Identification of a leucine-mediated threshold effect governing macrophage mTOR signalling and cardiovascular risk

High protein intake is common in western societies and is often promoted as part of a healthy lifestyle; however, amino-acid-mediated mammalian target of rapamycin (mTOR) signalling in macrophages has been implicated in the pathogenesis of ischaemic cardiovascular disease. In a series of clinical studies on male and female participants ( NCT03946774 and NCT03994367 ) that involved graded amounts of protein ingestion together with detailed plasma amino acid analysis and human monocyte/macrophage experiments, we identify leucine as the key activator of mTOR signalling in macrophages. We describe a threshold effect of high protein intake and circulating leucine on monocytes/macrophages wherein only protein in excess of ∼25 g per meal induces mTOR activation and functional effects. By designing specific diets modified in protein and leucine content representative of the intake in the general population, we confirm this threshold effect in mouse models and find ingestion of protein in excess of ∼22% of dietary energy requirements drives atherosclerosis in male mice. These data demonstrate a mechanistic basis for the adverse impact of excessive dietary protein on cardiovascular risk.

Loss of Macrophage mTORC2 Drives Atherosclerosis via FoxO1 and IL-1β Signaling

BACKGROUND

The mTOR (mechanistic target of rapamycin) pathway is a complex signaling cascade that regulates cellular growth, proliferation, metabolism, and survival. Although activation of mTOR signaling has been linked to atherosclerosis, its direct role in lesion progression and in plaque macrophages remains poorly understood. We previously demonstrated that mTORC1 (mTOR complex 1) activation promotes atherogenesis through inhibition of autophagy and increased apoptosis in macrophages.

METHODS

Using macrophage-specific Rictor- and mTOR-deficient mice, we now dissect the distinct functions of mTORC2 pathways in atherogenesis.

RESULTS

In contrast to the atheroprotective effect seen with blockade of macrophage mTORC1, macrophage-specific mTORC2-deficient mice exhibit an atherogenic phenotype, with larger, more complex lesions and increased cell death. In cultured macrophages, we show that mTORC2 signaling inhibits the FoxO1 (forkhead box protein O1) transcription factor, leading to suppression of proinflammatory pathways, especially the inflammasome/IL (interleukin)-1β response, a key mediator of vascular inflammation and atherosclerosis. In addition, administration of FoxO1 inhibitors efficiently rescued the proinflammatory response caused by mTORC2 deficiency both in vitro and in vivo. Interestingly, collective deletion of macrophage mTOR, which ablates mTORC1- and mTORC2-dependent pathways, leads to minimal change in plaque size or complexity, reflecting the balanced yet opposing roles of these signaling arms.

CONCLUSIONS

Our data provide the first mechanistic details of macrophage mTOR signaling in atherosclerosis and suggest that therapeutic measures aimed at modulating mTOR need to account for its dichotomous functions.

Abstract 433: Investigation Of The Divergent Roles Of Macrophage Mtor Signaling In Atherosclerosis

The mechanistic target of rapamycin (mTOR) pathway is considered a key regulator of cellular proliferation, survival, metabolism, and degradation. Yet its role in atherosclerosis and in macrophages of the atherosclerotic plaque remains poorly understood. Our previous study has demonstrated macrophage mTORC1 signaling promotes atherogenesis through inhibiting autophagy and increasing apoptosis. Using macrophage-specific Rictor- and mTOR-deficient mice, we now dissect the distinct functions of mTORC2 pathways in atherogenesis. In contrast to the protective effect against atherosclerosis from blocking macrophage mTORC1, macrophage-specific mTORC2-deficient mice have the opposite phenotype with the development of larger and more complex lesions. In cultured macrophages, we show that mTORC2 signaling inhibits the FoxO1 transcription factor leading to suppression of pro-inflammatory pathways, especially the inflammasome/IL-1β response, which also manifest in vivo. In addition, administration of FoxO1 inhibitors efficiently rescued the hyperinflammation caused by mTORC2 deficiency in vitro and in vivo. Interestingly, deletion of macrophage mTOR, which ablates both mTOR-dependent pathways leads to minimal change in plaques reflecting the apparently balanced and opposing roles of its signaling arms. Our data provide the first mechanistic details of macrophage mTOR signaling in atherosclerosis and suggest that therapeutic measures aimed at modulating mTOR need to account for its dichotomous functions.

MAP4K4 negatively regulates CD8 T cell-mediated antitumor and antiviral immunity

During cytotoxic T cell activation, lymphocyte function-associated antigen-1 (LFA-1) engages its ligands on antigen-presenting cells (APCs) or target cells to enhance T cell priming or lytic activity. Inhibiting LFA-1 dampens T cell-dependent symptoms in inflammation, autoimmune diseases, and graft-versus-host disease. However, the therapeutic potential of augmenting LFA-1 function is less explored. Here, we show that genetic deletion or inhibition of mitogen-activated protein kinase kinase kinase kinase 4 (MAP4K4) enhances LFA-1 activation on CD8 T cells and improves their adherence to APCs or LFA-1 ligand. In addition, loss of Map4k4 increases CD8 T cell priming, which culminates in enhanced antigen-dependent activation, proliferation, cytokine production, and cytotoxic activity, resulting in impaired tumor growth and improved response to viral infection. LFA-1 inhibition reverses these phenotypes. The ERM (ezrin, radixin, and moesin) proteins reportedly regulate T cell-APC conjugation, but the molecular regulator and effector of ERM proteins in T cells have not been defined. In this study, we demonstrate that the ERM proteins serve as mediators between MAP4K4 and LFA-1. Last, systematic analyses of many organs revealed that inducible whole-body deletion of Map4k4 in adult animals is tolerated under homeostatic conditions. Our results uncover MAP4K4 as a potential target to augment antitumor and antiviral immunity.

Author Correction: High-protein diets increase cardiovascular risk by activating macrophage mTOR to suppress mitophagy

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

High-protein diets increase cardiovascular risk by activating macrophage mTOR to suppress mitophagy

High protein diets are commonly utilized for weight loss, yet have been reported to raise cardiovascular risk. The mechanisms underlying this risk are unknown. Here, we show that dietary protein drives atherosclerosis and lesion complexity. Protein ingestion acutely elevates amino acid levels in blood and atherosclerotic plaques, stimulating macrophage mTOR signaling. This is causal in plaque progression as the effects of dietary protein are abrogated in macrophage-specific Raptor-null mice. Mechanistically, we find amino acids exacerbate macrophage apoptosis induced by atherogenic lipids, a process that involves mTORC1-dependent inhibition of mitophagy, accumulation of dysfunctional mitochondria, and mitochondrial apoptosis. Using macrophage-specific mTORC1- and autophagy-deficient mice we confirm this amino acid-mTORC1-autophagy signaling axis in vivo. Our data provide the first insights into the deleterious impact of excessive protein ingestion on macrophages and atherosclerotic progression. Incorporation of these concepts in clinical studies will be important to define the vascular effects of protein-based weight loss regimens.

TFEB and trehalose drive the macrophage autophagy-lysosome system to protect against atherosclerosis

In the atherosclerotic plaque, macrophages are the key catabolic workhorse responsible for clearing lipid and dead cell debris. To survive the highly proinflammatory and lipotoxic plaque environment, macrophages must adopt strategies for maintaining tight homeostasis and self-renewal. Macroautophagy/autophagy is a pro-survival cellular pathway wherein damaged or excess cellular cargoes are encapsulated by a double-membrane compartment and delivered to the lysosome for hydrolysis. Previously, macrophage-specific autophagy deficiency has been shown to be atherogenic through several complementary mechanisms including hyperactivation of the inflammasome, defective efferocytosis, accumulation of cytotoxic protein aggregates, and impaired lipid degradation. Conversely, in a recent study we hypothesized that enhancing the macrophage autophagy-lysosomal system through genetic or pharmacological means could protect against atherosclerosis. We demonstrated that TFEB, a transcription factor master regulator of autophagy and lysosome biogenesis, coordinately enhances the function of this system to reduce atherosclerotic plaque burden. Further, we characterized the disaccharide trehalose as a novel inducer of TFEB with similar atheroprotective effects. Overall, these findings mechanistically interrogate the importance and therapeutic promise of a functional autophagy-lysosome degradation system in plaque macrophage biology.

N-3 PUFAs induce inflammatory tolerance by formation of KEAP1-containing SQSTM1/p62-bodies and activation of NFE2L2

Inflammation is crucial in the defense against infections but must be tightly controlled to limit detrimental hyperactivation. Our diet influences inflammatory processes and omega-3 polyunsaturated fatty acids (n-3 PUFAs) have known anti-inflammatory effects. The balance of pro- and anti-inflammatory processes is coordinated by macrophages and macroautophagy/autophagy has recently emerged as a cellular process that dampens inflammation. Here we report that the n-3 PUFA docosahexaenoic acid (DHA) transiently induces cytosolic speckles of the autophagic receptor SQSTM1/p62 (sequestosome 1) (described as SQSTM1/p62-bodies) in macrophages. We suggest that the formation of SQSTM1/p62-bodies represents a fast mechanism of NFE2L2/Nrf2 (nuclear factor, erythroid 2 like 2) activation by recruitment of KEAP1 (kelch like ECH associated protein 1). Further, the autophagy receptor TAX1BP1 (Tax1 binding protein 1) and ubiquitin-editing enzyme TNFAIP3/A20 (TNF α induced protein 3) could be identified in DHA-induced SQSTM1/p62-bodies. Simultaneously, DHA strongly dampened the induction of pro-inflammatory genes including CXCL10 (C-X-C motif chemokine ligand 10) and we suggest that formation of SQSTM1/p62-bodies and activation of NFE2L2 leads to tolerance towards selective inflammatory stimuli. Finally, reduced CXCL10 levels were related to the improved clinical outcome in n-3 PUFA-supplemented heart-transplant patients and we propose CXCL10 as a robust marker for the clinical benefits mobilized by n-3 PUFA supplementation.

Abstract 384: Low Carbohydrate High Protein (LCHP) Diets are Atherogenic by Supplying Excess Amino Acids to Alter the Macrophage mTORC1-Autophagy Signaling Axis

Low carbohydrate high protein (LCHP) diets are commonly used in weight loss programs. However, the overall health benefits of such regimens are controversial with recent studies even suggesting an increased cardiovascular risk in certain populations. A few reports in animal models corroborate these concerns demonstrating increased LCHP-induced atherosclerosis. Interestingly, the downstream sequelae of such diets on tissues and cellular signaling are largely inferred with relevant mechanisms undefined. We first confirmed in the ApoE-null mouse model that LCHP diets are indeed atherogenic with the development of complex lesions. Using mass spectrometry, we find high protein feeding not only increases serum amino acid levels but increases amino acid load to tissues including the spleen and aorta with resultant activation of the mTORC1 signaling pathway particularly in macrophages. The involvement of mTORC1 is clearly causal as the atherogenic effect of LCHP-feeding is abrogated in macrophage-specific Raptor-null mice. Further mechanistic evaluation of the effects of amino acids on macrophages reveals dichotomous roles on a predominant mTORC1 target, autophagy. Certain amino acids such as Leucine potently activate mTORC1 via recruitment to lysosome and in turn suppress autophagy via ULK1 phosphorylation, whereas others such as glutamine act indirectly by downregulating the transcription of autophagy chaperones including p62/SQSTM1. This combined suppressive effect on autophagy leads to macrophage inflammasome activation and IL-1β release, accumulation of deleterious protein aggregates, and increased cell death. The in vivo relevance of this LCHP-amino acid-mTORC1-autophagy axis is supported by 1) the absence of increased atherosclerosis in macrophage autophagy-deficient (ATG5-/-) mice fed a LCHP diet, and 2) the absence of reduced atherosclerosis in mice dually deficient in macrophage mTORC1 and autophagy (Raptor/ATG5-/-). Our data provide the first mechanistic details of the deleterious effects of high protein diets on macrophages and the development of atherosclerosis. Incorporation of these concepts in clinical studies will be important to define the vascular effects of dietary protein.

Abstract 559: Exploiting Macrophage Autophagy-Lysosomal Biogenesis With the Natural Sugar Trehalose as a Therapy for Atherosclerosis

The autophagy-lysosome system is a catabolic cellular mechanism that degrades dysfunctional proteins and organelles. In atherosclerosis, there is mounting evidence that this process is rendered dysfunctional particularly in plaque macrophages and is an important trigger for plaque progression. In an effort to characterize practical inducers of macrophage degradative capacity, we now describe the unique vascular benefits of a natural sugar called trehalose, a recognized autophagy inducer with a currently unknown mechanism of action. Trehalose-treated macrophages display enhanced autophagy via a process that involves lysosomal stress and resultant activation of TFEB, the master transcriptional regulator of autophagy-lysosomal biogenesis. We find an important downstream effect of trehalose to be the induction of p62-dependent selective autophagy of cytotoxic polyubiquitinated protein aggregates and dampening of IL-1β/inflammasome function. We confirm the relevance of these in vitro observations in several pro-atherogenic (ApoE-/-) mouse models. Administration of trehalose during eight weeks of Western diet feeding potently induces autophagy and TFEB in plaque macrophages with concomitant reductions in polyubiquitinated protein aggregate burden along with significantly reduced plaque size and complexity. Importantly, these findings are completely abrogated in mice deficient in macrophage autophagy (ATG5-/-) or the selective autophagy chaperone (p62-/-). Further detailed pharmacokinetic evaluation of trehalose shows that physiologically relevant concentrations are indeed achievable in mice. Taken together, our data support the serious consideration of this safe and natural sugar as a potent inducer of macrophage degradative capacity in the treatment of atherosclerotic vascular disease.

Target acquired: Selective autophagy in cardiometabolic disease

The accumulation of damaged or excess proteins and organelles is a defining feature of metabolic disease in nearly every tissue. Thus, a central challenge in maintaining metabolic homeostasis is the identification, sequestration, and degradation of these cellular components, including protein aggregates, mitochondria, peroxisomes, inflammasomes, and lipid droplets. A primary route through which this challenge is met is selective autophagy, the targeting of specific cellular cargo for autophagic compartmentalization and lysosomal degradation. In addition to its roles in degradation, selective autophagy is emerging as an integral component of inflammatory and metabolic signaling cascades. In this Review, we focus on emerging evidence and key questions about the role of selective autophagy in the cell biology and pathophysiology of metabolic diseases such as obesity, diabetes, atherosclerosis, and steatohepatitis. Essential players in these processes are the selective autophagy receptors, defined broadly as adapter proteins that both recognize cargo and target it to the autophagosome. Additional domains within these receptors may allow integration of information about autophagic flux with critical regulators of cellular metabolism and inflammation. Details regarding the precise receptors involved, such as p62 and NBR1, and their predominant interacting partners are just beginning to be defined. Overall, we anticipate that the continued study of selective autophagy will prove to be informative in understanding the pathogenesis of metabolic diseases and to provide previously unrecognized therapeutic targets.

Sequences within the C Terminus of the Metabotropic Glutamate Receptor 5 (mGluR5) Are Responsible for Inner Nuclear Membrane Localization

Traditionally, G-protein-coupled receptors (GPCR) are thought to be located on the cell surface where they transmit extracellular signals to the cytoplasm. However, recent studies indicate that some GPCRs are also localized to various subcellular compartments such as the nucleus where they appear required for various biological functions. For example, the metabotropic glutamate receptor 5 (mGluR5) is concentrated at the inner nuclear membrane (INM) where it mediates Ca²⁺ changes in the nucleoplasm by coupling with G_q/11 Here, we identified a region within the C-terminal domain (amino acids 852-876) that is necessary and sufficient for INM localization of the receptor. Because these sequences do not correspond to known nuclear localization signal motifs, they represent a new motif for INM trafficking. mGluR5 is also trafficked to the plasma membrane where it undergoes re-cycling/degradation in a separate receptor pool, one that does not interact with the nuclear mGluR5 pool. Finally, our data suggest that once at the INM, mGluR5 is stably retained via interactions with chromatin. Thus, mGluR5 is perfectly positioned to regulate nucleoplasmic Ca²⁺in situ.

Anti-angiogenic Nanotherapy Inhibits Airway Remodeling and Hyper-responsiveness of Dust Mite Triggered Asthma in the Brown Norway Rat

Although angiogenesis is a hallmark feature of asthmatic inflammatory responses, therapeutic anti-angiogenesis interventions have received little attention. Objective: Assess the effectiveness of anti-angiogenic Sn2 lipase-labile prodrugs delivered via α_vβ₃-micellar nanotherapy to suppress microvascular expansion, bronchial remodeling, and airway hyper-responsiveness in Brown Norway rats exposed to serial house dust mite (HDM) inhalation challenges. Results: Anti-neovascular effectiveness of α_vβ₃-mixed micelles incorporating docetaxel-prodrug (Dxtl-PD) or fumagillin-prodrug (Fum-PD) were shown to robustly suppress neovascular expansion (p<0.01) in the upper airways/bronchi of HDM rats using simultaneous ¹⁹F/¹H MR neovascular imaging, which was corroborated by adjunctive fluorescent microscopy. Micelles without a drug payload (α_vβ₃-No-Drug) served as a carrier-only control. Morphometric measurements of HDM rat airway size (perimeter) and vessel number at 21d revealed classic vascular expansion in control rats but less vascularity (p<0.001) after the anti-angiogenic nanotherapies. CD31 RNA expression independently corroborated the decrease in airway microvasculature. Methacholine (MCh) induced respiratory system resistance (Rrs) was high in the HDM rats receiving α_vβ₃-No-Drug micelles while α_vβ₃-Dxtl-PD or α_vβ₃-Fum-PD micelles markedly and equivalently attenuated airway hyper-responsiveness and improved airway compliance. Total inflammatory BAL cells among HDM challenged rats did not differ with treatment, but α_vβ₃⁺ macrophages/monocytes were significantly reduced by both nanotherapies (p<0.001), most notably by the α_vβ₃-Dxtl-PD micelles. Additionally, α_vβ₃-Dxtl-PD decreased BAL eosinophil and α_vβ₃⁺ CD45⁺ leukocytes relative to α_vβ₃-No-Drug micelles, whereas α_vβ₃-Fum-PD micelles did not. Conclusion: These results demonstrate the potential of targeted anti-angiogenesis nanotherapy to ameliorate the inflammatory hallmarks of asthma in a clinically relevant rodent model.

Ursolic acid enhances macrophage autophagy and attenuates atherogenesis

Macrophage autophagy has been shown to be protective against atherosclerosis. We previously discovered that ursolic acid (UA) promoted cancer cell autophagy. In the present study, we aimed to examine whether UA enhances macrophage autophagy in the context of atherogenesis. Cell culture study showed that UA enhanced autophagy of macrophages by increasing the expression of Atg5 and Atg16l1, which led to altered macrophage function. UA reduced pro-interleukin (IL)-1β protein levels and mature IL-1β secretion in macrophages in response to lipopolysaccharide (LPS), without reducing IL-1β mRNA expression. Confocal microscopy showed that in LPS-treated macrophages, UA increased LC3 protein levels and LC3 appeared to colocalize with IL-1β. In cholesterol-loaded macrophages, UA increased cholesterol efflux to apoAI, although it did not alter mRNA or protein levels of ABCA1 and ABCG1. Electron microscopy showed that UA induced lipophagy in acetylated LDL-loaded macrophages, which may result in increased cholesterol ester hydrolysis in autophagolysosomes and presentation of free cholesterol to the cell membrane. In LDLR(-/-) mice fed a Western diet to induce atherogenesis, UA treatment significantly reduced atherosclerotic lesion size, accompanied by increased macrophage autophagy. In conclusion, the data suggest that UA promotes macrophage autophagy and, thereby, suppresses IL-1β secretion, promotes cholesterol efflux, and attenuates atherosclerosis in mice.

Abstract 608: Induction of Lysosomal Biogenesis in Adipose Tissue Macrophages Attenuates Inflammation and Cardiometabolic Disease

Adipose tissue inflammation is a fundamental feature of obesity and is strongly implicated in progression to overt cardiometabolic disease. In situ, this response is defined by a notable increase in number of adipose tissue macrophages (ATM’s) surrounding dying adipocytes and a phenotypic switch towards pro-inflammatory “M1” polarization. However, recent data suggest an equally notable upregulation of lysosomal catabolic pathways in these cells, and that inflammatory phenotypes may regulated by these processes. We and others speculate ATM lysosomal biogenesis is an adaptive response that can be harnessed for therapeutic benefit in treating cardiometabolic disease. To this end, we examined the effects of macrophage-specific overexpression of TFEB (mϕ-TFEB), a transcription factor master regulator of lysosomal biogenesis, in a murine model of diet induced obesity. We show broad physiological metabolic benefits including attenuated weight gain, improved body composition, and increased metabolic rate. Further, mϕ-TFEB mice demonstrate increased insulin sensitivity and glucose tolerance (Insulin and Glucose Tolerance Tests). These phenotypes are linked to profound changes in cellular, tissue, and systemic inflammation. mϕ-TFEB overexpression greatly reduces circulating interleukin 1β, and polarizes adipose tissue macrophages isolated from epididymal fat to an anti-inflammatory “M2” state in a manner that is dependent on lysosomal lipolysis. In cultured primary macrophages, TFEB overexpression attenuates LPS-induced proinflammatory M1 activation, and predisposes to IL-4 induced M2 polarization. Thus, harnessing the lysosomal biogenesis response in macrophages abrogates diet induced metabolic pathophysiology, potentially through regulation of inflammatory phenotypes. We postulate that ATM lysosomal biogenesis is a crucial, adaptive cellular response to obesity that can be preemptively induced to slow progression of cardiometabolic disease.

Abstract 468: Induction of Autophagy with Beclin-1 is not Sufficient to Reduce Atherosclerosis

Autophagy is a highly conserved cellular process designed to degrade long-lived and dysfunctional protein aggregates and organelles. Recent work has established an important role for macrophage autophagy in the control of inflammatory signaling, maintenance of efficient cholesterol homeostasis, and degradation of deleterious protein aggregates. The absence of macrophage autophagy renders many of these processes dysfunctional and leads to significant increases in atherosclerotic plaque formation. Thus, there has been intense interest in the cardiovascular community to devise strategies to induce autophagy as an atheroprotective measure. We approached this by pharmacologically manipulating a critical upstream activator of autophagy, Beclin-1, a protein involved in the formation of the autophagy initiation complex and biogenesis of the autophagosome. A cell-permeable Beclin-1 peptide (known to induce autophagy both in vitro and in vivo) was administered to pro-atherogenic ApoE-null mice fed a Western diet for a period of 8 weeks. Despite its autophagy-inducing properties in vivo, Beclin-1 peptide failed to reduce the development of atherosclerosis. Moreover, the peptide’s autophagy-inducing properties in vitro were also insufficient to blunt atherogenic lipid-induced hyperactivation of the inflammasome/IL-1β or stimulate the clearance of cytotoxic protein aggregates. These data lead us to conclude that the induction of autophagy is not sufficient to curb atherosclerotic progression. It is increasingly appreciated that macrophages of the atherosclerotic plaque also progressively develop a profound lysosomal dysfunction which likely precedes and exacerbates autophagy dysfunction. This raises that notion that combined stimulation of both autophagy and lysosomes would be necessary to ameliorate atherosclerosis and that efforts to stimulate autophagy in isolation are unlikely to be successful.

Inclusion bodies enriched for p62 and polyubiquitinated proteins in macrophages protect against atherosclerosis

Autophagy is a catabolic cellular mechanism that degrades dysfunctional proteins and organelles. Atherosclerotic plaque formation is enhanced in mice with macrophages deficient for the critical autophagy protein ATG5. We showed that exposure of macrophages to lipids that promote atherosclerosis increased the abundance of the autophagy chaperone p62 and that p62 colocalized with polyubiquitinated proteins in cytoplasmic inclusions, which are characterized by insoluble protein aggregates. ATG5-null macrophages developed further p62 accumulation at the sites of large cytoplasmic ubiquitin-positive inclusion bodies. Aortas from atherosclerotic mice and plaques from human endarterectomy samples showed increased abundance of p62 and polyubiquitinated proteins that colocalized with plaque macrophages, suggesting that p62-enriched protein aggregates were characteristic of atherosclerosis. The formation of the cytoplasmic inclusions depended on p62 because lipid-loaded p62-null macrophages accumulated polyubiquitinated proteins in a diffuse cytoplasmic pattern. Lipid-loaded p62-null macrophages also exhibited increased secretion of interleukin-1β (IL-1β) and had an increased tendency to undergo apoptosis, which depended on the p62 ubiquitin-binding domain and at least partly involved p62-mediated clearance of NLRP3 inflammasomes. Consistent with our in vitro observations, p62-deficient mice formed greater numbers of more complex atherosclerotic plaques, and p62 deficiency further increased atherosclerotic plaque burden in mice with a macrophage-specific ablation of ATG5. Together, these data suggested that sequestration of cytotoxic ubiquitinated proteins by p62 protects against atherogenesis, a condition in which the clearance of protein aggregates is disrupted.

Degradation and beyond: the macrophage lysosome as a nexus for nutrient sensing and processing in atherosclerosis

PURPOSE OF REVIEW

The ability of macrophage lysosomes to degrade both exogenous and internally derived cargo is paramount to handling the overabundance of lipid and cytotoxic material present in the atherosclerotic plaque. We will discuss recent insights in both classical and novel functions of the lysosomal apparatus, as it pertains to the pathophysiology of atherosclerosis.

RECENT FINDINGS

Lipid-mediated dysfunction in macrophage lysosomes appears to be a critical event in plaque progression. Consequences include enhanced inflammatory signalling [particularly the inflammasome/interleukin-1β axis] and an inability to interface with autophagy leading to a proatherogenic accumulation of dysfunctional organelles and protein aggregates. Aside from degradation, several novel functions have recently been ascribed to lysosomes, including involvement in macrophage polarization, generation of lipid signalling intermediates and serving as a nutrient depot for mechanistic target of rapamycin activation, each of which can have profound implications in atherosclerosis. Finally, the discovery of the transcription factor transcription factor EB as a mechanism of inducing lysosomal biogenesis can have therapeutic value by reversing lysosomal dysfunction in macrophages.

SUMMARY

Lysosomes are a central organelle in the processing of exogenous and intracellular biomolecules. Together with recent data that implicate the degradation products of lysosomes in modulation of signalling pathways, these organelles truly do lay at a nexus in nutrient sensing and processing. Dissecting the full repertoire of lysosome function and ensuing dysfunction in plaque macrophages is pivotal to our understanding of atherogenesis.

Abstract 163: Induction of Lysosomal Biogenesis in Macrophages Reduces Atherosclerosis in an Autophagy-dependent Manner

Recent reports of the proatherogenic phenotype of mice with a macrophage-specific autophagy deficiency have renewed interest in the role of the autophagy-lysosomal system in atherosclerosis. Lysosomes have the unique role of processing both exogenous material such as excess atherogenic lipids and endogenous cargo that includes dysfunctional proteins and organelles via autophagy. Previously we demonstrated that oxidized LDL and cholesterol crystals, two of the commonly encountered lipid species in the atherosclerotic plaque, create a profound lysosomal and autophagy dysfunction in cultured macrophages. Overexpression of TFEB, a transcription factor that is the only known master regulator of lysosomal and autophagy biogenesis, in macrophages initiates a robust prodegradative response including induction of lysosomal and autophagy genes. This in turn ameliorates several deleterious effects of the lipid-mediated dysfunction, namely the blunting of inflammasome activation, enhancing cholesterol efflux, and accelerating the degradation of protein aggregates. Our in vitro data suggest that the induction of a lysosomal biogenesis program in macrophages can have atheroprotective effects. Indeed, myeloid-specific TFEB overexpression in mice significantly reduces atherosclerotic plaque burden as well as plaque complexity as gauged by reduced necrotic core and markers of apoptosis. Interestingly, this protection is autophagy-dependent since these TFEB-overexpressing mice on a background of myeloid-specific autophagy (ATG5)-deficiency no longer demonstrate plaque reduction. Mechanistically, this indicates that suppression of the inflammasome and enhancement of cholesterol efflux and protein aggregate removal is dependent on the TFEB-autophagy axis. Taken together, our data support the notion that harnessing the prodegradative response in macrophages via TFEB can be atheroprotective and provides the impetus to evaluate mechanisms by which macrophage lysosomal and autophagy biogenesis can be modulated therapeutically.

Abstract 4: Macrophage p62/SQSTM1 Ameliorates Atherosclerosis by Sequestering Inclusion Bodies and Mediating Mitophagy

Protein and organelle turnover is critical for cellular homeostasis and is prominently mediated by autophagy. Disruptions in autophagy lead to accumulation of protein aggregates and dysfunctional organelles such as mitochondria. Recent evidence suggests that the chaperone protein p62 is a critical link for targeting polyubiquitinated protein aggregates/damaged mitochondria to autophagosomes for degradation. Herein we describe a p62-centric mechanism of handling protein aggregates and dysfunctional mitochondria in atherosclerosis. Macrophages deficient in autophagy (ATG5-/-) or rendered deficient by incubation with atherogenic lipids have significantly increased levels of p62. This coincides with 1) the accumulation of polyubiquitinated proteins co-localizing with p62 and present as cytoplasmic inclusion bodies, and 2) p62 co-localization with mitochondrial markers. Aortas from atherosclerotic (ApoE-/-) mice also have progressive and marked elevations in p62, polyubiquitinated proteins, and mitochondrial reactive oxygen species that predominantly co-localize with plaque macrophages, a process further exacerbated in the autophagy-deficient setting. The formation of cytoplasmic inclusions and maintenance of adequate mitochondrial function appears to be dependent on p62. Lipid-loaded p62-null macrophages show polyubiquitinated protein accumulation present in a diffuse/disrupted cytoplasmic pattern. These macrophages also develop larger dysmorphic mitochondria with increased polarization and decreased oxidative phosphorylation capacity. As a result, p62-null macrophages display apoptotic susceptibility to atherogenic lipids and increased IL-1β secretion likely through mitochondrial-dependent inflammasome activation. Consistent with our in vitro observations, mice with either whole-body p62-deficiency or transplanted with p62-deficient bone marrow show significantly increased atherosclerotic plaque burden and lesion complexity with increased apoptosis and necrotic cores. Taken together, these data demonstrate a previously unrecognized atheroprotective role for macrophage p62 by facilitating the formation of inclusion bodies and maintaining healthy mitochondria.

Location-dependent signaling of the group 1 metabotropic glutamate receptor mGlu5

Although G protein-coupled receptors are primarily known for converting extracellular signals into intracellular responses, some receptors, such as the group 1 metabotropic glutamate receptor, mGlu5, are also localized on intracellular membranes where they can mediate both overlapping and unique signaling effects. Thus, besides "ligand bias," whereby a receptor's signaling modality can shift from G protein dependence to independence, canonical mGlu5 receptor signaling can also be influenced by "location bias" (i.e., the particular membrane and/or cell type from which it signals). Because mGlu5 receptors play important roles in both normal development and in disorders such as Fragile X syndrome, autism, epilepsy, addiction, anxiety, schizophrenia, pain, dyskinesias, and melanoma, a large number of drugs are being developed to allosterically target this receptor. Therefore, it is critical to understand how such drugs might be affecting mGlu5 receptor function on different membranes and in different brain regions. Further elucidation of the site(s) of action of these drugs may determine which signal pathways mediate therapeutic efficacy.

Induction of lysosomal biogenesis in atherosclerotic macrophages can rescue lipid-induced lysosomal dysfunction and downstream sequelae

OBJECTIVE

Recent reports of a proatherogenic phenotype in mice with macrophage-specific autophagy deficiency have renewed interest in the role of the autophagy-lysosomal system in atherosclerosis. Lysosomes have the unique ability to process both exogenous material, including lipids and autophagy-derived cargo such as dysfunctional proteins/organelles. We aimed to understand the effects of an atherogenic lipid environment on macrophage lysosomes and to evaluate novel ways to modulate this system.

APPROACH AND RESULTS

Using a variety of complementary techniques, we show that oxidized low-density lipoproteins and cholesterol crystals, commonly encountered lipid species in atherosclerosis, lead to profound lysosomal dysfunction in cultured macrophages. Disruptions in lysosomal pH, proteolytic capacity, membrane integrity, and morphology are readily seen. Using flow cytometry, we find that macrophages isolated from atherosclerotic plaques also display features of lysosome dysfunction. We then investigated whether enhancing lysosomal function can be beneficial. Transcription factor EB (TFEB) is the only known transcription factor that is a master regulator of lysosomal biogenesis although its role in macrophages has not been studied. Lysosomal stress induced by chloroquine or atherogenic lipids leads to TFEB nuclear translocation and activation of lysosomal and autophagy genes. TFEB overexpression in macrophages further augments this prodegradative response and rescues several deleterious effects seen with atherogenic lipid loading as evidenced by blunted lysosomal dysfunction, reduced secretion of the proinflammatory cytokine interleukin-1β, enhanced cholesterol efflux, and decreased polyubiquitinated protein aggregation.

CONCLUSIONS

Taken together, these data demonstrate that lysosomal function is markedly impaired in atherosclerosis and suggest that induction of a lysosomal biogenesis program in macrophages has antiatherogenic effects.

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.

Abstract 28: p62-Enriched Inclusion Bodies in Macrophages Play a Protective Role in Atherosclerosis

Polyubiquitinated protein aggregates have been classically associated with human neurodegenerative, liver, and muscle disorders, but not atherosclerosis. Since autophagy-deficient atherosclerotic macrophages have markedly increased levels of p62, a chaperone protein critical for the autophagic turnover of polyubiquitinated proteins, we aimed to evaluate the role of p62 and protein aggregation in atherosclerosis. We first tested the effects of atherogenic lipid loading in peritoneal macrophages. Incubation with either cholesterol crystals or oxidized LDL leads to dramatically increased p62 protein that completely co-localizes with polyubiquitinated proteins as cytoplasmic inclusions or as aggregates partially sequestered by lysosomes. Such accumulation was intensified in autophagy-null (ATG5-/-) macrophages, where massive cytoplasmic ubiquitin-positive p62 aggregates form. Our in vitro observations are recapitulated in vivo. Aortas from atherosclerotic (ApoE-/-) mice have marked elevations in p62 and polyubiquitinated proteins that predominantly co-localize with plaque macrophages, a process that is further exacerbated in the autophagy-deficient setting. Remarkably, we find nearly identical results in human carotid endarterectomy samples, suggesting that p62-enriched protein aggregates are a characteristic feature of atherosclerosis. The homeostasis of cytoplasmic inclusions is dependent on the presence of p62 since lipid-loaded p62-null macrophages accumulate polyubiquitinated proteins in a diffuse and disrupted cytoplasmic pattern. The disruption of these aggregates has functional consequences manifested as increased secretion of IL-1β and enhanced macrophage apoptosis. Consistent with our in vitro observations, p62-deficient mice have increased atherosclerotic plaque formation. Furthermore, p62-deficiency increases plaque burden even in the highly atherosclerosis-prone macrophage-specific ATG5-null mice. Taken together, these data suggest that by sequestering cytotoxic ubiquitinated proteins, p62-enriched inclusion body formation is a protective response during atherogenesis, a setting where the clearance of protein aggregates is disrupted.

Embryonic and adult-derived resident cardiac macrophages are maintained through distinct mechanisms at steady state and during inflammation

Cardiac macrophages are crucial for tissue repair after cardiac injury but are not well characterized. Here we identify four populations of cardiac macrophages. At steady state, resident macrophages were primarily maintained through local proliferation. However, after macrophage depletion or during cardiac inflammation, Ly6c(hi) monocytes contributed to all four macrophage populations, whereas resident macrophages also expanded numerically through proliferation. Genetic fate mapping revealed that yolk-sac and fetal monocyte progenitors gave rise to the majority of cardiac macrophages, and the heart was among a minority of organs in which substantial numbers of yolk-sac macrophages persisted in adulthood. CCR2 expression and dependence distinguished cardiac macrophages of adult monocyte versus embryonic origin. Transcriptional and functional data revealed that monocyte-derived macrophages coordinate cardiac inflammation, while playing redundant but lesser roles in antigen sampling and efferocytosis. These data highlight the presence of multiple cardiac macrophage subsets, with different functions, origins, and strategies to regulate compartment size.

Abstract 371: An Intact Autophagy-lysosomal System is Required for the Clearance of p62-enriched Inclusion Bodies in Macrophages and has Implications for Atherosclerosis

Autophagy is an essential catabolic cellular mechanism that involves degradation of dysfunctional proteins and organelles through the lysosomal machinery. Recent studies suggest that autophagy- deficiency induces atherosclerotic plaque formation in mouse models. Although the exact mechanisms are unknown, hyperactivation of the inflammasome, enhanced apoptosis/oxidative stress, and disrupted lipid efflux have been proposed. Herein, we describe an alternative mechanism involving the clearance of protein aggregates. It is known that autophagy-deficient macrophages have significantly increased levels of the chaperone protein p62. Since p62 binds polyubiquitinated proteins and facilitates their turnover via autophagy, disruptions in this process might be pathogenic in atherosclerosis. We first tested the effects of atherogenic lipid loading in peritoneal macrophages. Incubation with either cholesterol crystals or oxidized-LDL dramatically increased p62 protein levels with little effect on transcription, suggesting defects in the clearance of p62 protein. This coincided with the accumulation of polyubiquitinated proteins colocalizing with p62 present either as cytoplasmic inclusions or partially sequestered by lysosomes. Such accumulation was intensified in autophagy-null (ATG5-/-) macrophages, where massive cytoplasmic ubiquitin-positive p62 aggregates form. Our in vitro observations are recapitulated in vivo ; aortas from atherosclerotic (ApoE-/-) mice have progressive and marked elevations in p62 and polyubiquitinated protein levels that predominantly colocalize with plaque macrophages, a process that is further exacerbated in the autophagy-deficient setting. The homeostasis of cytoplasmic inclusions is dependent on the presence of p62 as lipid loaded p62-null macrophages showed polyubiquitinated protein accumulation present in a diffuse and disrupted cytoplasmic pattern. This appears to have functional consequences with p62-null macrophages displaying increased secretion of IL-1β likely through inflammasome activation. Taken together these data suggest that p62-positive inclusion body formation is a marker of atherosclerotic progression with potential pathophysiological consequences.

Abstract 474: Atherosclerotic Macrophages Develop a Progressive Lysosomal Dysfunction that can be Rescued by Induction of a Lysosomal Biogenesis Program

Recent reports of the proatherogenic phenotype of mice with a macrophage-specific autophagy deficiency has renewed interest in the role of the autophagy-lysosomal system in atherosclerosis. Lysosomes have the unique role of serving to process both exogenous material including the excess of atherogenic lipids and endogenous cargo including dysfunctional proteins and organelles. Surprisingly, little is known about the effect of an atherogenic environment on macrophage lysosomes. To address this, we utilize a variety of complementary techniques to show that oxidized LDL and cholesterol crystals, two of the commonly encountered lipid species in the atherosclerotic plaque, create a profound lysosomal dysfunction in cultured peritoneal macrophages. Disruptions in lysosomal pH, enzyme activity, proteolytic capacity, membrane integrity, and morphology are readily seen when cells are incubated with such lipids. Using flow cytometry to isolate resident tissue macrophages, we show that atherosclerotic plaque macrophages show features of dysfunctional lysosomes, a process that appears to be progressive with advanced plaque formation. These observations lead us to investigate whether stimulation of lysosomal function can ameliorate some of these effects. TFEB is the only known transcription factor that acts as a master regulator of lysosomal biogenesis, although its role in macrophages has not been studied. We show that overexpression of TFEB in cultured macrophages initiates a robust prodegradative response including induction of lysosomal genes and the generation of nascent lysosomes. Interestingly, this response can rescue several deleterious effects seen with atherogenic lipid loading including reductions in the secretion of the proinflammatory cytokine IL-1β and reductions in foam cell formation. Taken together, these data demonstrate that lysosomal function is markedly impaired in atherosclerosis and suggest that induction of a lysosomal biogenesis program can have anti-atherogenic effects.