Muscle-specific overexpression of lipoprotein lipase causes a severe myopathy characterized by proliferation of mitochondria and peroxisomes in transgenic mice

Omega-3 Attenuates Disrupted Neurotransmission and Partially Protects Metabolic Dysfunction Caused by Obesity in Wistar Rats

Omega-3 (n3) is a polyunsaturated fatty acid well known for its anti-inflammatory and neuroprotective properties. Obesity is linked to chronic inflammation that disrupts metabolism, the intestine physiology and the central nervous system functioning. This study aims to determine if n3 supplementation can interfere with the effects of obesity on the mitochondrial activity, intestinal barrier, and neurotransmitter levels in the brain of Wistar rats that received cafeteria diet (CAF). We examined adipose tissue, skeletal muscle, plasma, intestine, and the cerebral cortex of four groups: CT (control diet), CTn3 (control diet with n3 supplementation), CAF, and CAFn3 (CAF and n3). Diets were offered for 13 weeks, with n3 supplementation in the final 5 weeks. Adipose tissue Electron Transport Chain complexes I, II, and III showed higher activity in CAF groups, as did complexes III and IV in skeletal muscle. Acetate levels in plasma were reduced in CAF groups, and Lipopolysaccharide (LPS) was higher in the CAF group but reduced in CAFn3 group. Claudin-5 in the intestine was lower in CAF groups, with no n3 supplementation effect. In the cerebral cortex, dopamine levels were decreased with CAF, which was reversed by n3. DOPAC, a dopamine metabolite, also showed a supplementation effect, and HVA, a diet effect. Serotonin levels increased in the CAF group that received supplementation. Therefore, we demonstrate disturbances in mitochondria, plasma, intestine and brain of rats submitted to CAF and the potential benefit of n3 supplementation in endotoxemia and neurotransmitter levels.

Mitochondrial-derived microprotein MOTS-c attenuates immobilization-induced skeletal muscle atrophy by suppressing lipid infiltration

Mitochondrial open reading frame of the 12S ribosomal RNA type-c (MOTS-c), a mitochondrial microprotein, has been described as a novel regulator of glucose and lipid metabolism. In addition to its role as a metabolic regulator, MOTS-c prevents skeletal muscle atrophy in high fat-fed mice. Here, we examined the preventive effect of MOTS-c on skeletal muscle mass, using an immobilization-induced muscle atrophy model, and explored its underlying mechanisms. Male C57BL/6J mice (10 wk old) were randomly assigned to one of the three experimental groups: nonimmobilization control group (sterilized water injection), immobilization control group (sterilized water injection), and immobilization and MOTS-c-treated group (15 mg/kg/day MOTS-c injection). We used casting tape for the immobilization experiment. After 8 days of the experimental period, skeletal muscle samples were collected and used for Western blotting, RNA sequencing, and lipid and collagen assays. Immobilization reduced ∼15% of muscle mass, whereas MOTS-c treatment attenuated muscle loss, with only a 5% reduction. MOTS-c treatment also normalized phospho-AKT, phospho-FOXO1, and phospho-FOXO3a expression levels and reduced circulating inflammatory cytokines, such as interleukin-1b (IL-1β), interleukin-6 (IL-6), chemokine C-X-C motif ligand 1 (CXCL1), and monocyte chemoattractant protein 1 (MCP-1), in immobilized mice. Unbiased RNA sequencing and its downstream analyses demonstrated that MOTS-c modified adipogenesis-modulating gene expression within the peroxisome proliferator-activated receptor (PPAR) pathway. Supporting this observation, muscle fatty acid levels were lower in the MOTS-c-treated group than in the casted control mice. These results suggest that MOTS-c treatment inhibits skeletal muscle lipid infiltration by regulating adipogenesis-related genes and prevents immobilization-induced muscle atrophy.NEW & NOTEWORTHY MOTS-c, a mitochondrial microprotein, attenuates immobilization-induced skeletal muscle atrophy. MOTS-c treatment improves systemic inflammation and skeletal muscle AKT/FOXOs signaling pathways. Furthermore, unbiased RNA sequencing and subsequent assays revealed that MOTS-c prevents lipid infiltration in skeletal muscle. Since lipid accumulation is one of the common pathologies among other skeletal muscle atrophies induced by aging, obesity, cancer cachexia, and denervation, MOTS-c treatment could be effective in other muscle atrophy models as well.

AAV-mediated hepatic LPL expression ameliorates severe hypertriglyceridemia and acute pancreatitis in Gpihbp1 deficient mice and rats

GPIHBP1 plays an important role in the hydrolysis of triglyceride (TG) lipoproteins by lipoprotein lipases (LPLs). However, Gpihbp1 knockout mice did not develop hypertriglyceridemia (HTG) during the suckling period but developed severe HTG after weaning on a chow diet. It has been postulated that LPL expression in the liver of suckling mice may be involved. To determine whether hepatic LPL expression could correct severe HTG in Gpihbp1 deficiency, liver-targeted LPL expression was achieved via intravenous administration of the adeno-associated virus (AAV)-human LPL gene, and the effects of AAV-LPL on HTG and HTG-related acute pancreatitis (HTG-AP) were observed. Suckling Gpihbp1-/- mice with high hepatic LPL expression did not develop HTG, whereas Gpihbp1-/- rat pups without hepatic LPL expression developed severe HTG. AAV-mediated liver-targeted LPL expression dose-dependently decreased plasma TG levels in Gpihbp1-/- mice and rats, increased post-heparin plasma LPL mass and activity, decreased mortality in Gpihbp1-/- rat pups, and reduced the susceptibility and severity of both Gpihbp1-/- animals to HTG-AP. However, the muscle expression of AAV-LPL had no significant effect on HTG. Targeted expression of LPL in the liver showed no obvious adverse reactions. Thus, liver-targeted LPL expression may be a new therapeutic approach for HTG-AP caused by GPIHBP1 deficiency.

The effects of pollutant mixture released from grafted adipose tissues on fatty acid and lipid metabolism in the skeletal muscles, kidney, heart, and lungs of male mice

Persistent organic pollutants (POPs) accumulated in the adipose tissue can affect the fatty acid and lipid metabolism in the body. Gas chromatography-mass spectrometry (GC-MS) metabolomics analysis was carried out to study the metabolic changes induced by internal exposure to the POPs in mouse skeletal muscle (soleus, plantaris, and gastrocnemius), kidney, heart, and lungs. Male donor mice were injected with a mixture of 10 POPs at concentrations of 0 × and 5 × lowest-observed-adverse-effect level (LOAEL). Their adipose tissue (AT) containing the POP was then grafted onto the host mice and the metabolic change of the host mice was monitored for 3 or 21 days. The metabolites related to fatty acid and lipid metabolism were studied. For the host mice engrafted with POP-containing fat pad, there was dysregulation of the fatty acids and glycerides observed in all the organs studied 3 days after the graft. However, there was no longer a significant change in the metabolites 21 days after the graft. The difference in significant values and metabolite regulation in each of the skeletal muscles showed that the POP mixture affects different types of skeletal muscle in a heterogeneous manner. Fold change analysis showed that certain metabolites in the kidney of host mice exposed to POPs for 3 days were greatly affected. Using multivariate analysis, apart from the plantaris, most treated groups exposed to POPs for 3 days are well distinguished from the control groups. However, for host mice exposed to POPs for 21 days, apart from the kidney and heart, groups are not well-distinguished from the control group. This study helps bring new insight into the effects of the pollutants mixture released from AT on fatty acid and lipid metabolism at different periods and how the dysregulation of metabolites might result in diseases associated with the organs.

Preclinical Development and Characterization of Novel Adeno-Associated Viral Vectors for the Treatment of Lipoprotein Lipase Deficiency

Lipoprotein lipase deficiency (LPLD) results from mutations within the lipoprotein lipase (LPL) gene that lead to a complete lack of catalytically active LPL protein. Glybera was one of the first adeno-associated virus (AAV) gene replacement therapy to receive European Medicines Agency regulatory approval for the treatment of LPLD. However, Glybera is no longer marketed potentially due to a combination of economical, manufacturing, and vector-related issues. The aim of this study was to develop a more efficacious AAV gene therapy vector for LPLD. Following preclinical biodistribution, efficacy and non-Good Laboratory Practice toxicity studies with novel AAV1 and AAV8-based vectors in mice, we identified AAV8 pVR59. AAV8 pVR59 delivered a codon-optimized, human gain-of-function hLPLS447X transgene driven by a CAG promoter in an AAV8 capsid. AAV8 pVR59 was significantly more efficacious, at 10- to 100-fold lower doses, compared with an AAV1 vector based on Glybera, when delivered intramuscularly or intravenously, respectively, in mice with LPLD. Efficient gene transfer was observed within the injected skeletal muscle and liver following delivery of AAV8 pVR59, with long-term correction of LPLD phenotypes, including normalization of plasma triglycerides and lipid tolerance, for up to 6 months post-treatment. While intramuscular delivery of AAV8 pVR59 was well tolerated, intravenous administration augmented liver pathology. These results highlight the feasibility of developing a superior AAV vector for the treatment of LPLD and provide critical insight for initiating studies in larger animal models. The identification of an AAV gene therapy vector that is more efficacious at lower doses, when paired with recent advances in production and manufacturing technologies, will ultimately translate to increased safety and accessibility for patients.

A coordinated multiorgan metabolic response contributes to human mitochondrial myopathy

Mitochondrial diseases are a heterogeneous group of monogenic disorders that result from impaired oxidative phosphorylation (OXPHOS). As neuromuscular tissues are highly energy-dependent, mitochondrial diseases often affect skeletal muscle. Although genetic and bioenergetic causes of OXPHOS impairment in human mitochondrial myopathies are well established, there is a limited understanding of metabolic drivers of muscle degeneration. This knowledge gap contributes to the lack of effective treatments for these disorders. Here, we discovered fundamental muscle metabolic remodeling mechanisms shared by mitochondrial disease patients and a mouse model of mitochondrial myopathy. This metabolic remodeling is triggered by a starvation-like response that evokes accelerated oxidation of amino acids through a truncated Krebs cycle. While initially adaptive, this response evolves in an integrated multiorgan catabolic signaling, lipid store mobilization, and intramuscular lipid accumulation. We show that this multiorgan feed-forward metabolic response involves leptin and glucocorticoid signaling. This study elucidates systemic metabolic dyshomeostasis mechanisms that underlie human mitochondrial myopathies and identifies potential new targets for metabolic intervention.

Deranged Myocardial Fatty Acid Metabolism in Heart Failure

The heart requires fatty acids to maintain its activity. Various mechanisms regulate myocardial fatty acid metabolism, such as energy production using fatty acids as fuel, for which it is known that coordinated control of fatty acid uptake, β-oxidation, and mitochondrial oxidative phosphorylation steps are important for efficient adenosine triphosphate (ATP) production without unwanted side effects. The fatty acids taken up by cardiomyocytes are not only used as substrates for energy production but also for the synthesis of triglycerides and the replacement reaction of fatty acid chains in cell membrane phospholipids. Alterations in fatty acid metabolism affect the structure and function of the heart. Recently, breakthrough studies have focused on the key transcription factors that regulate fatty acid metabolism in cardiomyocytes and the signaling systems that modify their functions. In this article, we reviewed the latest research on the role of fatty acid metabolism in the pathogenesis of heart failure and provide an outlook on future challenges.

The pleiotropic peroxisome proliferator activated receptors: Regulation and therapeutics

The Peroxisome proliferator-activated receptors (PPARs) are key regulators of metabolic events in our body. Owing to their implication in maintenance of homeostasis, both PPAR agonists and antagonists assume therapeutic significance. Understanding the molecular mechanisms of each of the PPAR isotypes in the healthy body and during disease is crucial to exploiting their full therapeutic potential. This article is an attempt to present a rational analysis of the multifaceted therapeutic effects and underlying mechanisms of isotype-specific PPAR agonists, dual PPAR agonists, pan PPAR agonists as well as PPAR antagonists. A holistic understanding of the mechanistic dimensions of these key metabolic regulators will guide future efforts to identify novel molecules in the realm of metabolic, inflammatory and immunotherapeutic diseases.

Lipoprotein Lipase and Its Delivery of Fatty Acids to the Heart

Ninety percent of plasma fatty acids (FAs) are contained within lipoprotein-triglyceride, and lipoprotein lipase (LPL) is robustly expressed in the heart. Hence, LPL-mediated lipolysis of lipoproteins is suggested to be a key source of FAs for cardiac use. Lipoprotein clearance by LPL occurs at the apical surface of the endothelial cell lining of the coronary lumen. In the heart, the majority of LPL is produced in cardiomyocytes and subsequently is translocated to the apical luminal surface. Here, vascular LPL hydrolyzes lipoprotein-triglyceride to provide the heart with FAs for ATP generation. This article presents an overview of cardiac LPL, explains how the enzyme works, describes key molecules that regulate its activity and outlines how changes in LPL are brought about by physiological and pathological states such as fasting and diabetes, respectively.

Distinct Roles of Perilipins in the Intramuscular Deposition of Lipids in Glutamine-Supplemented, Low-, and Normal-Birth-Weight Piglets

Piglets with low birth weight (LBW) usually have reduced muscle mass and increased lipid deposition compared with their normal-birth-weight (NBW) littermates. Supplementation of piglets with amino acids during the first days of life may improve muscle growth and simultaneously alter the intramuscular lipid deposition. The aim of the current study was to investigate the influence of glutamine (Gln) supplementation during the early suckling period on lipid deposition in the longissimus muscle (MLD) and the role of different perilipin (PLIN) family members in this process. Four groups were generated consisting of 72 male LBW piglets and 72 NBW littermates. Piglets were supplemented with either 1 g Gln/kg body weight or an isonitrogenous amount of alanine (Ala) between days post natum (dpn) 1 and 12. Twelve piglets per group were slaughtered at 5, 12, and 26 dpn, and muscle tissue was collected. Perilipins were localized by immunohistochemistry in muscle sections. The mRNA and protein abundances of PLIN family members and related lipases were quantified by quantitative RT-PCR (qPCR) and western blots, respectively. While PLIN1 was localized around lipid droplets in mature and developing adipocytes, PLIN2 was localized at intramyocellular lipid droplets, PLIN3 and 4 at cell membranes of muscle fibers and adipocytes, and PLIN5 in the cytoplasm of undefined cells. The western blot results indicated higher protein abundances of PLIN2, 3, 4, and 5 in LBW piglets (p < 0.05) at 5 dpn compared with their NBW littermates independent of supplementation, while not directly reflecting the mRNA expression levels. The mRNA abundance of PLIN2 was lower while PLIN4 was higher in piglets at 26 dpn in comparison with piglets at 5 dpn (p < 0.01). Relative mRNA expression of LPL and CGI-58 was lowest in piglets at 5 dpn (p < 0.001). However, ATGL mRNA was not influenced by birth weight or supplementation, but the Spearman correlation coefficient analysis revealed close correlations with PLIN2, 4, and 5 mRNA at 5 and 26 dpn (r > 0.5, p < 0.001). The results indicated the importance of birth weight and age for intramuscular lipid deposition and different roles of PLIN family members in this process, but no clear modulating effect of Gln supplementation.

Endurance Runners with Intramyocellular Lipid Accumulation and High Insulin Sensitivity Have Enhanced Expression of Genes Related to Lipid Metabolism in Muscle

Context: Endurance-trained athletes have high oxidative capacities, enhanced insulin sensitivities, and high intracellular lipid accumulation in muscle. These characteristics are likely due to altered gene expression levels in muscle. Design and setting: We compared intramyocellular lipid (IMCL), insulin sensitivity, and gene expression levels of the muscle in eight nonobese healthy men (control group) and seven male endurance athletes (athlete group). Their IMCL levels were measured by proton-magnetic resonance spectroscopy, and their insulin sensitivity was evaluated by glucose infusion rate (GIR) during a euglycemic–hyperinsulinemic clamp. Gene expression levels in the vastus lateralis were evaluated by quantitative RT-PCR (qRT-PCR) and microarray analysis. Results: IMCL levels in the tibialis anterior muscle were approximately 2.5 times higher in the athlete group compared to the control group, while the IMCL levels in the soleus muscle and GIR were comparable. In the microarray hierarchical clustering analysis, gene expression patterns were not clearly divided into control and athlete groups. In a gene set enrichment analysis with Gene Ontology gene sets, “RESPONSE TO LIPID” was significantly upregulated in the athlete group compared with the control group. Indeed, qRT-PCR analysis revealed that, compared to the control group, the athlete group had 2–3 times higher expressions of proliferator-activated receptor gamma coactivator-1 alpha (PGC1A), adiponectin receptors (AdipoRs), and fatty acid transporters including fatty acid transporter-1, plasma membrane-associated fatty acid binding protein, and lipoprotein lipase. Conclusions: Endurance runners with higher IMCL levels have higher expression levels of genes related to lipid metabolism such as PGC1A, AdipoRs, and fatty acid transporters in muscle.

Systemic MCPIP1 deficiency in mice impairs lipid homeostasis

Atherosclerosis involves interactions between inflammation system and dyslipidemia. MCPIP1 (Monocyte Chemotactic Protein induced Protein-1) is induced by proinflammatory molecules and serves as a negative feedback loop in regulating inflammatory responses. Our current study was designed to test the role of MCPIP1 in maintaining lipid homeostasis, the latter a pivotal factor that contributes to the pathogenesis of atherosclerosis. We found that MCPIP1 knockout mice displayed a decrease in levels of serum HDL-cholesterol and total triglycerides but an increase in serum LDL/VLDL-cholesterol levels when compared to wild-type mice. Additionally, ApoA-1 expression was reduced but LPL expression was upregulated in plasma from MCPIP1 knockout mice. The livers from the MCPIP1 knockout mice revealed a decrease in hepatocyte number and an increase in collagen deposition when compared to wild-type mice. These findings suggest that MCPIP1 deficiency can induce liver fibrosis, alter the expression of lipoproteins, and affect transportation and metabolism of lipids, indicating that MCPIP1 is involved in maintaining lipid homeostasis, possibly via negatively regulating inflammatory responses.

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Atherosclerosis is the result of interaction between inflammation and dyslipidemia.

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MCPIP1 is a negative regulator in inflammatory responses.

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MCPIP1 is upregulated in the atherosclerotic plaques.

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MCPIP1 deficiency induces dyslipidemia and hepatic remodeling.

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MCPIP1 deficiency may increase the risk of atherosclerosis.

Differences in macrophage polarization in the adipose tissue of obese mice under various levels of exercise intensity

Animal studies have demonstrated that the ratio of M1 (M1Φ) to M2 (M2Φ) macrophage-specific gene expression in adipose tissue (AT) may be altered by chronic exercise; however, whether macrophage polarization is induced under these conditions has not yet been reported. Therefore, this study aimed to investigate the effect of chronic exercise on M1Φ/M2Φ polarization in the AT of high-fat diet (HFD)-induced obese mice. Exercise-induced differences in M1Φ/M2Φ polarization were verified via an exercise intensity study (EIS) in which different levels of exercise intensity were evaluated. Obesity was induced in male C57BL/6 J mice by feeding them with an HFD for 6 weeks. The study consisted of four groups: control group (CON), HFD-fed group (HFD), HFD-fed with exercise group (HFD + EXE), dietary conversion from HFD to normal diet (ND) group (DC), and dietary conversion from HFD to ND group (DC + EXE). For EIS, the HFD + EXE group was divided into three subgroups: low- (LI), mid- (MI), and high- (HI) intensity exercise. The total intervention period was 8 weeks. M1Φ/M2Φ polarization was confirmed by flow cytometry. M2Φ polarization in the AT of obese mice was significantly higher in HFD + EXE mice than in HFD mice, despite the HFD intake. In the EIS, M2Φ polarization was most pronounced in HFD + EXE-HI mice than in HFD mice. It can be proposed that the enhanced insulin resistance and inflammation by obesity can be improved by the increase of M2Φ polarization which is achieved by relatively high-intensity exercise.

PPAR-Mediated Toxicology and Applied Pharmacology

Peroxisome proliferator-activated receptors (PPARs), members of the nuclear hormone receptor family, attract wide attention as promising therapeutic targets for the treatment of multiple diseases, and their target selective ligands were also intensively developed for pharmacological agents such as the approved drugs fibrates and thiazolidinediones (TZDs). Despite their potent pharmacological activities, PPARs are reported to be involved in agent- and pollutant-induced multiple organ toxicity or protective effects against toxicity. A better understanding of the protective and the detrimental role of PPARs will help to preserve efficacy of the PPAR modulators but diminish adverse effects. The present review summarizes and critiques current findings related to PPAR-mediated types of toxicity and protective effects against toxicity for a systematic understanding of PPARs in toxicology and applied pharmacology.

Myopathy reversion in mice after restauration of mitochondrial complex I

Myopathies are common manifestations of mitochondrial diseases. To investigate whether gene replacement can be used as an effective strategy to treat or cure mitochondrial myopathies, we have generated a complex I conditional knockout mouse model lacking NDUFS3 subunit in skeletal muscle. NDUFS3 protein levels were undetectable in muscle of 15‐day‐old smKO mice, and myopathy symptoms could be detected by 2 months of age, worsening over time. rAAV9‐Ndufs3 delivered systemically into 15‐ to 18‐day‐old mice effectively restored NDUFS3 levels in skeletal muscle, precluding the development of the myopathy. To test the ability of rAAV9‐mediated gene replacement to revert muscle function after disease onset, we also treated post‐symptomatic, 2‐month‐old mice. The injected mice showed a remarkable improvement of the mitochondrial myopathy and biochemical parameters, which remained for the duration of the study. Our results showed that muscle pathology could be reversed after restoring complex I, which was absent for more than 2 months. These findings have far‐reaching implications for the ability of muscle to tolerate a mitochondrial defect and for the treatment of mitochondrial myopathies.

Involvement of Heparanase in Endothelial Cell-Cardiomyocyte Crosstalk

Traditionally, the management of diabetes has focused mainly on controlling high blood glucose levels. Unfortunately, despite valiant efforts to normalize this blood glucose, poor medication management predisposes these patients to heart failure. Following diabetes, how the heart utilizes different sources of fuel for energy is key to the development of heart failure. The diabetic heart switches from using both glucose and fats, to predominately using fats as an energy resource for maintaining its activities. This transformation to using fats as an exclusive source of energy is helpful in the initial stages of the disease and is tightly controlled. However, over the progression of diabetes, there is a loss of this controlled supply and use of fats, which ultimately has terrible consequences since the uncontrolled use of fats produces toxic by-products which weaken heart function and cause heart disease. Heparanase is a key player that directs how much fats are provided to the heart and does so in association with several partners like LPL and VEGFs. Together, they regulate the amount of fats supplied, and their subsequent breakdown to provide energy. Following diabetes, there is a disruption in this network resulting in fat oversupply and cell death. Understanding how the heparanase-LPL-VEGFs "ensemble" cooperates, and its dysfunction in the diabetic heart would be useful in restoring metabolic equilibrium and limiting diabetes-related cardiac damage.

Maternal-Fetal Transfer of Vitamin A and Its Impact on Mammalian Embryonic Development

The placenta, a hallmark of mammalian embryogenesis, allows nutrients to be exchanged between the mother and the fetus. Vitamin A (VA), an essential nutrient, cannot be synthesized by the embryo, and must be acquired from the maternal circulation through the placenta. Our understanding of how this transfer is accomplished is still in its infancy. In this chapter, we recapitulate the early studies about the relationship between maternal dietary/supplemental VA intake and fetal VA levels. We then describe how the discovery of retinol-binding protein (RBP or RBP4), the development of labeling and detection techniques, and the advent of knockout mice shifted this field from a macroscopic to a molecular level. The most recent data indicate that VA and its derivatives (retinoids) and the pro-VA carotenoid, β-carotene, are transferred across the placenta by distinct proteins, some of which overlap with proteins involved in lipoprotein uptake. The VA status and dietary intake of the mother influence the expression of these proteins, creating feedback signals that control the uptake of retinoids and that may also regulate the uptake of lipids, raising the intriguing possibility of crosstalk between micronutrient and macronutrient metabolism. Many questions remain about the temporal and spatial patterns by which these proteins are expressed and transferred throughout gestation. The answers to these questions are highly relevant to human health, considering that those with either limited or excessive intake of retinoids/carotenoids during pregnancy may be at risk of obtaining improper amounts of VA that ultimately impact the development and health of their offspring.

Diabetes Mellitus Severity and a Switch From Using Lipoprotein Lipase to Adipose-Derived Fatty Acid Results in a Cardiac Metabolic Signature That Embraces Cell Death

Background

Fatty acid (FA) provision to the heart is from cardiomyocyte and adipose depots, plus lipoprotein lipase action. We tested how a graded reduction in insulin impacts the source of FA used by cardiomyocytes and the cardiac adaptations required to process these FA.

Methods and Results

Rats injected with 55 (D55) or 100 (D100) mg/kg streptozotocin were terminated after 4 days. Although D55 and D100 were equally hyperglycemic, D100 showed markedly lower pancreatic and plasma insulin and loss of lipoprotein lipase, which in D55 hearts had expanded. There was minimal change in plasma FA in D55. However, D100 exhibited a 2‐ to 3‐fold increase in various saturated, monounsaturated, and polyunsaturated FA in the plasma. D100 demonstrated dramatic cardiac transcriptomic changes with 1574 genes differentially expressed compared with only 49 in D55. Augmented mitochondrial and peroxisomal β‐oxidation in D100 was not matched by elevated tricarboxylic acid or oxidative phosphorylation. With increasing FA, although control myocytes responded by augmenting basal respiration, this was minimized in D55 and reversed in D100. Metabolomic profiling identified significant lipid accumulation in D100 hearts, which also exhibited sizeable change in genes related to apoptosis and terminal deoxynucleotidyl transferase dUTP nick‐end labeling–positive cells.

Conclusions

With increasing severity of diabetes mellitus, when the diabetic heart is unable to control its own FA supply using lipoprotein lipase, it undergoes dramatic reprogramming that is linked to handling of excess FA that arise from adipose tissue. This transition results in a cardiac metabolic signature that embraces mitochondrial FA overload, oxidative stress, triglyceride storage, and cell death.

GPIHBP1 expression in gliomas promotes utilization of lipoprotein-derived nutrients

GPIHBP1, a GPI-anchored protein of capillary endothelial cells, binds lipoprotein lipase (LPL) within the subendothelial spaces and shuttles it to the capillary lumen. GPIHBP1-bound LPL is essential for the margination of triglyceride-rich lipoproteins (TRLs) along capillaries, allowing the lipolytic processing of TRLs to proceed. In peripheral tissues, the intravascular processing of TRLs by the GPIHBP1-LPL complex is crucial for the generation of lipid nutrients for adjacent parenchymal cells. GPIHBP1 is absent from the capillaries of the brain, which uses glucose for fuel; however, GPIHBP1 is expressed in the capillaries of mouse and human gliomas. Importantly, the GPIHBP1 in glioma capillaries captures locally produced LPL. We use NanoSIMS imaging to show that TRLs marginate along glioma capillaries and that there is uptake of TRL-derived lipid nutrients by surrounding glioma cells. Thus, GPIHBP1 expression in gliomas facilitates TRL processing and provides a source of lipid nutrients for glioma cells.

High Fat With High Sucrose Diet Leads to Obesity and Induces Myodegeneration

Skeletal muscle utilizes both free fatty acids (FFAs) and glucose that circulate in the blood stream. When blood glucose levels acutely increase, insulin stimulates muscle glucose uptake, oxidation, and glycogen synthesis. Under these conditions, skeletal muscle preferentially oxidizes glucose while the oxidation of fatty acids (FAs) oxidation is reciprocally decreased. In metabolic disorders associated with insulin resistance, such as diabetes and obesity, both glucose uptake, and utilization muscle are significantly reduced causing FA oxidation to provide the majority of ATP for metabolic processes and contraction. Although the causes of this metabolic inflexibility or disrupted "glucose-fatty acid cycle" are largely unknown, a diet high in fat and sugar (HFS) may be a contributing factor. This metabolic inflexibility observed in models of obesity or with HFS feeding is detrimental because high rates of FA oxidation in skeletal muscle can lead to the buildup of toxic metabolites of fat metabolism and the accumulation of pro-inflammatory cytokines, which further exacerbate the insulin resistance. Further, HFS leads to skeletal muscle atrophy with a decrease in myofibrillar proteins and phenotypically characterized by loss of muscle mass and strength. Overactivation of ubiquitin proteasome pathway, oxidative stress, myonuclear apoptosis, and mitochondrial dysfunction are some of the mechanisms involved in muscle atrophy induced by obesity or in mice fed with HFS. In this review, we will discuss how HFS diet negatively impacts the various physiological and metabolic mechanisms in skeletal muscle.

CRISPR/Cas9-mediated Angptl8 knockout suppresses plasma triglyceride concentrations and adiposity in rats

Angiopoietin-like protein (ANGPTL)8 is a liver- and adipocyte-derived protein that controls plasma triglyceride (TG) levels. Most animal studies have used mouse models. Here, we generated an Angptl8 KO rat model using a clustered regulatory interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9) (CRISPR/Cas9) system to clarify the roles of ANGPTL8 in glucose and lipid metabolism. Compared with WT rats, Angptl8 KO rats had lower body weight and fat content, associated with impaired lipogenesis in adipocytes; no differences existed between the groups in food intake or rectal temperature. Plasma TG levels in both the fasted and refed states were significantly lower in KO than in WT rats, and an oral fat tolerance test showed decreased plasma TG excursion in Angptl8 KO rats. Higher levels of lipase activity in the heart and greater expression of genes related to β-oxidation in heart and skeletal muscle were observed in Angptl8 KO rats. However, there were no significant differences between KO and WT rats in glucose metabolism or the histology of pancreatic β-cells on both standard and high-fat diets. In conclusion, we demonstrated that Angptl8 KO in rats resulted in lower body weight and plasma TG levels without affecting glucose metabolism. ANGPTL8 might be an important therapeutic target for obesity and dyslipidemia.

Lipoprotein Lipase Deficiency Impairs Bone Marrow Myelopoiesis and Reduces Circulating Monocyte Levels

OBJECTIVE

Tissue macrophages induce and perpetuate proinflammatory responses, thereby promoting metabolic and cardiovascular disease. Lipoprotein lipase (LpL), the rate-limiting enzyme in blood triglyceride catabolism, is expressed by macrophages in atherosclerotic plaques. We questioned whether LpL, which is also expressed in the bone marrow (BM), affects circulating white blood cells and BM proliferation and modulates macrophage retention within the artery.

APPROACH AND RESULTS

We characterized blood and tissue leukocytes and inflammatory molecules in transgenic LpL knockout mice rescued from lethal hypertriglyceridemia within 18 hours of life by muscle-specific LpL expression (MCKL0 mice). LpL-deficient mice had ≈40% reduction in blood white blood cell, neutrophils, and total and inflammatory monocytes (Ly6C/G^hi). LpL deficiency also significantly decreased expression of BM macrophage-associated markers (F4/80 and TNF-α [tumor necrosis factor α]), master transcription factors (PU.1 and C/EBPα), and colony-stimulating factors (CSFs) and their receptors, which are required for monocyte and monocyte precursor proliferation and differentiation. As a result, differentiation of macrophages from BM-derived monocyte progenitors and monocytes was decreased in MCKL0 mice. Furthermore, although LpL deficiency was associated with reduced BM uptake and accumulation of triglyceride-rich particles and macrophage CSF-macrophage CSF receptor binding, triglyceride lipolysis products (eg, linoleic acid) stimulated expression of macrophage CSF and macrophage CSF receptor in BM-derived macrophage precursor cells. Arterial macrophage numbers decreased after heparin-mediated LpL cell dissociation and by genetic knockout of arterial LpL. Reconstitution of LpL-expressing BM replenished aortic macrophage density.

CONCLUSIONS

LpL regulates peripheral leukocyte levels and affects BM monocyte progenitor differentiation and aortic macrophage accumulation.

Striated muscle gene therapy for the treatment of lipoprotein lipase deficiency

Excessive circulating triglycerides due to reduction or loss of lipoprotein lipase activity contribute to hypertriglyceridemia and increased risk for pancreatitis. The only gene therapy treatment for lipoprotein lipase deficiency decreases pancreatitis but minimally reduces hypertriglyceridemia. Synthesized in multiple tissues including striated muscle and adipose tissue, lipoprotein lipase is trafficked to blood vessel endothelial cells where it is anchored at the plasma membrane and hydrolyzes triglycerides into free fatty acids. We conditionally knocked out lipoprotein lipase in differentiated striated muscle tissue lowering striated muscle lipoprotein lipase activity causing hypertriglyceridemia. We then crossed lipoprotein lipase striated muscle knockout mice with mice possessing a conditional avian retroviral receptor gene and injected mice with either a human lipoprotein lipase retrovirus or an mCherry control retrovirus. Post-heparin plasma lipoprotein lipase activity increased for three weeks following human lipoprotein lipase retroviral infection compared to mCherry infected mice. Human lipoprotein lipase infected mice had significantly lower blood triglycerides compared to mCherry controls and were comparable to wild-type blood triglyceride levels. Thus, targeted delivery of human lipoprotein lipase into striated muscle tissue identifies a potential therapeutic target for lipoprotein lipase deficiency.

Diabetes-Induced Dysfunction of Mitochondria and Stem Cells in Skeletal Muscle and the Nervous System

Diabetes mellitus is one of the most common metabolic diseases spread all over the world, which results in hyperglycemia caused by the breakdown of insulin secretion or insulin action or both. Diabetes has been reported to disrupt the functions and dynamics of mitochondria, which play a fundamental role in regulating metabolic pathways and are crucial to maintain appropriate energy balance. Similar to mitochondria, the functions and the abilities of stem cells are attenuated under diabetic condition in several tissues. In recent years, several studies have suggested that the regulation of mitochondria functions and dynamics is critical for the precise differentiation of stem cells. Importantly, physical exercise is very useful for preventing the diabetic alteration by improving the functions of both mitochondria and stem cells. In the present review, we provide an overview of the diabetic alterations of mitochondria and stem cells and the preventive effects of physical exercise on diabetes, focused on skeletal muscle and the nervous system. We propose physical exercise as a countermeasure for the dysfunction of mitochondria and stem cells in several target tissues under diabetes complication and to improve the physiological function of patients with diabetes, resulting in their quality of life being maintained.

Intramyocellular lipid droplets and insulin sensitivity, the human perspective

Skeletal muscle can store excess fat as subcellular lipid droplets (LDs). While originally viewed as uninteresting static balls of triacylglycerol, it is now clear that myocellular LDs play an active role in myocellular (patho)physiology. In this review we aim to discuss the role of LDs in muscle cell insulin sensitivity and identify parameters which appear to affect this relationship. Moreover, we discuss the application of novel tools permitting detailed examination of these parameters. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Establishment of a novel mouse model for pioglitazone-induced skeletal muscle injury

Skeletal muscle (SKM) injury is one of the major safety concerns in risk assessment for drug development. However, no appropriate pre-clinical animal model exists to evaluate drug-induced SKM injury except that caused by fibrates and statins. Thiazolidinedione, a PPARγ agonistic drug for type 2 diabetes mellitus, is widely used clinically but can induce adverse effects such as hepatotoxicity and SKM injury, as has been reported in recent decades. Moreover, thiazolidinedione-induced SKM injury has only been reported in humans, and no evidence of SKM injury has been observed in rodents. To establish a drug-induced SKM injury mouse model, we administered pioglitazone with a glutathione biosynthesis inhibitor, L-buthionine-S,R-sulfoximine, to C57BL/6J mice for 2days and subsequently observed prominent increases in plasma aspartate aminotransferase and creatinine phosphokinase, which were associated with SKM lesions. Furthermore, plasma miR-206 (SKM-specific microRNA) level was significantly increased, whereas plasma miR-208 (heart-specific microRNA) was not detected, indicating that pioglitazone specifically caused SKM, not cardiac, injury. Furthermore, we revealed that pioglitazone-induced SKM injury was caused by oxidative stress that was independent of the PPARγ agonistic effect. This study demonstrated for the first time that the glutathione-depleted C57BL/6J mouse is a novel model for assessing drug-induced SKM injury in drug development.

Cardiomyocyte-endothelial cell control of lipoprotein lipase

In people with diabetes, inadequate pharmaceutical management predisposes the patient to heart failure, which is the leading cause of diabetes related death. One instigator for this cardiac dysfunction is change in fuel utilization by the heart. Thus, following diabetes, when cardiac glucose utilization is impaired, the heart undergoes metabolic transformation wherein it switches to using fats as an exclusive source of energy. Although this switching is geared to help the heart initially, in the long term, this has detrimental effects on cardiac function. These include the generation of noxious byproducts, which damage the cardiomyocytes, and ultimately result in increased morbidity and mortality. A key perpetrator that may be responsible for organizing this metabolic disequilibrium is lipoprotein lipase (LPL), the enzyme responsible for providing fat to the hearts. Either exaggeration or reduction in its activity following diabetes could lead to heart dysfunction. Given the disturbing news that diabetes is rampant across the globe, gaining more insight into the mechanism(s) by which cardiac LPL is regulated may assist other researchers in devising new therapeutic strategies to restore metabolic equilibrium, to help prevent or delay heart disease seen during diabetes. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

A hepatic amino acid/mTOR/S6K-dependent signalling pathway modulates systemic lipid metabolism via neuronal signals

Metabolism is coordinated among tissues and organs via neuronal signals. Levels of circulating amino acids (AAs), which are elevated in obesity, activate the intracellular target of rapamycin complex-1 (mTORC1)/S6kinase (S6K) pathway in the liver. Here we demonstrate that hepatic AA/mTORC1/S6K signalling modulates systemic lipid metabolism via a mechanism involving neuronal inter-tissue communication. Hepatic expression of an AA transporter, SNAT2, activates the mTORC1/S6K pathway, and markedly elevates serum triglycerides (TGs), while downregulating adipose lipoprotein lipase (LPL). Hepatic Rheb or active-S6K expression have similar metabolic effects, whereas hepatic expression of dominant-negative-S6K inhibits TG elevation in SNAT2 mice. Denervation, pharmacological deafferentation and β-blocker administration suppress obesity-related hypertriglyceridemia with adipose LPL upregulation, suggesting that signals are transduced between liver and adipose tissue via a neuronal pathway consisting of afferent vagal and efferent sympathetic nerves. Thus, the neuronal mechanism uncovered here serves to coordinate amino acid and lipid levels and contributes to the development of obesity-related hypertriglyceridemia.

Differential expression of lipid metabolism-related genes and myosin heavy chain isoform genes in pig muscle tissue leading to different meat quality

The aim of this study was to investigate the variations in meat quality, lipid metabolism-related genes, myosin heavy chain (MyHC) isoform genes and peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) gene mRNA expressions in longissimus dorsi muscle (LM) of two different pig breeds. Six Rongchang and six Landrace barrows were slaughtered at 161 days of age. Subsequently, meat quality traits and gene expression levels in LM were observed. Results showed that Rongchang pigs not only exhibited greater pH, CIE a*24 h and intramuscular fat content but also exhibited lower body weight, carcass weight, dressing percentage, LM area and CIE b*24 h compared with Landrace pigs (P<0.05). Meanwhile, the mRNA expression levels of the lipogenesis (peroxisome proliferator-activated receptor gamma, acetyl-CoA carboxylase and fatty acid synthase) and fatty acid uptake (lipoprotein lipase)-related genes were greater in the Rongchang (P<0.05), whereas the lipolysis (adipose triglyceride lipase and hormone sensitive lipase) and fatty acid oxidation (carnitine palmitoyltransferase-1B)-related genes were better expressed in the Landrace. Moreover, compared with the Landrace, the mRNA expression levels of MyHCI, MyHCIIa and MyHCIIx were greater, whereas the mRNA expression levels of MyHCIIb were lower in the Rongchang pigs (P<0.05). In addition, the mRNA expression levels of PGC-1α were greater in Rongchang pigs than in the Landrace (P<0.05), which can partly explain the differences in MyHC isoform gene expressions between Rongchang and Landrace pigs. Although the small number of samples does not allow to obtain a definitive conclusion, we can suggest that Rongchang pigs possess better meat quality, and the underlying molecular mechanisms responsible for the better meat quality in fatty pigs may be partly due to the higher mRNA expression levels of lipogenesis and fatty acid uptake-related genes, as well as the oxidative and intermediate muscle fibers, and due to the lower mRNA expression levels of lipolysis and fatty acid oxidation-related genes, as well as the glycolytic muscle fibers.

The tissue distribution of lipoprotein lipase determines where chylomicrons bind

To determine the role of LPL for binding of lipoproteins to the vascular endothelium, and for the distribution of lipids from lipoproteins, four lines of induced mutant mice were used. Rat chylomicrons labeled in vivo with [(14)C]oleic acid (primarily in TGs, providing a tracer for lipolysis) and [(3)H]retinol (primarily in ester form, providing a tracer for the core lipids) were injected. TG label was cleared more rapidly than core label. There were no differences between the mouse lines in the rate at which core label was cleared. Two minutes after injection, about 5% of the core label, and hence chylomicron particles, were in the heart of WT mice. In mice that expressed LPL only in skeletal muscle, and had much reduced levels of LPL in the heart, binding of chylomicrons was reduced to 1%, whereas in mice that expressed LPL only in the heart, the binding was increased to over 10%. The same patterns of distribution were evident at 20 min when most of the label had been cleared. Thus, the amount of LPL expressed in muscle and heart governed both the binding of chylomicron particles and the assimilation of chylomicron lipids in the tissue.

Positive effect of exercise training at maximal fat oxidation intensity on body composition and lipid metabolism in overweight middle-aged women

The purpose of this study was to test the hypothesis that 10 weeks of supervised exercise training at the maximal fat oxidation (FATmax) intensity would improve important variables of body composition and lipid metabolism in overweight middle-aged women. A longitudinal study design was employed to evaluate the effects of FATmax exercise training. Thirty women (45-59 years old; BMI 28·2 ± 1·8 kg m(-2) ; body fat 38·9 ± 4·1%) were randomly allocated into the Exercise and Control groups, n = 15 in each group. Body composition, FATmax, predicted VO2 max, lipid profile, plasma lipoprotein lipase activity and serum leptin concentration were measured before and after the experimental period. The Exercise group was trained at the individualized FATmax intensity, 5 days per week and 1 h per day for 10 weeks. No diet control was introduced during the experimental period for all participants. Exercise group obtained significant decreases in body mass, BMI, body fat % and abdominal fat mass, as well as the concentrations of triglycerides, serum leptin and blood glucose. The activity of lipoprotein lipase was increased in trained participants. There were no changes in these variables in the Control group. In addition, there was no significant change in daily energy intake for all participants before and after the experimental period. In conclusion, the 10-week FATmax exercise training achieved improvements in body composition and lipid metabolism in overweight middle-aged women. This result suggests FATmax is an effective exercise training intensity for obesity treatment.

Intrinsic and extrinsic regulation of cardiac lipoprotein lipase following diabetes

Cardiac lipoprotein lipase (LPL) is a pivotal enzyme controlling heart metabolism by providing the majority of fatty acids required by this organ. From activation in cardiomyocytes to secretion to the vascular lumen, cardiac LPL is regulated by multiple pathways, which are altered during diabetes. Hence, dimerization/activation of LPL is modified following diabetes, a process controlled by lipase maturation factor 1. The role of AMP-activated protein kinase, protein kinase D, and heparan sulfate proteoglycans, intrinsic factors that regulate the intracellular transport of LPL is also shifted, and is discussed. More recent studies have identified several exogenous factors released from endothelial cells (EC) and adipose tissue that are required for proper functioning of LPL. In response to hyperglycemia, both active and latent heparanase are released from EC to facilitate LPL secretion. Diabetes also increased the expression of glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1) in EC, which mediates the transport of LPL across EC. Angiopoietin-like protein 4 secreted from the adipose tissue has the potential to reduce coronary LPL activity. Knowledge of these intrinsic and extrinsic factors could be used develop therapeutic targets to normalize LPL function, and maintain cardiac energy homeostasis after diabetes.

Lipolysis, and not hepatic lipogenesis, is the primary modulator of triglyceride levels in streptozotocin-induced diabetic mice

OBJECTIVE

Diabetic hypertriglyceridemia is thought to be primarily driven by increased hepatic de novo lipogenesis. However, experiments in animal models indicated that insulin deficiency should decrease hepatic de novo lipogenesis and reduce plasma triglyceride levels.

APPROACH AND RESULTS

To address the discrepancy between human data and genetically altered mouse models, we investigated whether insulin-deficient diabetic mice had triglyceride changes that resemble those in diabetic humans. Streptozotocin-induced insulin deficiency increased plasma triglyceride levels in mice. Contrary to the mouse models with impaired hepatic insulin receptor signaling, insulin deficiency did not reduce hepatic triglyceride secretion and de novo lipogenesis-related gene expression. Diabetic mice had a marked decrease in postprandial triglycerides clearance, which was associated with decreased lipoprotein lipase and peroxisome proliferator-activated receptor α mRNA levels in peripheral tissues and decreased lipoprotein lipase activity in skeletal muscle, heart, and brown adipose tissue. Diabetic heterozygous lipoprotein lipase knockout mice had markedly elevated fasting plasma triglyceride levels and prolonged postprandial triglycerides clearance.

CONCLUSIONS

Insulin deficiency causes hypertriglyceridemia by decreasing peripheral lipolysis and not by an increase in hepatic triglycerides production and secretion.

Peroxisomes take shape

Peroxisomes carry out various oxidative reactions that are tightly regulated to adapt to the changing needs of the cell and varying external environments. Accordingly, they are remarkably fluid and can change dramatically in abundance, size, shape and content in response to numerous cues. These dynamics are controlled by multiple aspects of peroxisome biogenesis that are coordinately regulated with each other and with other cellular processes. Ongoing studies are deciphering the diverse molecular mechanisms that underlie biogenesis and how they cooperate to dynamically control peroxisome utility. These important challenges should lead to an understanding of peroxisome dynamics that can be capitalized upon for bioengineering and the development of therapies to improve human health.

The effects of obesity on skeletal muscle regeneration

Obesity and metabolic disorders such as type 2 diabetes mellitus are accompanied by increased lipid deposition in adipose and non-adipose tissues including liver, pancreas, heart and skeletal muscle. Recent publications report impaired regenerative capacity of skeletal muscle following injury in obese mice. Although muscle regeneration has not been thoroughly studied in obese and type 2 diabetic humans and mechanisms leading to decreased muscle regeneration in obesity remain elusive, the initial findings point to the possibility that muscle satellite cell function is compromised under conditions of lipid overload. Elevated toxic lipid metabolites and increased pro-inflammatory cytokines as well as insulin and leptin resistance that occur in obese animals may contribute to decreased regenerative capacity of skeletal muscle. In addition, obesity-associated alterations in the metabolic state of skeletal muscle fibers and satellite cells may directly impair the potential for satellite cell-mediated repair. Here we discuss recent studies that expand our understanding of how obesity negatively impacts skeletal muscle maintenance and regeneration.

Safety profile of recombinant adeno-associated viral vectors: focus on alipogene tiparvovec (Glybera®)

There has been great interest over the past two decades in developing gene therapies (GTs) to treat a variety of diseases; however, translating research findings into clinical treatments have proved to be a challenge. A major milestone in the development of GT has been achieved with the approval of alipogene tiparvovec (Glybera(®)) in Europe for the treatment of familial lipoprotein lipase deficiency. At this important stage with the evolution of GT into the clinic, this review will examine the safety aspects GT with adeno-associated virus (AAV) vectors. The topics that will be covered include acute reactions, immunological reactions to the AAV capsid and expressed transgene, viral biodistribution and shedding, DNA integration and carcinogenicity. These safety aspects of GT will be discussed with a focus on alipogene tiparvovec, in addition to other AAV vector GT products currently in clinical development.

Hyperglycemia-induced secretion of endothelial heparanase stimulates a vascular endothelial growth factor autocrine network in cardiomyocytes that promotes recruitment of lipoprotein lipase

OBJECTIVE

During diabetes mellitus, coronary lipoprotein lipase increases to promote the predominant use of fatty acids. We have reported that high glucose stimulates active heparanase secretion from endothelial cells to cleave cardiomyocyte heparan sulfate and release bound lipoprotein lipase for transfer to the vascular lumen. In the current study, we examined whether heparanase also has a function to release cardiomyocyte vascular endothelial growth factor (VEGF), and whether this growth factor influences cardiomyocyte fatty acid delivery in an autocrine manner.

APPROACH AND RESULTS

Acute, reversible hyperglycemia was induced in rats, and a modified Langendorff heart perfusion was used to separate the coronary perfusate from the interstitial effluent. Coronary artery endothelial cells were exposed to high glucose to generate conditioned medium, and VEGF release from isolated cardiomyocytes was tested using endothelial cell conditioned medium or purified active and latent heparanase. Autocrine signaling of myocyte-derived VEGF on cardiac metabolism was studied. High glucose promoted latent and active heparanase secretion into endothelial cell conditioned medium, an effective stimulus for releasing cardiomyocyte VEGF. Intriguingly, latent heparanase was more efficient than active heparanase in releasing VEGF from a unique cell surface pool. VEGF augmented cardiomyocyte intracellular calcium and AMP-activated protein kinase phosphorylation and increased heparin-releasable lipoprotein lipase.

CONCLUSIONS

Our data suggest that the heparanase-lipoprotein lipase-VEGF axis amplifies fatty acid delivery, a rapid and adaptive mechanism that is geared to overcome the loss of glucose consumption by the diabetic heart. If prolonged, the resultant lipotoxicity could lead to cardiovascular disease in humans.

Mitochondrial deficiency is associated with insulin resistance

The specific cellular underpinnings or mechanisms of insulin resistance (IR) are not clear. Here I present evidence to support a causal association between mitochondrial energetics and IR. A large body of literature indicates that mitochondrial capacity for oxidative metabolism is lower in human obesity and type 2 diabetes. Whether or not mitochondria play a causal role in IR is hotly debated. First, IR can be caused by many factors, many of which may or may not involve mitochondria. These include lipid overload, oxidative stress, and inflammation. Thus the first tenet of an argument supporting a role for mitochondria in IR is that mitochondria derangements can cause IR, but IR does not have to involve mitochondria. The second tenet of this argument is that animal models in which oxidative metabolism are completely abolished are not always physiologically or pathologically relevant to human IR, in which small metabolic perturbations can have profound effects over a prolonged period. Lastly, mitochondria are complex organelles, with diverse functions, including links with cell signaling, oxidative stress, and inflammation, which in turn can be connected with IR. In summary, mitochondrial "deficiency" is not merely a reduced energy generation or low fatty acid oxidation; this concept should be expanded to numerous additional important functions, many of which can cause IR if perturbed.

Adipose-specific lipoprotein lipase deficiency more profoundly affects brown than white fat biology

Adipose fat storage is thought to require uptake of circulating triglyceride (TG)-derived fatty acids via lipoprotein lipase (LpL). To determine how LpL affects the biology of adipose tissue, we created adipose-specific LpL knock-out (ATLO) mice, and we compared them with whole body LpL knock-out mice rescued with muscle LpL expression (MCK/L0) and wild type (WT) mice. ATLO LpL mRNA and activity were reduced, respectively, 75 and 70% in gonadal adipose tissue (GAT), 90 and 80% in subcutaneous tissue, and 84 and 85% in brown adipose tissue (BAT). ATLO mice had increased plasma TG levels associated with reduced chylomicron TG uptake into BAT and lung. ATLO BAT, but not GAT, had altered TG composition. GAT from MCK/L0 was smaller and contained less polyunsaturated fatty acids in TG, although GAT from ATLO was normal unless LpL was overexpressed in muscle. High fat diet feeding led to less adipose in MCK/L0 mice but TG acyl composition in subcutaneous tissue and BAT reverted to that of WT. Therefore, adipocyte LpL in BAT modulates plasma lipoprotein clearance, and the greater metabolic activity of this depot makes its lipid composition more dependent on LpL-mediated uptake. Loss of adipose LpL reduces fat accumulation only if accompanied by greater LpL activity in muscle. These data support the role of LpL as the "gatekeeper" for tissue lipid distribution.

Cardiac-specific overexpression of perilipin 5 provokes severe cardiac steatosis via the formation of a lipolytic barrier

Cardiac triacylglycerol (TG) catabolism critically depends on the TG hydrolytic activity of adipose triglyceride lipase (ATGL). Perilipin 5 (Plin5) is expressed in cardiac muscle (CM) and has been shown to interact with ATGL and its coactivator comparative gene identification-58 (CGI-58). Furthermore, ectopic Plin5 expression increases cellular TG content and Plin5-deficient mice exhibit reduced cardiac TG levels. In this study we show that mice with cardiac muscle-specific overexpression of perilipin 5 (CM-Plin5) massively accumulate TG in CM, which is accompanied by moderately reduced fatty acid (FA) oxidizing gene expression levels. Cardiac lipid droplet (LD) preparations from CM of CM-Plin5 mice showed reduced ATGL- and hormone-sensitive lipase-mediated TG mobilization implying that Plin5 overexpression restricts cardiac lipolysis via the formation of a lipolytic barrier. To test this hypothesis, we analyzed TG hydrolytic activities in preparations of Plin5-, ATGL-, and CGI-58-transfected cells. In vitro ATGL-mediated TG hydrolysis of an artificial micellar TG substrate was not inhibited by the presence of Plin5, whereas Plin5-coated LDs were resistant toward ATGL-mediated TG catabolism. These findings strongly suggest that Plin5 functions as a lipolytic barrier to protect the cardiac TG pool from uncontrolled TG mobilization and the excessive release of free FAs.

Skeletal muscle damage and impaired regeneration due to LPL-mediated lipotoxicity

According to the concept of lipotoxicity, ectopic accumulation of lipids in non-adipose tissue induces pathological changes. The most prominent effects are seen in fatty liver disease, lipid cardiomyopathy, non-insulin-dependent diabetes mellitus, insulin resistance and skeletal muscle myopathy. We used the MCK(m)-hLPL mouse distinguished by skeletal and cardiac muscle-specific human lipoprotein lipase (hLPL) overexpression to investigate effects of lipid overload in skeletal muscle. We were intrigued to find that ectopic lipid accumulation induced proteasomal activity, apoptosis and skeletal muscle damage. In line with these findings we observed reduced Musculus gastrocnemius and Musculus quadriceps mass in transgenic animals, accompanied by severely impaired physical endurance. We suggest that muscle loss was aggravated by impaired muscle regeneration as evidenced by reduced cross-sectional area of regenerating myofibers after cardiotoxin-induced injury in MCK(m)-hLPL mice. Similarly, an almost complete loss of myogenic potential was observed in C2C12 murine myoblasts upon overexpression of LPL. Our findings directly link lipid overload to muscle damage, impaired regeneration and loss of performance. These findings support the concept of lipotoxicity and are a further step to explain pathological effects seen in muscle of obese patients, patients with the metabolic syndrome and patients with cancer-associated cachexia.

Lipoprotein lipase in the brain and nervous system

Lipoprotein lipase (LPL) is rate limiting in the provision of triglyceride-rich lipoprotein-derived lipids into tissues. LPL is also present in the brain, where its function has remained elusive. Recent evidence implicates a role of LPL in the brain in two processes: (a) the regulation of energy balance and body weight and (b) cognition. Mice with neuron-specific deletion of LPL have increases in food intake that lead to obesity, and then reductions in energy expenditure that further contribute to and sustain the phenotype. In other mice with LPL deficiency rescued from neonatal lethality by somatic gene transfer wherein LPL in the brain remains absent, altered cognition ensues. Taking into consideration data that associate LPL mutations with Alzheimer's disease, a role for LPL in learning and memory seems likely. Overall, the time is ripe for new insights into how LPL-mediated lipoprotein metabolism in the brain impacts CNS processes and systems biology.

Regulation of mitochondrial biogenesis by lipoprotein lipase in muscle of insulin-resistant offspring of parents with type 2 diabetes

Recent studies reveal a strong relationship between reduced mitochondrial content and insulin resistance in human skeletal muscle, although the underlying factors responsible for this association remain unknown. To address this question, we analyzed muscle biopsy samples from young, lean, insulin resistant (IR) offspring of parents with type 2 diabetes and control subjects by microarray analyses and found significant differences in expression of ~512 probe pairs. We then screened these genes for their potential involvement in the regulation of mitochondrial biogenesis using RNA interference and found that mRNA and protein expression of lipoprotein lipase (LPL) in skeletal muscle was significantly decreased in the IR offspring and was associated with decreased mitochondrial density. Furthermore, we show that LPL knockdown in muscle cells decreased mitochondrial content by effectively decreasing fatty acid delivery and subsequent activation of peroxisome proliferator-activated receptor (PPAR)-δ. Taken together, these data suggest that decreased mitochondrial content in muscle of IR offspring may be due in part to reductions in LPL expression in skeletal muscle resulting in decreased PPAR-δ activation.

Mitaplatin increases sensitivity of tumor cells to cisplatin by inducing mitochondrial dysfunction

Tumor resistance to chemotherapy is the major obstacle to employ cisplatin, one of the broadly used chemotherapeutic drugs, for effective treatment of various tumors in the clinic. Most acknowledged mechanisms of cancer resistance to cisplatin focus on increased nuclear DNA repair or detoxicity of cisplatin. We previously demonstrated that there was a unique metabolic profile in cisplatin-resistant (CP-r) human epidermoid adenocarcinoma KB-CP 20 and hepatoma BEL 7404-CP 20 cancer cells. In this study, we further defined hyperpolarized mitochondrial membrane potentials (Δψ(m)) in CP-r KB-CP 20 and BEL 7404-CP 20 cells compared to the cisplatin-sensitive (CP-s) KB-3-1 and BEL 7404 cells. Based on the mitochondrial dysfunction, mitaplatin was designed with two mitochondrial-targeting moieties [dichloroacetate (DCA) units] to the axial positions of a six-coordinate Pt(IV) center to sensitize cisplatin resistance. It was found that mitaplatin induced more apoptosis in CP-r KB-CP 20 and BEL 7404-CP 20 cells than that of cisplatin, DCA and cisplatin/DCA compared on an equal molar basis. There was more platinum accumulation in mitaplatin-treated CP-r cells due to enhanced transmembrane permeability of lipophilicity, and mitaplatin also showed special targeting to mitochondria. Moreover, in the case of treatment with mitaplatin, the dramatic collapse of Δψ(m) was shown in a dose-dependent manner, which was confirmed by FACS and confocal microscopic measurements. Reduced glucose utilization of CP-r cells was detected with specifically inhibited phosphorylation of pyruvate dehydrogenase (PDH) at Ser-232, Ser-293, and Ser-300 of the E1α subunit when treated with mitaplatin, which was indicated to modulate the abnormal glycolysis of resistant cells. The present study suggested novel mitochondrial mechanism of mitaplatin circumventing cisplatin resistance toward CP-r cells as a carrier across membrane to produce CP-like cytotoxicity and DCA-like mitochondria-dependent apoptosis. Therefore, mitochondria targeting compounds would be more vulnerable and selective to overcome cisplatin resistance due to the unique metabolic properties of CP-r cancer cells.

Lipoprotein lipase mediated fatty acid delivery and its impact in diabetic cardiomyopathy

Although cardiovascular disease is the leading cause of diabetes-related death, its etiology is still not understood. The immediate change that occurs in the diabetic heart is altered energy metabolism where in the presence of impaired glucose uptake, glycolysis, and pyruvate oxidation, the heart switches to exclusively using fatty acids (FA) for energy supply. It does this by rapidly amplifying its lipoprotein lipase (LPL-a key enzyme, which hydrolyzes circulating lipoprotein-triglyceride to release FA) activity at the coronary lumen. An abnormally high capillary LPL could provide excess fats to the heart, leading to a number of metabolic, morphological, and mechanical changes, and eventually to cardiac disease. Unlike the initial response, chronic severe diabetes "turns off" LPL, this is also detrimental to cardiac function. In this review, we describe a number of post-translational mechanisms that influence LPL vesicle formation, actin cytoskeleton rearrangement, and transfer of LPL from cardiomyocytes to the vascular lumen to hydrolyze lipoprotein-triglyceride following diabetes. Appreciating the mechanism of how the heart regulates its LPL following diabetes should allow the identification of novel targets for therapeutic intervention, to prevent heart failure. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Amelioration of hypertriglyceridemia with hypo-alpha-cholesterolemia in LPL deficient mice by hematopoietic cell-derived LPL

Background

Macrophage-derived lipoprotein lipase (LPL) has been shown uniformly to promote atherosclerotic lesion formation while the extent to which it affects plasma lipid and lipoprotein levels varies in wild-type and hypercholesterolemic mice. It is known that high levels of LPL in the bulk of adipose tissue and skeletal muscle would certainly mask the contribution of macrophage LPL to metabolism of plasma lipoprotein. Therefore, we chose LPL deficient (LPL^-/-) mice with severe hypertriglyceridemia as an alternative model to assess the role of macrophage LPL in plasma lipoprotein metabolism via bone marrow transplant, through which LPL will be produced mainly by hematopoietic cell-derived macrophages.

Methods and Results

Hypertriglyceridemic LPL^-/- mice were lethally irradiated, then transplanted with bone marrow from wild-type (LPL^+/+) or LPL^-/- mice, respectively. Sixteen weeks later, LPL^+/+ →LPL^-/- mice displayed significant reduction in plasma levels of triglyceride and cholesterol (408±44.9 vs. 2.7±0.5×10³ and 82.9±7.1 vs. 229.1±30.6 mg/dl, p<0.05, respectively), while a 2.7-fold increase in plasma high density lipoprotein- cholesterol (p<0.01) was observed, compared with LPL^-/-→LPL^-/- control mice. The clearance rate for the oral fat load test in LPL^+/+ →LPL^-/- mice was faster than that in LPL^-/-→LPL^-/- mice, but slower than that in wild-type mice. Liver triglyceride content in LPL^+/+→LPL^-/- mice was also significantly increased, compared with LPL^-/-→LPL^-/- mice (6.8±0.7 vs. 4.6±0.5 mg/g wet tissue, p<0.05, n = 6). However, no significant change was observed in the expression levels of genes involved in hepatic lipid metabolism between the two groups.

Conclusions

Hematopoietic cell-derived LPL could efficiently ameliorate severe hypertriglyceridemia and hypo-alpha-cholesterolemia at the compensation of increased triglyceride content of liver in LPL^-/- mice.

Transgenic rabbits expressing human lipoprotein lipase

To study the functions of lipoprotein lipase (LPL) in lipid and lipoprotein metabolism and the relationship between LPL and atherosclerosis, we generated transgenic rabbits expressing the human LPL gene. A total of 4045 Japanese whiterabbit embryos were microinjected with a 3.8-kb SalI/HindIII fragment containing the chicken beta-actin promoter, human LPL cDNA and rabbit beta-globin with poly (A) signals, and then transplanted into 116 recipient rabbits. Of the 166 pups born, six pups were transgenic as confirmed by Southern blot analysis. ANorthern blot analysis revealed that human LPL was expressed by a number of tissues including the heart, kidney, adrenal gland and intestine. One transgenic rabbit showed up to 3-foldincreased LPL activity in post-heparin plasma compared to thatin nontransgenic rabbits. Human LPL expression in various tissues of transgenic rabbits was further elucidated by in situ hybridization and immunostaining. Since rabbits are superior to mice as a model of atherosclerosis, this transgenicrabbit model should provide a valuable tool for the study of LPL in lipid metabolism and atherosclerosis.