Advanced atherosclerotic foam cell formation has features of an acquired lysosomal storage disorder

Single-particle rotational microrheology enables pathological staging of macrophage foaming and antiatherosclerotic studies

The formation of macrophage-derived foam cells has been recognized as the pathological hallmark of atherosclerotic diseases. However, the pathological evolution dynamics and underlying regulatory mechanisms remain largely unknown. Herein, we introduce a single-particle rotational microrheology method for pathological staging of macrophage foaming and antiatherosclerotic explorations by probing the dynamic changes of lysosomal viscous feature over the pathological evolution progression. The principle of this method involves continuous monitoring of out-of-plane rotation-caused scattering brightness fluctuations of the gold nanorod (AuNR) probe-based microrheometer and subsequent determination of rotational relaxation time to analyze the viscous feature in macrophage lysosomes. With this method, we demonstrated the lysosomal viscous feature as a robust pathological reporter and uncovered three distinct pathological stages underlying the evolution dynamics, which are highly correlated with a pathological stage-dependent activation of the NLRP3 inflammasome-involved positive feedback loop. We also validated the potential of this positive feedback loop as a promising therapeutic target and revealed the time window-dependent efficacy of NLRP3 inflammasome-targeted drugs against atherosclerotic diseases. To our knowledge, the pathological staging of macrophage foaming and the pathological stage-dependent activation of the NLRP3 inflammasome-involved positive feedback mechanism have not yet been reported. These findings provide insights into in-depth understanding of evolutionary features and regulatory mechanisms of macrophage foaming, which can benefit the analysis of effective therapeutical drugs as well as the time window of drug treatment against atherosclerotic diseases in preclinical studies.

The emerging crosstalk between atherosclerosis-related microRNAs and Bermuda triangle of foam cells: Cholesterol influx, trafficking, and efflux

In atherosclerosis, the gradual buildup of lipid particles into the sub-endothelium of damaged arteries leads to numerous lipid alterations. The absorption of these modified lipids by monocyte-derived macrophages in the arterial wall leads to cholesterol accumulation and increases the likelihood of foam cell formation and fatty streak, which is an early characteristic of atherosclerosis. Foam cell formation is related to an imbalance in cholesterol influx, trafficking, and efflux. The formation of foam cells is heavily regulated by various mechanisms, among them, the role of epigenetic factors like microRNA alteration in the formation of foam cells has been well studied. Recent studies have focused on the potential interplay between microRNAs and foam cell formation in the pathogenesis of atherosclerosis; nevertheless, there is significant space to progress in this attractive field. This review has focused to examine the underlying processes of foam cell formation and microRNA crosstalk to provide a deep insight into therapeutic implications in atherosclerosis.

Unravelling the role of macrophages in cardiovascular inflammation through imaging: a state-of-the-art review

Cardiovascular disease remains the leading cause of death and disability for patients across the world. Our understanding of atherosclerosis as a primary cholesterol issue has diversified, with a significant dysregulated inflammatory component that largely remains untreated and continues to drive persistent cardiovascular risk. Macrophages are central to atherosclerotic inflammation, and they exist along a functional spectrum between pro-inflammatory and anti-inflammatory extremes. Recent clinical trials have demonstrated a reduction in major cardiovascular events with some, but not all, anti-inflammatory therapies. The recent addition of colchicine to societal guidelines for the prevention of recurrent cardiovascular events in high-risk patients with chronic coronary syndromes highlights the real-world utility of this class of therapies. A highly targeted approach to modification of interleukin-1-dependent pathways shows promise with several novel agents in development, although excessive immunosuppression and resulting serious infection have proven a barrier to implementation into clinical practice. Current risk stratification tools to identify high-risk patients for secondary prevention are either inadequately robust or prohibitively expensive and invasive. A non-invasive and relatively inexpensive method to identify patients who will benefit most from novel anti-inflammatory therapies is required, a role likely to be fulfilled by functional imaging methods. This review article outlines our current understanding of the inflammatory biology of atherosclerosis, upcoming therapies and recent landmark clinical trials, imaging modalities (both invasive and non-invasive) and the current landscape surrounding functional imaging including through targeted nuclear and nanobody tracer development and their application.

The Unique Phenotype of Lipid-Laden Macrophages

Macrophages are key multi-talented cells of the innate immune system and are equipped with receptors involved in damage and pathogen recognition with connected immune response guiding signaling systems. In addition, macrophages have various systems that are involved in the uptake of extracellular and intracellular cargo. The lysosomes in macrophages play a central role in the digestion of all sorts of macromolecules and the entry of nutrients to the cytosol, and, thus, the regulation of endocytic processes and autophagy. Simplistically viewed, two macrophage phenotype extremes exist. On one end of the spectrum, the classically activated pro-inflammatory M1 cells are present, and, on the other end, alternatively activated anti-inflammatory M2 cells. A unique macrophage population arises when lipid accumulation occurs, either caused by flaws in the catabolic machinery, which is observed in lysosomal storage disorders, or as a result of an acquired condition, which is found in multiple sclerosis, obesity, and cardiovascular disease. The accompanying overload causes a unique metabolic activation phenotype, which is discussed here, and, consequently, a unifying phenotype is proposed.

Lysosome (Dys)function in Atherosclerosis-A Big Weight on the Shoulders of a Small Organelle

Atherosclerosis is a progressive insidious chronic disease that underlies most of the cardiovascular pathologies, including myocardial infarction and ischemic stroke. The malfunctioning of the lysosomal compartment has a central role in the etiology and pathogenesis of atherosclerosis. Lysosomes are the degradative organelles of mammalian cells and process endogenous and exogenous substrates in a very efficient manner. Dysfunction of these organelles and consequent inefficient degradation of modified low-density lipoproteins (LDL) and apoptotic cells in atherosclerotic lesions have, therefore, numerous deleterious consequences for cellular homeostasis and disease progression. Lysosome dysfunction has been mostly studied in the context of the inherited lysosomal storage disorders (LSDs). However, over the last years it has become increasingly evident that the consequences of this phenomenon are more far-reaching, also influencing the progression of multiple acquired human pathologies, such as neurodegenerative diseases, cancer, and cardiovascular diseases (CVDs). During the formation of atherosclerotic plaques, the lysosomal compartment of the various cells constituting the arterial wall is under severe stress, due to the tremendous amounts of lipoproteins being processed by these cells. The uncontrolled uptake of modified lipoproteins by arterial phagocytic cells, namely macrophages and vascular smooth muscle cells (VSMCs), is the initial step that triggers the pathogenic cascade culminating in the formation of atheroma. These cells become pathogenic "foam cells," which are characterized by dysfunctional lipid-laden lysosomes. Here, we summarize the current knowledge regarding the origin and impact of the malfunctioning of the lysosomal compartment in plaque cells. We further analyze how the field of LSD research may contribute with some insights to the study of CVDs, particularly how therapeutic approaches that target the lysosomes in LSDs could be applied to hamper atherosclerosis progression and associated mortality.

Macrophage Long Non-Coding RNAs in Pathogenesis of Cardiovascular Disease

Chronic inflammation is inextricably linked to cardiovascular disease (CVD). Macrophages themselves play important roles in atherosclerosis, as well as acute and chronic heart failure. Although the role of macrophages in CVD pathophysiology is well-recognized, little is known regarding the precise mechanisms influencing their function in these contexts. Long non-coding RNAs (lncRNAs) have emerged as significant regulators of macrophage function; as such, there is rising interest in understanding how these nucleic acids influence macrophage signaling, cell fate decisions, and activity in health and disease. In this review, we summarize current knowledge regarding lncRNAs in directing various aspects of macrophage function in CVD. These include foam cell formation, Toll-like receptor (TLR) and NF-kβ signaling, and macrophage phenotype switching. This review will provide a comprehensive understanding concerning previous, ongoing, and future studies of lncRNAs in macrophage functions and their importance in CVD.

7-Ketocholesterol in disease and aging

7-Ketocholesterol (7KC) is a toxic oxysterol that is associated with many diseases and disabilities of aging, as well as several orphan diseases. 7KC is the most common product of a reaction between cholesterol and oxygen radicals and is the most concentrated oxysterol found in the blood and arterial plaques of coronary artery disease patients as well as various other disease tissues and cell types. Unlike cholesterol, 7KC consistently shows cytotoxicity to cells and its physiological function in humans or other complex organisms is unknown. Oxysterols, particularly 7KC, have also been shown to diffuse through membranes where they affect receptor and enzymatic function. Here, we will explore the known and proposed mechanisms of pathologies that are associated with 7KC, as well speculate about the future of 7KC as a diagnostic and therapeutic target in medicine.

Macrophage Trafficking, Inflammatory Resolution, and Genomics in Atherosclerosis: JACC Macrophage in CVD Series (Part 2)

Atherosclerosis is characterized by the retention of modified lipoproteins in the arterial wall. These modified lipoproteins activate resident macrophages and the recruitment of monocyte-derived cells, which differentiate into mononuclear phagocytes that ingest the deposited lipoproteins to become "foam cells": a hallmark of this disease. In this Part 2 of a 4-part review series covering the macrophage in cardiovascular disease, we critically review the contributions and relevant pathobiology of monocytes, macrophages, and foam cells as relevant to atherosclerosis. We also review evidence that via various pathways, a failure of the resolution of inflammation is an additional key aspect of this disease process. Finally, we consider the likely role played by genomics and biological networks in controlling the macrophage phenotype in atherosclerosis. Collectively, these data provide substantial insights on the atherosclerotic process, while concurrently offering numerous molecular and genomic candidates that appear to hold great promise for selective targeting as clinical therapies.

Macrophage Plasticity and Function in the Eye and Heart

Macrophages are important mediators of inflammation and tissue remodeling. Recent insights into the heterogeneity of macrophage subpopulations have renewed interest in their functional diversity in homeostasis and disease. In addition, their plasticity enables them to perform a variety of functions in response to changing tissue contexts, such as those imposed by aging. These qualities make macrophages particularly intriguing cells given their dichotomous role in protecting against, or accelerating, diseases of the cardiovascular system and the eye, two tissues that are particularly susceptible to the effects of aging. We review novel perspectives on macrophage biology, as informed by recent studies detailing the diversity of macrophage identity and function, as well as mechanisms influencing macrophage behavior that might offer opportunities for new therapeutic strategies.

Cholesterol Acceptors Regulate the Lipidome of Macrophage Foam Cells

Arterial foam cells are central players of atherogenesis. Cholesterol acceptors, apolipoprotein A-I (apoA-I) and high-density lipoprotein (HDL), take up cholesterol and phospholipids effluxed from foam cells into the circulation. Due to the high abundance of cholesterol in foam cells, most previous studies focused on apoA-I/HDL-mediated free cholesterol (FC) transport. However, recent lipidomics of human atherosclerotic plaques also identified that oxidized sterols (oxysterols) and non-sterol lipid species accumulate as atherogenesis progresses. While it is known that these lipids regulate expression of pro-inflammatory genes linked to plaque instability, how cholesterol acceptors impact the foam cell lipidome, particularly oxysterols and non-sterol lipids, remains unexplored. Using lipidomics analyses, we found cholesterol acceptors remodel foam cell lipidomes. Lipid subclass analyses revealed various oxysterols, sphingomyelins, and ceramides, species uniquely enriched in human plaques were significantly reduced by cholesterol acceptors, especially by apoA-I. These results indicate that the function of lipid-poor apoA-I is not limited to the efflux of cholesterol and phospholipids but suggest that apoA-I serves as a major regulator of the foam cell lipidome and might play an important role in reducing multiple lipid species involved in the pathogenesis of atherosclerosis.

Lysosomal acid lipase and lipid metabolism: new mechanisms, new questions, and new therapies

PURPOSE OF REVIEW

Lysosomal acid lipase (LAL), encoded by the LIPA gene, is an essential lysosomal enzyme that hydrolyzes cholesteryl ester and triglyceride delivered to the lysosome. This review highlights the novel pathophysiological role of LAL, the functional genomic discoveries of LIPA as a risk locus for coronary heart diseases (CHD), and the clinical advance in therapies for LAL deficiency.

RECENT FINDINGS

The essential role of LAL in lipid metabolism has been confirmed in human and mice with LAL deficiency. In humans, loss-of-function mutations of LIPA cause rare lysosomal disorders, Wolman disease, and cholesteryl ester storage disease, in which LAL enzyme replacement therapy has shown significant benefits in a phase 3 clinical trial. Recent studies have revealed the role of LAL-mediated lysosomal lipolysis in regulating macrophage M2 polarization, lipid mediator production, VLDL secretion, lysosomal function and autophagy, extracellular degradation of aggregated-LDL, and adipose tissue lipolysis. Genome-wide association studies and functional genomic studies have identified LIPA as a risk locus for CHD, but the causal variants and mechanisms remain to be determined.

SUMMARY

Despite years of research, our understanding of LAL is incomplete. Future studies will continue to focus on the key pathophysiological functions of LAL in health and diseases including CHD.

Altered expression of lysosomal hydrolase, acid α-glucosidase, gene in coronary artery disease

BACKGROUND

Coronary artery disease (CAD) is a common complex disease caused by atherosclerosis. Autophagy is a cellular degradation process that delivers long-lived macromolecules and dysfunctional organelles into lysosomes for digestion. Autophagy regulates lipid and cholesterol metabolism. We have previously shown that expression of autophagic and lysosomal genes is altered in CAD patients. In this study, we investigated gene expression of a lysosomal hydrolase, acid α-glucosidase (GAA), in CAD patients and controls.

METHODS

GAA gene expression was examined in large cohorts of CAD patients (n=248) and ethnically matched controls (n=208). GAA enzymatic activity, protein levels, and transcript levels were determined and compared between CAD patients and controls.

RESULTS

GAA activities in CAD patients were significantly elevated (P<0.05) compared with controls. Consistently, GAA transcription levels were also significantly increased in CAD patients (P<0.01). Multivariate logistic regression analyses (GAA transcript level, hypertension, diabetes, and smoking) revealed that GAA transcript levels were strongly associated with CAD (odds ratio 5.93, 95% confidence interval 2.98-11.78, P=3.89×10(-7)). GAA protein levels were insignificantly increased in CAD patients (P>0.05), likely due to assay insensitivity.

CONCLUSION

Compared with controls, GAA gene expression levels in CAD patients were significantly increased, suggesting that GAA may be involved in the CAD development.

Autophagy is associated with cell fate in the process of macrophage-derived foam cells formation and progress

Background

Autophagy participates in plaque formation and progression; however, its association with foam cells’ fate is unknown. To investigate autophagy features and its effect on the fate of different-stage macrophage foam cells (FCs). Different-stage FCs were obtained through incubation of THP-1 macrophages (THP-M) with oxidized low-density lipoprotein LDL (oxLDL, 80 μg/mL) for various durations (0–72 h). Autophagy in THP-1 macrophage FCs and in apoE−/− mice was regulated by Rapamycin (80 ug/mL) or 3-MA (10 mM). Lipid droplet accumulation, LC3 I/II, P62 expression level, and autophagic flux were measured. Vascular ultrasound, TUNEL, IHC, and DHE staining were used to detect the artery plaques in apoE−/− mice.

Results

In early-stage FCs, the amount of autophagosomes gradually increased, and autophagic flux intensity accelerated, but in mid-late stage FCs, autophagic flux was suppressed. For early stage FCs, treatment with autophagy activator rapamycin markedly decreased intracellular lipid content and prevented them from transforming into foam cells, while the autophagy inhibitor 3-MA considerably increased the intracellular lipid-droplet accumulation. During the process of foam cell development, upregulating autophagy not only reduced intracellular lipid-droplet accumulation, but also inhibited cell apoptosis through clearing dysfunctional mitochondria and lowering intracellular ROS level. The in vivo experiments produced consistent results that rapamycin administration in apoE−/− mice reduced the death rate of macrophages and delayed plaque progression.

Conclusions

The fate of macrophage FCs was associated with autophagy. Early autophagy enhancement inhibits the formation and progression of macrophage FCs and prevents atherosclerosis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-016-0274-z) contains supplementary material, which is available to authorized users.

Weekly Treatment of 2-Hydroxypropyl-β-cyclodextrin Improves Intracellular Cholesterol Levels in LDL Receptor Knockout Mice

Recently, the importance of lysosomes in the context of the metabolic syndrome has received increased attention. Increased lysosomal cholesterol storage and cholesterol crystallization inside macrophages have been linked to several metabolic diseases, such as atherosclerosis and non-alcoholic fatty liver disease (NAFLD). Two-hydroxypropyl-β-cyclodextrin (HP-B-CD) is able to redirect lysosomal cholesterol to the cytoplasm in Niemann-Pick type C1 disease, a lysosomal storage disorder. We hypothesize that HP-B-CD ameliorates liver cholesterol and intracellular cholesterol levels inside Kupffer cells (KCs). Hyperlipidemic low-density lipoprotein receptor knockout (Ldlr^−/−) mice were given weekly, subcutaneous injections with HP-B-CD or control PBS. In contrast to control injections, hyperlipidemic mice treated with HP-B-CD demonstrated a shift in intracellular cholesterol distribution towards cytoplasmic cholesteryl ester (CE) storage and a decrease in cholesterol crystallization inside KCs. Compared to untreated hyperlipidemic mice, the foamy KC appearance and liver cholesterol remained similar upon HP-B-CD administration, while hepatic campesterol and 7α-hydroxycholesterol levels were back increased. Thus, HP-B-CD could be a useful tool to improve intracellular cholesterol levels in the context of the metabolic syndrome, possibly through modulation of phyto- and oxysterols, and should be tested in the future. Additionally, these data underline the existence of a shared etiology between lysosomal storage diseases and NAFLD.

Lysosomal acid lipase: at the crossroads of normal and atherogenic cholesterol metabolism

Unregulated cellular uptake of apolipoprotein B-containing lipoproteins in the arterial intima leads to the formation of foam cells in atherosclerosis. Lysosomal acid lipase (LAL) plays a crucial role in both lipoprotein lipid catabolism and excess lipid accumulation as it is the primary enzyme that hydrolyzes cholesteryl esters derived from both low density lipoprotein (LDL) and modified forms of LDL. Evidence suggests that as atherosclerosis progresses, accumulation of excess free cholesterol in lysosomes leads to impairment of LAL activity, resulting in accumulation of cholesteryl esters in the lysosome as well as the cytosol in foam cells. Impaired metabolism and release of cholesterol from lysosomes can lead to downstream defects in ATP-binding cassette transporter A1 regulation, needed to offload excess cholesterol from plaque foam cells. This review focuses on the role LAL plays in normal cholesterol metabolism and how the associated changes in its enzymatic activity may ultimately contribute to atherosclerosis progression.

Self-eating in the plaque: what macrophage autophagy reveals about atherosclerosis

Autophagy (or 'self-eating') is the process by which cellular contents are recycled to support downstream metabolism. An explosion in research in the past decade has implicated its role in both health and disease and established the importance of the autophagic response during periods of stress and nutrient deprivation. Atherosclerosis is a state where chronic exposure to cellular stressors promotes disease progression, and alterations in autophagy are predicted to be consequential. Recent reports linking macrophage autophagy to lipid metabolism, blunted inflammatory signaling, and an overall suppression of proatherogenic processes support this notion. We review these data and provide a framework for understanding the role of macrophage autophagy in the pathogenesis of atherosclerosis, one of the most formidable diseases of our time.

Reprogramming of lysosomal gene expression by interleukin-4 and Stat6

Background

Lysosomes play important roles in multiple aspects of physiology, but the problem of how the transcription of lysosomal genes is coordinated remains incompletely understood. The goal of this study was to illuminate the physiological contexts in which lysosomal genes are coordinately regulated and to identify transcription factors involved in this control.

Results

As transcription factors and their target genes are often co-regulated, we performed meta-analyses of array-based expression data to identify regulators whose mRNA profiles are highly correlated with those of a core set of lysosomal genes. Among the ~50 transcription factors that rank highest by this measure, 65% are involved in differentiation or development, and 22% have been implicated in interferon signaling. The most strongly correlated candidate was Stat6, a factor commonly activated by interleukin-4 (IL-4) or IL-13. Publicly available chromatin immunoprecipitation (ChIP) data from alternatively activated mouse macrophages show that lysosomal genes are overrepresented among Stat6-bound targets. Quantification of RNA from wild-type and Stat6-deficient cells indicates that Stat6 promotes the expression of over 100 lysosomal genes, including hydrolases, subunits of the vacuolar H⁺ ATPase and trafficking factors. While IL-4 inhibits and activates different sets of lysosomal genes, Stat6 mediates only the activating effects of IL-4, by promoting increased expression and by neutralizing undefined inhibitory signals induced by IL-4.

Conclusions

The current data establish Stat6 as a broadly acting regulator of lysosomal gene expression in mouse macrophages. Other regulators whose expression correlates with lysosomal genes suggest that lysosome function is frequently re-programmed during differentiation, development and interferon signaling.

Macrophages in atherosclerosis: a dynamic balance

Atherosclerosis is a chronic inflammatory disease that arises from an imbalance in lipid metabolism and a maladaptive immune response driven by the accumulation of cholesterol-laden macrophages in the artery wall. Through the analysis of the progression and regression of atherosclerosis in animal models, there is a growing understanding that the balance of macrophages in the plaque is dynamic and that both macrophage numbers and the inflammatory phenotype influence plaque fate. In this Review, we summarize recently identified pro- and anti-inflammatory pathways that link lipid and inflammation biology with the retention of macrophages in plaques, as well as factors that have the potential to promote their egress from these sites.

Trapping of oxidized LDL in lysosomes of Kupffer cells is a trigger for hepatic inflammation

BACKGROUND & AIMS

Non-alcoholic steatohepatitis (NASH) is characterized by steatosis and inflammation. The transition from steatosis towards NASH represents a key step in pathogenesis, as it will set the stage for further severe liver damage. Under normal conditions, lipoproteins that are endocytosed by Kupffer cells (KCs) are easily transferred from the lysosomes into the cytoplasm. Oxidized LDL (oxLDL) that is taken up by the macrophages in vitro is trapped within the lysosomes, while acetylated LDL (acLDL) is leading to normal lysosomal hydrolysis, resulting in cytoplasmic storage. We have recently demonstrated that hepatic inflammation is correlated with lysosomal trapping of lipids. So far, a link between lysosomal trapping of oxLDL and inflammation was not established. We hypothesized that lysosomal trapping of oxLDL in KCs will lead to hepatic inflammation.

METHODS

Ldlr(-/-) mice were injected with LDL, acLDL and oxLDL and sacrificed after 2, 6 and 24 h.

RESULTS

Electron microscopy of KCs demonstrated that after oxLDL injection, small lipid inclusions were present inside the lysosomes after all time points and were mostly pronounced after 6 and 24 h. In contrast, no lipid inclusions were present inside KCs after LDL or acLDL injection. Hepatic expression of several inflammatory genes and scavenger receptors was higher after oxLDL injections compared with LDL or acLDL.

CONCLUSIONS

These data suggest that trapping of oxLDL inside lysosomes of KCs in vivo is causally linked to increased hepatic inflammatory gene expression. Our novel observations provide new bases for prevention and treatment of NASH.

Non-alcoholic steatohepatitis: the role of oxidized low-density lipoproteins

Non-alcoholic steatohepatitis (NASH) is hallmarked by lipid accumulation in the liver (steatosis) along with inflammation (hepatitis). The transition from simple steatosis towards NASH represents a key step in pathogenesis, as it will set the stage for further severe liver damage. Yet, the pathogenesis behind hepatic inflammation is still poorly understood. It is of relevance to better understand the underlying mechanisms involved in NASH in order to apply new knowledge to potential novel therapeutic approaches. In the current review, we propose oxidized cholesterol as a novel risk factor for NASH. Here, we summarize mouse and human studies that provide possible mechanisms for the involvement of oxidized low-density lipoproteins in NASH and consequent potential novel diagnostic tools and treatment strategies for hepatic inflammation.

Changes of lysosomes in the earliest stages of the development of atherosclerosis

One of hypotheses of atherosclerosis is based on a presumption that the zones prone to the development of atherosclerosis contain lysosomes which are characterized by enzyme deficiency and thus, are unable to dispose of lipoproteins. The present study was undertaken to investigate the characteristics and changes of lysosomes in the earliest stages of the development of atherosclerosis. Electron microscopic immunocytochemistry revealed that there were certain changes in the distribution of CD68 antigen in lysosomes along the ‘normal intima-initial lesion-fatty streak’ sequence. There were no significant changes found in the key mRNAs encoding for the components of endosome/lysosome compartment in initial atherosclerotic lesions, but in fatty streaks, the contents of EEA1 and Rab5a mRNAs were found to be diminished while the contents of CD68 and p62 mRNAs were increased, compared with the intact tissue. The study reinforces a view that changes occurring in lysosomes play a role in atherogenesis from the very earlier stages of the disease.

Wip1-dependent regulation of autophagy, obesity, and atherosclerosis

Obesity and atherosclerosis-related diseases account for over one-third of deaths in the western world. Controlling these conditions remains a major challenge due to an incomplete understanding of the molecular pathways involved. Here, we show that Wip1 phosphatase, a known negative regulator of Atm-dependent signaling, plays a major role in controlling fat accumulation and atherosclerosis in mice; specifically, Wip1 deficiency prevents both conditions. In the course of atherosclerosis, deletion of Wip1 results in suppression of macrophage conversion into foam cells, thus preventing the formation of atherosclerotic plaques. This process appears to be independent of p53 but rely on a noncanonical Atm-mTOR signaling pathway and on selective autophagy in regulation of cholesterol efflux. We propose that the Wip1-dependent control of autophagy and cholesterol efflux may provide avenues for treating obesity and atherosclerosis.

Inhibition of lysosomal function in macrophages incubated with elevated glucose concentrations: a potential contributory factor in diabetes-associated atherosclerosis

OBJECTIVE

People with diabetes have an elevated risk of atherosclerosis. The accumulation of lipid within macrophage cells in the artery wall is believed to arise via the uptake and subsequent processing of modified low-density lipoproteins (LDL) via the endo-lysosomal system. In this study the effects of prolonged exposure to elevated glucose upon macrophage lysosomal function was examined to determine whether this contributes to modulated protein catabolism.

METHODS

Human monocytes were isolated from white-cell concentrates and differentiated, in vitro, into monocyte-derived macrophages over 11 days in medium containing 5-30 mmol/L glucose. Murine macrophage-like J774A.1 cells were incubated similarly. Lysosomal cathepsin (B, D, L and S) and acid lipase activities were assessed using fluorogenic substrates; cathepsin protein levels were examined by Western blotting. Lysosomal numbers were examined using the lysomotropic fluorescent dye LysoTracker DND-99, measurement of aryl sulfatase activity, and quantification of lysosome-associated membrane glycoprotein-1 (LAMP-1) by Western blotting.

RESULTS

Exposure to elevated glucose, but not mannitol, resulted in a concentration-dependent decrease in the activity, and to a lesser extent protein levels, of four lysosomal cathepsins. Acid lipase activity was also significantly reduced. Arysulfatase activity, LAMP-1 levels and lysosomal numbers were also decreased at the highest glucose concentrations, though to a lesser extent.

CONCLUSION

Long term exposure of human and murine macrophage cells to elevated glucose levels result in a depression of lysosomal proteolytic and lipase activities. This may result in decreased clearance and cellular accumulation of (lipo)proteins and contribute to the accumulation of modified proteins and lipids in diabetes-associated atherosclerosis.

Autophagy links inflammasomes to atherosclerotic progression

We investigated the role of autophagy in atherosclerosis. During plaque formation in mice, autophagic markers colocalized predominantly with macrophages (mφ). Atherosclerotic aortas had elevated levels of p62, suggesting that dysfunctional autophagy is characteristic of plaques. To determine whether autophagy directly influences atherogenesis, we characterized Beclin-1 heterozygous-null and mφ-specific ATG5-null (ATG5-mφKO) mice, commonly used models of autophagy haploinsufficiency and deficiency, respectively. Haploinsufficent Beclin-1 mice had no atherosclerotic phenotype, but ATG5-mφKO mice had increased plaques, suggesting an essential role for basal levels of autophagy in atheroprotection. Defective autophagy is associated with proatherogenic inflammasome activation. Classic inflammasome markers were robustly induced in ATG5-null mφ, especially when coincubated with cholesterol crystals. Moreover, cholesterol crystals appear to be increased in ATG5-mφKO plaques, suggesting a potentially vicious cycle of crystal formation and inflammasome activation in autophagy-deficient plaques. These results show that autophagy becomes dysfunctional in atherosclerosis and its deficiency promotes atherosclerosis in part through inflammasome hyperactivation.

A gene expression signature that classifies human atherosclerotic plaque by relative inflammation status

BACKGROUND

Atherosclerosis is a complex disease requiring improvements in diagnostic techniques and therapeutic treatments. Both improvements will be facilitated by greater exploration of the biology of atherosclerotic plaque. To this end, we carried out large-scale gene expression analysis of human atherosclerotic lesions.

METHODS AND RESULTS

Whole genome expression analysis of 101 plaques from patients with peripheral artery disease identified a robust gene signature (1514 genes) that is dominated by processes related to Toll-like receptor signaling, T-cell activation, cholesterol efflux, oxidative stress response, inflammatory cytokine production, vasoconstriction, and lysosomal activity. Further analysis of gene expression in microdissected carotid plaque samples revealed that this signature is differentially expressed in macrophage-rich and smooth muscle cell-containing regions. A quantitative PCR gene expression panel and inflammatory composite score were developed on the basis of the atherosclerotic plaque gene signature. When applied to serial sections of carotid plaque, the inflammatory composite score was observed to correlate with histological and morphological features related to plaque vulnerability.

CONCLUSIONS

The robust mRNA expression signature identified in the present report is associated with pathological features of vulnerable atherosclerotic plaque and may be useful as a source of biomarkers and targets of novel antiatherosclerotic therapies.

Alterations of autophagic-lysosomal system in the peripheral leukocytes of patients with myocardial infarction

BACKGROUND

Myocardial infarction (MI) is a common and multifactorial disease. To date, causal genes and underlying mechanisms remain largely unknown. Autophagic-lysosomal system, a highly conserved degradative process in cells, has been implicated in lipid metabolism. In this study, we explored the alterations of the autophagic-lysosomal system in patients with acute MI.

METHODS

Gene expression of lysosomal associated membrane protein 2 (LAMP-2), a lysosomal marker gene, and microtubule-associated protein 1 light chain 3 (LC3), an autophagy marker gene, in the peripheral leukocytes of MI patients were examined at transcription and protein levels by RT-PCR assay and western blot analysis, respectively.

RESULTS

Compared to age- and sex-matched healthy controls (n=146), levels of LC3 gene expression and LC3-II protein, a cleaved form of LC3 protein, were significantly decreased in MI patients (n=81). LAMP-2 gene expression and protein levels were significantly increased. Decreased LC3 gene expression (OR, 2.150, 95%CI, 1.050-4.405, P=0.036) or increased LAMP-2 gene expression (OR, 3.317, 95%CI, 1.588-6.931, P<0.001) levels were associated with MI.

CONCLUSIONS

Our findings indicated that in the peripheral leukocytes of MI patients, autophagy activity is reduced and lysosomal accumulation is increased, which may contribute to the MI pathogenesis. Further genetic analyses of autophagic-lysosomal genes are warranted.

Autophagy regulates cholesterol efflux from macrophage foam cells via lysosomal acid lipase

The lipid droplet (LD) is the major site of cholesterol storage in macrophage foam cells and is a potential therapeutic target for the treatment of atherosclerosis. Cholesterol, stored as cholesteryl esters (CEs), is liberated from this organelle and delivered to cholesterol acceptors. The current paradigm attributes all cytoplasmic CE hydrolysis to the action of neutral CE hydrolases. Here, we demonstrate an important role for lysosomes in LD CE hydrolysis in cholesterol-loaded macrophages, in addition to that mediated by neutral hydrolases. Furthermore, we demonstrate that LDs are delivered to lysosomes via autophagy, where lysosomal acid lipase (LAL) acts to hydrolyze LD CE to generate free cholesterol mainly for ABCA1-dependent efflux; this process is specifically induced upon macrophage cholesterol loading. We conclude that, in macrophage foam cells, lysosomal hydrolysis contributes to the mobilization of LD-associated cholesterol for reverse cholesterol transport.

Assessment of endothelial dysfunction in health and disease; using various parameters

Conditions like hypertension, atherosclerosis and diabetes are known to be the result of endothelial dysfunction which could begin early in the life of an individual. The markers of endothelial dysfunction studied in the present work are plasma hemoglobin, serum high sensitivity C-reactive protein (hsCRP) and plasma nitrite and nitrate. We studied the onset of endothelial dysfunction with increase in age as well as in disease condition like newly detected hypertensives and of hypertensive diabetic patients. The mean plasma hemoglobin and hsCRP values were found to increase with age. hsCRP and plasma hemoglobin levels were significantly higher in patient groups as compared to aged-matched controls. On the other hand nitrate and nitrite was significantly higher in patients who suffered from both diabetes and hypertension together and nitrate values were significantly lower in patients who suffered from hypertensive alone. Plasma hemoglobin and hsCRP are associated with endothelial dysfunction and increases both with age as well as in disease condition.

Macrophages in the pathogenesis of atherosclerosis

In atherosclerosis, the accumulation of apolipoprotein B-lipoproteins in the matrix beneath the endothelial cell layer of blood vessels leads to the recruitment of monocytes, the cells of the immune system that give rise to macrophages and dendritic cells. Macrophages derived from these recruited monocytes participate in a maladaptive, nonresolving inflammatory response that expands the subendothelial layer due to the accumulation of cells, lipid, and matrix. Some lesions subsequently form a necrotic core, triggering acute thrombotic vascular disease, including myocardial infarction, stroke, and sudden cardiac death. This Review discusses the central roles of macrophages in each of these stages of disease pathogenesis.

LAMP-2 gene expression in peripheral leukocytes is increased in patients with coronary artery disease

BACKGROUND

Coronary artery disease (CAD) is a common complex disease that is caused by interaction between genetic and environmental factors. Accumulating evidence indicates that foam cells in the atherosclerotic plaques exhibit the characteristics of lysosomal storage diseases, namely lysosomal accumulation of indigested materials. In patients with lysosomal storage diseases, lysosomal accumulation of lipids and cholesterols in atherosclerotic plaque cells has been observed. However, the roles of lysosomal hydrolases and proteins in the pathogenesis of atherosclerosis and CAD remain unclear.

HYPOTHESIS

Lysosomal hydrolases and proteins may be involved in the pathogenesis of atherosclerosis and CAD by affecting lipid and cholesterol metabolism.

METHODS

Expression levels of LAMP-2, a lysosomal membrane marker gene, in the peripheral leukocytes of CAD patients (n = 134) and age- and sex-matched healthy controls (n = 80) were examined at transcription and protein levels with reverse transcriptase-polymerase chain reaction and Western blot analyses, respectively. The results were compared between CAD patients and healthy controls.

RESULTS

LAMP-2 gene expression and LAMP-2 protein levels were significantly increased in the peripheral leukocytes of CAD patients, compared with healthy controls. Furthermore, multivariate logistic regression analyses revealed that CAD is significantly associated with LAMP-2 gene expression levels (odds ratio [OR] 8.84, 95% confidence interval [CI]: 2.15-36.40, P = 0.003) or LAMP-2 protein levels (OR 2.03, 95% CI: 1.15-3.59, P = 0.015).

CONCLUSIONS

In CAD patients, LAMP-2 gene expression in the peripheral leukocytes was significantly increased than were controls, which indicates lysosomal accumulation. These data suggest that insufficient lysosomal hydrolases and proteins may lead to abnormal lipid and cholesterol metabolism, which cause initiation and progression of atherosclerosis and CAD.

Lysosomes, cholesterol and atherosclerosis

Cholesterol-engorged macrophage foam cells are a critical component of the atherosclerotic lesion. Reducing the sterol deposits in lesions reduces clinical events. Sterol accumulations within lysosomes have proven to be particularly hard to mobilize out of foam cells. Moreover, excess sterol accumulation in lysosomes has untoward effects, including a complete disruption of lysosome function. Recently, we demonstrated that treatment of sterol-engorged macrophages in culture with triglyceride-containing particles can reverse many of the effects of cholesterol on lysosomes and dramatically reduce the sterol burden in these cells. This article describes what is known about lysosomal sterol engorgement, discusses the possible mechanisms by which triglyceride could produce its effects, and evaluates the possible positive and negative effects of reducing the lysosomal cholesterol levels in foam cells.

Oxidized LDL: diversity, patterns of recognition, and pathophysiology

Oxidative modification of LDL is known to elicit an array of pro-atherogenic responses, but it is generally underappreciated that oxidized LDL (OxLDL) exists in multiple forms, characterized by different degrees of oxidation and different mixtures of bioactive components. The variable effects of OxLDL reported in the literature can be attributed in large part to the heterogeneous nature of the preparations employed. In this review, we first describe the various subclasses and molecular composition of OxLDL, including the variety of minimally modified LDL preparations. We then describe multiple receptors that recognize various species of OxLDL and discuss the mechanisms responsible for the recognition by specific receptors. Furthermore, we discuss the contentious issues such as the nature of OxLDL in vivo and the physiological oxidizing agents, whether oxidation of LDL is a prerequisite for atherogenesis, whether OxLDL is the major source of lipids in foam cells, whether in some cases it actually induces cholesterol depletion, and finally the Janus-like nature of OxLDL in having both pro- and anti-inflammatory effects. Lastly, we extend our review to discuss the role of LDL oxidation in diseases other than atherosclerosis, including diabetes mellitus, and several autoimmune diseases, such as lupus erythematosus, anti-phospholipid syndrome, and rheumatoid arthritis.

Triglyceride alters lysosomal cholesterol ester metabolism in cholesteryl ester-laden macrophage foam cells

In late-stage atherosclerosis, much of the cholesterol in macrophage foam cells resides within enlarged lysosomes. Similarly, human macrophages incubated in vitro with modified LDLs contain significant amounts of lysosomal free cholesterol and cholesteryl ester (CE), which disrupts lysosomal function similar to macrophages in atherosclerotic lesions. The lysosomal cholesterol cannot be removed, even in the presence of strong efflux promoters. Thus, efflux of sterol is prevented. In the artery wall, foam cells interact with triglyceride-rich particles (TRPs) in addition to modified LDLs. Little is known about how TRP metabolism affects macrophage cholesterol. Therefore, we explored the effect of TRP on intracellular CE metabolism. Triglyceride (TG), delivered to lysosomes in TRP, reduced CE accumulation by 50%. Increased TG levels within the cell, particularly within lysosomes, correlated with reductions in CE content. The volume of cholesterol-engorged lysosomes decreased after TRP treatment, indicating cholesterol was cleared. Lysosomal TG also reduced the cholesterol-induced inhibition of lysosomal acidification allowing lysosomes to remain active. Enhanced degradation and clearance of CE may be explained by movement of cholesterol out of the lysosome to sites where it is effluxed. Thus, our results show that introduction of TG into CE-laden foam cells influences CE metabolism and, potentially, atherogenesis.-Ullery-Ricewick, J. C., B. E. Cox, E. E. Griffin, and W. G. Jerome. Triglyceride alters lysosomal cholesterol ester metabolism in cholesteryl ester-laden macrophage foam cells.

Endolysosomal phospholipidosis and cytosolic lipid droplet storage and release in macrophages

This review summarizes the current knowledge of endolysosomal and cytoplasmic lipid storage in macrophages induced by oxidized LDL (Ox-LDL), enzymatically degraded LDL (E-LDL) and other atherogenic lipoprotein modifications, and their relation to the adapter protein 3 (AP-3) dependent ABCA1 and ABCG1 cellular lipid efflux pathways. We compare endolysosomal lipid storage caused either through drug induced phospholipidosis, inheritable endolysosomal and cytosolic lipid storage disorders and Ox-LDL or E-LDL induced phagosomal uptake and cytosolic lipid droplet storage in macrophages. Ox-LDL is resistant to rapid endolysosomal hydrolysis and is trapped within the endolysosomal compartment generating lamellar bodies which resemble the characteristics of phospholipidosis. Various inherited lysosomal storage diseases including sphingolipidosis, glycosphingolipidosis and cholesterylester storage diseases also present a phospholipidosis phenotype. In contrast E-LDL resembling coreless unesterified cholesterol enriched LDL-particles, with a multilamellar, liposome-like structure, lead to rapid phagosomal degradation and cytosolic lipid droplet accumulation. As a consequence the uptake of E-LDL through type I and type II phagocytosis leads to increased lipid droplet formation and moderate upregulation of ABCA1 and ABCG1 while uptake of Ox-LDL leads to a rapid expansion of the lysosomal compartment and a pronounced upregulation of the ABCA1/ABCG1/AP-3 lipid efflux pathway.

Cultured porcine trabecular meshwork cells display altered lysosomal function when subjected to chronic oxidative stress

PURPOSE

To investigate the effects of chronic oxidative stress on lysosomal function in trabecular meshwork (TM) cells.

METHODS

Confluent cultures of porcine TM cells were grown for 2 weeks in physiological (5% O(2)) or hyperoxic conditions (40% O(2)) in the presence or absence of the protease inhibitor leupeptin (10 microM). The following parameters were quantified using the fluorogenic probes indicated within parentheses: autofluorescence, intracellular reactive oxygen species (ROS; H(2)DCFDA), mitochondrial membrane potential (JC-1), mitochondrial content (Mitotracker Red; Invitrogen-Molecular Probes, Eugene, OR), lysosomal content (acridine orange and Lysotracker Red [Invitrogen-Molecular Probes]), autophagic vacuole content (MDC), SA-beta-galactosidase (FDG), and cathepsin activities (z-FR-AMC). Cathepsin levels were quantified by qPCR and Western blot analysis. Ultrastructural analysis was performed by transmission electron microscopy.

RESULTS

Prolonged exposure of porcine TM cells to a hyperoxic environment led to an increase in ROS production and oxidized material. Electron micrographs revealed the cytoplasmic accumulation of lipofuscin-loaded lysosomes. Augmented lysosomal and autophagic vacuole content was confirmed with specific fluorophores. The mRNA and protein levels of several cathepsins were upregulated with oxidative stress. This upregulated expression did not correlate with increased lysosomal activity.

CONCLUSIONS

The results indicate that chronic exposure of TM cells to oxidative stress causes the accumulation of nondegradable material within the lysosomal compartment, leading to diminished lysosomal activity. Since the lysosomal system is responsible for the continuous turnover of cellular organelles, impaired lysosomal activity may lead to progressive failure of cellular TM function with age.

Lysosomal cholesterol accumulation inhibits subsequent hydrolysis of lipoprotein cholesteryl ester

Human macrophages incubated for prolonged periods with mildly oxidized LDL (oxLDL) or cholesteryl ester-rich lipid dispersions (DISP) accumulate free and esterified cholesterol within large, swollen lysosomes similar to those in foam cells of atherosclerosis. The cholesteryl ester (CE) accumulation is, in part, the result of inhibition of lysosomal hydrolysis due to increased lysosomal pH mediated by excessive lysosomal free cholesterol (FC). To determine if the inhibition of hydrolysis was long lived and further define the extent of the lysosomal defect, we incubated THP-1 macrophages with oxLDL or DISP to produce lysosome sterol engorgement and then chased with acetylated LDL (acLDL). Unlike oxLDL or DISP, CE from acLDL normally is hydrolyzed rapidly. Three days of incubation with oxLDL or DISP produced an excess of CE in lipid-engorged lysosomes, indicative of inhibition. After prolonged oxLDL or DISP pretreatment, subsequent hydrolysis of acLDL CE was inhibited. Coincident with the inhibition, the lipid-engorged lysosomes failed to maintain an acidic pH during both the initial pretreatment and subsequent acLDL incubation. This indicates that the alterations in lysosomes were general, long lived, and affected subsequent lipoprotein metabolism. This same phenomenon, occurring within atherosclerotic foam cells, could significantly affect lesion progression.

The yeast protein sorting pathway as an experimental model for lysosomal trafficking

Lysosomes are conserved organelles that are present in all eukaryotic cells. They are part of a complicated network of intracellular trafficking routes - the lysosomal transport system. Lysosomes are necessary for the maintenance of cellular homeostasis and for many specialized functions, including the activity of many components of the mammalian immune system. Dysfunctions of the lysosomal system are associated with numerous diseases, such as storage disorders, neuro- and myopathies, cancer and some types of albinism and immunological deficiencies. High conservation of the processes of lysosomal biogenesis and transport enables the use of yeast as a model for studying the mechanisms that underlie these diseases. In this review, we discuss several examples of such models in an attempt to present an overview of the most important experimental methods available in yeast research.

Effects of cellular cholesterol loading on macrophage foam cell lysosome acidification

Macrophages incubated with mildly oxidized low density lipoprotein (OxLDL), aggregated low density lipoprotein (AggLDL), or cholesteryl ester-rich lipid dispersions (DISPs) accumulate cholesterol in lysosomes followed by an inhibition of lysosomal cholesteryl ester (CE) hydrolysis. The variety of cholesterol-containing particles producing inhibition of hydrolysis suggests that inhibition may relate to general changes in lysosomes. Lysosome pH is a key mediator of activity and thus is a potential mechanism for lipid-induced inhibition. We investigated the effects of cholesterol accumulation on THP-1 macrophage lysosome pH. Treatment with OxLDL, AggLDL, and DISPs resulted in inhibition of the lysosome's ability to maintain an active pH and concomitant decreases in CE hydrolysis. Consistent with an overall disruption of lysosome function, exposure to OxLDL or AggLDL reduced lysosomal apolipoprotein B degradation. The lysosomal cholesterol sequestration and inactivation are not observed in cholesterol-equivalent cells loaded using acetylated low density lipoprotein (AcLDL). However, AcLDL-derived cholesterol in the presence of progesterone (to block cholesterol egression from lysosomes) inhibited lysosome acidification. Lysosome inhibition was not attributable to a decrease in the overall levels of vacuolar ATPase. However, augmentation of membrane cholesterol in isolated lysosomes inhibited vacuolar ATPase-dependent pumping of H+ ions into lysosomes. These data indicate that lysosomal cholesterol accumulation alters lysosomes in ways that could exacerbate foam cell formation and influence atherosclerotic lesion development.