Role of Tricellular Tight Junction Protein Lipolysis-Stimulated Lipoprotein Receptor (LSR) in Cancer Cells.

Maintaining a robust epithelial barrier requires the accumulation of tight junction proteins, LSR/angulin-1 and tricellulin, at the tricellular contacts. Alterations in the localization of these proteins temporarily cause epithelial barrier dysfunction, which is closely associated with not only physiological differentiation but also cancer progression and metastasis. In normal human endometrial tissues, the endometrial cells undergo repeated proliferation and differentiation under physiological conditions. Recent observations have revealed that the localization and expression of LSR/angulin-1 and tricellulin are altered in a menstrual cycle-dependent manner. Moreover, it has been shown that endometrial cancer progression affects these alterations. This review highlights the differences in the localization and expression of tight junction proteins in normal endometrial cells and endometrial cancers and how they cause functional changes in cells.

Abnormality of hepatic triglyceride metabolism in ApcMin/+ mice with colon cancer cachexia.

AIMS: Colorectal cancer syndrome has been one of the greatest concerns in the world. Although several epidemiological studies have shown that hepatic low lipoprotein lipase (LPL) mRNA expression may be associated with dyslipidemia and tumor progression, it is still not known whether the liver plays an essential role in hyperlipidemia of ApcMin/+ mice. MAIN METHODS: We measured the expression of metabolic enzymes that involved fatty acid uptake, de novo lipogenesis (DNL), ß-oxidation and investigated hepatic triglyceride production in the liver of wild-type and ApcMin/+ mice. KEY FINDINGS: We found that hepatic fatty acid uptake and DNL decreased, but there was no significant difference in fatty acid ß-oxidation. Interestingly, the production of hepatic very low-density lipoprotein-triglyceride (VLDL-TG) decreased at 20 weeks of age, but marked steatosis was observed in the livers of the ApcMin/+ mouse. To further explore hypertriglyceridemia, we assessed the function of hepatic glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 (GPIHBP1) for the first time. GPIHBP1 is governed by the transcription factor octamer-binding transcription factor-1 (Oct-1) which are involved in the nuclear factor-κB (NF-κB) signaling pathway in the liver of ApcMin/+ mice. Importantly, it was also confirmed that sn50 (100 µg/mL, an inhibitor of the NF-κB) reversed the tumor necrosis factor α (TNFα)-induced Oct-1 and GPIHBP1 reduction in HepG2 cells. SIGNIFICANCE: Altogether, these findings highlighted a novel role of GPIHBP1 that might be responsible for hypertriglyceridemia in ApcMin/+ mice. Hypertriglyceridemia in these mice may be associated with their hepatic lipid metabolism development.

Gpihbp1 deficiency accelerates atherosclerosis and plaque instability in diabetic Ldlr-/- mice.

BACKGROUND AND AIMS: Glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 (GPIHBP1) plays a crucial role in triglyceride hydrolysis, and GPIHBP1 deficiency leads to severe hypertriglyceridemia (HTG). Gpihbp1 knockout (GKO) mice develop mild lesions in the aortic root at the age of 11 months. Herein, we investigated the effect of Gpihbp1 deficiency on atherosclerosis (AS) under diabetic conditions. METHODS: For experiment 1, diabetes was induced in GKO and wild-type (WT) mice by injection of streptozotocin at 3 months of age and lasted for 4 months. For experiment 2, diabetes was induced in Gpihbp1/low-density lipoprotein receptor (Ldlr) double-knockout (GLDKO) mice, Ldlr knockout (LKO) mice were used as controls. The experiment was continued for 3 or 5 months. Plasma glucose and lipid levels were measured, and atherosclerotic lesions were analyzed at 3 and 5 months during the experiment. RESULTS: No atherosclerotic lesions were detected in the aorta in GKO mice after 4 months of diabetes. Compared with LKO mice, GLDKO mice manifested enhanced aortic atherosclerotic lesions, decreased plaque stability, and increased oxidative stress and inflammation in plaques at 3 and 5 months after diabetes. Atherosclerotic lesions in the coronary artery and dilated remodeling in the aortic root were also found in GLDKO diabetic mice. CONCLUSIONS: Gpihbp1 deficiency accelerates the development of AS in the aorta, and the instability of plaques in LKO mice and diabetes promotes these pathologic processes with coronary AS. These findings were probably associated with HTG caused by Gpihbp1 deficiency and with increased oxidative stress and inflammation in the atherosclerotic lesions.

The Behavior and Role of Lipolysis-stimulated Lipoprotein Receptor, a Component of Tricellular Tight Junctions, in Head and Neck Squamous Cell Carcinomas.

BACKGROUND/AIM: Lipolysis-stimulated lipoprotein receptor (LSR) knockdown has also been reported to increase the motility and invasiveness of certain cancer cells. Here, we describe, for the first time, the behavior and role of LSR in head and neck squamous cell carcinoma (HNSCC) in vivo and in vitro. MATERIALS AND METHODS: Samples of HNSCC, normal palatine tonsils, the pharynx carcinoma cell line Detroit562 and primary cultured HNSCC were characterized by immunostaining, western blot, real-time polymerase chain reaction (PCR), Matrigel invasion and proliferation assays. RESULTS: Protein and mRNA of LSR were strongly expressed, as well as claudin-1 in HNSCC tissues than in normal tissues, especially in invasive tissues. Knock-down of LSR and claudin-1 (CLDN-1), but not tricellulin (TRIC) by siRNAs, markedly induced invasiveness of Detroit562 cells and primary cultured HNSCC. LSR inhibited the development and progression of HNSCC. CONCLUSION: LSR is a potential target for new forms of head and neck cancer therapy.

LSR/angulin-1 is a tricellular tight junction protein involved in blood-brain barrier formation.

The blood-brain barrier (BBB) is a term used to describe the unique properties of central nervous system (CNS) blood vessels. One important BBB property is the formation of a paracellular barrier made by tight junctions (TJs) between CNS endothelial cells (ECs). Here, we show that Lipolysis-stimulated lipoprotein receptor (LSR), a component of paracellular junctions at points in which three cell membranes meet, is greatly enriched in CNS ECs compared with ECs in other nonneural tissues. We demonstrate that LSR is specifically expressed at tricellular junctions and that its expression correlates with the onset of BBB formation during embryogenesis. We further demonstrate that the BBB does not seal during embryogenesis in Lsr knockout mice with a leakage to small molecules. Finally, in mouse models in which BBB was disrupted, including an experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis and a middle cerebral artery occlusion (MCAO) model of stroke, LSR was down-regulated, linking loss of LSR and pathological BBB leakage.

Roles of lipoprotein and apolipoprotein in HCV infection.

Although chronic infection of hepatitis C virus (HCV) induces disorders of lipid metabolism, HCV is known to utilize lipid metabolism for efficient propagation in the liver. Due to the morphological and physiological similarities of HCV particles to lipoproteins, lipid-associated HCV particles are named lipoviroparticles. Previous reports have shown that lipoprotein receptors or cholesterol transporter participate in the entry of lipoviroparticles. In addition, recent analyses revealed that exchangeable apolipoproteins directly interact with the viral membrane to generate infectious HCV particles. In this review, we would like to discuss about involvement of lipoprotein and apolipoprotein in HCV lifecycle.

Linking nutritional regulation of Angptl4, Gpihbp1, and Lmf1 to lipoprotein lipase activity in rodent adipose tissue.

BACKGROUND: Lipoprotein lipase (LPL) hydrolyzes triglycerides in lipoproteins and makes fatty acids available for tissue metabolism. The activity of the enzyme is modulated in a tissue specific manner by interaction with other proteins. We have studied how feeding/fasting and some related perturbations affect the expression, in rat adipose tissue, of three such proteins, LMF1, an ER protein necessary for folding of LPL into its active dimeric form, the endogenous LPL inhibitor ANGPTL4, and GPIHBP1, that transfers LPL across the endothelium. RESULTS: The system underwent moderate circadian oscillations, for LPL in phase with food intake, for ANGPTL4 and GPIHBP1 in the opposite direction. Studies with cycloheximide showed that whereas LPL protein turns over rapidly, ANGPTL4 protein turns over more slowly. Studies with the transcription blocker Actinomycin D showed that transcripts for ANGPTL4 and GPIHBP1, but not LMF1 or LPL, turn over rapidly. When food was withdrawn the expression of ANGPTL4 and GPIHBP1 increased rapidly, and LPL activity decreased. On re-feeding and after injection of insulin the expression of ANGPTL4 and GPIHBP1 decreased rapidly, and LPL activity increased. In ANGPTL4(-/-) mice adipose tissue LPL activity did not show these responses. In old, obese rats that showed signs of insulin resistance, the responses of ANGPTL4 and GPIHBP1 mRNA and of LPL activity were severely blunted (at 26 weeks of age) or almost abolished (at 52 weeks of age). CONCLUSIONS: This study demonstrates directly that ANGPTL4 is necessary for rapid modulation of LPL activity in adipose tissue. ANGPTL4 message levels responded very rapidly to changes in the nutritional state. LPL activity always changed in the opposite direction. This did not happen in Angptl4(-/-) mice. GPIHBP1 message levels also changed rapidly and in the same direction as ANGPTL4, i.e. increased on fasting when LPL activity decreased. This was unexpected because GPIHBP1 is known to stabilize LPL. The plasticity of the LPL system is severely blunted or completely lost in insulin resistant rats.

The role of lipoprotein receptors on the physiological function of APP.

In this review, we will primarily focus on the role of members of the low-density lipoprotein receptor (LDL-R) family that are involved in trafficking and processing of the amyloid precursor protein (APP). We will discuss the role of the LDL-receptor family members, low-density lipoprotein receptor-related protein 1 (LRP1), LRP1b, apolipoprotein E receptor 2, sortilin-related receptor (SorLA/LR11) and megalin/LRP2 on the physiological function of APP and its cellular localization. Additionally, we will focus on adaptor proteins that have been shown to influence the physiological function of LDL-R family members in combination with APP processing. The results in this review emphasize that the physiological function of APP cannot be explained by the focus on the APP protein alone but rather in combination with various direct or indirect interaction partners within the cellular environment.

Reelin signals through apolipoprotein E receptor 2 and Cdc42 to increase growth cone motility and filopodia formation.

Lipoprotein receptor signaling regulates the positioning and differentiation of postmitotic neurons during development and modulates neuronal plasticity in the mature brain. Depending on the contextual situation, the lipoprotein receptor ligand Reelin can have opposing effects on cortical neurons. We show that Reelin increases growth cone motility and filopodia formation, and identify the underlying signaling cascade. Reelin activates the Rho GTPase Cdc42, known for its role in neuronal morphogenesis and directed migration, in an apolipoprotein E receptor 2-, Disabled-1-, and phosphatidylinositol 3-kinase-dependent manner. We demonstrate that neuronal vesicle trafficking, a Cdc42-controlled process, is increased after Reelin treatment and further provide evidence that the peptidergic VIP/PACAP38 system and Reelin can functionally interact to promote axonal branching. In conclusion, Reelin-induced activation of Cdc42 contributes to the regulation of the cytoskeleton of individual responsive neurons and converges with other signaling cascades to orchestrate Rho GTPase activity and promote neuronal development. Our data link the observation that defects in Rho GTPases and Reelin signaling are responsible for developmental defects leading to neurological and psychiatric disorders.

The metabolism of triglyceride-rich lipoproteins revisited: new players, new insight.

Peripheral lipoprotein lipase (LPL)-mediated lipolysis of triglycerides is the first step in chylomicron/VLDL clearance involving heparan sulfate proteoglycans (HSPGs) displayed at the cell surface of the capillaries in adipose tissue, heart and skeletal muscle. The newly generated chylomicron remnant particles are then cleared by the liver, whereas VLDL remnant particles are either further modified, through the action of hepatic lipase (HL) and cholesteryl ester transfer protein (CETP), into LDL particles or alternatively directly cleared by the liver. Two proteins, lipase maturation factor 1 (LMF1) and glycosylphosphatidylinositol-anchored high density lipoprotein binding protein 1 (GPIHBP1), have been recently identified and have revised our current understanding of LPL maturation and LPL-mediated lipolysis. Moreover, new insights have been gained with respect to hepatic remnant clearance using genetically modified mice targeting the sulfation of HSPGs and even deletion of the most abundant heparan sulfate proteoglycan: syndecan1. In this review, we will provide an overview of novel data on both peripheral TG hydrolysis and hepatic remnant clearance that will improve our knowledge of plasma triglyceride metabolism.

Nontraditional signaling mechanisms of lipoprotein receptors.

The low-density lipoprotein receptor family consists of a large number of single transmembrane proteins that are involved both in endocytosis of extracellular ligands and in intracellular signaling processes. New evidence ties these receptors to the transactivation of Trk receptors. Thus, this single receptor family demonstrates several distinct mechanisms for transducing information across the plasma membrane.

Liver-specific loss of lipolysis-stimulated lipoprotein receptor triggers systemic hyperlipidemia in mice.

OBJECTIVE: In mammals, proper storage and distribution of lipids in and between tissues is essential for the maintenance of energy homeostasis. In contrast, aberrantly high levels of triglycerides in the blood ("hypertriglyceridemia") represent a hallmark of the metabolic syndrome and type 2 diabetes. As hypertriglyceridemia has been identified as an important risk factor for cardiovascular complications, in this study we aimed to identify molecular mechanisms in aberrant triglyceride elevation under these conditions. RESEARCH DESIGN AND METHODS: To determine the importance of hepatic lipid handling for systemic dyslipidemia, we profiled the expression patterns of various hepatic lipid transporters and receptors under healthy and type 2 diabetic conditions. A differentially expressed lipoprotein receptor was functionally characterized by generating acute, liver-specific loss- and gain-of-function animal models. RESULTS: We show that the hepatic expression of lipid transporter lipolysis-stimulated lipoprotein receptor (LSR) is specifically impaired in mouse models of obesity and type 2 diabetes and can be restored by leptin replacement. Experimental imitation of this pathophysiological situation by liver-specific knockdown of LSR promotes hypertriglyceridemia and elevated apolipoprotein (Apo)B and E serum levels in lean wild-type and ApoE knockout mice. In contrast, genetic restoration of LSR expression in obese animals to wild-type levels improves serum triglyceride levels and serum profiles in these mice. CONCLUSIONS: The dysregulation of hepatic LSR under obese and diabetic conditions may provide a molecular rationale for systemic dyslipidemia in type 2 diabetes and the metabolic syndrome and represent a novel target for alternative treatment strategies in these patients.

Age-related influence of the HDL receptor SR-BI on synaptic plasticity and cognition.

Dysregulated cholesterol metabolism is a major risk factor for atherosclerosis and other late-onset disorders, such as Alzheimer's disease. The scavenger receptor, class B, type I (SR-BI) is critical in maintaining the homeostasis of cholesterol and alpha-tocopherol. SR-BI binds high-density lipoproteins (HDL) and mediates the selective transfer of cholesteryl esters and alpha-tocopherol from circulating HDL to cells. SR-BI is also involved in reverse cholesterol transport from peripheral tissues into the liver. Previous studies using SR-BI genetic knockout mice indicated that the deletion of SR-BI resulted in an accelerated onset of atherosclerosis. We hypothesized that SR-BI-dependent lipid dysregulation might disrupt brain function leading to cognitive impairment. Here, we report that very old SR-BI knockout mice show deficient synaptic plasticity (long-term potentiation) in the CA1 region of the hippocampus. Very old SR-BI KO mice also display selective impairments in recognition memory and spatial memory. Thus, SR-BI influences neural and cognitive processes, a finding that highlights the contribution of cholesterol and alpha-tocopherol homeostasis in proper cognitive function.

The Reelin receptors Apoer2 and Vldlr coordinate the patterning of Purkinje cell topography in the developing mouse cerebellum.

The adult cerebellar cortex is comprised of reproducible arrays of transverse zones and parasagittal stripes of Purkinje cells. Adult stripes are created through the perinatal rostrocaudal dispersion of embryonic Purkinje cell clusters, triggered by signaling through the Reelin pathway. Reelin is secreted by neurons in the external granular layer and deep cerebellar nuclei and binds to two high affinity extracellular receptors on Purkinje cells-the Very low density lipoprotein receptor (Vldlr) and apolipoprotein E receptor 2 (Apoer2). In mice null for either Reelin or double null for Vldlr and Apoer2, Purkinje cell clusters fail to disperse. Here we report that animals null for either Vldlr or Apoer2 individually, exhibit specific and parasagittally-restricted Purkinje cell ectopias. For example, in mice lacking Apoer2 function immunostaining reveals ectopic Purkinje cells that are largely restricted to the zebrin II-immunonegative population of the anterior vermis. In contrast, mice null for Vldlr have a much larger population of ectopic Purkinje cells that includes members from both the zebrin II-immunonegative and -immunopositive phenotypes. HSP25 immunoreactivity reveals that in Vldlr null animals a large portion of zebrin II-immunopositive ectopic cells are probably destined to become stripes in the central zone (lobules VI-VII). A small population of ectopic zebrin II-immunonegative Purkinje cells is also observed in animals heterozygous for both receptors (Apoer2(+/-): Vldlr(+/-)), but no ectopia is present in mice heterozygous for either receptor alone. These results indicate that Apoer2 and Vldlr coordinate the dispersal of distinct, but overlapping subsets of Purkinje cells in the developing cerebellum.

Lipoproteins and their receptors in embryonic development: more than cholesterol clearance.

Previously, the relevance of lipoproteins and their receptors has mainly been discussed in terms of cholesterol clearance in the adult organism. Now, findings from nematodes to fruit flies to mammals all point towards novel and unexpected roles for lipoprotein metabolism in the control of key regulatory pathways in the developing embryo, including signaling through steroid hormones and throughout the hedgehog and Wnt signaling pathways. Here, we discuss the emerging view of how lipoproteins and their receptors regulate embryogenesis.

GPIHBP1: an endothelial cell molecule important for the lipolytic processing of chylomicrons.

PURPOSE OF REVIEW: To summarize recent data indicating that glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1) plays a key role in the lipolytic processing of chylomicrons. RECENT FINDINGS: Lipoprotein lipase hydrolyses triglycerides in chylomicrons at the luminal surface of the capillaries in heart, adipose tissue, and skeletal muscle. The endothelial cell molecule that facilitates the lipolytic processing of chylomicrons has never been clearly defined. Mice lacking GPIHBP1 manifest chylomicronemia, with plasma triglyceride levels as high as 5000 mg/dl. In wild-type mice, GPIHBP1 is expressed on the luminal surface of capillaries in heart, adipose tissue, and skeletal muscle. Cells transfected with GPIHBP1 bind both chylomicrons and lipoprotein lipase avidly. SUMMARY: The chylomicronemia in Gpihbp1-deficient mice, the fact that GPIHBP1 is located within the lumen of capillaries, and the fact that GPIHBP1 binds lipoprotein lipase and chylomicrons suggest that GPIHBP1 is a key platform for the lipolytic processing of triglyceride-rich lipoproteins.