Effect of specific binding of high density lipoprotein to eel hepatocytes on their secretion of lipoprotein.

Specific binding of eel serum high-density lipoprotein (HDL) to eel hepatocytes was demonstrated by using a synthesized fluorescent lipophilic dye. HDL binding was inhibited by the addition of unlabeled HDL. The binding of HDL to the hepatocytes was saturated at concentrations over 100 micrograms HDL protein/ml and Kd value was 20 micrograms HDL protein/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 degrees C for 1 h. There are three remarkable characteristics of the effect of HDL on the cultured hepatocytes. One is that the addition of HDL to 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 HDL specifically stimulated the synthesis of the lipoprotein and had no effect on the synthesis of intracellular proteins and the secreted proteins except for the lipoprotein.