Effect of Specific Binding of High Density Lipoprotein to Eel Hepatocytes on Their Secretion of Lipoprotein.

Abstract

Key words: high-density lipoprotein/hepatocytes/eel/receptor/lipoprotein/fluorescent lipophilic dye ABSTRACT. Specific binding of eel serum high-density lipoprotein (HDL) to eel hepatocytes was demonstrated by using a synthesized fluorescent lipophilic dye. HDLbinding was inhibited by the addition of unlabeled HDL. The binding of HDLto the hepatocytes was saturated at concentrations over 100^g HDLprotein /ml and Kd value was 20 jug HDLprotein/ml. A fluorescent photomicrograph of the cultured eel hepatocytes which were incubated with the dye showed the bright, circumferential plasma membranes stained with the dye. 125I-HDL was incorporated into the acid insoluble-and soluble-fractions of the cultured hepatocytes during incubation at 28°Cfor 1 h. There are three remarkable characteristics of the effect of HDLon the cultured hepatocytes. One is that the addition of HDLto the hepatocytes induced the efflux of cholesterol, triacylglycerol, and phospholipid from the hepatocytes. The second characteristic is that the efflux of the intracellular lipids was carried out with very-low-density-like or chylomicron-like lipoprotein secreted by the hepatocytes. The third characteristic is that HDLspecifically stimulated the synthesis of the lipoprotein and had no effect on the synthesis of intracellular proteins and the secreted proteins except for the lipoprotein.Wereported that cultured eel hepatocytes secreted only one kind of lipoprotein (1). The secreted lipoprotein has someunique characteristics, whichare not observed in mammalianlipoproteins. Its main apoproteins consist of apo A and B and the main lipid is triacylglycerol. Since its density is between chylomicron and very-low-density lipoprotein (VLDL), we call it VLDL-(or chylomicron-) like lipoprotein. Although the main composition of eel serum lipoproteins is high-density lipoprotein (HDL), the cultured hepatocytes do not secrete HDL.In another previous report we investigated the effect of serum HDLon the synthesis of the lipoprotein in the cultured hepatocytes (2). Serum HDLhad a stimulatory effect on the synthesis of the secreted lipoprotein. Wefurther investigated the effect of HDLon