Carboxy-terminal deletion of the HDL receptor reduces receptor levels in liver and steroidogenic tissues, induces hypercholesterolemia, and causes fatal heart disease

SR-B1-/-ApoE-R61h/h Mice Mimic Human Coronary Heart Disease

Cardiovascular diseases are the leading cause of death in the modern world. Atherosclerosis underlies the majority of these pathologies and may result in sudden life-threatening events such as myocardial infarction or stroke. Current concepts consider a rupture (resp. erosion) of "unstable/vulnerable" atherosclerotic plaques as a primary cause leading to thrombus formation and subsequent occlusion of the artery lumen finally triggering an acute clinical event. We and others described SR-B1-/-ApoE-R61h/h mice mimicking clinical coronary heart disease in all major aspects: from coronary atherosclerosis through vulnerable plaque ruptures leading to thrombus formation/coronary artery occlusion, finally resulting in myocardial infarction/ischemia. SR-B1-/-ApoE-R61h/h mouse provides a valuable model to study vulnerable/occlusive plaques, to evaluate bioactive compounds as well as new anti-inflammatory and "anti-rupture" drugs, and to test new technologies in experimental cardiovascular medicine. This review summarizes and discuss our knowledge about SR-B1-/-ApoE-R61h/h mouse model based on recent publications and experimental observations from the lab.

Nanoparticle-Directed Antioxidant Therapy Can Ameliorate Disease Progression in a Novel, Diet-Inducible Model of Coronary Artery Disease

BACKGROUND

Oxidative stress plays a crucial role in the pathogenesis of coronary artery disease. In cardiovascular research using murine models, the generation and maintenance of models with robust coronary arterial atherosclerosis has been challenging.

METHODS

We characterized a new mouse model in which the last 3 amino acids of the carboxyl terminus of the HDL (high-density lipoprotein) receptor (SR-B1 [scavenger receptor, class B, type 1]) were deleted in a low-density lipoprotein receptor knockout (LDLR-/-) mouse model (SR-B1ΔCT/LDLR-/-) fed an atherogenic diet. We also tested the therapeutic effects of an oxidative stress-targeted nanoparticle in atherogenic diet-fed SR-B1ΔCT/LDLR-/- mice.

RESULTS

The SR-B1ΔCT/LDLR-/- mice fed an atherogenic diet had occlusive coronary artery atherosclerosis, impaired cardiac function, and a dramatically lower survival rate, compared with LDLR-/- mice fed the same diet. As SR-B1ΔCT/LDLR-/- mice do not exhibit female infertility or low pup yield, they are far easier and less costly to use than the previously described SR-B1-based models of coronary artery disease. We found that treatment with the targeted nanoparticles improved the cardiac functions and corrected hematologic abnormalities caused by the atherogenic diet in SR-B1ΔCT/LDLR-/- mice but did not alter the distinctive plasma lipid levels.

CONCLUSIONS

The SR-B1ΔCT/LDLR-/- mice developed diet-inducible, fatal atherosclerotic coronary artery disease, which could be ameliorated by targeted nanoparticle therapy. Our study provides new tools for the development of cardiovascular therapies.

The two major splice variants of scavenger receptor BI differ by their interactions with lipoproteins and cellular localization in endothelial cells

The scavenger receptor BI (SR-BI) facilitates the transport of both HDL and LDL through endothelial cells. Its two splice variants, SR-BIvar1 and SR-BIvar2, differ in their carboxy terminal domains. Only SR-BIvar1 contains the putative binding sites for the adapter proteins PDZ domain containing protein 1 (PDZK1) and dedicator of cytokinesis 4 (DOCK4), which limit the cell surface abundance and internalization of the receptor. To investigate the cellular localization of the SR-BI variants and their interaction with lipoproteins in endothelial cells, EA.hy926 cells were stably transfected with vectors encoding untagged, GFP- or mCherry-tagged constructs of the two SR-BI variants. Additionally, the cells were transfected with shRNAs against PDZK1 or DOCK4. Microscopy investigation showed that SR-BIvar1 was predominantly localized on the cell surface together with clathrin whereas SR-BIvar2 was absent from the cell surface but retrieved in endosomes and lysosomes. Accordingly, only SR-BIvar1 increased lipoprotein binding to endothelial while HDL and LDL uptake were enhanced by both variants. Silencing of PDZK1 or DOCK4 only reduced HDL association in SR-BIvar2 overexpressing cells while LDL association was reduced both in WT and SR-BIvar2 overexpressing cells. In conclusion, either SR-BI variant facilitates the uptake of HDL and LDL into endothelial cells, however by different mechanisms and trafficking routes. This dual role may explain why the loss of DOCK4 or PDZK1 differently affects the uptake of HDL and LDL in different endothelial cells.

Novel Mouse Model of Myocardial Infarction, Plaque Rupture, and Stroke Shows Improved Survival With Myeloperoxidase Inhibition

BACKGROUND

Thromboembolic events, including myocardial infarction (MI) or stroke, caused by the rupture or erosion of unstable atherosclerotic plaques are the leading cause of death worldwide. Although most mouse models of atherosclerosis develop lesions in the aorta and carotid arteries, they do not develop advanced coronary artery lesions. Moreover, they do not undergo spontaneous plaque rupture with MI and stroke or do so at such a low frequency that they are not viable experimental models to study late-stage thrombotic events or to identify novel therapeutic approaches for treating atherosclerotic disease. This has stymied the development of more effective therapeutic approaches for reducing these events beyond what has been achieved with aggressive lipid lowering. Here, we describe a diet-inducible mouse model that develops widespread advanced atherosclerosis in coronary, brachiocephalic, and carotid arteries with plaque rupture, MI, and stroke.

METHODS

We characterized a novel mouse model with a C-terminal mutation in the scavenger receptor class B, type 1 (SR-BI), combined with Ldlr knockout (designated SR-BI∆CT/∆CT/Ldlr-/-). Mice were fed Western diet (WD) for 26 weeks and analyzed for MI and stroke. Coronary, brachiocephalic, and carotid arteries were analyzed for atherosclerotic lesions and indices of plaque stability. To validate the utility of this model, SR-BI∆CT/∆CT/Ldlr-/- mice were treated with the drug candidate AZM198, which inhibits myeloperoxidase, an enzyme produced by activated neutrophils that predicts rupture of human atherosclerotic lesions.

RESULTS

SR-BI∆CT/∆CT/Ldlr-/- mice show high (>80%) mortality rates after 26 weeks of WD feeding because of major adverse cardiovascular events, including spontaneous plaque rupture with MI and stroke. Moreover, WD-fed SR-BI∆CT/∆CT/Ldlr-/- mice displayed elevated circulating high-sensitivity cardiac troponin I and increased neutrophil extracellular trap formation within lesions compared with control mice. Treatment of WD-fed SR-BI∆CT/∆CT/Ldlr-/- mice with AZM198 showed remarkable benefits, including >90% improvement in survival and >60% decrease in the incidence of plaque rupture, MI, and stroke, in conjunction with decreased circulating high-sensitivity cardiac troponin I and reduced neutrophil extracellular trap formation within lesions.

CONCLUSIONS

WD-fed SR-BI∆CT/∆CT/Ldlr-/- mice more closely replicate late-stage clinical events of advanced human atherosclerotic disease than previous models and can be used to identify and test potential new therapeutic agents to prevent major adverse cardiac events.

Stromal cell-derived factor-1 alpha improves cardiac function in a novel diet-induced coronary atherosclerosis model, the SR-B1ΔCT/LDLR KO mouse

BACKGROUND AND AIMS

There are a limited number of pharmacologic therapies for coronary artery disease, and few rodent models of occlusive coronary atherosclerosis and consequent myocardial infarction with which one can rapidly test new therapeutic approaches. Here, we characterize a novel, fertile, and easy-to-use HDL receptor (SR-B1)-based model of atherogenic diet-inducible, fatal coronary atherosclerosis, the SR-B1ΔCT/LDLR KO mouse. Additionally, we test intramyocardial injection of Stromal Cell-Derived Factor-1 alpha (SDF-1α), a potent angiogenic cytokine, as a possible therapy to rescue cardiac function in this mouse.

METHODS

SR-B1ΔCT/LDLR KO mice were fed the Paigen diet or standard chow diet, and we determined the effects of the diets on cardiac function, histology, and survival. After two weeks of feeding either the Paigen diet (n = 24) or standard chow diet (n = 20), the mice received an intramyocardial injection of either SDF-1α or phosphate buffered saline (PBS). Cardiac function and angiogenesis were assessed two weeks later.

RESULTS

When six-week-old mice were fed the Paigen diet, they began to die as early as 19 days later and 50% had died by 38 days. None of the mice maintained on the standard chow diet died by day 72. Hearts from mice on the Paigen diet showed evidence of cardiomegaly, myocardial infarction, and occlusive coronary artery disease. For the five mice that survived until day 28 that underwent an intramyocardial injection of PBS on day 15, the average ejection fraction (EF) decreased significantly from day 14 (the day before injection, 52.1 ± 4.3%) to day 28 (13 days after the injection, 30.6 ± 6.8%) (paired t-test, n = 5, p = 0.0008). Of the 11 mice fed the Paigen diet and injected with SDF-1α on day 15, 8 (72.7%) survived to day 28. The average EF for these 8 mice increased significantly from 48.2 ± 7.2% on day 14 to63.6 ± 6.9% on day 28 (Paired t-test, n = 8, p = 0.003).

CONCLUSIONS

This new mouse model and treatment with the promising angiogenic cytokine SDF-1α may lead to new therapeutic approaches for ischemic heart disease.

Integrated meta-omics reveals the regulatory landscape involved in lipid metabolism between pig breeds

BACKGROUND

Domesticated pigs serve as an ideal animal model for biomedical research and also provide the majority of meat for human consumption in China. Porcine intramuscular fat content associates with human health and diseases and is essential in pork quality. The molecular mechanisms controlling lipid metabolism and intramuscular fat accretion across tissues in pigs, and how these changes in response to pig breeds, remain largely unknown.

RESULTS

We surveyed the tissue-resident cell types of the porcine jejunum, colon, liver, and longissimus dorsi muscle between Lantang and Landrace breeds by single-cell RNA sequencing. Combining lipidomics and metagenomics approaches, we also characterized gene signatures and determined key discriminating markers of lipid digestibility, absorption, conversion, and deposition across tissues in two pig breeds. In Landrace, lean-meat swine mainly exhibited breed-specific advantages in lipid absorption and oxidation for energy supply in small and large intestinal epitheliums, nascent high-density lipoprotein synthesis for reverse cholesterol transport in enterocytes and hepatocytes, bile acid formation, and secretion for fat emulsification in hepatocytes, as well as intestinal-microbiota gene expression involved in lipid accumulation product. In Lantang, obese-meat swine showed a higher synthesis capacity of chylomicrons responsible for high serum triacylglycerol levels in small intestinal epitheliums, the predominant characteristics of lipid absorption in muscle tissue, and greater intramuscular adipcytogenesis potentials from muscular fibro-adipogenic progenitor subpopulation.

CONCLUSIONS

The findings enhanced our understanding of the cellular biology of lipid metabolism and opened new avenues to improve animal production and human diseases. Video Abstract.

HBV Infection-Related PDZK1 Plays an Oncogenic Role by Regulating the PI3K-Akt Pathway and Fatty Acid Metabolism and Enhances Immunosuppression

Background and Aim. Chronic hepatitis B virus (HBV) infection is the leading global cause of hepatocellular carcinoma (HCC). Few studies have been conducted concerning the HBV infection-related genes and their function. Methods. We compared differentially expressed genes (DGEs) in HBV-positive and -negative tumor samples and conducted a Spearman correlation study between the DGEs and HBV titers within The Cancer Genome Atlas (TCGA). Moreover, we validated the results of our in-house samples. Results. In this study, we discovered a series of genes that correlated statistically with HBV infection based on the TCGA database. These genes were related to increased inflammation and some oncogenic signaling pathways via Gene Set Enrichment Analysis (GSEA). PDZK1 is an ideal gene, which mostly relates positively to HBV infection; moreover, it is overexpressed in human HCC, especially in those HBV-infected HCCs. After analyzing the TCGA data and performing a verification study using our own samples, PDZK1 expression was investigated to be significantly associated with PI3K-Akt signaling and fatty acid metabolism. Further, single-sample GSEA analysis of tumor immune cell infiltration gene sets revealed that high PDZK1expression in HCC tissues was significantly associated with increased tumor-associated macrophages (TAMs) and regulatory T cells(Tregs). Conclusions. PDZK1 is an HBV infection-related gene, which plays oncogenic roles, possibly due to enhancing PI3K-Akt, fatty acid usage in tumor cells and TAMs, and Treg-induced immunosuppression.

How the immune system shapes atherosclerosis: roles of innate and adaptive immunity

Atherosclerosis is the root cause of many cardiovascular diseases. Extensive research in preclinical models and emerging evidence in humans have established the crucial roles of the innate and adaptive immune systems in driving atherosclerosis-associated chronic inflammation in arterial blood vessels. New techniques have highlighted the enormous heterogeneity of leukocyte subsets in the arterial wall that have pro-inflammatory or regulatory roles in atherogenesis. Understanding the homing and activation pathways of these immune cells, their disease-associated dynamics and their regulation by microbial and metabolic factors will be crucial for the development of clinical interventions for atherosclerosis, including potentially vaccination-based therapeutic strategies. Here, we review key molecular mechanisms of immune cell activation implicated in modulating atherogenesis and provide an update on the contributions of innate and adaptive immune cell subsets in atherosclerosis.

SR-B1's Next Top Model: Structural Perspectives on the Functions of the HDL Receptor

PURPOSE OF REVIEW

The binding of high-density lipoprotein (HDL) to its primary receptor, scavenger receptor class B type 1 (SR-B1), is critical for lowering plasma cholesterol levels and reducing cardiovascular disease risk. This review provides novel insights into how the structural elements of SR-B1 drive efficient function with an emphasis on bidirectional cholesterol transport.

RECENT FINDINGS

We have generated a new homology model of full-length human SR-B1 based on the recent resolution of the partial structures of other class B scavenger receptors. Interrogating this model against previously published observations allows us to generate structurally informed hypotheses about SR-B1's ability to mediate HDL-cholesterol (HDL-C) transport. Furthermore, we provide a structural perspective as to why human variants of SR-B1 may result in impaired HDL-C clearance. A comprehensive understanding of SR-B1's structure-function relationships is critical to the development of therapeutic agents targeting SR-B1 and modulating cardiovascular disease risk.

STARD1 Functions in Mitochondrial Cholesterol Metabolism and Nascent HDL Formation. Gene Expression and Molecular mRNA Imaging Show Novel Splicing and a 1:1 Mitochondrial Association

STARD1 moves cholesterol (CHOL) from the outer mitochondrial membrane (OMM) to the inner membrane (IMM) in steroidogenic cells. This activity is integrated into CHOL trafficking and synthesis homeostasis, involving uptake through SR-B1 and LDL receptors and distribution through endosomes, ER, and lipid droplets. In adrenal cells, STARD1 is imported into the mitochondrial matrix accompanied by delivery of several hundred CHOL molecules. This transfer limits CYP11A1-mediated generation of pregnenolone. CHOL transfer is coupled to translation of STARD1 mRNA at the OMM. In testis cells, slower CHOL trafficking seems to be limiting. STARD1 also functions in a slower process through ER OMM contacts. The START domain of STARD1 is utilized by a family of genes, which includes additional STARD (forms 3-6) and GRAMD1B proteins that transfer CHOL. STARD forms 2 and 7 deliver phosphatidylcholine. STARD1 and STARD7 target their respective activities to mitochondria, via N-terminal domains (NTD) of over 50 amino acids. The NTD is not essential for steroidogenesis but exerts tissue-selective enhancement (testis>>adrenal). Three conserved sites for cleavage by the mitochondrial processing protease (MPP) generate three forms, each potentially with specific functions, as demonstrated in STARD7. STARD1 is expressed in macrophage and cardiac repair fibroblasts. Additional functions include CHOL metabolism by CYP27A1 that directs activation of LXR and CHOL export processes. STARD1 generates 3.5- and 1.6-kb mRNA from alternative polyadenylation. The 3.5-kb form exclusively binds the PKA-induced regulator, TIS11b, which binds at conserved sites in the extended 3'UTR to control mRNA translation and turnover. STARD1 expression also exhibits a novel, slow splicing that delayed splicing delivery of mRNA to mitochondria. Stimulation of transcription by PKA is directed by suppression of SIK forms that activate a CRTC/CREB/CBP promoter complex. This process is critical to pulsatile hormonal activation in vivo. sm-FISH RNA imaging shows a flow of single STARD1 mRNA particles from asymmetric accumulations of primary transcripts at gene loci to 1:1 complex of 3.5-kb mRNA with peri-nuclear mitochondria. Adrenal cells are similar but distinguished from testis cells by appreciable basal expression prior to hormonal activation. This difference is conserved in culture and in vivo.

Scavenger receptor B type 1: expression, molecular regulation, and cholesterol transport function

Cholesterol is required for maintenance of plasma membrane fluidity and integrity and for many cellular functions. Cellular cholesterol can be obtained from lipoproteins in a selective pathway of HDL-cholesteryl ester (CE) uptake without parallel apolipoprotein uptake. Scavenger receptor B type 1 (SR-B1) is a cell surface HDL receptor that mediates HDL-CE uptake. It is most abundantly expressed in liver, where it provides cholesterol for bile acid synthesis, and in steroidogenic tissues, where it delivers cholesterol needed for storage or steroidogenesis in rodents. SR-B1 transcription is regulated by trophic hormones in the adrenal gland, ovary, and testis; in the liver and elsewhere, SR-B1 is subject to posttranscriptional and posttranslational regulation. SR-B1 operates in several metabolic processes and contributes to pathogenesis of atherosclerosis, inflammation, hepatitis C virus infection, and other conditions. Here, we summarize characteristics of the selective uptake pathway and involvement of microvillar channels as facilitators of selective HDL-CE uptake. We also present the potential mechanisms of SR-B1-mediated selective cholesterol transport; the transcriptional, posttranscriptional, and posttranslational regulation of SR-B1; and the impact of gene variants on expression and function of human SR-B1. A better understanding of this unique pathway and SR-B1's role may yield improved therapies for a wide variety of conditions.

SR-B1 and PDZK1: partners in HDL regulation

PURPOSE OF REVIEW

To outline the roles of SR-B1 and PDZK1 in hepatic selective HDL cholesterol uptake and reverse cholesterol transport and the consequences for atherosclerosis development.

RECENT FINDINGS

Much of our understanding of the physiological roles of SR-B1 and PDZK1 in HDL metabolism and atherosclerosis comes from studies of genetically manipulated mice. These show SR-B1 and PDZK1 play key roles in HDL metabolism and protection against atherosclerosis. The recent identification of rare loss of function mutations in the human SCARB1 gene verifies that it plays similar roles in HDL metabolism in humans. Other rare mutations in both the human SCARB1 and PDZK1 genes remain to be characterized but may have potentially devastating consequences to SR-B1 function.

SUMMARY

Identification of carriers of rare mutations in human SCARB1 and PDZK1 that impair the function of their gene products and characterization of the effects of these mutations on HDL cholesterol levels and atherosclerosis will add to our understanding of the importance of HDL function and cholesterol flux, as opposed to HDL-cholesterol levels, per se, for protection against cardiovascular disease.