BIOGENESIS AND FUNCTION OF LIPOLYSOSOMES IN DEVELOPING CHICK HEPATOCYTES

Proliferation of lipolysosomes is one of the characteristic aspects of embryonic chick hepatocytes. Formation of lipolysosomes is observed i n the well-developed trans-Golgi network, with the highest frequency o ccurring from 11 to 14 days of incubation. The lipolysosomes usually c ontain a small and electron-dense lipid inclusion; however, during dev elopment, they gradually enlarge with an accompanying reduction in the electron density of the inclusion. Lipolysosomes isolated from neonat al chick liver homogenates were mainly composed of esterified choleste rol and showed considerably high activity of lysosomal enzymes. Moreov er, the lipolysosome fraction is clearly shown to be a function of int ralysosomal lipolysis via acid lipase, This accumulation of esterified cholesterol within lipolysosomes might be attributed to an excessive uptake and conversion of plasma lipoproteins to lipolysosomes. This co ncept is supported by the appearance of an abundance of coated pits an d both ''early'' and ''late'' endosomes. The major components of plasm a lipoprotein are low density lipoprotein (LDL) and high density lipop rotein (HDL), the cholesterol-rich lipoproteins, whose cholesterol con tent increases during the last week of incubation when the lipolysosom es quickly enlarge. Plasma lipoprotein particles are produced in tile yolk sac epithelium from yolk very low density lipoprotein (VLDL) and transferred via the vitelline circulation to the chick liver. After ha tching, when the supply of nutrients from the yolk sac is terminated, lipolysosomes immediately decrease in size and number. The cholesterol and fatty acids released are useful as an energy source and lipid met abolism in general, especially after hatching. Food intake induces the use of and accelerates the disappearance of lipolysosomes. instead of lipolysosomes, lipid droplets appear and increase in number and size with concomitant increases of triglyceride concentrations in the liver homogenates, suggesting that lipogenesis has begun in the chick hepat ocyte. (C) 1997 Wiley-Liss, Inc.