Endothelial bound lipoprotein lipase (LpL) depletion in hypoalbuminemia results from decreased endothelial binding, not decreased secretion

Hypertriglyceridemia in nephrotic (NS) and Nagase analbuminemic rats (Analb) results from reduced triglyceride clearance. NS and Analb have reduced or absent albumin, reduced plasma oncotic pressure (pi), but Analb lack proteinuria. The heparin releasable lipoprotein lipase (LpL) pool in both models is greatly reduced, suggesting reduced LpL is related to low albumin or pi and not proteinuria. To determine the cause of endothelial LpL reduction, we studied effectors of endothelial LpL (eLpL) levels from gene expression, to delivery and endothelial binding. eLpL was measured as heparin releasable activity. eLpL and secretion rate was measured in isolated hearts perfused with heparin. mRNA levels were measured in rat hearts by kinetic RT-PCR. Finally, binding of (125)I-LpL by competition assays rat endothelial cells measured serum-induced changes in affinity. eLpL in vivo was reduced in nephrotic and Analb rats. While the eLpL pool was reduced in isolated perfused hearts, neither LpL secretion by isolated hearts nor myocardial mRNA was reduced in NS or Analb. Binding of LpL to RAEC preincubated with serum from either NS or Analb was reduced compared to control. LpL mRNA levels and release rate was not altered in hearts from NS rats, while eLpL is depleted, suggesting that reduced eLpL in NS is not the result of reduced delivery. The finding that NS serum alters LpL binding to RAEC suggests LpL depletion results from decreased binding rather than defective delivery. This in turn is a consequence of reduced serum albumin or pi but does not require proteinuria.

Identification of lipid binding and lipoprotein lipase activation domains of human apoAV

ApoAV, a newly discovered apolipoprotein, plays a key role in human triglyceride homeostasis; however, the structure-function correlation of apoAV is not clearly understood. To explore the relationship, wild type and six deletion mutants, that is (AV (Delta(1-51)), AV (Delta(51-128)), AV (Delta(132-188)), AV (Delta(192-238)), AV (Delta(246-299)), AV (Delta(301-343))), of human apoAV expressed in Escherichia coli were studied. All the deleted regions together encompass almost the entire 343 amino acid sequence of wild type apoAV. Circular dichroism spectroscopy showed that the alpha helical content of lipid-free wild type apoAV was 46%. In comparison with wild type apoAV, AV (Delta(192-238)) and AV (Delta(301-343)) displayed significantly decreased lipid binding activities, confirming the importance of these two regions in lipid binding function of apoAV. While, the LPL activation function of apoAV remarkably impaired after deletion of residues 192-238. These findings suggested that the domain (192-238) is absolutely necessary for apoAV in lipid binding and lipoprotein lipase activation.

Endothelial chylomicron binding is altered by interaction with high-density lipoprotein in Heymann's nephritis

Very-low-density lipoprotein (VLDL) catabolism is impaired in the nephrotic syndrome, partly as a result of structural changes that impair endothelial binding in the presence of lipoprotein lipase. Previous results suggested that postsynthetic modification of VLDL by high-density lipoprotein (HDL) in nephrotic syndrome rats causes their failure to bind endothelia normally. It is unknown (1) whether the structure of secreted lipoproteins is normal before exposure to nephrotic syndrome serum and (2) whether the same structural or functional defects are imparted to chylomicrons (CMs) through their interaction with HDL from nephrotic syndrome rats. CMs were isolated from thoracic duct lymph from rats with passive Heymann's nephritis (HN) and normal controls. CMs from control rats were incubated with HDL from either HN or control rats and reisolated, and apolipoprotein E (apo E) content and endothelial binding were determined. We found that CMs secreted by HN and control rats had similar apo E/B-48 ratios. HDL from HN rats had significantly lower apo E/A-I ratios than controls. Incubation of nascent control CMs with control HDL resulted in a 4-fold increase in CM apo E content, but binding was unaffected. Incubation with HDL from HN resulted in only a 50% increase in CM apo E content but reduced binding of these treated CMs by 50% compared either with nascent control CMs or with CMs incubated with control HDL. HDL from rats with HN alters CM binding to lipoprotein lipase by a mechanism that does not involve reducing the content of apo E already present on CMs at the time of secretion.

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

BACKGROUND

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.

METHODS

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.

RESULTS

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).

CONCLUSION

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.

Proteinuria and plasma compositional changes contribute to defective lipoprotein catabolism in the nephrotic syndrome by separate mechanisms

Triglyceride (TG)-rich lipoproteins are primarily increased in the nephrotic syndrome (NS) as a result of decreased catabolism. Lipoprotein lipase (LpL) is the rate limiting enzyme for lipolysis of TG. The biologically active endothelial bound LpL pool is reduced in NS providing one mechanism for decreased clearance of very low density lipoprotein (VLDL). LpL, however, is also reduced in the Nagase Analbuminemic Rat (NAR) to the same extent as in NS, suggesting that other factors contribute to decreased VLDL clearance. Hyperlipidemia worsens with the onset of proteinuria and is reduced when proteinuria abates. We established that while VLDL from NS rats bind poorly to bovine aortic endothelial cells (BAEC) in the presence of saturating LpL while, VLDL from NAR bind more avidly than control. We then established that rat aortic endothelial cells (RAEC) incubated with serum from NAR or from NS rats bind significantly less exogenous LpL. Thus decreased clearance of VLDL in NS results from: 1) reduced endothelial bound LpL; an effect of serum from animals with reduced oncotic pressure (pi) that makes cells unable to bind LpL; and 2) an alteration in VLDL binding to endothelial bound LpL. The former has no relationship to proteinuria while the latter occurs as a consequence of proteinuria. These effects combine to suppress VLDL clearance.

Nutritional regulation of binding sites for lipoprotein lipase in rat heart

Several laboratories have shown that when rats are fasted, the amount of lipoprotein lipase (LPL) at the vascular endothelium in heart (monitored as the amount released by heparin) increases severalfold without corresponding changes in the production of LPL. This suggests that there is a change in endothelial binding of LPL. To study this, (125)I-labeled bovine LPL was injected. The fraction that bound in the heart was more than twice as high in fasted than in fed rats, 4.3% compared with 1.9% of the injected dose. Refeeding reversed this in 5 h. When unlabeled LPL was injected before the tracer, the fraction of (125)I-LPL that bound in heart decreased, indicating that the binding was saturable. When isolated hearts were perfused at 4 degrees C with a single pass of labeled LPL, twice as much bound in hearts of fasted rats. We conclude that fasting causes a change in the vascular endothelium in heart such that its ability to bind LPL increases.

Lipoprotein lipase activity is stimulated by insulin and dexamethasone in cardiomyocytes from diabetic rats

Type 1 diabetes mellitus reduces lipoprotein lipase (LPL) activity in the heart. The diabetic phenotype of decreased LPL activity in freshly isolated cardiomyocytes persisted after overnight culture (16 h). Total cellular LPL activity was 311 ± 56 nmol oleate released·h-1·mg-1 cell protein in diabetic cultured cardiomyocytes compared with 661 ± 81 nmol oleate released·h-1·mg-1 cell protein for control cultured cells. Diabetes also resulted in lower heparin-releasable (HR) LPL activity compared with control cells (111 ± 25 vs. 432 ± 63 nmol·h-1·mg-1 cell protein). In kinetic experiments, the reduction in total cellular LPL and HR-LPL activities in cultured cells from diabetic hearts was due to a decrease in maximal velocity, with no change in apparent Km for substrate (triolein). LPL activity in primary cultures of cardiomyocytes from control rats is stimulated by the combination of insulin (Ins) and dexamethasone (Dex). Overnight treatment of cultured cardiomyocytes from diabetic rats with Ins+Dex elicited an 84% increase in cellular LPL activity (to 572 ± 65 nmol·h-1·mg-1 cell protein) and a 194% increase in HR-LPL activity (to 326 ± 46 nmol·h-1·mg-1 cell protein). This stimulation occurred at subnanomolar concentrations of the hormones, but neither hormone was effective alone. The amount of immunoreactive LPL protein mass in cultured cardiomyocytes from diabetic hearts was unchanged by Ins+Dex treatment. Addition of oleic acid (60 µM) to the overnight culture medium inhibited the already reduced HR-LPL activity in diabetic cultured cells by 73% (to 30 ± 4 nmol·h-1·mg-1 cell protein). The presence of oleic acid also reduced hormone-stimulated HR-LPL activity. Increasing the glucose concentration in the culture medium to 26 mM had no effect on total cellular LPL or HR-LPL activities.Key words: lipoprotein lipase, cardiomyocytes, diabetes.

Fatty acids reduce heparin-releasable LPL activity in cultured cardiomyocytes from rat heart

Varying glucose and fatty acid (FA) concentrations in the medium of cultured cardiomyocytes from adult rat hearts were tested for effects on lipoprotein lipase (LPL) activity. Glucose (5.5, 11, and 25 mM in the culture medium for 18-22 h) had no effect on either heparin-releasable LPL (HR-LPL) or on cellular LPL (C-LPL) activities. When cardiomyocytes were cultured overnight with 60 microM oleate, HR-LPL activity was reduced to 20% of control, with no change in C-LPL activity or total C-LPL mass. Similar results (HR-LPL and C-LPL activities) were obtained with 60 microM concentrations of palmitate and myristate; linoleate and eicosapentaenoate did reduce C-LPL activity, but the decrease in HR-LPL activity was much greater. Oxfenicine, an FA oxidation inhibitor, did not alter the inhibitory effect of 60 microM oleate on HR-LPL. Short-term incubations (1 and 3 h) of cultured cardiomyocytes with 60 microM oleate did not displace LPL into the medium. Immunodetectable LPL on the cell surface of oleate-treated cultured cardiomyocytes was increased compared with control cells, but heparin treatment released the same amount of LPL mass that had reduced catalytic activity.