The Degree of Sialylation of ApoC-III Is Altered by Diet

Abstract

ApoC-III, an apolipoprotein that contains from 0 to 3 or more moles of sialic acid per mole of protein, plays important roles in lipoprotein metabolism. A high carbohydrate diet alters the relative proportions of the variously sialylated isoforms of ApoC-III in the plasma VLDL of man and of rat. Hepatic perfusion experiments in the rat indicate that the diet produces its affects on plasma VLDL by altering the hepatic output of ApoC-III. Diet is a relatively long-term stimulus to VLDL secretion. Our aim was to ascertain whether the changes in ApoC-III sialylation patterns could be reproduced by an acute stimulus to increased hepatic VLDL secretion, i.e., the perfusion of rat livers with media containing high levels of free fatty acid (FFA). Male Sprague-Dawley rats (∼250 g) were fed either Purina rat meal or a high carbohydrate formula for 3 wk. Livers were perfused with Krebs-Ringers bicarbonate, 3% bovine serum albumin containing either no added FFA, or 200 mg of added oleate. ApoB and ApoC-III3 were measured by radioimmunoassay and lipids by enzymatic assays. Lipoproteins were isolated by preparative or zonal ultracentrifugation. The relative proportions of the various isoforms of ApoC-III were quantified by isoelectric focusing. The carbohydrate diet increased the plasma levels of triglycerides and ApoC-III3 ∼ twofold. The diet also increased the proportions of ApoC-III0 relative to total ApoC-III from 33 to 42%, in plasma VLDL (P < 0.02). Perfusates of livers of carbohydrate-fed rats accumulated VLDL-TG and ApoC-III3 at increased rates (93 versus 211, P < 0.001, and 12.7 versus 23.5, P < 0.05, μg/g/h, respectively) and the relative proportions of perfusate VLDL ApoC-lll were altered: ApoC-III0 rose from 37% to 59% and ApoC-III3 fell from 60% to 38% (P < 0.02 for each). Perfusion of livers of control (meal-fed) animals with 200 mg oleate yielded twofold increases in the rates of accumulation of VLDL-TG over meal fed non-FFA perfused controls and changes in gross VLDL composition that resembled those produced by carbohydrate feeding, but ApoC-III3 levels were not increased and the relative proportions of ApoC-III0, ApoC-III, 2 and ApoC-III3 were not altered. Perfusion of livers of carbohydrate-fed rats with oleate produced fivefold and 2.5-fold increases in the rates of accumulation of VLDL-TG and ApoC-III3, respectively. The proportion of ApoC-III isoforms were identical with those seen in the livers of carbohydrate-fed rats perfused without added oleate. Thus, the changes in ApoC-lll sialylation were related more to diet than to the presence of added FFA, i.e., an acute stimulus did not reproduce the changes seen with diet. These data indicate that the sialylation of ApoC-III is under metabolic control.