Cholesterol modulates LRP5 expression in the vessel wall

Canonical Wnt pathway and the LDL receptor superfamily in neuronal cholesterol homeostasis and function

AIMS

There is little information on the regulation of cholesterol homeostasis in the brain. Whether cholesterol crosses the blood-brain barrier is under investigation, but the present understanding is that cholesterol metabolism in the brain is independent from that in peripheral tissues. Lipoprotein receptors from the LDL receptor family (LRPs) have key roles in lipid particle accumulation in cells involved in vascular and cardiac pathophysiology; however, their function on neural cells is unknown.

METHODS AND RESULTS

The expression of LRP5 and the components and targets of its downstream signalling pathway, the canonical Wnt pathway, including β-catenin, LEF1, VEGF, OPN, MMP7, and ADAM10, is analysed in the brains of Wt and Lrp5-/- mice and in a neuroblastoma cell line. LRP5 expression is increased in a time- and dose-dependent manner after lipid loading in neuronal cells; however, it does not participate in cholesterol homeostasis as shown by intracellular lipid accumulation analyses. Neurons challenged with staurosporin and H2O2 display an anti-apoptotic protective role for LRP5.

CONCLUSIONS

For the first time, it has been shown that neurons can accumulate intracellular lipids and lipid uptake is performed mainly by the LDLR, while CD36, LRP1, and LRP5 do not play a major role. In addition, it has been shown that LRP5 triggers the canonical Wnt pathway in neuronal cells to generate pro-survival signals. Finally, Lrp5-/- mice have maintained expression of LRP5 only in the brain supporting the biological plausible concept of the need of brain LRP5 to elicit pro-survival processes and embryonic viability.

Atherosclerosis, Diabetes Mellitus, and Cancer: Common Epidemiology, Shared Mechanisms, and Future Management

The involvement of cardiovascular disease in cancer onset and development represents a contemporary interest in basic science. It has been recognized, from the most recent research, that metabolic syndrome-related conditions, ranging from atherosclerosis to diabetes, elicit many pathways regulating lipid metabolism and lipid signaling that are also linked to the same framework of multiple potential mechanisms for inducing cancer. Otherwise, dyslipidemia and endothelial cell dysfunction in atherosclerosis may present common or even interdependent changes, similar to oncogenic molecules elevated in many forms of cancer. However, whether endothelial cell dysfunction in atherosclerotic disease provides signals that promote the pre-clinical onset and proliferation of malignant cells is an issue that requires further understanding, even though more questions are presented with every answer. Here, we highlight the molecular mechanisms that point to a causal link between lipid metabolism and glucose homeostasis in metabolic syndrome-related atherosclerotic disease with the development of cancer. The knowledge of these breakthrough mechanisms may pave the way for the application of new therapeutic targets and for implementing interventions in clinical practice.

Differential cholesterol uptake in liver cells: A role for PCSK9

The clearance of low-density lipoprotein (LDL) particles from the circulation is regulated by the LDL receptor (LDLR) and proprotein convertase subtilisin/kexin 9 (PCSK9) interaction. Its disruption reduces blood cholesterol levels and delays atherosclerosis progression. Whether other members of the LDLR superfamily are in vivo targets of PCSK9 has been poorly explored. The aim of this work was to study the interaction between PCSK9 and members of the LDLR superfamily in the regulation of liver cholesterol homeostasis in an in vivo low-density lipoprotein receptor related protein 5 (LRP5) deficient mice model challenged with high-fat diet. Our results show that Wt and Lrp5^-/- mice fed a hypercholesterolemic diet (HC) have increased cholesterol ester accumulation and decreased liver LDLR and LRP5 gene and protein expression. Very low-density lipoprotein receptor (VLDLR), LRP6, LRP2, and LRP1 expression levels were analyzed in liver samples and show that they do not participate in Lrp5^-/- liver cholesterol uptake. Immunoprecipitation experiments show that LRP5 forms a complex with PCSK9 in liver-specific fat-storing stellate cells but not in structural HepG2 cells. Hepatic stellate cells silenced for LRP5 and/or PCSK9 expression and challenged with lipids show reduced cholesterol ester accumulation, indicating that both proteins are involved in lipid processing in the liver. Our results indicate that cholesterol esters accumulate in livers of Wt mice in a LDLR-family-members dependent manner as VLDLR, LRP2, and LRP6 show increased expression in HC mice. However, this increase is lost in livers of Lrp5^-/- mice, where scavenger receptors are involved in cholesterol uptake. PCSK9 expression is strongly downregulated in mice livers after HC feeding. However PCSK9 and LRP5 bind in the cytoplasm of fat storing liver cells, indicating that this PCSK9-LRP5 interaction is cell-type specific and that both proteins contribute to lipid uptake.

PCSK9 and LRP5 in macrophage lipid internalization and inflammation

AIMS

Atherosclerosis, the leading cause of cardiovascular diseases, is driven by high blood cholesterol levels and chronic inflammation. Low-density lipoprotein receptors (LDLR) play a critical role in regulating blood cholesterol levels by binding to and clearing LDLs from the circulation. The disruption of the interaction between proprotein convertase subtilisin/kexin 9 (PCSK9) and LDLR reduces blood cholesterol levels. It is not well known whether other members of the LDLR superfamily may be targets of PCSK9. The aim of this work was to determine if LDLR-related protein 5 (LRP5) is a PCSK9 target and to study the role of PCSK9 and LRP5 in foam cell formation and lipid accumulation.

METHODS AND RESULTS

Primary cultures of human inflammatory cells (monocytes and macrophages) were silenced for LRP5 or PCSK9 and challenged with LDLs. We first show that LRP5 is needed for macrophage lipid uptake since LRP5-silenced macrophages show less intracellular CE accumulation. In macrophages, internalization of LRP5-bound LDL is already highly evident after 5 h of LDL incubation and lasts up to 24 h; however, in the absence of both LRP5 and PCSK9, there is a strong reduction of CE accumulation indicating a role for both proteins in lipid uptake. Immunoprecipitation experiments show that LRP5 forms a complex with PCSK9 in lipid-loaded macrophages. Finally, PCSK9 participates in TLR4/NFkB signalling; a decreased TLR4 protein expression levels and a decreased nuclear translocation of NFκB were detected in PCSK9 silenced cells after lipid loading, indicating a downregulation of the TLR4/NFκB pathway.

CONCLUSION

Our results show that both LRP5 and PCSK9 participate in lipid uptake in macrophages. In the absence of LRP5, there is a reduced release of PCSK9 indicating that LRP5 also participates in the mechanism of release of soluble PCSK9. Furthermore, PCSK9 up-regulates TLR4/NFκB favouring inflammation.

The Role of Sclerostin in Bone and Ectopic Calcification

Sclerostin, a 22-kDa glycoprotein that is mainly secreted by the osteocytes, is a soluble inhibitor of canonical Wnt signaling. Therefore, when present at increased concentrations, it leads to an increased bone resorption and decreased bone formation. Serum sclerostin levels are known to be increased in the elderly and in patients with chronic kidney disease. In these patient populations, there is a high incidence of ectopic cardiovascular calcification. These calcifications are strongly associated with cardiovascular morbidity and mortality. Although data are still controversial, it is likely that there is a link between ectopic calcification and serum sclerostin levels. The main question, however, remains whether sclerostin exerts either a protective or deleterious role in the ectopic calcification process.

GSK3β inhibition and canonical Wnt signaling in mice hearts after myocardial ischemic damage

Aims

Myocardial infarction induces myocardial injury and tissue damage. During myocardial infarction strong cellular response is initiated to salvage the damaged tissues. This response is associated with the induction of different signaling pathways. Of these, the canonical Wnt signaling is increasingly important for its prosurvival cellular role, making it a good candidate for the search of new molecular targets to develop therapies to prevent heart failure in infarcted patients.

Methods

Herein we report that GSK3β regulates the canonical Wnt signaling in C57Bl6 mice hearts. GSK3β is a canonical Wnt pathway inhibitor. Using GSK3β inhibitors and inducing myocardial injury (MI) in Lrp5^-/- mice model we show that GSK3β phosphorylation levels regulate downstream canonical Wnt pathway genes in the ischemic heart. In the setting of MI, myocardial damage assessment usually correlates with functional and clinical outcomes. Therefore, we measured myocardial injury size in Wt and Lrp5^-/- mice in the presence and absence of two different GSK3 inhibitors prior to MI. Myocardial injury was independent of GSK3 inhibitor treatments and GSK3β expression levels.

Results

These studies support a central role for GSK3β in the activation of the canonical Wnt pathway in the Wt heart. Although LRP5 is protective against myocardial injury, GSK3β expression levels do not regulate heart damage.

Clinical advantages and disadvantages of anabolic bone therapies targeting the WNT pathway

The WNT signalling pathway is a key regulator of bone metabolism, particularly bone formation, which has helped to define the role of osteocytes - the most abundant bone cells - as orchestrators of bone remodelling. Several molecules involved in the control of the WNT signalling pathway have been identified as potential targets for the development of bone-building therapeutics for patients with osteoporosis. Several of these molecules have been investigated in animal models, but only inhibitors of sclerostin (which is produced by osteocytes) have been investigated in phase III clinical studies. Here, we review the rationale for these developments and the specificity and potential off-target actions of WNT-based therapeutics. We also describe the available preclinical and clinical studies and discuss the benefits and risks of using sclerostin inhibitors for the management of patients with osteoporosis.

Myeloid β-Catenin Deficiency Exacerbates Atherosclerosis in Low-Density Lipoprotein Receptor-Deficient Mice

Supplemental Digital Content is available in the text.

Objective—

The Wnt/β-catenin signaling is an ancient and evolutionarily conserved pathway that regulates essential aspects of cell differentiation, proliferation, migration and polarity. Canonical Wnt/β-catenin signaling has also been implicated in the pathogenesis of atherosclerosis. Macrophage is one of the major cell types involved in the initiation and progression of atherosclerosis, but the role of macrophage β-catenin in atherosclerosis remains elusive. This study aims to investigate the impact of β-catenin expression on macrophage functions and atherosclerosis development.

Approach and Results—

To investigate the role of macrophage canonical Wnt/β-catenin signaling in atherogenesis, we generated β-catenin^ΔmyeLDLR^−/− mice (low-density lipoprotein receptor–deficient mice with myeloid-specific β-catenin deficiency). As expected, deletion of β-catenin decreased macrophage adhesion and migration properties in vitro. However, deficiency of β-catenin significantly increased atherosclerotic lesion areas in the aortic root of LDLR^−/− (low-density lipoprotein receptor–deficient) mice without affecting the plasma lipid levels and atherosclerotic plaque composition. Mechanistic studies revealed that β-catenin can regulate activation of STAT (signal transducer and activator of transcription) pathway in macrophages, and ablation of β-catenin resulted in STAT3 downregulation and STAT1 activation, leading to elevated macrophage inflammatory responses and increased atherosclerosis.

Conclusions—

This study demonstrates a critical role of myeloid β-catenin expression in atherosclerosis by modulating macrophage inflammatory responses.

Identification of Smoking-Associated Differentially Methylated Regions Using Reduced Representation Bisulfite Sequencing and Cell type-Specific Enhancer Activation and Gene Expression

BACKGROUND

Cigarette smoke is a causal factor in cancers and cardiovascular disease. Smoking-associated differentially methylated regions (SM-DMRs) have been observed in disease studies, but the causal link between altered DNA methylation and transcriptional change is obscure.

OBJECTIVE

Our objectives were to finely resolve SM-DMRs and to interrogate the mechanistic link between SM-DMRs and altered transcription of enhancer noncoding RNA (eRNA) and mRNA in human circulating monocytes.

METHOD

We integrated SM-DMRs identified by reduced representation bisulfite sequencing (RRBS) of circulating CD14+ monocyte DNA collected from two independent human studies [n=38 from Clinical Research Unit (CRU) and n=55 from the Multi-Ethnic Study of Atherosclerosis (MESA), about half of whom were active smokers] with gene expression for protein-coding genes and noncoding RNAs measured by RT-PCR or RNA sequencing. Candidate SM-DMRs were compared with RRBS of purified CD4+ T cells, CD8+ T cells, CD15+ granulocytes, CD19+ B cells, and CD56+ NK cells (n=19 females, CRU). DMRs were validated using pyrosequencing or bisulfite amplicon sequencing in up to 85 CRU volunteers, who also provided saliva DNA.

RESULTS

RRBS identified monocyte SM-DMRs frequently located in putative gene regulatory regions. The most significant monocyte DMR occurred at a poised enhancer in the aryl-hydrocarbon receptor repressor gene (AHRR) and it was also detected in both granulocytes and saliva DNA. To our knowledge, we identify for the first time that SM-DMRs in or near AHRR, C5orf55-EXOC-AS, and SASH1 were associated with increased noncoding eRNA as well as mRNA in monocytes. Functionally, the AHRR SM-DMR appeared to up-regulate AHRR mRNA through activating the AHRR enhancer, as suggested by increased eRNA in the monocytes, but not granulocytes, from smokers compared with nonsmokers.

CONCLUSIONS

Our findings suggest that AHRR SM-DMR up-regulates AHRR mRNA in a monocyte-specific manner by activating the AHRR enhancer. Cell type-specific activation of enhancers at SM-DMRs may represent a mechanism driving smoking-related disease. https://doi.org/10.1289/EHP2395.

WNT Signaling in Cardiac and Vascular Disease

WNT signaling is an elaborate and complex collection of signal transduction pathways mediated by multiple signaling molecules. WNT signaling is critically important for developmental processes, including cell proliferation, differentiation and tissue patterning. Little WNT signaling activity is present in the cardiovascular system of healthy adults, but reactivation of the pathway is observed in many pathologies of heart and blood vessels. The high prevalence of these pathologies and their significant contribution to human disease burden has raised interest in WNT signaling as a potential target for therapeutic intervention. In this review, we first will focus on the constituents of the pathway and their regulation and the different signaling routes. Subsequently, the role of WNT signaling in cardiovascular development is addressed, followed by a detailed discussion of its involvement in vascular and cardiac disease. After highlighting the crosstalk between WNT, transforming growth factor-β and angiotensin II signaling, and the emerging role of WNT signaling in the regulation of stem cells, we provide an overview of drugs targeting the pathway at different levels. From the combined studies we conclude that, despite the sometimes conflicting experimental data, a general picture is emerging that excessive stimulation of WNT signaling adversely affects cardiovascular pathology. The rapidly increasing collection of drugs interfering at different levels of WNT signaling will allow the evaluation of therapeutic interventions in the pathway in relevant animal models of cardiovascular diseases and eventually in patients in the near future, translating the outcomes of the many preclinical studies into a clinically relevant context.

Systems biology approaches to understand the effects of nutrition and promote health

Within the last years the implementation of systems biology in nutritional research has emerged as a powerful tool to understand the mechanisms by which dietary components promote health and prevent disease as well as to identify the biologically active molecules involved in such effects. Systems biology, by combining several '-omics' disciplines (mainly genomics/transcriptomics, proteomics and metabolomics), creates large data sets that upon computational integration provide in silico predictive networks that allow a more extensive analysis of the individual response to a nutritional intervention and provide a more global comprehensive understanding of how diet may influence health and disease. Numerous studies have demonstrated that diet and particularly bioactive food components play a pivotal role in helping to counteract environmental-related oxidative damage. Oxidative stress is considered to be strongly implicated in ageing and the pathophysiology of numerous diseases including neurodegenerative disease, cancers, metabolic disorders and cardiovascular diseases. In the following review we will provide insights into the role of systems biology in nutritional research and focus on transcriptomic, proteomic and metabolomics studies that have demonstrated the ability of functional foods and their bioactive components to fight against oxidative damage and contribute to health benefits.

LRP5/canonical Wnt signalling and healing of ischemic myocardium

LRP5 (low-density lipoprotein receptor-related protein 5) activates canonical Wnt signalling. LRP5 plays multiple roles including regulation of lipoprotein and cholesterol homeostasis as well as innate immunity cell function. However, it is not known whether LRP5 has a role in the myocardium. The aim of this study was to investigate LRP5 and Wnt signalling in myocardial remodelling after acute myocardial infarction (MI). Wnt protein levels were determined in a hypercholesterolemic porcine model of MI, in Lrp5 ^-/- C57Bl6 mice, in cultured cardiomyocytes and in human explanted hearts with previous MI episodes. 21 days post-MI, there was upregulation of LRP5 in the ischemic myocardium of hypercholesterolemic pigs as well as an upregulated expression of proteins of the Wnt pathway. We demonstrate via overexpression and silencing experiments that LRP5 induces Wnt pathway activation in isolated cardiomyocytes. Hypoxia and lipid-loading induced the expression of Wnt proteins, whereas this effect is blocked in LRP5-silenced cardiomyocytes. To characterize the function of the LRP5-Wnt axis upregulation in the heart, we induced MI in wild-type and Lrp5 ^-/- mice. Lrp5 ^-/- mice had significantly larger infarcts than Wt mice, indicating a protective role of LRP5 in injured myocardium. The LRP5 upregulation in post-MI hearts seen in pigs and mice was also evident in human hearts as dyslipidemic patients with previous episodes of ischemia have higher expression of LRP5 and Wnt-signalling genes than non-ischemic dilated hearts. We demonstrate an upregulation of LRP5 and the Wnt signalling pathway that it is a prosurvival healing response of cardiomyocytes upon injury.

Macrophages of genetically characterized familial hypercholesterolaemia patients show up-regulation of LDL-receptor-related proteins

Familial hypercholesterolaemia (FH) is a major risk for premature coronary heart disease due to severe long‐life exposure to high LDL levels. Accumulation of LDL in the vascular wall triggers atherosclerosis with activation of the innate immunity system. Here, we have investigated (i) gene expression of LDLR and LRPs in peripheral blood cells (PBLs) and in differentiated macrophages of young FH‐patients; and (ii) whether macrophage from FH patients have a differential response when exposed to high levels of atherogenic LDL. PBLs in young heterozygous genetically characterized FH patients have higher expression of LRP5 and LRP6 than age‐matched healthy controls or patients with secondary hypercholesterolaemia. LRP1 levels were similar among groups. In monocyte‐derived macrophages (MACs), LRP5 and LRP1 transcript levels did not differ between FHs and controls in resting conditions, but when exposed to agLDL, FH‐MAC showed a highly significant up‐regulation of LRP5, while LRP1 was unaffected. PBL and MAC cells from FH patients had significantly lower LDLR expression than control cells, independently of the lipid‐lowering therapy. Furthermore, exposure of FH‐MAC to agLDL resulted in a reduced expression of CD163, scavenger receptor with anti‐inflammatory and atheroprotective properties. In summary, our results show for first time that LRPs, active lipid‐internalizing receptors, are up‐regulated in innate immunity cells of young FH patients that have functional LDLR mutations. Additionally, their reduced CD163 expression indicates less atheroprotection. Both mechanisms may play a synergic effect on the onset of premature atherosclerosis in FH patients.

Wnt Signaling in Cardiac Disease

Wnt signaling encompasses multiple and complex signaling cascades and is involved in many developmental processes such as tissue patterning, cell fate specification, and control of cell division. Consequently, accurate regulation of signaling activities is essential for proper embryonic development. Wnt signaling is mostly silent in the healthy adult organs but a reactivation of Wnt signaling is generally observed under pathological conditions. This has generated increasing interest in this pathway from a therapeutic point of view. In this review article, the involvement of Wnt signaling in cardiovascular development will be outlined, followed by its implication in myocardial infarct healing, cardiac hypertrophy, heart failure, arrhythmias, and atherosclerosis. The initial experiments not always offer consensus on the effects of activation or inactivation of the pathway, which may be attributed to (i) the type of cardiac disease, (ii) timing of the intervention, and (iii) type of cells that are targeted. Therefore, more research is needed to determine the exact implication of Wnt signaling in the conditions mentioned above to exploit it as a powerful therapeutic target.

The role of Wnt regulation in heart development, cardiac repair and disease: A tissue engineering perspective

Wingless-related integration site (Wnt) signaling has proven to be a fundamental mechanism in cardiovascular development as well as disease. Understanding its particular role in heart formation has helped to develop pluripotent stem cell differentiation protocols that produce relatively pure cardiomyocyte populations. The resultant cardiomyocytes have been used to generate heart tissue for pharmaceutical testing, and to study physiological and disease states. Such protocols in combination with induced pluripotent stem cell technology have yielded patient-derived cardiomyocytes that exhibit some of the hallmarks of cardiovascular disease and are therefore being used to model disease states. While FDA approval of new treatments typically requires animal experiments, the burgeoning field of tissue engineering could act as a replacement. This would necessitate the generation of reproducible three-dimensional cardiac tissues in a well-controlled environment, which exhibit native heart properties, such as cellular density, composition, extracellular matrix composition, and structure-function. Such tissues could also enable the further study of Wnt signaling. Furthermore, as Wnt signaling has been found to have a mechanistic role in cardiac pathophysiology, e.g. heart attack, hypertrophy, atherosclerosis, and aortic stenosis, its strategic manipulation could provide a means of generating reproducible and specific, physiological and pathological cardiac models.

Wnt signaling in atherosclerosis

Atherosclerosis is a disease of the vascular wall that forms the basis for a large spectrum of pathologies of various organs and tissues. Although massive research efforts in the last decades have yielded valuable information about its underlying molecular mechanisms, this has not led to a translation into effective therapeutic interventions that can stop the progression or even can induce regression of atherosclerosis. This underscores the importance of investigations on the involvement of novel signaling pathways in the development and progression of this condition. In this review we focus on the role of Wnt signaling in atherosclerosis. Experimental evidence is presented that Wnt signaling is involved in many aspects of the development and progression of vascular lesions including endothelial dysfunction, macrophage activation and the proliferation and migration of vascular smooth muscle cells. Subsequently, we will discuss the role of Wnt signaling in myocardial infarction and stroke, two common pathologies resulting from the progression of atherosclerotic lesions towards an unstable phenotype. Despite the fact that the published data sometimes are ambiguous or even conflicting, a picture is emerging that an attenuation of Wnt signaling is beneficial for the cardiovascular system that is compromised by atherosclerosis.