Biochemistry and pathophysiology of intravascular and intracellular lipolysis

Targeting lipid metabolism of cancer cells: A promising therapeutic strategy for cancer

One of the most important metabolic hallmarks of cancer cells is deregulation of lipid metabolism. In addition, enhancing de novo fatty acid (FA) synthesis, increasing lipid uptake and lipolysis have also been considered as means of FA acquisition in cancer cells. FAs are involved in various aspects of tumourigenesis and tumour progression. Therefore, targeting lipid metabolism is a promising therapeutic strategy for human cancer. Recent studies have shown that reprogramming lipid metabolism plays important roles in providing energy, macromolecules for membrane synthesis, and lipid signals during cancer progression. Moreover, accumulation of lipid droplets in cancer cells acts as a pivotal adaptive response to harmful conditions. Here, we provide a brief review of the crucial roles of FA metabolism in cancer development, and place emphasis on FA origin, utilization and storage in cancer cells. Understanding the regulation of lipid metabolism in cancer cells has important implications for exploring a new therapeutic strategy for management and treatment of cancer.

A 1-month-old infant with chylomicronemia due to GPIHBP1 gene mutation treated by plasmapheresis

Chylomicronemia is a severe type of hypertriglyceridemia characterized by chylomicron accumulation that arises from a genetic defect in intravascular lipolysis. It requires urgent and proper management, because serious cases can be accompanied by pancreatic necrosis or persistent multiple organ failure. We present the case of a 1-month-old infant with chylomicronemia treated by plasmapheresis. His chylomicronemia was discovered incidentally when lactescent plasma was noticed during routine blood sampling during a hospital admission for fever and irritability. Laboratory investigation revealed marked triglyceridemia (>5,000 mg/dL) with high chylomicron levels. We therefore decided to perform a therapeutic plasmapheresis to prevent acute pancreatitis. Sequence analysis revealed a homozygous novel mutation in exon 4 of GPIHBP1: c.476delG (p.Gly159Alafs). Glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1) stabilizes the binding of chylomicrons near lipoprotein lipase and supports lipolysis. Mutations of GPIHBP1, the most recently discovered gene, can lead to severe hyperlipidemia and are known to make up only 2% of the monogenic mutations associated with chylomicronemia. The patient maintains mild hypertriglyceridemia without rebound after single plasmapheresis and maintenance fibrate medication so far. Here, we report an infant with chylomicronemia due to GPIHBP1 mutation, successfully treated by plasmapheresis.

A novel frameshift mutation in the lipoprotein lipase gene is rescued by alternative messenger RNA splicing

BACKGROUND

Type I hyperlipoproteinemia, manifesting as chylomicronemia and severe hypertriglyceridemia, is a rare autosomal recessive disorder usually caused by mutations in the lipoprotein lipase gene (LPL).

OBJECTIVE

We sought to determine whether mutations in LPL could explain the clinical indications of a patient presenting with pancreatitis and hypertriglyceridemia.

METHODS

Coding regions of LPL were amplified by polymerase chain reaction and analyzed by nucleotide sequencing. The LPL messenger RNA transcript was also analyzed to investigate whether alternative splicing was occurring.

RESULTS

The patient was homozygous for the mutation c.767_768insTAAATATT in exon 5 of the LPL gene. This mutation is predicted to result in either a truncated nonfunctional LPL, or alternatively a new 5' donor splice site may be used, resulting in a full-length LPL with an in-frame deletion of 3 amino acids. Analysis of messenger RNA from the patient showed that the new splice site is used in vivo.

CONCLUSION

Homozygosity for a mutation in the LPL gene was consistent with the clinical findings. Use of the new splice site created by the insertion mutation rescues an otherwise damaging frameshift mutation, resulting in expression of an almost full-length LPL that is predicted to be partially functional. The patient therefore has a less severe form of type I hyperlipoproteinemia than would be expected if she lacked any functional LPL.

Lipid droplet dynamics and insulin sensitivity upon a 5-day high-fat diet in Caucasians and South Asians

A 5-day High-Fat High-Calorie diet (HFHC-diet) reduces insulin-stimulated glucose disposal (Rd) in South Asian, but not Caucasian healthy lean males. We aimed to investigate if differences in myocellular lipid handling are underlying this differential response. A two-step hyperinsulinemic-euglycemic clamp and muscle biopsies were performed in 12 healthy lean Caucasian and South Asian males (BMI < 25 kg/m², 19-25 years) before and after a 5-day HFHC-diet (regular diet + 375 mL cream/day; 1275 kcal/day; 94% fat). Triglyceride extractions and Western Blots for lipid droplet and mitochondrial proteins were performed. Intramyocellular lipid content and HFHC-diet response were similar between ethnicities (group effect: P = 0.094; diet effect: +~30%, P = 0.044). PLIN5 protein content increased upon the HFHC-diet (P = 0.031) and tended to be higher in South Asians (0.87 ± 0.42 AU vs. 1.35 ± 0.58 AU, P = 0.07). 4-HNE tended to increase in South Asians upon the HFHC-diet (interaction effect: P = 0.057). In Caucasians ΔPLIN5 content correlated with ΔR_d (Caucasians: r = 0.756, P = 0.011; South Asians: r = -0.085, P = 0.816), while in South Asians Δ4-HNE associated with ΔPLIN5 content (Caucasians: r = 0.312, P = 0.380; South Asians: r = 0.771, P = 0.003). These data indicate that in Caucasians, PLIN5 may be protective against HFHC-diet induced insulin resistance, which for reasons not yet understood is not observed in South Asians, who possess increased lipid peroxidation levels.

Lipid Droplet Biogenesis and Function in the Endothelium

RATIONALE

Fatty acids (FA) are transported across the capillary endothelium to parenchymal tissues. However, it is not known how endothelial cells (EC) from large vessels process a postprandial surge of FA.

OBJECTIVE

This study was designed to characterize lipid droplet (LD) formation in EC by manipulating pathways leading to the formation and degradation of LD. In addition, several functions of LD-derived FA were assessed.

METHODS AND RESULTS

LD were present in EC lining the aorta after the peak in plasma triglycerides initiated by a gavage of olive oil in mice, in vivo. Similarly, in isolated aorta, oleic acid treatment generates LD in EC ex vivo. Cultured EC readily form LD largely via the enzyme DGAT (diacylglycerol O-acyltransferase 1) and degrade LD via ATGL (adipocyte triglyceride lipase) after FA loading. Functionally, LD-derived FA are dynamically regulated and function to protect EC from lipotoxic stress and provide FA for metabolic needs.

CONCLUSIONS

Our results delineate endothelial LD dynamics for the first time in vivo and in vitro. Moreover, LD formation protects EC from lipotoxic stress, regulates EC glycolysis, and provides a source of FA for adjacent cells in the vessel wall or tissues.

Investigating Sitosterolemia to Understand Lipid Physiology

The cholesterol molecule is at the center of the pathophysiology of many vascular diseases. Whole-body cholesterol pools are maintained by a balance of endogenous synthesis, dietary absorption and elimination from our bodies. While the cellular aspects of cholesterol metabolism received significant impetus from the seminal work of Goldstein and Brown investigating LDL receptor trafficking, how dietary cholesterol was absorbed and eliminated was relatively neglected. The identification of the molecular defect a rare human disorder, Sitosterolemia, led to elucidation of a key mechanism of how we regulate the excretory pathway in the liver and in the intestine. Two proteins, ABCG5 and ABCG8, constitute a heterodimeric transporter that facilitates the extrusion of sterols from the cell into the biliary lumen, with a preference for xenosterols. This mechanism explained how dietary xenosterols are prevented from accumulating in our bodies. In addition, this disease has also highlighted the potential harm of xenosterols; macrothrombocytopenia, liver disease and endocrine disruption are seen when xenosterols accumulate. Mouse models of this disease suggest that there are more dramatic alterations of physiology, suggesting that these highly conserved mechanisms have evolved to prevent these xenosterols from accumulating in our bodies.

Tissue Immunometabolism: Development, Physiology, and Pathobiology

Evolution of metazoans resulted in the specialization of cellular and tissue function. This was accomplished by division of labor, which allowed tissue parenchymal cells to prioritize their core functions while ancillary functions were delegated to tissue accessory cells, such as immune, stromal, and endothelial cells. In metabolic organs, the accessory cells communicate with their clients, the tissue parenchymal cells, to optimize cellular processes, allowing organisms to adapt to changes in their environment. Here, we discuss tissue immunometabolism from this vantage point and use examples from adipose tissues (white, beige, and brown) and liver to outline the general principles by which accessory cells support metabolic homeostasis in parenchymal cells. A corollary of this model is that disruption of communication between client and accessory cells might predispose metabolic organs to the development of disease.

Lipoquality control by phospholipase A2 enzymes

The phospholipase A₂ (PLA₂) family comprises a group of lipolytic enzymes that typically hydrolyze the sn-2 position of glycerophospholipids to give rise to fatty acids and lysophospholipids. The mammalian genome encodes more than 50 PLA₂s or related enzymes, which are classified into several subfamilies on the basis of their structures and functions. From a general viewpoint, the PLA₂ family has mainly been implicated in signal transduction, producing bioactive lipid mediators derived from fatty acids and lysophospholipids. Recent evidence indicates that PLA₂s also contribute to phospholipid remodeling for membrane homeostasis or energy production for fatty acid β-oxidation. Accordingly, PLA₂ enzymes can be regarded as one of the key regulators of the quality of lipids, which I herein refer to as lipoquality. Disturbance of PLA₂-regulated lipoquality hampers tissue and cellular homeostasis and can be linked to various diseases. Here I overview the current state of understanding of the classification, enzymatic properties, and physiological functions of the PLA₂ family.

Lipoprotein lipase activity and interactions studied in human plasma by isothermal titration calorimetry

LPL hydrolyzes triglycerides in plasma lipoproteins. Due to the complex regulation mechanism, it has been difficult to mimic the physiological conditions under which LPL acts in vitro. We demonstrate that isothermal titration calorimetry (ITC), using human plasma as substrate, overcomes several limitations of previously used techniques. The high sensitivity of ITC allows continuous recording of the heat released during hydrolysis. Both initial rates and kinetics for complete hydrolysis of plasma lipids can be studied. The heat rate was shown to correspond to the release of fatty acids and was linearly related to the amount of added enzyme, either purified LPL or postheparin plasma. Addition of apoC-III reduced the initial rate of hydrolysis by LPL, but the inhibition became less prominent with time when the lipoproteins were triglyceride poor. Addition of angiopoietin-like protein (ANGPTL)3 or ANGPTL4 caused reduction of the activity of LPL via a two-step mechanism. We conclude that ITC can be used for quantitative measurements of LPL activity and interactions under in vivo-like conditions, for comparisons of the properties of plasma samples from patients and control subjects as substrates for LPL, as well as for testing of drug candidates developed with the aim to affect the LPL system.

The ANGPTL3-4-8 model, a molecular mechanism for triglyceride trafficking

Lipoprotein lipase (LPL) is a rate-limiting enzyme for hydrolysing circulating triglycerides (TG) into free fatty acids that are taken up by peripheral tissues. Postprandial LPL activity rises in white adipose tissue (WAT), but declines in the heart and skeletal muscle, thereby directing circulating TG to WAT for storage; the reverse is true during fasting. However, the mechanism for the tissue-specific regulation of LPL activity during the fed–fast cycle has been elusive. Recent identification of lipasin/angiopoietin-like 8 (Angptl8), a feeding-induced hepatokine, together with Angptl3 and Angptl4, provides intriguing, yet puzzling, insights, because all the three Angptl members are LPL inhibitors, and the deficiency (overexpression) of any one causes hypotriglyceridaemia (hypertriglyceridaemia). Then, why does nature need all of the three? Our recent data that Angptl8 negatively regulates LPL activity specifically in cardiac and skeletal muscles suggest an Angptl3-4-8 model: feeding induces Angptl8, activating the Angptl8–Angptl3 pathway, which inhibits LPL in cardiac and skeletal muscles, thereby making circulating TG available for uptake by WAT, in which LPL activity is elevated owing to diminished Angptl4; the reverse is true during fasting, which suppresses Angptl8 but induces Angptl4, thereby directing TG to muscles. The model suggests a general framework for how TG trafficking is regulated.

Divergent functions of endotrophin on different cell populations in adipose tissue

Endotrophin is a cleavage product of collagen 6 (Col6) in adipose tissue (AT). Previously, we demonstrated that endotrophin serves as a costimulator to trigger fibrosis and inflammation within the unhealthy AT milieu. However, how endotrophin affects lipid storage and breakdown in AT and how different cell types in AT respond to endotrophin stimulation remain unknown. In the current study, by using a doxycycline-inducible mouse model, we observed significant upregulation of adipogenic genes in the white AT (WAT) of endotrophin transgenic mice. We further showed that the mice exhibited inhibited lipolysis and accelerated hypertrophy and hyperplasia in WAT. To investigate the effects of endotrophin in vitro, we incubated different cell types from AT with conditioned medium from endotrophin-overexpressing 293T cells. We found that endotrophin activated multiple pathological pathways in different cell types. Particularly in 3T3-L1 adipocytes, endotrophin triggered a fibrotic program by upregulating collagen genes and promoted abnormal lipid accumulation by downregulating hormone-sensitive lipolysis gene and decreasing HSL phosphorylation levels. In macrophages isolated from WAT, endotrophin stimulated higher expression of the collagen-linking enzyme lysyl oxidase and M1 proinflammatory marker genes. In the stromal vascular fraction isolated from WAT, endotrophin induced upregulation of both profibrotic and proinflammatory genes. In conclusion, our study provides a new perspective on the effect of endotrophin in abnormal lipid accumulation and a mechanistic insight into the roles played by adipocytes and a variety of other cell types in AT in shaping the unhealthy microenvironment upon endotrophin treatment.

Oleic acid-induced ANGPTL4 enhances head and neck squamous cell carcinoma anoikis resistance and metastasis via up-regulation of fibronectin

Obese patients have higher levels of free fatty acids (FFAs) in their plasma and a higher risk of cancer than their non-obese counterparts. However, the mechanisms involved in the regulation of cancer metastasis by FFAs remain unclear. In this study, we found that oleic acid (OA) induced angiopoietin-like 4 (ANGPTL4) protein expression and secretion and conferred anoikis resistance to head and neck squamous cell carcinomas (HNSCCs). The autocrine production of OA-induced ANGPTL4 further promoted HNSCC migration and invasion. In addition, the expression of peroxisome proliferator-activated receptor (PPAR) was essential for the OA-induced ANGPTL4 expression and invasion. The levels of OA-induced epithelial-mesenchymal transition markers, such as vimentin, MMP-9, and fibronectin and its downstream effectors Rac1/Cdc42, were significantly reduced in ANGPTL4-depleted cells. Knocking down fibronectin inhibited the expression of MMP-9 and repressed OA- and recombinant ANGPTL4-induced HNSCC invasion. On the other hand, ANGPTL4 siRNA inhibited OA-induced MMP-9 expression, which was reversed in fibronectin-overexpressing cells. Furthermore, the depletion of ANGPTL4 impeded the OA-primed metastatic seeding of tumor cells in the lungs. These results demonstrate that OA enhances HNSCC metastasis through the ANGPTL4/fibronectin/Rac1/Cdc42 and ANGPTL4/fibronectin/MMP-9 signaling axes.

Adipose Snail1 Regulates Lipolysis and Lipid Partitioning by Suppressing Adipose Triacylglycerol Lipase Expression

Lipolysis provides metabolic fuel; however, aberrant adipose lipolysis results in ectopic lipid accumulation and lipotoxicity. While adipose triacylglycerol lipase (ATGL) catalyzes the first step of lipolysis, its regulation is not fully understood. Here, we demonstrate that adipocyte Snail1 suppresses both ATGL expression and lipolysis. Adipose Snail1 levels are higher in fed mice than in fasted mice and higher in obese mice as opposed to lean mice. Insulin increases Snail1 levels in both murine and human adipocytes, wherein Snail1 binds to the ATGL promoter to repress its expression. Importantly, adipocyte-specific deletion of Snail1 increases adipose ATGL expression and lipolysis, resulting in decreased fat mass and increased liver fat content in mice fed either a normal chow diet or a high-fat diet. Thus, we have identified a Snail1-ATGL axis that regulates adipose lipolysis and fatty acid release, thereby governing lipid partitioning between adipose and non-adipose tissues.

An LPL-specific monoclonal antibody, 88B8, that abolishes the binding of LPL to GPIHBP1

LPL contains two principal domains: an amino-terminal catalytic domain (residues 1-297) and a carboxyl-terminal domain (residues 298-448) that is important for binding lipids and binding glycosylphosphatidylinositol-anchored high density lipoprotein binding protein 1 (GPIHBP1) (an endothelial cell protein that shuttles LPL to the capillary lumen). The LPL sequences required for GPIHBP1 binding have not been examined in detail, but one study suggested that sequences near LPL's carboxyl terminus (residues ∼403-438) were crucial. Here, we tested the ability of LPL-specific monoclonal antibodies (mAbs) to block the binding of LPL to GPIHBP1. One antibody, 88B8, abolished LPL binding to GPIHBP1. Consistent with those results, antibody 88B8 could not bind to GPIHBP1-bound LPL on cultured cells. Antibody 88B8 bound poorly to LPL proteins with amino acid substitutions that interfered with GPIHBP1 binding (e.g., C418Y, E421K). However, the sequences near LPL's carboxyl terminus (residues ∼403-438) were not sufficient for 88B8 binding; upstream sequences (residues 298-400) were also required. Additional studies showed that these same sequences are required for LPL binding to GPIHBP1. In conclusion, we identified an LPL mAb that binds to LPL's GPIHBP1-binding domain. The binding of both antibody 88B8 and GPIHBP1 to LPL depends on large segments of LPL's carboxyl-terminal domain.

Implications of thermogenic adipose tissues for metabolic health

Excess and ectopic fat accumulation in obesity is a major risk factor for developing hyperlipidemia, type 2 diabetes and cardiovascular disease. The activation of brown and/or beige adipocytes is a promising target for the treatment of metabolic disorders as the combustion of excess energy by these thermogenic adipocytes may help losing weight and improving plasma parameters including triglyceride, cholesterol and glucose levels. The regulation of heat production by thermogenic adipose tissues is based on a complex crosstalk between the autonomous nervous system, intracellular and secreted factors. This multifaceted alignment regulates thermogenic demands to environmental circumstances in dependence on available energy resources. This review summarizes the current knowledge how thermogenic tissues can be targeted to combat the burden of diseases with a special focus on lipid metabolism and diseases related to lipoprotein metabolism.

Mechanisms of metabolic dysfunction in cancer-associated cachexia

Metabolic dysfunction contributes to the clinical deterioration observed in advanced cancer patients and is characterized by weight loss, skeletal muscle wasting, and atrophy of the adipose tissue. This systemic syndrome, termed cancer-associated cachexia (CAC), is a major cause of morbidity and mortality. While once attributed solely to decreased food intake, the present description of cancer cachexia is a disorder of multiorgan energy imbalance. Here we review the molecules and pathways responsible for metabolic dysfunction in CAC and the ideas that led to the current understanding.

Peroxisome proliferator-activated receptor γ activation favours selective subcutaneous lipid deposition by coordinately regulating lipoprotein lipase modulators, fatty acid transporters and lipogenic enzymes

AIM

Peroxisome proliferator-activated receptor (PPAR) γ activation is associated with preferential lipoprotein lipase (LPL)-mediated fatty acid storage in peripheral subcutaneous fat depots. How PPARγ agonism acts upon the multi-level modulation of depot-specific lipid storage remains incompletely understood.

METHODS

We evaluated herein triglyceride-derived lipid incorporation into adipose tissue depots, LPL mass and activity, mRNA levels and content of proteins involved in the modulation of LPL activity and fatty acid transport, and the expression/activity of enzymes defining adipose tissue lipogenic potential in rats treated with the PPARγ ligand rosiglitazone (30 mg kg(-1)  day(-1) , 23 days) after either a 10-h fasting period or a 17-h fast followed by 6 h of ad libitum refeeding.

RESULTS

Rosiglitazone stimulated lipid accretion in subcutaneous fat (SF) ~twofold and significantly reduced that of visceral fat (VF) to nearly half. PPARγ activation selectively increased LPL mass, activity and the expression of its chaperone LMF1 in SF. In VF, rosiglitazone had no effect on LPL activity and downregulated the mRNA levels of the transendothelial transporter GPIHBP1. Overexpression of lipid uptake and fatty acid transport proteins (FAT/CD36, FATP1 and FABP4) and stimulation of lipogenic enzyme activities (GPAT, AGPAT and DGAT) upon rosiglitazone treatment were of higher magnitude in SF.

CONCLUSIONS

Together these findings demonstrate that the depot-specific transcriptional control of LPL induced by PPARγ activation extends to its key interacting proteins and post-translational modulators to favour subcutaneous lipid storage.

Inositol hexakisphosphate kinase-1 interacts with perilipin1 to modulate lipolysis

Lipolysis leads to the breakdown of stored triglycerides (TAG) to release free fatty acids (FFA) and glycerol which is utilized by energy expenditure pathways to generate energy. Therefore, a decrease in lipolysis augments fat accumulation in adipocytes which promotes weight gain. Conversely, if lipolysis is not complemented by energy expenditure, it leads to FFA induced insulin resistance and type-2 diabetes. Thus, lipolysis is under stringent physiological regulation, although the precise mechanism of the regulation is not known. Deletion of inositol hexakisphosphate kinase-1 (IP6K1), the major inositol pyrophosphate biosynthetic enzyme, protects mice from high fat diet (HFD) induced obesity and insulin resistance. IP6K1-KO mice are lean due to enhanced energy expenditure. Therefore, IP6K1 is a target in obesity and type-2 diabetes. However, the mechanism/s by which IP6K1 regulates adipose tissue lipid metabolism is yet to be understood. Here, we demonstrate that IP6K1-KO mice display enhanced basal lipolysis. IP6K1 modulates lipolysis via its interaction with the lipolytic regulator protein perilipin1 (PLIN1). Furthermore, phosphorylation of IP6K1 at a PKC/PKA motif modulates its interaction with PLIN1 and lipolysis. Thus, IP6K1 is a novel regulator of PLIN1 mediated lipolysis.

Lipids, lysosomes, and autophagy

Lipids are essential components of a cell providing energy substrates for cellular processes, signaling intermediates, and building blocks for biological membranes. Lipids are constantly recycled and redistributed within a cell. Lysosomes play an important role in this recycling process that involves the recruitment of lipids to lysosomes via autophagy or endocytosis for their degradation by lysosomal hydrolases. The catabolites produced are redistributed to various cellular compartments to support basic cellular function. Several studies demonstrated a bidirectional relationship between lipids and lysosomes that regulate autophagy. While lysosomal degradation pathways regulate cellular lipid metabolism, lipids also regulate lysosome function and autophagy. In this review, we focus on this bidirectional relationship in the context of dietary lipids and provide an overview of recent evidence of how lipid-overload lipotoxicity, as observed in obesity and metabolic syndrome, impairs lysosomal function and autophagy that may eventually lead to cellular dysfunction or cell death.

Endothelial cell-cardiomyocyte crosstalk in diabetic cardiomyopathy

The incidence of diabetes is increasing globally, with cardiovascular disease accounting for a substantial number of diabetes-related deaths. Although atherosclerotic vascular disease is a primary reason for this cardiovascular dysfunction, heart failure in patients with diabetes might also be an outcome of an intrinsic heart muscle malfunction, labelled diabetic cardiomyopathy. Changes in cardiomyocyte metabolism, which encompasses a shift to exclusive fatty acid utilization, are considered a leading stimulus for this cardiomyopathy. In addition to cardiomyocytes, endothelial cells (ECs) make up a significant proportion of the heart, with the majority of ATP generation in these cells provided by glucose. In this review, we will discuss the metabolic machinery that drives energy metabolism in the cardiomyocyte and EC, its breakdown following diabetes, and the research direction necessary to assist in devising novel therapeutic strategies to prevent or delay diabetic heart disease.

Role of lipoprotein lipase in lipid metabolism

PURPOSE OF REVIEW

A major step in energy metabolism is hydrolysis of triacylglycerol-rich lipoproteins (TRLs) to release fatty acids that can be used or stored. This is accomplished by lipoprotein lipase (LPL) at 'binding lipolysis sites' at the vascular endothelium. A multitude of interactions are involved in this seemingly simple reaction. Recent advances in the understanding of some of these factors will be discussed in an attempt to build a comprehensive picture.

RECENT FINDINGS

The first event in catabolism of TRLs is that they dock at the vascular endothelium. This requires LPL and GPIHBP1, the endothelial transporter of LPL.Kinetic studies in rats with labeled chylomicrons showed that once a chylomicron has docked in the heart it stays for minutes and a large number of triacylglycerol molecules are split. The distribution of binding between tissues reflects the amount of LPL, as evident from studies with mutant mice.Clearance of TRLs is often slowed down in metabolic disease, as was demonstrated both in mice and men. In mice, this was directly connected to decreased amounts of endothelial LPL.

SUMMARY

The LPL system is central in energy metabolism and results from interplay between several factors. Rapid and exciting progress is being made.

Metabolic interplay between white, beige, brown adipocytes and the liver

In mammalian evolution, three types of adipocytes have developed, white, brown and beige adipocytes. White adipocytes are the major constituents of white adipose tissue (WAT), the predominant store for energy-dense triglycerides in the body that are released as fatty acids during catabolic conditions. The less abundant brown adipocytes, the defining parenchymal cells of brown adipose tissue (BAT), internalize triglycerides that are stored intracellularly in multilocular lipid droplets. Beige adipocytes (also known as brite or inducible brown adipocytes) are functionally very similar to brown adipocytes and emerge in specific WAT depots in response to various stimuli including sustained cold exposure. The activation of brown and beige adipocytes (together referred to as thermogenic adipocytes) causes both the hydrolysis of stored triglycerides as well as the uptake of lipids and glucose from the circulation. Together, these fuels are combusted for heat production to maintain body temperature in mammals including adult humans. Given that heating by brown and beige adipocytes is a very-well controlled and energy-demanding process which entails pronounced shifts in energy fluxes, it is not surprising that an intensive interplay exists between the various adipocyte types and parenchymal liver cells, and that this influences systemic metabolic fluxes and endocrine networks. In this review we will emphasize the role of hepatic factors that regulate the metabolic activity of white and thermogenic adipocytes. In addition, we will discuss the relevance of lipids and hormones that are secreted by white, brown and beige adipocytes regulating liver metabolism in order to maintain systemic energy metabolism in health and disease.

Recent discoveries on absorption of dietary fat: Presence, synthesis, and metabolism of cytoplasmic lipid droplets within enterocytes

Dietary fat provides essential nutrients, contributes to energy balance, and regulates blood lipid concentrations. These functions are important to health, but can also become dysregulated and contribute to diseases such as obesity, diabetes, cardiovascular disease, and cancer. Within enterocytes, the digestive products of dietary fat are re-synthesized into triacylglycerol, which is either secreted on chylomicrons or stored within cytoplasmic lipid droplets (CLDs). CLDs were originally thought to be inert stores of neutral lipids, but are now recognized as dynamic organelles that function in multiple cellular processes in addition to lipid metabolism. This review will highlight recent discoveries related to dietary fat absorption with an emphasis on the presence, synthesis, and metabolism of CLDs within this process.

Role of sulfatase 2 in lipoprotein metabolism and angiogenesis

PURPOSE OF REVIEW

This article summarizes the current evidence to support a role of sulfatase 2 (SULF2) in triglyceride-rich lipoprotein (TRL) metabolism and angiogenesis.

RECENT FINDINGS

Heparan sulfate proteoglycans (HSPG) are involved in the hepatic clearance of TRLs in mice and in humans. Different genetically modified mouse models have been instrumental to provide evidence that syndecan1, the core protein of HSPG, but also the degree of sulfation of the heparin sulfate chain, attached to syndecan 1, is important for hepatic TRL metabolism. Studies in humans demonstrate the regulating role of SULF2 in the hepatic uptake of TRL by HSPG and demonstrate the importance of 6-O-sulfation, modulated by SULF2, for HSPG function. The role of SULF2 in angiogenesis is illustrated by increased SULF2 mRNA expression in the stalk cells of angiogenic vascular sprouts that use fatty acids derived from TRL as a source for biomass production. Interestingly, SULF2 also interferes with HSPG-vascular endothelial growth factor binding, which impacts upon the angiogenic properties of stalk cells.

SUMMARY

SULF2 is a multifaceted protein involved in TRL homeostasis and angiogenesis. Future investigations should focus on the potential benefits of targeting SULF2 in atherosclerosis and angiogenesis.

Angiopoietin-like 4 promotes intracellular degradation of lipoprotein lipase in adipocytes

LPL hydrolyzes triglycerides in triglyceride-rich lipoproteins along the capillaries of heart, skeletal muscle, and adipose tissue. The activity of LPL is repressed by angiopoietin-like 4 (ANGPTL4) but the underlying mechanisms have not been fully elucidated. Our objective was to study the cellular location and mechanism for LPL inhibition by ANGPTL4. We performed studies in transfected cells, ex vivo studies, and in vivo studies with Angptl4(-/-) mice. Cotransfection of CHO pgsA-745 cells with ANGPTL4 and LPL reduced intracellular LPL protein levels, suggesting that ANGPTL4 promotes LPL degradation. This conclusion was supported by studies of primary adipocytes and adipose tissue explants from wild-type and Angptl4(-/-) mice. Absence of ANGPTL4 resulted in accumulation of the mature-glycosylated form of LPL and increased secretion of LPL. Blocking endoplasmic reticulum (ER)-Golgi transport abolished differences in LPL abundance between wild-type and Angptl4(-/-) adipocytes, suggesting that ANGPTL4 acts upon LPL after LPL processing in the ER. Finally, physiological changes in adipose tissue ANGPTL4 expression during fasting and cold resulted in inverse changes in the amount of mature-glycosylated LPL in wild-type mice, but not Angptl4(-/-) mice. We conclude that ANGPTL4 promotes loss of intracellular LPL by stimulating LPL degradation after LPL processing in the ER.

Conserved Amphipathic Helices Mediate Lipid Droplet Targeting of Perilipins 1-3

Perilipins (PLINs) play a key role in energy storage by orchestrating the activity of lipases on the surface of lipid droplets. Failure of this activity results in severe metabolic disease in humans. Unlike all other lipid droplet-associated proteins, PLINs localize almost exclusively to the phospholipid monolayer surrounding the droplet. To understand how they sense and associate with the unique topology of the droplet surface, we studied the localization of human PLINs inSaccharomyces cerevisiae,demonstrating that the targeting mechanism is highly conserved and that 11-mer repeat regions are sufficient for droplet targeting. Mutations designed to disrupt folding of this region into amphipathic helices (AHs) significantly decreased lipid droplet targetingin vivoandin vitro Finally, we demonstrated a substantial increase in the helicity of this region in the presence of detergent micelles, which was prevented by an AH-disrupting missense mutation. We conclude that highly conserved 11-mer repeat regions of PLINs target lipid droplets by folding into AHs on the droplet surface, thus enabling PLINs to regulate the interface between the hydrophobic lipid core and its surrounding hydrophilic environment.

Clinical and genetic features of 3 patients with familial chylomicronemia due to mutations in GPIHBP1 gene

BACKGROUND

Familial chylomicronemia is a recessive disorder that may be due to mutations in lipoprotein lipase (LPL) and in other proteins such as apolipoprotein C-II and apolipoprotein A-V (activators of LPL), GPIHBP1 (the molecular platform required for LPL activity on endothelial surface), and LMF1 (a factor required for intracellular formation of active LPL).

METHODS

We sequenced the familial chylomicronemia candidate genes in 2 adult females presenting long-standing hypertriglyceridemia and a history of acute pancreatitis.

RESULTS

Both probands had plasma triglyceride >10 mmol/L but no mutations in the LPL gene. The sequence of the other candidate genes showed that one patient was homozygous for a novel missense mutation p.(Cys83Arg), and the other was homozygous for a previously reported nonsense mutation p.(Cys 89*), respectively, in GPIHBP1. Family screening showed that the hypertriglyceridemic brother of the p.(Cys83Arg) homozygote was also homozygous for this mutation. He had no history of pancreatitis. The p.(Cys83Arg) heterozygous carriers had normal triglyceride levels. The substitution of a cysteine residue in the Ly6 domain of GPIHBP1 is predicted to abolish one of the disulfide bridges required to maintain the structure of GPIHBP1. The p.(Cys89*) mutation results in a truncated protein devoid of function.

CONCLUSIONS

Both mutant GPIHBP1 proteins are expected to be incapable of transferring LPL from the subendothelial space to the endothelial surface.

Identification and characterization of two novel mutations in the LPL gene causing type I hyperlipoproteinemia

BACKGROUND

Type 1 hyperlipoproteinemia is a rare autosomal recessive disorder most often caused by mutations in the lipoprotein lipase (LPL) gene resulting in severe hypertriglyceridemia and pancreatitis.

OBJECTIVES

The aim of this study was to identify novel mutations in the LPL gene causing type 1 hyperlipoproteinemia and to understand the molecular mechanisms underlying the severe hypertriglyceridemia.

METHODS

Three patients presenting classical features of type 1 hyperlipoproteinemia were recruited for DNA sequencing of the LPL gene. Pre-heparin and post-heparin plasma of patients were used for protein detection analysis and functional test. Furthermore, in vitro experiments were performed in HEK293 cells. Protein synthesis and secretion were analyzed in lysate and medium fraction, respectively, whereas medium fraction was used for functional assay.

RESULTS

We identified two novel mutations in the LPL gene causing type 1 hyperlipoproteinemia: a two base pair deletion (c.765_766delAG) resulting in a frameshift at position 256 of the protein (p.G256TfsX26) and a nucleotide substitution (c.1211 T > G) resulting in a methionine to arginine substitution (p.M404 R). LPL protein and activity were not detected in pre-heparin or post-heparin plasma of the patient with p.G256TfsX26 mutation or in the medium of HEK293 cells over-expressing recombinant p.G256TfsX26 LPL. A relatively small amount of LPL p.M404 R was detected in both pre-heparin and post-heparin plasma and in the medium of the cells, whereas no LPL activity was detected.

CONCLUSIONS

We conclude that these two novel mutations cause type 1 hyperlipoproteinemia by inducing a loss or reduction in LPL secretion accompanied by a loss of LPL enzymatic activity.

Molecular and biochemical characterizations of the monoacylglycerol lipase gene family of Arabidopsis thaliana

Monoacylglycerol lipase (MAGL) catalyzes the last step of triacylglycerol breakdown, which is the hydrolysis of monoacylglycerol (MAG) to fatty acid and glycerol. Arabidopsis harbors over 270 genes annotated as 'lipase', the largest class of acyl lipid metabolism genes that have not been characterized experimentally. In this study, computational modeling suggested that 16 Arabidopsis putative MAGLs (AtMAGLs) have a three-dimensional structure that is similar to a human MAGL. Heterologous expression and enzyme assays indicated that 11 of the 16 encoded proteins indeed possess MAG lipase activity. Additionally, AtMAGL4 displayed hydrolase activity with lysophosphatidylcholine and lysophosphatidylethanolamine (LPE) substrates and AtMAGL1 and 2 utilized LPE as a substrate. All recombinant AtMAGLs preferred MAG substrates with unsaturated fatty acids over saturated fatty acids and AtMAGL8 exhibited the highest hydrolase activities with MAG containing 20:1 fatty acids. Except for AtMAGL4, -14 and -16, all AtMAGLs showed similar activity with both sn-1 and sn-2 MAG isomers. Spatial, temporal and stress-induced expression of the 16 AtMAGL genes was analyzed by transcriptome analyses. AtMAGL:eYFP fusion proteins provided initial evidence that AtMAGL1, -3, -6, -7, -8, -11, -13, -14 and -16 are targeted to the endoplasmic reticulum and/or Golgi network, AtMAGL10, -12 and -15 to the cytosol and AtMAGL2, -4 and -5 to the chloroplasts. Furthermore, AtMAGL8 was associated with the surface of oil bodies in germinating seeds and leaves accumulating oil bodies. This study provides the broad characterization of one of the least well-understood groups of Arabidopsis lipid-related enzymes and will be useful for better understanding their roles in planta.

Antioxidant Role for Lipid Droplets in a Stem Cell Niche of Drosophila

Stem cells reside in specialized microenvironments known as niches. During Drosophila development, glial cells provide a niche that sustains the proliferation of neural stem cells (neuroblasts) during starvation. We now find that the glial cell niche also preserves neuroblast proliferation under conditions of hypoxia and oxidative stress. Lipid droplets that form in niche glia during oxidative stress limit the levels of reactive oxygen species (ROS) and inhibit the oxidation of polyunsaturated fatty acids (PUFAs). These droplets protect glia and also neuroblasts from peroxidation chain reactions that can damage many types of macromolecules. The underlying antioxidant mechanism involves diverting PUFAs, including diet-derived linoleic acid, away from membranes to the core of lipid droplets, where they are less vulnerable to peroxidation. This study reveals an antioxidant role for lipid droplets that could be relevant in many different biological contexts.

•

Oxidative stress stimulates lipid droplet biosynthesis in a neural stem cell niche

•

Lipid droplets protect niche and neural stem cells from damaging PUFA peroxidation

•

PUFAs are less vulnerable to peroxidation in lipid droplets than in cell membranes

Endocannabinoid regulation in white and brown adipose tissue following thermogenic activation

The endocannabinoids and their main receptor, cannabinoid type-1 (CB1), suppress intracellular cyclic AMP levels and have emerged as key players in the control of energy metabolism. CB1 agonists and blockers have been reported to influence the thermogenic function of white and brown adipose tissue (WAT and BAT), affecting body weight through the inhibition and stimulation of energy expenditure, respectively. The purpose of the current study was to investigate the regulation of the endocannabinoid system in WAT and BAT following exposure to either cold or specific agonism of β3-adrenoceptors using CL316,243 (CL), conditions known to cause BAT activation and WAT browning. To address this question, we performed quantitative PCR-based mRNA profiling of genes important for endocannabinoid synthesis, degradation, and signaling, and determined endocannabinoid levels by LC-MS in WAT and BAT of control, cold-exposed, and CL-treated wild-type mice as well as primary brown adipocytes. Treatment with CL and exposure to cold caused an upregulation of endocannabinoid levels and biosynthetic enzymes in WAT. Acute β3-adrenoceptor activation increased endocannabinoids and a subset of genes of biosynthesis in BAT and primary brown adipocytes. We suggest that the cold-mediated increase in endocannabinoid tone is part of autocrine negative feed-back mechanisms controlling β3-adrenoceptor-induced BAT activation and WAT browning.

The acidic domain of the endothelial membrane protein GPIHBP1 stabilizes lipoprotein lipase activity by preventing unfolding of its catalytic domain

GPIHBP1 is a glycolipid-anchored membrane protein of capillary endothelial cells that binds lipoprotein lipase (LPL) within the interstitial space and shuttles it to the capillary lumen. The LPL•GPIHBP1 complex is responsible for margination of triglyceride-rich lipoproteins along capillaries and their lipolytic processing. The current work conceptualizes a model for the GPIHBP1•LPL interaction based on biophysical measurements with hydrogen-deuterium exchange/mass spectrometry, surface plasmon resonance, and zero-length cross-linking. According to this model, GPIHBP1 comprises two functionally distinct domains: (1) an intrinsically disordered acidic N-terminal domain; and (2) a folded C-terminal domain that tethers GPIHBP1 to the cell membrane by glycosylphosphatidylinositol. We demonstrate that these domains serve different roles in regulating the kinetics of LPL binding. Importantly, the acidic domain stabilizes LPL catalytic activity by mitigating the global unfolding of LPL's catalytic domain. This study provides a conceptual framework for understanding intravascular lipolysis and GPIHBP1 and LPL mutations causing familial chylomicronemia.

DOI:http://dx.doi.org/10.7554/eLife.12095.001

Fat is an important part of our diet. The intestines absorb fats and package them into particles called lipoproteins. After reaching the bloodstream, the fat molecules (lipids) in the lipoproteins are broken down by an enzyme called lipoprotein lipase (LPL), which is located along the surface of small blood vessels. This releases nutrients that can be used by vital tissues – mainly the heart, skeletal muscle, and adipose tissues. LPL is produced by muscle and adipose tissue, but it is quickly swept up by a protein called GPIHBP1 and shuttled to its site of action inside the blood vessels.

Mutations that alter the structure of LPL or GPIHBP1 can prevent the breakdown of lipids, resulting in high levels of lipids in the blood. This can lead to inflammation in the pancreas and also increases the risk of heart attacks and strokes. Many earlier studies have examined the properties of LPL, but our understanding of GPIHBP1 has been limited, mainly because it has been difficult to purify GPIHBP1 for analysis.

Using genetically altered insect cells, Mysling et al. were able to purify two different forms of GPIHBP1 – a full-length version and a shorter version that lacked a small section at the end of the molecule known as the acidic domain. This revealed that the opposite end of the molecule – called the carboxyl-terminal domain – is primarily responsible for binding LPL and anchoring it inside blood vessels. Once LPL is bound to GPIHBP1, the acidic domain of GPIHBP1 helps to stabilize LPL. If GPIHBP1’s acidic domain is missing then LPL is more susceptible to losing its structure, rendering it incapable of breaking down the lipids in the blood.

Mysling et al. describe a new model for how LPL and GPIHBP1 interact that explains how specific mutations in the genes that encode these proteins interfere with the delivery of LPL to small blood vessels. In the future, this could help researchers to develop new strategies to treat people with high levels of lipids in their blood.

DOI:http://dx.doi.org/10.7554/eLife.12095.002

Zebrafish Models for Dyslipidemia and Atherosclerosis Research

Atherosclerotic cardiovascular disease is the leading cause of death. Elevated circulating concentrations of lipids are a central pathogenetic driver of atherosclerosis. While numerous effective therapies for this condition have been developed, there is substantial unmet need for this pandemic illness. Here, I will review nutritional, physiological, genetic, and pathological discoveries in the emerging zebrafish model for studying dyslipidemia and atherosclerosis. The technical and physiological advantages and the pharmacological potential of this organism for discovery and validation of dyslipidemia and atherosclerosis targets are stressed through summary of recent findings. An emerging literature shows that zebrafish, through retention of a cetp ortholog gene and high sensitivity to ingestion of excess cholesterol, rapidly develops hypercholesterolemia, with a pattern of distribution of lipid species in lipoprotein particles similar to humans. Furthermore, recent studies leveraging the optical transparency of zebrafish larvae to monitor the fate of these ingested lipids have provided exciting insights to the development of dyslipidemia and atherosclerosis. Future directions for investigation are considered, with particular attention to the potential for in vivo cell biological study of atherosclerotic plaques.

Pathogenic classification of LPL gene variants reported to be associated with LPL deficiency

BACKGROUND

Lipoprotein lipase (LPL) deficiency is a serious lipid disorder of severe hypertriglyceridemia (SHTG) with chylomicronemia. A large number of variants in the LPL gene have been reported but their influence on LPL activity and SHTG has not been completely analyzed. Gaining insight into the deleterious effect of the mutations is clinically essential.

METHODS

We used gene sequencing followed by in-vivo/in-vitro and in-silico tools for classification. We classified 125 rare LPL mutations in 33 subjects thought to have LPL deficiency and in 314 subjects selected for very SHTG.

RESULTS

Of the 33 patients thought to have LPL deficiency, only 13 were homozygous or compound heterozygous for deleterious mutations in the LPL gene. Among the 314 very SHTG patients, 3 were compound heterozygous for pathogenic mutants. In a third group of 51,467 subjects, from a general population, carriers of common variants, Asp9Asn and Asn291Ser, were associated with mild increase in triglyceride levels (11%-35%).

CONCLUSION

In total, 39% of patients clinically diagnosed as LPL deficient had 2 deleterious variants. Three patients selected for very SHTG had LPL deficiency. The deleterious mutations associated with LPL deficiency will assist in the diagnosis and selection of patients as candidates for the presently approved LPL gene therapy.

Endogenous and Synthetic ABHD5 Ligands Regulate ABHD5-Perilipin Interactions and Lipolysis in Fat and Muscle

Fat and muscle lipolysis involves functional interactions of adipose triglyceride lipase (ATGL), α-β hydrolase domain-containing protein 5 (ABHD5), and tissue-specific perilipins 1 and 5 (PLIN1 and PLIN5). ABHD5 potently activates ATGL, but this lipase-promoting activity is suppressed when ABHD5 is bound to PLIN proteins on lipid droplets. In adipocytes, protein kinase A (PKA) phosphorylation of PLIN1 rapidly releases ABHD5 to activate ATGL, but mechanisms for rapid regulation of PLIN5-ABHD5 interaction in muscle are unknown. Here, we identify synthetic ligands that release ABHD5 from PLIN1 or PLIN5 without PKA activation and rapidly activate adipocyte and muscle lipolysis. Molecular imaging and affinity probe labeling demonstrated that ABHD5 is directly targeted by these synthetic ligands and additionally revealed that ABHD5-PLIN interactions are regulated by endogenous ligands, including long-chain acyl-CoA. Our results reveal a new locus of lipolysis control and suggest ABHD5 ligands might be developed into novel therapeutics that directly promote fat catabolism.

ANGPTL4 mediates shuttling of lipid fuel to brown adipose tissue during sustained cold exposure

Brown adipose tissue (BAT) activation via cold exposure is increasingly scrutinized as a potential approach to ameliorate cardio-metabolic risk. Transition to cold temperatures requires changes in the partitioning of energy substrates, re-routing fatty acids to BAT to fuel non-shivering thermogenesis. However, the mechanisms behind the redistribution of energy substrates to BAT remain largely unknown. Angiopoietin-like 4 (ANGPTL4), a protein that inhibits lipoprotein lipase (LPL) activity, is highly expressed in BAT. Here, we demonstrate that ANGPTL4 is part of a shuttling mechanism that directs fatty acids derived from circulating triglyceride-rich lipoproteins to BAT during cold. Specifically, we show that cold markedly down-regulates ANGPTL4 in BAT, likely via activation of AMPK, enhancing LPL activity and uptake of plasma triglyceride-derived fatty acids. In contrast, cold up-regulates ANGPTL4 in WAT, abolishing a cold-induced increase in LPL activity. Together, our data indicate that ANGPTL4 is an important regulator of plasma lipid partitioning during sustained cold.

DOI:http://dx.doi.org/10.7554/eLife.08428.001

The body stores energy in the form of fat molecules. Most of these molecules are stored in white fat cells. Other fat cells, the so-called brown fat cells, consume fats and produce heat to maintain body temperature in cold conditions. The capacity of brown fat cells to consume fats has led researchers to investigate whether brown fat cells might be a key to combat obesity.

When an organism is cold, fat is shuttled to the brown fat cells. An enzyme called lipoprotein lipase is involved in a process that allows these fat molecules to be taken up by brown fat cells. However, it was not clear exactly how this process works.

A protein called Angiopoietin-like 4 (ANGPTL4) inhibits the activity of lipoprotein lipase in white fat cells and is also found at high levels in brown fat cells. Here, Dijk et al. used genetic and biochemical approaches to study the role of ANGPTL4 in the fat cells of mice. The experiments show that when mice are exposed to cold, the levels of ANGPTL4 decrease in the brown fat cells. This allows the activity of lipoprotein lipase to increase so that these cells are able to take up more fat molecules.

However, the opposite happens in white fat cells during cold exposure. The levels of ANGPTL4 increase, which decreases the activity of lipoprotein lipase in white fat cells to allow fat molecules to be shuttled specifically to the brown fat cells. Further experiments suggest that the opposite regulation of ANGPTL4 in brown and white fat cells could be due to a protein called AMPK. This protein is found at higher levels in brown fat cells than in white fat cells and is produced by brown fat cells during cold exposure.

Taken together, Dijk et al. show that organs and cells work together to ensure that fat molecules are appropriately distributed to cells in need of energy, such as to brown fat cells during cold. How these findings could be used to stimulate fat consumption by brown fat cells in humans remains open for further investigation.

DOI:http://dx.doi.org/10.7554/eLife.08428.002

Preclinical Investigation of the Novel Histone Deacetylase Inhibitor AR-42 in the Treatment of Cancer-Induced Cachexia

BACKGROUND

Cancer cachexia is a debilitating condition that impacts patient morbidity, mortality, and quality of life and for which effective therapies are lacking. The anticachectic activity of the novel HDAC inhibitor AR-42 was investigated in murine models of cancer cachexia.

METHODS

The effects of AR-42 on classic features of cachexia were evaluated in the C-26 colon adenocarcinoma and Lewis lung carcinoma (LLC) models. Effects on survival in comparison with approved HDAC inhibitors (vorinostat, romidepsin) were determined. The muscle metabolome and transcriptome (by RNA-seq), as well as serum cytokine profile, were evaluated. Data were analyzed using mixed effects models, analysis of variance, or log-rank tests. All statistical tests were two-sided.

RESULTS

In the C-26 model, orally administered AR-42 preserved body weight (23.9±2.6 grams, AR-42-treated; 20.8±1.3 grams, vehicle-treated; P = .005), prolonged survival (P < .001), prevented reductions in muscle and adipose tissue mass, muscle fiber size, and muscle strength and restored intramuscular mRNA expression of the E3 ligases MuRF1 and Atrogin-1 to basal levels (n = 8). This anticachectic effect, confirmed in the LLC model, was not observed after treatment with vorinostat and romidepsin. AR-42 suppressed tumor-induced changes in inflammatory cytokine production and multiple procachexia drivers (IL-6, IL-6Rα, leukemia inhibitory factor, Foxo1, Atrogin-1, MuRF1, adipose triglyceride lipase, uncoupling protein 3, and myocyte enhancer factor 2c). Metabolomic analysis revealed cachexia-associated changes in glycolysis, glycogen synthesis, and protein degradation in muscle, which were restored by AR-42 to a state characteristic of tumor-free mice.

CONCLUSIONS

These findings support further investigation of AR-42 as part of a comprehensive therapeutic strategy for cancer cachexia.

DAPK2 Downregulation Associates With Attenuated Adipocyte Autophagic Clearance in Human Obesity

Adipose tissue dysfunction in obesity has been linked to low-grade inflammation causing insulin resistance. Transcriptomic studies have identified death-associated protein kinase 2 (DAPK2) among the most strongly downregulated adipose tissue genes in human obesity, but the role of this kinase is unknown. We show that mature adipocytes rather than the stromal vascular cells in adipose tissue mainly expressed DAPK2 and that DAPK2 mRNA in obese patients gradually recovered after bariatric surgery-induced weight loss. DAPK2 mRNA is also downregulated in high-fat diet-induced obese mice. Adenoviral-mediated DAPK2 overexpression in 3T3-L1 adipocytes did not affect lipid droplet size or cell viability but did increase autophagic clearance in nutrient-rich conditions, dependent on protein kinase activity. Conversely, DAPK2 inhibition in human preadipocytes by small interfering RNA decreased LC3-II accumulation rates with lysosome inhibitors. This led us to assess autophagic clearance in adipocytes freshly isolated from subcutaneous adipose tissue of obese patients. Severe reduction in autophagic flux was observed in obese adipocytes compared with control adipocytes, inversely correlated to fat cell lipids. After bariatric surgery, adipocyte autophagic clearance partially recovered proportional to the extent of fat cell size reduction. This study links adipocyte expression of an autophagy-regulating kinase, lysosome-mediated clearance and fat cell lipid accumulation; it demonstrates obesity-related attenuated autophagy in adipocytes, and identifies DAPK2 dependence in this regulation.

Simplified assays of lipolysis enzymes for drug discovery and specificity assessment of known inhibitors

Lipids are used as cellular building blocks and condensed energy stores and also act as signaling molecules. The glycerolipid/ fatty acid cycle, encompassing lipolysis and lipogenesis, generates many lipid signals. Reliable procedures are not available for measuring activities of several lipolytic enzymes for the purposes of drug screening, and this resulted in questionable selectivity of various known lipase inhibitors. We now describe simple assays for lipolytic enzymes, including adipose triglyceride lipase (ATGL), hormone sensitive lipase (HSL), sn-1-diacylglycerol lipase (DAGL), monoacylglycerol lipase, α/β-hydrolase domain 6, and carboxylesterase 1 (CES1) using recombinant human and mouse enzymes either in cell extracts or using purified enzymes. We observed that many of the reported inhibitors lack specificity. Thus, Cay10499 (HSL inhibitor) and RHC20867 (DAGL inhibitor) also inhibit other lipases. Marked differences in the inhibitor sensitivities of human ATGL and HSL compared with the corresponding mouse enzymes was noticed. Thus, ATGListatin inhibited mouse ATGL but not human ATGL, and the HSL inhibitors WWL11 and Compound 13f were effective against mouse enzyme but much less potent against human enzyme. Many of these lipase inhibitors also inhibited human CES1. Results describe reliable assays for measuring lipase activities that are amenable for drug screening and also caution about the specificity of the many earlier described lipase inhibitors.

Structure of a CGI-58 motif provides the molecular basis of lipid droplet anchoring

Triacylglycerols (TGs) stored in lipid droplets (LDs) are hydrolyzed in a highly regulated metabolic process called lipolysis to free fatty acids that serve as energy substrates for β-oxidation, precursors for membrane lipids and signaling molecules. Comparative gene identification-58 (CGI-58) stimulates the enzymatic activity of adipose triglyceride lipase (ATGL), which catalyzes the hydrolysis of TGs to diacylglycerols and free fatty acids. In adipose tissue, protein-protein interactions between CGI-58 and the LD coating protein perilipin 1 restrain the ability of CGI-58 to activate ATGL under basal conditions. Phosphorylation of perilipin 1 disrupts these interactions and mobilizes CGI-58 for the activation of ATGL. We have previously demonstrated that the removal of a peptide at the N terminus (residues 10–31) of CGI-58 abrogates CGI-58 localization to LDs and CGI-58-mediated activation of ATGL. Here, we show that this tryptophan-rich N-terminal peptide serves as an independent LD anchor, with its three tryptophans serving as focal points of the left (harboring Trp²¹ and Trp²⁵) and right (harboring Trp²⁹) anchor arms. The solution state NMR structure of a peptide comprising the LD anchor bound to dodecylphosphocholine micelles as LD mimic reveals that the left arm forms a concise hydrophobic core comprising tryptophans Trp²¹ and Trp²⁵ and two adjacent leucines. Trp²⁹ serves as the core of a functionally independent anchor arm. Consequently, simultaneous tryptophan alanine permutations in both arms abolish localization and activity of CGI-58 as opposed to tryptophan substitutions that occur in only one arm.

Structure-activity relationships for lipoprotein lipase agonists that lower plasma triglycerides in vivo

The risk of cardiovascular events increases in individuals with elevated plasma triglyceride (TG) levels, therefore advocating the need for efficient TG-lowering drugs. In the blood circulation, TG levels are regulated by lipoprotein lipase (LPL), an unstable enzyme that is only active as a non-covalently associated homodimer. We recently reported on a N-phenylphthalimide derivative (1) that stabilizes LPL in vitro, and moderately lowers triglycerides in vivo (Biochem. Biophys. Res. Commun.2014, 450, 1063). Herein, we establish structure-activity relationships of 51 N-phenylphthalimide analogs of the screening hit 1. In vitro evaluation highlighted that modifications on the phthalimide moiety were not tolerated and that lipophilic substituents on the central phenyl ring were functionally essential. The substitution pattern on the central phenyl ring also proved important to stabilize LPL. However, in vitro testing demonstrated rapid degradation of the phthalimide fragment in plasma which was addressed by replacing the phthalimide scaffold with other heterocyclic fragments. The in vitro potency was retained or improved and substance 80 proved stable in plasma and efficiently lowered plasma TGs in vivo.

Ginkgolide C Suppresses Adipogenesis in 3T3-L1 Adipocytes via the AMPK Signaling Pathway

Ginkgolide C, isolated from Ginkgo biloba leaves, is a diterpene lactone derivative [corrected] reported to have multiple biological functions, from decreased platelet aggregation to ameliorating Alzheimer disease. The study aim was to evaluate the antiadipogenic effect of ginkgolide C in 3T3-L1 adipocytes. Ginkgolide C was used to treat differentiated 3T3-L1 cells. Cell supernatant was collected to assay glycerol release, and cells were lysed to measure protein and gene expression related to adipogenesis and lipolysis by western blot and real-time PCR, respectively. Ginkgolide C significantly suppressed lipid accumulation in differentiated adipocytes. It also decreased adipogenesis-related transcription factor expression, including peroxisome proliferator-activated receptor and CCAAT/enhancer-binding protein. Furthermore, ginkgolide C enhanced adipose triglyceride lipase and hormone-sensitive lipase production for lipolysis and increased phosphorylation of AMP-activated protein kinase (AMPK), resulting in decreased activity of acetyl-CoA carboxylase for fatty acid synthesis. In coculture with an AMPK inhibitor (compound C), ginkgolide C also improved activation of sirtuin 1 and phosphorylation of AMPK in differentiated 3T3-L1 cells. The results suggest that ginkgolide C is an effective flavone for increasing lipolysis and inhibiting adipogenesis in adipocytes through the activated AMPK pathway.