As a hepatic receptor for triglyceride-rich lipoproteins, the lipolysis-stimulated lipoprotein receptor (LSR) may be involved in the dynamics of lipid distribution between the liver and peripheral tissues. Here, we explore the potential role of leptin in regulating LSR. At physiological concentrations (1-10 ng/ml), leptin increased LSR protein and mRNA levels in Hepa1-6 cells through an ERK1/2-dependent and α-amanitin-sensitive pathway. In vivo, leptin treatment of C57BL6/Rj mice (1 μg 2×/d, 8 d) led to a significant increase in hepatic LSR mRNA and protein, decreased liver triglycerides and increased VLDL secretion as compared to controls. LSR(+/-) mice with elevated postprandial lipemia placed on a high-fat (60% kcal) diet exhibited accelerated weight gain and increased fat mass as compared to controls. While plasma leptin levels were increased 3-fold, hepatic leptin receptor protein levels and phosphorylation of ERK1/2 were significantly reduced. Therefore, leptin is an important regulator of LSR protein levels providing the means for the control of hepatic uptake of lipids during the postprandial phase. However, this may no longer be functional in LSR(+/-) mice placed under a chronic dietary fat load, suggesting that this animal model could be useful for the study of molecular mechanisms involved in peripheral leptin resistance.

Yeast Dop1p is an essential protein that is highly conserved in evolution and whose function is largely unknown. Here, we provide evidence that Dop1p localizes to endosomes and exists in a complex with two other conserved proteins: Neo1p, a P 4 -ATPase and putative flippase, and the scaffolding protein Ysl2p/Mon2p. The latter operates during membrane budding at the tubular endosomal network/trans-Golgi network (TEN/TGN) in a process that includes clathrin recruitment via adaptor proteins. Consistent with a role for Dop1p during this process, temperature-sensitive dop1-3 cells accumulate multivesicular, elongated tubular and ring-like structures similar to those displayed by neo1 and ysl2 mutants. In further agreement with the concept of Dop1p-Neo1p-Ysl2p complex formation and co-operation, we show that dop1-3 cells exhibit reduced levels of Neo1p and Ysl2p at steady state. Conversely, mutations or deletions in NEO1 and YSL2 lead to a decrease in Dop1p levels. In addition to binding to Neo1p and Ysl2p, Dop1p can form dimers or multimers. A critical region for dimerization resides in the C-terminus with leucine zipper-like domains. Dop1p's membrane association is largely mediated by its internal region, but Ysl2p might not be crucial for membrane recruitment.

The lipolysis-stimulated lipoprotein receptor, LSR, is a multimeric protein complex in the liver that undergoes conformational changes upon binding of free fatty acids, thereby revealing a binding site(s) that recognizes both apoB and apoE. Complete inactivation of the LSR gene is embryonic lethal in mice. Here we show that removal of a single LSR allele (LSR ؊/؉ ) caused statistically significant increases in both plasma triglyceride and cholesterol levels, a 2-fold increase in plasma triglyceride changes during the post-prandial phase, and delayed clearance of lipid emulsions or a high fat meal. The longer postprandial lipoprotein clearance time observed in LSR ؊/؉ mice was further increased in LSR ؊/؉ mice lacking functional low density lipoprotein (LDL) receptors. LSR ؊/؉ mice placed on a Westerntype diet displayed higher plasma triglycerides and cholesterol levels, increased triglyceride-rich lipoproteins and LDL, and increased aorta lipid content, as compared with control mice on the same diet. Furthermore, a direct correlation was observed between the hyperlipidemia and weight gain but only in the LSR ؊/؉ mice. Knockdown of LSR expression by small interfering RNA in mouse Hepa1-6 cells led to decreased internalization of both DiI-labeled cyclohexanedione-LDL and very low density lipoprotein in the presence of oleate. These data led us to conclude that LSR contributes to the physiological clearance of atherogenic triglyceride-rich lipoproteins and LDL. We propose that LSR cooperates with the LDL receptor in the final hepatic processing of apoB-containing lipoproteins and represents a novel therapeutic target for the treatment of hyperlipidemia associated with obesity and atherosclerosis. Triglyceride (TG)2 -rich apoB-containing lipoproteins are produced by the liver as VLDL or by the intestine as chylomicrons (1). Both types of particles deliver their TG load to peripheral tissues through interactions with lipoprotein lipase (LpL) anchored to capillary endothelium heparan sulfate proteoglycans (HSPGs) (1). The residual cholesterol enriched inter-mediate density lipoprotein (IDL) and LDL for VLDL, and chylomicrons remnants are then removed from the circulation by hepatocytes. Clearance of these lipoproteins in the liver is a complex process through which the particles enter the space of Disse, bind to HSPGs, acquire apoE, and interact with hepatic lipase and LpL (2). After this, the residual particles are delivered to endocytic receptors for cellular internalization and degradation (3).The LDL receptor (LDL-R) accounts for most of LDL removal and contributes to part of the uptake of chylomicron remnants (4, 5). This highly informative endocytic receptor prototype nevertheless leaves two unsolved issues regarding apoB-containing lipoprotein clearance. First, in the absence of functional LDL-R, the amount of LDL cleared daily is 2-fold greater than that of subjects with normal LDL-R activity (6) and has been shown to take place in the liver (7). Second, the clearance of intestinally derived chylomicron remnan...

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