A novel functional assay for simultaneous determination of total fatty acid beta-oxidation flux and acylcarnitine profiling in human skin fibroblasts using (2)H(31)-palmitate by isotope ratio mass spectrometry and electrospray tandem mass spectrometry

MLX plays a key role in lipid and glucose metabolism in humans: Evidence from in vitro and in vivo studies

BACKGROUND AND AIM

Enhanced hepatic de novo lipogenesis (DNL) has been proposed as an underlying mechanism for the development of NAFLD and insulin resistance. Max-like protein factor X (MLX) acts as a heterodimer binding partner for glucose sensing transcription factors and inhibition of MLX or downstream targets has been shown to alleviate intrahepatic triglyceride (IHTG) accumulation in mice. However, its effect on insulin sensitivity remains unclear. As human data is lacking, the aim of the present work was to investigate the role of MLX in regulating lipid and glucose metabolism in primary human hepatocytes (PHH) and in healthy participants with and without MLX polymorphisms.

METHODS

PHH were transfected with non-targeting or MLX siRNA to assess the effect of MLX knockdown on lipid and glucose metabolism, insulin signalling and the hepatocellular transcriptome. A targeted association analysis on imputed genotype data for MLX on healthy individuals was undertaken to assess associations between specific MLX SNPs (rs665268, rs632758 and rs1474040), plasma biochemistry, IHTG content, DNL and gluconeogenesis.

RESULTS

MLX knockdown in PHH altered lipid metabolism (decreased DNL (p < 0.05), increased fatty acid oxidation and ketogenesis (p < 0.05), and reduced lipid accumulation (p < 0.001)). Additionally, MLX knockdown increased glycolysis, lactate secretion and glucose production (p < 0.001) and insulin-stimulated pAKT levels (p < 0.01) as assessed by transcriptomic, steady-state and dynamic measurements. Consistent with the in vitro data, individuals with the rs1474040-A and rs632758-C variants had lower fasting plasma insulin (p < 0.05 and p < 0.01, respectively) and TG (p < 0.05 and p < 0.01, respectively). Although there was no difference in IHTG or gluconeogenesis, individuals with rs632758 SNP had notably lower hepatic DNL (p < 0.01).

CONCLUSION

We have demonstrated using human in vitro and in vivo models that MLX inhibition favored lipid catabolism over anabolism and increased glucose production, despite increased glycolysis and phosphorylation of Akt, suggesting a metabolic mechanism that involves futile cycling.

Determining the temporal, dose, and composition effects of nutritional substrates in an in vitro model of intrahepatocellular triglyceride accumulation

Pathological accumulation of intrahepatic triglyceride underpins the early stages of nonalcoholic fatty liver disease (NAFLD) and can progress to fibrosis, cirrhosis, and cancer of the liver. Studies in humans suggest that consumption of a diet enriched in saturated compared to unsaturated fatty acids (FAs), is more detrimental to liver fat accumulation and metabolism. However, the reasons for the divergence remain unclear and physiologically-relevant cellular models are required. Therefore, the aims of this study were to investigate the effect of modifying media composition, concentration, and treatment frequency of sugars, FAs and insulin on intrahepatocellular triglyceride content and intracellular glucose, FA and circadian function. Huh7 cells were treated with 2% human serum and a combination of sugars and FAs (low fat low sugar [LFLS], high fat low sugar [HFLS], or high fat high sugar [HFHS]) enriched in either unsaturated (OPLA) or saturated (POLA) FAs for 2, 4, or 7 days with a daily or alternating treatment regime. Stable isotope tracers were utilized to investigate basal and/or insulin-responsive changes in hepatocyte metabolism in response to different treatment regimes. Cell viability, media biochemistry, intracellular metabolism, and circadian biology were quantified. The FA composition of the media (OPLA vs. POLA) did not influence cell viability or intracellular triglyceride content in hepatocytes. In contrast, POLA-treated cells had lower FA oxidation and media acetate, and with higher FA concentrations, displayed lower intracellular glycogen content and diminished insulin stimulation of glycogenesis, compared to OPLA-treated cells. The addition of HFHS also had profound effects on circadian oscillation and gene expression. Cells treated daily with HFHS for at least 4 days resulted in a cellular model displaying characteristics of early stage NAFLD seen in humans. Repeated treatment for longer durations (≥7 days) may provide opportunities to investigate lipid and glucose metabolism in more severe stages of NAFLD.

Prolyl-4-hydroxylase 3 maintains β cell glucose metabolism during fatty acid excess in mice

The α-ketoglutarate–dependent dioxygenase, prolyl-4-hydroxylase 3 (PHD3), is an HIF target that uses molecular oxygen to hydroxylate peptidyl prolyl residues. Although PHD3 has been reported to influence cancer cell metabolism and liver insulin sensitivity, relatively little is known about the effects of this highly conserved enzyme in insulin-secreting β cells in vivo. Here, we show that the deletion of PHD3 specifically in β cells (βPHD3KO) was associated with impaired glucose homeostasis in mice fed a high-fat diet. In the early stages of dietary fat excess, βPHD3KO islets energetically rewired, leading to defects in the management of pyruvate fate and a shift from glycolysis to increased fatty acid oxidation (FAO). However, under more prolonged metabolic stress, this switch to preferential FAO in βPHD3KO islets was associated with impaired glucose-stimulated ATP/ADP rises, Ca²⁺ fluxes, and insulin secretion. Thus, PHD3 might be a pivotal component of the β cell glucose metabolism machinery in mice by suppressing the use of fatty acids as a primary fuel source during the early phases of metabolic stress.

Untargeted LC/MS-Based Metabolic Phenotyping of Hypopituitarism in Young Males

Objective: Hypopituitarism (Hypo-Pit) is partial or complete insufficiency of anterior pituitary hormones. Besides hormone metabolism, the global metabolomics in Hypo-Pit are largely unknown. We aimed to explore potential biomarkers to aid in diagnosis and personalized treatment.

Methods: Using both univariate and multivariate statistical methods, we identified 72 differentially abundant features through liquid chromatography coupled to high-resolution mass spectrometry, obtained in 134 males with Hypo-Pit and 90 age matched healthy controls.

Results: Hypopituitarism exhibits an increased abundance of metabolites involved in amino acid degradation and glycerophospholipid synthesis, but decreased content of metabolites in steroid hormone synthesis and fatty acid beta-oxidation. Significantly changed metabolites included creatine, creatinine, L-alanine, phosphocholines, androstenedione, hydroprenenolone, and acylcarnitines. In Hypo-Pit patients, the increased ratio of creatine/creatinine suggested reduced creatine uptake and impaired creatine utilization, whereas the decreased level of beta-hydroxybutyrate, acetylcarnitine (C2) and a significantly decreased ratio of decanoylcarnitine (C10) to free carnitine suggested an impaired beta-oxidation. Furthermore, the creatine/creatinine and decanoylcarnitine/carnitine ratio were identified as diagnostic biomarkers for Hypo-Pit with AUCs of 0.976 and 0.988, respectively. Finally, we found that the creatinine and decanoylcarnitine/carnitine ratio could distinguish cases that were sensitive vs. resistant to human chorionic gonadotropin therapy.

Conclusion: We provided a global picture of altered metabolic pathways in Hypo-Pit, and the identified biomarkers in creatine metabolism and beta-oxidation might be useful for the preliminary screening and diagnosis of Hypo-Pit.

Modifying nutritional substrates induces macrovesicular lipid droplet accumulation and metabolic alterations in a cellular model of hepatic steatosis

BACKGROUND AND AIMS

Nonalcoholic fatty liver disease (NAFLD) begins with steatosis, where a mixed macrovesicular pattern of large and small lipid droplets (LDs) develops. Since in vitro models recapitulating this are limited, the aims of this study were to develop mixed macrovesicular steatosis in immortalized hepatocytes and investigate effects on intracellular metabolism by altering nutritional substrates.

METHODS

Huh7 cells were cultured in 11 mM glucose and 2% human serum (HS) for 7 days before additional sugars and fatty acids (FAs), either with 200 µM FAs (low fat low sugar; LFLS), 5.5 mM fructose + 200 µM FAs (low fat high sugar; LFHS), or 5.5 mM fructose + 800 µM FAs (high fat high sugar; HFHS), were added for 7 days. FA metabolism, lipid droplet characteristics, and transcriptomic signatures were investigated.

RESULTS

Between the LFLS and LFHS conditions, there were few notable differences. In the HFHS condition, intracellular triacylglycerol (TAG) was increased and the LD pattern and distribution was similar to that found in primary steatotic hepatocytes. HFHS-treated cells had lower levels of de novo-derived FAs and secreted larger, TAG-rich lipoprotein particles. RNA sequencing and gene set enrichment analysis showed changes in several pathways including those involved in metabolism and cell cycle.

CONCLUSIONS

Repeated doses of HFHS treatment resulted in a cellular model of NAFLD with a mixed macrovesicular LD pattern and metabolic dysfunction. Since these nutrients have been implicated in the development of NAFLD in humans, the model provides a good physiological basis for studying NAFLD development or regression in vitro.

Hepatic de novo lipogenesis is suppressed and fat oxidation is increased by omega-3 fatty acids at the expense of glucose metabolism

OBJECTIVE

Increased hepatic de novo lipogenesis (DNL) is suggested to be an underlying cause in the development of nonalcoholic fatty liver disease and/or insulin resistance. It is suggested that omega-3 fatty acids (FA) lower hepatic DNL. We investigated the effects of omega-3 FA supplementation on hepatic DNL and FA oxidation using a combination of human in vivo and in vitro studies.

RESEARCH DESIGN AND METHODS

Thirty-eight healthy men were randomized to take either an omega-3 supplement (4 g/day eicosapentaenoic acid (EPA)+docosahexaenoic acid (DHA) as ethyl esters) or placebo (4 g/day olive oil) and fasting measurements were made at baseline and 8 weeks. The metabolic effects of omega-3 FAs on intrahepatocellular triacylglycerol (IHTAG) content, hepatic DNL and FA oxidation were investigated using metabolic substrates labeled with stable-isotope tracers. In vitro studies, using a human liver cell-line was undertaken to gain insight into the intrahepatocellular effects of omega-3 FAs.

RESULTS

Fasting plasma TAG concentrations significantly decreased in the omega-3 group and remained unchanged in the placebo group. Eight weeks of omega-3 supplementation significantly decreased IHTAG, fasting and postprandial hepatic DNL while significantly increasing dietary FA oxidation and fasting and postprandial plasma glucose concentrations. In vitro studies supported the in vivo findings of omega-3 FAs (EPA+DHA) decreasing intracellular TAG through a shift in cellular metabolism away from FA esterification toward oxidation.

CONCLUSIONS

Omega-3 supplementation had a potent effect on decreasing hepatic DNL and increasing FA oxidation and plasma glucose concentrations. Attenuation of hepatic DNL may be considered advantageous; however, consideration is required as to what the potential excess of nonlipid substrates (eg, glucose) will have on intrahepatic and extrahepatic metabolic pathways.

TRIAL REGISTRATION NUMBER

NCT01936779.

In Vitro Monitoring of the Mitochondrial Beta-Oxidation Flux of Palmitic Acid and Investigation of Its Pharmacological Alteration by Therapeutics

BACKGROUND AND OBJECTIVE

The present study was designed to validate the functional assay that enables rapid screening of therapeutic candidates for their effect on mitochondrial fatty acid oxidation.

METHODS

The two whole-cell systems (tissue homogenates and hepatocytes) have been evaluated to monitor the total beta-oxidation flux of physiologically important ³H-palmitic acid by measurement of tritiated water enrichment in incubations using UPLC coupled on-line to radioactivity monitoring and mass spectrometry.

RESULTS

Our results with several known inhibitors of fatty acid oxidation showed that this simple assay could correctly predict a potential in alteration of mitochondrial function by drug candidates. Since the beta-oxidation of palmitic acid takes place almost exclusively in mitochondria of human hepatocytes, this model can be also utilized to distinguish between the mitochondrial and peroxisomal routes of this essential metabolic pathway in some cases.

CONCLUSIONS

The present work offers a new in vitro screen of changes in mitochondrial beta-oxidation by xenobiotics as well as a model to study the mechanism of this pathway.

Cardiac function and exercise adaptation in 8 children with LPIN1 mutations

INTRODUCTION

Lipin-1 deficiency is a major cause of rhabdomyolysis that are precipitated by febrile illness. The prognosis is poor, with one-third of patients dying from cardiac arrest during a crisis episode. Apart from acute rhabdomyolysis, most patients are healthy, showing normal clinical and cardiac ultrasound parameters.

PATIENTS AND METHODS

We report cardiac and exercise examinations of 8 children carrying two LPIN1 mutations. The examinations were performed outside of a myolysis episode, but one patient presented with fever during one examination.

RESULTS

All but one patient displayed normal resting cardiac function, as determined by echocardiography. One patient exhibited slight left ventricular dysfunction at rest and a lack of increased stroke volume during cycle ramp exercise. During exercise, peripheral muscle adaptation was impaired in 2 patients compared to healthy controls: they presented an abnormal increase in cardiac output relative to oxygen uptake: dQ/dVO₂=8.2 and 9.5 (>2DS of controls population). One patient underwent 2 exercise tests; during one test, the patient was febrile, leading to acute rhabdomyolysis in the following hours. He exhibited changes in recovery muscle reoxygenation parameters and an increased dQ/dVO₂ during exercise compared with that under normothermia (7.9 vs 6), which did not lead to acute rhabdomyolysis. The four patients assessed by cardiac ¹H-magnetic resonance spectroscopy exhibited signs of intracardiac steatosis.

CONCLUSION

We observed abnormal haemodynamic profiles during exercise in 3/8 patients with lipin-1 deficiency, suggesting impaired muscle oxidative phosphorylation during exercise. Fever appeared to be an aggravating factor. One patient exhibited moderate cardiac dysfunction, which was possibly related to intracardiac stored lipid toxicity.

Characterization of lipid metabolism in a novel immortalized human hepatocyte cell line

The development of hepatocyte cell models that represent fatty acid partitioning within the human liver would be beneficial for the study of the development and progression of nonalcoholic fatty liver disease (NAFLD). We sought to develop and characterize a novel human liver cell line (LIV0APOLY) to establish a model of lipid accumulation using a physiological mixture of fatty acids under low- and high-glucose conditions. LIV0APOLY cells were compared with a well-established cell line (HepG2) and, where possible, primary human hepatocytes. LIV0APOLY cells were found to proliferate and express some mature liver markers and were wild type for the PNPLA3 (rs738409) gene, whereas HepG2 cells carried the Ile(148)Met variant that is positively associated with liver fat content. Intracellular triglyceride content was higher in HepG2 than in LIV0APOLY cells; exposure to high glucose and/or exogenous fatty acids increased intracellular triglyceride in both cell lines. Triglyceride concentrations in media were higher from LIV0APOLY compared with HepG2 cells. Culturing LIV0APOLY cells in high glucose increased a marker of endoplasmic reticulum stress and attenuated insulin-stimulated Akt phosphorylation whereas low glucose and exogenous fatty acids increased AMPK phosphorylation. Although LIV0APOLY cells and primary hepatocytes stored similar amounts of exogenous fatty acids as triglyceride, more exogenous fatty acids were partitioned toward oxidation in the LIV0APOLY cells than in primary hepatocytes. LIV0APOLY cells offer the potential to be a renewable cellular model for studying the effects of exogenous metabolic substrates on fatty acid partitioning; however, their usefulness as a model of lipoprotein metabolism needs to be further explored.

Biosynthetic Studies of the Male Marking Pheromone in Bumblebees by Using Labelled Fatty Acids and Two-Dimensional Gas Chromatography with Mass Detection

Two-dimensional comprehensive gas chromatography (GC×GC) coupled with mass detection was used as a tool for biosynthetic studies of bumblebee pheromones. Prior to biosynthetic experiments, the chromatographic behaviour of isotopically modified esters in the GC×GC system as well as their behaviour in mass detection was studied. The male marking pheromones of Bombus lucorum, Bombus lapidarius and Bombus terrestris were investigated. Main pheromonal components are ethyl tetradec-9-enoate (53 %) and ethyl dodecanoate (6 %) in B. lucorum, hexadec-9-en-1-ol (52 %) and hexadecan-1-ol (31 %) in B. lapidarius, and 2,3-dihydrofarnesol (58 %) and ethyl dodecanoate (15 %) in B. terrestris. The research strategy was based on 1) in vivo incubation of isotopically (² H, ¹³ C) modified fatty acids (FAs) and analysis of their metabolites and 2) feeding experiments with ² H- and ¹³ C-labelled FAs mixed with food. It was observed that labelled FAs were modified into the most abundant aliphatic compounds present in labial gland secretions. In feeding experiments, the labelled FAs were transformed into pheromone components. Transport of the FA precursors from the fat body through haemolymph was confirmed. The results show that FAs, stored in the form of triacylglycerols in the fat body, are likely to participate in the biosynthesis of some aliphatic pheromone components.

Combination of lipid metabolism alterations and their sensitivity to inflammatory cytokines in human lipin-1-deficient myoblasts

Lipin-1 deficiency is associated with massive rhabdomyolysis episodes in humans, precipitated by febrile illnesses. Despite well-known roles of lipin-1 in lipid biosynthesis and transcriptional regulation, the pathogenic mechanisms leading to rhabdomyolysis remain unknown. Here we show that primary myoblasts from lipin-1-deficient patients exhibit a dramatic decrease in LPIN1 expression and phosphatidic acid phosphatase 1 activity, and a significant accumulation of lipid droplets (LD). The expression levels of LPIN1-target genes [peroxisome proliferator-activated receptors delta and alpha (PPARδ, PPARα), peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), acyl-coenzyme A dehydrogenase, very long (ACADVL), carnitine palmitoyltransferase IB and 2 (CPT1B and CPT2)] were not affected while lipin-2 protein level, a closely related member of the family, was increased. Microarray analysis of patients' myotubes identified 19 down-regulated and 51 up-regulated genes, indicating pleiotropic effects of lipin-1 deficiency. Special attention was paid to the up-regulated ACACB (acetyl-CoA carboxylase beta), a key enzyme in the fatty acid synthesis/oxidation balance. We demonstrated that overexpression of ACACB was associated with free fatty acid accumulation in patients' myoblasts whereas malonyl-carnitine (as a measure of malonyl-CoA) and CPT1 activity were in the normal range in basal conditions accordingly to the normal daily activity reported by the patients. Remarkably ACACB invalidation in patients' myoblasts decreased LD number and size while LPIN1 invalidation in controls induced LD accumulation. Further, pro-inflammatory treatments tumor necrosis factor alpha+Interleukin-1beta(TNF1α+IL-1ß) designed to mimic febrile illness, resulted in increased malonyl-carnitine levels, reduced CPT1 activity and enhanced LD accumulation, a phenomenon reversed by dexamethasone and TNFα or IL-1ß inhibitors. Our data suggest that the pathogenic mechanism of rhabdomyolysis in lipin-1-deficient patients combines the predisposing constitutive impairment of lipid metabolism and its exacerbation by pro-inflammatory cytokines.

The effect of valinomycin in fibroblasts from patients with fatty acid oxidation disorders

Disorders of the carnitine cycle and of the beta oxidation spiral impair the ability to obtain energy from fats at time of fasting and stress. This can result in hypoketotic hypoglycemia, cardiomyopathy, cardiac arrhythmia and other chronic medical problems. The in vitro study of fibroblasts from patients with these conditions is impaired by their limited oxidative capacity. Here we evaluate the capacity of valinomycin, a potassium ionophore that increases mitochondrial respiration, to increase the oxidation of fatty acids in cells from patients with inherited fatty acid oxidation defects. The addition of valinomycin to fibroblasts decreased the accumulation of the lipophilic cation tetraphenylphosphonium (TPP(+)) at low concentrations due to the dissipation of the mitochondrial membrane potential. At higher doses, valinomycin increased TPP(+) accumulation due to the increased potassium permeability of the plasma membrane and subsequent cellular hyperpolarization. The incubation of normal fibroblasts with valinomycin increased [(14)C]-palmitate oxidation (measured as [(14)C]O2 release) in a dose-dependent manner. By contrast, valinomycin failed to increase palmitate oxidation in fibroblasts from patients with very long chain acyl CoA dehydrogenase (VLCAD) deficiency. This was not observed in fibroblasts from patients heterozygous for this condition. These results indicate that valinomycin can increase fatty acid oxidation in normal fibroblasts and could be useful to differentiate heterozygotes from patients affected with VLCAD deficiency.

In situ assay of fatty acid β-oxidation by metabolite profiling following permeabilization of cell membranes

Quantitative analysis of mitochondrial FA β-oxidation (FAO) has drawn increasing interest for defining lipid-induced metabolic dysfunctions, such as in obesity-induced insulin resistance, and evaluating pharmacologic strategies to improve β-oxidation function. The aim was to develop a new assay to quantify β-oxidation function in intact mitochondria and with a low amount of cell material. Cell membranes of primary human fibroblasts were permeabilized with digitonin prior to a load with FFA substrate. Following 120 min of incubation, the various generated acylcarnitines were extracted from both cells and incubation medium by protein precipitation/desalting and subjected to solid-phase extraction. A panel of 30 acylcarnitines per well was quantified by MS/MS and normalized to citrate synthase activity to analyze mitochondrial metabolite flux. Pretreatment with bezafibrate and etomoxir revealed stimulating and inhibiting regulatory effects on β-oxidation function, respectively. In addition to the advantage of a much shorter assay time due to in situ permeabilization compared with whole-cell incubation systems, the method allows the detection of multiple acylcarnitines from an only limited amount of intact cells, particularly relevant to the use of primary cells. This novel approach facilitates highly sensitive, simple, and fast monitoring of pharmacological effects on FAO.

The enzymology of mitochondrial fatty acid beta-oxidation and its application to follow-up analysis of positive neonatal screening results

Oxidation of fatty acids in mitochondria is a key physiological process in higher eukaryotes including humans. The importance of the mitochondrial beta-oxidation system in humans is exemplified by the existence of a group of genetic diseases in man caused by an impairment in the mitochondrial oxidation of fatty acids. Identification of patients with a defect in mitochondrial beta-oxidation has long remained notoriously difficult, but the introduction of tandem-mass spectrometry in laboratories for genetic metabolic diseases has revolutionalized the field by allowing the rapid and sensitive analysis of acylcarnitines. Equally important is that much progress has been made with respect to the development of specific enzyme assays to identify the enzyme defect in patients subsequently followed by genetic analysis. In this review, we will describe the current state of knowledge in the field of fatty acid oxidation enzymology and its application to the follow-up analysis of positive neonatal screening results.