Autophagy Is Dispensable for Macrophage-Mediated Lipid Homeostasis in Adipose Tissue

Interactions of Autophagy and the Immune System in Health and Diseases

Autophagy is a highly conserved process that utilizes lysosomes to selectively degrade a variety of intracellular cargo, thus providing quality control over cellular components and maintaining cellular regulatory functions. Autophagy is triggered by multiple stimuli ranging from nutrient starvation to microbial infection. Autophagy extensively shapes and modulates the inflammatory response, the concerted action of immune cells, and secreted mediators aimed to eradicate a microbial infection or to heal sterile tissue damage. Here, we first review how autophagy affects innate immune signaling, cell-autonomous immune defense, and adaptive immunity. Then, we discuss the role of non-canonical autophagy in microbial infections and inflammation. Finally, we review how crosstalk between autophagy and inflammation influences infectious, metabolic, and autoimmune disorders.

Lactate Is a Key Mediator That Links Obesity to Insulin Resistance via Modulating Cytokine Production From Adipose Tissue

Numerous evidence indicates that inflammation in adipose tissue is the primary cause of systemic insulin resistance induced by obesity. Obesity-associated changes in circulating LPS level and hypoxia/HIF-1α activation have been proposed to be involved in boosting obesity-induced inflammation. However, there is poor understanding of what triggers obesity-induced inflammation. In this study, we pinpoint lactate as a key trigger to mediate obesity-induced inflammation and systemic insulin resistance. Specific deletion of Slc16a1 that encodes MCT1, the primary lactate transporter in adipose tissues, robustly elevates blood levels of proinflammatory cytokines and aggravates systemic insulin resistance without alteration of adiposity in mice fed high-fat diet. Slc16a1 deletion in adipocytes elevates intracellular lactate level while reducing circulating lactate concentration. Mechanistically, lactate retention due to Slc16a1 deletion initiates adipocyte apoptosis and cytokine release. The locally recruited macrophages amplify the inflammation by release of proinflammatory cytokines to the circulation, leading to insulin resistance in peripheral tissues. This study, therefore, indicates that lactate within adipocytes has a key biological function linking obesity to insulin resistance, and harnessing lactate in adipocytes can be a promising strategy to break this link.

Phenotypical Conversions of Dermal Adipocytes as Pathophysiological Steps in Inflammatory Cutaneous Disorders

Adipocytes from the superficial layer of subcutaneous adipose tissue undergo cyclic de- and re-differentiation, which can significantly influence the development of skin inflammation under different cutaneous conditions. This inflammation can be connected with local loading of the reticular dermis with lipids released due to de-differentiation of adipocytes during the catagen phase of the hair follicle cycle. Alternatively, the inflammation parallels a widespread release of cathelicidin, which typically takes place in the anagen phase (especially in the presence of pathogens). Additionally, trans-differentiation of dermal adipocytes into myofibroblasts, which can occur under some pathological conditions, can be responsible for the development of collateral scarring in acne. Here, we provide an overview of such cellular conversions in the skin and discuss their possible involvement in the pathophysiology of inflammatory skin conditions, such as acne and psoriasis.

New Insights Into the Interplay Among Autophagy, the NLRP3 Inflammasome and Inflammation in Adipose Tissue

Obesity is a feature of metabolic syndrome with chronic inflammation in obese subjects, characterized by adipose tissue (AT) expansion, proinflammatory factor overexpression, and macrophage infiltration. Autophagy modulates inflammation in the enlargement of AT as an essential step for maintaining the balance in energy metabolism and waste elimination. Signaling originating from dysfunctional AT, such as AT containing hypertrophic adipocytes and surrounding macrophages, activates NOD-like receptor family 3 (NLRP3) inflammasome. There are interactions about altered autophagy and NLRP3 inflammasome activation during the progress in obesity. We summarize the current studies and potential mechanisms associated with autophagy and NLRP3 inflammasome in AT inflammation and aim to provide further evidence for research on obesity and obesity-related complications.

ANT2 drives proinflammatory macrophage activation in obesity

Macrophage proinflammatory activation is an important etiologic component of the development of insulin resistance and metabolic dysfunction in obesity. However, the underlying mechanisms are not clearly understood. Here, we demonstrate that a mitochondrial inner membrane protein, adenine nucleotide translocase 2 (ANT2), mediates proinflammatory activation of adipose tissue macrophages (ATMs) in obesity. Ant2 expression was increased in ATMs of obese mice compared with lean mice. Myeloid-specific ANT2-knockout (ANT2-MKO) mice showed decreased adipose tissue inflammation and improved insulin sensitivity and glucose tolerance in HFD/obesity. At the molecular level, we found that ANT2 mediates free fatty acid–induced mitochondrial permeability transition, leading to increased mitochondrial reactive oxygen species production and damage. In turn, this increased HIF-1α expression and NF-κB activation, leading to proinflammatory macrophage activation. Our results provide a previously unknown mechanism for how obesity induces proinflammatory activation of macrophages with propagation of low-grade chronic inflammation (metaflammation).

Understanding the heterogeneity and functions of metabolic tissue macrophages

Growing evidence places tissue-resident macrophages as essential gatekeepers of metabolic organ homeostasis, including the adipose tissue and the pancreatic islets. Therein, macrophages may adopt specific phenotypes and ensure local functions. Recent advances in single cell genomic analyses provide a comprehensive map of adipose tissue macrophage subsets and their potential roles are now better apprehended. Whether they are beneficial or detrimental, macrophages overall contribute to the proper adipose tissue expansion under steady state and during obesity. By contrast, macrophages residing inside pancreatic islets, which may exert fundamental functions to fine tune insulin secretion, have only started to attract attention and their cellular heterogeneity remains to be established. The present review will focus on the latest findings exploring the phenotype and the properties of macrophages in adipose tissue and pancreatic islets, questioning early beliefs and future perspectives in the field of immunometabolism.

Remodeling of Macrophages in White Adipose Tissue under the Conditions of Obesity as well as Lipolysis

Adipose tissue macrophages (ATM) are a major source of low-grade inflammation in obesity, and yet reasons driving ATM accumulation in white adipose tissue (WAT) are not fully understood. Emerging evidence suggested that ATM underwent extensive remodeling in obesity. In addition to abundance, ATM in obesity were lipid-laden and metabolically reprogrammed, which in turn was tightly related to their functional alterations and persistence in obesity. Herein, we aimed to discuss that activation of lipid sensing signaling associated with metabolic reprogramming in ATM was indispensible for their migration, retention, or proliferation in obesity. Likewise, lipolysis also induced similar but transient ATM remodeling. Therefore, we assumed that obesity might share overlapping mechanisms with lipolysis in remodeling ATM. Formation of crown-like structures (CLS) in WAT was presumably a common event initiating ATM remodeling, with a spectrum of lipid metabolites released from adipocytes being potential signaling molecules. Moreover, adipose interlerkin-6 (IL-6) exhibited homologous alterations by obesity and lipolysis. Thus, we postulated a positive feedback loop between ATM and adipocytes via IL-6 signaling backing ATM persistence by comparison of ATM remodeling under obesity and lipolysis. An elucidation of ATM persistence could help to provide novel therapeutic targets for obesity-associated inflammation.

TFEB-GDF15 axis protects against obesity and insulin resistance as a lysosomal stress response

TFEB, a key regulator of lysosomal biogenesis and autophagy, is induced not only by nutritional deficiency but also by organelle stress. Here, we find that Tfeb and its downstream genes are upregulated together with lipofuscin accumulation in adipose tissue macrophages (ATMs) of obese mice or humans, suggestive of obesity-associated lysosomal dysfunction/stress in ATMs. Macrophage-specific TFEB-overexpressing mice display complete abrogation of diet-induced obesity, adipose tissue inflammation and insulin resistance, which is independent of autophagy, but dependent on TFEB-induced GDF15 expression. Palmitic acid induces Gdf15 expression through lysosomal Ca²⁺-mediated TFEB nuclear translocation in response to lysosomal stress. In contrast, mice fed a high-fat diet with macrophage-specific Tfeb deletion show aggravated adipose tissue inflammation and insulin resistance, accompanied by reduced GDF15 level. Finally, we observe activation of TFEB-GDF15 in ATMs of obese humans as a consequence of lysosomal stress. These findings highlight the importance of the TFEB-GDF15 axis as a lysosomal stress response in obesity or metabolic syndrome and as a promising therapeutic target for treatment of these conditions.

Autophagy in liver diseases

Autophagy is the liver cell energy recycling system regulating a variety of homeostatic mechanisms. Damaged organelles, lipids and proteins are degraded in the lysosomes and their elements are re-used by the cell. Investigations on autophagy have led to the award of two Nobel Prizes and a health of important reports. In this review we describe the fundamental functions of autophagy in the liver including new data on the regulation of autophagy. Moreover we emphasize the fact that autophagy acts like a two edge sword in many occasions with the most prominent paradigm being its involvement in the initiation and progress of hepatocellular carcinoma. We also focused to the implication of autophagy and its specialized forms of lipophagy and mitophagy in the pathogenesis of various liver diseases. We analyzed autophagy not only in well studied diseases, like alcoholic and nonalcoholic fatty liver and liver fibrosis but also in viral hepatitis, biliary diseases, autoimmune hepatitis and rare diseases including inherited metabolic diseases and also acetaminophene hepatotoxicity. We also stressed the different consequences that activation or impairment of autophagy may have in hepatocytes as opposed to Kupffer cells, sinusoidal endothelial cells or hepatic stellate cells. Finally, we analyzed the limited clinical data compared to the extensive experimental evidence and the possible future therapeutic interventions based on autophagy manipulation.

Hypoglycemic effects of polysaccharides from Gomphidiaceae rutilus fruiting bodies and their mechanisms

Insulin resistance is the main cause of type 2 diabetes, fatty liver and obesity. Our previous study found that mushroom polysaccharides improved insulin resistance in vitro, but the underlying mechanisms were still unknown. Thus, we investigate the hypoglycemic effects of polysaccharides from Gomphidiaceae rutilus fruiting bodies and their mechanisms. The total polysaccharides (AGRP) from Gomphidiaceae rutilus fruiting bodies and the neutral polysaccharide (AGRP-N) fraction both enhance insulin-stimulated glucose uptake in an autophagy-dependent manner in high glucose and fatty acid-treated hepatic cells, but not the acidic polysaccharide (AGRP-A) fraction. Further, we elucidate the oral hypoglycemic effects of polysaccharides on ob/ob mice. AGRP and AGRP-N lower blood glucose and improve insulin sensitivity. They inhibit liver lipid deposition, not only by activating AMPK to increase autophagy but also by increasing the expressions of PPARα and CPT-1a to enhance lipolysis. Our results provide a basis for the development of polysaccharides from Gomphidiaceae rutilus as a hypoglycemic healthy food.

Rewiring of Lipid Metabolism in Adipose Tissue Macrophages in Obesity: Impact on Insulin Resistance and Type 2 Diabetes

Obesity and its two major comorbidities, insulin resistance and type 2 diabetes, represent worldwide health issues whose incidence is predicted to steadily rise in the coming years. Obesity is characterized by an accumulation of fat in metabolic tissues resulting in chronic inflammation. It is now largely accepted that adipose tissue inflammation underlies the etiology of these disorders. Adipose tissue macrophages (ATMs) represent the most enriched immune fraction in hypertrophic, chronically inflamed adipose tissue, and these cells play a key role in diet-induced type 2 diabetes and insulin resistance. ATMs are triggered by the continuous influx of dietary lipids, among other stimuli; however, how these lipids metabolically activate ATM depends on their nature, composition and localization. This review will discuss the fate and molecular programs elicited within obese ATMs by both exogenous and endogenous lipids, as they mediate the inflammatory response and promote or hamper the development of obesity-associated insulin resistance and type 2 diabetes.

Interorgan communication by exosomes, adipose tissue, and adiponectin in metabolic syndrome

Adipose tissue plays important roles in regulating whole-body energy metabolism through its storage function in white adipocytes and its dissipating function in brown and beige adipocytes. Adipose tissue also produces a variety of secreted factors called adipocytokines, including leptin and adiponectin. Furthermore, recent studies have suggested the important roles of extracellular vesicles of endosomal origin termed exosomes, which are secreted from adipocytes and other cells in adipose tissue and influence whole-body glucose and lipid metabolism. Adiponectin is known to be a pleiotropic organ-protective protein that is exclusively produced by adipocytes and decreased in obesity. Adiponectin accumulates in tissues such as heart, muscle, and vascular endothelium through binding with T-cadherin, a glycosylphosphatidylinositol-anchored (GPI-anchored) cadherin. Recently, adiponectin was found to enhance exosome biogenesis and secretion, leading to a decrease in cellular ceramides, excess of which is known to cause insulin resistance and cardiovascular disease phenotypes. These findings support the hypothesis that adipose tissue metabolism systemically regulates exosome production and whole-body metabolism through exosomes. This review focuses on intra-adipose and interorgan communication by exosomes, adiponectin-stimulated exosome production, and their dysregulation in metabolic diseases.

The Role of Autophagy in White Adipose Tissue Function: Implications for Metabolic Health

White adipose tissue (WAT) is a highly adaptive endocrine organ that continuously remodels in response to nutritional cues. WAT expands to store excess energy by increasing adipocyte number and/or size. Failure in WAT expansion has serious consequences on metabolic health resulting in altered lipid, glucose, and inflammatory profiles. Besides an impaired adipogenesis, fibrosis and low-grade inflammation also characterize dysfunctional WAT. Nevertheless, the precise mechanisms leading to impaired WAT expansibility are yet unresolved. Autophagy is a conserved and essential process for cellular homeostasis, which constitutively allows the recycling of damaged or long-lived proteins and organelles, but is also highly induced under stress conditions to provide nutrients and remove pathogens. By modulating protein and organelle content, autophagy is also essential for cell remodeling, maintenance, and survival. In this line, autophagy has been involved in many processes affected during WAT maladaptation, including adipogenesis, adipocyte, and macrophage function, inflammatory response, and fibrosis. WAT autophagy dysregulation is related to obesity and diabetes. However, it remains unclear whether WAT autophagy alteration in obese and diabetic patients are the cause or the consequence of WAT malfunction. In this review, current data regarding these issues are discussed, focusing on evidence from human studies.

Adipose tissue macrophages: Unique polarization and bioenergetics in obesity

Macrophages comprise a majority of the resident immune cells in adipose tissue (AT) and regulate both tissue homeostasis in the lean state and metabolic dysregulation in obesity. Since the AT environment rapidly changes based upon systemic energy status, AT macrophages (ATMs) must adapt phenotypically and metabolically. There is a distinct dichotomy in the polarization and bioenergetics of in vitro models, with M2 macrophages utilizing oxidative phosphorylation (OX PHOS) and M1 macrophages utilizing glycolysis. Early studies suggested differential polarization of ATMs, with M2-like macrophages predominant in lean AT and M1-like macrophages in obese AT. However, recent studies show that the phenotypic plasticity of ATMs is far more complicated, which is also reflected in their bioenergetics. Multiple ATM populations exist along the M2 to M1 continuum and appear to utilize both glycolysis and OX PHOS in obesity. The significance of the dual fuel bioenergetics is unclear and may be related to an intermediate polarization, their buffering capacity, or the result of a mixed population of distinct polarized ATMs. Recent evidence also suggests that ATMs of lean mice serve as a substrate buffer or reservoir to modulate lipid, catecholamine, and iron availability. Furthermore, recent models of weight loss and weight cycling reveal additional roles for ATMs in systemic metabolism. Evaluating ATM phenotype and intracellular metabolism together may more accurately illuminate the consequences of ATM accumulation in obese AT, lending further insight into obesity-related comorbidities in humans.

The regulation of TFEB in lipid homeostasis of non-alcoholic fatty liver disease: Molecular mechanism and promising therapeutic targets

Nonalcoholic fatty liver disease (NAFLD), which is characterized by disruption of lipid homeostasis, has been the leading cause of chronic liver disease worldwide. However, currently there is no effective therapy for NAFLD. Consequently, it is extremely urgent to explore the specific and effective target functioned as lipids regulator during the pathological process of NAFLD for the drug development. Transcription factor EB (TFEB) plays a crucial role in the regulation of lipid homeostasis through linking autophagy to energy metabolism at the transcriptional level. In this review, we summarize the currently available information regarding the mediation of TFEB in lipid metabolism during the pathological process of NAFLD, and the specific regulatory mechanism of TFEB activity. We further recapitulate TFEB as a promising therapeutic target for NAFLD, primarily through the regulation of lipid homeostasis, energy metabolism as well as immune defense. A better understanding of these key issues will be helpful to promote the development of therapeutic agents which specifically target TFEB to halt or reverse the pathological progression of NAFLD.

Lysosomal Acid Lipase in Lipid Metabolism and Beyond

Lysosomal acid lipase (LAL), encoded by the lipase A ( LIPA) gene, hydrolyzes cholesteryl esters and triglycerides to generate free fatty acids and cholesterol in the cell. The essential role of LAL in lipid metabolism has been confirmed in mice and human 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 regulatory role of lipolytic products of lysosomal lipid hydrolysis in catabolic, anabolic, and signaling pathways. In vivo studies in mice with knockout of Lipa highlight the systemic impact of Lipa deficiency on metabolic homeostasis and immune cell function. Genome-wide association studies and functional genomic studies have identified LIPA as a risk locus for coronary heart disease, but the causal variants and mechanisms remain to be determined. Future studies will continue to focus on the role of LAL in the crosstalk between lipid metabolism and cellular function in health and diseases including coronary heart disease.

Homocysteine accelerates atherosclerosis by inhibiting scavenger receptor class B member1 via DNMT3b/SP1 pathway

Homocysteine (Hcy) is an independent risk factor for atherosclerosis, which is characterized by lipid accumulation in the atherosclerotic plaque. Increasing evidence supports that as the main receptor of high-density lipoprotein, scavenger receptor class B member 1 (SCARB1) is protective against atherosclerosis. However, the underlying mechanism regarding it in Hcy-mediated atherosclerosis remains unclear. Here, we found the remarkable inhibition of SCARB1 expression in atherosclerotic plaque and Hcy-treated foam cells, whereas overexpression of SCARB1 can suppress lipid accumulation in foam cells following Hcy treatment. Analysis of SCARB1 promoter showed that no significant change of methylation level was observed both in vivo and in vitro under Hcy treatment. Moreover, it was found that the negative regulation of DNMT3b on SCARB1 was due to the decreased recruitment of SP1 to SCARB1 promoter. Thus, we concluded that inhibition of SCARB1 expression induced by DNMT3b at least partly accelerated Hcy-mediated atherosclerosis through promoting lipid accumulation in foam cells, which was attributed to the decreased binding of SP1 to SCARB1 promoter. In our point, these findings will provide novel insight into an epigenetic mechanism for atherosclerosis.

A lipase-independent pathway of lipid release and immune modulation by adipocytes

To meet systemic metabolic needs, adipocytes release fatty acids and glycerol through the action of neutral lipases. Here, we describe a secondary pathway of lipid release from adipocytes that is independent of canonical lipolysis. We found that adipocytes release exosome-sized, lipid-filled vesicles (AdExos) that become a source of lipid for local macrophages. Adipose tissue from lean mice released ~1% of its lipid content per day via exosomes ex vivo, a rate that more than doubles in obese animals. AdExos and associated factors were sufficient to induce in vitro differentiation of bone marrow precursors into adipose tissue macrophage-like cells. Thus, AdExos are both an alternative pathway of local lipid release and a mechanism by which parenchymal cells can modulate tissue macrophage differentiation and function.

Role of autophagy in the regulation of adipose tissue biology

Autophagy plays a key role in cellular homeostasis since it allows optimal cellular functioning and provides energy under conditions of stress. Initial is revealed that alterations of macroautophagy disturb adipogenic differentiation in cultured cells, and in mice, leading to a drastic reduction of adipose tissue depots. Nevertheless, more recent studies indicate that autophagy participates in the control of adipose tissue biology in a more complex manner. The protein TP53INP2 activates the formation of autophagosomes by binding to ATG8 proteins such as LC3 or GATE16, and its genetic elimination reduces but does not cancel this activity. TP53INP2 deficiency increases adipogenic differentiation and induces a gain in adiposity in the mouse. At the cellular level, TP53INP2 promotes the sequestration of the regulatory protein GSK3β in multivesicular bodies (MVBs) by a process that involves autophagic activity and the participation of the endosomal sorting complexes required for transport (ESCRT) machinery. Through this mechanism, TP53INP2 stabilizes and activates β-catenin, which in turn triggers the inhibition of adipogenesis. In summary, autophagic pathways provide a whole set of mechanisms that may regulate in an opposite way the biology of adipose tissue, and consequently, have a variable impact on the whole body adiposity. This concept may be extensible to other cell types.

Lysosomal Acid Lipase: From Cellular Lipid Handler to Immunometabolic Target

Lysosomal acid lipase (LAL) hydrolyzes cholesteryl esters (CEs) and triglycerides (TGs) to free cholesterol (FC) and free fatty acids (FFAs), which are then used for metabolic purposes in the cell. The process also occurs in immune cells that adapt their metabolic machinery to cope with the different energetic requirements associated with cell activation, proliferation, and polarization. LAL deficiency (LALD) causes severe lipid accumulation and affects the immunometabolic signature in animal models. In humans, LAL deficiency is associated with a peculiar clinical immune phenotype, secondary hemophagocytic lymphohistiocytosis. These observations suggest that LAL might play an important role in cellular immunometabolic modulation, and availability of an effective enzyme replacement strategy makes LAL an attractive target to rewire the metabolic machinery of immune cells beyond its role in controlling cellular lipid metabolism.

Deconstructing Adipogenesis Induced by β3-Adrenergic Receptor Activation with Single-Cell Expression Profiling

Recruitment of brown/beige adipocytes (BAs) in white adipose tissue (WAT) involves proliferation and differentiation of adipocyte stem cells (ASCs) in concert with close interactions with resident immune cells. To deconvolve stromal cell heterogeneity in a comprehensive and unbiased fashion, we performed single-cell RNA sequencing (scRNA-seq) of >33,000 stromal/vascular cells from epididymal WAT (eWAT) and inguinal WAT (iWAT) under control conditions and during β3-adrenergic receptor (ADRB3) activation. scRNA-seq identified distinct ASC subpopulations in eWAT and iWAT that appeared to be differentially poised to enter the adipogenic pathway. ADRB3 activation triggered the dramatic appearance of proliferating ASCs in eWAT, whose differentiation into BAs could be inferred from a single time point. scRNA-seq identified various immune cell types in eWAT, including a proliferating macrophage subpopulation that occupies adipogenic niches. These results demonstrate the power of scRNA-seq to deconstruct adipogenic niches and suggest novel functional interactions among resident stromal cell subpopulations.

Targeting autophagy in obesity: from pathophysiology to management

Obesity poses a severe threat to human health, including the increased prevalence of hypertension, insulin resistance, diabetes mellitus, cancer, inflammation, sleep apnoea and other chronic diseases. Current therapies focus mainly on suppressing caloric intake, but the efficacy of this approach remains poor. A better understanding of the pathophysiology of obesity will be essential for the management of obesity and its complications. Knowledge gained over the past three decades regarding the aetiological mechanisms underpinning obesity has provided a framework that emphasizes energy imbalance and neurohormonal dysregulation, which are tightly regulated by autophagy. Accordingly, there is an emerging interest in the role of autophagy, a conserved homeostatic process for cellular quality control through the disposal and recycling of cellular components, in the maintenance of cellular homeostasis and organ function by selectively ridding cells of potentially toxic proteins, lipids and organelles. Indeed, defects in autophagy homeostasis are implicated in metabolic disorders, including obesity, insulin resistance, diabetes mellitus and atherosclerosis. In this Review, the alterations in autophagy that occur in response to nutrient stress, and how these changes alter the course of obesogenesis and obesity-related complications, are discussed. The potential of pharmacological modulation of autophagy for the management of obesity is also addressed.

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.

Myeloid ATG16L1 does not affect adipose tissue inflammation or body mass in mice fed high fat diet

BACKGROUND

An influx of lipid-loaded macrophages characterizes visceral adipose tissue (VAT) inflammation, which is an important factor in the development of insulin resistance (IR) in obesity. Depletion of macrophage lipids accompanies increased whole body insulin sensitivity, but the underlying mechanism is unknown. Deficiency of autophagy protein ATG16L1 is associated with increases in inflammatory diseases and lipid metabolism, but the connection between ATG16L1, IR, and obesity remains elusive. We hypothesize that myeloid ATG16L1 contributes to lipid loading in macrophages and to IR.

METHODS

Wild-type (WT) bone marrow derived macrophages (BMDMs) were treated with fatty acids and assessed for markers of autophagy. Myeloid-deficient Atg16l1 and littermate control male mice were fed high fat diet (HFD) or low fat diet (LFD) for 3 months starting at 8 weeks of age. Mice were assessed for body mass, fat and lean mass, glucose and insulin sensitivity, food consumption and adipose inflammation. Fluorescence-activated cell sorted VAT macrophages were assessed for lipid content and expression of autophagy related genes.

RESULTS

VAT and VAT macrophages from HFD-fed WT mice did not show differences in autophagy protein and gene expression compared to tissue from LFD-fed mice. Fatty acid-treated BMDMs increased neutral lipid content but did not change autophagy protein expression. HFD-fed Atg16l1 myeloid-deficient and littermate mice demonstrated no differences in body mass, glucose or insulin sensitivity, food consumption, fat or lean mass, macrophage lipid content, or adipose tissue inflammation.

CONCLUSION

ATG16L1 does not contribute to obesity, IR, adipose tissue inflammation or lipid loading in macrophages in mice fed HFD.

Macrophage VLDLR mediates obesity-induced insulin resistance with adipose tissue inflammation

Obesity is closely associated with increased adipose tissue macrophages (ATMs), which contribute to systemic insulin resistance and altered lipid metabolism by creating a pro-inflammatory environment. Very low-density lipoprotein receptor (VLDLR) is involved in lipoprotein uptake and storage. However, whether lipid uptake via VLDLR in macrophages affects obesity-induced inflammatory responses and insulin resistance is not well understood. Here we show that elevated VLDLR expression in ATMs promotes adipose tissue inflammation and glucose intolerance in obese mice. In macrophages, VLDL treatment upregulates intracellular levels of C16:0 ceramides in a VLDLR-dependent manner, which potentiates pro-inflammatory responses and promotes M1-like macrophage polarization. Adoptive transfer of VLDLR knockout bone marrow to wild-type mice relieves adipose tissue inflammation and improves insulin resistance in diet-induced obese mice. These findings suggest that increased VLDL-VLDLR signaling in ATMs aggravates adipose tissue inflammation and insulin resistance in obesity.

VLDLR regulates cellular lipoprotein uptake and storage. Here, the authors show that VLDLR, expressed on adipose tissue macrophages, is upregulated in obesity and promotes adipose tissue inflammation by upregulating ceramide production and facilitating M1-like macrophage polarization.

Lipid Droplets in Health and Disease

Lipids are essential building blocks synthesized by complex molecular pathways and deposited as lipid droplets (LDs) in cells. LDs are evolutionary conserved organelles found in almost all organisms, from bacteria to mammals. They are composed of a hydrophobic neutral lipid core surrounding by a phospholipid monolayer membrane with various decorating proteins. Degradation of LDs provide metabolic energy for divergent cellular processes such as membrane synthesis and molecular signaling. Lipolysis and autophagy are two main catabolic pathways of LDs, which regulate lipid metabolism and, thereby, closely engaged in many pathological conditons. In this review, we first provide an overview of the current knowledge on the structural properties and the biogenesis of LDs. We further focus on the recent findings of their catabolic mechanism by lipolysis and autophagy as well as their connection ragarding the regulation and function. Moreover, we discuss the relevance of LDs and their catabolism-dependent pathophysiological conditions.

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.

Adipocyte Fatty Acid Binding Protein Potentiates Toxic Lipids-Induced Endoplasmic Reticulum Stress in Macrophages via Inhibition of Janus Kinase 2-dependent Autophagy

Lipotoxicity is implicated in the pathogenesis of obesity-related inflammatory complications by promoting macrophage infiltration and activation. Endoplasmic reticulum (ER) stress and adipocyte fatty acid binding protein (A-FABP) play key roles in obesity and mediate inflammatory activity through similar signaling pathways. However, little is known about their interplay in lipid-induced inflammatory responses. Here, we showed that prolonged treatment of palmitic acid (PA) increased ER stress and expression of A-FABP, which was accompanied by reduced autophagic flux in macrophages. Over-expression of A-FABP impaired PA-induced autophagy associating with enhanced ER stress and pro-inflammatory cytokine production, while genetic ablation or pharmacological inhibition of A-FABP reversed the conditions. PA-induced expression of autophagy-related protein (Atg)7 was attenuated in A-FABP over-expressed macrophages, but was elevated in A-FABP-deficient macrophages. Mechanistically, A-FABP potentiated the effects of PA by inhibition of Janus Kinase (JAK)2 activity, thus diminished PA-induced Atg7 expression contributing to impaired autophagy and further augmentation of ER stress. These findings suggest that A-FABP acts as autophagy inhibitor to instigate toxic lipids-induced ER stress through inhibition of JAK2-dependent autophagy, which in turn triggers inflammatory responses in macrophages. A-FABP-JAK2 axis may represent an important pathological pathway contributing to obesity-related inflammatory diseases.

Regulation and Functions of Autophagic Lipolysis

The selective breakdown by autophagy of lipid droplet (LD)-stored lipids, termed lipophagy, is a lysosomal lipolytic pathway that complements the actions of cytosolic neutral lipases. The physiological importance of lipophagy has been demonstrated in multiple mammalian cell types, as well as in lower organisms, and this pathway has many functions in addition to supplying free fatty acids to maintain cellular energy stores. Recent studies have begun to delineate the molecular mechanisms of the selective recognition of LDs by the autophagic machinery, as well as the intricate crosstalk between the different forms of autophagy and neutral lipases. These studies have led to increased interest in the role of lipophagy in both human disease pathogenesis and therapy.

Social isolation induces autophagy in the mouse mammary gland: link to increased mammary cancer risk

Social isolation is a strong predictor of early all-cause mortality and consistently increases breast cancer risk in both women and animal models. Because social isolation increases body weight, we compared its effects to those caused by a consumption of obesity-inducing diet (OID) in C57BL/6 mice. Social isolation and OID impaired insulin and glucose sensitivity. In socially isolated, OID-fed mice (I-OID), insulin resistance was linked to reduced Pparg expression and increased neuropeptide Y levels, but in group-housed OID fed mice (G-OID), it was linked to increased leptin and reduced adiponectin levels, indicating that the pathways leading to insulin resistance are different. Carcinogen-induced mammary tumorigenesis was significantly higher in I-OID mice than in the other groups, but cancer risk was also increased in socially isolated, control diet-fed mice (I-C) and G-OID mice compared with that in controls. Unfolded protein response (UPR) signaling (GRP78; IRE1) was upregulated in the mammary glands of OID-fed mice, but not in control diet-fed, socially isolated I-C mice. In contrast, expression of BECLIN1, ATG7 and LC3II were increased, and p62 was downregulated by social isolation, indicating increased autophagy. In the mammary glands of socially isolated mice, but not in G-OID mice, mRNA expressions of p53 and the p53-regulated autophagy inducer Dram1 were upregulated, and nuclear p53 staining was strong. Our findings further indicated that autophagy and tumorigenesis were not increased in Atg7(+/-) mice kept in social isolation and fed OID. Thus, social isolation may increase breast cancer risk by inducing autophagy, independent of changes in body weight.