CHOLESTEROL DERIVATIVE OF A NEW TRIANTENNARY CLUSTER GALACTOSIDE DIRECTS LOW-DENSITY AND HIGH-DENSITY LIPOPROTEINS TO THE PARENCHYMAL LIVER-CELL

We have developed a new triantennary galactoside. in which the termina l galactose moieties are connected to the branching point of the clust er galactoside via a 20 Angstrom (2 nm) spacer [TG(20 Angstrom)]. In v itro binding studies have demonstrated that introduction of a 20 Angst rom spacer resulted in avid and specific binding of the triantennary g alactoside to the asialoglycoprotein receptor on the parenchymal liver cell. Derivatization of this galactoside with a cholesterol moiety af forded a compound [TG(20 Angstrom)C] that lowered the serum cholestero l concentration when injected into rats. In the present study we have evaluated the direct effect of TG(20 Angstrom)C on the in vivo fate of high-density lipoprotein (HDL) and low-density lipoprotein (LDL). A d irect association of TG(20 Angstrom)C with HDL and LDL was observed on mixing these components. Incorporation of TG(20 Angstrom)C into I-125 -HDL and I-125-LDL significantly accelerated the serum decay and conco mitantly stimulated the hepatic uptake of these lipoproteins in rats. The liver uptake of TG(20 Angstrom)C-loaded I-125-HDL or I-125- LDL co uld be inhibited by 81% and 82% respectively by preinjection of 150 mg of N-acetylgalactosamine, indicating that the enhanced liver uptake p roceeded via galactose-specific receptors. More than 96% of the hepati c uptake of TG(20 Angstrom)C-loaded I-125-HDL could be attributed to t he parenchymal cell. Surprisingly, the parenchymal cell also accounted for 93% of the liver association of TG(20 Angstrom)C-loaded I-125-LDL , suggesting that TG(20 Angstrom)C stimulates the uptake and processin g of both lipoproteins by the asialoglycoprotein receptor on the paren chymal liver cell. This contrasts with earlier data indicating that a triantennary cluster galactoside provided with a 4 A spacer between th e terminal galactose moieties and the branching point of the dendrite stimulated hepatic uptake of LDL via the Kupffer cells. The parenchyma l cell is the only liver cell type that is capable of irreversibly rem oving cholesterol from the body in the form of bile acids. The above r esults imply that administration of TG(20 Angstrom)C not only facilita tes the hepatic uptake of lipoprotein-derived cholesterol (esters) but also their elimination from the body. In addition, it might be possib le to utilize TG(20 Angstrom)C as a targeting device to selectively de liver large drug carriers and possibly genes to the parenchymal liver cell.