Regulation of very-low-density lipoprotein receptor in hypertrophic rat heart

Soluble very low-density lipoprotein receptor (sVLDLR) inhibits fibrosis in neovascular age-related macular degeneration

Subretinal fibrosis is a key pathological feature in neovascular age‐related macular degeneration (nAMD). Previously, we identified soluble very low‐density lipoprotein receptor (sVLDLR) as an endogenous Wnt signaling inhibitor. This study investigates whether sVLDLR plays an anti‐fibrogenic role in nAMD models, including Vldlr^−/− mice and laser‐induced choroidal neovascularization (CNV). We found that fibrosis factors including P‐Smad2/3, α‐SMA, and CTGF were upregulated in the subretinal area of Vldlr^−/− mice and the laser‐induced CNV model. The antibody blocking Wnt co‐receptor LRP6 significantly attenuated the overexpression of fibrotic factors in these two models. Moreover, there was a significant reduction of sVLDLR in the interphotoreceptor matrix (IPM) in the laser‐induced CNV model. A transgenic strain (sVLDLR‐Tg) with sVLDLR overexpression in the IPM was generated. Overexpression of sVLDLR ameliorated the profibrotic changes in the subretinal area of the laser‐induced CNV model. In addition, Wnt and TGF‐β signaling synergistically promoted fibrogenesis in human primary retinal pigment epithelium (RPE) cells. CRISPR/Cas9‐mediated LRP6 gene knockout (KO) attenuated this synergistic effect. The disruption of VLDLR expression promoted, while the overexpression of sVLDLR inhibited TGF‐β‐induced fibrosis. These findings suggest that overactivated Wnt signaling enhances the TGF‐β pathway in subretinal fibrosis. sVLDLR confers an antifibrotic effect, at least partially, through the inhibition of Wnt signaling and thus, has therapeutic potential for fibrosis.

The Pathogenic Role of Very Low Density Lipoprotein on Atrial Remodeling in the Metabolic Syndrome

Atrial fibrillation (AF) is the most common persistent arrhythmia, and can lead to systemic thromboembolism and heart failure. Aging and metabolic syndrome (MetS) are major risks for AF. One of the most important manifestations of MetS is dyslipidemia, but its correlation with AF is ambiguous in clinical observational studies. Although there is a paradoxical relationship between fasting cholesterol and AF incidence, the benefit from lipid lowering therapy in reduction of AF is significant. Here, we reviewed the health burden from AF and MetS, the association between two disease entities, and the metabolism of triglyceride, which is elevated in MetS. We also reviewed scientific evidence for the mechanistic links between very low density lipoproteins (VLDL), which primarily carry circulatory triglyceride, to atrial cardiomyopathy and development of AF. The effects of VLDL to atria suggesting pathogenic to atrial cardiomyopathy and AF include excess lipid accumulation, direct cytotoxicity, abbreviated action potentials, disturbed calcium regulation, delayed conduction velocities, modulated gap junctions, and sarcomere protein derangements. The electrical remodeling and structural changes in concert promote development of atrial cardiomyopathy in MetS and ultimately lead to vulnerability to AF. As VLDL plays a major role in lipid metabolism after meals (rather than fasting state), further human studies that focus on the effects/correlation of postprandial lipids to atrial remodeling are required to determine whether VLDL-targeted therapy can reduce MetS-related AF. On the basis of our scientific evidence, we propose a pivotal role of VLDL in MetS-related atrial cardiomyopathy and vulnerability to AF.

Impaired stimulation of glucose transport in cardiac myocytes exposed to very low-density lipoproteins

We recently observed that free fatty acids impair the stimulation of glucose transport into cardiomyocytes in response to either insulin or metabolic stress. In vivo, fatty acids for the myocardium are mostly obtained from triglyceride-rich lipoproteins (chylomicrons and Very Low-Density Lipoproteins). We therefore determined whether exposure of cardiac myocytes to VLDL resulted in impaired basal and stimulated glucose transport. Primary adult rat cardiac myocytes were chronically exposed to VLDL before glucose uptake was measured in response to insulin or metabolic stress, provoked by the mitochondrial ATP synthase inhibitor oligomycin. Exposure of cardiac myocytes to VLDL reduced both insulin-and oligomycin-stimulated glucose uptake. The reduction of glucose uptake was associated with a moderately reduced tyrosine phosphorylation of the insulin receptor. No reduction of the phosphorylation of the downstream effectors of insulin signaling Akt and AS160 was however observed. Similarly only a modest reduction of the activating phosphorylation of the AMP-activated kinase (AMPK) was observed in response to oligomycin. Similar to our previous observations with free fatty acids, inhibition of fatty acid oxidation restored oligomycin-stimulated glucose uptake. In conclusions, VLDL-derived fatty acids impair stimulated glucose transport in cardiac myocytes by a mechanism that seems to be mediated by a fatty acid oxidation intermediate. Thus, in the clinical context of the metabolic syndrome high VLDL may contribute to enhancement of ischemic injury by reduction of metabolic stress-stimulated glucose uptake.

Cardiac lipotoxicity: molecular pathways and therapeutic implications

Diabetes and obesity are both associated with lipotoxic cardiomyopathy exclusive of coronary artery disease and hypertension. Lipotoxicities have become a public health concern and are responsible for a significant portion of clinical cardiac disease. These abnormalities may be the result of a toxic metabolic shift to more fatty acid and less glucose oxidation with concomitant accumulation of toxic lipids. Lipids can directly alter cellular structures and activate downstream pathways leading to toxicity. Recent data have implicated fatty acids and fatty acyl coenzyme A, diacylglycerol, and ceramide in cellular lipotoxicity, which may be caused by apoptosis, defective insulin signaling, endoplasmic reticulum stress, activation of protein kinase C, MAPK activation, or modulation of PPARs.

AMPK signalling and the control of substrate use in the heart

All mammalian cells rely on adenosine triphosphate (ATP) to maintain function and for survival. The heart has the highest basal ATP demand of any organ due to the necessity for continuous contraction. As such, the ability of the cardiomyocyte to monitor cellular energy status and adapt the supply of substrates to match the energy demand is crucial. One important serine/threonine protein kinase that monitors cellular energy status in the heart is adenosine monophosphate activated protein kinase (AMPK). AMPK is also a key enzyme that controls multiple catabolic and anabolic biochemical pathways in the heart and indirectly plays a crucial role in regulating cardiac function in both physiological and pathophysiological conditions. Herein, we review the involvement of AMPK in myocardial fatty acid and glucose transport and utilization, as it relates to basal cardiac function. We also assess the literature amassed on cardiac AMPK and discuss the controversies surrounding the role of AMPK in physiological and pathophysiological processes in the heart. The work reviewed herein also emphasizes areas that require further investigation for the purpose of eventually translating this information into improved patient care.

Vldlr overexpression causes hyperactivity in rats

BACKGROUND

Reelin regulates neuronal positioning in cortical brain structures and neuronal migration via binding to the lipoprotein receptors Vldlr and Lrp8. Reeler mutant mice display severe brain morphological defects and behavioral abnormalities. Several reports have implicated reelin signaling in the etiology of neurodevelopmental and psychiatric disorders, including autism, schizophrenia, bipolar disorder, and depression. Moreover, it has been reported that VLDLR mRNA levels are increased in the post-mortem brain of autistic patients.

METHODS

We generated transgenic (Tg) rats overexpressing Vldlr, and examined their histological and behavioral features.

RESULTS

Spontaneous locomotor activity was significantly increased in Tg rats, without detectable changes in brain histology. Additionally, Tg rats tended to show performance deficits in the radial maze task, suggesting that their spatial working memory was slightly impaired. Thus, Vldlr levels may be involved in determining locomotor activity and memory function.

CONCLUSIONS

Unlike reeler mice, patients with neurodevelopmental or psychiatric disorders do not show striking neuroanatomical aberrations. Therefore, it is notable, from a clinical point of view, that we observed behavioral phenotypes in Vldlr-Tg rats in the absence of neuroanatomical abnormalities.

Very-low-density lipoprotein: complex particles in cardiac energy metabolism

The heart is a major consumer of energy and is able to utilise a wide range of substrates including lipids. Nonesterified fatty acids (NEFA) were thought to be a favoured carbon source, but their quantitative contribution is limited because of their relative histotoxicity. Circulating triacylglycerols (TAGs) in the form of chylomicrons (CMs) and very-low-density lipoprotein (VLDL) are an alternative source of fatty acids and are now believed to be important in cardiac metabolism. However, few studies on cardiac utilisation of VLDL have been performed and the role of VLDL in cardiac energy metabolism remains unclear. Hearts utilise VLDL to generate ATP, but the oxidation rate of VLDL-TAG is relatively low under physiological conditions; however, in certain pathological states switching of energy substrates occurs and VLDL may become a major energy source for hearts. We review research regarding myocardial utilisation of VLDL and suggest possible roles of VLDL in cardiac energy metabolism: metabolic regulator and extracardiac energy storage for hearts.

Up-regulated expression of type II very low density lipoprotein receptor correlates with cancer metastasis and has a potential link to β-catenin in different cancers

Background

Very low density lipoprotein receptor (VLDLR) has been considered as a multiple function receptor due to binding numerous ligands, causing endocytosis and regulating cellular signaling. Our group previously reported that enhanced activity of type II VLDLR (VLDLR II), one subtype of VLDLR, promotes adenocarcinoma SGC7901 cells proliferation and migration. The aim of this study is to explore the expression levels of VLDLR II in human gastric, breast and lung cancer tissues, and to investigate its relationship with clinical characteristics and β-catenin expression status.

Methods

VLDLR II expression was examined using immunohistochemistry (IHC) and Western blot in tumor tissues from 213 gastric, breast and lung cancer patients, tumor adjacent noncancerous tissues by same methods. Correlations between VLDLR II and clinical features, as well as β-catenin expression status were evaluated by statistical analysis.

Results

The immunohistochemical staining of VLDLR II showed statistical difference between tumor tissues and tumor adjacent noncancerous tissues in gastric, breast and lung cancers (P = 0.034, 0.018 and 0.043, respectively). Moreover, using Western, we found higher VLDLR II expression levels were associated with lymph node and distant metastasis in gastric and breast cancer (P < 0.05). Furthermore, highly significant positive correlations were found between VLDLR II and β-catenin in gastric cancer (r = 0.689; P < 0.001)breast cancer (r = 0.594; P < 0.001).

Conclusions

According to the results of the current study, high VLDLR II expression is correlated with lymph node and distant metastasis in gastric and breast cancer patients, the data suggest that VLDLR II may be a clinical marker in cancers, and has a potential link with β-catenin signaling pathway. This is the first to reveal the closer relationship of VLDLR II with clinical information.

Variation in yolk precursor receptor mRNA expression is a key determinant of reproductive phenotype in the zebra finch (Taeniopygia guttata)

The vitellogenin/very low density lipoprotein receptor (VTG/VLDL-R), a 95 kDa protein that belongs to the low density lipoprotein receptor gene family,mediates the uptake of yolk precursors by developing follicles during oocyte growth. However, the extent to which variation in VTG/VLDL-R expression plays a role in determining inter-individual variation in reproductive phenotype(e.g. follicle or egg size) is not known. Here we show that the mRNA sequence of the zebra finch (Taeniopygia guttata) VTG/VLDL-R shows a high degree of sequence identity (92%) with chicken VTG/VLDL-R mRNA. Using quantitative real-time PCR we measured transcriptional expression of VTG/VLDL-R mRNA in various tissues, and for different stages of oocyte growth,in individual female zebra finches. VTG/VLDL-R mRNA was expressed at high levels in vitellogenic oocytes and in skeletal muscle, and was also detectable in liver, but these tissues expressed different splice variants: the short-form LR8–in oocytes and liver, and the LR8+ form in skeletal muscle. There was significant temporal variation in VTG/VLDL-R expression during follicle growth, with highest levels in ovary and a gradual decrease from pre-F3 to F1 vitellogenic follicles. Variation in ovary mRNA expression was correlated with inter-individual variation in clutch size and laying interval. Furthermore, variation in F3 follicle VTG/VLDL-R mRNA expression was correlated with inter-individual variation in egg mass and F1 follicle mass,suggesting that VTG/VLDL receptor mRNA expression is a key determinant of inter-individual variation in reproductive phenotype.

The lipoprotein lipase gene serine 447 stop variant influences hypertension-induced left ventricular hypertrophy and risk of coronary heart disease

LVH [LV (left ventricular) hypertrophy] is an independent risk factor for CHD (coronary heart disease). During LVH, the preferred cardiac energy substrate switches from FAs (fatty acids) to glucose. LPL (lipoprotein lipase) is the key enzyme in triacylglycerol (triglyceride) hydrolysis and supplies FAs to the heart. To investigate whether substrate utilization influences cardiac growth and CHD risk, we examined the association between the functional LPL S447X (rs328) variant and hypertension-induced LV growth and CHD risk. LPL-X447 has been shown to be more hydrolytically efficient and would therefore release more free FAs than LPL-S477. In a cohort of 190 hypertensive subjects, LPL X447 was associated with a greater LV mass index [85.2 (1.7) in S/S compared with 91.1 (3.4) in S/X+X/X; P=0.01], but no such association was seen in normotensive controls (n=60). X447 allele frequency was higher in hypertensives with than those without LVH {0.14 [95% CI (confidence interval), 0.08-0.19] compared with 0.07 (95% CI, 0.05-0.10) respectively; odds ratio, 2.52 (95% CI, 1.17-5.40), P=0.02}. The association of LPL S447X with CHD risk was then examined in a prospective study of healthy middle-aged U.K. men (n=2716). In normotensive individuals, compared with S447 homozygotes, X447 carriers were protected from CHD risk [HR (hazard ratio), 0.48 (95% CI, 0.23-1.00); P=0.05], whereas, in the hypertensives, X447 carriers had increased risk [HR, 1.54 (95% CI, 1.13-2.09) for S/S (P=0.006) and 2.30 (95% CI, 1.53-3.45) for X447+ (P<0.0001)] and had a significant interaction with hypertension in CHD risk determination (P=0.007). In conclusion, hypertensive LPL X447 carriers have increased risk of LVH and CHD, suggesting that altered FA delivery constitutes a mechanism through which LVH and CHD are associated in hypertensive subjects.

Variation in the lipoprotein lipase gene influences exercise-induced left ventricular growth

The adult heart relies predominantly on fatty acids (FA) for energy generation, and defects in FA catabolism cause dramatic left ventricular (LV) growth in early age. Since lipoprotein lipase (LPL) is the key enzyme in plasma triglyceride catabolism and is highly expressed in the myocardium, we investigated an association between the functional LPL gene serine 447 stop (S447X) variant and exercise-induced LV growth. The S447X variant was genotyped in 146 British Army recruits undergoing a 10-week exercise programme. Over the training period, X447 allele carriers showed less LV growth than S447 homozygotes (SS, 5.8+/-0.7%; SX, 2.2+/-1.5%; P=0.03) and a decrease in systolic blood pressure (DeltaSBP: SS, 1.9+/-1.3 mmHg; SX, -5.7+/-2.2 mmHg; P=0.015). Although LPL genotype did not significantly predict LV growth with DeltaSBP in statistical modelling (LPL, P=0.14; DeltaSBP, P=0.06), regression analysis indicated that LPL S447X genotype effect on DeltaSBP accounted for only 20% of the effect on LV growth. In multivariate analysis, LPL, peroxisome-proliferator-activated receptor alpha and angiotensin-converting enzyme genotypes were independent predictors of cardiac growth. Thus, LPL S447X genotype influenced exercise-induced changes in LV mass and SBP. Change in blood pressure accounted for a proportion of LV growth. These data suggest that increased myocardial FA availability may reduce exercise-induced LV growth.

Transgenic expression of mutant peroxisome proliferator-activated receptor gamma in liver precipitates fasting-induced steatosis but protects against high-fat diet-induced steatosis in mice

Steatosis is one of the most common liver diseases and is associated with the metabolic syndrome. A line of evidence suggests that peroxisome proliferator-activated receptor (PPAR) alpha and PPARgamma are involved in its pathogenesis. Hepatic overexpression of PPARgamma1 in mice provokes steatosis, whereas liver-specific PPARgamma disruption ameliorates steatosis in ob/ob mice, suggesting that hepatic PPARgamma functions as an aggravator of steatosis. In contrast, PPARalpha-null mice are susceptible to steatosis because of reduced hepatic fatty acid oxidation. PPARgamma with mutations in its C-terminal ligand-binding domain (L468A/E471A mutant PPARgamma1) have been reported as a constitutive repressor of both PPARalpha and PPARgamma activities in vitro. To elucidate the effect of co-suppression of PPARalpha and PPARgamma on steatosis, we generated mutant PPARgamma transgenic mice (Liver mt PPARgamma Tg) under the control of liver-specific human serum amyloid P component promoter. In the liver of transgenic mice, PPARalpha and PPARgamma agonist-induced augmentation of the expression of downstream target genes of PPARalpha and PPARgamma, respectively, was significantly attenuated, suggesting PPARalpha and PPARgamma co-suppression in vivo. Suppression of PPARalpha and PPARgamma target genes was also observed in the fasted and high-fat-fed conditions. Liver mt PPARgamma Tg were susceptible to fasting-induced steatosis while being protected against high-fat diet-induced steatosis. The opposite hepatic outcomes in Liver mt PPARgamma Tg as a result of fasting and high-fat feeding may indicate distinct roles of PPARalpha and PPARgamma in 2 different types of nutritionally provoked steatosis.

Effects of oleic acid rich oils on aorta lipids and lipoprotein lipase activity of spontaneously hypertensive rats

Hypertension development in the spontaneously hypertensive rat (SHR) leads to vascular wall widening by smooth muscle cell proliferation. In these cells, triglycerides (TG) and cholesteryl esters (CE) can accumulate until they become foam cells. We administrated two oleic rich oils, virgin olive (VOO) and high oleic sunflower oils (HOSO), to Wistar-Kyoto rats (WKY) and SHR because these oils have been reported to reduce the risk for coronary heart disease in hypertensive patients and SHR. After 12 weeks of feeding, we analyzed the TG and CE composition and the lipolytic (lipoprotein lipase, LPL, and non-LPL) activity in aortas of these animals. HOSO increased the content of linoleic acid in CE and TG of aortas from both WKY and SHR as compared with animals fed VOO by proportionally decreasing the content of oleic acid. Conversely, VOO reduced the LPL and non-LPL lipolytic activities, hence limiting the free fatty acids available for the synthesis of TG and CE in the vascular wall.

Utilization of triacylglycerol-rich lipoproteins by the working rat heart: routes of uptake and metabolic fates

Very-low-density lipoprotein (VLDL) and chylomicrons (CMs) transport triacylglycerol (TAG) to peripheral tissues. Lipoprotein-TAG may gain access to target cells by lipoprotein lipase (LPL) hydrolysis or via receptor-mediated uptake; the principal routes of entry of VLDL and CM into heart are unknown, and different routes of entry may result in different metabolic fates. To examine this, isolated working rat hearts were perfused with rat VLDL and CMs, dual-labelled with [3H]TAG and [14C]cholesterol. Uptake and utilization of CM-TAG were significantly greater than VLDL-TAG, but both were decreased significantly (more than halved) by tetrahydrolipstatin (THL, an inhibitor of lipoprotein lipase). By contrast, uptake of VLDL-cholesterol was much higher than CM-cholesterol (P < 0.01), and suramin (a lipoprotein receptor antagonist) decreased cholesterol uptake of both forms. CM-TAG oxidation rate was more than 4-fold higher than VLDL-TAG oxidation. However, suramin decreased TAG oxidation from both VLDL and CM without affecting TAG uptake or total utilization, suggesting that the TAG gaining access through receptor-mediated pathways is preferentially 'channelled' towards oxidation. Most (79%) CM-TAG was oxidized whilst the proportion of VLDL-TAG oxidized was only about half (49%). In the presence of suramin, there was a significant increase in esterification (incorporation of assimilated [3H]TAG into myocardial tissue [3H]lipids, mainly TAG) of assimilated TAG from both VLDL and CMs, again suggesting that receptor-mediated TAG uptake is directed towards oxidation rather than esterification. The importance of this relatively small pool of TAG is indicated by the fact that cardiac mechanical function declined markedly when lipoprotein receptors were inhibited. These results suggest that CMs, most fatty acids of which gain access into cardiomyocytes through LPL-mediated hydrolysis, are the major supplier of TAG for hearts to oxidize; however, the metabolic fate of VLDL was split evenly between oxidation and deposition as myocardial tissue lipid. Most importantly, VLDL may play a regulatory role in heart lipid metabolism through a lipoprotein receptor-mediated mechanism.

Transgenic amplification of glucocorticoid action in adipose tissue causes high blood pressure in mice

Obesity is closely associated with the metabolic syndrome, a combination of disorders including insulin resistance, diabetes, dyslipidemia, and hypertension. A role for local glucocorticoid reamplification in obesity and the metabolic syndrome has been suggested. The enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) regenerates active cortisol from inactive 11-keto forms, and aP2-HSD1 mice with relative transgenic overexpression of this enzyme in fat cells develop visceral obesity with insulin resistance and dyslipidemia. Here we report that aP2-HSD1 mice also have high arterial blood pressure (BP). The mice have increased sensitivity to dietary salt and increased plasma levels of angiotensinogen, angiotensin II, and aldosterone. This hypertension is abolished by selective angiotensin II receptor AT-1 antagonist at a low dose that does not affect BP in non-Tg littermates. These findings suggest that activation of the circulating renin-angiotensin system (RAS) develops in aP2-HSD1 mice. The long-term hypertension is further reflected by an appreciable hypertrophy and hyperplasia of the distal tubule epithelium of the nephron, resembling salt-sensitive or angiotensin II-mediated hypertension. Taken together, our findings suggest that overexpression of 11beta-HSD1 in fat is sufficient to cause salt-sensitive hypertension mediated by an activated RAS. The potential role of adipose 11beta-HSD1 in mediating critical features of the metabolic syndrome extends beyond obesity and metabolic complications to include the most central cardiovascular feature of this disorder.

The very low density lipoprotein (VLDL) receptor--a peripheral lipoprotein receptor for remnant lipoproteins into fatty acid active tissues

The VLDL (very low density lipoprotein) receptor is a member of the LDL (low density lipoprotein) receptor family. The VLDL receptor binds apolipoprotein (apo) E but not apo B, and is expressed in fatty acid active tissues (heart, muscle, adipose) and macrophages abundantly. Lipoprotein lipase (LPL) modulates the binding of triglyceride (TG)-rich lipoprotein particles to the VLDL receptor. By the unique ligand specificity, VLDL receptor practically appeared to function as IDL (intermediate density lipoprotein) and chylomicron remnant receptor in peripheral tissues in concert with LPL. In contrast to LDL receptor, the VLDL receptor expression is not down regulated by lipoproteins. Recently several possible functions of the VLDL receptor have been reported in lipoprotein metabolism, atherosclerosis, obesity/insulin resistance, cardiac fatty acid metabolism and neuronal migration. The gene therapy of VLDL receptor into the LDL receptor knockout mice liver showed a benefit effect for lipoprotein metabolism and atherosclerosis. Further researches about the VLDL receptor function will be needed in the future.

Remnant lipoprotein particles are taken up into myocardium through VLDL receptor--a possible mechanism for cardiac fatty acid metabolism

The VLDL (very low-density lipoprotein) receptor is a peripheral lipoprotein receptor expressing in fatty acid active tissues abundantly. In the Balb/c fasting mice, VLDL receptor as well as LPL (lipoprotein lipase), FAT (fatty acid translocase)/CD36, H-FABP (heart-type fatty acid-binding protein), ACS (acyl-CoA synthetase) and LCAD (long-chain acyl-CoA dehydrogenase) expressions increased. An electron microscopic examination indicated the lipid droplets that accumulated in the hearts of fasting Balb/c mice. During the development of SD (Sprague-Dawley) rats, VLDL receptor, LPL, FAT/CD36, H-FABP, ACS, and LCAD mRNAs concomitantly increased with growth. However, PK (pyruvate kinase) mRNA expression was negligible. In cultured neonatal rat cardiomyocytes, VLDL receptor expression increased with days in culture. Oil red-O staining showed that cardiomyocytes after 7 days in culture (when the VLDL receptor protein is present) accumulated beta-migrating VLDL. Thereby, we showed that the cardiac VLDL receptor pathway for delivery of remnant lipoprotein particles might be part of a cardiac fatty acid metabolism.

Metabolic cardiomyopathies

The energy needed by cardiac muscle to maintain proper function is supplied by adenosine Ariphosphate primarily (ATP) production through breakdown of fatty acids. Metabolic cardiomyopathies can be caused by disturbances in metabolism, for example diabetes mellitus, hypertrophy and heart failure or alcoholic cardiomyopathy. Deficiency in enzymes of the mitochondrial beta-oxidation show a varying degree of cardiac manifestation. Aberrations of mitochondrial DNA lead to a wide variety of cardiac disorders, without any obvious correlation between genotype and phenotype. A completely different pathogenetic model comprises cardiac manifestation of systemic metabolic diseases caused by deficiencies of various enzymes in a variety of metabolic pathways. Examples of these disorders are glycogen storage diseases (e.g. glycogenosis type II and III), lysosomal storage diseases (e.g. Niemann-Pick disease, Gaucher disease, I-cell disease, various types of mucopolysaccharidoses, GM1 gangliosidosis, galactosialidosis, carbohydrate-deficient glycoprotein syndromes and Sandhoff's disease). There are some systemic diseases which can also affect the heart, for example triosephosphate isomerase deficiency, hereditary haemochromatosis, CD 36 defect or propionic acidaemia.

Very low-density lipoprotein receptor in fetal intestine and gastric adenocarcinoma cells

BACKGROUND

A very low-density lipoprotein receptor (VLDLR) was recently identified. This receptor reportedly binds specifically to very low-density lipoproteins; however, its distribution and functions in vivo have yet to be elucidated. We investigated the expression and regulation of VLDLR in fetal and carcinoma cells.

OBJECTIVE

The expression of VLDLR was examined by immunohistochemistry and reverse-transcriptase polymerase chain reaction using several specimens, including a fetus of 12 to 15 weeks' gestation, various tumors, AGS cells, and INT407 cells.

RESULTS

Immunoreactive VLDLR was abundantly present in human fetal intestinal epithelial and gastric adenocarcinoma cells. This receptor was also noted in the intestinal cell line, INT407, and gastric cancer cell line, AGS. In addition, the VLDLR that was expressed in INT407 cells, AGS cells, and gastric adenocarcinoma tissue was present mainly in a variant form lacking the O-linked sugar domain.

CONCLUSIONS

These data suggest that an important function of VLDLR may be the mediation of cell growth in developing tissues, such as fetal intestinal and cancer cells. The INT407 and AGS cell lines appear to be useful for examining the regulation of VLDLR expression.

The mammalian low-density lipoprotein receptor family

The low-density lipoprotein (LDL) receptor (LDL-R) family consists of cell-surface receptors that recognize extracellular ligands and internalize them for degradation by lysosomes. The LDL-R is the prototype of this family, which also contains very-low-density lipoprotein receptors (VLDL-R), apolipoprotein E receptor 2, LRP, and megalin. The family members contain four major structural modules: the cysteine-rich complement-type repeats, epidermal growth factor precursor-like repeats, a transmembrane domain, and a cytoplasmic domain. Each structural module serves distinct and important functions. These receptors bind several structurally dissimilar ligands. It is proposed that instead of a primary sequence, positive electrostatic potential in different ligands constitutes a receptor binding domain. This family of receptors plays crucial roles in various physiologic functions. LDL-R plays an important role in cholesterol homeostasis. Mutations cause familial hypercholesterolemia and premature coronary artery disease. LDL-R-related protein plays an important role in the clearance of plasma-activated alpha 2-macroglobulin and apolipoprotein E-enriched lipoproteins. It is essential for fetal development and has been associated with Alzheimer's disease. Megalin is the major receptor in absorptive epithelial cells of the proximal tubules and an antigenic determinant for Heymann nephritis in rats. Mutations in a chicken homolog of VLDL-R cause female sterility and premature atherosclerosis. This receptor is not expressed in liver tissue; however, transgenic expression of VLDL-R in liver corrects hypercholesterolemia in experiment animals, which suggests that it can be a candidate for gene therapy for various hyperlipidemias. The functional importance of individual receptors may lie in their differential tissue expression. The regulation of expression of these receptors occurs at the transcriptional level. Expression of the LDL-R is regulated by intracellular sterol levels involving novel membrane-bound transcription factors. Other members of the family are not regulated by sterols. All the members are, however, regulated by hormones and growth factors, but the mechanisms of regulation by hormones have not been elucidated. Studies of these receptors have provided important insights into receptor structure-function and mechanisms of ligand removal and catabolism. It is anticipated that increased knowledge about the LDL-R family members will open new avenues for the treatment of many disorders.

Mouse very low-density lipoprotein receptor (VLDLR): gene structure, tissue-specific expression and dietary and developmental regulation

The very low density lipoprotein receptor (VLDLR) is a multifunctional apolipoprotein (apo) E receptor that shares a common structural feature as well as some ligand specificity to apo E with members of the low density lipoprotein receptor gene family. We have isolated and characterized the mouse VLDLR gene. The mouse VLDLR gene contains 19 exons spanning approximately 50 kb. The exon-intron organization of the gene is completely conserved between mouse and human. Since the 5'-flanking region of the mouse VLDLR gene contains two copies of a sterol regulatory element-1 like sequence (SRE-1), we next studied regulation of the VLDLR mRNA expression in heart, skeletal muscle and adipose tissue in C57BL/6, LDLR-/-, apo E-/- and LDLR-/-apo E-/- mice fed normal chow or atherogenic diet. The VLDLR mRNA expression was down-regulated 3-fold by feeding atherogenic diet in heart and skeletal muscle only in LDLR-/- mice. In contrast, VLDLR mRNA expression was up-regulated by atherogenic diet in adipose tissue in all animal models except double knockout mice. These results suggest that SRE-1 may be functional and VLDLR plays a role in cholesterol homeostasis in heart and skeletal muscle when LDLR is absent and that apo E is required for this modulation. Developmental regulation of the VLDLR mRNA expression was also tissue-specific. VLDLR mRNA expression in heart displayed significant up and down regulation during development. Maximal level was detected on post-natal day 3. However, the VLDLR mRNA levels in skeletal muscle remained relatively constant except a slight dip on post-natal day 7. In kidney and brain, VLDLR mRNA also peaked on post-natal day 3 but remained relatively constant thereafter. In liver, VLDLR mRNA expression was very low; it was barely detectable at day 19 of gestation and was decreased further thereafter. In adipose tissue, the VLDLR mRNA level showed an increase on post-natal day 13, went down again during weaning and then continued to increase afterwards. This developmental pattern as well as dietary regulation in adipose tissue supports the notion that VLDLR plays a role in lipid accumulation in this tissue. Although the primary role of VLDLR in heart, muscle and adipose tissue is likely in lipid metabolism, developmental pattern of this receptor in other tissues suggests that VLDLR has functions that are unrelated to lipid metabolism.

Effect of increased afterload on cardiac lipoprotein lipase and VLDL receptor expression

Fatty acids are a major source of fuel for energy production by myocytes. Lipoprotein lipase (LPL) and very low density lipoprotein (VLDL) receptor are abundantly expressed by the heart and skeletal muscles. LPL and possibly VLDL receptor represent the primary route of access to fatty acids contained in circulating triglyceride-rich lipoproteins. Physical exercise and thyroid hormone, which promote energy consumption, upregulate LPL expression in skeletal muscles. This study tested the hypothesis that increased cardiac workload might modulate myocardial LPL and/or VLDL receptor expressions. Accordingly, cardiac tissue LPL activity, LPL and VLDL receptor proteins and mRNA abundance were studied in Sprague-Dawley rats 4 weeks after induction of severe thoracic aorta constriction or sham operation. Elevation of afterload with thoracic aortic constriction led to a significant cardiomegaly and a marked upregulation of cardiac LPL activity, LPL mRNA and LPL protein abundance, but did not modify VLDL receptor mRNA or protein abundance. Thus, increased cardiac workload in this model results in upregulation of myocardial LPL expression which can enhance fatty acid availability to accommodate the heart's increased energy requirement.

The VLDL receptor: an LDL receptor relative with eight ligand binding repeats, LR8

The discovery in 1992 of a member of the low density lipoprotein receptor (LDLR) family with eight ligand binding repeats (LR8) has raised more questions than have been answered to date. Here, we summarize the current status of knowledge about this intriguing molecule, generally termed VLDL receptor, at the molecular biological, cell biological, and physiological levels. On one hand, the wealth of reports concerning the role(s) of this receptor in lipoprotein metabolism in mammalian systems has revealed partially conflicting details, particularly in regards to its natural ligand(s) and site of action. On the other hand, molecular genetic and biochemical studies in the chicken have clearly demonstrated the multiple roles of LR8 in the physiology and reproduction of egg-laying species, and have generated insights into the evolutionary aspects of the LDLR gene family.

Dynamic regulation of beta-adrenergic receptors by endothelin-1 in smooth-muscle cells

Elevated endothelin-1 (ET-1) levels are found in atherosclerosis, myocardial ischemia, and heart failure, and are correlated with increased mortality rates. Contrary to expectations, elevations of endogenous ET-1 levels in transgenic mice are not associated with increases in arterial blood pressure or with vasospasm, although these effects can be observed after i.v. ET-1 administration. The aim of this study was to determine the regulatory effects of ET-1 on the expression of vasodilator beta-adrenergic receptors and their ability to activate adenylyl cyclase. Smooth-muscle cells were incubated with ET-1 (10(-7) mol/L) for 3 days. The density of ET-1 or beta-adrenergic receptor binding sites was determined using a radioligand binding procedure. Adenylyl cyclase activity was measured to assess any functional changes in the beta-adrenergic receptor density. ET-1 incubation reduced ET-1 binding sites by 70%. In contrast, the beta-adrenergic receptor density increased from 354 +/- 35 to 538 +/- 50 fmol/mg protein (p < 0.01; n = 7) after 3 days. ET-1 increased beta-adrenergic receptors dose-dependently. Incubation with ET-1 for different periods of time showed an initial decrease of 30% after 6 h of ET-1 incubation. However, after 24 h ET-1 induced an increase of beta-adrenergic receptors, reaching a maximal amount after 48 h. An increased stimulation of beta-adrenergic receptor-activated adenylyl cyclase was observed after 3-day ET-1 incubation compared to controls. These data demonstrate that chronic ET receptor activation by ET-1 results in a functionally significant increase in beta-adrenergic receptor density and adenylyl cyclase activity.