Hypoalbuminemia and proteinuria contribute separately to reduced lipoprotein catabolism in the nephrotic syndrome

Abstract

Hypertriglyceridemia is a result of reduced triglyceride (TG)-rich lipoprotein (TRL) catabolism and occurs in rats with nephrotic syndrome (NS) and in Nagase analbuminemic rats (NARs). While the heparin-releasable lipoprotein lipase (LpL) pool in NAR and in NS is similar, TG levels are significantly greater in NS, suggesting that factors other than reduced LpL alone act in NS but not in NARs. Furthermore, clearance of chylomicrons (CM) and very low-density lipoprotein (VLDL) is normal in vivo in NAR despite low LpL levels. We tested the hypotheses that impaired binding of VLDL and impaired VLDL-high density lipoprotein (HDL) interactions contribute to hyperlipidemia in NS. TG and apoB secretion was measured using Triton WR 1339. Clearance of CMs by perfused hearts from NS and NAR was determined. Binding of VLDL from control, NS and NAR to rat aortic endothelial cells (RAECs) was measured prior to and following incubation with HDL from NS, NARs, and control. ApoE, protein, and TG content was determined. TG levels were greatest in NS (516 +/- 95 mg/dL), intermediate in NAR (193 +/- 20), and least in control (97 +/- 16, P = 0.05), while in contrast, TG secretion was least in NS (178 +/- 33 mg/dL/hour) versus 212 +/- 17 in NAR and 294 +/- 15 in control (P < 0.001 vs. NS). Clearance of CMs by NS and NAR hearts was the same and significantly reduced versus control (P < 0.005). Binding of NS-VLDL to endothelial cells was reduced, while NAR-VLDL binding was increased versus control (P < 0.001). Incubation of NS-VLDL with control or NAR HDL increased VLDL binding compared with binding following incubation with NS HDL (P < 0.001). Increased TG levels in both NS and NAR are the result of decreased TRL clearance. TG levels are greater in NS because of the presence of a combined defect: (1) a decrease in endothelial-bound LpL that occurs as a consequence of reduced serum albumin concentration, and (2) a defect in VLDL binding to endothelial-bound LpL. This latter defect occurs only in the presence of proteinuria and is conferred by HDL.