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To explore mechanisms underlying triglyceride (TG) accumulation in livers of chow-fed apo E-deficient mice (Kuipers, F., J.M. van Ree, M.H. Hofker, H. Wolters, G. In't Veld, R.J. Vonk, H.M.G. Princen, and L.M. Havekes. 1996. Hepatology. 24:241-247), we investigated the effects of apo E deficiency on secretion of VLDL-associated TG (a) in vivo in mice, (b) in isolated perfused mouse livers, and (c) in cultured mouse hepatocytes. (a) Hepatic VLDL-TG production rate in vivo, determined after Triton WR1339 injection, was reduced by 46% in apo E-deficient mice compared with controls. To eliminate the possibility that impaired VLDL secretion is caused by aspecific changes in hepatic function due to hypercholesterolemia, VLDL-TG production rates were also measured in apo E-deficient mice after transplantation of wild-type mouse bone marrow. Bone marrow- transplanted apo E-deficient mice, which do not express apo E in hepatocytes, showed normalized plasma cholesterol levels, but VLDL-TG production was reduced by 59%. (b) VLDL-TG production by isolated perfused livers from apo E-deficient mice was 50% lower than production by livers from control mice. Lipid composition of nascent VLDL particles isolated from the perfusate was similar for both groups. (c) Mass VLDL-TG secretion by cultured apo E-deficient hepatocytes was reduced by 23% compared with control values in serum-free medium, and by 61% in the presence of oleate in medium (0. 75 mM) to stimulate lipogenesis. Electron microscopic evaluation revealed a smaller average size for VLDL particles produced by apo E-deficient cells compared with control cells in the presence of oleate (38 and 49 nm, respectively). In short-term labeling studies, apo E-deficient and control cells showed a similar time-dependent accumulation of [3H]TG formed from [3H]glycerol, yet secretion of newly synthesized VLDL-associated [3H]TG by apo E-deficient cells was reduced by 60 and 73% in the absence and presence of oleate, respectively. We conclude that apo E, in addition to its role in lipoprotein clearance, has a physiological function in the VLDL assembly-secretion cascade.

In patients with type 2 diabetes, a strong correlation between accumulation of intramuscular triclycerides (TGs) and insulin resistance has been found. The aim of the present study was to determine whether there is a causal relation between intramuscular TG accumulation and insulin sensitivity. Therefore, in mice with musclespecific overexpression of human lipoprotein lipase (LPL) and control mice, muscle TG content was measured in combination with glucose uptake in vivo, under hyperinsulinemic-euglycemic conditions. Overexpression of LPL in muscle resulted in accumulation of TGs in skeletal muscle (85.5 ؎ 33.3 vs. 25.7 ؎ 23.1 mol/g tissue in LPL and control mice, respectively; P < 0.05). During the hyperinsulinemic clamp study, there were no differences in plasma glucose, insulin, and FFA concentrations between the two groups. Moreover, wholebody, as well as skeletal muscle, insulin-mediated glucose uptake did not differ between LPL-overexpressing and wild-type mice. Surprisingly, whole-body glucose oxidation was decreased by ϳ60% (P < 0.05), whereas nonoxidative glucose disposal was increased by ϳ50% (P < 0.05) in LPL-overexpressing versus control mice. In conclusion, overexpression of human LPL in muscle increases intramuscular TG accumulation, but does not affect whole-body or muscle-specific insulinmediated uptake, findings that argue against a simple causal relation between intramuscular TG content and insulin resistance.

To study the role of apoC1 in lipoprotein metabolism, we have generated transgenic mice expressing the human APOC1 gene. On a sucrose-rich diet, male transgenic mice with high APOC1 expression in the liver showed elevated levels of serum cholesterol and triglyceride compared with control mice (5.7 Ϯ 0.7 and 3.3 Ϯ 2.1 vs. 2.7 Ϯ 0.1 and 0.4 Ϯ 0.1 mmol/liter, respectively). These elevated levels were mainly confined to the VLDL fraction. Female APOC1 transgenic mice showed less pronounced elevated serum lipid levels. In vivo VLDL turnover studies revealed that, in hyperlipidemic APOC1 transgenic mice, VLDL particles are cleared less efficiently from the circulation as compared with control mice. No differences were observed in the hepatic production and extrahepatic lipolysis of VLDL-triglyceride. Also, VLDL isolated from control and APOC1 transgenic mice were found to be equally good substrates for bovine lipoprotein lipase in vitro. These data indicate that the hyperlipidemia in APOC1 transgenic mice results primarily from impaired hepatic VLDL particle clearance, rather than a defect in the hydrolysis of VLDL-triglyceride.To

Abstract-It has previously been reported that mice lacking the VLDL receptor (VLDLRϪ/Ϫ) exhibit normal plasma lipid levels and a modest decrease in adipose tissue mass. In the present study, the effect of VLDLR deficiency on profound weight gain was studied in mice. Obesity was induced either by feeding of a high-fat, high-calorie (HFC) diet or by crossbreeding mice onto the genetically obese ob/ob background. After 17 weeks of HFC feeding, VLDLRϪ/Ϫ mice remained lean, whereas their wild-type littermates (VLDLRϩ/ϩ) became obese. Similarly, the weight gain of ob/ob mice was less profound in the absence of the VLDLR. Moreover, VLDLR deficiency led to increased plasma triglycerides after HFC feeding. The protection from obesity in VLDLRϪ/Ϫ mice involved decreased peripheral uptake of fatty acids, because VLDLRϪ/Ϫ mice exhibited a significant reduction in whole-body free fatty acid uptake, with no clear differences in food intake and fat absorption. These observations were supported by a strong decrease in average adipocyte size in VLDLRϪ/Ϫ mice of both obesity models, implying reduced adipocyte triglyceride storage in the absence of the VLDLR. These results suggest that the VLDLR plays a role in the delivery of VLDL-derived fatty acids into adipose tissue. The most striking features that distinguish the VLDLR from the LDLR are (1) 8 ligandbinding repeats instead of 7 and (2) its expression pattern among tissues. The VLDLR is highly expressed in skeletal muscle, heart, and adipose tissue and only in trace amounts in the liver, whereas the LDLR is abundantly expressed in the liver. 1,2 A role for the VLDLR in lipoprotein metabolism has been suggested by in vitro experiments showing that the VLDLR binds and internalizes particles that are rich in apolipoprotein (apo) E, such as VLDL, IDL, and chylomicrons. 1,3 The binding of these lipoprotein particles to the VLDLR was stimulated by lipoprotein lipase (LPL) 3,4 and inhibited by a 39-kDa protein named the receptor-associated protein (RAP). 5 In addition to lipoproteins, the VLDLR has been shown to bind several other ligands, including urokinase complexed to its inhibitor, plasminogen activator inhibitor type 1, 6 and thrombospondin-1. 7 Based on its binding characteristics, endothelial localization, 8 and tissue expression pattern, it is hypothesized that the VLDLR facilitates the binding of triglyceride (TG)-rich particles in the capillary bed and subsequent delivery of free fatty acids (FFAs) to tissues active in fatty acid metabolism. 3,9,10 In line with this hypothesis, it was shown in mice that VLDLR mRNA levels are upregulated in heart and downregulated in adipose tissue after prolonged fasting. 11Reciprocally, VLDLR mRNA levels were downregulated in heart and upregulated in adipose tissue of LDLR-deficient mice fed an atherogenic diet. 12 To directly investigate a role for the VLDLR in lipid metabolism, mice were generated lacking the VLDLR by gene targeting. 13 VLDLRϪ/Ϫ mice exhibited no differences in plasma lipoproteins, and the sole abnormality detected was...