Relationship between skeletal muscle lipoprotein lipase activity and 24-hour macronutrient oxidation

Sedentary time associates detrimentally and physical activity beneficially with metabolic flexibility in adults with metabolic syndrome

Metabolic flexibility (MetFlex) describes the ability to respond and adapt to changes in metabolic demand and substrate availability. The relationship between physical (in)activity and MetFlex is unclear. This study aimed to determine whether sedentary time, physical activity (PA), and cardiorespiratory fitness associate with MetFlex. Sedentary time, standing, and PA were measured with accelerometers for 4 weeks in 64 sedentary adults with metabolic syndrome [37 women, 27 men; 58.3 (SD 6.8) years]. Fitness (V̇o2max; mL·kg-1·min-1) was measured with graded maximal cycle ergometry. MetFlex was assessed with indirect calorimetry as the change in respiratory exchange ratio (ΔRER) from fasting to insulin stimulation with hyperinsulinemic-euglycemic clamp and from low-intensity to maximal exercise. Carbohydrate (CHOox) and fat oxidation (FATox) were calculated from respiratory gases. High sedentary time associated with higher fasting RER [β = 0.35 (95% confidence interval: 0.04, 0.67)], impaired insulin-stimulated MetFlex (ΔRER) [β=-0.41 (-0.72, -0.09)], and lower fasting FATox [β=-0.36 (-0.67, -0.04)]. Standing associated with lower fasting RER [β=-0.32 (-0.62, -0.02)]. Higher standing time and steps/day associated with higher fasting FATox [β = 0.31 (0.01, 0.61), and β = 0.26 (0.00, 0.53)]. Light-intensity and total PA associated with better insulin-stimulated MetFlex [β = 0.33 (0.05, 0.61)], and β = 0.33 (0.05, 0.60)]. Higher V̇o2max associated with higher CHOox during maximal exercise [β = 0.81 (0.62, 1.00)], as well as during insulin stimulation [β = 0.43 (0.13, 0.73)]. P values are less than 0.05 for all associations. Sedentary time and PA associate with MetFlex. Reducing sitting and increasing PA of even light intensity might aid in the prevention of metabolic diseases in risk populations through their potential effects on energy metabolism.NEW & NOTEWORTHY High accelerometer-assessed sedentary time associates with metabolic inflexibility measured during hyperinsulinemic-euglycemic clamp in adults with metabolic syndrome, and more light-intensity and total physical activity associate with more metabolic flexibility. Physical activity behaviors may thus play an important role in the regulation of fuel metabolism. This highlights the potential of reduced sedentary time and increased physical activity of any intensity to induce metabolic health benefits and help in disease prevention in risk populations.

Lipoprotein Lipase Overexpression in Skeletal Muscle Attenuates Weight Regain by Potentiating Energy Expenditure

Moderate weight loss improves numerous risk factors for cardiometabolic disease; however, long-term weight loss maintenance (WLM) is often thwarted by metabolic adaptations that suppress energy expenditure and facilitate weight regain. Skeletal muscle has a prominent role in energy homeostasis; therefore, we investigated the effect of WLM and weight regain on skeletal muscle in rodents. In skeletal muscle of obesity-prone rats, WLM reduced fat oxidative capacity and downregulated genes involved in fat metabolism. Interestingly, even after weight was regained, genes involved in fat metabolism were also reduced. We then subjected mice with skeletal muscle lipoprotein lipase overexpression (mCK-hLPL), which augments fat metabolism, to WLM and weight regain and found that mCK-hLPL attenuates weight regain by potentiating energy expenditure. Irrespective of genotype, weight regain suppressed dietary fat oxidation and downregulated genes involved in fat metabolism in skeletal muscle. However, mCK-hLPL mice oxidized more fat throughout weight regain and had greater expression of genes involved in fat metabolism and lower expression of genes involved in carbohydrate metabolism during WLM and regain. In summary, these results suggest that skeletal muscle fat oxidation is reduced during WLM and regain, and therapies that improve skeletal muscle fat metabolism may attenuate rapid weight regain.

Microbial lipases and their industrial applications: a comprehensive review

Lipases are very versatile enzymes, and produced the attention of the several industrial processes. Lipase can be achieved from several sources, animal, vegetable, and microbiological. The uses of microbial lipase market is estimated to be USD 425.0 Million in 2018 and it is projected to reach USD 590.2 Million by 2023, growing at a CAGR of 6.8% from 2018. Microbial lipases (EC 3.1.1.3) catalyze the hydrolysis of long chain triglycerides. The microbial origins of lipase enzymes are logically dynamic and proficient also have an extensive range of industrial uses with the manufacturing of altered molecules. The unique lipase (triacylglycerol acyl hydrolase) enzymes catalyzed the hydrolysis, esterification and alcoholysis reactions. Immobilization has made the use of microbial lipases accomplish its best performance and hence suitable for several reactions and need to enhance aroma to the immobilization processes. Immobilized enzymes depend on the immobilization technique and the carrier type. The choice of the carrier concerns usually the biocompatibility, chemical and thermal stability, and insolubility under reaction conditions, capability of easy rejuvenation and reusability, as well as cost proficiency. Bacillus spp., Achromobacter spp., Alcaligenes spp., Arthrobacter spp., Pseudomonos spp., of bacteria and Penicillium spp., Fusarium spp., Aspergillus spp., of fungi are screened large scale for lipase production. Lipases as multipurpose biological catalyst has given a favorable vision in meeting the needs for several industries such as biodiesel, foods and drinks, leather, textile, detergents, pharmaceuticals and medicals. This review represents a discussion on microbial sources of lipases, immobilization methods increased productivity at market profitability and reduce logistical liability on the environment and user.

Phenotype of carnitine palmitoyltransferase II (CPT II) deficiency: A questionnaire-based survey

Wide spectrums of symptoms besides muscle weakness, different triggering factors and varied muscles involvement are associated with CPT II deficiency. However, systematic clinical characterization of CPT II deficiency is not known. A Questionnaire-based retrospective study on 13 biochemically and genetically confirmed CPT II deficient patients was performed to analyze these aspects. Attacks of myalgia (13/13 patients), weakness (13/13) and rhabdomyolysis (10/13 patients) were most frequently reported. The number of attacks ranged from 1 to 85/year. Common triggers were exercise (13/13), fasting (13/13), cold (12/13) and infections (12/13). Exercise lasting from 15 to 60 min was sufficient for attacks in 9/13 patients, 1-4 h in 3/13 patients and more than 4 h in 1/13 patient. 2/13 patients required dialysis. Limb muscles were affected slightly more often than other muscles. Mean intensity of pain in visual analogue scale (VAS) during regular attack was 4.77 (±1.36). Frequency and severity of attacks did not increase during the course of disease in 10/13 patients. 7/13 patients quit sports after the symptoms emerged. 3/13 patients changed their profession permanently. Increased number of attacks were positively correlated with higher BMI (P = 0.05). Body rest, carbohydrate-rich nutrients and fluid-supplement mitigated the pain. After the first attack [Mean: 9.7 (±4.46) years], diagnosis took an average of 26.7 (± 13.06) years. In myopathic CPT II deficiency, frequencies of attacks are highly variable. Generally, the myopathic form is a mild form. However, severe patients requiring dialysis due to kidney failure could be present. Individuals with higher BMI are at risk of developing more frequent attacks.

The childhood obesity epidemic as a result of nongenetic evolution: the maternal resources hypothesis

Over the past century, socioenvironmental evolution (eg, reduced pathogenic load, decreased physical activity, and improved nutrition) led to cumulative increments in maternal energy resources (ie, body mass and adiposity) and decrements in energy expenditure and metabolic control. These decrements reduced the competition between maternal and fetal energy demands and increased the availability of energy substrates to the intrauterine milieu. This perturbation of mother-conceptus energy partitioning stimulated fetal pancreatic β-cell and adipocyte hyperplasia, thereby inducing an enduring competitive dominance of adipocytes over other tissues in the acquisition and sequestering of nutrient energy via intensified insulin secretion and hyperplastic adiposity. At menarche, the competitive dominance of adipocytes was further amplified via hormone-induced adipocyte hyperplasia and weight-induced decrements in physical activity. These metabolic and behavioral effects were propagated progressively when obese, inactive, metabolically compromised women produced progressively larger, more inactive, metabolically compromised children. Consequently, the evolution of human energy metabolism was markedly altered. This phenotypic evolution was exacerbated by increments in the use of cesarean sections, which allowed both the larger fetuses and the metabolically compromised mothers who produced them to survive and reproduce. Thus, natural selection was iatrogenically rendered artificial selection, and the frequency of obese, inactive, metabolically compromised phenotypes increased in the global population. By the late 20th century, a metabolic tipping point was reached at which the postprandial insulin response was so intense, the relative number of adipocytes so large, and inactivity so pervasive that the competitive dominance of adipocytes in the sequestering of nutrient energy was inevitable and obesity was unavoidable.

Effects of carbohydrate ingestion during exercise on substrate oxidation in physically active women with different body compositions

We examined the effects of consuming a 6% carbohydrate (CHO) beverage during exercise on substrate oxidation in active, young Caucasian women with higher body fat (HF) (>25%) and lower body fat (LF) (<25%). The HF group (n = 9) had 32.4% ± 5.6% fat and the LF group (n = 8) had 20.0% ± 3.0% fat. Subjects completed 2 sessions of 45 min of treadmill exercise at 55% of maximal oxygen consumption. Immediately prior to and at 15-min intervals during exercise, subjects consumed 25% of a total volume of a CHO beverage (1 g CHO·kg(-1)) or a placebo (PLC). During exercise and for 2 h after exercise, expired gases were analyzed to determine oxidation rates for CHO (CHO-OX) and fat (FAT-OX). During exercise, significant differences (p < 0.05) in CHO-OX (mg·kg fat-free mass(-1)·min(-1)) were found between groups for the CHO trial (LF, 35.4 ± 4.7 vs. HF, 29.8 ± 3.6) and the PLC trial (LF, 33.7 ± 6.4 vs. HF, 26.3 ± 4.3). CHO-OX was significantly higher during the first hour of recovery in both the LF group (CHO, 9.3 ± 2.1 vs. PLC, 5.3 ± 2.4) and the HF group (CHO, 8.7 ± 2.0 vs. PLC, 4.2 ± 1.0), but during the second hour of recovery, only the HF group had a higher CHO-OX (CHO, 5.3 ± 1.8 vs. PLC, 3.9 ± 1.1). FAT-OX was significantly lower during the first hour of recovery in both the LF group (CHO, 0.6 ± 0.4 vs. PLC, 1.0 ± 0.4) and the HF group (CHO, 0.4 ± 0.4 vs. PLC, 1.4 ± 0.4), as well as during the second hour of recovery in both the LF group (CHO, 0.8 ± 0.4 vs. PLC, 1.3 ± 0.5) and the HF group (CHO, 0.9 ± 0.6 vs. PLC, 1.6 ± 0). CHO ingestion promotes CHO-OX and suppresses FAT-OX during and following exercise in physically active women with low and high levels of body fat.

The nuclear receptor REV-ERBα is required for the daily balance of carbohydrate and lipid metabolism

Mutations of clock genes can lead to diabetes and obesity. REV-ERBα, a nuclear receptor involved in the circadian clockwork, has been shown to control lipid metabolism. To gain insight into the role of REV-ERBα in energy homeostasis in vivo, we explored daily metabolism of carbohydrates and lipids in chow-fed, unfed, or high-fat-fed Rev-erbα(-/-) mice and their wild-type littermates. Chow-fed Rev-erbα(-/-) mice displayed increased adiposity (2.5-fold) and mild hyperglycemia (∼10%) without insulin resistance. Indirect calorimetry indicates that chow-fed Rev-erbα(-/-) mice utilize more fatty acids during daytime. A 24-h nonfeeding period in Rev-erbα(-/-) animals favors further fatty acid mobilization at the expense of glycogen utilization and gluconeogenesis, without triggering hypoglycemia and hypothermia. High-fat feeding in Rev-erbα(-/-) mice amplified metabolic disturbances, including expression of lipogenic factors. Lipoprotein lipase (Lpl) gene, critical in lipid utilization/storage, is triggered in liver at night and constitutively up-regulated (∼2-fold) in muscle and adipose tissue of Rev-erbα(-/-) mice. We show that CLOCK, up-regulated (2-fold) at night in Rev-erbα(-/-) mice, can transactivate Lpl. Thus, overexpression of Lpl facilitates muscle fatty acid utilization and contributes to fat overload. This study demonstrates the importance of clock-driven Lpl expression in energy balance and highlights circadian disruption as a potential cause for the metabolic syndrome.

Postprandial metabolism of meal triglyceride in humans

The intake of dietary fat above energy needs has contributed to the growing rates of obesity worldwide. The concept of disease development occurring in the fed state now has much support and dysregulation of substrate flux may occur due to poor handling of dietary fat in the immediate postprandial period. The present paper will review recent observations implicating cephalic phase events in the control of enterocyte lipid transport, the impact of varying the composition of meals on subsequent fat metabolism, and the means by which dietary lipid carried in chylomicrons can lead to elevated postprandial non-esterified fatty acid concentrations. This discussion is followed by an evaluation of the data on quantitative meal fat oxidation at the whole body level and an examination of dietary fat clearance to peripheral tissues - with particular attention paid to skeletal muscle and liver given the role of ectopic lipid deposition in insulin resistance. Estimates derived from data of dietary-TG clearance show good agreement with clearance to the liver equaling 8-12% of meal fat in lean subjects and this number appears higher (10-16%) in subjects with diabetes and fatty liver disease. Finally, we discuss new methods with which to study dietary fatty acid partitioning in vivo. Future research is needed to include a more comprehensive understanding of 1) the potential for differential oxidation of saturated versus unsaturated fatty acids which might lead to meaningful energy deficit and whether this parameter varies based on insulin sensitivity, 2) whether compartmentalization exists for diet-derived fatty acids within tissues vs. intracellular pools, and 3) the role of reduced peripheral fatty acid clearance in the development of fatty liver disease. Further advancements in the quantitation of dietary fat absorption and disposal will be central to the development of therapies designed to treat diet-induced obesity. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Exercise attenuates the association of body weight with diet in 106,737 runners

PURPOSE

The high prevalence of obesity in Western societies has been attributed in part to high-fat low-CHO food consumption. However, people have also become less active, and inactivity may have increased the risk for weight gain from poor dietary choices. Analyses were performed to test whether diet-weight relationships were attenuated by vigorous exercise.

METHODS

Age- and education-adjusted cross-sectional regression analyses of 62,042 men and 44,695 women recruited for the National Runners' Health Study were conducted. Reported meat and fruit intakes were analyzed separately and as indicators of high-risk diets.

RESULTS

The runners were generally lean (mean ± SD: males = 24.15 ± 2.81 kg·m(-2), females = 21.63 ± 2.70 kg·m(-2)) as measured by body mass index (BMI), educated (males = 16.42 ± 2.47 yr, females = 16.04 ± 2.32 yr), and middle-aged (males = 44.40 ± 10.83 yr, females = 38.21 ± 10.08 yr), who ran 5.30 ± 3.23 km·d(-1) if male and 4.79 ± 3.00 km·d(-1) if female. Running significantly attenuated BMI's relationship to reported meat and fruit intakes in men (P < 10(-8) and P < 10(-12), respectively) and women (P < 10(-15) and P < 10(-6), respectively). Specifically, compared with running <2 km·d(-1), running >8 km·d(-1) reduced the apparent BMI increase per serving of meat by 43% in men (slope ± SE = from 0.74 ± 0.10 to 0.42 ± 0.06) and 55% in women (from 1.26 ± 0.13 to 0.57 ± 0.09) and reduced the apparent BMI reduction per serving of fruit by 75% in men (from -0.28 ± 0.04 to -0.07 ± 0.02) and 94% in women (from -0.16 ± 0.05 to -0.01 ± 0.02). Running also significantly attenuated the concordant relationship between reported meat intake and waist and chest circumferences in men (P < 10(-9) and P = 0.0002, respectively) and women (P = 0.0004 and P < 10(-5), respectively) and the concordant relationship between meat intake and hip circumference in women (P < 10(-6)).

CONCLUSIONS

Vigorous exercise may mitigate diet-induced weight gain, albeit not guaranteeing protection from poor dietary choices.

Lipoprotein lipase: from gene to obesity

Lipoprotein lipase (LPL) is a multifunctional enzyme produced by many tissues, including adipose tissue, cardiac and skeletal muscle, islets, and macrophages. LPL is the rate-limiting enzyme for the hydrolysis of the triglyceride (TG) core of circulating TG-rich lipoproteins, chylomicrons, and very low-density lipoproteins (VLDL). LPL-catalyzed reaction products, fatty acids, and monoacylglycerol are in part taken up by the tissues locally and processed differentially; e.g., they are stored as neutral lipids in adipose tissue, oxidized, or stored in skeletal and cardiac muscle or as cholesteryl ester and TG in macrophages. LPL is regulated at transcriptional, posttranscriptional, and posttranslational levels in a tissue-specific manner. Nutrient states and hormonal levels all have divergent effects on the regulation of LPL, and a variety of proteins that interact with LPL to regulate its tissue-specific activity have also been identified. To examine this divergent regulation further, transgenic and knockout murine models of tissue-specific LPL expression have been developed. Mice with overexpression of LPL in skeletal muscle accumulate TG in muscle, develop insulin resistance, are protected from excessive weight gain, and increase their metabolic rate in the cold. Mice with LPL deletion in skeletal muscle have reduced TG accumulation and increased insulin action on glucose transport in muscle. Ultimately, this leads to increased lipid partitioning to other tissues, insulin resistance, and obesity. Mice with LPL deletion in the heart develop hypertriglyceridemia and cardiac dysfunction. The fact that the heart depends increasingly on glucose implies that free fatty acids are not a sufficient fuel for optimal cardiac function. Overall, LPL is a fascinating enzyme that contributes in a pronounced way to normal lipoprotein metabolism, tissue-specific substrate delivery and utilization, and the many aspects of obesity and other metabolic disorders that relate to energy balance, insulin action, and body weight regulation.

Effect of high-intensity interval exercise on lipid oxidation during postexercise recovery

PURPOSE

The aim of this study was to examine whether lipid oxidation predominates during 3 h of postexercise recovery in high-intensity interval exercise as compared with moderate-intensity continuous exercise on a cycle ergometer in fit young men (n = 12; 24.6 +/- 0.6 yr).

METHODS

The energy substrate partitioning was evaluated during and after high-intensity submaximal interval exercise (INT, 1-min intervals at 80% of maximal aerobic power output [Wmax] with an intervening 1 min of active recovery at 40% Wmax) and 60-min moderate-intensity continuous exercise at 45% of maximal oxygen uptake (C45%) as well as a time-matched resting control trial (CON). Exercise bouts were matched for mechanical work output.

RESULTS

During exercise, a significantly greater contribution of CHO and a lower contribution of lipid to energy expenditure were found in INT (512.7 +/- 26.6 and 41.0 +/- 14.0 kcal, respectively) than in C45% (406.3 +/- 21.2 and 170.3 +/- 24.0 kcal, respectively; P < 0.001) despite similar overall energy expenditure in both exercise trials (P = 0.13). During recovery, there were no significant differences between INT and C45% in substrate turnover and oxidation (P > 0.05). On the other hand, the mean contribution of lipids to energy yield was significantly higher after exercise trials (C45% = 61.3 +/- 4.2 kcal; INT = 66.7 +/- 4.7 kcal) than after CON (51.5 +/- 3.4 kcal; P < 0.05).

CONCLUSIONS

These findings show that lipid oxidation during postexercise recovery was increased by a similar amount on two isoenergetic exercise bouts of different forms and intensities compared with the time-matched no-exercise control trial.

Skeletal muscle-specific deletion of lipoprotein lipase enhances insulin signaling in skeletal muscle but causes insulin resistance in liver and other tissues

OBJECTIVE

Skeletal muscle-specific LPL knockout mouse (SMLPL(-/-)) were created to study the systemic impact of reduced lipoprotein lipid delivery in skeletal muscle on insulin sensitivity, body weight, and composition.

RESEARCH DESIGN AND METHODS

Tissue-specific insulin sensitivity was assessed using a hyperinsulinemic-euglycemic clamp and 2-deoxyglucose uptake. Gene expression and insulin-signaling molecules were compared in skeletal muscle and liver of SMLPL(-/-) and control mice.

RESULTS

Nine-week-old SMLPL(-/-) mice showed no differences in body weight, fat mass, or whole-body insulin sensitivity, but older SMLPL(-/-) mice had greater weight gain and whole-body insulin resistance. High-fat diet feeding accelerated the development of obesity. In young SMLPL(-/-) mice, insulin-stimulated glucose uptake was increased 58% in the skeletal muscle, but was reduced in white adipose tissue (WAT) and heart. Insulin action was also diminished in liver: 40% suppression of hepatic glucose production in SMLPL(-/-) vs. 90% in control mice. Skeletal muscle triglyceride was 38% lower, and insulin-stimulated phosphorylated Akt (Ser473) was twofold greater in SMLPL(-/-) mice without changes in IRS-1 tyrosine phosphorylation and phosphatidylinositol 3-kinase activity. Hepatic triglyceride and liver X receptor, carbohydrate response element-binding protein, and PEPCK mRNAs were unaffected in SMLPL(-/-) mice, but peroxisome proliferator-activated receptor (PPAR)-gamma coactivator-1alpha and interleukin-1beta mRNAs were higher, and stearoyl-coenzyme A desaturase-1 and PPARgamma mRNAs were reduced.

CONCLUSIONS

LPL deletion in skeletal muscle reduces lipid storage and increases insulin signaling in skeletal muscle without changes in body composition. Moreover, lack of LPL in skeletal muscle results in insulin resistance in other key metabolic tissues and ultimately leads to obesity and systemic insulin resistance.

Dietary fat oxidation as a function of body fat

BACKGROUND

It is hypothesized that low dietary fat oxidation makes subjects prone to weight gain.

OBJECTIVE

The aim of the study was to determine dietary fat oxidation in normal, overweight, and obese subjects.

DESIGN

The subjects were 38 women and 18 men with a mean (+/-SD) age of 30+/-12 y and a body mass index (in kg/m2) of 25+/-4 (range: 18-39). Dietary fat oxidation was measured with deuterated palmitic acid, given simultaneously with breakfast, while the subjects were fed under controlled conditions in a respiration chamber. Body composition was measured by hydrodensitometry and deuterium dilution.

RESULTS

Dietary fat oxidation, measured over 12 h after breakfast, ranged from 4% to 28% with a mean (+/-SD) of 16+/-6%. Dietary fat oxidation was negatively related to percentage body fat, and lean subjects had the highest and obese subjects the lowest values (r=-0.65, P<0.001).

CONCLUSION

The observed reduction in dietary fat oxidation in subjects with a higher percentage body fat may play a role in human obesity.

Higher post-absorptive skeletal muscle LPL activity in African American vs. non-Hispanic White pre-menopausal women

OBJECTIVE

Higher post-absorptive post-heparin plasma lipoprotein lipase (LPL) activity has been reported in African Americans as compared to non-Hispanic whites but differences in tissue-specific LPL activity are unclear.

METHODS AND PROCEDURES

Post-absorptive skeletal muscle (SM)-LPL (vastus lateralis ) and subcutaneous abdominal adipose tissue (AT)-LPL activity was measured in overweight, sedentary African American females (n = 11) as well as in their non-Hispanic white counterparts (n = 6) during a period of controlled low fat (30%) diet (for 10 days) combined with physical activity (for days 8-10). Post-absorptive substrate utilization was measured on day 10; fasting blood levels and SM and AT biopsies were obtained on day 11.

RESULTS

African Americans had significantly greater post-absorptive SM-LPL activity (P = 0.04) when compared to non-Hispanic whites. There were no significant differences in post-absorptive AT-LPL activity, free fatty acids, and systemic fat oxidation or respiratory quotient between African American and white non-Hispanic women in this study (P > 0.2 for all).

DISCUSSION

During a controlled low fat (30%) diet post-absorptive vastus lateralis SM-LPL activity is higher in sedentary pre-menopausal African American as compared to non-Hispanic white women.

Trafficking of dietary fat in obesity-prone and obesity-resistant rats

The trafficking of dietary fat was assessed in obesity-prone (OP) and obesity-resistant (OR) male and female rats. Test meals containing [1-(14)C]palmitate were delivered through gastric feeding tubes while rats consumed a high-carbohydrate diet (HCD) or after 5 days of a high-fat diet (HFD). Over the subsequent 24 h, the appearance of (14)C was followed in the GI tract, skeletal muscles (SM), liver, adipose tissues (AT), and expired CO(2). There was no difference in the production of (14)CO(2) between OP and OR rats consuming a HCD. However, after 5 days on HFD, OR rats produced significantly more (14)CO(2) after the test meal than OP rats (P < 0.001 females, P = 0.03 males). The differential oxidation of dietary fat between OP and OR rats on HFD was not due to differences in absorption but rather was associated with preferential disposition of tracer to AT in OP rats. Measurements of lipoprotein lipase in part explained increased tracer uptake by AT in OP rats but were not consistent with increased SM tracer uptake in OR rats. Surprisingly, female rats oxidized more tracer than male rats irrespective of phenotype or diet. These results are consistent with the notion that differences in the partitioning of dietary fat between storage in AT and oxidation in SM and liver that develop shortly after the introduction of a HFD may in part underlie the differential tendency for OR and OP rats to gain weight on this diet.

Regulation of lipoprotein lipase activity in rainbow trout (Oncorhynchus mykiss) tissues

Lipoprotein lipase (LPL) is considered as a key enzyme in the lipid deposition and metabolism of many tissues. Information on LPL activity and its regulation in fish remains very scarce. In the present study, we have examined the nutritional regulation of LPL activity by conducting post-feeding and fasting experiments in rainbow trout (Oncorhynchus mykiss). As insulin plays an important role in the nutritional regulation of LPL activity in mammals, the effects of this hormone were tested in vivo by intraperitoneal administration. Moreover, we conducted in vitro studies using fat pads of rainbow trout to better clarify the direct role of insulin and tumor necrosis factor-alpha (TNFalpha) as possible regulators of LPL activity in rainbow trout. LPL activity in adipose tissue increased in response to feeding, 4h after ingestion of food, then decreasing to basal levels at 6h. No clear response was found in either red or white muscles, where LPL values were lower. Moreover, fasting produced a down-regulation of LPL activity in adipose tissue, concomitant with low levels of plasma insulin. While insulin administration stimulated LPL activity of adipose tissue 3h after injection, no response was observed in red or white muscles. Finally, in vitro studies using fat pads revealed that insulin significantly stimulated the proportion of LPL in active conformation at the extracellular level. On the other hand, TNFalpha did not greatly affect LPL activity using this in vitro model. These data indicate that LPL activity is regulated in a tissue-specific manner following food intake, and suggest that insulin is an important regulator of LPL activity in the adipose tissue of rainbow trout.

Activity and tissue-specific expression of lipases and tumor-necrosis factor alpha in lean and obese cats

Post-heparin plasma activity of lipoprotein lipase (LPL) and hepatic lipase (HL), and fat and muscle activity of LPL were measured in neutered lean and obese cats. Lipoprotein lipase, hormone-sensitive lipase (HSL), and tumor necrosis factor a (TNF) mRNA were measured in muscle and fat tissue with real-time PCR using primers for feline LPL, HSL, and TNF. Lipoprotein lipase plasma and fat activity and fat mRNA levels were significantly lower (50, 80, and 50%, respectively) in obese cats than lean cats, whereas the muscle/fat ratio of LPL was significantly higher in obese compared to lean cats. The activity of HL was not different between the groups. Hormone-sensitive lipase mRNA levels were significantly higher in obese than lean cats. The level of fat TNF also was significantly higher in obese cats than in lean cats, whereas the level in muscle was not different. The lower LPL activity and mRNA expression in fat and the higher LPL and HSL mRNA expression in muscle in obese cats compared to lean cats expectedly favor a redistribution of fatty acids from fat to muscle tissue where they can be deposited or used for energy in times of need. Tumor necrosis factor alpha may regulate this repartitioning process through suppression of adipocyte LPL.

Adipocyte differentiation-related protein in human skeletal muscle: relationship to insulin sensitivity

OBJECTIVE

To determine whether adipocyte differentiation-related protein (ADRP), a lipid droplet-associated protein that binds to and sequesters intracellular fatty acids, is 1) expressed in human skeletal muscle and 2) differentially regulated in human skeletal muscle obtained from obese non-diabetic (OND) and obese diabetic (OD) subjects.

RESEARCH METHODS AND PROCEDURES

Ten OND subjects and 15 OD subjects underwent a weight loss or pharmacological intervention program to improve insulin sensitivity. Anthropometric data, hemoglobin A(1C), fasting glucose, lipids, and glucose disposal rate were determined at baseline and at completion of studies. Biopsies of the vastus lateralis muscle (SkM) were obtained in the fasting state from OND and OD subjects. Protein expression was determined by Western blotting.

RESULTS

ADRP was highly expressed in SkM from OND (4.4 +/- 1.54 AU/10 microg, protein, n = 10) and OD (5.02 +/- 1.33 AU/10 microg, n = 12) subjects. OND subjects undergoing weight loss had decreased triglyceride levels and improved insulin action. SkM ADRP content increased with weight loss from 5.14 +/- 2.15 AU/10 microg to 9.92 +/- 1.57 AU/10 microg (p < 0.025). OD subjects were treated with either troglitazone or metformin, together with glyburide, for 3 to 4 months. Both treatments attained similar levels of glycemic control. OD subjects with lower baseline ADRP content (2.85 +/- 1.07 AU/10 microg, n = 6) displayed up-regulation of ADRP expression (to 9.27 +/- 2.76 AU/10 microg, p < 0.025).

DISCUSSION

ADRP is the predominant lipid droplet-associated protein in SkM, and low ADRP expression is up-regulated in circumstances of improved glucose tolerance. Up-regulation of ADRP may act to sequester fatty acids as triglycerides in discrete lipid droplets that could protect muscle from the detrimental effects of fatty acids on insulin action and glucose tolerance.

Long-chain fatty acid uptake and FAT/CD36 translocation in heart and skeletal muscle

Cellular long-chain fatty acid (LCFA) uptake constitutes a process that is not yet fully understood. LCFA uptake likely involves both passive diffusion and protein-mediated transport. Several lines of evidence support the involvement of a number of plasma membrane-associated proteins, including fatty acid translocase (FAT)/CD36, plasma membrane-bound fatty acid binding protein (FABPpm), and fatty acid transport protein (FATP). In heart and skeletal muscle primary attention has been given to unravel the mechanisms by which FAT/CD36 expression and function are regulated. It appears that both insulin and contractions induce the translocation of intracellular stored FAT/CD36 to the plasma membrane to increase cellular LCFA uptake. This review focuses on this novel mechanism of regulation of LCFA uptake in heart and skeletal muscle in health and disease. The distinct signaling pathways underlying insulin-induced and contraction-induced FAT/CD36 translocation will be discussed and a comparison will be made with the well-defined glucose transport system involving the glucose transporter GLUT4. Finally, it is hypothesized that malfunctioning of recycling of these transporters may lead to intracellular triacylglycerol (TAG) accumulation and cellular insulin resistance. Current data indicate a pivotal role for FAT/CD36 in the regulation of LCFA utilization in heart and skeletal muscle under normal conditions as well as during the altered LCFA utilization observed in obesity and insulin resistance. Hence, FAT/CD36 might provide a useful therapeutic target for the prevention or treatment of insulin resistance.

Genetic determinants of energy expenditure and insulin resistance in diet-induced obesity in mice

Diet-induced obesity is the primary determinant of the current epidemic of diabetes. We have explored the role of genetics in this phenomenon, using C57Bl/6 (B6), 129S6/SvEvTac (129), and intercross (B6 x 129)F2 mice on a low- or high-fat diet. Over an 18-week period, B6 and F2 mice gained more weight, had higher levels of insulin and leptin, and showed greater glucose intolerance than 129 mice, despite lower food intake. By contrast, metabolic rate and diet-induced thermogenesis were significantly higher in the 129 mice. Genome-wide scans identified several quantitative trait loci, including a quantitative trait locus that was linked with hyperinsulinemia/insulin resistance on chromosome 14 in a region similar to that seen in mice with genetically induced insulin resistance. Microarray analysis indicated significant changes in expression levels between B6 and 129 mice in the identified chromosomal area of Wnt5a and protein kinase Cdelta (PKCdelta). Thus, caloric efficiency, i.e., the "thrifty gene," is a dominant-acting genetic determinant of diet-induced obesity in mice and can be linked to a locus on chromosome 14, including genes linked to adipose development and insulin sensitivity.

Mechanisms mediating insulin resistance in transgenic mice overexpressing mouse apolipoprotein A-II

We previously demonstrated that transgenic mice overexpressing mouse apolipoprotein A-II (apoA-II) exhibit several traits associated with the insulin resistance (IR) syndrome, including increased atherosclerosis, hypertriglyceridemia, obesity, and IR. The skeletal muscle appeared to be the insulin-resistant tissue in the apoA-II transgenic mice. We now demonstrate a decrease in FA oxidation in skeletal muscle of apoA-II transgenic mice, consistent with reports that decreased skeletal muscle FA oxidation is associated with increased skeletal muscle triglyceride accumulation, skeletal muscle IR, and obesity. The decrease in FA oxidation is not due to decreased carnitine palmitoyltransferase 1 activity, because oxidation of palmitate and octanoate were similarly decreased. Quantitative RT-PCR analysis of gene expression demonstrated that the decrease in FA oxidation may be explained by a decrease in medium chain acyl-CoA dehydrogenase. We previously demonstrated that HDLs from apoA-II transgenic mice exhibit reduced binding to CD36, a scavenger receptor involved in FA metabolism. However, studies of combined apoA-II transgenic and CD36 knockout mice suggest that the major effects of apoA-II are independent of CD36. Rosiglitazone treatment significantly ameliorated IR in the apoA-II transgenic mice, suggesting that the underlying mechanisms of IR in this animal model may share common features with certain types of human IR.

Greater rise in fat oxidation with medium-chain triglyceride consumption relative to long-chain triglyceride is associated with lower initial body weight and greater loss of subcutaneous adipose tissue

OBJECTIVE

Medium-chain triglyceride (MCT) consumption has been shown to increase energy expenditure (EE) and lead to greater losses of the adipose tissue in animals and humans. The objective of this research was to examine the relationship between body composition and thermogenic responsiveness to MCT treatment.

DESIGN

Randomized, crossover, controlled feeding trial, with diets rich in either MCT or long-chain triglyceride (LCT) (as olive oil) for periods of 4 weeks each.

SUBJECTS

A total of 19 healthy overweight men aged (x+/-s.e.m.) 44.5+/-2.5 y with a body mass index of 27.8+/-0.5 kg/m(2).

MEASUREMENTS

EE and body composition were measured using indirect calorimetry and magnetic resonance imaging, respectively, at the baseline and end point of each feeding period. EE was measured for 30 min before consumption of a standard meal and for 5.5 h following the meal.

RESULTS

Body weight (BW) decreased (P<0.05) by 1.03+/-0.25 kg with MCT consumption compared to 0.62+/-0.29 kg with LCT consumption. The difference in average EE between MCT and LCT consumptions was related to initial BW, such that men with lower initial BW had a greater rise in EE with MCT consumption relative to LCT on day 28 (r=-0.472, P=0.04) but not day 2 (r=-0.368, P=0.12). Similar results were obtained with fat oxidation on day 28 (r=-0.553, P=0.01). The greater rise in fat oxidation with MCT compared to LCT consumption on day 2 tended to be related to greater loss of BW after MCT vs LCT consumption (r=-0.4075, P=0.08).

CONCLUSION

These data suggest that shunting of dietary fat towards oxidation results in diminished fat storage, as reflected by the loss of BW and subcutaneous adipose tissue. Furthermore, MCT consumption may stimulate EE and fat oxidation to a lower extent in men of greater BW compared to men of lower BW, indicative of the lower responsiveness to a rapidly oxidized fat by overweight men.International

Lipoprotein lipase activator NO-1886 (ibrolipim) accelerates the mRNA expression of fatty acid oxidation-related enzymes in rat liver

The lipoprotein lipase (LPL) activator NO-1886 (ibrolipim) has been shown to have potential benefits for the treatment of obesity in rats. However, the anti-obesity mechanism of NO-1886 has not been clearly understood. To address this, we studied the effects of NO-1886 on the mRNA expression of fatty acid oxidation-related enzymes in rats. The respiratory quotient (RQ) in rats administered a single oral dose of NO-1886 was significantly lower than control rats under both fed and fasted conditions. NO-1886 orally administered to rats for 7 days caused 1.54-fold increase in carnitine palmitoyl transferase II (CPTII) mRNA in the carnitine palmitoyl transferase system. Furthermore, NO-1886 caused a 1.47-fold increase in long-chain acyl-CoA dehydrogenase (LCAD) mRNA, a 1.49-fold increase in acetyl-CoA acyltransferase 2 (ACAA2) mRNA, and a 1.24-fold increase in enoyl-CoA hydratase (ECH) mRNA in rats, all which are liver beta-oxidation enzymes. NO-1886 also increased uncoupling protein-2 (UCP2) mRNA levels in liver by 1.42-fold when compared to the control group. These results suggest that the LPL activator NO-1886 may accelerate the expression of fatty acid oxidation-related enzymes, resulting in a reduction of RQ.

Contribution of fatty acids released from lipolysis of plasma triglycerides to total plasma fatty acid flux and tissue-specific fatty acid uptake

There is controversy over the extent to which fatty acids (FAs) derived from plasma free FAs (FFAs) or from hydrolysis of plasma triglycerides (TGFAs) form communal or separate pools and what the contribution of each FA source is to cellular FA metabolism. Chylomicrons and lipid emulsions were labeled with [(3)H]triolein, injected into mice, and appearance in plasma of [(3)H]oleic acid was estimated, either through a steady-state approach or by compartmental modeling. [(14)C]oleic acid was included to trace plasma FFA. Eighty to 90% of triglyceride (TG) label was recovered in plasma, irrespective of tracer method or TG source. The contribution of TG lipolysis to total plasma FA turnover was 10-20%. After infusion of [(3)H]TG and [(14)C]FA, the retention of these labels varied substantially among liver, adipose tissue, and skeletal and heart muscle. Retention of TG label changed during fasting in the same direction as lipoprotein lipase (LPL) activity is regulated. We propose a model that reconciles the paradoxical 80-90% loss of TG label into plasma with LPL-directed differential uptake of TGFA in tissues. In this model, TGFAs mix locally at the capillaries with plasma FFAs, where they would lead to an increase in the local FA concentration, and hence, FA uptake. Our data indicate that a distinction between TG-derived FA and plasma FFA cannot be made.

Effects of obesity on lipid profiles in neutered male and female cats

OBJECTIVE

To examine whether obese cats, compared with lean cats, have alterations in lipoprotein metabolism that might lead to a decrease in glucose metabolism and insulin secretion.

ANIMALS

10 lean and 10 obese adults cats (5 neutered males and 5 neutered females each).

PROCEDURE

Intravenous glucose tolerance tests with measurements of serum glucose, insulin, and nonesterified fatty acid (NEFA) concentrations were performed. Lipoprotein fractions were examined in serum by isopycnic density gradient ultracentrifugation.

RESULTS

Obese cats had insulin resistance. Plasma triglyceride and cholesterol concentrations were significantly increased in obese cats, compared with lean cats. Very low density lipoprotein (VLDL) concentrations were increased in obese cats, compared with lean cats; however, the composition of various fractions remained unchanged between obese and lean cats, indicating greater synthesis and catabolism of VLDL in obese cats. Serum high density lipoprotein (HDL) cholesterol concentrations were increased in obese cats, compared with lean cats. Serum NEFA concentrations were only significantly different between obese and lean cats when separated by sex; obese male cats had higher baseline serum NEFA concentrations and greater NEFA suppression in response to insulin, compared with lean male cats.

CONCLUSIONS AND CLINICAL RELEVANCE

Lipid metabolism changes in obese cats, compared with lean cats. The increase in VLDL turnover in obese cats might contribute to insulin resistance of glucose metabolism, whereas the increase in serum HDL cholesterol concentration might reflect a protective effect against atherosclerosis in obese cats.

Molecular mechanisms of diabetic cardiovascular disease

Cardiovascular disease is a major cause of morbidity and mortality in persons with diabetes mellitus. This population represents an important target for preventive therapies aimed at reducing atherosclerosis. Recent molecular research has uncovered many of the cellular mechanisms that lead to atherosclerosis in the diabetic patient. This review, part 1 of a 2-part series, is geared toward clinicians and discusses these mechanisms as they pertain to prevention of cardiovascular disease in patients with diabetes.

Muscle triglyceride and insulin resistance

Skeletal muscle contains the majority of the body's glycogen stores and a similar amount of readily accessible energy as intramyocellular triglyceride (imTG). While a number of factors have been considered to contribute to the pathogenesis of insulin resistance (IR) in obesity and type 2 diabetes mellitus (DM), this review will focus on the potential role of skeletal muscle triglyceride content. In obesity and type 2 DM, there is an increased content of lipid within and around muscle fibers. Changes in muscle in fuel partitioning of lipid, between oxidation and storage of fat calories, almost certainly contribute to accumulation of imTG and to the pathogenesis of both obesity and type 2 DM. In metabolic health, skeletal muscle physiology is characterized by the capacity to utilize either lipid or carbohydrate fuels, and to effectively transition between these fuels. We will review recent findings that indicate that in type 2 DM and obesity, skeletal muscle manifests inflexibility in the transition between lipid and carbohydrate fuels. This inflexibility in fuel selection by skeletal muscle appears to be related to the accumulation of imTG and is an important aspect of IR of skeletal muscle in obesity and type 2 DM.

The relationship between lipoprotein lipase activity and respiratory quotient of rats in circadian rhythms

Plasma lipid levels and lipoprotein lipase (LPL) are known to follow circadian rhythms in rats. However, very little information is available on the variations in respiratory quotient (RQ) during the 24-h period in rats. The aims of this study were to provide an overall view of the effects of circadian rhythm on RQ and to determine the relationship of LPL and RQ with metabolic parameters in these animals. Male rats were fed ad libitum and were kept under a 12 :12-h light-dark cycle. Rats were killed every 2 h over a 24-h period for measurement of metabolic parameters and tissue LPL activity. The RQ was measured every 4 h over the same 24-h period. The gastric contents increased during the dark phase and decreased during the light phase. For the metabolic parameters, circadian rhythms were detected for plasma glucose, triglycerides, high-density lipoprotein cholesterol and non esterified free fatty acids, but not for plasma total cholesterol or phospholipids. The RQ and adipose tissue LPL activity increased during the dark phase, while skeletal muscle LPL activity decreased during this phase. The RQ was inversely correlated with skeletal muscle LPL activity (r = -0.880) and positively correlated with adipose tissue LPL activity (r = 0.937). These results appear to show that rats tend toward consumption of fat by accelerating fat oxidation, resulting in suppression of fat accumulation in the light phase, while tending toward fat accumulation by the suppression of fat oxidation in the dark phase.

Developmental changes in adipose and muscle lipoprotein lipase activity in the atherosclerosis-prone JCR:LA-corpulent rat

OBJECTIVE

To characterize the developmental changes in adipose and muscle lipoprotein lipase (LPL) activity in the atherosclerosis-prone JCR:LA-corpulent rat, and to test the hypothesis that tissue-specific abnormalities in LPL activity precede the establishment of obesity.

DESIGN

Lean (+/?) and obese cp/cp male JCR:LA rats were studied at 4, 5 and 8 weeks of age, that is at the onset of obesity, and at a time when obesity is well established. Assessment was made of plasma variables related to glucose and lipid metabolism and of LPL activity in several adipose depots, skeletal muscles and the heart.

RESULTS

At week 4, body weights were identical in both genotypes and began to diverge at week 5. Eight-week-old cp/cp rats weighed 35% more than their lean counterparts. Perirenal and epididymal adipose depot weights were also identical in both genotypes at week 4 and began to increase in cp/cp rats at week 5, whereas the subcutaneous depot of 4-week-old cp/cp rats was slightly enlarged. At week 4, the cp/cp rats were hyperinsulinemic (5-fold), hyperleptinemic (30-fold) and hypertriglyceridemic (3-fold) compared to their lean counterparts, and their liver contained twice as much triglyceride. The 4-week-old cp/cp rats displayed 2-7-fold higher LPL specific activity in the various adipose depots compared to lean rats, and enzyme activity remained higher in obese than in lean rats at all subsequent ages. In contrast, LPL activity in the vastus lateralis, gastrocnemius and heart muscles of 4-week-old obese rats was approximately half that observed in lean animals.

CONCLUSION

Profound, persistent alterations in the tissue-specific modulation of LPL activity are established in the JCR:LA cp/cp rat prior to the development of frank obesity. The increase in adipose tissue LPL activity and its decrease in muscle tissues are likely to be related to the concomitant alterations in insulinemia and triglyceridemia, respectively. The pre-obesity, tissue-specific alterations in LPL activity may be considered as an integrated adaptation to increased lipid flux aimed at driving lipids toward storage sites and limiting their uptake by triglyceride-laden muscles.

Effect of diet on adipose tissue and skeletal muscle VLDL receptor and LPL: implications for obesity and hyperlipidemia

This study was designed to examine the effect of a high-fat (primarily saturated), refined-carbohydrate (sucrose) diet (HFS), which is known to induce obesity and hyperlipidemia, on adipose tissue and skeletal muscle lipoprotein lipase (LPL) and very-low density lipoprotein receptor (VLDL-R) protein expressions. Female Fischer rats were placed on either a HFS or a low-fat, complex-carbohydrate (LFCC) diet for 22 months beginning at 2 months of age. After 20 months, a subgroup of the HFS rats were switched to the LFCC diet for 2 months (HFS/LFCC). Body weight, feed efficiency, plasma total cholesterol, VLDL-C, low-density lipoprotein cholesterol (LDL-C) and triglyceride (TG) concentrations and LDL-C to high-density lipoprotein cholesterol ratio were all significantly raised by the HFS diet and improved by conversion to the LFCC diet. Adipose tissue heparin-releasable, extractable and total LPL activity expressed per cell were significantly increased in the HFS-fed group. However, LPL protein abundance normalized against total cellular protein was unchanged in the HFS group. This observation is consistent with the presence of adipose tissue hypertrophy. Skeletal muscle LPL protein abundance and heparin-releasable activity were reduced by the HFS diet and improved after switching to the LFCC diet. Both adipose tissue and skeletal muscle VLDL-R protein levels were significantly reduced by the HFS diet and increased after conversion to the LFCC diet. We conclude that an HFS diet induces changes in LPL and VLDL-R in a manner which favors shunting of dietary fat from skeletal muscle to adipose tissue and decreases TG-rich lipoprotein clearance contributing to increased plasma lipids and obesity. Conversion to a LFCC diet can ameliorate the dyslipidemia and tissue changes induced by long-term HFS diet consumption.

Effect of exercise training at different intensities on fat metabolism of obese men

The present study investigated the effect of exercise training at different intensities on fat oxidation in obese men. Twenty-four healthy male obese subjects were randomly divided in either a low- [40% maximal oxygen consumption (VO(2 max))] or high-intensity exercise training program (70% VO(2 max)) for 12 wk, or a non-exercising control group. Before and after the intervention, measurements of fat metabolism at rest and during exercise were performed by using indirect calorimetry, [U-(13)C]palmitate, and [1,2-(13)C]acetate. Furthermore, body composition and maximal aerobic capacity were measured. Total fat oxidation did not change at rest in any group. During exercise, after low-intensity exercise training, fat oxidation was increased by 40% (P < 0.05) because of an increased non-plasma fatty acid oxidation (P < 0.05). High-intensity exercise training did not affect total fat oxidation during exercise. Changes in fat oxidation were not significantly different among groups. It was concluded that low-intensity exercise training in obese subjects seemed to increase fat oxidation during exercise but not at rest. No effect of high-intensity exercise training on fat oxidation could be shown.

Weight loss-induced rise in plasma pollutant is associated with reduced skeletal muscle oxidative capacity

In this study, we examined whether weight loss-induced changes in plasma organochlorine compounds (OC) were associated with those in skeletal muscle markers of glycolytic and oxidative metabolism. Vastus lateralis skeletal muscle enzyme activities and plasma OC (Aroclor 1260, polychlorinated biphenyl 153, p,p'-DDE, beta-hexachlorocyclohexane, and hexachlorobenzene) were measured before and after a weight loss program in 17 men and 20 women. Both sexes showed a similar reduction in body weight (approximately 11 kg) in response to treatment, although men lost significantly more fat mass than women (P < 0.05). Enzymatic markers of glycolysis, phosphofructokinase (PFK) activity, and oxidative metabolism, beta-hydroxyacyl-CoA dehydrogenase (HADH), citrate synthase (CS), and cytochrome c oxidase (COX) activities, remained unchanged after weight loss. A significant increase in plasma OC levels was observed in response to weight loss, an effect that was more pronounced in men. No relationship was observed between changes in OC and those in PFK activity in either sex [-0.31 < r < 0.12, not significant (NS)]. However, the greater the increase in plasma OC levels, the greater the reduction in oxidative enzyme (HADH, CS, COX) activities was in response to weight loss in men (-0.75 < r < -0.50, P < 0.05) but not in women (-0.33 < r < 0.33, NS). These results suggest that the weight loss-induced increase in plasma pollutant levels is likely to be associated with reduced skeletal muscle oxidative metabolism in men but not in women.

In muscle-specific lipoprotein lipase-overexpressing mice, muscle triglyceride content is increased without inhibition of insulin-stimulated whole-body and muscle-specific glucose uptake

In patients with type 2 diabetes, a strong correlation between accumulation of intramuscular triclycerides (TGs) and insulin resistance has been found. The aim of the present study was to determine whether there is a causal relation between intramuscular TG accumulation and insulin sensitivity. Therefore, in mice with muscle-specific overexpression of human lipoprotein lipase (LPL) and control mice, muscle TG content was measured in combination with glucose uptake in vivo, under hyperinsulinemic-euglycemic conditions. Overexpression of LPL in muscle resulted in accumulation of TGs in skeletal muscle (85.5 +/- 33.3 vs. 25.7 +/- 23.1 micromol/g tissue in LPL and control mice, respectively; P < 0.05). During the hyperinsulinemic clamp study, there were no differences in plasma glucose, insulin, and FFA concentrations between the two groups. Moreover, whole-body, as well as skeletal muscle, insulin-mediated glucose uptake did not differ between LPL-overexpressing and wild-type mice. Surprisingly, whole-body glucose oxidation was decreased by approximately 60% (P < 0.05), whereas nonoxidative glucose disposal was increased by approximately 50% (P < 0.05) in LPL-overexpressing versus control mice. In conclusion, overexpression of human LPL in muscle increases intramuscular TG accumulation, but does not affect whole-body or muscle-specific insulin-mediated uptake, findings that argue against a simple causal relation between intramuscular TG content and insulin resistance.

Overexpressing human lipoprotein lipase in mouse skeletal muscle is associated with insulin resistance

Lipoprotein lipase (LPL) plays a rate-limiting role in triglyceride-rich lipoprotein metabolism and is expressed in most tissues. Overexpression of LPL in skeletal muscle has been linked with higher plasma glucose levels suggesting insulin resistance (Jensen et al., Am J Physiol 273:R683-R689, 1997). The aim of our study was to ascertain whether the overexpression of human LPL in skeletal muscle leads to insulin resistance and to investigate the mechanism. Respiratory quotient measurements in both transgenic (MCKhLPL) and nontransgenic mice on a high-carbohydrate diet were conducted and showed a shift in fuel usage in transgenic mice when fasting but not when actively feeding. An increase in citrate and glucose 6-phosphate levels in fasted MCKhLPL mice further supports this preferential use of lipids. When challenged with an intraperitoneal injection of glucose (1 g/kg), MCKhLPL mice had a higher plasma glycemic excursion than nontransgenic mice. No differences in insulin response were observed between the two groups. Further investigation using hyperinsulinemic-euglycemic clamps revealed insulin resistance in MCKhLPL mice. Despite signs of insulin resistance, there was no associated increase in free fatty acids, hypertriglyceridemia, or hyperinsulinemia in MCKhLPL mice. In conclusion, MCKhLPL mice are insulin resistant, presumably due to increased delivery of lipoprotein-derived fatty acids to muscle.

Skeletal muscle triglyceride. An aspect of regional adiposity and insulin resistance

Recent evidence derived from four independent methods indicates that an excess triglyceride storage within skeletal muscle is linked to insulin resistance. Potential mechanisms for this association include apparent defects in fatty acid metabolism that are centered at the mitochondria in obesity and in type 2 diabetes. Specifically, defects in the pathways for fatty acid oxidation during postabsorptive conditions are prominent, leading to diminished use of fatty acids and increased esterification and storage of lipid within skeletal muscle. These impairments in fatty acid metabolism during fasting conditions may be related to a metabolic inflexibility in insulin resistance that is not limited to defects in glucose metabolism during insulin-stimulated conditions. Thus, there is substantial evidence implicating perturbations in fatty acid metabolism during accumulation of skeletal muscle triglyceride and in the pathogenesis of insulin resistance. Weight loss by caloric restriction improves insulin sensitivity, but the effects on fatty acid metabolism are less conspicuous. Nevertheless, weight loss decreases the content of triglyceride within skeletal muscle, perhaps contributing to the improvement in Insulin action with weight loss. Alterations in skeletal muscle substrate metabolism provide insight into the link between skeletal muscle triglyceride accumulation and insulin resistance, and they may lead to more appropriate therapies to improve glucose and fatty acid metabolism in obesity and in type 2 diabetes.

Studies with apolipoprotein A-II transgenic mice indicate a role for HDLs in adiposity and insulin resistance

Apolipoprotein A-II (apoA-II) is the second most abundant protein in HDLs. Genetic studies in humans have provided evidence of linkage of the apoA-II gene locus to plasma free fatty acid (FFA) levels and to type 2 diabetes, and transgenic mice overexpressing mouse apoA-II have elevated levels of both FFA and triglycerides. We now show that apoA-II promotes insulin resistance and has diverse effects on fat homeostasis. ApoA-II transgenic mice have increased adipose mass and higher plasma leptin levels than C57BL/6J control mice. Fasting glucose levels were similar between apoA-II transgenic and control mice, but plasma insulin levels were elevated approximately twofold in the apoA-II transgenic mice. Compared with control mice, apoA-II transgenic mice exhibited a delay in plasma clearance of a glucose bolus. Adipose tissue isolated from fasted apoA-II transgenic mice exhibited a 50% decrease in triglyceride hydrolysis compared with adipose tissue from control mice. This is consistent with a normal response of adipose tissue to the increased insulin levels in the apoA-II transgenic mice and may partially explain the increased fat deposition. Skeletal muscle isolated from fasted apoA-II transgenic mice exhibited reduced uptake of 2-deoxyglucose compared with muscles isolated from control mice. Our observations indicate that a primary disturbance in lipoprotein metabolism can result in several traits associated with insulin resistance, consistent with the hypothesis that insulin resistance and type 2 diabetes can, under certain circumstances, be related primarily to altered lipid metabolism rather than glucose metabolism.

The role of high-fat diets and physical activity in the regulation of body weight

The prevalence of obesity is increasing in westernized societies. In the USA the age-adjusted prevalence of BMI > 30 kg/m2 increased between 1960 and 1994 from 13% to 23% for people over 20 years of age. This increase in the prevalence of obesity has been attributed to an increased fat intake and a decreased physical activity. However, the role of the impact of the level of dietary fat intake on human obesity has been challenged. High-fat diets, due to their high energy density, stimulate voluntary energy intake. An increased fat intake does not stimulate its own oxidation but the fat is stored in the human body. When diet composition is isoenergetically switched from low to high fat, fat oxidation only slowly increases, resulting in positive fat balances on the short term. Together with a diminished fat oxidation capacity in pre-obese subjects, high-fat diets can therefore be considered to be fattening. Another environmental factor which could explain the increasing prevalence of obesity is a decrease in physical activity. The percentage of body fat is negatively associated with physical activity and exercise has pronounced effects on energy expenditure and substrate oxidation. High-intensity exercise, due to a lowering of glycogen stores, can lead to a rapid increase in fat oxidation, which could compensate for the consumption of high-fat diets in westernized societies. Although the consumption of high-fat diets and low physical activity will easily lead to the development of obesity, there is still considerable inter-individual variability in body composition in individuals on similar diets. This can be attributed to the genetic background, and some candidate genes have been discovered recently. Both leptin and uncoupling protein have been suggested to play a role in the prevention of diet-induced obesity. Indeed, leptin levels are increased on a high-fat diet but this effect can be attributed to the increased fat mass observed on the high-fat diet. No effect of a high-fat diet per se on leptin levels is observed. Uncoupling proteins are increased by high-fat diets in rats but no data are available in human subjects yet. In conclusion, the increased intake of dietary fat and a decreasing physical activity level are the most important environmental factors explaining the increased prevalence of obesity in westernized societies.

Influence of topiramate in the regulation of energy balance

Topiramate (TPM) is a novel neurotherapeutic agent currently indicated for the treatment of epilepsy and undergoing development for other central nervous system indications including neuropathic pain, bipolar disorder, and migraine prophylaxis. TPM is synthesized from D-fructose and contains a sulfamate moiety that is essential for its pharmacologic activity. TPM has been observed to significantly reduce body weight in patients treated for seizure, which has prompted the realization of preclinical studies to characterize the effects of TPM in the regulation of energy balance. Studies carried out in various strains of rats have provided good evidence for the ability of TPM to blunt energy deposition. Body composition analyses from rat trials have demonstrated that TPM inhibits fat deposition while reducing the activity of lipoprotein lipase (LPL) in various white adipose tissue depots. High doses of TPM (likely above the therapeutic dose range) have also been observed to reduce protein gain without catabolic effects. Although TPM cannot be described as a potent anorectic agent, it seems to have the ability to reduce food intake; significant reductions in food intake have been observed in female obese (fa/fa) Zucker rats and in female Wistar rats. TPM can also reduce energy deposition in the absence of alterations in food intake. This effect has been clearly emphasized in female lean (Fa/?) Zucker rats. In female Sprague-Dawley rats, TPM also increased energy expenditure and it has been observed to increase LPL activity in brown adipose tissue, which could indicate that TPM has the ability to enhance regulatory thermogenesis. In addition, TPM stimulates LPL activity in skeletal muscles, further emphasizing its potential to promote substrate oxidation. The mechanisms whereby TPM affects the regulation of energy balance have yet to be understood. TPM represents an antiepileptic drug (AED) with complex biochemical/pharmacologic actions. Its negative effects on energy deposition cannot be readily predicted from these actions, as AEDs are generally expected to stimulate body weight gain. Recent data, obtained from investigations aimed at assessing the effects of TPM on neuropeptidergic systems involved in the regulation of energy balance, have failed to demonstrate any significant effects of TPM on the neuropeptide Y and proopiomelanocortin systems. In conclusion, it is clear that TPM can reduce fat deposition by either reducing food intake or stimulating energy expenditure. The mechanisms whereby an AED such as TPM controls food intake and energy expenditure remains to be delineated. Copyright1999 ASCRS and ESCRS

Contribution of hyperinsulinemia to modulation of lipoprotein lipase activity in the obese Zucker rat

This study was designed to assess the contribution of hyperinsulinemia to the maintenance of high adipose and low muscle lipoprotein lipase (LPL) activity in the obese Zucker fa/fa rat. Insulinemia in obese Zucker rats was reduced for 4 days with a single injection of low-dose streptozotocin (STZ). Saline-injected intact obese (obese-INT) and STZ-injected obese (obese-STZ) rats were compared with a lean Fa/? reference group. LPL activity was assessed after a 12-hour fast, with or without a 1-hour refeeding period. Fasting serum insulin levels were 17-fold higher in obese-INT versus lean rats and were reduced to 60% of obese-INT levels in obese-STZ animals. In the postprandial state, serum insulin levels remained low in obese-STZ rats and were similar to the values in lean animals, whereas insulinemia increased in the obese-INT group to 18-fold the levels in lean rats. Serum glucose, nonesterified fatty acid (NEFA), and triglyceride levels, which were higher in obese-INT versus lean rats, were further increased in the obese-STZ group. Tissue weights of obese rats were unaffected by STZ treatment. Fasting LPL specific activity was higher in white adipose tissue ([WAT] +87%) and brown adipose tissue ([BAT] +167%) of obese-INT versus lean rats. Reducing the insulinemia in obese-STZ rats reduced fasting enzyme activity to the levels in lean animals in both WAT and BAT. Insulinemia and adipose LPL activity were positively correlated in the fasted state. Acute food intake increased WAT LPL activity in lean animals, but not in obese animals. Soleus LPL activity was lower in obese-INT compared with lean rats and was further decreased in obese-STZ animals. Heart LPL was decreased only in obese-STZ rats compared with the lean group. LPL in muscle tissue was not correlated with insulinemia, but an inverse relationship was found between serum NEFA levels and enzyme activity. It is concluded that in the obese Zucker rat, hyperinsulinemia is responsible for the maintenance of elevated basal LPL activity in adipose tissue independently of fat mass, whereas muscle enzyme activity appears to be more strongly and inversely related to the availability or tissue utilization of lipid substrates.

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.

Markers of capacity to utilize fatty acids in human skeletal muscle: relation to insulin resistance and obesity and effects of weight loss

A number of biochemical defects have been identified in glucose metabolism within skeletal muscle in obesity, and positive effects of weight loss on insulin resistance are also well established. Less is known about the capacity of skeletal muscle for the metabolism of fatty acids in obesity-related insulin resistance and of the effects of weight loss, though it is evident that muscle contains increased triglyceride. The current study was therefore undertaken to profile markers of human skeletal muscle for fatty acid metabolism in relation to obesity, in relation to the phenotype of insulin-resistant glucose metabolism, and to examine the effects of weight loss. Fifty-five men and women, lean and obese, with normal glucose tolerance underwent percutaneous biopsy of vastus lateralis skeletal muscle for determination of HADH, CPT, heparin-releasable (Hr) and tissue-extractable (Ext) LPL, CS, COX, PFK, and GAPDH enzyme activities, and content of cytosolic and plasma membrane FABP. Insulin sensitivity was measured using the euglycemic clamp method. DEXA was used to measure FM and FFM. In skeletal muscle of obese individuals, CPT, CS, and COX activities were lower while, conversely, they had a higher or similar content of FABP(C) and FABP(PM) than in lean individuals. Hr and Ext LPL activities were similar in both groups. In multivariate and simple regression analyses, there were significant correlations between insulin resistance and several markers of FA metabolism, notably, CPT and FABP(PM). These data suggest that in obesity-related insulin resistance, the metabolic capacity of skeletal muscle appears to be organized toward fat esterification rather than oxidation and that dietary-induced weight loss does not correct this disposition.

Racial differences in metabolic predictors of obesity among postmenopausal women

OBJECTIVE

This study determined whether there are racial differences in resting metabolic rate (RMR), fat oxidation, and maximal oxygen consumption (VO2max) in obese [body mass index (BMI = 34+/-2 kg/m2)], postmenopausal (58+/-2 years) women.

RESEARCH METHODS AND PROCEDURES

Twenty black and 20 white women were matched for fat mass and lean mass (LM), as determined by dual energy X-ray absorptiometry. RMR and fat oxidation were measured by indirect calorimetry in the early morning after a 12-hour fast using the ventilated hood technique. VO2max was measured on a treadmill during a progressive exercise test to voluntary exhaustion.

RESULTS

RMR, adjusted for differences in LM, was 5% higher in white than black women (1566+/-27 and 1490+/-26 kcal/day, respectively; p<0.05); and fat oxidation rate was 17% higher in white than black women (87+/-4 and 72+/-3 g/day, respectively; p<0.01). VO2max (L/minute) was 150 mL per minute (8%) higher (p<0.05) in white than black women. VO2max correlated with LM in black (r=0.44, p=0.05) and white (r=0.53, p<0.05) women, but the intercept of the regression line was higher in white than black women (p<0.05), with no significant difference in slopes. In a multiple regression model including race, body weight, LM, and age, LM was the only independent predictor of RMR (r2 = 0.46, p<0.0001), whereas race was the only independent predictor of fat oxidation (r2 = 0.18, p<0.05). The best predictors of VO2max were LM (r2 = 0.22, p<0.05) and race (cumulative r2 = 0.30, p<0.05).

DISCUSSION

These results show there are racial differences in metabolic predictors of obesity. Determination of whether these ethnic differences lead to, or are an effect of, obesity status or other lifestyle factors requires further study.

Interaction of physical activity and diet: implications for lipoprotein metabolism

OBJECTIVE

To consider how physical activity interacts with diet to modify lipoprotein metabolism and comment on implications for human health.

DESIGN

An overview of lipoprotein metabolism is followed by a summary of the main effects of physical activity on lipoprotein metabolism. Interactions with dietary practice and the disposition of dietary lipid are reviewed, with comment on links with body fatness.

SETTING

Literature is reviewed in relation to the risk of atherosclerotic disease.

SUBJECTS

Although some data are presented on athletic groups, evidence relating to individuals with normal physical activity habits is mainly discussed.

RESULTS

Physical inactivity is a risk factor for cardiovascular disease and one mechanism may involve changes to lipoprotein metabolism. The consensus is that aerobic activity involving an expenditure of > or = 8 MJ week(-1) results in an increase in HDL cholesterol and probably decreases in fasting triacylglycerol. These changes Occur despite the spontaneous increase in the proportion of dietary energy from carbohydrate which accompanies increased exercise. For this reason, exercise may be a means of reducing the hypertriglyceridaemic and HDL-lowering effects of low fat (high carbohydrate) diets. Decreases in total and low density lipoprotein cholesterol are sometimes, but not always, reported in sedentary individuals beginning exercise. One mechanism linking all these changes may be alterations to the dynamics of triacylglycerol-rich particles, particularly in the fed state.

CONCLUSION

The expenditure of considerable amounts of energy through regular, frequent physical activity increases the turnover of lipid substrates, with effects on their transport and disposition which may reduce the progression of atherosclerosis.

Modulation of triglyceride metabolism by glucocorticoids in diet-induced obesity

The involvement of glucocorticoids (GC) in the development of diet-induced obesity and in the concomitant adaptations of triglyceride (TG)-rich lipoprotein metabolism were examined. Rats were fed either rodent chow, which maintains a low lipid flux, or a diet high in sucrose and fat (HSF) that increases lipid flux, leading to metabolic perturbations similar to those that define the plurimetabolic syndrome in humans. The GC status was manipulated through adrenalectomy (ADX) and corticosterone (Cort) replacement. Compared with chow, the HSF diet increased energy intake (17%) and whole body (8%) and adipose tissue (80%) weights. The HSF diet also increased the acute postprandial rise in plasma insulin (4-fold) and TG (3-fold), fasting liver TG content (3-fold), triglyceridemia (54%), and adipose tissue lipoprotein lipase (LPL) activity (2-fold). ADX decreased energy intake and whole body and adipose tissue weights in both dietary cohorts, but more so in HSF-fed than in chow-fed animals. These ADX-induced effects were totally prevented by Cort replacement in rats fed chow, but only partially so in those fed the HSF diet in proportion to the degree of restoration of energy intake. In the chow-fed cohort, the above indexes of TG metabolism remained unaffected by the Cort status, whereas in the HSF-fed cohort, these variables were decreased by ADX to levels of chow-fed animals. Cort replacement in the HSF-fed animals restored indexes of TG metabolism to intact levels and reestablished the diet-related differences observed in intact animals. These findings indicate that GC modulate fasting TG metabolism only minimally when a diet that maintains a low lipid flux is fed. In contrast, their presence is a necessary condition for the development of diet-induced obesity and the concomitant alterations in insulin sensitivity and TG-rich lipoprotein metabolism.

Effect of protein intake and physical activity on 24-h pattern and rate of macronutrient utilization

Effects of moderate physical activity (90 min at 45-50% of maximal O2 uptake 2 times daily) and "high" (2.5 g protein. kg-1. day-1, n = 6) or "normal" protein intake (1.0 g protein. kg-1. day-1, n = 8) on the pattern and rate of 24-h macronutrient utilization in healthy adult men were compared after a diet-exercise-adjustment period of 6 days. Energy turnover (ET) was determined by indirect and direct (suit) calorimetry, and "protein oxidation" was determined by a 24-h continuous intravenous infusion of [1-13C]leucine. Subjects were in slight positive energy balance during both studies. Protein contributed to a higher (22 vs. 10%) and carbohydrate (CHO) a lower (33 vs. 58%) proportion of total 24-h ET on the high- vs. normal-protein intake. The highest contribution of fat to ET was seen postexercise during fasting (73 and 61% of ET for high and normal, respectively). With the high-protein diet the subjects were in a positive protein (P < 0.001) and CHO balance (P < 0.05) and a negative fat balance (P < 0.05). The increased ET postexercise was not explained by increased rates of urea production and/or protein synthesis.

Epinephrine stimulates human muscle lipoprotein lipase activity in vivo

Lipoprotein lipase (LPL) is involved in lipoprotein metabolism and nutrient partitioning in both adipose tissue and skeletal muscle, and LPL activity is regulated by various hormones and the nutritional state. However, the action of catecholamines has not been thoroughly investigated in humans. Therefore, the effects of exogenous epinephrine on skeletal muscle LPL (SM-LPL) activity and whole-body lipid oxidation were studied. Muscle biopsies were obtained from eight healthy subjects before, during, and after epinephrine infusion. Somatostatin was infused to suppress endogenous insulin production and insulin was infused at a constant rate to maintain basal insulin levels throughout the study. After an equilibrium period (120 minutes), epinephrine (0.05 microg/kg/min) was infused for another 120 minutes. Epinephrine stimulated SM-LPL activity by 21.8%+/-6.8% above basal levels from 1.44+/-0.25 to 1.69+/-0.28 micromol free fatty acid (FFA)/h/g muscle (P<.02), increased plasma FFA 270% from 0.147 to 0.544 mmol/L (P<.05), and increased lipid oxidation 45% from 4.37 to 6.36 mg/kg/min (P<.05). The increase in SM-LPL activity was positively correlated with the increase in whole-body lipid oxidation (R=.75, P<.05). Finally, lipid oxidation and SM-LPL activity were negatively correlated with whole-body glucose oxidation. Overall, the results demonstrate that epinephrine is able to stimulate SM-LPL activity in humans, and thus may have opposite effects on adipose tissue and SM-LPL activity.

The influence of exercise on postprandial triacylglycerol metabolism

Trained people exhibit low plasma concentrations of triacylglcyeride (TAG) in both fasted and postprandial states. This mainly reflects enhanced uptake of TAG into skeletal muscle, via enhanced activity of lipoprotein lipase, the rate-limiting step in TAG removal. Endurance athletes possess a large, well-vascularised muscle mass and this may contribute through the increased availability of endothelial binding sites for LPL. However, each session of exercise stimulates a delayed increase in LPL activity so that prior exercise enhances uptake into muscle. Intramuscular TAG is one source of energy for muscular contraction so this may serve to replenish muscle nutrient stores which have been diminished by exercise. Regular, frequent aerobic exercise may oppose the atherogenic disturbances to lipoprotein metabolism evident during the postprandial period. It may also, by favouring the disposition of dietary fatty acids in muscle, improve the matching of fat oxidation to fat intake and hence help with maintenance of a desirable level of body fatness.

Lower activity of oxidative key enzymes and smaller fiber areas in skeletal muscle of postobese women

Muscle fiber morphology and activities of four key enzymes, as well as energy metabolism, were determined in nine normal-weight postobese women and nine matched control subjects. No differences in fiber type composition, but a smaller mean fiber area and area of fiber types I and IIb, were found in postobese compared with control subjects (P < 0.05). The activities of beta-hydroxyacyl-CoA dehydrogenase (HADH) and citrate synthase (CS) were 20% lower in postobese than in control subjects (P < 0.05). However, the activities of lactate dehydrogenase and lipoprotein lipase were not significantly different between postobese and control subjects. Basal metabolic rate and respiratory exchange ratio were also similar, but maximal oxygen uptake (VO2 max) tended to be lower in postobese than in control subjects (P = 0.06). When adjustments were made for differences in VO2 max, HADH and CS were not different between postobese and control subjects. In conclusion, these data suggest that smaller fiber areas and lower enzyme activities, i.e., markers of aerobic capacity of skeletal muscle, but not fiber composition, may be factors predisposing to obesity.