbits as well as chow-fed controls (14, 17). Our objectives were: i) to determine the contribution of different cell types to the hepatic uptake of 6-VLDL; ii) to investigate the effect of cholesterol feeding on the morphology of each liver cell type by transmission electron microscopy; and ii;) to determine the specificity of the receptors involved in the in vitro uptake of 0-VLDL in liver parenchymal cells from cholesterol-fed and controlfed rabbits. The results obtained were compared with those previously found for the hepatic uptake of cholesterol-rich LDL (LDL(cr)) (17).