Fuel selection in white adipose tissue

Changing lanes: seasonal differences in cellular metabolism of adipocytes in grizzly bears (Ursus arctos horribilis)

Obesity is among the most prevalent of health conditions in humans leading to a multitude of metabolic pathologies such as type 2 diabetes and hyperglycemia. However, there are many wild animals that have large seasonal cycles of fat accumulation and loss that do not result in the health consequences observed in obese humans. One example is the grizzly bear (Ursus arctos horribilis) that can have body fat content > 40% that is then used as the energy source for hibernation. Previous in vitro studies found that hibernation season adipocytes exhibit insulin resistance and increased lipolysis. Yet, other aspects of cellular metabolism were not addressed, leaving this in vitro model incomplete. Thus, the current studies were performed to determine if the cellular energetic phenotype-measured via metabolic flux-of hibernating bears was retained in cultured adipocytes and to what extent that was due to serum or intrinsic cellular factors. Extracellular acidification rate and oxygen consumption rate were used to calculate proton efflux rate and total ATP defined as both ATP from glycolysis and from mitochondrial respiration. Hibernation adipocytes treated with hibernation serum produced less ATP and exhibited lower maximal respiration and glycolysis rates than active season adipocytes. These effects were reversed with serum from the opposite season. Insulin had little influence on total ATP production and lipolysis in both hibernation and active serum-treated adipocytes. Together, these results suggest that the metabolic suppression occurring in hibernation adipocytes are downstream of insulin signaling and likely due to a combined reduction in mitochondria number and/or function and glycolytic processes. Future elucidation of the serum components and the cellular mechanisms that enable alterations in mitochondrial function could provide a novel avenue for the development of treatments for human metabolic diseases.

Interactive effects of aging and aerobic capacity on energy metabolism-related metabolites of serum, skeletal muscle, and white adipose tissue

Aerobic capacity is a strong predictor of longevity. With aging, aerobic capacity decreases concomitantly with changes in whole body metabolism leading to increased disease risk. To address the role of aerobic capacity, aging, and their interaction on metabolism, we utilized rat models selectively bred for low and high intrinsic aerobic capacity (LCRs/HCRs) and compared the metabolomics of serum, muscle, and white adipose tissue (WAT) at two time points: Young rats were sacrificed at 9 months of age, and old rats were sacrificed at 21 months of age. Targeted and semi-quantitative metabolomics analysis was performed on the ultra-pressure liquid chromatography tandem mass spectrometry (UPLC-MS) platform. The effects of aerobic capacity, aging, and their interaction were studied via regression analysis. Our results showed that high aerobic capacity is associated with an accumulation of isovalerylcarnitine in muscle and serum at rest, which is likely due to more efficient leucine catabolism in muscle. With aging, several amino acids were downregulated in muscle, indicating more efficient amino acid metabolism, whereas in WAT less efficient amino acid metabolism and decreased mitochondrial β-oxidation were observed. Our results further revealed that high aerobic capacity and aging interactively affect lipid metabolism in muscle and WAT, possibly combating unfavorable aging-related changes in whole body metabolism. Our results highlight the significant role of WAT metabolism for healthy aging.

Increased mitochondrial respiration of adipocytes from metabolically unhealthy obese compared to healthy obese individuals

Among obese subjects, metabolically healthy (MHO) and unhealthy obese (MUHO) subjects exist, the latter being characterized by whole-body insulin resistance, hepatic steatosis, and subclinical inflammation. Insulin resistance and obesity are known to associate with alterations in mitochondrial density, morphology, and function. Therefore, we assessed mitochondrial function in human subcutaneous preadipocytes as well as in differentiated adipocytes derived from well-matched donors. Primary subcutaneous preadipocytes from 4 insulin-resistant (MUHO) versus 4 insulin-sensitive (MHO), non-diabetic, morbidly obese Caucasians (BMI > 40 kg/m²), matched for sex, age, BMI, and percentage of body fat, were differentiated in vitro to adipocytes. Real-time cellular respiration was measured using an XF24 Extracellular Flux Analyzer (Seahorse). Lipolysis was stimulated by forskolin (FSK) treatment. Mitochondrial respiration was fourfold higher in adipocytes versus preadipocytes (p = 1.6*10^–9). In adipocytes, a negative correlation of mitochondrial respiration with donors’ insulin sensitivity was shown (p = 0.0008). Correspondingly, in adipocytes of MUHO subjects, an increased basal respiration (p = 0.002), higher proton leak (p = 0.04), elevated ATP production (p = 0.01), increased maximal respiration (p = 0.02), and higher spare respiratory capacity (p = 0.03) were found, compared to MHO. After stimulation with FSK, the differences in ATP production, maximal respiration and spare respiratory capacity were blunted. The differences in mitochondrial respiration between MUHO/MHO were not due to altered mitochondrial content, fuel switch, or lipid metabolism. Thus, despite the insulin resistance of MUHO, we could clearly show an elevated mitochondrial respiration of MUHO adipocytes. We suggest that the higher mitochondrial respiration reflects a compensatory mechanism to cope with insulin resistance and its consequences. Preserving this state of compensation might be an attractive goal for preventing or delaying the transition from insulin resistance to overt diabetes.

Deletion of intestinal Hdac3 remodels the lipidome of enterocytes and protects mice from diet-induced obesity

Histone deacetylase 3 (Hdac3) regulates the expression of lipid metabolism genes in multiple tissues, however its role in regulating lipid metabolism in the intestinal epithelium is unknown. Here we demonstrate that intestine-specific deletion of Hdac3 (Hdac3^IKO) protects mice from diet induced obesity. Intestinal epithelial cells (IECs) from Hdac3^IKO mice display co-ordinate induction of genes and proteins involved in mitochondrial and peroxisomal β-oxidation, have an increased rate of fatty acid oxidation, and undergo marked remodelling of their lipidome, particularly a reduction in long chain triglycerides. Many HDAC3-regulated fatty oxidation genes are transcriptional targets of the PPAR family of nuclear receptors, Hdac3 deletion enhances their induction by PPAR-agonists, and pharmacological HDAC3 inhibition induces their expression in enterocytes. These findings establish a central role for HDAC3 in co-ordinating PPAR-regulated lipid oxidation in the intestinal epithelium, and identify intestinal HDAC3 as a potential therapeutic target for preventing obesity and related diseases.

Histone deacetylase 3 (HDAC3) is a regulator of lipid homeostasis in several tissues, however, its role in intestinal lipid metabolism was not yet known. Here the authors study intestine specific HDAC3 knock out mice and report that these animals have increased fatty acid oxidation and undergo remodeling of the intestinal epithelial cell lipidome.

High released lactate by epicardial fat from coronary artery disease patients is reduced by dapagliflozin treatment

BACKGROUND AND AIMS

Dapagliflozin, a sodium-glucose co-transporter 2 inhibitor, improves glucose uptake by epicardial adipose tissue (EAT). However, its metabolism might raise the lactate production and acidosis under hypoxia conditions, i.e. coronary artery disease (CAD), or lipogenesis and, in consequence, expand adipose tissue. Since lactate secreted by adipose tissue is correlated with tissue stress and inflammation, our aim was to study glucose metabolism by epicardial fat in CAD and its regulation by dapagliflozin.

METHODS

Paired EAT and subcutaneous adipose tissue (SAT) biopsies from 49 patients who underwent open-heart surgery were cultured and split into three equal pieces, some treated with and others without dapagliflozin at 10 or 100 μM for 6 h. Anaerobic glucose metabolites were measured in supernatants of fat pads, and acidosis on adipogenesis-induced primary culture cells was analysed by colorimetric or fluorescence assays. Gene expression levels were assessed by real-time polymerase chain reaction.

RESULTS

Our results showed that dapagliflozin reduced the released lactate and acidosis in epicardial fat (p < 0.05) without changes in lipid storage-involved genes. In addition, this drug induced gene expression levels of peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC1α), a mitochondrial biogenesis-involved gene in both EAT and SAT (p < 0.05). After splitting the population regarding the presence of CAD, we observed higher lactate production in EAT from these patients (2.46 [1.75-3.47] mM), which was reduced after treatment with dapagliflozin 100 μM (1.99 [1.08-2.99] mM, p < 0.01).

CONCLUSIONS

Dapagliflozin improved glucose metabolism without lipogenesis-involved gene regulation or lactate production, mainly in patients with CAD. These results suggest an improvement of glucose oxidation metabolism that can contribute to cardiovascular benefits.

Non-alcoholic fatty liver disease as a consequence of autonomic imbalance and circadian desynchronization

The circadian system, headed by the suprachiasmatic nucleus, synchronizes behaviour and metabolism according to the external light-dark cycle through neuroendocrine and autonomic signals. Metabolic diseases, such as steatosis, obesity and glucose intolerance, have been associated with conditions of circadian misalignment wherein the feeding schedule has been moved to the resting phase. Here we describe the physiological processes involved in liver lipid accumulation and show how they follow a circadian pattern importantly regulated by both the autonomic nervous system and the feeding-fasting cycle. We propose that an unbalanced activity of the sympathetic-parasympathetic branches between organs induced by circadian misalignment provides the conditions for the development and progression of non-alcoholic fatty liver disease.

Cellular and molecular remodeling of inguinal adipose tissue mitochondria by dietary methionine restriction

Dietary methionine restriction (MR) produces a coordinated series of biochemical and physiological responses that improve biomarkers of metabolic health, increase energy expenditure, limit fat accretion and improve overall insulin sensitivity. Inguinal white adipose tissue (IWAT) is a primary target and site of action where the diet initiates transcriptional programs linked to enhancing both synthesis and oxidation of lipid. Using a combination of ex vivo approaches to assess dietary effects on cell morphology and function, we report that dietary MR produced a fourfold increase in multilocular, UCP1-expressing cells within this depot in conjunction with significant increases in mitochondrial content, size and cristae density. Dietary MR increased expression of multiple enzymes within the citric acid cycle, as well as respiratory complexes I, II and III. The physiological significance of these responses, evaluated in isolated mitochondria by high-resolution respirometry, was a significant increase in respiratory capacity measured using multiple substrates. The morphological, transcriptional and biochemical remodeling of IWAT mitochondria enhances the synthetic and oxidative capacity of this tissue and collectively underlies its expanded role as a significant contributor to the overall increase in metabolic flexibility and uncoupled respiration produced by the diet.

Clinically Relevant Mechanisms of Lipid Synthesis, Transport, and Storage

Lipids not only are fundamental nutrients but also serve as basic structural components of cells and as multifunctional signaling molecules. Lipid metabolism pathways underlie basic processes in health and disease and are the targets of novel therapeutics. In this review, we explore the molecular control of lipid synthesis, trafficking, and storage, with a focus on clinically relevant pathways. To illustrate the clinical relevance of molecular lipid regulation, we highlight how these biochemical processes contribute to the pathogenesis of nonalcoholic fatty liver disease, a component of the metabolic syndrome and a paradigmatic example of lipid dysregulation.

PLIN5 deletion remodels intracellular lipid composition and causes insulin resistance in muscle

Defective control of lipid metabolism leading to lipotoxicity causes insulin resistance in skeletal muscle, a major factor leading to diabetes. Here, we demonstrate that perilipin (PLIN) 5 is required to couple intramyocellular triacylglycerol lipolysis with the metabolic demand for fatty acids. PLIN5 ablation depleted triacylglycerol stores but increased sphingolipids including ceramide, hydroxylceramides and sphingomyelin. We generated perilipin 5 (Plin5)(-/-) mice to determine the functional significance of PLIN5 in metabolic control and insulin action. Loss of PLIN5 had no effect on body weight, feeding or adiposity but increased whole-body carbohydrate oxidation. Plin5 (-/-) mice developed skeletal muscle insulin resistance, which was associated with ceramide accumulation. Liver insulin sensitivity was improved in Plin5 (-/-) mice, indicating tissue-specific effects of PLIN5 on insulin action. We conclude that PLIN5 plays a critical role in coordinating skeletal muscle triacylglycerol metabolism, which impacts sphingolipid metabolism, and is requisite for the maintenance of skeletal muscle insulin action.

Increased β-oxidation with improved glucose uptake capacity in adipose tissue from obese after weight loss and maintenance

OBJECTIVE

We investigated protein markers for pathways of the fatty acids (FAs) and glucose metabolism in human adipose tissue after a weight loss program by calorie restriction.

METHODS

Overweight/obese subjects underwent an intervention of 5 weeks of a very low-calorie diet followed by a 3-week weight maintenance diet. Abdominal subcutaneous adipose tissue biopsies were sampled before and after the intervention. Seventeen target proteins as markers of metabolic pathways for the uptake and handling of FAs and glucose were quantified by Western blotting and 11 were retrieved from previous proteomics work. Correlation coefficients were calculated among changes of these proteins.

RESULTS

Short-chain 3-hydroxyacyl-CoA dehydrogenase, catalase, fatty acid translocase, fatty acid transporter protein 3, adipose triglyceride lipase, fatty acid-binding protein 4, aldolase-C, tubulin-β-5, and annexin A2 changed significantly, and lipoprotein lipase, perilipin 1, and hormone-sensitive lipase tended to change. On an average, increased glucose transporter type 4 translocation was observed, supported by a consistent increase of tyr-24 phosphorylated annexin A2.

CONCLUSIONS

Our findings suggest that after weight loss by calorie restriction and a short period of maintenance, adipose tissue has an increased capacity for glucose uptake, and lipid mobilization and oxidation. Such metabolic profile may relate to the health benefit of weight loss.

The succinated proteome

The post-translational modifications (PTMs) of cysteine residues include oxidation, S-glutathionylation, S-nitrosylation, and succination, all of which modify protein function or turnover in response to a changing intracellular redox environment. Succination is a chemical modification of cysteine in proteins by the Krebs cycle intermediate, fumarate, yielding S-(2-succino)cysteine (2SC). Intracellular fumarate concentration and succination of proteins are increased by hyperpolarization of the inner mitochondrial membrane, in concert with mitochondrial, endoplasmic reticulum (ER) and oxidative stress in 3T3 adipocytes grown in high glucose medium and in adipose tissue in obesity and diabetes in mice. Increased succination of proteins is also detected in the kidney of a fumarase deficient conditional knock-out mouse which develops renal cysts. A wide range of proteins are subject to succination, including enzymes, adipokines, cytoskeletal proteins, and ER chaperones with functional cysteine residues. There is also some overlap between succinated and glutathionylated proteins, suggesting that the same low pKa thiols are targeted by both. Succination of adipocyte proteins in diabetes increases as a result of nutrient excess derived mitochondrial stress and this is inhibited by uncouplers, which discharge the mitochondrial membrane potential (ΔΨm) and relieve the electron transport chain. 2SC therefore serves as a biomarker of mitochondrial stress or dysfunction in chronic diseases, such as obesity, diabetes, and cancer, and recent studies suggest that succination is a mechanistic link between mitochondrial dysfunction, oxidative and ER stress, and cellular progression toward apoptosis. In this article, we review the history of the succinated proteome and the challenges associated with measuring this non-enzymatic PTM of proteins by proteomics approaches.

Fatty acid metabolism, energy expenditure and insulin resistance in muscle

Fatty acids (FAs) are essential elements of all cells and have significant roles as energy substrates, components of cellular structure and signalling molecules. The storage of excess energy intake as fat in adipose tissue is an evolutionary advantage aimed at protecting against starvation, but in much of today's world, humans are faced with an unlimited availability of food, and the excessive accumulation of fat is now a major risk for human health, especially the development of type 2 diabetes (T2D). Since the first recognition of the association between fat accumulation, reduced insulin action and increased risk of T2D, several mechanisms have been proposed to link excess FA availability to reduced insulin action, with some of them being competing or contradictory. This review summarises the evidence for these mechanisms in the context of excess dietary FAs generating insulin resistance in muscle, the major tissue involved in insulin-stimulated disposal of blood glucose. It also outlines potential problems with models and measurements that may hinder as well as help improve our understanding of the links between FAs and insulin action.

Obesity and diabetes: lipids, 'nowhere to run to'

Although specific pathogenic entities contributing to diabetic risk, such as central adiposity, ectopic fat accumulation, hyperlipidaemia and inflammation, are well-characterized, the response of cellular systems to such insults are less well understood. This short review highlights the effect of increasing fat mass on ectopic fat accumulation, the role of triacylglycerols (triglycerides) in Type 2 diabetes mellitus and cardiovascular disease pathogenesis, and selected current therapeutic strategies used to ameliorate these risk factors.

Disturbed cholesterol homeostasis in hormone-sensitive lipase-null mice

Transcriptomics analysis revealed that genes involved in hepatic de novo cholesterol synthesis were downregulated in fed HSL-null mice that had been on a high-fat diet (HFD) for 6 mo. This finding prompted a further analysis of cholesterol metabolism in HSL-null mice, which was performed in fed and 16-h-fasted mice on a normal chow diet (ND) or HFD regimen. Plasma cholesterol was elevated in HSL-null mice, in all tested conditions, as a result of cholesterol enrichment of HDL and VLDL. Hepatic esterified cholesterol content and ATP-binding cassette transporter A1 (ABCA1) mRNA and protein levels were increased in HSL-null mice regardless of the dietary regimen. Unsaturated fatty acid composition of hepatic triglycerides was modified in fasted HSL-null mice on ND and HFD. The increased ABCA1 expression had no major effect on cholesterol efflux from HSL-null mouse hepatocytes. Taken together, the results of this study suggest that HSL plays a critical role in the hydrolysis of cytosolic cholesteryl esters and that increased levels of hepatic cholesteryl esters, due to lack of action of HSL in the liver, are the main mechanism underlying the imbalance in cholesterol metabolism in HSL-null mice.

Plasma free fatty acid storage in subcutaneous and visceral adipose tissue in postabsorptive women

OBJECTIVE

We assessed the direct (VLDL-triglycerides [VLDL-TG] independent) storage of circulating free fatty acids (FFAs) in visceral and subcutaneous fat in postabsorptive women.

RESEARCH DESIGN AND METHODS

Twelve women (BMI 29.6 +/- 6.6 kg/m(2)) received an identical, intravenous bolus dose of [1-(14)C]oleate followed by timed subcutaneous fat biopsies (abdominal and femoral) and then omental fat biopsy during tubal ligation surgery. Regional fat masses were assessed by combining dual-energy X-ray absorptiometry and computed tomography scanning. Separately, we assessed the fraction of FFA tracer entering VLDL-TG over the time representing the delay in collecting omental fat.

RESULTS

Site-specific fat specific activity (SA) (dpm/g lipid) decreased as a function of fat mass in both upper-body subcutaneous (UBSQ) and visceral fat depots. These patterns are consistent with dilution of a relatively fixed amount of FFA tracer within progressively greater amounts of fat. Interestingly, femoral SA did not vary as a function of lower-body subcutaneous (LBSQ) fat mass. [1-(14)C]oleate storage per million LBSQ adipocytes was positively associated with LBSQ fat mass, but no significant relationships were observed in UBSQ or visceral fat depot. The fraction of [1-(14)C]oleate stored in UBSQ, LBSQ, and visceral fat was 6.7 +/- 3.2, 4.9 +/- 3.4, and 1.0 +/- 0.3%, respectively. Only approximately 4% of the tracer traversed VLDL-TG over 9.5 h.

CONCLUSIONS

The increase in FFA tracer storage per adipocyte as a function of LBSQ fat mass implies that LBSQ adipocytes, in contrast to UBSQ and omental adipocytes, store more FFA in women with greater adiposity. The direct FFA storage pathway might play a role in favoring lower-body fat accumulation in women.

Fatty acid-induced mitochondrial uncoupling in adipocytes is not a promising target for treatment of insulin resistance unless adipocyte oxidative capacity is increased

The release of fatty acids from white adipose tissue is regulated at several levels. We have examined the suggestion that fatty acid release might be diminished by upregulation of mitochondrial fatty acid oxidation in the adipocyte, through increasing mitochondrial uncoupling. The intrinsic oxidative capacity of white adipose tissue is low, and older studies suggest that there is little fatty acid oxidation in white adipocytes, human or rodent. We have examined data on fatty acid metabolism and O(2) consumption in human white adipose tissue in vivo, and conclude that increasing fatty acid oxidation within the oxidative capacity of the tissue would produce only small changes (a few percent) in fatty acid release. The major locus of control of fatty acid release beyond the stimulation of lipolysis is the pathway of fatty acid esterification, already probably targeted by the thiazolidinedione insulin-sensitising agents. An alternative approach would be to upregulate the mitochondrial capacity of the adipocyte. We review proof-of-concept studies in which the phenotype of the white adipocyte has been changed to resemble that of the brown adipocyte by expression of peroxisome proliferator-activated receptor coactivator-1alpha. This increases oxidative capacity and also leads to fatty acid retention through upregulation of glycerol-3-phosphate production, and hence increased fatty acid re-esterification. We conclude that prevention or treatment of insulin resistance through alteration of adipocyte fatty acid handling will require more than a simple alteration of the activity of mitochondrial beta-oxidation within normal limits.

A computational model of adipose tissue metabolism: evidence for intracellular compartmentation and differential activation of lipases

Regulation of lipolysis in adipose tissue is critical to whole body fuel homeostasis and to the development of insulin resistance. Due to the challenging nature of laboratory investigations of regulatory mechanisms in adipose tissue, mathematical models could provide a valuable adjunct to such experimental work. We have developed a computational model to analyze key components of adipose tissue metabolism in vivo in human in the fasting state. The various key components included triglyceride-fatty acid cycling, regulation of lipolytic reactions, and glyceroneogenesis. The model, consisting of spatially lumped blood and cellular compartments, included essential transport processes and biochemical reactions. Concentration dynamics for major substrates were described by mass balance equations. Model equations were solved numerically to simulate dynamic responses to intravenous epinephrine infusion. Model simulations were compared with the corresponding experimental measurements of the arteriovenous difference across the abdominal subcutaneous fat bed in humans. The model can simulate physiological responses arising from the different expression levels of lipases. Key findings of this study are as follows: (1) Distinguishing the active metabolic subdomain ( approximately 3% of total tissue volume) is critical for simulating data. (2) During epinephrine infusion, lipases are differentially activated such that diglyceride breakdown is approximately four times faster than triglyceride breakdown. (3) Glyceroneogenesis contributes more to glycerol-3-phosphate synthesis during epinephrine infusion when pyruvate oxidation is inhibited by a high acetyl-CoA/free-CoA ratio.

Adipose tissue metabolism, diabetes and vascular disease--lessons from in vivo studies

There is an epidemic of obesity, insulin resistance and cardiovascular disease. Adipose tissue plays a major metabolic role and produces hormones with important physiological effects. In vitro studies remove regulatory factors, such as blood flow, making results difficult to interpret, and animal studies cannot necessarily be extrapolated to humans. Fortunately, adipose tissue can be studied in vivo with microdialysis, adipose tissue vein cannulation, measurement of blood flow using 133Xenon washout, stable isotope tracers and biopsies. In vivo studies have shown that adipose tissue is an efficient buffer against the postprandial flux of non-esterified fatty acids (NEFA) in the circulation, protecting other tissues. When there is excess adipose tissue, this buffering effect may be impaired. The postprandial blood flow response is also reduced, potentially causing an atherogenic lipid profile and atheroma. A systems biology approach, combining in vivo techniques with genomics, proteomics and metabolomics, will clarify links between adipose tissue and vascular disease.

Metabolism of lipids in human white adipocyte

Adipose tissue is considered as the body's largest storage organ for energy in the form of triacylglycerols, which are mobilized through lipolysis process, to provide fuel to other organs and to deliver substrates to liver for gluconeogenesis (glycerol) and lipoprotein synthesis (free fatty acids). The release of glycerol and free fatty acids from human adipose tissue is mainly dependent on hormone-sensitive lipase which is intensively regulated by hormones and agents, such as insulin (inhibition of lipolysis) and catecholamines (stimulation of lipolysis). A special attention is paid to the recently discovered perilipins which could regulate the activity of the lipase hormono-sensible. Most of the plasma triacylglycerols are provided by dietary lipids, secreted from the intestine in the form of chylomicron or from the liver in the form of VLDL. Released into circulation as non-esterified fatty acids by lipoprotein lipase, those are taken up by adipose tissue via specific plasma fatty acid transporters (CD36, FATP, FABPpm) and used for triacylglycerol synthesis. A small part of triacylglycerols is synthesized into adipocytes from carbohydrates (lipogenesis) but its regulation is still debated in human. Physiological factors such as dieting/fasting regulate all these metabolic pathways, which are also modified in pathological conditions e.g. obesity.

Does peroxisome proliferator-activated receptor gamma genotype (Pro12ala) modify the association of physical activity and dietary fat with fasting insulin level?

Peroxisome proliferator-activated receptor gamma (PPARgamma) has a role in controlling adipogenesis and insulin sensitivity. Previous studies have suggested that a common polymorphism (Pro12Ala) in the PPARgamma-2 isoform of this gene may be associated with markers of insulin resistance. We have previously shown that in combination, the relationships with fasting insulin of dietary polyunsaturated to saturated fatty acid ratio (P:S ratio) and physical activity are additive. We have also demonstrated that the association between P:S ratio and fasting insulin level is modified by the Pro12Ala genotype. The purpose of the present study was to investigate whether the Pro12Ala genotype modified the combined relationships of P:S ratio and physical activity level (PAL) on fasting insulin concentration. A population-based cohort of 506 Caucasian men and women aged 31 to 71 years was genotyped for the Pro12Ala polymorphism. P:S ratio was assessed by food-frequency questionnaire (FFQ) and PAL was estimated from 4 days of free-living heart rate monitoring following individual calibration of heart rate against energy expenditure during an exercise stress test. The combined associations of PAL and P:S ratio on fasting insulin level were examined stratified by Pro12Ala genotypes in a dominant model for the Ala allele. Among Pro allele homozygotes, there was no interaction between PAL and P:S ratio on fasting insulin (P =.929). However, in carriers of the Ala allele the association of P:S ratio with fasting insulin was modified by activity level (interaction P = 0.038). In those who were inactive and carried the Ala allele, the age-, sex-, and body mass-adjusted relationship between P:S ratio and log insulin was not significant (beta = -0.03, P =.93). In contrast, in physically active Ala carriers, the association of P:S ratio with log fasting insulin was highly significant (beta = -0.93, P =.004). In conclusion, this study examined the modification by PPARgamma genotype of the association between energy expenditure, P:S ratio, and fasting insulin level, a measure of insulin resistance. These data show that in Pro allele homozygotes the combined associations of P:S ratio and PAL are additive. In contrast, in Ala allele carriers, PAL modifies the association between P:S ratio and fasting insulin level in a multiplicative manner.

Hormone-sensitive lipase deficiency in mice changes the plasma lipid profile by affecting the tissue-specific expression pattern of lipoprotein lipase in adipose tissue and muscle

Hormone-sensitive lipase (HSL) is believed to play an important role in the mobilization of fatty acids from triglycerides (TG), diglycerides, and cholesteryl esters in various tissues. Because HSL-mediated lipolysis of TG in adipose tissue (AT) directly feeds non-esterified fatty acids (NEFA) into the vascular system, the enzyme is expected to affect many metabolic processes including the metabolism of plasma lipids and lipoproteins. In the present study we examined these metabolic changes in induced mutant mouse lines that lack HSL expression (HSL-ko mice). During fasting, when HSL is normally strongly induced in AT, HSL-ko animals exhibited markedly decreased plasma concentrations of NEFA (-40%) and TG (-63%), whereas total cholesterol and HDL cholesterol levels were increased (+34%). Except for the increased HDL cholesterol concentrations, these differences were not observed in fed animals, in which HSL activity is generally low. Decreased plasma TG levels in fasted HSL-ko mice were mainly caused by decreased hepatic very low density lipid lipoprotein (VLDL) synthesis as a result of decreased NEFA transport from the periphery to the liver. Reduced NEFA transport was also indicated by a depletion of hepatic TG stores (-90%) and strongly decreased ketone body concentrations in plasma (-80%). Decreased plasma NEFA and TG levels in fasted HSL-ko mice were associated with increased fractional catabolic rates of VLDL-TG and an induction of the tissue-specific lipoprotein lipase (LPL) activity in cardiac muscle, skeletal muscle, and white AT. In brown AT, LPL activity was decreased. Both increased VLDL fractional catabolic rates and increased LPL activity in muscle were unable to provide the heart with sufficient NEFA, which led to decreased tissue TG levels in cardiac muscle. Our results demonstrate that HSL deficiency markedly affects the metabolism of TG-rich lipoproteins by the coordinate down-regulation of VLDL synthesis and up-regulation of LPL in muscle and white adipose tissue. These changes result in an "anti-atherogenic" lipoprotein profile.

A computational framework for a nutrient flow representation of energy utilization by growing monogastric animals

A computational framework to represent nutrient utilization for body protein and lipid accretion by growing monogastric animals is presented. Nutrient and metabolite flows, and the biochemical and biological processes which transform these, are explicitly represented. A minimal set of calibration parameters is determined to provide five degrees of freedom in the adjustment of the marginal input-output response of this nutritional process model for a particular (monogastric) animal species. These parameters reflect the energy requirements to support the main biological processes: nutrient intake, faecal and urinary excretion, and production in terms of protein and lipid accretion. Complete computational details are developed and presented for these five nutritional processes, as well as a representation of the main biochemical transformations in the metabolic processing of nutrient intake. Absolute model response is determined as the residual nutrient requirements for basal processes. This model can be used to improve the accuracy of predicting the energetic efficiency of utilizing nutrient intake, as this is affected by independent diet and metabolic effects. Model outputs may be used to generate mechanistically predicted values for the net energy of a diet at particular defined metabolic states.

The adipocyte: a model for integration of endocrine and metabolic signaling in energy metabolism regulation

The ability to ensure continuous availability of energy despite highly variable supplies in the environment is a major determinant of the survival of all species. In higher organisms, including mammals, the capacity to efficiently store excess energy as triglycerides in adipocytes, from which stored energy could be rapidly released for use at other sites, was developed. To orchestrate the processes of energy storage and release, highly integrated systems operating on several physiological levels have evolved. The adipocyte is no longer considered a passive bystander, because fat cells actively secrete many members of the cytokine family, such as leptin, tumor necrosis factor-alpha, and interleukin-6, among other cytokine signals, which influence peripheral fuel storage, mobilization, and combustion, as well as energy homeostasis. The existence of a network of adipose tissue signaling pathways, arranged in a hierarchical fashion, constitutes a metabolic repertoire that enables the organism to adapt to a wide range of different metabolic challenges, such as starvation, stress, infection, and short periods of gross energy excess.

Skeletal muscle fatty acid metabolism in association with insulin resistance, obesity, and weight loss

The current study was undertaken to investigate fatty acid metabolism by skeletal muscle to examine potential mechanisms that could lead to increased muscle triglyceride in obesity. Sixteen lean and 40 obese research volunteers had leg balance measurement of glucose and free fatty acid (FFA) uptake (fractional extraction of [9,10 (3)H]oleate) and indirect calorimetry across the leg to determine substrate oxidation during fasting and insulin-stimulated conditions. Muscle obtained by percutaneous biopsy had lower carnitine palmitoyl transferase (CPT) activity and oxidative enzyme activity in obesity (P < 0.05). During fasting conditions, obese subjects had an elevated leg respiratory quotient (RQ, 0.83 +/- 0.02 vs. 0.90 +/- 0.01; P < 0.01) and reduced fat oxidation but similar FFA uptake across the leg. During insulin infusions, fat oxidation by leg tissues was suppressed in lean but not obese subjects; rates of FFA uptake were similar. Fasting values for leg RQ correlated with insulin sensitivity (r = -0.57, P < 0.001). Thirty-two of the obese subjects were restudied after weight loss (WL, -14.0 +/- 0.9 kg); insulin sensitivity and insulin suppression of fat oxidation improved (P < 0.01), but fasting leg RQ (0.90 +/- 0.02 vs. 0.90 +/- 0.02, pre-WL vs. post-WL) and muscle CPT activity did not change. The findings suggest that triglyceride accumulation in skeletal muscle in obesity derives from reduced capacity for fat oxidation and that inflexibility in regulating fat oxidation, more than fatty acid uptake, is related to insulin resistance.

Macronutrient metabolism of adipose tissue at rest and during exercise: a methodological viewpoint

The metabolism of white adipose tissue is regulated by many factors, including hormones and substrates delivered in the blood, the activity of the autonomic nervous system and the rate of flow of blood through the tissue. An integrated view of adipose tissue metabolism can only be gained, therefore, from studies in vivo. Of the various techniques available for studying adipose tissue metabolism in vivo, the measurement of arterio-venous differences offers some unique possibilities. In human subjects this technique has been performed mostly by catheterization of the venous drainage of the subcutaneous abdominal depot. Studies using this technique indicate that adipose tissue has an active pattern of metabolism, responding rapidly to meal ingestion by suppressing the release of non-esterified fatty acids, or to exercise with an increase in fat mobilization. Adipose tissue blood flow may also change rapidly in these situations; for instance, it increases markedly after a meal, potentially increasing the delivery of triacylglycerol to the enzyme lipoprotein lipase (EC 3.1.1.34) for hydrolysis. During exercise, there is evidence that adipose tissue blood flow does not increase sufficiently to allow delivery of all the fatty acids released into the systemic circulation. The various adipose tissue depots have their own characteristic metabolic properties, although in human subjects these are difficult to study with the arterio-venous difference technique. A combination of tracer infusion with selective catheterization allows measurements of leg, splanchnic and non-splanchnic upper-body fat mobilization and triacylglycerol clearance. Development of such techniques may open up new possibilities in the future for obtaining an integrated picture of adipose tissue function and its depot-specific variations.

Coordinated regulation of hormone-sensitive lipase and lipoprotein lipase in human adipose tissue in vivo: implications for the control of fat storage and fat mobilization

The enzymes lipoprotein lipase (LPL, EC 3.1.1.34) and hormone-sensitive lipase (HSL, EC 3.1.1.3) apparently catalyze opposing functions in white adipose tissue: the former is concerned with fat storage, the latter with fat mobilization. We have studied their regulation in vivo in normal subjects in the postabsorptive state and after eating meals of different compositions, by measurement of arteriovenous concentration differences for triacylglycerol, non-esterified fatty acids and glycerol across a subcutaneous adipose depot. The two enzymes are regulated in a broadly reciprocal manner: in the overnight-fasted state, HSL is more active, but after a meal HSL is suppressed whilst LPL is activated. The movement of fatty acids in and out of adipose tissue appears to be driven by concentration gradients generated by regulation of these two enzymes, and also by activation, in the postprandial period, of the process of fatty acid esterification. The results show some interesting and perhaps unexpected features of metabolic regulation. Of the fatty acids generated by the action of LPL on circulating TAG, a large proportion is released directly into the venous plasma: close to 100% in the overnight-fasted state, and 50% or more at the peak of LPL action after a meal, making what appear reasonable assumptions. We suggest that this apparent 'inefficiency' of fat storage reflects the energetic cost of maintaining precise control over such a fundamental process. Although LPL is usually thought of as the enzyme regulating fat deposition, in fact the fatty acids and glycerol it releases from circulating TAG represent a substantial proportion of those released from adipose tissue, especially in the postprandial state. In addition, although HSL is considered the enzyme responsible for fat mobilization, suppression of its activity is essential to normal regulation of fat deposition. Thus, fat storage and fat mobilization during normal daily life are controlled by coordinated regulation of a number of enzymatic processes in white adipose tissue.