Skeletal muscle lipid accumulation in type 2 diabetes may involve the liver X receptor pathway

Energy metabolism in skeletal muscle cells from donors with different body mass index

Obesity and physical inactivity have a profound impact on skeletal muscle metabolism. In the present work, we have investigated differences in protein expression and energy metabolism in primary human skeletal muscle cells established from lean donors (BMI<25 kg/m2) and individuals with obesity (BMI>30 kg/m2). Furthermore, we have studied the effect of fatty acid pretreatment on energy metabolism in myotubes from these donor groups. Alterations in protein expression were investigated using proteomic analysis, and energy metabolism was studied using radiolabeled substrates. Gene Ontology enrichment analysis showed that glycolytic, apoptotic, and hypoxia pathways were upregulated, whereas the pentose phosphate pathway was downregulated in myotubes from donors with obesity compared to myotubes from lean donors. Moreover, fatty acid, glucose, and amino acid uptake were increased in myotubes from individuals with obesity. However, fatty acid oxidation was reduced, glucose oxidation was increased in myotubes from subjects with obesity compared to cells from lean. Pretreatment of myotubes with palmitic acid (PA) or eicosapentaenoic acid (EPA) for 24 h increased glucose oxidation and oleic acid uptake. EPA pretreatment increased the glucose and fatty acid uptake and reduced leucine fractional oxidation in myotubes from donors with obesity. In conclusion, these results suggest that myotubes from individuals with obesity showed increased fatty acid, glucose, and amino acid uptake compared to cells from lean donors. Furthermore, myotubes from individuals with obesity had reduced fatty acid oxidative capacity, increased glucose oxidation, and a higher glycolytic reserve capacity compared to cells from lean donors. Fatty acid pretreatment enhances glucose metabolism, and EPA reduces oleic acid and leucine fractional oxidation in myotubes from donor with obesity, suggesting increased metabolic flexibility after EPA treatment.

SENP2 is vital for optimal insulin signaling and insulin-stimulated glycogen synthesis in human skeletal muscle cells

Sentrin-specific protease (SENP) 2 has been suggested as a possible novel drug target for the treatment of obesity and type 2 diabetes mellitus after observations of a palmitate-induced increase in SENP2 that lead to increased fatty acid oxidation and improved insulin sensitivity in skeletal muscle cells from mice. However, no precedent research has examined the role of SENP2 in human skeletal muscle cells. In the present work, we have investigated the impact of SENP2 on fatty acid and glucose metabolism as well as insulin sensitivity in human skeletal muscle using cultured primary human myotubes. Acute (4 ​h) oleic acid oxidation was reduced in SENP2-knockdown (SENP2-KD) cells compared to control cells, with no difference in uptake. After prelabeling (24 ​h) with oleic acid, total lipid content and incorporation into triacylglycerol was decreased, while incorporation into other lipids, as well as complete oxidation and β-oxidation was increased in SENP2-KD cells. Basal glucose uptake (i.e., not under insulin-stimulated conditions) was higher in SENP2-KD cells, whereas oxidation was similar to control myotubes. Further, basal glycogen synthesis was not different in SENP2-KD myotubes, but both insulin-stimulated glycogen synthesis and Akt^Ser473 phosphorylation was completely blunted in SENP2-KD cells. In conclusion, SENP2 plays an important role in fatty acid and glucose metabolism in human myotubes. Interestingly, it also appears to have a pivotal role in regulating myotube insulin sensitivity. Future studies should examine the role of SENP2 in regulation of insulin sensitivity in other tissues and in vivo, defining the potential for SENP2 as a drug target.

Skeletal muscle energy metabolism in obesity

Comparing energy metabolism in human skeletal muscle and primary skeletal muscle cells in obesity, while focusing on glucose and fatty acid metabolism, shows many common changes. Insulin-mediated glucose uptake in skeletal muscle and primary myotubes is decreased by obesity, whereas differences in basal glucose metabolism are inconsistent among studies. With respect to fatty acid metabolism, there is an increased uptake and storage of fatty acids and a reduced complete lipolysis, suggesting alterations in lipid turnover. In addition, fatty acid oxidation is decreased, probably at the level of complete oxidation, as β -oxidation may be enhanced in obesity, which indicates mitochondrial dysfunction. Metabolic changes in skeletal muscle with obesity promote metabolic inflexibility, ectopic lipid accumulation, and formation of toxic lipid intermediates. Skeletal muscle also acts as an endocrine organ, secreting myokines that participate in interorgan cross talk. This review highlights interventions and some possible targets for treatment through action on skeletal muscle energy metabolism. Effects of exercise in vivo on obesity have been compared with simulation of endurance exercise in vitro on myotubes (electrical pulse stimulation). Possible pharmaceutical targets, including signaling pathways and drug candidates that could modify lipid storage and turnover or increase mitochondrial function or cellular energy expenditure through adaptive thermogenic mechanisms, are discussed.

Estrogen-related receptor α is involved in angiogenesis and skeletal muscle revascularization in hindlimb ischemia

Skeletal muscle ischemia is a major consequence of peripheral arterial disease (PAD) or critical limb ischemia (CLI). Although therapeutic options for resolving muscle ischemia in PAD/CLI are limited, the issue is compounded by poor understanding of the mechanisms driving muscle vascularization. We found that nuclear receptor estrogen-related receptor alpha (ERRα) expression is induced in murine skeletal muscle by hindlimb ischemia (HLI), and in cultured myotubes by hypoxia, suggesting a potential role for ERRα in ischemic response. To test this, we generated skeletal muscle-specific ERRα transgenic (TG) mice. In these mice, ERRα drives myofiber type switch from glycolytic type IIB to oxidative type IIA/IIX myofibers, which are typically associated with more vascular supply in muscle. Indeed, RNA sequencing and functional enrichment analysis of TG muscle revealed that "paracrine angiogenesis" is the top-ranked transcriptional program activated by ERRα in the skeletal muscle. Immunohistochemistry and angiography showed that ERRα overexpression increases baseline capillarity, arterioles and non-leaky blood vessel formation in the skeletal muscles. Moreover, ERRα overexpression facilitates ischemic neo-angiogenesis and perfusion recovery in hindlimb musculature of mice subjected to HLI. Therefore, ERRα is a hypoxia inducible nuclear receptor that is involved in skeletal muscle angiogenesis and could be potentially targeted for treating PAD/CLI.

Advances in Understanding of the Role of Lipid Metabolism in Aging

During aging, body adiposity increases with changes in the metabolism of lipids and their metabolite levels. Considering lipid metabolism, excess adiposity with increased lipotoxicity leads to various age-related diseases, including cardiovascular disease, cancer, arthritis, type 2 diabetes, and Alzheimer's disease. However, the multifaceted nature and complexities of lipid metabolism make it difficult to delineate its exact mechanism and role during aging. With advances in genetic engineering techniques, recent studies have demonstrated that changes in lipid metabolism are associated with aging and age-related diseases. Lipid accumulation and impaired fatty acid utilization in organs are associated with pathophysiological phenotypes of aging. Changes in adipokine levels contribute to aging by modulating changes in systemic metabolism and inflammation. Advances in lipidomic techniques have identified changes in lipid profiles that are associated with aging. Although it remains unclear how lipid metabolism is regulated during aging, or how lipid metabolites impact aging, evidence suggests a dynamic role for lipid metabolism and its metabolites as active participants of signaling pathways and regulators of gene expression. This review describes recent advances in our understanding of lipid metabolism in aging, including established findings and recent approaches.

Regulation of glycolytic genes in human macrophages by oxysterols: a potential role for liver X receptors

BACKGROUND AND PURPOSE

Subset of macrophages within the atheroma plaque displays a high glucose uptake activity. Nevertheless, the molecular mechanisms and the pathophysiological significance of this high glucose need remain unclear. While the role for hypoxia and hypoxia inducible factor 1α has been demonstrated, the contribution of lipid micro-environment and more specifically oxysterols is yet to be explored.

EXPERIMENTAL APPROACH

Human macrophages were conditioned in the presence of homogenates from human carotid plaques, and expression of genes involved in glucose metabolism was quantified. Correlative analyses between gene expression and the oxysterol composition of plaques were performed.

KEY RESULTS

Conditioning of human macrophages by plaque homogenates induces expression of several genes involved in glucose uptake and glycolysis including glucose transporter 1 (SLC2A1) and hexokinases 2 and 3 (HK2 and HK3). This activation is significantly correlated to the oxysterol content of the plaque samples and is associated with a significant increase in the glycolytic activity of the cells. Pharmacological inverse agonist of the oxysterol receptor liver X receptor (LXR) partially reverses the induction of glycolysis genes without affecting macrophage glycolytic activity. Chromatin immunoprecipitation analysis confirms the implication of LXR in the regulation of SLC2A1 and HK2 genes.

CONCLUSION AND IMPLICATIONS

While our work supports the role of oxysterols and the LXR in the modulation of macrophage metabolism in atheroma plaques, it also highlights some LXR-independent effects of plaques samples. Finally, this study identifies hexokinase 3 as a promising target in the context of atherosclerosis.

LINKED ARTICLES

This article is part of a themed issue on Oxysterols, Lifelong Health and Therapeutics. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.16/issuetoc.

Dietary high calories from sunflower oil, sucrose and fructose sources alters lipogenic genes expression levels in liver and skeletal muscle in rats

INTRODUCTION AND OBJECTIVES

The objectives of this study were to investigate the underlying mechanism of PPARα, LXRα, ChREBP, and SREBP-1c at the level of gene and protein expression with high-energy diets in liver and skeletal muscle.

MATERIALS AND METHODS

Metabolic changes with consumption of high fat (Hfat), high sucrose (Hsuc) and high fructose (Hfru) diets were assessed. Levels of mRNA and protein of PPARα, LXRα, ChREBP, and SREBP-1c were investigated. Body weight changes, histological structure of liver and plasma levels of some parameters were also examined.

RESULTS

In Hfru group, body weights were higher than other groups (P<0.05). In liver, LXRα levels of Hsuc and Hfru groups were upregulated as 1.87±0.30 (P<0.05) and 2.01±0.29 (P<0.01). SREBP-1c levels were upregulated as 4.52±1.25 (P<0.05); 4.05±1.11 (P<0.05) and 3.85±1.04 (P<0.05) in Hfat, Hsuc, and Hfru groups, respectively. In skeletal muscle, LXRα and SREBP-1c were upregulated as 1.77±0.30 (P<0.05) and 2.71±0.56 (P<0.05), in the Hfru group. Protein levels of ChREBP (33.92±8.84ng/mg protein (P<0.05)) and SREBP-1c (135.16±15.57ng/mg protein (P<0.001)) in liver were higher in Hfru group. In skeletal muscle, LXRα, ChREBP and SREBP-1c in Hfru group were 6.67±0.60, 7.11±1.29 and 43.17±6.37ng/mg, respectively (P<0.05; P<0.01; P<0.05). The rats in Hfru group had the most damaged livers.

CONCLUSION

Besides liver, fructose consumption significantly effects skeletal muscle and leads to weight gain, triggers lipogenesis and metabolic disorders.

Role of HSD17B13 in the liver physiology and pathophysiology

17β-Hydroxysteroid dehydrogenases (HSD17Bs) comprise a large family of 15 members that are mainly involved in sex hormone metabolism. Some HSD17Bs enzymes also play key roles in cholesterol and fatty acid metabolism. Recent study showed that hydroxysteroid 17β-dehydrogenase 13 (HSD17B13), an enzyme with unknown biological function, is a novel liver-specific lipid droplet (LD)-associated protein in mouse and humans. HSD17B13 expression is markedly upregulated in patients and mice with non-alcoholic fatty liver disease (NAFLD). Hepatic overexpression of HSD17B13 promotes lipid accumulation in the liver. In this review, we summarize recent progress regarding the role of HSD17B13 in the regulation of hepatic lipid homeostasis and discuss genetic, genomic and proteomic evidence supporting the pathogenic role of HSD17B13 in NAFLD. We also emphasize its potential as a biomarker of advanced liver disease, such as non-alcoholic steatohepatitis (NASH) and liver cancer.

Challenges and perspectives in the treatment of diabetes associated breast cancer

Type 2 diabetes mellitus is one of the most common chronic disease worldwide and affects all cross-sections of the society including children, women, youth and adults. Scientific evidence has linked diabetes to higher incidence, accelerated progression and increased aggressiveness of different cancers. Among the different forms of cancer, research has reinforced a link between diabetes and the risk of breast cancer. Some studies have specifically linked diabetes to the highly aggressive, triple negative breast cancers (TNBCs) which do not respond to conventional hormonal/HER2 targeted interventions, have chances of early recurrence, metastasize, tend to be more invasive in nature and develop drug resistance. Commonly used anti-diabetic drugs, such as metformin, have recently gained importance in the treatment of breast cancer due to their proposed anti-cancer properties. Here we discuss the link between diabetes and breast cancer, the metabolic disturbances in diabetes that support the development of breast cancer, the challenges involved and future perspective and directions. We link the three main metabolic disturbances (dyslipidemia, hyperinsulinemia and hyperglycemia) that occur in diabetes to potential aberrant molecular pathways that may lead to the development of an oncogenic phenotype of the breast tissue, thereby leading to acceleration of cell growth, proliferation, migration, inflammation, angiogenesis, EMT and metastasis and inhibition of apoptosis in breast cancer cells. Furthermore, managing diabetes and treating cancer using a combination of anti-diabetic and classical anti-cancer drugs should prove to be more efficient in the treatment diabetes associated cancers.

Deficiency of liver-X-receptor-α reduces glucose uptake and worsens post-myocardial infarction remodeling

Liver X receptor α (LXRα) is an endogenous protective receptor against ischemic heart diseases. However, whether LXRα regulated glucose metabolism in ischemic heart diseases has not been investigated. In this study we investigated the involvement of LXRα on glucose metabolism in cardiac remodeling after myocardial infarction (MI). MI was induced in mice by permanent ligation of the left anterior descending coronary artery (LCA). Genetic LXRα deletion significantly worsened cardiac remodeling and impaired cardiac function at 4 weeks after MI. Cardiac 18F-fluorodeoxyglucose (FDG) uptake by positron emission tomography (PET) demonstrated that the FDG standardized uptake value (SUV) was significantly lower in LXRα^-/- mice as compared to WT mice. Mechanistically, GLUT1/4 and AMPK phosphorylation were significantly downregulated while CD36 expression was markedly upregulated in LXRα^-/- mice. This study demonstrated that deficiency of LXRα decreased glucose uptake after MI, resulting in a metabolic shift that suppressed glucose metabolism, which was in association with adverse cardiac remodeling.

Exercise in vivo marks human myotubes in vitro: Training-induced increase in lipid metabolism

BACKGROUND AND AIMS

Physical activity has preventive as well as therapeutic benefits for overweight subjects. In this study we aimed to examine effects of in vivo exercise on in vitro metabolic adaptations by studying energy metabolism in cultured myotubes isolated from biopsies taken before and after 12 weeks of extensive endurance and strength training, from healthy sedentary normal weight and overweight men.

METHODS

Healthy sedentary men, aged 40-62 years, with normal weight (body mass index (BMI) < 25 kg/m2) or overweight (BMI ≥ 25 kg/m2) were included. Fatty acid and glucose metabolism were studied in myotubes using [14C]oleic acid and [14C]glucose, respectively. Gene and protein expressions, as well as DNA methylation were measured for selected genes.

RESULTS

The 12-week training intervention improved endurance, strength and insulin sensitivity in vivo, and reduced the participants' body weight. Biopsy-derived cultured human myotubes after exercise showed increased total cellular oleic acid uptake (30%), oxidation (46%) and lipid accumulation (34%), as well as increased fractional glucose oxidation (14%) compared to cultures established prior to exercise. Most of these exercise-induced increases were significant in the overweight group, whereas the normal weight group showed no change in oleic acid or glucose metabolism.

CONCLUSIONS

12 weeks of combined endurance and strength training promoted increased lipid and glucose metabolism in biopsy-derived cultured human myotubes, showing that training in vivo are able to induce changes in human myotubes that are discernible in vitro.

Liver X receptor α induces 17β-hydroxysteroid dehydrogenase-13 expression through SREBP-1c

Liver X receptors, including LXRα and LXRβ, are known to be master regulators of liver lipid metabolism. Activation of LXRα increases hepatic lipid storage in lipid droplets (LDs). 17β-Hydroxysteroid dehydrogenase-13 (17β-HSD13), a recently identified liver-specific LD-associated protein, has been reported to be involved in the development of nonalcoholic fatty liver disease. However, little is known about its transcriptional regulation. In the present study, we aimed at determining whether 17β-HSD13 gene transcription is controlled by LXRs. We found that treatment with T0901317, a nonspecific LXR agonist, increased both 17β-HSD13 mRNA and protein levels in cultured hepatocytes. It also significantly upregulated hepatic 17β-HSD13 expression in wild-type (WT) and LXRβ^-/- mice but not in LXRα^-/- mice. Basal expression of 17β-HSD13 in the livers of LXRα^-/- mice was lower than that in the livers of WT and LXRβ^-/- mice. Moreover, induction of hepatic 17β-HSD13 expression by T0901317 was almost completely abolished in SREBP-1c^-/- mice. Bioinformatics analysis revealed a consensus sterol regulatory element (SRE)-binding site in the promoter region of the 17β-HSD13 gene. A 17β-HSD13 gene promoter-driven luciferase reporter and ChIP assays further confirmed that the 17β-HSD13 gene was under direct control of SREBP-1c. Collectively, these findings demonstrate that LXRα activation induces 17β-HSD13 expression in a SREBP-1c-dependent manner. 17β-HSD13 may be involved in the development of LXRα-mediated fatty liver.

Effect of Phenolic Compounds from Elderflowers on Glucose- and Fatty Acid Uptake in Human Myotubes and HepG2-Cells

Type 2 diabetes (T2D) is manifested by progressive metabolic impairments in tissues such as skeletal muscle and liver, and these tissues become less responsive to insulin, leading to hyperglycemia. In the present study, stimulation of glucose and oleic acid uptake by elderflower extracts, constituents and metabolites were tested in vitro using the HepG2 hepatocellular liver carcinoma cell line and human skeletal muscle cells. Among the crude extracts, the 96% EtOH extract showed the highest increase in glucose and oleic acid uptake in human skeletal muscle cells and HepG2-cells. The flavonoids and phenolic acids contained therein were potent stimulators of glucose and fatty acid uptake in a dose-dependent manner. Most of the phenolic constituents and several of the metabolites showed high antioxidant activity and showed considerably higher α-amylase and α-glucosidase inhibition than acarbose. Elderflower might therefore be valuable as a functional food against diabetes.

Synthesis, in vitro and in vivo biological evaluation of new oxysterols as modulators of the liver X receptors

Liver X Receptor (LXR) modulators have shown potential as drugs since they target genes affecting metabolism and fatty acid synthesis. LXR antagonists are of particular interest since they are able to reduce the synthesis of complex fatty acids and glucose uptake. Based on molecular modeling, five new cholesterol mimics were synthesized, where four contained a hydroxyl group in the 22-S-position. The new compounds were screened in vitro against several genes affecting lipid metabolism. The compound that performed best in vitro was a dimethylamide derivative of 22(S)-hydroxycholesterol and it was chosen for in vivo testing. However, the blood plasma analysis from the in vivo tests revealed a concentration lower than needed to give any response, indicating either rapid metabolism or low bioavailability.

Pyruvate dehydrogenase kinase 4 deficiency attenuates cisplatin-induced acute kidney injury

Clinical prescription of cisplatin, one of the most widely used chemotherapeutic agents, is limited by its side effects, particularly tubular injury-associated nephrotoxicity. Since details of the underlying mechanisms are not fully understood, we investigated the role of pyruvate dehydrogenase kinase (PDK) in cisplatin-induced acute kidney injury. Among the PDK isoforms, PDK4 mRNA and protein levels were markedly increased in the kidneys of mice treated with cisplatin, and c-Jun N-terminal kinase activation was involved in cisplatin-induced renal PDK4 expression. Treatment with the PDK inhibitor sodium dichloroacetate (DCA) or genetic knockout of PDK4 attenuated the signs of cisplatin-induced acute kidney injury, including apoptotic morphology of the kidney tubules along with numbers of TUNEL-positive cells, cleaved caspase-3, and renal tubular injury markers. Cisplatin-induced suppression of the mitochondrial membrane potential, oxygen consumption rate, expression of electron transport chain components, cytochrome c oxidase activity, and disruption of mitochondrial morphology were noticeably improved in the kidneys of DCA-treated or PDK4 knockout mice. Additionally, levels of the oxidative stress marker 4-hydroxynonenal and mitochondrial reactive oxygen species were attenuated, whereas superoxide dismutase 2 and catalase expression and glutathione synthetase and glutathione levels were recovered in DCA-treated or PDK4 knockout mice. Interestingly, lipid accumulation was considerably attenuated in DCA-treated or PDK4 knockout mice via recovered expression of peroxisome proliferator-activated receptor-α and coactivator PGC-1α, which was accompanied by recovery of mitochondrial biogenesis. Thus, PDK4 mediates cisplatin-induced acute kidney injury, suggesting that PDK4 might be a therapeutic target for attenuating cisplatin-induced acute kidney injury.

Cardiac LXRα protects against pathological cardiac hypertrophy and dysfunction by enhancing glucose uptake and utilization

Pathological cardiac hypertrophy is characterized by a shift in metabolic substrate utilization from fatty acids to glucose, but the molecular events underlying the metabolic remodeling remain poorly understood. Here, we investigated the role of liver X receptors (LXRs), which are key regulators of glucose and lipid metabolism, in cardiac hypertrophic pathogenesis. Using a transgenic approach in mice, we show that overexpression of LXRα acts to protect the heart against hypertrophy, fibrosis, and dysfunction. Gene expression profiling studies revealed that genes regulating metabolic pathways were differentially expressed in hearts with elevated LXRα. Functionally, LXRα overexpression in isolated cardiomyocytes and murine hearts markedly enhanced the capacity for myocardial glucose uptake following hypertrophic stress. Conversely, this adaptive response was diminished in LXRα-deficient mice. Transcriptional changes induced by LXRα overexpression promoted energy-independent utilization of glucose via the hexosamine biosynthesis pathway, resulting in O-GlcNAc modification of GATA4 and Mef2c and the induction of cytoprotective natriuretic peptide expression. Our results identify LXRα as a key cardiac transcriptional regulator that helps orchestrate an adaptive metabolic response to chronic cardiac stress, and suggest that modulating LXRα may provide a unique opportunity for intervening in myocyte metabolism.

The association of NR1H3 gene with lipid deposition in the pig

Background

Nuclear receptor subfamily 1, group H, member 3 (NR1H3, an alias for Liver X receptor α, LXRα) is a member of the LXR nuclear receptor super family and is an important regulator of lipid and fatty acid accumulation in the liver, adipose and skeletal muscle.

Methods

In this study, single-nucleotide polymorphisms (SNPs) from six populations of pig (Sus scrofa) were screened by PCR-sequencing and genotyped, and its association with backfat thickness was analyzed in a population of New Huai line (NHP, n = 117). In addition, quantitative real-time PCR and western blot were used to measure expression of NR1H3 in the liver tissue, backfat and longissimus dorsi muscle of DSP (n = 10), TP (n = 10) and YY (n = 10) pigs.

Results

Three SNPs (exon2-C105T, exon2-G106C, and exon5-A201C) were screened and exon5-A201C was identified; the genotype frequencies were significantly different between indigenous and introduced breeds. The CC genotype was associated with higher backfat thickness than the AA and AC genotypes in the NYP. NR1H3 mRNA and protein expression were higher in the liver and longissimus dorsi of DSP and TP than in those of YY. This increased NR1H3 expression might be associated with higher lipid deposition. NR1H3 expression in the backfat of YY was not lower than that in DSP or TP, which might because NR1H3 has an alternative regulatory function for lipid metabolism in the subcutaneous fat of pigs.

Conclusions

Our results suggest that allele A of the exon5-A201C in NR1H3 may promote a reduction in backfat thickness, and differences in NR1H3 expression may be associated with differences in lipid deposition capacity among pigs.

A PUFA-rich diet improves fat oxidation following saturated fat-rich meal

PURPOSE

To determine substrate oxidation responses to saturated fatty acid (SFA)-rich meals before and after a 7-day polyunsaturated fatty acid (PUFA)-rich diet versus control diet.

METHODS

Twenty-six, normal-weight, adults were randomly assigned to either PUFA or control diet. Following a 3-day lead-in diet, participants completed the pre-diet visit where anthropometrics and resting metabolic rate (RMR) were measured, and two SFA-rich HF meals (breakfast and lunch) were consumed. Indirect calorimetry was used to determine fat oxidation (Fox) and energy expenditure (EE) for 4 h after each meal. Participants then consumed a PUFA-rich diet (50 % carbohydrate, 15 % protein, 35 % fat, of which 21 % of total energy was PUFA) or control diet (50 % carbohydrate, 15 % protein, 35 % fat, of which 7 % of total energy was PUFA) for the next 7 days. Following the 7-day diet, participants completed the post-diet visit.

RESULTS

From pre- to post-PUFA-rich diet, there was no change in RMR (16.3 ± 0.8 vs. 16.4 ± 0.8 kcal/20 min) or in incremental area under the curve for EE (118.9 ± 20.6-126.9 ± 14.1 kcal/8h, ns). Fasting respiratory exchange ratio increased from pre- to post-PUFA-rich diet only (0.83 ± 0.1-0.86 ± 0.1, p < 0.05). The postprandial change in Fox increased from pre- to post-visit in PUFA-rich diet (0.03 ± 0.1-0.23 ± 0.1 g/15 min for cumulative Fox; p < 0.05), whereas controls showed no change.

CONCLUSIONS

Adopting a PUFA-rich diet initiates greater fat oxidation after eating occasional high SFA meals compared to a control diet, an effect achieved in 7 days.

Liver X receptors as regulators of metabolism

The liver X receptors (LXR) are crucial regulators of metabolism. After ligand binding, they regulate gene transcription and thereby mediate changes in metabolic pathways. Modulation of LXR and their downstream targets has appeared to be a promising treatment for metabolic diseases especially atherosclerosis and cholesterol metabolism. However, the complexity of LXR action in various metabolic tissues and the liver side effect of LXR activation have slowed down the interest for LXR drugs. In this review, we summarized the role of LXR in the main metabolically active tissues with a special focus on obesity and associated diseases in mammals. We will also discuss the dual interplay between the two LXR isoforms suggesting that they may collaborate to establish a fine and efficient system for the maintenance of metabolism homeostasis.

Emerging role of liver X receptors in cardiac pathophysiology and heart failure

Liver X receptors (LXRs) are master regulators of metabolism and have been studied for their pharmacological potential in vascular and metabolic disease. Besides their established role in metabolic homeostasis and disease, there is mounting evidence to suggest that LXRs may exert direct beneficial effects in the heart. Here, we aim to provide a conceptual framework to explain the broad mode of action of LXRs and how LXR signaling may be an important local and systemic target for the treatment of heart failure. We discuss the potential role of LXRs in systemic conditions associated with heart failure, such as hypertension, diabetes, and renal and vascular disease. Further, we expound on recent data that implicate a direct role for LXR activation in the heart, for its impact on cardiomyocyte damage and loss due to ischemia, and effects on cardiac hypertrophy, fibrosis, and myocardial metabolism. Taken together, the accumulating evidence supports the notion that LXRs may represent a novel therapeutic target for the treatment of heart failure.

Oleic and linoleic fatty acids downregulate Slc2a4/GLUT4 expression via NFKB and SREBP1 in skeletal muscle cells

Oleic (OA) and linoleic (LA) fatty acids may be important regulators of Slc2a4 gene (GLUT4 protein) in skeletal muscle, thus participating in insulin resistance. We investigated the effect of OA and LA on the Slc2a4/GLUT4 expression in L6 muscle cells; as well as potential transcriptional regulators. OA and LA (50-400 µM) decreased the Slc2a4/GLUT4 expression in a dose-dependent way (maximum of ~50%, P < 0.001). OA and LA did not alter the Slc2a4-binding activity of oxysterols-receptor-LXR-alpha and peroxisome-proliferator-activated-receptor-gamma; but decreased the Slc2a4-binding activity of the sterol-regulatory-element-binding-protein-1 (SREBP1) enhancer (50%, P < 0.001), and increased (~30%, P < 0.001) the nuclear proteins binding into the Slc2a4-nuclear-factor-NF-kappa-B-binding site (repressor), and the phosphorylation of the inhibitors of nuclear-factor-kappa-B-kinase alpha/beta (150-300%, P < 0.001). In sum, OA and LA are potent inhibitors of the Slc2a4/GLUT4 expression in muscle cells; an effect involving reduced SREBP1 and increased NFKB transcriptional activity. These regulations may participate in the fatty acid-related pathophysiology of insulin resistance.

A systems biology approach reveals the physiological origin of hepatic steatosis induced by liver X receptor activation

Liver X receptor (LXR) agonists exert potent antiatherosclerotic actions but simultaneously induce excessive triglyceride (TG) accumulation in the liver. To obtain a detailed insight into the underlying mechanism of hepatic TG accumulation, we used a novel computational modeling approach called analysis of dynamic adaptations in parameter trajectories (ADAPT). We revealed that both input and output fluxes to hepatic TG content are considerably induced on LXR activation and that in the early phase of LXR agonism, hepatic steatosis results from only a minor imbalance between the two. It is generally believed that LXR-induced hepatic steatosis results from increased de novo lipogenesis (DNL). In contrast, ADAPT predicted that the hepatic influx of free fatty acids is the major contributor to hepatic TG accumulation in the early phase of LXR activation. Qualitative validation of this prediction showed a 5-fold increase in the contribution of plasma palmitate to hepatic monounsaturated fatty acids on acute LXR activation, whereas DNL was not yet significantly increased. This study illustrates that complex effects of pharmacological intervention can be translated into distinct patterns of metabolic regulation through state-of-the-art mathematical modeling.

The pivotal role of pyruvate dehydrogenase kinases in metabolic flexibility

Metabolic flexibility is the capacity of a system to adjust fuel (primarily glucose and fatty acids) oxidation based on nutrient availability. The ability to alter substrate oxidation in response to nutritional state depends on the genetically influenced balance between oxidation and storage capacities. Competition between fatty acids and glucose for oxidation occurs at the level of the pyruvate dehydrogenase complex (PDC). The PDC is normally active in most tissues in the fed state, and suppressing PDC activity by pyruvate dehydrogenase (PDH) kinase (PDK) is crucial to maintain energy homeostasis under some extreme nutritional conditions in mammals. Conversely, inappropriate suppression of PDC activity might promote the development of metabolic diseases. This review summarizes PDKs’ pivotal role in control of metabolic flexibility under various nutrient conditions and in different tissues, with emphasis on the best characterized PDK4. Understanding the regulation of PDC and PDKs and their roles in energy homeostasis could be beneficial to alleviate metabolic inflexibility and to provide possible therapies for metabolic diseases, including type 2 diabetes (T2D).

Herpesvirus-associated ubiquitin-specific protease (HAUSP) modulates peroxisome proliferator-activated receptor γ (PPARγ) stability through its deubiquitinating activity

The peroxisome proliferator-activated receptor γ (PPARγ) is a central regulator of adipogenesis and modulates glucose and lipid metabolism. In this study, herpesvirus-associated ubiquitin-specific protease (HAUSP) was isolated as a binding partner of PPARγ. Both endogenous and exogenous PPARγ associated with HAUSP in co-immunoprecipitation analysis. HAUSP, but not the catalytically inactive HAUSP C223S mutant, increased the stability of both endogenous and exogenous PPARγ through its deubiquitinating activity. Site-directed mutagenesis experiments showed that the Lys(462) residue of PPARγ is critical for ubiquitination. HBX 41,108, a specific inhibitor of HAUSP, abolished the increase in PPARγ stability induced by HAUSP. In addition, knockdown of endogenous HAUSP using siRNA decreased PPARγ protein levels. HAUSP enhanced the transcriptional activity of both exogenous and endogenous PPARγ in luciferase activity assays. Quantitative RT-PCR analysis showed that HAUSP increased the transcript levels of PPARγ target genes in HepG2 cells, resulting in the enhanced uptake of glucose and fatty acids, and vice versa, upon siRNA knockdown of HAUSP. In vivo analysis using adenoviruses confirmed that HAUSP, but not the HAUSP C223S mutant, decreased blood glucose and triglyceride levels, which are associated with the increased expression of endogenous PPARγ and lipid accumulation in the liver. Our results demonstrate that the stability and activity of PPARγ are modulated by the deubiquitinating activity of HAUSP, which may be a target for the development of anti-diabetic drugs.

LXR is a negative regulator of glucose uptake in human adipocytes

AIMS/HYPOTHESIS

Obesity increases the risk of developing type 2 diabetes mellitus, characterised by impaired insulin-mediated glucose uptake in peripheral tissues. Liver X receptor (LXR) is a positive regulator of adipocyte glucose transport in murine models and a possible target for diabetes treatment. However, the levels of LXRα are increased in obese adipose tissue in humans. We aimed to investigate the transcriptome of LXR and the role of LXR in the regulation of glucose uptake in primary human adipocytes.

METHODS

The insulin responsiveness of human adipocytes differentiated in vitro was characterised, adipocytes were treated with the LXR agonist GW3965 and global transcriptome profiling was determined by microarray, followed by quantitative RT-PCR (qRT-PCR), western blot and ELISA. Basal and insulin-stimulated glucose uptake was measured and the effect on plasma membrane translocation of glucose transporter 4 (GLUT4) was assayed.

RESULTS

LXR activation resulted in transcriptional suppression of several insulin signalling genes, such as AKT2, SORBS1 and CAV1, but caused only minor changes (<15%) in microRNA expression. Activation of LXR impaired the plasma membrane translocation of GLUT4, but not the expression of its gene, SLC2A4. LXR activation also diminished insulin-stimulated glucose transport and lipogenesis in adipocytes obtained from overweight individuals. Furthermore, AKT2 expression was reduced in obese adipose tissue, and AKT2 and SORBS1 expression was inversely correlated with BMI and HOMA index.

CONCLUSIONS/INTERPRETATION

In contrast to murine models, LXR downregulates insulin-stimulated glucose uptake in human adipocytes from overweight individuals. This could be due to suppression of Akt2, c-Cbl-associated protein and caveolin-1. These findings challenge the idea of LXR as a drug target in the treatment of diabetes.

Are cultured human myotubes far from home?

Satellite cells can be isolated from skeletal muscle biopsies, activated to proliferating myoblasts and differentiated into multinuclear myotubes in culture. These cell cultures represent a model system for intact human skeletal muscle and can be modulated ex vivo. The advantages of this system are that the most relevant genetic background is available for the investigation of human disease (as opposed to rodent cell cultures), the extracellular environment can be precisely controlled and the cells are not immortalized, thereby offering the possibility of studying innate characteristics of the donor. Limitations in differentiation status (fiber type) of the cells and energy metabolism can be improved by proper treatment, such as electrical pulse stimulation to mimic exercise. This review focuses on the way that human myotubes can be employed as a tool for studying metabolism in skeletal muscles, with special attention to changes in muscle energy metabolism in obesity and type 2 diabetes.

Selective liver X receptor modulators (SLiMs): what use in human health?

Liver X receptors (LXR) are members of the nuclear receptor family. As activated transcription factors, their putative association with human diseases makes them promising pharmacological targets because of the large potential to develop ligands. LXR are mainly considered as intracellular cholesterol "sensors" whose activation leads to decreased plasma cholesterol. They also modulate numerous physiological functions: fatty acid synthesis and metabolism, glucose homeostasis, steroidogenesis, immunity, and neurological homeostasis. LXR-deficiency in mouse results in several phenotypes mimicking pathological conditions in humans. This review will be focused on the various natural and synthetic LXR agonists and antagonists. Putative clinical targets including atherosclerosis, diabetes, Alzheimer's disease, skin disorders, and cancer will be covered.

Electrical pulse stimulation of cultured human skeletal muscle cells as an in vitro model of exercise

Background and Aims

Physical exercise leads to substantial adaptive responses in skeletal muscles and plays a central role in a healthy life style. Since exercise induces major systemic responses, underlying cellular mechanisms are difficult to study in vivo. It was therefore desirable to develop an in vitro model that would resemble training in cultured human myotubes.

Methods

Electrical pulse stimulation (EPS) was applied to adherent human myotubes. Cellular contents of ATP, phosphocreatine (PCr) and lactate were determined. Glucose and oleic acid metabolism were studied using radio-labeled substrates, and gene expression was analyzed using real-time RT-PCR. Mitochondrial content and function were measured by live imaging and determination of citrate synthase activity, respectively. Protein expression was assessed by electrophoresis and immunoblotting.

Results

High-frequency, acute EPS increased deoxyglucose uptake and lactate production, while cell contents of both ATP and PCr decreased. Chronic, low-frequency EPS increased oxidative capacity of cultured myotubes by increasing glucose metabolism (uptake and oxidation) and complete fatty acid oxidation. mRNA expression level of pyruvate dehydrogenase complex 4 (PDK4) was significantly increased in EPS-treated cells, while mRNA expressions of interleukin 6 (IL-6), cytochrome C and carnitin palmitoyl transferase b (CPT1b) also tended to increase. Intensity of MitoTracker®Red FM was doubled after 48 h of chronic, low-frequency EPS. Protein expression of a slow fiber type marker (MHCI) was increased in EPS-treated cells.

Conclusions

Our results imply that in vitro EPS (acute, high-frequent as well as chronic, low-frequent) of human myotubes may be used to study effects of exercise.

The liver X receptor modulator 22(S)-hydroxycholesterol exerts cell-type specific effects on lipid and glucose metabolism

The aim of this study was to explore the effects of 22(S)-hydroxycholesterol (22(S)-HC) on lipid and glucose metabolism in human-derived cells from metabolic active tissues. Docking of T0901317 and 22(S)-HC showed that both substances fitted into the ligand binding domain of liver X receptors (LXR). Results show that while several lipogenic genes were induced by T0901317 in myotubes, HepG2 cells and SGBS cells, effect of 22(S)-HC varied more between cell types. In myotubes, most lipogenic genes were downregulated or unchanged by 22(S)-HC, whereas a more diverse pattern was found in HepG2 and SGBS cells. Treatment with 22(S)-HC induced sterol regulatory element binding transcription factor 1 in SGBS and HepG2 cells, but not in myotubes. Fatty acid synthase was downregulated by 22(S)-HC in myotubes, upregulated in SGBS and unchanged in HepG2 cells. De novo lipogenesis was increased by T0901317 in all cell models, whereas differently affected by 22(S)-HC depending on the cell type; decreased in myotubes and HepG2 cells, whereas increased in SGBS cells. Oxidation of linoleic acid was reduced by 22(S)-HC in all cell models while glucose uptake increased and tended to increase in myotubes and SGBS cells, respectively. Cholesterol efflux was unaffected by 22(S)-HC treatment. These results show that 22(S)-HC affects LXR-regulated processes differently in various cell types. Ability of 22(S)-HC to reduce lipogenesis and lipid accumulation in myotubes and hepatocytes indicate that 22(S)-HC might reduce lipid accumulation in non-adipose tissues, suggesting a potential role for 22(S)-HC or a similar LXR modulator in the treatment of type 2 diabetes.

Metabolic switching of human skeletal muscle cells in vitro

In this review we will focus on external factors that may modify energy metabolism in human skeletal muscle cells (myotubes) and the ability of the myotubes to switch between lipid and glucose oxidation. We describe the metabolic parameters suppressibility, adaptability and substrate-regulated flexibility, and show the influence of nutrients such as fatty acids and glucose (chronic hyperglycemia), and some pharmacological agents modifying nuclear receptors (PPAR and LXR), on these parameters in human myotubes. Possible cellular mechanisms for changes in these parameters will also be highlighted.

Sterol regulation of metabolism, homeostasis, and development

Sterol metabolites are critical signaling molecules that regulate metabolism, development, and homeostasis. Oxysterols, bile acids (BAs), and steroids work primarily through cognate sterol-responsive nuclear hormone receptors to control these processes through feed-forward and feedback mechanisms. These signaling pathways are conserved from simple invertebrates to mammals. Indeed, results from various model organisms have yielded fundamental insights into cholesterol and BA homeostasis, lipid and glucose metabolism, protective mechanisms, tissue differentiation, development, reproduction, and even aging. Here, we review how sterols act through evolutionarily ancient mechanisms to control these processes.

Overexpression of PGC-1α increases fatty acid oxidative capacity of human skeletal muscle cells

We investigated the effects of PGC-1α (peroxisome proliferator-activated receptor γ coactivator-1α) overexpression on the oxidative capacity of human skeletal muscle cells ex vivo. PGC-1α overexpression increased the oxidation rate of palmitic acid and mRNA expression of genes regulating lipid metabolism, mitochondrial biogenesis, and function in human myotubes. Basal and insulin-stimulated deoxyglucose uptake were decreased, possibly due to upregulation of PDK4 mRNA. Expression of fast fiber-type gene marker (MHCIIa) was decreased. Compared to skeletal muscle in vivo, PGC-1α overexpression increased expression of several genes, which were downregulated during the process of cell isolation and culturing. In conclusion, PGC-1α overexpression increased oxidative capacity of cultured myotubes by improving lipid metabolism, increasing expression of genes involved in regulation of mitochondrial function and biogenesis, and decreasing expression of MHCIIa. These results suggest that therapies aimed at increasing PGC-1α expression may have utility in treatment of obesity and obesity-related diseases.

PPARα–LXR as a novel metabolostatic signalling axis in skeletal muscle that acts to optimize substrate selection in response to nutrient status

LXR (liver X receptor) and PPARα (peroxisome-proliferator-activated receptor α) are nuclear receptors that control the expression of genes involved in glucose and lipid homoeostasis. Using wild-type and PPARα-null mice fed on an LXR-agonist-supplemented diet, the present study analysed the impact of pharmacological LXR activation on the expression of metabolically important genes in skeletal muscle, testing the hypothesis that LXR activation can modulate PPAR action in skeletal muscle in a manner dependent on nutritional status. In the fed state, LXR activation promoted a gene profile favouring lipid storage and glucose oxidation, increasing SCD1 (stearoyl-CoA desaturase 1) expression and down-regulating PGC-1α (PPARγ co-activator-1α) and PDK4 (pyruvate dehydrogenase kinase 4) expression. PPARα deficiency enhanced LXR stimulation of SCD1 expression, and facilitated elevated SREBP-1 (sterol-regulatory-element-binding protein-1) expression. However, LXR-mediated down-regulation of PGC-1α and PDK4 was opposed and reversed by PPARα deficiency. During fasting, prior LXR activation augmented PPARα signalling to heighten FA (fatty acid) oxidation and decrease glucose oxidation by augmenting fasting-induced up-regulation of PGC-1α and PDK4 expression, effects opposed by PPARα deficiency. Starvation-induced down-regulation of SCD1 expression was opposed by antecedent LXR activation in wild-type mice, an effect enhanced further by PPARα deficiency, which may elicit increased channelling of FA into triacylglycerol to limit lipotoxicity. Our results also identified potential regulatory links between the protein deacetylases SIRT1 (sirtuin 1) and SIRT3 and PDK4 expression in muscle from fasted mice, with a requirement for PPARα. In summary, we therefore propose that a LXR–PPARα signalling axis acts as a metabolostatic regulatory mechanism to optimize substrate selection and disposition in skeletal muscle according to metabolic requirement.

LXR activation prevents exhaustive exercise-induced hypoglycaemia and spares muscle glycogen but does not enhance running endurance in untrained rats

AIM

Liver X receptors (LXRs) are ligand-activated transcription factors that play an important role in regulation of hepatic lipid and carbohydrate metabolism. However, to date there is very few information on the role of LXRs in skeletal muscle. Moreover, it remains obscure whether LXR activation affects physical endurance. Therefore, we aimed to examine effects of selective LXR activator--T0901317--on running endurance and skeletal muscle exercise metabolism in rats.

METHODS

The animals were assigned to two groups (n=20) receiving either vehicle or T0901317 (10 mg kg(-1) day(-1) ) for 1 week. One day after the final administration, half of the rats in each group were exercised until exhaustion on the electrically driven treadmill. All animals were then anaesthetized and samples of the soleus, red and white sections of the gastrocnemius muscle, epididymal fat pad and liver were excised.

RESULTS

We found that LXR activation prevented exhaustive exercise-induced hypoglycaemia. T0901317 also shifted substrate utilization in working muscles in favour of fatty acids as indicated by its glycogen sparing effect, enhanced consumption of intramuscular triacylglycerol and upregulation of genes promoting fatty acid oxidation and suppressing carbohydrate oxidation. However, running time to exhaustion was not improved.

CONCLUSION

We conclude that LXR activation increases fatty acid utilization during exercise which, however, does not translate into measurable enhancement of exercise endurance.

Beyond glucose: metabolic shifts in responses to the effects of the oral glucose tolerance test and the high-fructose diet in rats

High-fructose diet-fed rats as one of the insulin resistant models was used widely for understanding the mechanisms of type 2 diabetes mellitus. Systems-level metabolic profiling of the rat model, however, has not been deciphered clearly. To address this issue, mass spectrometry-based metabolomics was employed to unlock the metabolic snapshots of the oral glucose tolerance test (oGTT) effect in either healthy or diabetic rats, as well as to delineate the metabolic signatures in tissues of rats fed with high-fructose diet. Several differentiating metabolites were highlighted to reveal the metabolic perturbation of the oGTT effects in healthy and diabetic rats, which involved amino acid biosynthesis, polyunsaturated fatty acids, phospholipids and purine metabolism. Surprisingly, the patterns of relationships for the metabolic phenotypes by using data mining revealed that glucose ingestion might induce the healthy group to display its trajectory towards diabetic status, while only a very slight influence was observed on the high-fructose diet-fed rats 120 min after glucose ingestion. The data treatment for liver, skeletal muscle and brain tissues suggested that oxidative stress, such as lipid peroxidation and the declined antioxidant, the elevated amino acids and the perturbation of fatty acids, were caused by the high-fructose diet in liver and skeletal muscle tissues. On the other hand, the up-regulation in purine biosynthesis and the decreased concentrations for amino acids were observed in the cerebral cortex and hippocampus tissues. Collectively, the obtained results might provide a new insight not only for the impairment of glucose tolerance but also for the dietary style in rats.

Gene regulation mediating fiber-type transformation in skeletal muscle cells is partly glucose- and ChREBP-dependent

Adaptations in the oxidative capacity of skeletal muscle cells can occur under several physiological or pathological conditions. We investigated the effect of increasing extracellular glucose concentration on the expression of markers of energy metabolism in primary skeletal muscle cells and the C2C12 muscle cell line. Growth of myotubes in 25mM glucose (high glucose, HG) compared with 5.55mM led to increases in the expression and activity of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a marker of glycolytic energy metabolism, while oxidative markers peroxisome proliferator-activated receptor γ coactivator 1α and citrate synthase decreased. HG induced metabolic adaptations as are seen during a slow-to-fast fiber transformation. Furthermore, HG increased fast myosin heavy chain (MHC) IId/x but did not change slow MHCI/β expression. Protein phosphatase 2A (PP2A) was shown to mediate the effects of HG on GAPDH and MHCIId/x. Carbohydrate response element-binding protein (ChREBP), a glucose-dependent transcription factor downstream of PP2A, partially mediated the effects of glucose on metabolic markers. The glucose-induced increase in PP2A activity was associated with an increase in p38 mitogen-activated protein kinase activity, which presumably mediates the increase in MHCIId/x promoter activity. Liver X receptor, another possible mediator of glucose effects, induced only an incomplete metabolic shift, mainly increasing the expression of the glycolytic marker. Taken together, HG induces a partial slow-to-fast transformation comprising metabolic enzymes together with an increased expression of MHCIId/x. This work demonstrates a functional role for ChREBP in determining the metabolic type of muscle fibers and highlights the importance of glucose as a signaling molecule in muscle.

Metabolic switching of human myotubes is improved by n-3 fatty acids

The aim of the present study was to examine whether pretreatment with different fatty acids, as well as the liver X receptor (LXR) agonist T0901317, could modify metabolic switching of human myotubes. The n-3 FA eicosapentaenoic acid (EPA) increased suppressibility, the ability of glucose to suppress FA oxidation. Substrate-regulated flexibility, the ability to increase FA oxidation when changing from a high glucose, low fatty acid condition ("fed") to a high fatty acid, low glucose ("fasted") condition, was increased by EPA and other n-3 FAs. Adaptability, the capacity to increase FA oxidation with increasing FA availability, was enhanced after pretreatment with EPA, linoleic acid (LA), and palmitic acid (PA). T0901317 counteracted the effect of EPA on suppressibility and adaptability, but it did not affect these parameters alone. EPA per se accumulated less, however, EPA, LA, oleic acid, and T0901317 treatment increased the number of lipid droplets (LD) in myotubes. LD volume and intensity, as well as mitochondrial mass, were independent of FA pretreatment. Microarray analysis showed that EPA regulated more genes than the other FAs and that specific pathways involved in carbohydrate metabolism were induced only by EPA. The present study suggests a favorable effect of n-3 FAs on skeletal muscle metabolic switching and glucose utilization.

Lipogenesis in arterial wall and vascular smooth muscle cells of Psammomys obesus: its regulation and abnormalities in diabetes

AIM

Lipogenesis is expressed in vascular smooth muscle cells (VSMCs), and such in situ lipogenesis could be providing the fatty acids for triglyceride synthesis and cholesterol esterification, and contributing to lipid accumulation in the arterial wall. This study investigated both the expression and regulation of lipogenesis in VSMCs to determine if they are modified in Psammomys obesus gerbils fed a high-fat diet as a model of insulin resistance and diabetes.

METHODS

Aortas were collected from diabetic and non-diabetic P. obesus for histological examination, measurement of lipogenic gene expression and VSMC culture.

RESULTS

The aortas of diabetic animals exhibited lipid deposits and foam cells as well as disorganization of elastic fibres. However, lipogenic gene expression was not modified. VSMCs in vitro from the aortas of diabetic animals had, compared with cells from non-diabetic animals, lower mRNA levels of SREBP-1c and ChREBP. An adipogenic medium stimulated moderate FAS and ACC1 expression in cells from both diabetic and non-diabetic animals, but glucose and insulin on their own had no such stimulatory action. Also, triiodothyronine (T3) had a clear stimulatory action, while angiotensin II had a moderate effect, in cells from non-diabetic P. obesus, but not from diabetic animals, whereas LXR agonists stimulated lipogenesis in cells from both animal groups.

CONCLUSION

Lipogenesis is expressed in the arterial walls and VSMCs of P. obesus. However, its expression was not increased in diabetes, and did not respond to either T3 or angiotensin II. Therefore, lipogenesis in situ is unlikely to contribute to the accumulation of lipids in the arterial walls of diabetic P. obesus gerbils.

Activation of LXR increases acyl-CoA synthetase activity through direct regulation of ACSL3 in human placental trophoblast cells

Placental fatty acid transport and metabolism are important for proper growth and development of the feto-placental unit. The nuclear receptors, liver X receptors alpha and beta (LXRalpha and LXRbeta), are key regulators of lipid metabolism in many tissues, but little is known about their role in fatty acid transport and metabolism in placenta. The current study investigates the LXR-mediated regulation of long-chain acyl-CoA synthetase 3 (ACSL3) and its functions in human placental trophoblast cells. We demonstrate that activation of LXR increases ACSL3 expression, acyl-CoA synthetase activity, and fatty acid uptake in human tropholast cells. Silencing of ACSL3 in these cells attenuates the LXR-mediated increase in acyl-CoA synthetase activity. Furthermore, we show that ACSL3 is directly regulated by LXR through a conserved LXR responsive element in the ACSL3 promoter. Our results suggest that LXR plays a regulatory role in fatty acid metabolism by direct regulation of ACSL3 in human placental trophoblast cells.