Hepatic Secretion of Lipoproteins in the Rat and the Effect of Experimental Nephrosis

Abstract

Livers from normal and nephrotic rats were perfused by the nonrecirculating technique. Nephrosis was studied on the 7th d after the injection of puromycin animonucleoside. Amino acid-labeled lipoproteins (d < 1.21) were isolated from the perfusion medium by agarose column chromatography or by sequential density ultracentrifugation. In both groups of animals, in addition to very low density lipoproteins and nascent high density lipoproteins, column chromatography revealed the presence of a peak of 2-3 × 10⁶ daltons. This peak contained lipoproteins of densities corresponding to d < 1.02, and 1.02 < d < 1.06, which indicated that rat liver secretes a heterogeneous mixture of triglyceride-rich lipoproteins. The amount of these lipoprotein density classes was measured and their lipid and apoprotein composition and their apoprotein specific activity were determined. In both groups of rats there was a progressive rise in phospholipid and decrease in triglyceride content as the isolation density increased from 1.006 and 1.06. The lipoproteins from the nephrotics had higher amounts of cholesterol. The livers from the nephrotic rats secreted two to three times as much lipoprotein as controls in all density classes in the first 20 min, but during the next 40 min only the 1.02 < d < 1.06 and nascent high density lipoproteins remained at this high level compared to controls. A larger total liver pool of apolipoproteins in nephrotic livers was inferred from their lower specific activities during the first 20 min. The apoprotein composition of liver perfusate lipoproteins from nephrotics differed from controls. There was a 40% decrease in the amount of low molecular weight apoproteins in all density classes, with corresponding increases in apo B and apo E in the triglyceride-rich fractions. The apo A-1 content of nascent HDL was increased from 16% in controls to 52% in nephrotics, with corresponding decreases in apo C and apo E. When these results were combined with specific activity measurements of the individual apoproteins and the net secretion rate of total protein in each lipoprotein class, it was possible to estimate the total amount of each apoprotein secreted and the total incorporation of labeled amino acids into each. The incorporation of label gave results similar to those obtained by direct measurement of the amounts of apoproteins. Apo E secretion was increased by a factor of 1.8, apo B by 2.8, and apo A-1 by 8.4, whereas the secretion of apo C was not significantly altered. We explain these results by postulating that the primary stimulus to hepatic plasma protein synthesis in response to proteinuria is general and that subsequent negative feedback regulation affects individual apolipoprotein synthesis rates. A corollary of this hypothesis is that the biosynthesis and secretion of an apoprotein may be regulated independently of the lipoprotein density class in which it is found.