DGAT1-Dependent Lipid Droplet Biogenesis Protects Mitochondrial Function during Starvation-Induced Autophagy

LACC1 deficiency links juvenile arthritis with autophagy and metabolism in macrophages

Juvenile idiopathic arthritis is the most common chronic rheumatic disease in children, and its etiology remains poorly understood. Here, we explored four families with early-onset arthritis carrying homozygous loss-of-expression mutations in LACC1. To understand the link between LACC1 and inflammation, we performed a functional study of LACC1 in human immune cells. We showed that LACC1 was primarily expressed in macrophages upon mTOR signaling. We found that LACC1 deficiency had no obvious impact on inflammasome activation, type I interferon response, or NF-κB regulation. Using bimolecular fluorescence complementation and biochemical assays, we showed that autophagy-inducing proteins, RACK1 and AMPK, interacted with LACC1. Autophagy blockade in macrophages was associated with LACC1 cleavage and degradation. Moreover, LACC1 deficiency reduced autophagy flux in primary macrophages. This was associated with a defect in the accumulation of lipid droplets and mitochondrial respiration, suggesting that LACC1-dependent autophagy fuels macrophage bioenergetics metabolism. Altogether, LACC1 deficiency defines a novel form of genetically inherited juvenile arthritis associated with impaired autophagy in macrophages.

Lipid droplets and autophagy-links and regulations from yeast to humans

Recent advances in the yeast Saccharomyces cerevisiae and higher eukaryotes have been increasingly connecting lipid droplet (LD) dynamics to the regulation of autophagy. In this review we will discuss implications that connect LD de novo synthesis and LD mobilization to autophagy and how autophagy is regulated by these mechanisms. Elucidating these connections might pose a chance to further understand autophagy induction and membrane biogenesis for the growing autophagosome under different conditions. Increasing our understanding of these mechanisms might provide a chance to understand several conditions that might be related to LD dysregulation and, possibly, as a consequence of this, dysregulation of autophagy.

Myriocin modulates the altered lipid metabolism and storage in cystic fibrosis

Cystic fibrosis (CF) is a hereditary disease mostly related to ΔF508 CFTR mutation causing a proteinopathy that is characterized by multiple organ dysfunction, primarily lungs chronic inflammation, and infection. Defective autophagy and accumulation of the inflammatory lipid ceramide have been proposed as therapeutic targets. Accumulation of lipids and cholesterol was reported in the airways of CF patients, together with altered triglycerides and cholesterol levels in plasma, thus suggesting a disease-related dyslipidemia. Myriocin, an inhibitor of sphingolipids synthesis, significantly reduces inflammation and activates TFEB-induced response to stress, enhancing fatty acids oxidation and promoting autophagy. Myriocin ameliorates the response against microbial infection in CF models and patients' monocytes. Here we show that CF broncho-epithelial cells exhibit an altered distribution of intracellular lipids. We demonstrated that lipid accumulation is supported by an enhanced synthesis of fatty acids containing molecules and that Myriocin is able to reduce such accumulation. Moreover, Myriocin modulated the transcriptional profile of CF cells in order to restore autophagy, activate an anti-oxidative response, stimulate lipid metabolism and reduce lipid peroxidation. Moreover, lipid storage may be altered in CF cells, since we observed a reduced expression of lipid droplets related proteins named perilipin 3 and 5 and seipin. To note, Myriocin up-regulates the expression of genes that are involved in lipid droplets biosynthesis and maturation. We suggest that targeting sphingolipids de novo synthesis may counteract lipids accumulation by modulating CF altered transcriptional profile, thus restoring autophagy and lipid metabolism homeostasis.

Exploring Metabolic Adaptations to the Acidic Microenvironment of Osteosarcoma Cells Unveils Sphingosine 1-Phosphate as a Valuable Therapeutic Target

Simple Summary

By studying the role of tumor acidosis in osteosarcoma, we have identified a novel lipid signaling pathway that is selectively activated in acid-induced highly metastatic cell subpopulation. Furthermore, when combined to low-serine/glycine diet, the targeting of this acid-induced lipid pathway by the FDA-approved drug FTY720 significantly impaired tumor growth. This new knowledge will provide a giant leap in the understanding of the molecular mechanisms responsible for sarcoma relapses and metastasis. Finally, we paved the way to the recognition of a novel biomarker, as our data provided evidence of significantly high circulating levels in the serum of osteosarcoma patients of S1P, a lipid member of the identified acid-driven metabolic pathway.

Abstract

Acidity is a key player in cancer progression, modelling a microenvironment that prevents immune surveillance and enhances invasiveness, survival, and drug resistance. Here, we demonstrated in spheroids from osteosarcoma cell lines that the exposure to acidosis remarkably caused intracellular lipid droplets accumulation. Lipid accumulation was also detected in sarcoma tissues in close proximity to tumor area that express the acid-related biomarker LAMP2. Acid-induced lipid droplets-accumulation was not functional to a higher energetic request, but rather to cell survival. As a mechanism, we found increased levels of sphingomyelin and secretion of the sphingosine 1-phosphate, and the activation of the associated sphingolipid pathway and the non-canonical NF-ĸB pathway, respectively. Moreover, decreasing sphingosine 1-phosphate levels (S1P) by FTY720 (Fingolimod) impaired acid-induced tumor survival and migration. As a confirmation of the role of S1P in osteosarcoma, we found S1P high circulating levels (30.8 ± 2.5 nmol/mL, n = 17) in the serum of patients. Finally, when we treated osteosarcoma xenografts with FTY720 combined with low-serine/glycine diet, both lipid accumulation (as measured by magnetic resonance imaging) and tumor growth were greatly inhibited. For the first time, this study profiles the lipidomic rearrangement of sarcomas under acidic conditions, suggesting the use of anti-S1P strategies in combination with standard chemotherapy.

ApoE4 Impairs Neuron-Astrocyte Coupling of Fatty Acid Metabolism

Alzheimer’s disease (AD) risk gene ApoE4 perturbs brain lipid homeostasis and energy transduction. However, the cell-type-specific mechanism of ApoE4 in modulating brain lipid metabolism is unclear. Here, we describe a detrimental role of ApoE4 in regulating fatty acid (FA) metabolism across neuron and astrocyte in tandem with their distinctive mitochondrial phenotypes. ApoE4 disrupts neuronal function by decreasing FA sequestering in lipid droplets (LDs). FAs in neuronal LDs are exported and internalized by astrocytes, with ApoE4 diminishing the transport efficiency. Further, ApoE4 lowers FA oxidation and leads to lipid accumulation in both astrocyte and the hippocampus. Importantly, diminished capacity of ApoE4 astrocytes in eliminating neuronal lipids and degrading FAs accounts for their compromised metabolic and synaptic support to neurons. Collectively, our findings reveal a mechanism of ApoE4 disruption to brain FA and bioenergetic homeostasis that could underlie the accelerated lipid dysregulation and energy deficits and increased AD risk for ApoE4 carriers.

Qi et al. reveal a role of Alzheimer’s disease risk gene ApoE4 in disrupting fatty acid metabolism coupled between neuron and astrocyte. A systematic impairment in neuronal sequestering, neuron-to-astrocyte transport, and astrocytic degradation of fatty acids could contribute to lipid dysregulation, bioenergetic deficits, and increased Alzheimer’s risk in ApoE4 carriers.

Getting in Touch Is an Important Step: Control of Metabolism at Organelle Contact Sites

Metabolic pathways are often spread over several organelles and need to be functionally integrated by controlled organelle communication. Physical organelle contact-sites have emerged as critical hubs in the regulation of cellular metabolism, but the molecular understanding of mechanisms that mediate formation or regulation of organelle interfaces was until recently relatively limited. Mitochondria are central organelles in anabolic and catabolic pathways and therefore interact with a number of other cellular compartments including the endoplasmic reticulum (ER) and lipid droplets (LDs). An interesting set of recent work has shed new light on the molecular basis forming these contact sites. This brief overview describes the discovery of unanticipated functions of contact sites between the ER, mitochondria and LDs in de novo synthesis of storage lipids of brown and white adipocytes. Interestingly, the factors involved in mediating the interaction between these organelles are subject to unexpected modes of regulation through newly uncovered Phospho-FFAT motifs. These results suggest dynamic regulation of contact sites between organelles and indicate that spatial organization of organelles within the cell contributes to the control of metabolism.

Direct lysosome-based autophagy of lipid droplets in hepatocytes

Hepatocytes metabolize energy-rich cytoplasmic lipid droplets (LDs) in the lysosome-directed process of autophagy. An organelle-selective form of this process (macrolipophagy) results in the engulfment of LDs within double-membrane delimited structures (autophagosomes) before lysosomal fusion. Whether this is an exclusive autophagic mechanism used by hepatocytes to catabolize LDs is unclear. It is also unknown whether lysosomes alone might be sufficient to mediate LD turnover in the absence of an autophagosomal intermediate. We performed live-cell microscopy of hepatocytes to monitor the dynamic interactions between lysosomes and LDs in real-time. We additionally used a fluorescent variant of the LD-specific protein (PLIN2) that exhibits altered fluorescence in response to LD interactions with the lysosome. We find that mammalian lysosomes and LDs undergo interactions during which proteins and lipids can be transferred from LDs directly into lysosomes. Electron microscopy (EM) of primary hepatocytes or hepatocyte-derived cell lines supports the existence of these interactions. It reveals a dramatic process whereby the lipid contents of the LD can be "extruded" directly into the lysosomal lumen under nutrient-limited conditions. Significantly, these interactions are not affected by perturbations to crucial components of the canonical macroautophagy machinery and can occur in the absence of double-membrane lipoautophagosomes. These findings implicate the existence of an autophagic mechanism used by mammalian cells for the direct transfer of LD components into the lysosome for breakdown. This process further emphasizes the critical role of lysosomes in hepatic LD catabolism and provides insights into the mechanisms underlying lipid homeostasis in the liver.

Two Types of Contact Between Lipid Droplets and Mitochondria

Lipid droplets (LDs) and mitochondria are essential organelles involved in cellular lipid metabolism and energy homeostasis. Accumulated studies have revealed that the physical contact between these two organelles is important for their functions. Current understanding of the contact between cellular organelles is highly dynamic, fitting a "kiss-and-run" model. The same pattern of contact between LDs and mitochondria has been reported and several proteins are found to mediate this contact, such as perilipin1 (PLIN1) and PLIN5. Another format of the contact has also been found and termed anchoring. LD-anchored mitochondria (LDAM) are identified in oxidative tissues including brown adipose tissue (BAT), skeletal muscle, and heart muscle, and this anchoring between these two organelles is conserved from mouse to monkey. Moreover, this anchoring is generated during the brown/beige adipocyte differentiation. In this review, we will summarize previous studies on the interaction between LDs and mitochondria, categorize the types of the contacts into dynamic and stable/anchored, present their similarities and differences, discuss their potential distinct molecular mechanism, and finally propose a working hypothesis that may explain why and how cells use two patterns of contact between LDs and mitochondria.

Importance of lipids for upper motor neuron health and disease

Building evidence reveals the importance of maintaining lipid homeostasis for the health and function of neurons, and upper motor neurons (UMNs) are no exception. UMNs are critically important for the initiation and modulation of voluntary movement as they are responsible for conveying cerebral cortex' input to spinal cord targets. To maintain their unique cytoarchitecture with a prominent apical dendrite and a very long axon, UMNs require a stable cell membrane, a lipid bilayer. Lipids can act as building blocks for many biomolecules, and they also contribute to the production of energy. Therefore, UMNs require sustained control over the production, utilization and homeostasis of lipids. Perturbations of lipid homeostasis lead to UMN vulnerability and progressive degeneration in diseases such as hereditary spastic paraplegia (HSP) and primary lateral sclerosis (PLS). Here, we discuss the importance of lipids, especially for UMNs.

Lipid Droplet Nucleation

All living organisms can make lipid droplets (LDs), intracellular oil-in-water droplets, surrounded by a phospholipid and protein monolayer. LDs are at the nexus of cellular lipid metabolism and function in diverse biological processes. During the past decade, multidisciplinary approaches have shed light on LD assembly steps from the endoplasmic reticulum (ER): nucleation, growth, budding, and formation of a separate organelle. However, the molecular mechanisms underpinning these steps remain elusive. In this review, we focus on the nucleation step, defining where and how LD assembly is initiated. We present how membrane biophysical and physicochemical properties control this step and how proteins act on it to orchestrate LD biogenesis.

Identification of potential post-ethylene events in the signaling cascade induced by stimuli of bud dormancy release in grapevine

Ethylene signaling appears critical for grape bud dormancy release. We therefore focused on identification and characterization of potential downstream targets and events, assuming that they participate in the regulation of dormancy release. Because ethylene responding factors (ERF) are natural candidates for targets of ethylene signaling, we initially characterized the behavior of two VvERF-VIIs, which we identified within a gene set induced by dormancy release stimuli. As expected, these VvERF-VIIs are localized within the nucleus, and are stabilized upon decreases in oxygen availability within the dormant buds. Less expected, the proteins are also stabilized upon hydrogen cyanamide (HC) application under normoxic conditions, and their levels peak at deepest dormancy under vineyard conditions. We proceeded to catalog the response of all bud-expressed ERFs, and identified additional ERFs that respond similarly to ethylene, HC, azide and hypoxia. We also identified a core set of genes that are similarly affected by treatment with ethylene and with various dormancy release stimuli. Interestingly, the functional annotations of this core set center around response to energy crisis and renewal of energy resources via autophagy-mediated catabolism. Because ERF-VIIs are stabilized under energy shortage and reshape cell metabolism to allow energy regeneration, we propose that: (i) the availability of VvERF-VIIs is a consequence of an energy crisis within the bud; (ii) VvERF-VIIs function as part of an energy-regenerating mechanism, which activates anaerobic metabolism and autophagy-mediated macromolecule catabolism; and (iii) activation of catabolism serves as the mandatory switch and the driving force for activation of the growth-inhibited meristem during bud-break.

Surface-associated lipid droplets: an intermediate site for lipid transport in human adipocytes?

Adipose tissue plays a major role in regulating whole-body energy metabolism. While the biochemical processes regulating storage and release of excess energy are well known, the temporal organization of these events is much less defined. In this study, we have characterized the presence of small surface-associated lipid droplets, distinct from the central droplet, in primary human adipocytes. Based on microscopy analyses, we illustrate the distribution of mitochondria, endoplasmic reticulum and lysosomes in the vicinity of these specialized lipid droplets. Ultrastructure analysis confirmed the presence of small droplets in intact adipose tissue. Further, CIDEC, known to bind and regulate lipid droplet expansion, clearly localized at these lipid droplets. Neither acute or prolonged stimulation with insulin or isoprenaline, or pharmacologic intervention to suppress lipid flux, affected the presence of these lipid droplets. Still, phosphorylated perilipin and hormone-sensitive lipase accumulated at these droplets following adrenergic stimuli, which supports metabolic activity at these locations. Altogether, we propose these lipid droplet clusters represent an intermediate site involved in lipid transport in primary adipocytes.

Recent Advances in the Pathophysiology of Fatty Acid Oxidation Defects: Secondary Alterations of Bioenergetics and Mitochondrial Calcium Homeostasis Caused by the Accumulating Fatty Acids

Deficiencies of medium-chain acyl-CoA dehydrogenase, mitochondrial trifunctional protein, isolated long-chain 3-hydroxyacyl-CoA dehydrogenase, and very long-chain acyl-CoA dehydrogenase activities are considered the most frequent fatty acid oxidation defects (FAOD). They are biochemically characterized by the accumulation of medium-chain, long-chain hydroxyl, and long-chain fatty acids and derivatives, respectively, in tissues and biological fluids of the affected patients. Clinical manifestations commonly include hypoglycemia, cardiomyopathy, and recurrent rhabdomyolysis. Although the pathogenesis of these diseases is still poorly understood, energy deprivation secondary to blockage of fatty acid degradation seems to play an important role. However, recent evidence indicates that the predominant fatty acids accumulating in these disorders disrupt mitochondrial functions and are involved in their pathophysiology, possibly explaining the lactic acidosis, mitochondrial morphological alterations, and altered mitochondrial biochemical parameters found in tissues and cultured fibroblasts from some affected patients and also in animal models of these diseases. In this review, we will update the present knowledge on disturbances of mitochondrial bioenergetics, calcium homeostasis, uncoupling of oxidative phosphorylation, and mitochondrial permeability transition induction provoked by the major fatty acids accumulating in prevalent FAOD. It is emphasized that further in vivo studies carried out in tissues from affected patients and from animal genetic models of these disorders are necessary to confirm the present evidence mostly achieved from in vitro experiments.

The Functional Impact of Mitochondrial Structure Across Subcellular Scales

Mitochondria are key determinants of cellular health. However, the functional role of mitochondria varies from cell to cell depending on the relative demands for energy distribution, metabolite biosynthesis, and/or signaling. In order to support the specific needs of different cell types, mitochondrial functional capacity can be optimized in part by modulating mitochondrial structure across several different spatial scales. Here we discuss the functional implications of altering mitochondrial structure with an emphasis on the physiological trade-offs associated with different mitochondrial configurations. Within a mitochondrion, increasing the amount of cristae in the inner membrane improves capacity for energy conversion and free radical-mediated signaling but may come at the expense of matrix space where enzymes critical for metabolite biosynthesis and signaling reside. Electrically isolating individual cristae could provide a protective mechanism to limit the spread of dysfunction within a mitochondrion but may also slow the response time to an increase in cellular energy demand. For individual mitochondria, those with relatively greater surface areas can facilitate interactions with the cytosol or other organelles but may be more costly to remove through mitophagy due to the need for larger phagophore membranes. At the network scale, a large, stable mitochondrial reticulum can provide a structural pathway for energy distribution and communication across long distances yet also enable rapid spreading of localized dysfunction. Highly dynamic mitochondrial networks allow for frequent content mixing and communication but require constant cellular remodeling to accommodate the movement of mitochondria. The formation of contact sites between mitochondria and several other organelles provides a mechanism for specialized communication and direct content transfer between organelles. However, increasing the number of contact sites between mitochondria and any given organelle reduces the mitochondrial surface area available for contact sites with other organelles as well as for metabolite exchange with cytosol. Though the precise mechanisms guiding the coordinated multi-scale mitochondrial configurations observed in different cell types have yet to be elucidated, it is clear that mitochondrial structure is tailored at every level to optimize mitochondrial function to meet specific cellular demands.

Hepatic lipid droplets: A balancing act between energy storage and metabolic dysfunction in NAFLD

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is defined by the abundance of lipid droplets (LDs) in hepatocytes. While historically considered simply depots for energy storage, LDs are increasingly recognized to impact a wide range of biological processes that influence cellular metabolism, signaling, and function. While progress has been made toward understanding the factors leading to LD accumulation (i.e. steatosis) and its progression to advanced stages of NAFLD and/or systemic metabolic dysfunction, much remains to be resolved.

SCOPE OF REVIEW

This review covers many facets of LD biology. We provide a brief overview of the major pathways of lipid accretion and degradation that contribute to steatosis and how they are altered in NAFLD. The major focus is on the relationship between LDs and cell function and the detailed mechanisms that couple or uncouple steatosis from the severity and progression of NAFLD and systemic comorbidities. The importance of specific lipids and proteins within or on LDs as key components that determine whether LD accumulation is linked to cellular and metabolic dysfunction is presented. We discuss emerging areas of LD biology and future research directions that are needed to advance our understanding of the role of LDs in NAFLD etiology.

MAJOR CONCLUSIONS

Impairments in LD breakdown appear to contribute to disease progression, but inefficient incorporation of fatty acids (FAs) into LD-containing triacylglycerol (TAG) and the consequential changes in FA partitioning also affect NAFLD etiology. Increased LD abundance in hepatocytes does not necessarily equate to cellular dysfunction. While LD accumulation is the prerequisite step for most NAFLD cases, the protein and lipid composition of LDs are critical factors in determining the progression from simple steatosis. Further defining the detailed molecular mechanisms linking LDs to metabolic dysfunction is important for designing effective therapeutic approaches targeting NAFLD and its comorbidities.

The yeast FIT2 homologs are necessary to maintain cellular proteostasis and membrane lipid homeostasis

Lipid droplets (LDs) are implicated in conditions of lipid and protein dysregulation. The fat storage-inducing transmembrane (FIT; also known as FITM) family induces LD formation. Here, we establish a model system to study the role of the Saccharomyces cerevisiae FIT homologues (ScFIT), SCS3 and YFT2, in the proteostasis and stress response pathways. While LD biogenesis and basal endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR) remain unaltered in ScFIT mutants, SCS3 was found to be essential for proper stress-induced UPR activation and for viability in the absence of the sole yeast UPR transducer IRE1 Owing to not having a functional UPR, cells with mutated SCS3 exhibited an accumulation of triacylglycerol within the ER along with aberrant LD morphology, suggesting that there is a UPR-dependent compensatory mechanism that acts to mitigate lack of SCS3 Additionally, SCS3 was necessary to maintain phospholipid homeostasis. Strikingly, global protein ubiquitylation and the turnover of both ER and cytoplasmic misfolded proteins is impaired in ScFITΔ cells, while a screen for interacting partners of Scs3 identifies components of the proteostatic machinery as putative targets. Together, our data support a model where ScFITs play an important role in lipid metabolism and proteostasis beyond their defined roles in LD biogenesis.This article has an associated First Person interview with the first author of the paper.

Multifaceted Roles of Plant Autophagy in Lipid and Energy Metabolism

Together with sugars and proteins, lipids constitute the main carbon reserves in plants. Lipids are selectively recycled and catabolized for energy production during development and in response to environmental stresses. Autophagy is a major catabolic pathway, operating in the recycling of cellular components in eukaryotes. Although the autophagic degradation of lipids has been mainly characterized in mammals and yeast, growing evidence has highlighted the role of autophagy in several aspects of lipid metabolism in plants. Here, we summarize recent findings focusing on autophagy functions in lipid droplet (LD) metabolism. We further provide novel insights regarding the relevance of autophagy in the maintenance and clearance of mitochondria and peroxisomes and its consequences for proper lipid usage and energy homeostasis in plants.

The ins-and-outs of podocyte lipid metabolism

Obesity and hyperlipidemia are the most prevalent independent risk factors of ESRD, suggesting that lipid accumulation is detrimental to renal function. The origin of lipid accumulation (a common feature in podocyte injury) and its pathophysiological relevance are unknown. This commentary discusses the finding by Liu et al. that deficiency of the endoplasmic reticulum enzyme SOAT1, which metabolizes cholesterol to cholesterol esters, attenuates renal/podocyte injury in murine models of diabetes and Alport's syndrome.

Adipocyte lipolysis: from molecular mechanisms of regulation to disease and therapeutics

Fatty acids (FAs) are stored safely in the form of triacylglycerol (TAG) in lipid droplet (LD) organelles by professional storage cells called adipocytes. These lipids are mobilized during adipocyte lipolysis, the fundamental process of hydrolyzing TAG to FAs for internal or systemic energy use. Our understanding of adipocyte lipolysis has greatly increased over the past 50 years from a basic enzymatic process to a dynamic regulatory one, involving the assembly and disassembly of protein complexes on the surface of LDs. These dynamic interactions are regulated by hormonal signals such as catecholamines and insulin which have opposing effects on lipolysis. Upon stimulation, patatin-like phospholipase domain containing 2 (PNPLA2)/adipocyte triglyceride lipase (ATGL), the rate limiting enzyme for TAG hydrolysis, is activated by the interaction with its co-activator, alpha/beta hydrolase domain-containing protein 5 (ABHD5), which is normally bound to perilipin 1 (PLIN1). Recently identified negative regulators of lipolysis include G0/G1 switch gene 2 (G0S2) and PNPLA3 which interact with PNPLA2 and ABHD5, respectively. This review focuses on the dynamic protein-protein interactions involved in lipolysis and discusses some of the emerging concepts in the control of lipolysis that include allosteric regulation and protein turnover. Furthermore, recent research demonstrates that many of the proteins involved in adipocyte lipolysis are multifunctional enzymes and that lipolysis can mediate homeostatic metabolic signals at both the cellular and whole-body level to promote inter-organ communication. Finally, adipocyte lipolysis is involved in various diseases such as cancer, type 2 diabetes and fatty liver disease, and targeting adipocyte lipolysis is of therapeutic interest.

EGFR Signaling Stimulates Autophagy to Regulate Stem Cell Maintenance and Lipid Homeostasis in the Drosophila Testis

Although typically upregulated upon cellular stress, autophagy can also be utilized under homeostatic conditions as a quality control mechanism or in response to developmental cues. Here, we report that autophagy is required for the maintenance of somatic cyst stem cells (CySCs) in the Drosophila testis. Disruption of autophagy in CySCs and early cyst cells (CCs) by the depletion of autophagy-related (Atg) genes reduced early CC numbers and affected CC function, resembling decreased epidermal growth factor receptor (EGFR) signaling. Indeed, our data indicate that EGFR acts to stimulate autophagy to preserve early CC function, whereas target of rapamycin (TOR) negatively regulates autophagy in the differentiating CCs. Finally, we show that the EGFR-mediated stimulation of autophagy regulates lipid levels in CySCs and CCs. These results demonstrate a key role for autophagy in regulating somatic stem cell behavior and tissue homeostasis by integrating cues from both the EGFR and TOR signaling pathways to control lipid metabolism.

Sênos Demarco et al. demonstrate that Drosophila somatic cyst stem cells (CySCs) rely on basal levels of autophagy, which is regulated by EGFR signaling, for maintenance under homeostatic conditions. Blocking autophagy results in the accumulation of lipid droplets; therefore, one role of autophagy is to regulate lipid homeostasis in the soma.

S100A11 Promotes Liver Steatosis via FOXO1-Mediated Autophagy and Lipogenesis

BACKGROUND & AIMS

Nonalcoholic fatty liver disease (NAFLD) is becoming a severe liver disorder worldwide. Autophagy plays a critical role in liver steatosis. However, the role of autophagy in NAFLD remains exclusive and under debate. In this study, we investigated the role of S100 calcium binding protein A11 (S100A11) in the pathogenesis of hepatic steatosis.

METHODS

We performed liver proteomics in a well-established tree shrew model of NAFLD. The expression of S100A11 in different models of NAFLD was detected by Western blot and/or quantitative polymerase chain reaction. Liver S100A11 overexpression mice were generated by injecting a recombinant adenovirus gene transfer vector through the tail vein and then induced by a high-fat and high-cholesterol diet. Cell lines with S100a11 stable overexpression were established with a recombinant lentiviral vector. The lipid content was measured with either Bodipy staining, Oil Red O staining, gas chromatography, or a triglyceride kit. The autophagy and lipogenesis were detected in vitro and in vivo by Western blot and quantitative polymerase chain reaction. The functions of Sirtuin 1, histone deacetylase 6 (HDAC6), and FOXO1 were inhibited by specific inhibitors. The interactions between related proteins were analyzed by a co-immunoprecipitation assay and immunofluorescence analysis.

RESULTS

The expression of S100A11 was up-regulated significantly in a time-dependent manner in the tree shrew model of NAFLD. S100A11 expression was induced consistently in oleic acid-treated liver cells as well as the livers of mice fed a high-fat diet and NAFLD patients. Both in vitro and in vivo overexpression of S100A11 could induce hepatic lipid accumulation. Mechanistically, overexpression of S100A11 activated an autophagy and lipogenesis process through up-regulation and acetylation of the transcriptional factor FOXO1, consequently promoting lipogenesis and lipid accumulation in vitro and in vivo. Inhibition of HDAC6, a deacetylase of FOXO1, showed similar phenotypes to S100A11 overexpression in Hepa 1-6 cells. S100A11 interacted with HDAC6 to inhibit its activity, leading to the release and activation of FOXO1. Under S100A11 overexpression, the inhibition of FOXO1 and autophagy could alleviate the activated autophagy as well as up-regulated lipogenic genes. Both FOXO1 and autophagy inhibition and Dgat2 deletion could reduce liver cell lipid accumulation significantly.

CONCLUSIONS

A high-fat diet promotes liver S100A11 expression, which may interact with HDAC6 to block its binding to FOXO1, releasing or increasing the acetylation of FOXO1, thus activating autophagy and lipogenesis, and accelerating lipid accumulation and liver steatosis. These findings indicate a completely novel S100A11-HDAC6-FOXO1 axis in the regulation of autophagy and liver steatosis, providing potential possibilities for the treatment of NAFLD.

Molecular Basis for Autosomal-Dominant Renal Fanconi Syndrome Caused by HNF4A

HNF4A is a nuclear hormone receptor that binds DNA as an obligate homodimer. While all known human heterozygous mutations are associated with the autosomal-dominant diabetes form MODY1, one particular mutation (p.R85W) in the DNA-binding domain (DBD) causes additional renal Fanconi syndrome (FRTS). Here, we find that expression of the conserved fly ortholog dHNF4 harboring the FRTS mutation in Drosophila nephrocytes caused nuclear depletion and cytosolic aggregation of a wild-type dHNF4 reporter protein. While the nuclear depletion led to mitochondrial defects and lipid droplet accumulation, the cytosolic aggregates triggered the expansion of the endoplasmic reticulum (ER), autophagy, and eventually cell death. The latter effects could be fully rescued by preventing nuclear export through interfering with serine phosphorylation in the DBD. Our data describe a genomic and a non-genomic mechanism for FRTS in HNF4A-associated MODY1 with important implications for the renal proximal tubule and the regulation of other nuclear hormone receptors.

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HNF4 controls lipid metabolism in Drosophila nephrocytes

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The kidney disease mutation R85W shows dominant-negative effects in nephrocytes

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Dephosphorylation at S87 prevents the dominant-negative effects

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R85W mutation causes mitochondrial dysfunction in reprogrammed renal epithelial cells

Inhibiting diacylglycerol acyltransferase-1 reduces lipid biosynthesis in bovine blastocysts produced in vitro

Diacylglycerol acyltransferase-1 (DGAT1) is one of the DGAT enzymes that catalyzes the final step in the synthesis of triacylglycerol, which is a major component of the lipid droplets in embryos. Intracellular lipids accumulated in embryos produced in vitro have been associated with reduced cryotolerance and quality. The objective of the present study was to investigate the influence of DGAT1 inhibition on embryo development, quality, and post-vitrification survival, in addition to expression profiles of selected lipid metabolism-regulating and oxidative stress genes. Bovine cumulus-oocyte complexes were matured and fertilized in vitro and were cultured in synthetic oviduct fluid (SOF) supplemented with 5% fetal calf serum (FCS) alone (Control) or with 1, 5, 10 or 50 μM DGAT1 inhibitor (A922500®; D1, D5, D10, and D50, respectively) or 0.1% dimethyl sulfoxide (CDMSO: vehicle for DGAT1 inhibitor dilution) from 54 h post-insemination until Day 8 post insemination. No differences were found in blastocyst yield on days 7 and 8 in Control, CDMSO, D10, and D50 groups. Embryos cultured with 10 or 50 μM DGAT1 inhibitor had greater mitochondrial activity (P < 0.01), and increased number of cells (P < 0.05), while the cytoplasmic lipid content was reduced (P < 0.01), the latter associated with altered expression profiles of selected genes regulating lipid metabolism or genes related with oxidative stress (transcript abundance increased for SLC2A1 and SLC2A5 and decreased for DGAT1 and GPX1). Importantly, the survival rate of blastocysts produced with 10 μM DGAT1 was higher than that of Control, CDMSO and D50 groups at 72 h after vitrification and warming (73.8 vs 57.1, 55.9 and 56.1%, respectively, P < 0.001). In conclusion, inhibition of DGAT1 synthesis in bovine embryos produced in vitro abrogates the negative effect of FCS by decreasing their lipid content, increasing mitochondria activity and improving embryo cryotolerance, as well as favoring the expression of lipid metabolism regulating and oxidative stress-related transcripts.

Long-term autophagy is sustained by activation of CCTβ3 on lipid droplets

Macroautophagy initiates by formation of isolation membranes, but the source of phospholipids for the membrane biogenesis remains elusive. Here, we show that autophagic membranes incorporate newly synthesized phosphatidylcholine, and that CTP:phosphocholine cytidylyltransferase β3 (CCTβ3), an isoform of the rate-limiting enzyme in the Kennedy pathway, plays an essential role. In starved mouse embryo fibroblasts, CCTβ3 is initially recruited to autophagic membranes, but upon prolonged starvation, it concentrates on lipid droplets that are generated from autophagic degradation products. Omegasomes and isolation membranes emanate from around those lipid droplets. Autophagy in prolonged starvation is suppressed by knockdown of CCTβ3 and is enhanced by its overexpression. This CCTβ3-dependent mechanism is also present in U2OS, an osteosarcoma cell line, and autophagy and cell survival in starvation are decreased by CCTβ3 depletion. The results demonstrate that phosphatidylcholine synthesis through CCTβ3 activation on lipid droplets is crucial for sustaining autophagy and long-term cell survival.

Sustained activation of autophagy suppresses adipocyte maturation via a lipolysis-dependent mechanism

Dysregulation of macroautophagy/autophagy is implicated in obesity and insulin resistance. However, it remains poorly defined how autophagy regulates adipocyte development. Using adipose-specific rptor/raptor knockout (KO), atg7 KO and atg7 rptor double-KO mice, we show that inhibiting MTORC1 by RPTOR deficiency led to autophagic sequestration of lipid droplets, formation of LD-containing lysosomes, and elevation of basal and isoproterenol-induced lipolysis in vivo and in primary adipocytes. Despite normal differentiation at an early phase, progressive degradation and shrinkage of cellular LDs and downregulation of adipogenic markers PPARG and PLIN1 occurred in terminal differentiation of rptor KO adipocytes, which was rescued by inhibiting lipolysis or lysosome. In contrast, inactivating autophagy by depletion of ATG7 protected adipocytes against RPTOR deficiency-induced formation of LD-containing lysosomes, LD degradation, and downregulation of adipogenic markers in vitro. Ultimately, atg7 rptor double-KO mice displayed decreased lipolysis, restored adipose tissue development, and upregulated thermogenic gene expression in brown and inguinal adipose tissue compared to RPTOR-deficient mice in vivo. Collectively, our study demonstrates that autophagy plays an important role in regulating adipocyte maturation via a lipophagy and lipolysis-dependent mechanism.

ABBREVIATIONS

ATG7: autophagy related 7; BAT: brown adipose tissue; CEBPB/C/EBPβ: CCAAT enhancer binding protein beta; DGAT1: diacylglycerol O-acyltransferase 1; eWAT: epididymal white adipose tissue; iWAT: inguinal white adipose tissue; KO: knockout; LD: lipid droplet; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; MTORC1: mechanistic target of rapamycin kinase complex 1; PLIN1: perepilin 1; PNPLA2/ATGL: patatin-like phospholipase domain containing 2; PPARG/PPARγ: peroxisome proliferator activated receptor gamma; RPTOR: regulatory associated protein of MTOR complex1; TG: triglyceride; ULK1: unc-51 like kinase 1; UCP1: uncoupling protein 1; WAT: white adipose tissue.

Excess diacylglycerol at the endoplasmic reticulum disrupts endomembrane homeostasis and autophagy

Background

When stressed, eukaryotic cells produce triacylglycerol (TAG) to store nutrients and mobilize autophagy to combat internal damage. We and others previously reported that in yeast, elimination of TAG synthesizing enzymes inhibits autophagy under nitrogen starvation, yet the underlying mechanism has remained elusive.

Results

Here, we show that disruption of TAG synthesis led to diacylglycerol (DAG) accumulation and its relocation from the vacuolar membrane to the endoplasmic reticulum (ER). We further show that, beyond autophagy, ER-accumulated DAG caused severe defects in the endomembrane system, including disturbing the balance of ER-Golgi protein trafficking, manifesting in bulging of ER and loss of the Golgi apparatus. Genetic or chemical manipulations that increase consumption or decrease supply of DAG reversed these defects. In contrast, increased amounts of precursors of glycerolipid synthesis, including phosphatidic acid and free fatty acids, did not replicate the effects of excess DAG. We also provide evidence that the observed endomembrane defects do not rely on Golgi-produced DAG, Pkc1 signaling, or the unfolded protein response.

Conclusions

This work identifies DAG as the critical lipid molecule responsible for autophagy inhibition under condition of defective TAG synthesis and demonstrates the disruption of ER and Golgi function by excess DAG as the potential cause of the autophagy defect.

Triacylglycerol synthesis enhances macrophage inflammatory function

Foamy macrophages, which have prominent lipid droplets (LDs), are found in a variety of disease states. Toll-like receptor agonists drive triacylglycerol (TG)-rich LD development in macrophages. Here we explore the basis and significance of this process. Our findings indicate that LD development is the result of metabolic commitment to TG synthesis on a background of decreased fatty acid oxidation. TG synthesis is essential for optimal inflammatory macrophage activation as its inhibition, which prevents LD development, has marked effects on the production of inflammatory mediators, including IL-1β, IL-6 and PGE2, and on phagocytic capacity. The failure of inflammatory macrophages to make PGE2 when TG-synthesis is inhibited is critical for this phenotype, as addition of exogenous PGE2 is able to reverse the anti-inflammatory effects of TG synthesis inhibition. These findings place LDs in a position of central importance in inflammatory macrophage activation.

As macrophages switch to a proinflammatory gylcolytic state they start to generate triglyceride-rich lipid droplets, but what function these droplets have in this context is not clear. Here the authors show that this triglyceride synthesis is requisite for prostaglandin E2 production and subsequent inflammatory activation.

Pyrroline-5-carboxylate synthase senses cellular stress and modulates metabolism by regulating mitochondrial respiration

Pyrroline-5-carboxylate synthase (P5CS) catalyzes the synthesis of pyrroline-5-carboxylate (P5C), a key precursor for the synthesis of proline and ornithine. P5CS malfunction leads to multiple human diseases; however, the molecular mechanism underlying these diseases is unknown. We found that P5CS localizes in mitochondria in rod- and ring-like patterns but diffuses inside the mitochondria upon cellular starvation or exposure to oxidizing agents. Some of the human disease-related mutant forms of P5CS also exhibit diffused distribution. Multimerization (but not the catalytic activity) of P5CS regulates its localization. P5CS mutant cells have a reduced proliferation rate and are sensitive to cellular stresses. Flies lacking P5CS have reduced eclosion rates. Lipid droplets accumulate in the eyes of the newly eclosed P5CS mutant flies, which degenerate with aging. The loss of P5CS in cells leads to abnormal purine metabolism and lipid-droplet accumulation. The reduced lipid-droplet consumption is likely due to decreased expression of the fatty acid transporter, CPT1, and few β-oxidation-related genes following P5CS knockdown. Surprisingly, we found that P5CS is required for mitochondrial respiratory complex organization and that the respiration defects in P5CS knockout cells likely contribute to the metabolic defects in purine synthesis and lipid consumption. This study links amino acid synthesis with mitochondrial respiration and other key metabolic processes, whose imbalance might contribute to P5CS-related disease conditions.

Targeting DGAT1 Ameliorates Glioblastoma by Increasing Fat Catabolism and Oxidative Stress

Glioblastoma (GBM), a mostly lethal brain tumor, acquires large amounts of free fatty acids (FAs) to promote cell growth. But how the cancer avoids lipotoxicity is unknown. Here, we identify that GBM upregulates diacylglycerol-acyltransferase 1 (DGAT1) to store excess FAs into triglycerides and lipid droplets. Inhibiting DGAT1 disrupted lipid homeostasis and resulted in excessive FAs moving into mitochondria for oxidation, leading to the generation of high levels of reactive oxygen species (ROS), mitochondrial damage, cytochrome c release, and apoptosis. Adding N-acetyl-cysteine or inhibiting FA shuttling into mitochondria decreased ROS and cell death induced by DGAT1 inhibition. We show in xenograft models that targeting DGAT1 blocked lipid droplet formation, induced tumor cell apoptosis, and markedly suppressed GBM growth. Together, our study demonstrates that DGAT1 upregulation protects GBM from oxidative damage and maintains lipid homeostasis by facilitating storage of excess FAs. Targeting DGAT1 could be a promising therapeutic approach for GBM.

Lipid Droplets in Neurodegenerative Disorders

Knowledge of lipid droplets (LDs) has evolved from simple depots of lipid storage to dynamic and functionally active organelles involved in a variety of cellular functions. Studies have now informed significant roles for LDs in cellular signaling, metabolic disease, and inflammation. While lipid droplet biology has been well explored in peripheral organs such as the liver and heart, LDs within the brain are relatively understudied. The presence and function of these dynamic organelles in the central nervous system has recently gained attention, especially in the context of neurodegeneration. In this review, we summarize the current understanding of LDs within the brain, with an emphasis on their relevance in neurodegenerative diseases.

NOXA upregulation by the prohibitin-binding compound fluorizoline is transcriptionally regulated by integrated stress response-induced ATF3 and ATF4

Fluorizoline is a new synthetic molecule that induces p53-independent apoptosis, in several tumor cell lines and in primary leukemia cells, by selectively targeting prohibitins (PHBs). In this study, we describe how fluorizoline induces BCL-2 homology 3-only protein NOXA, without modulating the protein levels of anti-apoptotic B-cell lymphoma-2 (BCL-2) family members prior to caspase activation, as well as how it synergizes with the BCL-2 and BCL-X_L inhibitor ABT-737 to induce apoptosis. Interestingly, fluorizolinetreatment triggers the activation of the integrated stress response (ISR) in HeLa and HAP1 cells, with increased eukaryotic translation initiation factor 2α phosphorylation, and induction of ATF3, ATF4, and CHOP. Moreover, PHB downregulation induces similar ISR activation and apoptosis as with fluorizoline treatment. In addition, we studied the essential role of the pro-apoptotic protein NOXA in fluorizoline-induced apoptosis and we describe its mechanism of induction in HeLa and HAP1 cells. Moreover, we identified ATF3 and ATF4 as the transcription factors that bind to NOXA promoter upon fluorizoline treatment. Furthermore, using ATF3 and ATF4 CRISPR HeLa and HAP1 cells, we confirmed that both factors mediate the induction of NOXA and apoptosis by fluorizoline. In conclusion, fluorizoline treatment triggers the activation of the ISR that results in the induction of ATF3 and ATF4, important regulators of NOXA transcription in fluorizoline-induced apoptosis.

Maintaining social contacts: The physiological relevance of organelle interactions

Membrane-bound organelles in eukaryotic cells form an interactive network to coordinate and facilitate cellular functions. The formation of close contacts, termed "membrane contact sites" (MCSs), represents an intriguing strategy for organelle interaction and coordinated interplay. Emerging research is rapidly revealing new details of MCSs. They represent ubiquitous and diverse structures, which are important for many aspects of cell physiology and homeostasis. Here, we provide a comprehensive overview of the physiological relevance of organelle contacts. We focus on mitochondria, peroxisomes, the Golgi complex and the plasma membrane, and discuss the most recent findings on their interactions with other subcellular organelles and their multiple functions, including membrane contacts with the ER, lipid droplets and the endosomal/lysosomal compartment.

Mitochondria-Associated ER Membranes - The Origin Site of Autophagy

Autophagy is a process of intracellular self-recycling and degradation that plays an important role in maintaining cell homeostasis. However, the molecular mechanism of autophagy remains to be further studied. Mitochondria-associated endoplasmic reticulum membranes (MAMs) are the region of the ER that mediate communication between the ER and mitochondria. MAMs have been demonstrated to be involved in autophagy, Ca²⁺ transport and lipid metabolism. Here, we discuss the composition and function of MAMs, more specifically, to emphasize the role of MAMs in regulating autophagy. Finally, some key information that may be useful for future research is summarized.

Identification and analysis of lipid droplet-related proteome in the adipose tissue of grass carp (Ctenopharyngodon idella) under fed and starved conditions

Fat accumulation in the mesenteric adipose tissue is a serious problem in grass carp (Ctenopharyngodon idella) culture. Lipid droplet-related proteins (LDRPs) are involved in the formation, degradation, and biological functions of lipid droplets. In this study, we aimed to provide reference proteomics data to study lipid droplet regulation in fish. We isolated LDRPs from the mesenteric adipose tissue of grass carp (1-year-old) after normal feeding and 7 days of starvation, and identified and analysed them using isobaric tags for relative and absolute quantitation (iTRAQ) technology. Short-term starvation had no significant effect on the body weight, condition factor, visceral index, hepatopancreas index, intraperitoneal fat index, adipose tissue triglyceride content, and adipocyte size of grass carp. Nine hundred and fifty proteins were identified and annotated using the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases; they are involved in a variety of metabolic and signalling pathways, including amino acid, lipid, and carbohydrate metabolism, and the PI3K-Akt signalling pathway. There were 296 differentially expressed proteins (DEPs), with 143 up-regulated and 153 down-regulated proteins. Three proteins involved in triglyceride and fatty acid syntheses and two proteins involved in autophagy were up-regulated, and six proteins involved in lipid catabolism were down-regulated. These results indicate that under short-term starvation, lipid droplets in the adipose tissue of grass carp may maintain their shape by promoting fat production and inhibiting lipolysis, and autophagy may be one of the main strategies for coping with short-term energy deprivation.

Grease on-Perspectives in lipid droplet biology

Lipid droplets (LDs) are now recognized as omnipresent and dynamic subcellular organelles of amazing morphological and functional diversity. Beyond the obvious benefit of having molecules full of chemical energy stored in a dedicated structural entity, LDs may also be viewed as a safe harbor for potentially damaging metabolites. This protective function might in many cases even supersede the relevance of lipid storage for eventual energy gain and membrane biogenesis. Furthermore, the LD surface constitutes a unique membrane environment, creating a platform for hosting specific proteins and thus enabling their interactions. These metabolic hotspots would contribute decisively to compartmentalized metabolism in the cytosol. LDs are also communicating extensively with other subcellular organelles in directing and regulating lipid metabolism. Deciphering the relevance of LD storage and regulation at the organismic level will be essential for the understanding of widespread and serious metabolic complications in humans. Increasing attention is also devoted to pathogens appropriating LDs for their own benefit. LD biology is still considered an emerging research area in rapid and vibrant development, attracting scientists from all disciplines of the life sciences and beyond, which is mirrored by the accompanying review collection. Here, we present our personal views on areas we believe are especially exciting and hold great potential for future developments. Particularly, we address issues relating to LD biogenesis and heterogeneity, required technological advances, and the complexity of human physiology.

Eicosapentaenoic acid attenuates renal lipotoxicity by restoring autophagic flux

Recently, we identified a novel mechanism of lipotoxicity in the kidney proximal tubular cells (PTECs); lipid overload stimulates macroautophagy/autophagy for the renovation of plasma and organelle membranes to maintain the integrity of the PTECs. However, this autophagic activation places a burden on the lysosomal system, leading to a downstream suppression of autophagy, which manifests as phospholipid accumulation and inadequate acidification in lysosomes. Here, we investigated whether pharmacological correction by eicosapentaenoic acid (EPA) supplementation could restore autophagic flux and alleviate renal lipotoxicity. EPA supplementation to high-fat diet (HFD)-fed mice reduced several hallmarks of lipotoxicity in the PTECs, such as phospholipid accumulation in the lysosome, mitochondrial dysfunction, inflammation, and fibrosis. In addition to improving the metabolic syndrome, EPA alleviated renal lipotoxicity via several mechanisms. EPA supplementation to HFD-fed mice or the isolated PTECs cultured in palmitic acid (PA) restored lysosomal function with significant improvements in the autophagic flux. The PA-induced redistribution of phospholipids from cellular membranes into lysosomes and the HFD-induced accumulation of SQSTM1/p62 (sequestosome 1), an autophagy substrate, during the temporal and genetic ablation of autophagy were significantly reduced by EPA, indicating that EPA attenuated the HFD-mediated increases in autophagy demand. Moreover, a fatty acid pulse-chase assay revealed that EPA promoted lipid droplet (LD) formation and transfer from LDs to the mitochondria for beta-oxidation. Noteworthy, the efficacy of EPA on lipotoxicity is autophagy-dependent and cell-intrinsic. In conclusion, EPA counteracts lipotoxicity in the proximal tubule by alleviating autophagic numbness, making it potentially suitable as a novel treatment for obesity-related kidney diseases.Abbreviations: 4-HNE: 4-hydroxy-2-nonenal; ACTB: actin beta; ADGRE1/F4/80: adhesion G protein-coupled receptor E1; ATG: autophagy-related; ATP: adenosine triphosphate; BODIPY: boron-dipyrromethene; BSA: bovine serum albumin; cKO: conditional knockout; CML: N-carboxymethyllysine; COL1A1: collagen type I alpha 1 chain; COX: cytochrome c oxidase; CTRL: control; DGAT: diacylglycerol O-acyltransferase; EPA: eicosapentaenoic acid; FA: fatty acid; FFA: free fatty acid; GFP: green fluorescent protein; HFD: high-fat diet; iKO: inducible knockout; IRI: ischemia-reperfusion injury; LAMP1: lysosomal-associated membrane protein 1; LD: lipid droplet; LRP2: low density lipoprotein receptor-related protein 2; MAP1LC3: microtubule-associated protein 1 light chain 3; MTORC1: mechanistic target of rapamycin kinase complex 1; OA: oleic acid; PAS: periodic-acid Schiff; PPAR: peroxisome proliferator activated receptor; PPARGC1/PGC1: peroxisome proliferator activated receptor, gamma, coactivator 1; PTEC: proximal tubular epithelial cell; ROS: reactive oxygen species; RPS6: ribosomal protein S6; SDH: succinate dehydrogenase complex; SFC/MS/MS: supercritical fluid chromatography triple quadrupole mass spectrometry; SQSTM1/p62: sequestosome 1; TFEB: transcription factor EB; TG: triglyceride; TUNEL: terminal deoxynucleotidyl transferase dUTP nick end labeling.

Implications of lipid droplets in lung cancer: Associations with drug resistance

Cancer cells usually show different metabolic patterns compared with healthy cells due to the reprogramming of metabolic processes. The process of lipid metabolism undergoes notable changes, leading to the accumulation of lipid droplets in cells. Additionally, this phenotype is considered an important marker of cancer cells. Lipid droplets are a highly dynamic type of organelle in the cell, which is composed of a neutral lipid core, a monolayer phospholipid membrane and lipid droplet-related proteins. Lipid droplets are involved in several biological processes, including cell proliferation, apoptosis, lipid metabolism, stress, immunity, signal transduction and protein trafficking. Epidermal growth factor receptor (EGFR)-activating mutations are currently the most effective therapeutic targets for non-small cell lung cancer. Several EGFR tyrosine kinase inhibitors (EGFR-TKIs) that target these mutations, including gefitinib, erlotinib, afatinib and osimertinib, have been widely used clinically. However, the development of acquired resistance has a major impact on the efficacy of these drugs. A number of previous studies have reported that the expression of lipid droplets in the tumor tissues of patients with lung cancer are elevated, whereas the association between elevated numbers of lipid droplets and drug resistance has received little attention. The present review describes the potential association between lipid droplets and drug resistance. Furthermore, the mechanisms and implications of lipid droplet accumulation in cancer cells are analyzed, as wells as the mechanism by which lipid droplets suppress endoplasmic reticulum stress and apoptosis, which are essential for the development and treatment of lung cancer.

β-cell autophagy: Mechanism and role in β-cell dysfunction

Background

Elucidation of the basic molecular mechanism of autophagy was a breakthrough in understanding various physiological events and pathogenesis of diverse diseases. In the fields of diabetes and metabolism, many cellular events associated with the development of disease or its treatment cannot be explained well without taking autophagy into account. While a grand picture of autophagy has been established, detailed aspects of autophagy, particularly that of selective autophagy responsible for homeostasis of specific organelles or metabolic intermediates, are still ambiguous and currently under intensive research.

Scope of review

Here, results from previous and current studies on the role of autophagy and its dysregulation in the physiology of metabolism and pathogenesis of diabetes are summarized, with an emphasis on the pancreatic β-cell autophagy. In addition to nonselective (bulk) autophagy, machinery and significance of selective autophagy such as mitophagy of pancreatic β-cells is discussed. Novel findings regarding autophagy types other than macroautophagy are also covered, since several types of autophagy or lysosomal degradation pathways other than macroautophagy coexist in pancreatic β-cells.

Major conclusion

Autophagy plays a critical role in cellular metabolism, homeostasis of the intracellular environment and function of organelles such as mitochondria and endoplasmic reticulum. Impaired autophagic activity due to aging, obesity or genetic predisposition could be a factor in the development of β-cell dysfunction and diabetes associated with lipid overload or human-type diabetes characterized by islet amyloid deposition. Modulation of autophagy of pancreatic β-cells is likely to be possible in the near future, which would be valuable in the treatment of diabetes associated with lipid overload or accumulation of islet amyloid.

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Autophagy is critical for cellular metabolism, homeostasis and organelle function.

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Impaired autophagic activity could predispose to β-cell dysfunction and diabetes.

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Several types of autophagy coexist in pancreatic β-cells.

Metabolic reprogramming by Zika virus provokes inflammation in human placenta

The recent outbreak of Zika virus (ZIKV) was associated with birth defects and pregnancy loss when maternal infection occurs in early pregnancy, but specific mechanisms driving placental insufficiency and subsequent ZIKV-mediated pathogenesis remain unclear. Here we show, using large scale metabolomics, that ZIKV infection reprograms placental lipidome by impairing the lipogenesis pathways. ZIKV-induced metabolic alterations provide building blocks for lipid droplet biogenesis and intracellular membrane rearrangements to support viral replication. Furthermore, lipidome reprogramming by ZIKV is paralleled by the mitochondrial dysfunction and inflammatory immune imbalance, which contribute to placental damage. In addition, we demonstrate the efficacy of a commercially available inhibitor in limiting ZIKV infection, provides a proof-of-concept for blocking congenital infection by targeting metabolic pathways. Collectively, our study provides mechanistic insights on how ZIKV targets essential hubs of the lipid metabolism that may lead to placental dysfunction and loss of barrier function.

Zika virus (ZIKV) infection of pregnant women is associated with pregnancy loss and birth defects, but molecular insights for the aetiology are scarce. Here the authors show that ZIKV reprograms the host lipidome to facilitate viral replication, induce mitochondria dysfunction, and cause immune imbalance, thereby identifying a potential target for ZIKV therapy.

Dysregulation of Lipid Metabolism in Macrophages Is Responsible for Severe Endotoxin Tolerance in FcgRIIB-Deficient Lupus Mice

FcgRIIB dysfunction is commonly found in patients with lupus, especially in Asia. LPS-tolerance is prominent in FcgRIIB–/– lupus mice. LPS-tolerant macrophages demonstrate cell energy depletion, which might affect lipid metabolism. Therefore, to explore lipid metabolism, LPS-tolerance was induced twice by LPS administration in macrophages and in mice. LPS-tolerant FcgRIIB–/– macrophages demonstrated lesser mitochondrial DNA (mtDNA), more severe ATP depletion, lower cytokine production, and higher lipid accumulation (oil red O staining) compared to LPS-tolerant WT cells. Mass-spectrometry-based lipidomic analysis demonstrated a higher abundance of phosphatidylethanolamine (PE) phospholipid in LPS-tolerant FcgRIIB–/– macrophages than WT cells. This was at least in part due to the lower expression of phosphatidylethanolamine N-methyltransferase (pemt), an enzyme that converts PE to phosphatidylcholine (PC). Aminoimidazole-4-carboxamide ribonucleotide (AICAR), a pemt inhibitor, worsens LPS-tolerance in WT macrophages and supports the impact of pemt upon LPS-tolerant FcgRIIB–/– macrophages. Additionally, phosphorylated AMP-activated protein kinase (AMPK-p), a molecule for ATP-restoration associated with pemt, and phosphorylated acetyl CoA carboxylase, a downstream signaling of AMPK-p, were higher in LPS-tolerant FcgRIIB–/– macrophages than WT. Furthermore, Compound C, an AMPK inhibitor, attenuated LPS-tolerance in both FcgRIIB–/– macrophages and mice. Taken together, the intense decrease in cytokine production after the second LPS stimulation (LPS-tolerance) in FcgRIIB–/– macrophages was possibly due to the impact of an immense cytokine synthesis after the first dose of LPS. This includes using up PEMT, an enzyme of phospholipid synthesis during cytokine production, and AMPK-p induction in response to profound ATP-depletion. Therefore, the manipulation of the AMPK/PEMT axis provides a novel therapeutic candidate for the treatment of severe LPS-tolerance in lupus.

Regulation of lipid metabolism in pancreatic beta cells by interferon gamma: A link to anti-viral function

Interferons (IFN) have been shown to alter lipid metabolism in immune and some non-hematopoietic cells and this affects host cell response to pathogens. In type 1 diabetes, IFNγ acts as a proinflammatory cytokine that, along with other cytokines, is released during pancreatic beta cell autoinflammation and contributes to immune response and beta cell dysfunction. The hypothesis tested herein is that IFN modifies beta cell lipid metabolism and this is associated with enhanced anti-viral response and beta cell stress. Treatment of INS-1 cells with IFNγ for 6 to 24 h led to a dynamic change in TAG and lipid droplet (LD) levels, with a decrease at 6 h and an increase at 24 h. The later accumulation of TAG was associated with increased de novo lipogenesis (DNL), and impaired mitochondrial fatty acid oxidation (FAO). Gene expression results suggested that IFNγ regulates lipolytic, lipogenic, LD and FAO genes in a temporal manner. The changes in lipid gene expression are dependent on the classical Janus kinase (JAK) pathway. Pretreatment with IFNγ robustly enhanced anti-viral gene expression induced by the viral mimetic polyinosinic: polycytidylic acid (PIC), and this potentiating effect of IFNγ was markedly attenuated by inhibitors of DNL. The IFNγ-induced accumulation of lipid, however, was insufficient to cause endoplasmic reticulum (ER) stress. These studies demonstrated a non-canonical effect of IFNγ in regulation of pancreatic beta cell lipid metabolism that is intimately linked with host cell defense and might alter cellular function early in the progression to type 1 diabetes.

Reserve lipids and plant autophagy

Autophagy is a universal mechanism that facilitates the degradation of unwanted cytoplasmic components in eukaryotic cells. In this review, we highlight recent developments in the investigation of the role of autophagy in lipid homeostasis in plants by comparison with algae, yeast, and animals. We consider the storage compartments that form the sources of lipids in plants, and the roles that autophagy plays in the synthesis of triacylglycerols and in the formation and maintenance of lipid droplets. We also consider the relationship between lipids and the biogenesis of autophagosomes, and the role of autophagy in the degradation of lipids in plants.

Hepatic lipid droplet homeostasis and fatty liver disease

In cells, lipids are stored in lipid droplets, dynamic organelles that adapt their size, abundance, lipid and protein composition and organelle interactions to metabolic changes. Lipid droplet accumulation in the liver is the hallmark of non-alcoholic fatty liver disease (NAFLD). Due to the prevalence of obesity, the strongest risk factor for steatosis, NAFLD and its associated complications are currently affecting more than 1 billion people worldwide. Here, we review how triglyceride and phospholipid homeostasis are regulated in hepatocytes and how imbalances between lipid storage, degradation and lipoprotein secretion lead to NAFLD. We discuss how organelle interactions are altered in NAFLD and provide insights how NAFLD progression is associated with changes in hepatocellular signaling and organ-crosstalk. Finally, we highlight unsolved questions in hepatic LD and lipoprotein biology and give an outlook on therapeutic options counteracting hepatic lipid accumulation.

The biology of lipid droplet-bound mitochondria

Proper regulation of cellular lipid storage and oxidation is indispensable for the maintenance of cellular energy homeostasis and health. Mitochondrial function has been shown to be a main determinant of functional lipid storage and oxidation, which is of particular interest for the adipose tissue, as it is the main site of triacylglyceride storage in lipid droplets (LDs). Recent studies have identified a subpopulation of mitochondria attached to LDs, peridroplet mitochondria (PDM) that can be separated from cytoplasmic mitochondria (CM) by centrifugation. PDM have distinct bioenergetics, proteome, cristae organization and dynamics that support LD build-up, however their role in adipose tissue biology remains largely unexplored. Therefore, understanding the molecular basis of LD homeostasis and their relationship to mitochondrial function and attachment in adipocytes is of major importance.

Pemafibrate, a selective PPARα modulator, prevents non-alcoholic steatohepatitis development without reducing the hepatic triglyceride content

Non-alcoholic steatohepatitis (NASH) is characterized by macrovesicular steatosis with ballooning degeneration of hepatocytes, diffused lobular inflammation, and fibrosis. PPAR ligands are promising therapeutic agents in NASH; accordingly, we evaluated the effects of the first clinically available selective PPARα modulator, pemafibrate. We found that pemafibrate improves F4/80-positive macrophage accumulation, ballooning degeneration of hepatocytes, and the non-alcoholic fatty liver disease (NAFLD) activity score without affecting triglyceride (TG) accumulation in the liver of a mouse model of NASH (STAM). A global gene expression analysis indicated that pemafibrate enhances TG hydrolysis and fatty acid β-oxidation as well as re-esterification from dihydroxyacetone 3-phosphate and monoacylglycerol to TG. These changes are accompanied by the induction of genes involved in lipolysis and lipid droplet formation, along with an increased number and reduced size of lipid droplets in pemafibrate-treated livers. Pemafibrate reduced the expression of the cell adhesion molecule Vcam-1, myeloid cell markers, and inflammation- and fibrosis-related genes in STAM mice. Furthermore, pemafibrate significantly reduced VCAM-1 expression induced by high glucose in cultured human umbilical vein endothelial cells. These results suggest that pemafibrate prevents NASH development by reducing myeloid cell recruitment via interactions with liver sinusoidal endothelial cells, without altering hepatic TG accumulation.

Lipid droplets throughout the evolutionary tree

Intracellular lipid droplets are utilized for lipid storage and metabolism in organisms as evolutionarily diverse as animals, fungi, plants, bacteria, and archaea. These lipid droplets demonstrate great diversity in biological functions and protein and lipid compositions, yet fundamentally share common molecular and ultrastructural characteristics. Lipid droplet research has been largely fragmented across the diversity of lipid droplet classes and sub-classes. However, we suggest that there is great potential benefit to the lipid community in better integrating the lipid droplet research fields. To facilitate such integration, we survey the protein and lipid compositions, functional roles, and mechanisms of biogenesis across the breadth of lipid droplets studied throughout the natural world. We depict the big picture of lipid droplet biology, emphasizing shared characteristics and unique differences seen between different classes. In presenting the known diversity of lipid droplets side-by-side it becomes necessary to offer for the first time a consistent system of categorization and nomenclature. We propose a division into three primary classes that reflect their sub-cellular location: i) cytoplasmic lipid droplets (CYTO-LDs), that are present in the eukaryotic cytoplasm, ii) prokaryotic lipid droplets (PRO-LDs), that exist in the prokaryotic cytoplasm, and iii) plastid lipid droplets (PL-LDs), that are found in plant plastids, organelles of photosynthetic eukaryotes. Within each class there is a remarkable array of sub-classes displaying various sizes, shapes and compositions. A more integrated lipid droplet research field will provide opportunities to better build on discoveries and accelerate the pace of research in ways that have not been possible.

Blood cytokine patterns suggest a modest inflammation phenotype in subjects with long-chain fatty acid oxidation disorders

Excessive cellular accumulation or exposure to lipids such as long‐chain acylcarnitines (LCACs), ceramides, and others is implicated in cell stress and inflammation. Such a situation might manifest when there is a significant mismatch between long‐chain fatty acid (LCFA) availability versus storage and oxidative utilization; for example, in cardiac ischemia, increased LCACs may contribute to tissue cell stress and infarct damage. Perturbed LCFAβ‐oxidation is also seen in fatty acid oxidation disorders (FAODs). FAODs typically manifest with fasting‐ or stress‐induced symptoms, and patients can manage many symptoms through control of diet and physical activity. However, episodic clinical events involving cardiac and skeletal muscle myopathies are common and can present without an obvious molecular trigger. We have speculated that systemic or tissue‐specific lipotoxicity and activation of inflammation pathways contribute to long‐chain FAOD pathophysiology. With this in mind, we characterized inflammatory phenotype (14 blood plasma cytokines) in resting, overnight‐fasted (~10 h), or exercise‐challenged subjects with clinically well‐controlled long‐chain FAODs (n = 12; 10 long‐chain 3‐hydroxyacyl‐CoA dehydrogenase [LCHAD]; 2 carnitine palmitoyltransferase 2 [CPT2]) compared to healthy controls (n = 12). Across experimental conditions, concentrations of three cytokines were modestly but significantly increased in FAOD (IFNγ, IL‐8, and MDC), and plasma levels of IL‐10 (considered an inflammation‐dampening cytokine) were significantly decreased. These novel results indicate that while asymptomatic FAOD patients do not display gross body‐wide inflammation even after moderate exercise, β‐oxidation deficiencies might be associated with chronic and subtle activation of “sterile inflammation.” Further studies are warranted to determine if inflammation is more apparent in poorly controlled long‐chain FAOD or when long‐chain FAOD‐associated symptoms are present.

Microtubule-dependent and independent roles of spastin in lipid droplet dispersion and biogenesis

Lipid droplets (LDs) are metabolic organelles that store neutral lipids and dynamically respond to changes in energy availability by accumulating or mobilizing triacylglycerols (TAGs). How the plastic behavior of LDs is regulated is poorly understood. Hereditary spastic paraplegia is a central motor axonopathy predominantly caused by mutations in SPAST, encoding the microtubule-severing protein spastin. The spastin-M1 isoform localizes to nascent LDs in mammalian cells; however, the mechanistic significance of this targeting is not fully explained. Here, we show that tightly controlled levels of spastin-M1 are required to inhibit LD biogenesis and TAG accumulation. Spastin-M1 maintains the morphogenesis of the ER when TAG synthesis is prevented, independent from microtubule binding. Moreover, spastin plays a microtubule-dependent role in mediating the dispersion of LDs from the ER upon glucose starvation. Our results reveal a dual role of spastin to shape ER tubules and to regulate LD movement along microtubules, opening new perspectives for the pathogenesis of hereditary spastic paraplegia.