Deficiency of LRP1 in Mature Adipocytes Promotes Diet-Induced Inflammation and Atherosclerosis-Brief Report

LRP1 is the cell-surface endocytosis receptor for vaspin in adipocytes

Vaspin is a serine protease inhibitor that protects against adipose tissue inflammation and insulin resistance, two key drivers of adipocyte dysfunction and metabolic disorders in obesity. Inhibition of target proteases such as KLK7 has been shown to reduce adipose tissue inflammation in obesity, while vaspin binding to cell surface GRP78 has been linked to reduced obesity-induced ER stress and insulin resistance in the liver. However, the molecular mechanisms by which vaspin directly affects cellular processes in adipocytes remain unknown. Using fluorescently labeled vaspin, we found that vaspin is rapidly internalized by mouse and human adipocytes, but less efficiently by endothelial, kidney, liver, and neuronal cells. Internalization occurs by active, clathrin-mediated endocytosis, which is dependent on vaspin binding to the LRP1 receptor, rather than GRP78 as previously thought. This was demonstrated by competition experiments and RNAi-mediated knock-down in adipocytes and by rescuing vaspin internalization in LRP1-deficient Pea13 cells after transfection with a functional LRP1 minireceptor. Vaspin internalization is further increased in mature adipocytes after insulin-stimulated translocation of LRP1. Although vaspin has nanomolar affinity for LRP1 clusters II-IV, binding to cell surface heparan sulfates is required for efficient LRP1-mediated internalization. Native, but not cleaved vaspin, and also vaspin polymers are efficiently endocytosed, and ultimately targeted for lysosomal degradation. Our study provides mechanistic insight into the uptake and degradation of vaspin in adipocytes, thereby broadening our understanding of its functional repertoire. We hypothesize the vaspin-LRP1 axis to be an important mediator of vaspin effects not only in adipose tissue but also in other LRP1-expressing cells.

Fat and inflammation: adipocyte-myeloid cell crosstalk in atherosclerosis

Adipose tissue inflammation has been implicated in various chronic inflammatory diseases and cancer. Perivascular adipose tissue (PVAT) surrounds the aorta as an extra layer and was suggested to contribute to atherosclerosis development. PVAT regulates the function of endothelial and vascular smooth muscle cells in the aorta and represent a reservoir for various immune cells which may participate in aortic inflammation. Recent studies demonstrate that adipocytes also express various cytokine receptors and, therefore, may directly respond to inflammatory stimuli. Here we will summarize current knowledge on immune mechanisms regulating adipocyte activation and the crosstalk between myeloid cells and adipocytes in pathogenesis of atherosclerosis.

Thymosin β4 preserves vascular smooth muscle phenotype in atherosclerosis via regulation of low density lipoprotein related protein 1 (LRP1)

Atherosclerosis is a progressive, degenerative vascular disease and a leading cause of morbidity and mortality. In response to endothelial damage, platelet derived growth factor (PDGF)-BB induced phenotypic modulation of medial smooth muscle cells (VSMCs) promotes atherosclerotic lesion formation and destabilisation of the vessel wall. VSMC sensitivity to PDGF-BB is determined by endocytosis of Low density lipoprotein receptor related protein 1 (LRP1)-PDGFR β complexes to balance receptor recycling with lysosomal degradation. Consequently, LRP1 is implicated in various arterial diseases. Having identified Tβ4 as a regulator of LRP1-mediated endocytosis to protect against aortic aneurysm, we sought to determine whether Tβ4 may additionally function to protect against atherosclerosis, by regulating LRP1-mediated growth factor signalling. By single cell transcriptomic analysis, Tmsb4x, encoding Tβ4, strongly correlated with contractile gene expression and was significantly down-regulated in cells that adopted a modulated phenotype in atherosclerosis. We assessed susceptibility to atherosclerosis of global Tβ4 knockout mice using the ApoE-/- hypercholesterolaemia model. Inflammation, elastin integrity, VSMC phenotype and signalling were analysed in the aortic root and descending aorta. Tβ4KO; ApoE-/- mice develop larger atherosclerotic plaques than control mice, with medial layer degeneration characterised by accelerated VSMC phenotypic modulation. Defects in Tβ4KO; ApoE-/- mice phenocopied those in VSMC-specific LRP1 nulls and, moreover, were underpinned by hyperactivated LRP1-PDGFRβ signalling. We identify an atheroprotective role for endogenous Tβ4 in maintaining differentiated VSMC phenotype via LRP1-mediated PDGFRβ signalling.

Autosomal recessive LRP1-related syndrome featuring cardiopulmonary dysfunction, bone dysmorphology, and corneal clouding

We provide the first study of two siblings with a novel autosomal recessive LRP1-related syndrome identified by rapid genome sequencing and overlapping multiple genetic models. The patients presented with respiratory distress, congenital heart defects, hypotonia, dysmorphology, and unique findings, including corneal clouding and ascites. Both siblings had compound heterozygous damaging variants, c.11420G > C (p.Cys3807Ser) and c.12407T > G (p.Val4136Gly) in LRP1, in which segregation analysis helped dismiss additional variants of interest. LRP1 analysis using multiple human/mouse data sets reveals a correlation to patient phenotypes of Peters plus syndrome with additional severe cardiomyopathy and blood vessel development complications linked to neural crest cells.

Identification of a biomarker and immune infiltration in perivascular adipose tissue of abdominal aortic aneurysm

Objective: Abdominal aortic aneurysm (AAA) refers to unusual permanent dilation of the abdominal aorta, and gradual AAA expansion can lead to fatal rupture. However, we lack clear understanding of the pathogenesis of this disease. The effect of perivascular adipose tissue (PVAT) on vascular functional status has attracted increasing attention. Here, we try to identify the potential mechanisms linking AAA and PVAT.

Methods: We downloaded dataset GSE119717, including 30 dilated AAA PVAT samples and 30 non-dilated aorta PVAT samples from AAA cases, from Gene Expression Omnibus to identify differentially expressed genes (DEGs). We performed pathway enrichment analysis by Metascape, ClueGo and DAVID to annotate PVAT functional status according to the DEGs. A protein-protein interaction network, the support vector machine (SVM)-recursive feature elimination and the least absolute shrinkage and selection operator regression model were constructed to identify feature genes. Immune infiltration analysis was explored by CIBERSORT. And the correlation between feature gene and immune cells was also calculated. Finally, we used the angiotensin II (Ang II)-ApoE−/− mouse model of AAA to verify the effect of feature gene expression by confirming protein expression using immunohistochemistry and western blot.

Results: We identified 22 DEGs, including 21 upregulated genes and 1 downregulated gene. The DEGs were mainly enriched in neutrophil chemotaxis and IL-17 signaling pathway. FOS was identified as a good diagnostic feature gene (AUC = 0.964). Immune infiltration analysis showed a higher level of T cells follicular helper, activated NK cells, Monocytes, activated Mast cells in AAA group. And FOS was correlated with immune cells. Immunohistochemistry and western blot confirmed higher FOS expression in PVAT of the AAA mouse model compared to control group.

Conclusion: The differentially expressed genes and pathways identified in this study provide further understanding of how PVAT affects AAA development. FOS was identified as the diagnostic gene. There was an obvious difference in immune cells infiltration between normal and AAA groups.

The pathobiology of perivascular adipose tissue (PVAT), the fourth layer of the blood vessel wall

The perivascular adipose tissue (PVAT) is an adipose tissue depot which surrounds most human blood vessels. It is metabolically active and has both a protective and a pathogenic role in vascular biology and pathobiology. It regulates vascular homeostasis and promotes vascular dysfunction. The purpose of this review is to consider the origin, structure, function, and dysfunction of this unique adipose depot consisting of white (WAT), brown (BAT) and beige adipose tissue, to support the concept that PVAT may be considered the fourth layer of the normal arterial wall (tunica adiposa), in which dysfunction creates a microenvironment that regulates, in part, the initiation and growth of the fibro-inflammatory lipid atherosclerotic plaque. Experimental in-vivo and in-vitro studies and human investigations show that the adipocytes, extracellular matrix, nerve fibers and vasa vasorum found in PVAT form a functional adipose tissue unit adjacent to, but not anatomically separated from, the adventitia. PVAT maintains and regulates the structure and function of the normal arterial wall through autocrine and paracrine mechanisms, that include modulation of medial smooth muscle cell contractility and secretion of anti-inflammatory molecules. PVAT shows regional phenotypic heterogeneity which may be important in its effect on the wall of specific sections of the aorta and its muscular branches during perturbations and various injuries including obesity and diabetes. In atherosclerosis, a pan-vascular microenvironment is created that functionally links the intima-medial atherosclerotic plaque to the adventitia and PVAT beneath the plaque, highlighting the local impact of PVAT on atherogenesis. PVAT adipocytes have inflammatory effects which in response to injury show activation and phenotypic changes, some of which are considered to have direct and indirect effects on the intima and media during the initiation, growth, and development of complicated atherosclerotic plaques. Thus, it is important to maintain the integrity of the full vascular microenvironment so that design of experimental and human studies include investigation of PVAT. The era of discarding PVAT tissue in both experimental and human research and clinical vascular studies should end.

Key Chemokine Pathways in Atherosclerosis and Their Therapeutic Potential

The search to improve therapies to prevent or treat cardiovascular diseases (CVDs) rages on, as CVDs remain a leading cause of death worldwide. Here, the main cause of CVDs, atherosclerosis, and its prevention, take center stage. Chemokines and their receptors have long been known to play an important role in the pathophysiological development of atherosclerosis. Their role extends from the initiation to the progression, and even the potential regression of atherosclerotic lesions. These important regulators in atherosclerosis are therefore an obvious target in the development of therapeutic strategies. A plethora of preclinical studies have assessed various possibilities for targeting chemokine signaling via various approaches, including competitive ligands and microRNAs, which have shown promising results in ameliorating atherosclerosis. Developments in the field also include detailed imaging with tracers that target specific chemokine receptors. Lastly, clinical trials revealed the potential of various therapies but still require further investigation before commencing clinical use. Although there is still a lot to be learned and investigated, it is clear that chemokines and their receptors present attractive yet extremely complex therapeutic targets. Therefore, this review will serve to provide a general overview of the connection between various chemokines and their receptors with atherosclerosis. The different developments, including mouse models and clinical trials that tackle this complex interplay will also be explored.

The Dual Role of Low-Density Lipoprotein Receptor-Related Protein 1 in Atherosclerosis

Low-density lipoprotein receptor–related protein-1 (LRP1) is a large endocytic and signaling receptor belonging to the LDL receptor (LDLR) gene family and that is widely expressed in several tissues. LRP1 comprises a large extracellular domain (ECD; 515 kDa, α chain) and a small intracellular domain (ICD; 85 kDa, β chain). The deletion of LRP1 leads to embryonic lethality in mice, revealing a crucial but yet undefined role in embryogenesis and development. LRP1 has been postulated to participate in numerous diverse physiological and pathological processes ranging from plasma lipoprotein homeostasis, atherosclerosis, tumor evolution, and fibrinolysis to neuronal regeneration and survival. Many studies using cultured cells and in vivo animal models have revealed the important roles of LRP1 in vascular remodeling, foam cell biology, inflammation and atherosclerosis. However, its role in atherosclerosis remains controversial. LRP1 not only participates in the removal of atherogenic lipoproteins and proatherogenic ligands in the liver but also mediates the uptake of aggregated LDL to promote the formation of macrophage- and vascular smooth muscle cell (VSMC)-derived foam cells, which causes a prothrombotic transformation of the vascular wall. The dual and opposing roles of LRP1 may also represent an interesting target for atherosclerosis therapeutics. This review highlights the influence of LRP1 during atherosclerosis development, focusing on its dual role in vascular cells and immune cells.

Roles of Perivascular Adipose Tissue in Hypertension and Atherosclerosis

Significance: Perivascular adipose tissue (PVAT), which is present surrounding most blood vessels, from the aorta to the microvasculature of the dermis, is mainly composed of fat cells, fibroblasts, stem cells, mast cells, and nerve cells. Although the PVAT is objectively present, its physiological and pathological significance has long been ignored. Recent Advances: PVAT was considered as a supporting component of blood vessels and a protective cushion to the vessel wall from the neighboring tissues during relaxation and contraction. Nonetheless, further extensive research found that PVAT actively regulates blood vessel tone through PVAT-derived vasoactive factors, including both relaxing and contracting factors. In addition, PVAT contributes to atherosclerosis through paracrine secretion of a large number of bioactive factors such as adipokines and cytokines. Thereby, PVAT regulates the functions of blood vessels through various mechanisms operating directly on PVAT or on the underlying vessel layers, including vascular smooth muscle cells (VSMCs) and endothelial cells (ECs). Critical Issues: PVAT is a unique adipose tissue that plays an essential role in maintaining the vascular structure and regulating vascular function and homeostasis. This review focuses on recent updates on the various PVAT roles in hypertension and atherosclerosis. Future Directions: Future studies should further investigate the actual contribution of alterations in PVAT metabolism to the overall systemic outcomes of cardiovascular disease, which remains largely unknown. In addition, the messengers and underlying mechanisms responsible for the crosstalk between PVAT and ECs and VSMCs in the vascular wall should be systematically addressed, as well as the contributions of PVAT aging to vascular dysfunction.

Hepatic LDL receptor-related protein-1 deficiency alters mitochondrial dynamics through phosphatidylinositol 4,5-bisphosphate reduction

The LDL receptor-related protein 1 (LRP1) is a multifunctional transmembrane protein with endocytosis and signal transduction functions. Previous studies have shown that hepatic LRP1 deficiency exacerbates diet-induced steatohepatitis and insulin resistance via mechanisms related to increased lysosome and mitochondria permeability and dysfunction. The current study examined the impact of LRP1 deficiency on mitochondrial function in the liver. Hepatocytes isolated from liver-specific LRP1 knockout (hLrp1^−/−) mice showed reduced oxygen consumption compared with control mouse hepatocytes. The mitochondria in hLrp1^−/− mouse livers have an abnormal morphology and their membranes contain significantly less anionic phospholipids, including lower levels of phosphatidylethanolamine and cardiolipin that increase mitochondrial fission and impair fusion. Additional studies showed that LRP1 complexes with phosphatidylinositol 4-phosphate 5-kinase like protein-1 (PIP5KL1) and phosphatidylinositol 4-phosphate 5-kinase-1β (PIP5K1β). The absence of LRP1 reduces the levels of both PIP5KL1 and PIP5K1β in the plasma membrane and also lowers phosphatidylinositol(4,5) bisphosphate (PI(4,5)P₂) levels in hepatocytes. These data indicate that LRP1 recruits PIP5KL1 and PIP5K1β to the plasma membrane for PI(4,5)P₂ biosynthesis. The lack of LRP1 reduces lipid kinase expression, leading to lower PI(4,5)P₂ levels, thereby decreasing the availability of this lipid metabolite in the cardiolipin biosynthesis pathway to cause cardiolipin reduction and the impairment in mitochondria homeostasis. Taken together, the current study identifies another signaling mechanism by which LRP1 regulates cell functions: binding and recruitment of PIP5KL1 and PIP5K1β to the membrane for PI(4,5)P₂ synthesis. In addition, it highlights the importance of this mechanism for maintaining the integrity and functions of intracellular organelles.

Mutation in the distal NPxY motif of LRP1 alleviates dietary cholesterol-induced dyslipidemia and tissue inflammation

The impairment of LDL receptor-related protein-1 (LRP1) in numerous cell types is associated with obesity, diabetes, and fatty liver disease. Here, we compared the metabolic phenotype of C57BL/6J wild-type and LRP1 knock-in mice carrying an inactivating mutation in the distal NPxY motif after feeding a low-fat diet or high-fat (HF) diet with cholesterol supplementation (HFHC) or HF diet without cholesterol supplementation. In response to HF feeding, both groups developed hyperglycemia, hyperinsulinemia, hyperlipidemia, increased adiposity, and adipose tissue inflammation and liver steatosis. However, LRP1 NPxY mutation prevents HFHC diet-induced hypercholesterolemia, reduces adipose tissue and brain inflammation, and limits liver progression to steatohepatitis. Nevertheless, this mutation does not protect against HFHC diet-induced insulin resistance. The selective metabolic improvement observed in HFHC diet-fed LRP1 NPxY mutant mice is due to an apparent increase of hepatic LDL receptor levels, leading to an elevated rate of plasma lipoprotein clearance and lower hepatic cholesterol levels. The unique metabolic phenotypes displayed by LRP1 NPxY mutant mice indicate an LRP1-cholesterol axis in modulating tissue inflammation. The LRP1 NPxY mutant mouse phenotype differs from phenotypes observed in mice with tissue-specific LRP1 inactivation, thus highlighting the importance of an integrative approach to evaluate how global LRP1 dysfunction contributes to metabolic disease development.

Adipose Tissue Immunomodulation: A Novel Therapeutic Approach in Cardiovascular and Metabolic Diseases

Adipose tissue is a critical regulator of systemic metabolism and bodily homeostasis as it secretes a myriad of adipokines, including inflammatory and anti-inflammatory cytokines. As the main storage pool of lipids, subcutaneous and visceral adipose tissues undergo marked hypertrophy and hyperplasia in response to nutritional excess leading to hypoxia, adipokine dysregulation, and subsequent low-grade inflammation that is characterized by increased infiltration and activation of innate and adaptive immune cells. The specific localization, physiology, susceptibility to inflammation and the heterogeneity of the inflammatory cell population of each adipose depot are unique and thus dictate the possible complications of adipose tissue chronic inflammation. Several lines of evidence link visceral and particularly perivascular, pericardial, and perirenal adipose tissue inflammation to the development of metabolic syndrome, insulin resistance, type 2 diabetes and cardiovascular diseases. In addition to the implication of the immune system in the regulation of adipose tissue function, adipose tissue immune components are pivotal in detrimental or otherwise favorable adipose tissue remodeling and thermogenesis. Adipose tissue resident and infiltrating immune cells undergo metabolic and morphological adaptation based on the systemic energy status and thus a better comprehension of the metabolic regulation of immune cells in adipose tissues is pivotal to address complications of chronic adipose tissue inflammation. In this review, we discuss the role of adipose innate and adaptive immune cells across various physiological and pathophysiological states that pertain to the development or progression of cardiovascular diseases associated with metabolic disorders. Understanding such mechanisms allows for the exploitation of the adipose tissue-immune system crosstalk, exploring how the adipose immune system might be targeted as a strategy to treat cardiovascular derangements associated with metabolic dysfunctions.

Potential role of perivascular adipose tissue in modulating atherosclerosis

Perivascular adipose tissue (PVAT) directly juxtaposes the vascular adventitia and contains a distinct mixture of mature adipocytes, preadipocytes, stem cells, and inflammatory cells that communicate via adipocytokines and other signaling mediators with the nearby vessel wall to regulate vascular function. Cross-talk between perivascular adipocytes and the cells in the blood vessel wall is vital for normal vascular function and becomes perturbed in diseases such as atherosclerosis. Perivascular adipocytes surrounding coronary arteries may be primed to promote inflammation and angiogenesis, and PVAT phenotypic changes occurring in the setting of obesity, hyperlipidemia etc., are fundamentally important in determining a pathogenic versus protective role of PVAT in vascular disease. Recent discoveries have advanced our understanding of the role of perivascular adipocytes in modulating vascular function. However, their impact on cardiovascular disease (CVD), particularly in humans, is yet to be fully elucidated. This review will highlight the complex mechanisms whereby PVAT regulates atherosclerosis, with an emphasis on clinical implications of PVAT and emerging strategies for evaluation and treatment of CVD based on PVAT biology.

Drug Targeting of Plasminogen Activator Inhibitor-1 Inhibits Metabolic Dysfunction and Atherosclerosis in a Murine Model of Metabolic Syndrome

OBJECTIVE

Enhanced expression of PAI-1 (plasminogen activator inhibitor-1) has been implicated in atherosclerosis formation in humans with obesity and metabolic syndrome. However, little is known about the effects of pharmacological targeting of PAI-1 on atherogenesis. This study examined the effects of pharmacological PAI-1 inhibition on atherosclerosis formation in a murine model of obesity and metabolic syndrome. Approach and Results: LDL receptor-deficient (ldlr-/-) mice were fed a Western diet high in cholesterol, fat, and sucrose to induce obesity, metabolic dysfunction, and atherosclerosis. Western diet triggered significant upregulation of PAI-1 expression compared with normal diet controls. Addition of a pharmacological PAI-1 inhibitor (either PAI-039 or MDI-2268) to Western diet significantly inhibited obesity and atherosclerosis formation for up to 24 weeks without attenuating food consumption. Pharmacological PAI-1 inhibition significantly decreased macrophage accumulation and cell senescence in atherosclerotic plaques. Recombinant PAI-1 stimulated smooth muscle cell senescence, whereas a PAI-1 mutant defective in LRP1 (LDL receptor-related protein 1) binding did not. The prosenescent effect of PAI-1 was blocked by PAI-039 and R2629, a specific anti-LRP1 antibody. PAI-039 significantly decreased visceral adipose tissue inflammation, hyperglycemia, and hepatic triglyceride content without altering plasma lipid profiles.

CONCLUSIONS

Pharmacological targeting of PAI-1 inhibits atherosclerosis in mice with obesity and metabolic syndrome, while inhibiting macrophage accumulation and cell senescence in atherosclerotic plaques, as well as obesity-associated metabolic dysfunction. PAI-1 induces senescence of smooth muscle cells in an LRP1-dependent manner. These results help to define the role of PAI-1 in atherosclerosis formation and suggest a new plasma-lipid-independent strategy for inhibiting atherogenesis.

B Lymphocytes and Adipose Tissue Inflammation

The immune system plays an important role in obesity-induced adipose tissue inflammation and the resultant metabolic dysfunction, which can lead to hypertension, dyslipidemia, and insulin resistance and their downstream sequelae of type 2 diabetes mellitus and cardiovascular disease. While macrophages are the most abundant immune cell type in adipose tissue, other immune cells are also present, such as B cells, which play important roles in regulating adipose tissue inflammation. This brief review will overview B-cell subsets, describe their localization in various adipose depots and summarize our knowledge about the function of these B-cell subsets in regulating adipose tissue inflammation, obesity-induced metabolic dysfunction and atherosclerosis.

Metabolism, Obesity, and Diabetes Mellitus

Obesity and diabetes remain leading causes of reduced health span and life span throughout the world. Hence, it is not surprising that these areas are at the center of highly active areas of research. The identification of novel mechanisms underlying these metabolic disorders sets the stage for uncovering new potential therapeutic strategies. In this issue of Highlights in Arteriosclerosis, Thrombosis and Vascular Biology, we review recently published papers in the journal that add to our understanding of causes and consequences of obesity and diabetes and how these disorders impact metabolic function. Collectively, these studies in cultured cells to in vivo animal models to human subjects add to the growing body of evidence that both cell-intrinsic and cell-cell communication mechanisms collaborate in metabolic disorders to cause obesity, insulin resistance and diabetes and its complications.

Hyperlipidemias and Obesity

The organs require oxygen and other types of nutrients (amino acids, sugars, and lipids) to function, the heart consuming large amounts of fatty acids for oxidation and adenosine triphosphate (ATP) generation.

The Role of Low-Density Lipoprotein Receptor-Related Protein 1 in Lipid Metabolism, Glucose Homeostasis and Inflammation

Metabolic syndrome (MetS) is a highly prevalent disorder which can be used to identify individuals with a higher risk for cardiovascular disease and type 2 diabetes. This metabolic syndrome is characterized by a combination of physiological, metabolic, and molecular alterations such as insulin resistance, dyslipidemia, and central obesity. The low-density lipoprotein receptor-related protein 1 (LRP1—A member of the LDL receptor family) is an endocytic and signaling receptor that is expressed in several tissues. It is involved in the clearance of chylomicron remnants from circulation, and has been demonstrated to play a key role in the lipid metabolism at the hepatic level. Recent studies have shown that LRP1 is involved in insulin receptor (IR) trafficking and intracellular signaling activity, which have an impact on the regulation of glucose homeostasis in adipocytes, muscle cells, and brain. In addition, LRP1 has the potential to inhibit or sustain inflammation in macrophages, depending on its cellular expression, as well as the presence of particular types of ligands in the extracellular microenvironment. In this review, we summarize existing perspectives and the latest innovations concerning the role of tissue-specific LRP1 in lipoprotein and glucose metabolism, and examine its ability to mediate inflammatory processes related to MetS and atherosclerosis.

LRP1 integrates murine macrophage cholesterol homeostasis and inflammatory responses in atherosclerosis

Low-density lipoprotein receptor-related protein 1 (LRP1) is a multifunctional cell surface receptor with diverse physiological roles, ranging from cellular uptake of lipoproteins and other cargo by endocytosis to sensor of the extracellular environment and integrator of a wide range of signaling mechanisms. As a chylomicron remnant receptor, LRP1 controls systemic lipid metabolism in concert with the LDL receptor in the liver, whereas in smooth muscle cells (SMC) LRP1 functions as a co-receptor for TGFβ and PDGFRβ in reverse cholesterol transport and the maintenance of vascular wall integrity. Here we used a knockin mouse model to uncover a novel atheroprotective role for LRP1 in macrophages where tyrosine phosphorylation of an NPxY motif in its intracellular domain initiates a signaling cascade along an LRP1/SHC1/PI3K/AKT/PPARγ/LXR axis to regulate and integrate cellular cholesterol homeostasis through the expression of the major cholesterol exporter ABCA1 with apoptotic cell removal and inflammatory responses.

Atherosclerosis is a disease in which “plaques” build up inside the walls of arteries. Plaques consist of a fatty substance called cholesterol, together with immune cells such as macrophages and other material from the blood. Over time, the plaque narrows and hardens the arteries. This restricts the flow of blood to vital parts of the body, which increases the risk of heart attacks, strokes and other severe conditions.

Macrophages play an important role in atherosclerosis. At the early stage of the disease, macrophages enter the developing plaques to take up the excess cholesterol. Cholesterol taken up by macrophages needs to be exported out of the cell and sent to the liver for removal. Yet, these processes can go awry. Macrophages can fill up with too much cholesterol and become trapped in the arteries. These cholesterol-laden macrophages can also start dying. These problems enable the plaques to grow and worsen the disease.

LRP1 is an important protein present on the surface of many types of cells. In macrophages, LRP1 helps to export excess cholesterol out of the cell, thus lowering the risk of atherosclerosis. LRP1 also reduces cell death in the plaque, which slows the plaques’ progression. Previous research has shown that the region of LRP1 present inside the cell can be modified by the attachment of a phosphate group – a process termed phosphorylation. Whether phosphorylation of LRP1 plays a role in preventing atherosclerosis is not understood.

To address this question, Xian, Ding, Dieckmann et al. engineered mice in which LRP1 was unable to get phosphorylated. The results show that phosphorylated LRP1 – but not the non-phosphorylated version – turns on a signaling pathway in macrophages. This pathway increases the expression of a transporter protein that exports cholesterol out of the cell. This reduces the amount of cholesterol that accumulates in macrophages. Lastly, mice with problems with LRP1 phosphorylation developed more severe atherosclerotic plaques with more dying cells present in the affected areas compared to normal mice.

These findings show how phosphorylation of LRP1 protects against atherosclerosis. Understanding this process in further detail may help scientists to devise new ways to treat this disease.

Perivascular Adipose Tissue Harbors Atheroprotective IgM-Producing B Cells

Adipose tissue surrounding major arteries (Perivascular adipose tissue or PVAT) has long been thought to exist to provide vessel support and insulation. Emerging evidence suggests that PVAT regulates artery physiology and pathology, such as, promoting atherosclerosis development through local production of inflammatory cytokines. Yet the immune subtypes in PVAT that regulate inflammation are poorly characterized. B cells have emerged as important immune cells in the regulation of visceral adipose tissue inflammation and atherosclerosis. B cell-mediated effects on atherosclerosis are subset-dependent with B-1 cells attenuating and B-2 cells aggravating atherosclerosis. While mechanisms whereby B-2 cells aggravate atherosclerosis are less clear, production of immunoglobulin type M (IgM) antibodies is thought to be a major mechanism whereby B-1 cells limit atherosclerosis development. B-1 cell-derived IgM to oxidation specific epitopes (OSE) on low density lipoproteins (LDL) blocks oxidized LDL-induced inflammatory cytokine production and foam cell formation. However, whether PVAT contains B-1 cells and whether atheroprotective IgM is produced in PVAT is unknown. Results of the present study provide clear evidence that the majority of B cells in and around the aorta are derived from PVAT. Interestingly, a large proportion of these B cells belong to the B-1 subset with the B-1/B-2 ratio being 10-fold higher in PVAT relative to spleen and bone marrow. Moreover, PVAT contains significantly greater numbers of IgM secreting cells than the aorta. ApoE^−/− mice with B cell-specific knockout of the gene encoding the helix-loop-helix factor Id3, known to have attenuated diet-induced atherosclerosis, have increased numbers of B-1b cells and increased IgM secreting cells in PVAT relative to littermate controls. Immunostaining of PVAT on human coronary arteries identified fat associated lymphoid clusters (FALCs) harboring high numbers of B cells, and flow cytometry demonstrated the presence of T cells and B cells including B-1 cells. Taken together, these results provide evidence that murine and human PVAT harbor B-1 cells and suggest that local IgM production may serve to provide atheroprotection.