Urinary Excretion of Apolipoproteins Bound to HDL-Like Particles in Rat Nephrotic Syndrome and Their Relation to Plasma HDL

Abstract

Lipoprotein excretion was investigated in the urine of hyperlipidemic rats with nephrotic syndrome induced by aminonucleoside of puromycin. Incubation with phosphatidylcholine liposomes was employed to float apoproteins not bound to lipids, by ultracentrifugation at d = 1.21 g/ml. On ultracentrifugation of whole, untreated urine, the amount of protein floated was 6-fold greater in nephrotic vs. control rats and consisted mainly of HDL-like particles. Sodium dodecylsulfate-polyacrylamide gel electrophoresis showed that control urine contained apoproteins A-I, A-II, E and traces of C, whereas in the nephrotic urine apo-E and a large amount of apo-C was found. Addition of liposomes to the ultracentrifugal d = 1.21 g/ml infranate and reflotation at the same density resulted only in slight increment in the floated apoproteins, mainly C and A-I. Addition of liposomes to the whole urine and centrifugation at d = 1.21 g/ml also did not produce a greater yield in the floated apoproteins of control or nephrotic urine. These results indicated that the urine is virtually devoid of lipid-free apoproteins and those floated from both the nephrotic and control urine are complexed with lipids. The plasma VLDL+LDL fraction of nephrotic rats, though increased in quantity, did not differ markedly in composition from that of control rats. The HDL, ∼ 3-fold elevated in nephrotic rats, were poorer in esterified cholesterol and richer in phospholipids. Relative to plasma HDL, the nephrotic urine HDL were protein-rich and phospholipid-poor and appeared to be larger in particle size as suggested by the lower estimated specific volume. The modified plasma HDL in nephrosis may have a pathophysiological implication.

Regulation of Apolipoprotein A-1 and E Gene Expression in Liver and Intestine of Nephrotic and Pair-Fed Rats

“…However, we cannot rule out the possibility that intestinal apo A-l m R N A is inactive and therefore untranslated, since a de crease in apo A-l protein in intestinal lymph chylomicrons has been reported in this experimental model [29]. A re duced catabolic rate o f plasma apo A-l in nephrotic rats [30], together with a light increase of H D L observed in this study and others [29,31,32], also suggests that perhaps not all o f the apo A-l m R N A is active.…”

“…With the exception o f albumin, urinary loss o f apolipoproteins is not substantial in nephrotic syndrome [31,32], This could suggest that factors other than proteinuria and oncotic pressure are involved in the regulation o f apo A-l and E gene expression at the transcriptional or posttranscriptional level. Yedgar et al [33] have shown that plasma viscosity regulates lipoprotein levels in vivo.…”