Quantification of soluble very low-density lipoprotein receptor in human serum using a sandwich enzyme-linked immunosorbent assay.

To investigate the regulation of soluble very low-density lipoprotein receptor (sVLDL-R), which is cleaved mostly from the extracellular domain of VLDL-R II, we generated two rat monoclonal antibodies (mAbs) against human sVLDL-R, and used them to develop a sandwich enzyme-linked immunosorbent assay (ELISA) to measure sVLDL-R levels in human serum or plasma. The ELISA had a linear range from 0.20 ng/mL to 13.02 ng/mL and allowed for the quantification of sVLDL-R in serum and culture cell medium. The coefficient of variation (CV) was less than 10% for both the intra- and inter-assays. The bililubin F, and C, triglyceride (TG), and hemoglobin levels did not affect assay precision. The sVLDL-R concentration was negatively associated with body fat percentage, TG, and HbA1c, suggesting the possibility of obesity and diabetes in middle-aged Japanese women.

Role of VLDL receptor in atherogenesis.

PURPOSE OF REVIEW: My group previously discovered and characterized the rabbit and human VLDL receptors. For more than 30 years, I have focused on research regarding the functions of VLDL receptors in the fields of lipoprotein metabolism and atherogenesis. In this review, I introduce the roles of VLDL receptors in lipoprotein metabolism under physiological conditions and in atherogenesis under nonphysiological conditions. RECENT FINDINGS: I propose that the VLDL receptor plays key roles in the metabolism of postprandial remnant lipoproteins in concert with lipoprotein lipase (LPL). Furthermore, I propound a new mechanism for macrophage foam cell formation via VLDL receptors by remnant lipoproteins and lipoprotein(a) [Lp(a)] in addition to scavenger receptor pathways. SUMMARY: The VLDL receptor is a so-called macrophage ß-VLDL receptor, which is involved in macrophage foam cell formation by remnant lipoproteins. Furthermore, Lp(a) is a VLDL receptor ligand and is directly taken up through macrophage VLDL receptors for macrophage foam cell formation. Additionally, the roles of VLDL receptors in atherogenesis are canvassed. SUPPLEMENTARY VIDEO ABSTRACT: http://links.lww.com/COL/A21.

Cardiac overexpression of perilipin 2 induces atrial steatosis, connexin 43 remodeling, and atrial fibrillation in aged mice.

Atrial fibrillation (AF) is prevalent in patients with obesity and diabetes, and such patients often exhibit cardiac steatosis. Since the role of cardiac steatosis per se in the induction of AF has not been elucidated, the present study was designed to explore the relation between cardiac steatosis and AF. Transgenic (Tg) mice with cardiac-specific overexpression of perilipin 2 (PLIN2) were housed in the laboratory for more than 12 mo before the study. Electron microscopy of the atria of PLIN2-Tg mice showed accumulation of small lipid droplets around mitochondrial chains, and five- to ninefold greater atrial triacylglycerol (TAG) content compared with wild-type (WT) mice. Electrocardiography showed significantly longer RR intervals in PLIN2-Tg mice than in WT mice. Transesophageal electrical burst pacing resulted in significantly higher prevalence of sustained (>5 min) AF (69%) in PLIN2-Tg mice than in WT mice (24%), although it was comparable in younger (4-mo-old) mice. Connexin 43 (Cx43), a gap junction protein, was localized at the intercalated disks in WT atria but was heterogeneously distributed on the lateral side of cardiomyocytes in PLIN2-Tg atria. Langendorff-perfused hearts using the optical mapping technique showed slower and heterogeneous impulse propagation in PLIN2-Tg atria compared with WT atria. Cardiac overexpression of hormone-sensitive lipase in PLIN2-Tg mice resulted in atrial TAG depletion and amelioration of AF susceptibility. The results suggest that PLIN2-induced steatosis is associated with Cx43 remodeling, impaired conduction propagation, and higher incidence of AF in aged mice. Therapies targeting cardiac steatosis could be potentially beneficial against AF in patients with obesity or diabetes.

The VLDL receptor plays a key role in the metabolism of postprandial remnant lipoproteins.

A new concept to account for the process of postprandial remnant lipoprotein metabolism is proposed based on the characteristics of lipoprotein particles and their receptors. The characteristics of remnant lipoprotein (RLP) were investigated using an immuno-separation method. The majority of the postprandial lipoproteins increased after fat intake was shown to be VLDL remnants, not chylomicron (CM) remnants, based on the significantly high ratio of apoB100/apoB48 in the RLP and the high degree of similarity in the particle size of the apoB48 and apoB100 carrying lipoproteins, which fluctuate in parallel during a 6 h period after fat intake. The VLDL receptor was discovered as a receptor for TG-rich lipoprotein metabolism and is located in peripheral tissues such as skeletal muscle, adipose tissue, etc., but not in the liver. Postprandial VLDL particles are strongly bound and internalized into cells expressing the VLDL receptor. Ligands that bind to VLDL receptor, such as LPL and Lp(a), present in RLP. The presence of various specific ligands in VLDL remnants may enhance the capacity for binding to the VLDL receptor, which play the role primarily for energy delivery to the peripheral tissues, but is also a causal factor in atherogenic diseases when excessively and/or continuously remained in plasma.

Cardiac overexpression of perilipin 2 induces dynamic steatosis: prevention by hormone-sensitive lipase.

Cardiac intracellular lipid accumulation (steatosis) is a pathophysiological phenomenon observed in starvation and diabetes mellitus. Perilipin 2 (PLIN2) is a lipid droplet (LD)-associated protein expressed in nonadipose tissues, including the heart. To explore the pathophysiological function of myocardial PLIN2, we generated transgenic (Tg) mice by cardiac-specific overexpression of PLIN2. Tg hearts showed accumulation of numerous small LDs associated with mitochondrial chains and high cardiac triacylglycerol (TAG) content [8-fold greater than wild-type (WT) mice]. Despite massive steatosis, cardiac uptake of glucose, fatty acids and VLDL, systolic function, and expression of metabolic genes were comparable in the two genotypes, and no morphological changes were observed by electron microscopy in the Tg hearts. Twenty-four hours of fasting markedly reduced steatosis in Tg hearts, whereas WT mice showed accumulation of LDs. Although activity of adipose triglyceride lipase in heart homogenate was comparable between WT and Tg mice, activity of hormone-sensitive lipase (HSL) was 40-50% less in Tg than WT mice under both feeding and fasting conditions, suggesting interference of PLIN2 with HSL. Mice generated through crossing of PLIN2-Tg mice and HSL-Tg mice showed cardiac-specific HSL overexpression and complete lack of steatosis. The results suggest that cardiac PLIN2 plays an important pathophysiological role in the development of dynamic steatosis and that the latter was prevented by upregulation of intracellular lipases, including HSL.

Triglyceride Rich Lipoprotein -LPL-VLDL Receptor and Lp(a)-VLDL Receptor Pathways for Macrophage Foam Cell Formation.

Very low-density lipoprotein (VLDL) receptor is a member of the low-density lipoprotein (LDL) receptor family. It binds triglyceride rich lipoprotein (TGRL) but not LDL, because it recognizes apolipoprotein (apo)E only but not apoB. The VLDL receptor functions as a peripheral lipoprotein receptor in concert with lipoprotein lipase (LPL) in heart, muscle, adipose tissue and macrophages. In contrast to the LDL receptor, VLDL receptor binds apo E2/2 VLDL and apoE3/3 VLDL particles, and its expression is not down-regulated by intracellular lipoproteins. It has been reported that both LDL-cholesterol (LDL-C) and postprandial triglyceride (chyromicron and VLDL remnants) are risk factors for human atherosclerotic cardiovascular disease (ASCVD). True ligands such as lipoprotein particles of the VLDL receptor are chyromicron remnant (CMR) and VLDL remnant (postprandial hyperlipidemia). Although the oxidized LDL (oxLDL)-scavenger receptors pathway is considered to be the main mechanism for macrophage foam cell formation, it seems that the TGRL-LPL-VLDL receptor pathway is also involved. Since Lp(a) is one of the ligands for the VLDL receptor, the Lp(a)-VLDL receptor pathway is another potential alternative. The expression of VLDL receptor protein in mouse macrophages is modest compared to that in rabbit and human macrophages, both in vitro and in vivo. Therefore, we need to elucidate the mechanism of human ASCVD not by using the mouse model and scavenger receptors pathway but instead using the rabbit model and VLDL receptor pathway, respectively.

Differential effects of α-glucosidase inhibitors on postprandial plasma glucose and lipid profile in patients with type 2 diabetes under control with insulin lispro mix 50/50.

BACKGROUND: The additive effect of α-glucosidase inhibitors (α-GIs) was investigated in patients with type 2 diabetes (T2D) under control with rapid-acting insulin analog. SUBJECTS AND METHODS: Thirty-six poorly controlled T2D patients were recruited, and plasma glucose (PG) was controlled by three times daily injection of insulin lispro mix 50/50 (Mix50) to maintain fasting PG <130 mg/dL and 2-h postprandial PG (PPG) <180 mg/dL. Another group of 20 patients was randomly assigned to either 0.3 mg of voglibose or 50 mg of miglitol, which was administered at breakfast every other day. Another group of 16 patients was assigned to a crossover study, in which each α-GI was switched every day during the 6-day study. PPG, C-peptide, and lipid profile were analyzed. RESULTS: The addition of voglibose had no effect on PPG, but miglitol blunted the PPG rise and significantly decreased 1-h and 2-h postprandial C-peptide levels compared with Mix50 alone. In addition, miglitol significantly decreased the 1-h postprandial triglyceride rise and the remnant-like particle-cholesterol rise, while it increased the 1-h postprandial high-density lipoprotein-cholesterol and apolipoprotein A-I levels in the crossover study. CONCLUSIONS: Miglitol appears to have rapid action, which appears earlier than that of lispro. The combination of miglitol and Mix50 seems effective for the control of PPG and lipid profile in T2D.

Comparative reactivity of remnant-like lipoprotein particles (RLP) and low-density lipoprotein (LDL) to LDL receptor and VLDL receptor: effect of a high-dose statin on VLDL receptor expression.

BACKGROUND: Comparison of the reactivity of remnant-like lipoprotein particles (RLP) and LDL particles to LDL receptor and VLDL receptor has not been investigated. METHODS: LDL receptor- or VLDL receptor-transfected ldlA-7, HepG2 and L6 cells were used. Human LDL and rabbit ß-VLDL were isolated by ultracentrifugation. Human RLP was isolated using an immunoaffinity mixed gel. The effect of statin on lipoprotein receptors was examined. RESULTS: Both LDL receptor and VLDL receptor recognized RLP. In LDL receptor transfectants, RLP, ß-VLDL and LDL all bound to LDL receptor. Cold RLP competed efficiently with DiI-ß-VLDL; however, cold LDL competed weakly. In VLDL receptor transfectants, RLP and ß-VLDL bound to VLDL receptor, but not LDL. RLP bound to VLDL receptor with higher affinity than ß-VLDL because of higher apolipoprotein E in RLP. LDL receptor expression was induced in HepG2 by the low concentration of statin while VLDL receptor expression was induced in L6 myoblasts at higher concentration. CONCLUSIONS: RLP are bound to hepatic LDL receptor more efficiently than LDL, which may explain the mechanism by which statins prevent cardiovascular risk by primarily reducing plasma RLP rather than by reducing LDL. Additionally, a high-dose of statins also may reduce plasma RLP through muscular VLDL receptor.

Species differences of macrophage very low-density-lipoprotein (VLDL) receptor protein expression.

Triglyceride-rich lipoproteins (TGRLs) and low-density-lipoprotein (LDL) cholesterol are independent risk factors for coronary artery disease. We have previously proposed that the very low-density-lipoprotein (VLDL) receptor is one of the receptors required for foam cell formation by TGRLs in human macrophages. However, the VLDL receptor proteins have not been detected in atherosclerotic lesions of several animal models. Here we showed no VLDL receptor protein was detected in mouse macrophage cell lines (Raw264.7 and J774.2) or in mouse peritoneal macrophages in vitro. Furthermore, no VLDL receptor protein was detected in macrophages in atherosclerotic lesions of chow-fed apolipoprotein E-deficient or cholesterol-fed LDL receptor-deficient mice in vivo. In contrast, macrophage VLDL receptor protein was clearly detected in human macrophages in vitro and in atherosclerotic lesions in myocardial infarction-prone Watanabe-heritable hyperlipidemic (WHHLMI) rabbits in vivo. There are species differences in the localization of VLDL receptor protein in vitro and in vivo. Since VLDL receptor is expressed on macrophages in atheromatous plaques of both rabbit and human but not in mouse models, the mechanisms of atherogenesis and/or growth of atherosclerotic lesions in mouse models may be partly different from those of humans and rabbits.

Effects of hormone-sensitive lipase disruption on cardiac energy metabolism in response to fasting and refeeding.

Increased fatty acid (FA) flux and intracellular lipid accumulation (steatosis) give rise to cardiac lipotoxicity in both pathological and physiological conditions. Since hormone-sensitive lipase (HSL) contributes to intracellular lipolysis in adipose tissue and heart, we investigated the impact of HSL disruption on cardiac energy metabolism in response to fasting and refeeding. HSL-knockout (KO) mice and wild-type (WT) littermates were fasted for 24 h, followed by ∼6 h of refeeding. Plasma FA concentration in WT mice was elevated twofold with fasting, whereas KO mice lacked this elevation, resulting in twofold lower cardiac FA uptake compared with WT mice. Echocardiography showed that fractional shortening was 15% decreased during fasting in WT mice and was associated with steatosis, whereas both of these changes were absent in KO mice. Compared with Langendorff-perfused hearts isolated from fasted WT mice, the isolated KO hearts also displayed higher contractile function and a blunted response to FA. Although cardiac glucose uptake in KO mice was comparable with WT mice under all conditions tested, cardiac VLDL uptake and lipoprotein lipase (LPL) activity were twofold higher in KO mice during fasting. The KO hearts showed undetectable activity of neutral cholesteryl esterase and 40% lower non-LPL triglyceride lipase activity compared with WT hearts in refed conditions accompanied by overt steatosis, normal cardiac function, and increased mRNA expression of adipose differentiation-related protein. Thus, the dissociation between cardiac steatosis and functional sequelae observed in HSL-KO mice suggests that excess FA influx, rather than steatosis per se, appears to play an important role in the pathogenesis of cardiac lipotoxicity.

Remnant lipoproteins induced proliferation of human prostate cancer cell, PC-3 but not LNCaP, via low density lipoprotein receptor.

BACKGROUND: Hypertriglyceridemia has been shown to be one of the risk factors for prostate cancer. In this study, we investigated the effect of remnant lipoproteins on cell growth in prostate cancer cell lines. METHODS: Remnant lipoproteins were isolated as remnant like particles (RLP) from human plasma. We used RLP for TG-rich lipoproteins and low density lipoproteins (LDL) for cholesterol-rich lipoproteins respectively and examined the effect of lipoproteins on proliferation of PC-3 and LNCaP cells using MTS assays. Moreover, we studied the effect of RLP and LDL treatment on the regulation of lipoprotein receptors in prostate cancer cells to investigate the relationship between lipoprotein-induced cell proliferation and lipoprotein receptor expression using real-time PCR, Western blotting assays and siRNA. RESULTS: RLP effectively induced PC-3 cell proliferation more than LDL, whereas both RLP and LDL could not induce LNCaP cell proliferation except at a higher concentration of RLP. LDL receptor (LDLr) was expressed in both prostate cancer cells but there was a sharp difference of sterol regulation between two cells. In PC-3 cells, LDL decreased the LDLr expression in some degree, but RLP did not. Meanwhile LDLr expression in LNCaP was easily downregulated by RLP and LDL. Blocking LDLr function significantly inhibited both RLP- and LDL-induced PC-3 cell proliferation. CONCLUSIONS: This study demonstrated that RLP-induced PC-3 cell proliferation more than LDL; however, both RLP and LDL hardly induced LNCaP cell proliferation. The differences of proliferation by lipoproteins might be involved in the regulation of LDLr expression.

[A case of type 2 diabetes mellitus associated with familial hypercholesterolemia confirming the improvement of blood glucose control by colestimide].

A 65-year-old woman had been treated for type 2 diabetes mellitus, familial hypercholesterolemia and old myocardial infarction. Combination therapy of atorvastatin (40 mg/day), ethyl icosapentate (1,200 mg/day), probucol (500 mg/day) and colestimide (1 g/day) had never reached an ideal low-density lipoprotein cholesterol (LDL-C) level. However the conversion to high dose of colestimide (4 g/day) with the same dose of atorvastatin, ethyl icosapentate, and probucol obviously decreased her LDL-C level from 181.2 mg/dl to 148 mg/dl. Reduction of LDL-C level was also associated with the lowering of glycohemoglobin A1c from 10.7% to 8.7% simultaneously. Challenge tests by the cessation and resumption of only colestimide treatment clearly indicated that colestimide has both cholesterol and blood glucose lowering effect. Her body weight and appetite did not change by colestimide treatment. We think that colestimide therapy might provide a beneficial effect on atherosclerotic disease in diabetes mellitus with dyslipidemia through reduction of cholesterol and blood glucose.

Cardiac overexpression of hormone-sensitive lipase inhibits myocardial steatosis and fibrosis in streptozotocin diabetic mice.

Intracellular lipid accumulation (steatosis) and resultant lipotoxicity are key features of diabetic cardiomyopathy. Since cardiac hormone-sensitive lipase (HSL) is activated in diabetic mice, we sought to explore a pathophysiological function of cardiac HSL in the development of diabetic cardiomyopathy. Transgenic (Tg) mice with heart-specific HSL overexpression were generated, and cardiac histology, function, lipid profile, and gene expressions were analyzed after induction of diabetes by streptozotocin. Electron microscopy showed numerous lipid droplets in wild-type (Wt) hearts after 3 wk of diabetes, whereas Tg mice showed no lipid droplet accumulation. Cardiac content of acylglycerides was increased approximately 50% with diabetes in Wt mice, whereas this was blunted in Tg hearts. Cardiac lipid peroxide content was twofold lower in Tg hearts than in Wt hearts. The mRNA expressions for peroxisome proliferator-activated receptor-alpha, genes for triacylglycerol synthesis, and lipoprotein lipase were increased with diabetes in Wt hearts, whereas this induction was absent in Tg hearts. Expression of genes associated with lipoapoptosis was decreased, whereas antioxidant protein metallothioneins were increased in diabetic Tg hearts. Diabetic Wt hearts showed interstitial fibrosis and increased collagen content. However, Tg hearts displayed no overt fibrosis, concomitant with decreased expression of collagens, transforming growth factor-beta, and matrix metalloproteinase 2. Notably, mortality during the experimental period was approximately twofold lower in diabetic Tg mice compared with Wt mice. In conclusion, since HSL overexpression inhibits cardiac steatosis and fibrosis by apparently hydrolyzing toxic lipid metabolites, cardiac HSL could be a therapeutic target for regulating diabetic cardiomyopathy.

Glucose deprivation accelerates VLDL receptor-mediated TG-rich lipoprotein uptake by AMPK activation in skeletal muscle cells.

Glucose and fatty acids are major energy sources in skeletal muscle. Very low-density lipoprotein receptor (VLDL-R), which is highly expressed in heart, skeletal muscle and adipose tissue, plays a crucial role in metabolism of triglyceride (TG)-rich lipoproteins. To explore energy switching between glucose and fatty acids, we studied expression of VLDL-R and lipoprotein uptake in rat L6 myoblasts. l-Glucose or d-glucose deprivation in the medium noticeably induced the AMPK (AMP-activated protein kinase) activation and VLDL-R expression. Dose-dependent induction of VLDL-R expression was observed when d-glucose was less than 4.2mM. The same phenomenon was also observed in rat primary skeletal myoblasts and cultured vascular smooth muscle cells. The uptake of beta-VLDL but not LDL was accompanied by induction of VLDL-R expression. Our study suggests that the VLDL-R-mediated uptake of TG-rich lipoproteins might compensate for glucose shortfall through AMPK activation in skeletal muscle.

Peroxisome proliferator-activated receptor delta (PPARdelta), a novel target site for drug discovery in metabolic syndrome.

The development of new treatments for metabolic syndrome is urgent project for decreasing the prevalence of coronary heart disease and diabetes mellitus in the advanced countries. Peroxisome proliferator-activated receptor (PPAR)alpha and gamma agonists have shed light on the treatment of hypertriglyceridemia and type 2 diabetes mellitus, respectively. Among PPARs, analysis of the PPARdelta functions is lagging behind because specific PPARdelta agonists have not been developed. The appearance of new PPARdelta agonists is brightening the prospects for elucidating the physiological role of PPARdelta. PPARdelta is a new target for the treatment of metabolic syndrome. In particular, the fact that fatty acid oxidation and energy dissipation in skeletal muscle and adipose tissue by PPARdelta agonists lead to improved lipid profile, reduced adiposity and insulin sensitivity is a breakthrough. It seems that treatment of PPARdelta agonists operate similarly to the caloric restriction and prolonged exercise. We suggest that the physiological role of PPARdelta may be an indicator for switching from glucose metabolism to fatty acid metabolism. To receive new benefits of PPARdelta agonists against metabolic syndrome by increasing fatty acid consumption in skeletal muscle and adipose tissue, we need to unveil more details on the functions of PPARdelta itself and its agonists in the future.

Changes in cardiac lipid metabolism during sepsis: the essential role of very low-density lipoprotein receptors.

OBJECTIVE: Sepsis accompanies myocardial dysfunction and dynamic alterations of cardiac metabolism. We have recently demonstrated that the very low-density lipoprotein receptor (VLDL-R), which is abundantly expressed in the heart, plays a key role in energy metabolism of the fasting heart. However, little is known about the function and regulation of the VLDL-R during sepsis. In the present study, we explored lipid accumulation and VLDL-R expression in the lipopolysaccharide (LPS)-stimulated heart in vivo and regulation of VLDL-R expression in vitro. METHODS AND RESULTS: Electron microscopy and immunohistochemistry demonstrated that LPS significantly decreased both lipid accumulation and VLDL-R expression in the hearts of fasting mice. Treatment with LPS also downregulated VLDL-R in rat neonatal cardiac myocytes, and this downregulation was completely reversed by interleukin (IL)-1beta receptor antagonist. IL-1beta downregulated the expression of VLDL-R in a time- and dose-dependent manner and markedly reduced the uptake of DiI-labeled beta-VLDL but not DiI-labeled low-density lipoprotein (LDL). Use of specific pharmacologic inhibitors and short interference RNA revealed that Hsp90 was required for IL-1beta to downregulate VLDL-R expression. CONCLUSIONS: These findings suggest that IL-1beta is a principle mediator of changes in cardiac lipid and energy metabolism during sepsis through the downregulation of myocardial VLDL-R expression.

Deficiency of the very low-density lipoprotein (VLDL) receptors in streptozotocin-induced diabetic rats: insulin dependency of the VLDL receptor.

Hyperlipidemia is a common feature of diabetes and is related to cardiovascular disease. The very low-density lipoprotein receptor (VLDL-R) is a member of the low-density lipoprotein receptor (LDL-R) family. It binds and internalizes triglyceride-rich lipoproteins with high specificity. We examined the etiology of hyperlipidemia in the insulin-deficient state. VLDL-R expression in heart and skeletal muscle were measured in rats with streptozotocin (STZ)-induced diabetes. STZ rats showed severe hyperlipidemia on d 21 and 28, with a dramatic decline in VLDL-R protein in skeletal muscle (>90%), heart (approximately 50%) and a loss of adipose tissues itself on d 28. The reduction of VLDL-R protein in skeletal muscle could not be explained simply by a decrease at the transcriptional level, because a dissociation between VLDL-R protein and mRNA expression was observed. The expression of LDL-R and LDL-R-related protein in liver showed no consistent changes. Furthermore, no effect on VLDL-triglyceride production in liver was observed in STZ rats. A decrease in postheparin plasma lipoprotein lipase activity started on d 7 and continued to d 28 at the 50% level even though severe hyperlipidemia was detected only on d 21 and 28. In rat myoblast cells, serum deprivation for 24 h induced a reduction in VLDL-R proteins. Insulin (10(-6) m), but not IGF-I (10 ng/ml), restored the decreased VLDL-R proteins by serum deprivation. These results suggest that the combination of VLDL-R deficiency and reduced plasma lipoprotein lipase activity may be responsible for severe hyperlipidemia in insulin-deficient diabetes.