What does the measurement of whole-body fatty acid rate of appearance in plasma by using a fatty acid tracer really mean?

Human obesity is characterized by defective fat storage and enhanced muscle fatty acid oxidation, and trimetazidine gradually counteracts these abnormalities

An impaired ability to store fatty acids (FA) in subcutaneous adipose tissue (SAT) may be implicated in the pathogenesis of obesity-related diseases via overexposure of lean tissues and production of free radicals from FA oxidation (FAO). We studied regional FA metabolism in skeletal muscle and adipose tissue in humans and investigated the long-term effects of the FAO inhibitor trimetazidine on glucose and FA metabolism. Positron emission tomography (PET) and [(11)C]palmitate were used to compare FA metabolism in SAT and skeletal muscle between eight obese and eight nonobese subjects (BMI ≥/< 30 kg/m(2)). A subgroup of nine subjects underwent a 1-mo trimetazidine administration. PET with [(11)C]palmitate and [(18)F]fluorodeoxyglucose, indirect calorimetry, and MRI before and after this period were performed to characterize glucose and FA metabolism, fat masses, skeletal muscle triglyceride, and creatine contents. Obesity was characterized by a 100% elevation in FAO and a defect in the FA esterification rate constant (P < 0.05) in skeletal muscle. FA esterification was reduced by ~70% in SAT (P < 0.001) in obese vs. control subjects. The degrees of obesity and insulin resistance were both negatively associated with esterification-related parameters and positively with FAO (P < 0.05). Trimetazidine increased skeletal muscle FA esterification (P < 0.01) and mildly upregulated glucose phosphorylation (P = 0.066). Our data suggest that human obesity is characterized by a defect in tissue FA storage capability, which is accompanied by a (potentially compensatory) elevation in skeletal muscle FAO; trimetazidine diverted FA from oxidative to nonoxidative pathways and provoked an initial activation of glucose metabolism in skeletal muscle.

Palmitate induces C-reactive protein expression in human aortic endothelial cells. Relevance to fatty acid-induced endothelial dysfunction

Circulating levels of free fatty acids are commonly elevated in patients with the metabolic syndrome and exert, through activating proinflammatory pathways, harmful effects of the vascular endothelium. In this study, we examined the effect of palmitate (PA) on endothelial C-reactive protein (CRP) expression and the role of CRP in PA-induced nitric oxide (NO) inhibition. Palmitate increased, in a dose-dependent manner, CRP protein expression and production in human aortic endothelial cells (HAECs). Induction of CRP protein was mimicked by ceramide, whereas bromopalmitate and other common free fatty acids such as oleate or linoleate were ineffective. Palmitate also elicited reactive oxygen species production in HAECs, an effect prevented by protein kinase C (PKC) inhibition and adenosine monophosphate-activated kinase (AMPK) activation. Palmitate-treated HAECs showed increased CRP messenger RNA expression and nuclear factor (NF)-κB activation. Induction of CRP expression by PA was prevented by antioxidants and normalized by PKC and mitogen-activated protein kinase inhibitors. Disrupting NF-κB and Janus kinase/signal transducers and activators of transcription pathways or inducing AMPK activation also suppressed the stimulatory effect of PA on CRP messenger RNA expression. Finally, in HAECs, PA reduced NO release, an effect reversed by anti-CRP antibody. These data demonstrate that PA-induced endothelial CRP expression involves PKC-driven oxidative stress, possibly through AMPK inhibition, and activation of downstream redox-sensitive signaling pathways, including NF-κB. They further support a role for endothelial cell-derived CRP as mediator of the suppressive effect of PA on NO production.

The use of stable-isotopically labeled oleic acid to interrogate lipid assembly in vivo: assessing pharmacological effects in preclinical species

The use of stable isotopically labeled substrates and analysis by mass spectrometry have provided substantial insight into rates of synthesis, disposition, and utilization of lipids in vivo. The information to be gained from such studies is of particular benefit to therapeutic research where the underlying causes of disease may be related to the production and utilization of lipids. When studying biology through the use of isotope tracers, care must be exercised in interpreting the data to ensure that any response observed can truly be interpreted as biological and not as an artifact of the experimental design or a dilutional effect on the isotope. We studied the effects of dosing route and tracer concentration on the mass isotopomer distribution profile as well as the action of selective inhibitors of microsomal tri-glyceride transfer protein (MTP) in mice and diacylglycerol acyltransferase 1 (DGAT1) in nonhuman primates, using a stable-isotopically labeled approach. Subjects were treated with inhibitor and subsequently given a dose of uniformly ¹³C-labeled oleic acid. Samples were analyzed using a rapid LC-MS technique, allowing the effects of the intervention on the assembly and disposition of triglycerides, cholesteryl esters, and phospholipids to be determined in a single 3 min run from just 10 μl of plasma.

Palmitate induces apoptosis in Schwann cells via both ceramide-dependent and independent pathways

Peripheral neuropathy has been reported to prevail in obese or pre-diabetic individuals, yet its etiology remains unknown. Palmitate, a saturated fatty acid increased in obesity and diabetes, is known to induce apoptosis in multiple types of cells and this effect may be mediated by ceramide, a member of the sphingolipid family. To clarify whether de novo ceramide synthesis from palmitate contributes to apoptosis of Schwann cells, we cultured immortalized mouse Schwann cells (IMS) and rat primary Schwann cells with palmitate, a ceramide analogue C2-ceramide as well as inhibitors of the de novo ceramide synthesis (myriocin and fumonisin B1). Apoptosis of IMS detected by nuclear staining and cell membrane inversion was significantly increased by incubation with palmitate for 48 h in a dose-dependent fashion. This enhanced apoptosis was partially but significantly suppressed by myriocin and fumonisin B1. Western blot analysis and immunostaining revealed that palmitate clearly activated caspase-3 in IMS. Unexpectedly, the ceramide synthesis inhibitors failed to suppress the palmitate-induced caspase-3 activation in spite of complete restoration in ceramide accumulation. The results seemed relevant to the observations that C2-ceramide did not activate caspase-3 while provoking apoptosis with a clear dose-dependency. In agreement, the pro-apoptotic action of C2-ceramide was not attenuated by caspase inhibitors that partially suppressed palmitate-induced apoptosis. These results in IMS were well reproducible in rat primary Schwann cells, indicating that the observed phenomena are not specific to the cell line. Collectively, we have reached a conclusion that palmitate induces apoptosis in Schwann cells via both a ceramide-mediated, caspase-3-independent pathway and ceramide-independent, caspase-3-dependent pathways. Given the fact that palmitate and ceramide are increased in obese or pre-diabetic subjects, these lipids may be implicated in the pathogenesis of peripheral neuropathy observed in these disorders.

Atherosclerosis induced by a high-fat diet is alleviated by lithium chloride via reduction of VCAM expression in ApoE-deficient mice

Endothelial cell dysfunction may play an important role in the development of various vascular diseases, including atherosclerosis. Here we investigated whether lithium chloride (LiCl), an inhibitor of glycogen synthase kinase-3β (GSK-3β), could counteract atherosclerosis induced by a high-fat diet in ApoE⁻/⁻ mice. Ten-week-old male mice were randomly divided into four groups: normal chow diet, high-fat diet (i.e., 20% fat and 0.5% cholesterol), high-fat diet with LiCl treatment for 6 weeks and high-fat diet with LiCl treatment for 14 weeks. Examination of plasma profiles indicated that blood glucose levels were significantly decreased by LiCl treatment. Supplementation with LiCl dramatically reduced atherosclerotic lesion formation in the aorta and aortic root. LiCl treatment also decreased vascular cell adhesion molecule (VCAM)-1 expression and macrophage infiltration into atherosclerotic lesion areas within the aortic valve. In addition, inhibition of GSK-3β by TDZD-8, SB216763, and LiCl, as well as adenoviral transduction with a catalytically inactive GSK-3β, reduced palmitate-induced VCAM-1 expression through inhibition of JNK activity and degradation of Iκ-Bα in human umbilical vein endothelial cells (HUVECs). The results of the present study suggest that LiCl alleviates palmitate-induced cell adhesion molecule expression in HUVECs and decreases atherosclerosis induced by a high-fat diet in ApoE⁻/⁻ mice. Thus, GSK-3β may be involved in the development of atherosclerosis induced by a high-fat diet in ApoE⁻/⁻ mice.

Estrogen deficiency after menopause does not result in male very-low-density lipoprotein metabolism phenotype

CONTEXT

Sex differences in lipid metabolism result in a less proatherogenic plasma lipid profile in premenopausal women than men. The mechanisms responsible for this are unclear but are thought to be related to differences in the sex hormone milieu in men and women.

OBJECTIVE

Our objective was to evaluate the effect of endogenous sex hormones on very-low-density lipoprotein (VLDL) triglyceride (TG) and apolipoprotein B-100 (apoB-100) metabolism. EXPERIMENTAL DESIGN AND MAIN OUTCOME MEASURES: We measured basal VLDL-TG and VLDL-apoB-100 concentrations and kinetics by using stable isotope-labeled tracers.

SETTING AND PARTICIPANTS

Eight premenopausal women [age, 43 + or - 8 yr; body mass index (BMI), 35 + or - 4 kg/m(2); mean + or - sd], eight postmenopausal women (age, 55 + or - 4 yr; BMI, 34 + or - 4 kg/m(2)), and eight men (age, 41 + or - 13 yr; BMI, 34 + or - 4 kg/m(2)) were studied at Washington University School of Medicine, St. Louis, MO.

RESULTS

VLDL-TG secretion rate was approximately double (P < 0.05) in postmenopausal women and men compared with premenopausal women but not different in postmenopausal women and men. The secretion rate of VLDL-apoB-100 was not different in pre- and postmenopausal women but was greater (P < 0.05) in men than in women.

CONCLUSIONS

Endogenous ovarian sex steroids are responsible for sexual dimorphism in VLDL-TG secretion, whereas VLDL-apoB-100 secretion is not regulated by female reproductive hormones.

Decrease in hepatic very-low-density lipoprotein-triglyceride secretion after weight loss is inversely associated with changes in circulating leptin

AIM

Although weight loss usually decreases very-low-density lipoprotein-triglyceride (VLDL-TG) secretion rate, the change in VLDL-TG kinetics is not directly related to the change in body weight. Circulating leptin also declines with weight loss and can affect hepatic lipid metabolism. The aim of this study was to determine whether circulating leptin is associated with weight loss-induced changes in VLDL-TG secretion.

METHODS

Ten extremely obese subjects were studied. VLDL-TG secretion rate and the contribution of systemic (derived from lipolysis of subcutaneous adipose tissue TG) and non-systemic fatty acids (derived primarily from lipolysis of intrahepatic and intraperitoneal TG, and de novo lipogenesis) to VLDL-TG production were determined by using stable isotopically labelled tracer methods before and 1 year after gastric bypass surgery.

RESULTS

Subjects lost 33 +/- 12% of body weight, and VLDL-TG secretion rate decreased by 46 +/- 23% (p = 0.001), primarily because of a decrease in the secretion of VLDL-TG from non-systemic fatty acids (p = 0.002). Changes in VLDL-TG secretion rates were not significantly related to reductions in body weight, body mass index, plasma palmitate flux, free fatty acid or insulin concentrations. The change in VLDL-TG secretion was inversely correlated with the change in plasma leptin concentration (r = -0.72, p = 0.013), because of a negative association between changes in leptin and VLDL-TG secretion from non-systemic fatty acids (r = -0.95, p < 0.001).

CONCLUSIONS

Weight loss-induced changes in plasma leptin concentration are inversely associated with changes in VLDL-TG secretion rate. Additional studies are needed to determine whether the correlation between circulating leptin and VLDL-TG secretion represents a cause-and-effect relationship.

Increased postprandial nonesterified fatty acid appearance and oxidation in type 2 diabetes is not fully established in offspring of diabetic subjects

Background

It has been proposed that abnormal postprandial plasma nonesterified fatty acid (NEFA) metabolism may participate in the development of tissue lipotoxicity and type 2 diabetes (T2D). We previously found that non-diabetic offspring of two parents with T2D display increased plasma NEFA appearance and oxidation rates during intravenous administration of a fat emulsion. However, it is currently unknown whether plasma NEFA appearance and oxidation are abnormal during the postprandial state in these subjects at high-risk of developing T2D.

Methodology

Palmitate appearance and oxidation rates and glycerol appearance rate were determined in eleven healthy offspring of two parents with T2D (positive family history, FH+), 13 healthy subjects without first-degree relatives with T2D (FH-) and 12 subjects with T2D at fasting, during normoglycemic hyperinsulinemic clamp and during continuous oral intake of a standard liquid meal to achieve steady postprandial NEFA and triacylglycerols (TG) without and with insulin infusion to maintain similar glycemia in all three groups.

Principal Findings

Plasma palmitate appearance and oxidation were higher at fasting and during the clamp conditions in the T2D group (all P<0.05). In the postprandial state, palmitate appearance, oxidative and non oxidative rates were all elevated in T2D (all P<0.05) but not in FH+. Both T2D and FH+ displayed elevated postprandial TG vs. FH- (P<0.001). Acute correction of hyperglycemia during the postprandial state did not affect these group differences. Increased waist circumference and BMI were positively associated with elevated postprandial plasma palmitate appearance and oxidation.

Conclusions/Significance

Postprandial plasma NEFA intolerance observed in subjects with T2D is not fully established in non-diabetic offspring of both parents with T2D, despite the presence of increased postprandial plasma TG in the later. Elevated postprandial plasma NEFA appearance and oxidation in T2D is observed despite acute correction of the exaggerated glycemic excursion in this group.

Pathogenesis and pathophysiology of the cardiometabolic syndrome

The cardiometabolic syndrome represents a cluster of metabolic abnormalities that are risk factors for cardiovascular disease. The mechanism(s) responsible for developing the cardiometabolic syndrome is not known, but it is likely that multi-organ insulin resistance, which is a common feature of the cardiometabolic syndrome, is involved. Insulin resistance is an important risk factor for type 2 diabetes and can cause vasoconstriction and renal sodium reabsorption, leading to increased blood pressure. Alterations in adipose tissue fatty acid and adipokine metabolism are involved in the pathogenesis of insulin resistance. Excessive rates of fatty acid release into the bloodstream can impair the ability of insulin to stimulate muscle glucose uptake and suppress hepatic glucose production. Noninfectious systemic inflammation associated with adipocyte and adipose tissue macrophage cytokine production can also cause insulin resistance. In addition, increased free fatty acid delivery to the liver can stimulate hepatic very low-density lipoprotein triglyceride production, leading to dyslipidemia.

Improved insulin sensitivity after weight loss and exercise training is mediated by a reduction in plasma fatty acid mobilization, not enhanced oxidative capacity

Obesity is characterized by excessive rates of plasma fatty acid mobilization and uptake, which play a key role in mediating insulin resistance. While weight loss via diet-only or a diet + exercise program clearly improves insulin sensitivity, the precise mechanisms modulating this improvement are not completely understood. The purpose of the present study was to determine the role of the reduced fatty acid mobilization and uptake after weight loss in obese women who were randomly assigned to lifestyle interventions of either weight loss without exercise (WL) (n = 7) or a weight loss + exercise program (WL + EX) (n = 10). Before and after losing 12% of their body weight, we measured insulin sensitivity (S(I)), systemic fatty acid rate of appearance (Ra) and disappearance (Rd), oxidative capacity, and markers for pro-inflammatory pathways in skeletal muscle. Fatty acid Ra and Rd were reduced by 30% after both interventions (P < 0.05). While oxidative capacity increased 25% in WL + EX (compared with no increase after WL), the improvement in S(I) was identical in both groups (60%; P < 0.05), and skeletal muscle pro-inflammatory pathways were reduced (P < 0.05) similarly in both groups. When we artificially increased fatty acid mobilization after weight loss to pre-weight-loss levels via an overnight lipid infusion, the improvement in S(I) was almost completely reversed. Importantly, WL + EX did not protect against this lipid-induced reversal in S(I) despite a significant increase in resting whole-body fat oxidation and a marked increase in skeletal muscle oxidative capacity. In conclusion, reduced fatty acid mobilization and uptake appears to be a primary mediator of improved insulin sensitivity after weight loss. Moreover, enhancing fatty acid oxidative capacity via exercise training is not sufficient to prevent the insulin resistance caused by high fatty acid mobilization, such as that found in obesity.

Relationship between body fat mass and free fatty acid kinetics in men and women

An increased release of free fatty acids (FFAs) into plasma likely contributes to the metabolic complications associated with obesity. However, the relationship between body fat and FFA metabolism is unclear because of conflicting results from different studies. The goal of our study was to determine the inter-relationships between body fat, sex, and plasma FFA kinetics. We determined FFA rate of appearance (Ra) in plasma, by using stable isotopically labeled tracer techniques, during basal conditions in 106 lean, overweight, and obese, nondiabetic subjects (43 men and 63 women who had 7.0-56.0% body fat). Correlation analyses demonstrated: (i) no differences between men and women in the relationship between fat mass (FM) and total FFA Ra (micromol/min); (ii) total FFA Ra increased linearly with increasing FM (r=0.652, P<0.001); (iii) FFA Ra per kg FM decreased in a curvilinear fashion with increasing FM (r=-0.806; P<0.001); (iv) FFA Ra in relationship to fat-free mass (FFM) was greater in obese than lean subjects and greater in women than in men; and (v) abdominal fat itself was not an important determinant of total FFA Ra. We conclude that total body fat, not regional fat distribution or sex, is an important modulator of the rate of FFA release into plasma. Although increased adiposity is associated with a decrease in fatty acid release in relationship to FM, this downregulation is unable to completely compensate for the increase in FM, so total FFA Ra and FFA Ra with respect to FFM are greater in women than in men and in obese than in lean subjects.

Free fatty acid kinetics in the late phase of postexercise recovery: importance of resting fatty acid metabolism and exercise-induced energy deficit

Free fatty acid (FFA) availability increases several-fold during exercise and remains significantly elevated for at least 3 to 6 hours after exercise cessation. Little, however, is known regarding the duration of the postexercise rise in FFA flux. In the present study, we used stable isotope-labeled palmitate infusion to examine fatty acid metabolism in 27 healthy untrained men and women (age, 29 +/- 7 years; body mass index, 25 +/- 4 kg/m2) between 13 to 16 hours and 21 to 24 hours after a single bout of moderate-intensity endurance exercise (1-2 hours at 60% of peak oxygen consumption), performed in the evening, and after a time-matched resting trial. Postabsorptive FFA rate of appearance (Ra) and FFA concentration in plasma were significantly greater after exercise than rest throughout the recovery period (P < .015), but the exercise-induced increases declined from approximately 40% at 13 to 16 hours to approximately 10% at 21 to 24 hours postexercise (P = .001). The magnitude of the exercise-induced increase in plasma FFA concentration was proportional to the increase in FFA Ra. Correlation analysis demonstrated that exercise-induced changes in plasma FFA Ra at 13 to 16 hours are (1) negatively associated with resting plasma FFA Ra and (2) positively associated with the net energy expenditure of exercise and the exercise-induced changes in whole-body fat oxidation rate (all P values < .05). In multivariate stepwise linear regression analysis, baseline plasma FFA Ra (P < or = .008) and net energy expenditure of exercise (P < or = .005) independently predicted the exercise-induced change in plasma FFA Ra at 13 to 16 hours. We conclude that the exercise-induced increase in FFA mobilization is (1) long-lived, persisting for 12 to 24 hours after exercise, with a progressive decline with time; (2) greater in subjects with low than high resting plasma FFA availability; and (3) greater after exercise with high than low energy demand.

Lipolysis and lipid mobilization in human adipose tissue

Triacylglycerol (TAG) stored in adipose tissue (AT) can be rapidly mobilized by the hydrolytic action of the three main lipases of the adipocyte. The non-esterified fatty acids (NEFA) released are used by other tissues during times of energy deprivation. Until recently hormone-sensitive lipase (HSL) was considered to be the key rate-limiting enzyme responsible for regulating TAG mobilization. A novel lipase named adipose triglyceride lipase/desnutrin (ATGL) has been identified as playing an important role in the control of fat cell lipolysis. Additionally perilipin and other proteins of the surface of the lipid droplets protecting or exposing the TAG core of the droplets to lipases are also potent regulators of lipolysis. Considerable progress has been made in understanding the mechanisms of activation of the various lipases. Lipolysis is under tight hormonal regulation. The best understood hormonal effects on AT lipolysis concern the opposing regulation by insulin and catecholamines. Heart-derived natriuretic peptides (i.e., stored in granules in the atrial and ventricle cardiomyocytes and exerting stimulating effects on diuresis and natriuresis) and numerous autocrine/paracrine factors originating from adipocytes and other cells of the stroma-vascular fraction may also participate in the regulation of lipolysis. Endocrine and autocrine/paracrine factors cooperate and lead to a fine regulation of lipolysis in adipocytes. Age, anatomical site, sex, genotype and species differences all play a part in the regulation of lipolysis. The manipulation of lipolysis has therapeutic potential in the metabolic disorders frequently associated with obesity and probably in several inborn errors of metabolism.

Stable isotope-labeled tracers for the investigation of fatty acid and triglyceride metabolism in humans in vivo

Understanding lipid metabolism and its regulation requires information on the rates at which lipids are produced within the body, absorbed (dietary lipids) into the body, transported within the body, and utilized by various tissues. This article focuses on the use of stable isotope-labeled tracers for the quantitative evaluation of major pathways of fatty acid and triglyceride metabolism in humans in vivo. Adipose tissue lipolysis and free fatty acid appearance in plasma, fatty acid tissue uptake and oxidation, and hepatic very low-density lipoprotein triglyceride secretion are among the metabolic pathways that can be studied by using stable isotope labeled tracers, and will be discussed in detail. The methodology has been in use for many years and is constantly being refined. A variety of tracers and analytical approaches are available and can be used; knowing the advantages, assumptions, and limitations of each is essential for the planning of studies and the interpretation of data, which can provide unique insights into human lipid metabolism.

Endoplasmic reticulum stress is reduced in tissues of obese subjects after weight loss

OBJECTIVE

Obesity is associated with insulin resistance and type 2 diabetes, although the mechanisms linking these pathologies remain undetermined. Recent studies in rodent models revealed endoplasmic reticulum (ER) stress in adipose and liver tissues and demonstrated that ER stress could cause insulin resistance. Therefore, we tested whether these stress pathways were also present in obese human subjects and/or regulated by weight loss.

RESEARCH DESIGN AND METHODS

Eleven obese men and women (BMI 51.3 +/- 3.0 kg/m2) were studied before and 1 year after gastric bypass (GBP) surgery. We examined systemic insulin sensitivity using hyperinsulinemic-euglycemic clamp studies before and after surgery and collected subcutaneous adipose and liver tissues to examine ER stress markers.

RESULTS

Subjects lost 39 +/- 9% body wt at 1 year after GBP surgery (P < 0.001), which was associated with a marked improvement in hepatic, skeletal muscle, and adipose tissue insulin sensitivity. Markers of ER stress in adipose tissue significantly decreased with weight loss. Specifically, glucose-regulated protein 78 (Grp78) and spliced X-box binding protein-1 (sXBP-1) mRNA levels were reduced, as were phosphorylated elongation initiation factor 2alpha (eIF2alpha) and stress kinase c-Jun NH2-terminal kinase 1 (JNK1) (all P values <0.05). Liver sections from a subset of subjects showed intense staining for Grp78 and phosphorylated eIF2alpha before surgery, which was reduced in post-GBP sections.

CONCLUSIONS

This study presents important evidence that ER stress pathways are present in selected tissues of obese humans and that these signals are regulated by marked weight loss and metabolic improvement. Hence, this suggests the possibility of a relationship between obesity-related ER stress and metabolic dysfunction in obese humans.

Relationship between total and high molecular weight adiponectin levels and plasma nonesterified fatty acid tolerance during enhanced intravascular triacylglycerol lipolysis in men

CONTEXT

Increased plasma nonesterified fatty acid (NEFA) appearance during enhanced intravascular triacylglycerol (TG) lipolysis is a marker of metabolic adipose tissue dysfunction and may lead to the development of insulin resistance. The relationship between total and high molecular weight (HMW) adiponectin levels, NEFA appearance, and total TG lipolytic capacity has not been previously studied in humans.

OBJECTIVES

Our objective was to determine whether total and HMW adiponectin plasma levels are associated with plasma NEFA level and appearance, and with total TG lipolytic rate during enhanced intravascular TG lipolysis in men.

DESIGN

This was a cross-sectional metabolic study.

SETTING

The study was performed at an academic clinical research center.

PARTICIPANTS

There were 15 healthy men (mean +/- sd body mass index 25.5 +/- 4.7 kg/m(2)) aged 21-50 yr (mean +/- sd 31.1 +/- 10.2) without first-degree relatives with type 2 diabetes included in the study.

INTERVENTIONS

Pancreatic clamps and iv infusion of stable isotopic tracers ([1,1,2,3,3-(2)H(5)]glycerol and [U-(13)C]palmitate) were performed, whereas intravascular TG lipolysis was clamped by iv infusion of heparin plus Intralipid at low (fasting) and high insulin levels. Total and HMW adiponectin levels were measured using an ELISA.

MAIN OUTCOME MEASURES

Levels of total and HMW adiponectin, palmitate appearance (plasma palmitate appearance rate), and glycerol appearance (plasma glycerol appearance rate) were calculated.

RESULTS

During heparin plus Intralipid infusion, total and HMW adiponectin was inversely correlated with plasma palmitate appearance rate (r = -0.65; P = 0.01), but this association was lost when expressed per nonlean weight. Adiponectin levels were positively associated with plasma glycerol appearance rate per nonlean weight (r = 0.71 and r = 0.66, respectively; P < or = 0.01).

CONCLUSIONS

Increased adipose tissue mass likely explains the association between low adiponectin and reduced NEFA tolerance. Adiponectin level is a marker of total TG lipolytic rate per adipose tissue mass in men.

Basal adipose tissue and hepatic lipid kinetics are not affected by a single exercise bout of moderate duration and intensity in sedentary women

Hypertriacylglycerolaemia is an important risk factor for cardiovascular disease. In men, we have shown that the effects of evening exercise on basal VLDL (very-low-density lipoprotein) metabolism are dose-dependent: a single prolonged bout of aerobic exercise [2 h at 60% of VO(2 peak) (peak oxygen consumption)] reduces fasting plasma TAG [triacylglycerol (triglyceride)] concentrations, via enhanced clearance of VLDL-TAG from the circulation, whereas the same exercise performed for 1 h has no effect on VLDL-TAG metabolism and concentration. We hypothesized that women are more sensitive to the TAG-lowering effect of exercise because they reportedly use more intramuscular TAG as an energy source during exercise, and depletion of muscle TAG stores has been linked to reciprocal changes in skeletal muscle LPL (lipoprotein lipase) activity. To test our hypothesis, we measured basal VLDL-TAG and VLDL-apoB-100 (apolipoprotein B-100), and plasma NEFA [non-esterified fatty acid ('free fatty acid')] kinetics, by using stable isotope-labelled tracer techniques, on the morning after a single session of evening exercise of moderate duration and intensity (1 h at 60% of VO(2 peak)) in eight sedentary pre-menopausal women (age, 28+/-3 years; body mass index, 27+/-2 kg/m(2); body fat, 34+/-3%; values are means+/-S.E.M.). Compared with an equivalent period of evening rest, exercise had no effect on post-absorptive NEFA concentrations and the rate of appearance in plasma, VLDL-TAG and VLDL-apoB-100 concentrations, hepatic VLDL-TAG and VLDL-apoB-100 secretion and plasma clearance rates (all P>0.05). We conclude that, in women, as in men, a single session of exercise of moderate intensity and duration is not sufficient to bring about the alterations in VLDL metabolism that have been linked to post-exercise hypotriacylglycerolaemia.

Changes in the serum composition of free-fatty acids during an intravenous glucose tolerance test

Recent studies suggest that measuring the free-fatty acids (FFA) during an intravenous glucose tolerance test (IVGTT) may provide information about the metabolic associations between serum FFA and carbohydrate and insulin metabolism. We evaluated the FFA profile during an IVGTT and determined whether this test changes the composition and concentration of FFA. An IVGTT was given to 38 severely obese persons before and 7 months after undergoing bariatric surgery and also to 12 healthy, nonobese persons. The concentration and composition of the FFA were studied at different times during the test. The concentration of FFA fell significantly faster during the IVGTT in the controls and in the severely obese persons with normal-fasting glucose (NFG) than in the severely obese persons with impaired-fasting glucose (IFG) or type 2 diabetes mellitus (T2DM) (P < 0.05). Significant differences were found in the time to minimum serum concentrations of FFA (control = NFG < IFG < T2DM) (P < 0.001). These variables improved after bariatric surgery in the three groups. The percentage of monounsaturated and n-6 polyunsaturated FFA in the control subjects and in the obese persons, both before and after surgery, decreased significantly during the IVGTT. In conclusion, during an IVGTT, severely obese persons with IFG or T2DM experienced a lower fall in the FFA than the severely obese persons with NFG and the controls, becoming normal after bariatric surgery.

The lowering of hepatic fatty acid uptake improves liver function and insulin sensitivity without affecting hepatic fat content in humans

Lipolysis may regulate liver free fatty acid (FFA) uptake and triglyceride accumulation; both are potential causes of insulin resistance and liver damage. We evaluated whether 1) systemic FFA release is the major determinant of liver FFA uptake in fasting humans in vivo and 2) the beneficial metabolic effects of FFA lowering can be explained by a reduction in liver triglyceride content. Sixteen healthy subjects were subdivided in two groups of similar characteristics to undergo positron emission tomography with [(11)C]acetate and [(11)C]palmitate to quantify liver FFA metabolism (n = 8), or magnetic resonance spectroscopy (MRS) to measure hepatic fat content (n = 8), before and after the acute lowering of circulating FFAs by using the antilipolytic agent acipimox. MRS was again repeated after a 1-wk treatment period. Acipimox suppressed FFA levels while stimulating hepatic fractional extraction of FFAs (P < 0.05). As a result, fasting liver FFA uptake was decreased by 79% (P = 0.0002) in tight association with lipolysis (r = 0.996, P < 0.0001). The 1-wk treatment induced a significant improvement in systemic (+30%) and liver (+70%) insulin sensitivity (P < 0.05) and decreased circulating triglycerides (-20%, P = 0.06) and liver enzymes (ALT -20%, P = 0.03). No change in liver fat content was observed after either acute or sustained FFA suppression. We conclude that acute and sustained inhibitions of lipolysis and liver FFA uptake fail to deplete liver fat in healthy human subjects. Liver FFA uptake was decreased in proportion to FFA delivery. As a consequence, liver and systemic insulin sensitivity were improved, together with liver function, independently of changes in hepatic triglyceride accumulation.

Liver, muscle, and adipose tissue insulin action is directly related to intrahepatic triglyceride content in obese subjects

BACKGROUND & AIMS

Nonalcoholic fatty liver disease is associated with insulin resistance and diabetes. The purpose of this study was to determine the relationship between intrahepatic triglyceride (IHTG) content and insulin action in liver (suppression of glucose production), skeletal muscle (stimulation of glucose uptake), and adipose tissue (suppression of lipolysis) in nondiabetic obese subjects.

METHODS

A euglycemic-hyperinsulinemic clamp procedure and stable isotopically labeled tracer infusions were used to assess insulin action, and magnetic resonance spectroscopy was used to determine IHTG content, in 42 nondiabetic obese subjects (body mass index, 36 +/- 4 kg/m(2)) who had a wide range of IHTG content (1%-46%).

RESULTS

Hepatic insulin sensitivity, assessed as a function of glucose production rate and plasma insulin concentration, was inversely correlated with IHTG content (r = -0.599; P < .001). The ability of insulin to suppress fatty acid release from adipose tissue and to stimulate glucose uptake by skeletal muscle were also inversely correlated with IHTG content (adipose tissue: r = -0.590, P < .001; skeletal muscle: r = -0.656, P < .001). Multivariate linear regression analyses found that IHTG content was the best predictor of insulin action in liver, skeletal muscle, and adipose tissue, independent of body mass index and percent body fat, and accounted for 34%, 42%, and 44% of the variability in these tissues, respectively (P < .001 for each model).

CONCLUSIONS

These results show that progressive increases in IHTG content are associated with progressive impairment of insulin action in liver, skeletal muscle, and adipose tissue in nondiabetic obese subjects. Therefore, nonalcoholic fatty liver disease should be considered part of a multiorgan system derangement in insulin sensitivity.

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.

Impaired plasma nonesterified fatty acid tolerance is an early defect in the natural history of type 2 diabetes

CONTEXT

Abnormal plasma nonesterified fatty acid (NEFA) metabolism may play a role in the development of type 2 diabetes.

OBJECTIVES

Our objectives were to demonstrate whether there is a defect in insulin-mediated suppression of plasma NEFA appearance (RaNEFA) and oxidation (OxNEFA) during enhanced intravascular triacylglycerol lipolysis early in the natural history of type 2 diabetes, and if so, to determine whether other mechanisms than reduced insulin-mediated suppression of intracellular lipolysis are involved.

DESIGN

These are cross-sectional studies.

SETTING

The studies were performed at an academic clinical research center.

PARTICIPANTS

Nine healthy subjects with both parents with type 2 diabetes (FH+) and nine healthy subjects with no first-degree relatives with type 2 diabetes (FH-) with similar anthropometric features were included in the studies.

INTERVENTIONS

Pancreatic clamps and iv infusion of stable isotopic tracers ([1,1,2,3,3-(2)H5]-glycerol and [U-(13)C]-palmitate or [1,2-(13)C]-acetate) were performed while intravascular triacylglycerol lipolysis was simultaneously clamped by iv infusion of heparin plus Intralipid at low (fasting) and high insulin levels. Oral nicotinic acid (NA) was used to inhibit intracellular lipolysis.

MAIN OUTCOME MEASURES

RaNEFA and OxNEFA were determined.

RESULTS

During heparin plus Intralipid infusion at high plasma insulin levels, and despite similar intravascular lipolytic rates, FH+ had higher RaNEFA and OxNEFA than FH- (RaNEFA: 17.4+/-6.3 vs. 9.2+/-4.2; OxNEFA: 4.5+/-1.8 vs. 2.3+/-1.5 micromol/kg lean body mass/min), independent of NA intake, gender, age, and body composition. In the presence of NA, insulin-mediated suppression of RaNEFA was still observed in FH-, but not in FH+.

CONCLUSIONS

Increased RaNEFA and OxNEFA during intravascular lipolysis at high insulin levels occur early in the natural history of type 2 diabetes.

Alterations in adipose tissue and hepatic lipid kinetics in obese men and women with nonalcoholic fatty liver disease

BACKGROUND & AIMS

Steatosis in patients with nonalcoholic fatty liver disease (NAFLD) is due to an imbalance between intrahepatic triglyceride (IHTG) production and export. The purpose of this study was to evaluate TG metabolism in adipose tissue and liver in NAFLD.

METHODS

Fatty acid, VLDL-TG, and VLDL-apolipoprotein B-100 (apoB100) kinetics were assessed by using stable isotope tracers in 14 nondiabetic obese subjects with NAFLD (IHTG, 22.7% +/- 2.0%) and 14 nondiabetic obese subjects with normal IHTG content (IHTG, 3.4% +/- 0.4%), matched on age, sex, body mass index, and percent body fat.

RESULTS

Compared with the normal IHTG group, the NAFLD group had greater rates of palmitate release from adipose tissue into plasma (85.4 +/- 6.6 and 114.1 +/- 8.1 micromol/min, respectively; P = .01) and VLDL-TG secretion (11.4 +/- 1.1 and 24.3 +/- 3.1 micromol/min, respectively; P = .001); VLDL-apoB100 secretion rates were not different between groups. The increase in VLDL-TG secretion was primarily due to an increased contribution from "nonsystemic" fatty acids, presumably derived from lipolysis of intrahepatic and intra-abdominal fat and de novo lipogenesis. VLDL-TG secretion rate increased linearly with increasing IHTG content in subjects with normal IHTG but reached a plateau when IHTG content was >/=10% (r = 0.618, P < .001).

CONCLUSIONS

Obese persons with NAFLD have marked alterations in both adipose tissue (increased lipolytic rates) and hepatic (increased VLDL-TG secretion) TG metabolism. Fatty acids derived from nonsystemic sources are responsible for the increase in VLDL-TG secretion. However, the increase in hepatic TG export is not adequate to normalize IHTG content.

Acute exposure to long-chain fatty acids impairs {alpha}2-adrenergic receptor-mediated antilipolysis in human adipose tissue

The acute in vitro and in vivo effects of long-chain fatty acids (LCFAs) on the regulation of adrenergic lipolysis were investigated in human adipose tissue. The effect of a 2 h incubation, without or with LCFA (200 mumol/l), on basal and hormonally induced lipolysis was tested in vitro on isolated fat cells. The lipolytic response to epinephrine was enhanced by suppression of the antilipolytic alpha(2)-adrenergic effect. Then, healthy lean and obese male subjects performed a 45 min exercise bout at 50% of their heart rate reserve either after an overnight fast or 3 h after a high-fat meal (HFM: 95% fat, 5% carbohydrates). Subcutaneous adipose tissue lipolysis was measured by microdialysis in the presence or absence of an alpha-antagonist (phentolamine). In vivo, a HFM increased plasma levels of nonesterified fatty acids in lean and obese subjects. In both groups, the HFM did not alter hormonal responses to exercise. Under fasting conditions, the alpha(2)-adrenergic antilipolytic effect was more pronounced in obese than in lean subjects. The HFM totally suppressed the alpha(2)-adrenergic antilipolytic effect in lean and obese subjects during exercise. LCFAs per se, in vitro as well as in vivo, suppress alpha(2)-adrenergic-mediated antilipolysis in adipose tissue. LCFA-mediated suppression of antilipolytic pathways represents another mechanism whereby a high fat content in the diet might increase adipose tissue lipolysis.

Triglyceride uptake and lipoprotein lipase-generated fatty acid spillover in the splanchnic bed of dogs

The action of lipoprotein lipase on triglyceride-rich lipoproteins generates fatty acids that are either transported into tissues or mix with circulating free fatty acids (FFAs) via a process known as spillover. In the present study, arterial, portal vein, and hepatic vein sampling catheters were surgically placed in nine mongrel dogs. The animals were subsequently studied after a 42-h fast during infusion of [14C]oleate and a lipid emulsion containing [3H]triolein; the emulsion was used as a surrogate for the study of chylomicron metabolism. More than one-half of splanchnic [3H]triglyceride uptake occurred in the liver, and substantial fractional spillover of [3H]oleate was observed in both liver and nonhepatic tissues (approximately 50% each). There was a significant correlation between FFA release from nonhepatic tissues (presumably visceral fat) and nonhepatic fractional spillover (R = 0.81, P < 0.01), consistent with a model in which the rate of intracellular lipolysis influences spillover by determining the direction of net fatty acid flow between the cell and the interstitium. There was a significant correlation between "true" and "net" splanchnic spillover (R = 0.84, P < 0.005), the latter representing calculation of spillover between arterial and hepatic venous blood without portal venous data. Metabolism of chylomicron triglycerides in visceral fat may be an important source of portal venous FFAs.

A single 1-h bout of evening exercise increases basal FFA flux without affecting VLDL-triglyceride and VLDL-apolipoprotein B-100 kinetics in untrained lean men

Our group (Magkos F, Wright DC, Patterson BW, Mohammed BS, Mittendorfer B, Am J Physiol Endocrinol Metab 290: E355-E362, 2006) has recently demonstrated that a single, prolonged bout of moderate-intensity cycling (2 h at 60% of peak oxygen consumption) in the evening increases basal whole-body free fatty acid (FFA) flux and fat oxidation, decreases hepatic VLDL-apolipoprotein B-100 (apoB-100) secretion, and enhances removal efficiency of VLDL-triglyceride (TG) from the circulation the following day in untrained, healthy, lean men. In the present study, we investigated the effect of a single, shorter-duration bout of the same exercise (1 h cycling at 60% of peak oxygen consumption) on basal FFA, VLDL-TG, and VLDL-apoB-100 kinetics in seven untrained, healthy, lean men by using stable isotope-labeled tracer techniques. Basal FFA rate of appearance in plasma and plasma FFA concentration were approximately 55% greater (P < 0.05) the morning after exercise than rest, whereas resting metabolic rate and whole-body substrate oxidation rates were not different after rest and exercise. Exercise had no effect on plasma VLDL-TG and VLDL-apoB-100 concentrations, hepatic VLDL-TG and VLDL-apoB-100 secretion rates, and VLDL-TG and VLDL-apoB-100 plasma clearance rates (all P > 0.05). We conclude that in untrained, healthy, lean men 1) the exercise-induced changes in basal whole-body fat oxidation, VLDL-TG, and VLDL-apoB-100 metabolism during the late phase of recovery from exercise are related to the duration of the exercise bout; 2) single sessions of typical recreational activities appear to have little effect on basal, fasting plasma TG homeostasis; and 3) there is a dissociation between systemic FFA availability and VLDL-TG and VLDL-apoB-100 secretion by the liver.

Involvement of Glycogen Synthase Kinase-3β in Palmitate-Induced Human Umbilical Vein Endothelial Cell Apoptosis

BACKGROUND/AIMS

The death of endothelial cells may play a critical role in the development of various vascular diseases, including atherosclerosis. While free fatty acids (FFAs) may stimulate endothelial apoptosis, the molecular and cellular mechanisms of this effect have not been studied intensively. To elucidate the mechanisms involved in FFA-induced endothelial cell apoptosis, we investigated the effect of different pharmacological inhibitors on palmitate-induced apoptosis in human umbilical vein endothelial cells (HUVECs). Interestingly, lithium, a glycogen synthase kinase-3 (GSK-3) inhibitor, showed a strong protective effect.

METHODS AND RESULTS

To examine the involvement of GSK-3beta in palmitate-induced HUVEC apoptosis, its dephosphorylation at Ser9 and enzymatic activation in response to palmitate treatment were monitored by immunoblotting and in vitro kinase assays, respectively. GSK-3beta was dephosphorylated and its enzymatic activity increased in palmitate-treated HUVECs. In addition, pretreatment with other GSK-3beta inhibitors, e.g. SB216763 or TDZD-8, as well as adenoviral transduction with a catalytically inactive GSK-3beta had significant protective effects against palmitate-induced HUVEC apoptosis.

CONCLUSION

These results demonstrate that the GSK-3beta signalling pathway is involved in palmitate-induced HUVEC apoptosis.

Women produce fewer but triglyceride-richer very low-density lipoproteins than men

CONTEXT

Very low-density lipoproteins (VLDL) are a major risk factor for cardiovascular disease. The concentrations of VLDL particles and VLDL-triglyceride (TG) in plasma are lower in women than men, but the mechanisms responsible for these differences are unclear.

OBJECTIVE

The objective of the study was to investigate the effects of sex on VLDL-TG and VLDL-apolipoprotein B-100 (apoB-100) metabolism. EXPERIMENTAL DESIGN AND MAIN OUTCOME MEASURES: We measured basal VLDL-TG and VLDL-apoB-100 kinetics by using stable isotope labeled tracers.

SETTING/PARTICIPANTS

Twenty-six healthy, lean subjects (13 men, aged 29+/-5 yr; 13 women, aged 28+/-6 yr) were studied in the General Clinical Research Center at Washington University School of Medicine.

RESULTS

VLDL-TG and VLDL-apoB-100 concentrations were less in women than men (P<0.05). The secretion rate of VLDL-TG was approximately 70% greater (P<0.05), whereas the secretion rate of VLDL-apoB-100 (i.e. VLDL particles) was approximately 20% less (P<0.05) in women than men. The molar ratio of VLDL-TG and VLDL-apoB-100 secretion rates was therefore more than double (P<0.05) in women than men. VLDL-TG plasma clearance rate was approximately 70% greater in women than men (P<0.05), whereas VLDL-apoB-100 plasma clearance rate was not different between sexes. However, VLDL-TG and VLDL-apoB-100 mean residence times in plasma were both shorter (by 45 and 25%, respectively; P<0.05) in women than men.

CONCLUSIONS

Increased VLDL-TG plasma clearance is responsible for lower VLDL-TG concentration, whereas decreased VLDL-apoB-100 secretion rate, combined with shorter VLDL-apoB-100 residence time in plasma, is responsible for lower VLDL-apoB-100 concentration in women than men. The greater molar ratio of VLDL-TG and VLDL-apoB-100 secretion rates suggests that the liver in women secretes fewer but TG-richer VLDL particles than the liver in men.

p38 mitogen-activated protein kinase mediates palmitate-induced apoptosis but not inhibitor of nuclear factor-kappaB degradation in human coronary artery endothelial cells

Plasma free fatty acids are elevated in patients with type 2 diabetes and contribute to the pathogenesis of insulin resistance and endothelial dysfunction. The p38 MAPK mediates stress, inflammation, and apoptosis. Whether free fatty acids induce apoptosis and/or activate nuclear factor-kappaB inflammatory pathway in human coronary artery endothelial cells (hCAECs) and, if so, whether this involves the p38 MAPK pathway is unknown. hCAECs (passages 4-6) were grown to 70% confluence and then incubated with palmitate at concentrations of 0-300 microm for 6-48 h. Palmitate at 100, 200, or 300 microm markedly increased apoptosis after 12 h of incubation. This apoptotic effect was time (P=0.008) and dose (P=0.006) dependent. Palmitate (100 microm for 24 h) induced a greater than 2-fold increase in apoptosis, which was accompanied with a 4-fold increase in p38 MAPK activity (P<0.001). Palmitate did not affect the phosphorylation of Akt1 or ERK1/2. SB203580 (a specific inhibitor of p38 MAPK) alone did not affect cellular apoptosis; however, it abolished palmitate-induced apoptosis and p38 MAPK activation. Palmitate significantly reduced the level of inhibitor of nuclear factor-kappaB (IkappaB). However, treatment of cells with SB203580 did not restore IkappaB to baseline. We conclude that palmitate induces hCAEC apoptosis via a p38 MAPK-dependent mechanism and may participate in coronary endothelial injury in diabetes. However, palmitate-mediated IkappaB degradation in hCAECs is independent of p38 MAPK activity.

Mechanism of insulin-stimulated clearance of plasma nonesterified fatty acids in humans

Insulin increases plasma nonesterified fatty acid (NEFA) clearance in humans, but whether this is independent of change in plasma NEFA appearance is currently unknown. Nine nondiabetic men (age: 28+/-3 yr, body mass index: 27.2+/-1.7 kg/m2) underwent euglycemic clamps to maintain low (LINS) vs. high (HINS) physiological insulin levels for 6 h. An intravenous infusion of heparin+Intralipid (HI) was performed during 4 of the 6 h of the clamps (in the last 4 h at LINS and in the first 4 h at HINS), whereas saline infusion (SAL) was administered in the remaining 2 h to modulate plasma NEFA levels independently of plasma insulin levels. Four experimental conditions were obtained in each individual: LINS with saline (LINS/SAL) and with HI infusion (LINS/HI) and HINS with saline (HINS/SAL) and with HI infusion (HINS/HI). Plasma palmitate appearance during HINS/SAL was lower than during the three other experimental conditions (P<0.05). In contrast, plasma linoleate appearance, as expected, was increased by HI independently of insulin level (P<0.02). Plasma palmitate clearance during HINS/SAL was higher than LINS/SAL and LINS/HI (P<0.008), and this increase was blunted during HINS/HI. We observed a linear decrease in plasma palmitate clearance with increasing plasma NEFA appearance independent of insulin levels. Plasma NEFA levels increased exponentially with increase in plasma NEFA appearance. We conclude that insulin stimulates plasma NEFA clearance by reducing the endogenous appearance rate of NEFA. The relationship between plasma NEFA level and appearance rate is nonlinear.

Reproducibility of stable isotope-labeled tracer measures of VLDL-triglyceride and VLDL-apolipoprotein B-100 kinetics

To gain insight into the mechanisms regulating plasma lipid homeostasis, FFA, VLDL-triglyceride (TG), and VLDL-apolipoprotein B-100 (apoB-100) kinetics are commonly assessed using stable isotope-labeled tracer methods. The reproducibility of these measurements, which is critical for the experimental design, is unknown. Therefore, we investigated the repeatability of plasma FFA, VLDL-TG, and VLDL-apoB-100 kinetics in eight healthy men using stable isotope-labeled tracer techniques. There were no systematic differences in plasma FFA, VLDL-TG, and VLDL-apoB-100 concentrations and kinetics between the two studies. Intraindividual day-to-day variability for various outcome variables ranged from 15% to 25%, and almost all of this was of biological origin. The most robust outcome variables were FFA rate of appearance and hepatic VLDL-TG and VLDL-apoB-100 secretion rates; the least robust were VLDL-TG and VLDL-apoB-100 plasma clearance rates and mean residence times. Overall, physiologically meaningful differences in mean values (i.e., 25-30% in magnitude) can be obtained with a sample size of 6-10 subjects for paired studies and 12-20 subjects per group for cross-sectional studies, assuming a type I error rate of 0.05 and a type II error rate of 0.20 (i.e., 80% power). These findings will be useful for future studies investigating FFA, VLDL-TG, and VLDL-apoB-100 kinetics with the methods described.

No effect of menstrual cycle phase on basal very-low-density lipoprotein triglyceride and apolipoprotein B-100 kinetics

Dyslipidemia, manifested by increased plasma triglyceride (TG), increased total and LDL-cholesterol concentrations and decreased HDL-cholesterol concentration, is an important risk factor for cardiovascular disease. Premenopausal women have a less atherogenic plasma lipid profile and a lower risk of cardiovascular disease than men, but this female advantage disappears after menopause. This suggests that female sex steroids affect lipoprotein metabolism. The impact of variations in the availability of ovarian hormones during the menstrual cycle on lipoprotein metabolism is not known. We therefore investigated whether very-low-density lipoprotein (VLDL)-TG and VLDL-apolipoprotein B-100 (apoB-100) kinetics are different during the follicular (FP) and luteal phases (LP) of the menstrual cycle. We studied seven healthy, premenopausal women (age 27 +/- 2 yr, BMI 25 +/- 2 kg/m(2)) once during FP and once during LP. We measured VLDL-TG, VLDL-apoB-100, and plasma free fatty acid (FFA) kinetics by using stable isotope-labeled tracers, VLDL subclass profile by nuclear magnetic resonance spectroscopy, whole body fat oxidation by indirect calorimetry, and the plasma concentrations of lipoprotein lipase (LPL) and hepatic lipase (HL) by ELISA. VLDL-TG and VLDL-apoB-100 concentrations in plasma, VLDL-TG and VLDL-apoB-100 secretion rates and mean residence times, VLDL subclass distribution, FFA concentration and rate of appearance in plasma, whole body substrate oxidation, and LPL and HL concentrations in plasma were not different during the FP and the LP. We conclude that VLDL-TG and VLDL-apoB-100 metabolism is not affected by menstrual cycle phase.

Endurance training has little effect on active muscle free fatty acid, lipoprotein cholesterol, or triglyceride net balances

We evaluated the hypothesis that net leg total FFA, LDL-C, and TG uptake and HDL-C release during moderate-intensity cycling exercise would be increased following endurance training. Eight sedentary men (26 +/- 1 yr, 77.4 +/- 3.7 kg) were studied in the postprandial state during 90 min of rest and 60 min of exercise twice before (45% and 65% V(O2 peak)) and twice after 9 wk of endurance training (55% and 65% posttraining V(O2 peak)). Measurements across an exercising leg were taken to be a surrogate for active skeletal muscle. To determine limb lipid exchange, femoral arterial and venous blood samples drawn simultaneously at rest and during exercise were analyzed for total and individual FFA (e.g., palmitate, oleate), LDL-C, HDL-C, and TG concentrations, and limb blood flow was determined by thermodilution. The transition from rest to exercise resulted in a shift from net leg total FFA release (-44 +/- 16 micromol/min) to uptake (193 +/- 49 micromol/min) that was unaffected by either exercise intensity or endurance training. The relative net leg release and uptake of individual FFA closely resembled their relative abundances in the plasma with approximately 21 and 41% of net leg total FFA uptake during exercise accounted for by palmitate and oleate, respectively. Endurance training resulted in significant changes in arterial concentrations of HDL-C (49 +/- 5 vs. 52 +/- 5 mg/dl, pre vs. post) and LDL-C (82 +/- 9 vs. 76 +/- 9 mg/dl, pre vs. post), but there was no net TG or LDL-C uptake or HDL-C release across the resting or active leg before or after endurance training. In conclusion, endurance training favorably affects blood lipoprotein profiles, even in young, healthy normolipidemic men, but muscle contractions per se have little effect on net leg LDL-C, or TG uptake or HDL-C release during moderate-intensity cycling exercise. Therefore, the favorable effects of physical activity on the lipid profiles of young, healthy normolipidemic men in the postprandial state are not attributable to changes in HDL-C or LDL-C exchange across active skeletal muscle.

Gastric bypass surgery improves metabolic and hepatic abnormalities associated with nonalcoholic fatty liver disease

BACKGROUND & AIMS

Most patients with extreme obesity have nonalcoholic fatty liver disease (NAFLD). Although gastric bypass (GBP) surgery is the most common bariatric operation performed in obese patients in the United States, the effect of GBP surgery-induced weight loss on the metabolic and hepatic abnormalities associated with NAFLD are not clear.

METHODS

Whole-body glucose, fatty acid and lipoprotein kinetics, liver histology, and hepatic cellular factors involved in inflammation and fibrogenesis were evaluated in 7 extremely obese subjects (body mass index, 58 +/- 4 kg/m(2)) before and 1 year after GBP surgery.

RESULTS

At 1 year after surgery, subjects lost 29% +/- 5% of initial body weight (P < .01); palmitate rate of appearance in plasma, an index of adipose tissue lipolysis, decreased by 47% +/- 4% (P < .01); endogenous glucose production rate decreased by 27% +/- 7% (P < .01); and very-low-density lipoprotein-triglyceride secretion rate decreased by 44% +/- 9% (P < .05). In addition, GBP surgery-induced weight loss decreased hepatic steatosis but did not change standard histologic assessments of inflammation and fibrosis. However, there was a marked decrease in hepatic factors involved in regulating fibrogenesis (collagen-alpha1(I), transforming growth factor-beta1, alpha-smooth muscle actin, and tissue inhibitor of metalloproteinase 1 expression and alpha-smooth muscle actin content) and inflammation (macrophage chemoattractant protein 1 and interleukin 8 expression) (P < .05, compared with values before weight loss).

CONCLUSIONS

These data demonstrate that weight loss induced by GBP surgery normalizes the metabolic abnormalities involved in the pathogenesis and pathophysiology of NAFLD and decreases the hepatic expression of factors involved in the progression of liver inflammation and fibrosis.

Lipid metabolism response to a single, prolonged bout of endurance exercise in healthy young men

To discover the alterations in lipid metabolism linked to postexercise hypotriglyceridemia, we measured lipid kinetics, lipoprotein subclass distribution and lipid transfer enzymes in seven healthy, lean, young men the day after 2 h of cycling and rest. Compared with rest, exercise increased fatty acid rate of appearance and whole body fatty acid oxidation by approximately 65 and 40%, respectively (P < 0.05); exercise had no effect on VLDL-triglyceride (TG) secretion rate, increased VLDL-TG plasma clearance rate by 40 +/- 8%, and reduced VLDL-TG mean residence time by approximately 40 min and VLDL-apolipoprotein B-100 (apoB-100) secretion rate by 24 +/- 8% (all P < 0.05). Exercise also reduced the number of VLDL but almost doubled the number of IDL particles in plasma (P < 0.05). Muscle lipoprotein lipase content was not different after exercise and rest, but plasma lipoprotein lipase concentration increased by approximately 20% after exercise (P < 0.05). Plasma hepatic lipase and lecithin:cholesterol acyltransferase concentrations were not affected by exercise, whereas cholesterol ester transfer protein concentration was approximately 10% lower after exercise than after rest (P = 0.052). We conclude that 1) greater fatty acid availability after exercise does not stimulate VLDL-TG secretion, probably because of the increase in fatty acid oxidation and possibly also fatty acid use for restoration of tissue TG stores; 2) reduced secretion of VLDL-apoB-100 lowers plasma VLDL particle concentration; and 3) increased VLDL-TG plasma clearance maintains low plasma TG concentration but is not accompanied by similar increases in subsequent steps of the delipidation cascade. Acutely, therefore, the cardioprotective lowering of plasma TG and VLDL concentrations by exercise is counteracted by a proatherogenic increase in IDL concentration.

On the suppression of plasma nonesterified fatty acids by insulin during enhanced intravascular lipolysis in humans

During the fasting state, insulin reduces nonesterified fatty acid (NEFA) appearance in the systemic circulation mostly by suppressing intracellular lipolysis in the adipose tissue. In the postprandial state, insulin may also control NEFA appearance through enhanced trapping into the adipose tissue of NEFA derived from intravascular triglyceride lipolysis. To determine the contribution of suppression of intracellular lipolysis in the modulation of plasma NEFA metabolism by insulin during enhanced intravascular triglyceride lipolysis, 10 healthy nonobese subjects underwent pancreatic clamps at fasting vs. high physiological insulin level with intravenous infusion of heparin plus Intralipid. Nicotinic acid was administered orally during the last 2 h of each 4-h clamp to inhibit intracellular lipolysis and assess insulin's effect on plasma NEFA metabolism independently of its effect on intracellular lipolysis. Stable isotope tracers of palmitate, acetate, and glycerol were used to assess plasma NEFA metabolism and total triglyceride lipolysis in each participant. The glycerol appearance rate was similar during fasting vs. high insulin level, but plasma NEFA levels were significantly lowered by insulin. Nicotinic acid significantly blunted the insulin-mediated suppression of plasma palmitate appearance and oxidation rates by approximately 60 and approximately 70%, respectively. In contrast, nicotinic acid did not affect the marked stimulation of palmitate clearance by insulin. Thus most of the insulin-mediated reduction of plasma NEFA appearance and oxidation can be explained by suppression of intracellular lipolysis during enhanced intravascular triglyceride lipolysis in healthy humans. Our results also suggest that insulin may affect plasma NEFA clearance independently of the suppression of intracellular lipolysis.

Fatty acid reesterification but not oxidation is increased by oral contraceptive use in women

We evaluated the hypothesis that fatty acid reesterification would be increased during rest and exercise in the midluteal menstrual cycle phase and during oral contraceptive use, when ovarian hormone concentrations are high, compared with the early follicular phase. Subjects were eight moderately active, weight-stable, eumenorrheic women (24.8 ± 1.2 yr, peak oxygen consumption = 42.0 ± 2.3 ml·kg−1·min−1) who had not taken oral contraceptives for at least 6 mo. Plasma free fatty acid (FFA) kinetics were assessed in the 3-h postprandial state by continuous infusion of [1-13C]palmitate and [1,1,2,3,3-2H]glycerol during 90 min of rest and 60 min of exercise at 45% and 65% peak oxygen consumption in the early follicular and midluteal menstrual cycle phases and during the inactive- and high-dose phases following 4 mo of oral contraceptive use. Plasma FFA rates of appearance, disappearance, and oxidation increased significantly from rest to exercise with no differences noted between menstrual cycle or oral contraceptive phases or exercise intensities. Compared with either menstrual cycle phase, oral contraceptive use resulted in an increase in plasma-derived fatty acid reesterification and a decrease in the proportion of plasma FFA rate of disappearance that was oxidized at rest and during exercise. Endogenous and exogenous synthetic ovarian hormones do not exert a measurable influence on plasma FFA turnover or oxidation at rest or during moderate-intensity exercise in the 3-h postprandial state when carbohydrate use predominates. The increase in whole body lipolytic rate during exercise noted previously with oral contraceptive use is not matched by an increase in fatty acid oxidation and results in an increase in reesterification. Synthetic ovarian hormones contained in oral contraceptives increase lipolytic rate, but fatty acid oxidation during exercise is determined by exercise intensity and its metabolic and endocrine consequences.

Lipoprotein transport in the metabolic syndrome: methodological aspects of stable isotope kinetic studies

The metabolic syndrome encapsulates visceral obesity, insulin resistance, diabetes, hypertension and dyslipidaemia. Dyslipidaemia is a cardinal feature of the metabolic syndrome that accelerates the risk of cardiovascular disease. It is usually characterized by high plasma concentrations of triacylglycerol (triglyceride)-rich and apoB (apolipoprotein B)-containing lipoproteins, with depressed concentrations of HDL (high-density lipoprotein). However, lipoprotein metabolism is complex and abnormal plasma concentrations can result from alterations in the rates of production and/or catabolism of these lipoprotein particles. Our in vivo understanding of kinetic defects in lipoprotein metabolism in the metabolic syndrome has been achieved chiefly by ongoing developments in the use of stable isotope tracers and mathematical modelling. This review deals with the methodological aspects of stable isotope kinetic studies. The design of in vivo turnover studies requires considerations related to stable isotope tracer administration, duration of sampling protocol and interpretation of tracer data, all of which are critically dependent on the kinetic properties of the lipoproteins under investigation. Such models provide novel insight that further understanding of metabolic disorders and effects of treatments. Future investigations of the pathophysiology and therapy of the dyslipoproteinaemia of the metabolic syndrome will require the development of novel kinetic methodologies. Specifically, new stable isotope techniques are required for investigating in vivo the turnover of the HDL subpopulation of particles, as well as the cellular efflux of cholesterol into the extracellular space and its subsequent transport in plasma and metabolic fate in the liver.

Excess body fat in men decreases plasma fatty acid availability and oxidation during endurance exercise

The effect of relative body fat mass on exercise-induced stimulation of lipolysis and fatty acid oxidation was evaluated in 15 untrained men (5 lean, 5 overweight, and 5 obese with body mass indexes of 21 +/- 1, 27 +/- 1, and 34 +/- 1 kg/m2, respectively, and %body fat ranging from 12 to 32%). Palmitate and glycerol kinetics and substrate oxidation were assessed during 90 min of cycling at 50% peak aerobic capacity (VO2 peak) by use of stable isotope-labeled tracer infusion and indirect calorimetry. An inverse relationship was found between %body fat and exercise-induced increase in glycerol appearance rate relative to fat mass (r2 = 0.74; P < 0.01). The increase in total fatty acid uptake during exercise [(micromol/kg fat-free mass) x 90 min] was approximately 50% smaller in obese (181 +/- 70; P < 0.05) and approximately 35% smaller in overweight (230 +/- 71; P < 0.05) than in lean (354 +/- 34) men. The percentage of total fatty acid oxidation derived from systemic plasma fatty acids decreased with increasing body fat, from 49 +/- 3% in lean to 39 +/- 4% in obese men (P < 0.05); conversely, the percentage of nonsystemic fatty acids, presumably derived from intramuscular and possibly plasma triglycerides, increased with increasing body fat (P < 0.05). We conclude that the lipolytic response to exercise decreases with increasing adiposity. The blunted increase in lipolytic rate in overweight and obese men compared with lean men limits the availability of plasma fatty acids as a fuel during exercise. However, the rate of total fat oxidation was similar in all groups because of a compensatory increase in the oxidation of nonsystemic fatty acids.