Induction by leptin of uncoupling protein-2 and enzymes of fatty acid oxidation

Role of UCP2 in the Energy Metabolism of the Cancer Cell Line A549

The uncoupling protein UCP2 is a mitochondrial carrier for which transport activity remains controversial. The physiological contexts in which UCP2 is expressed have led to the assumption that, like UCP1, it uncouples oxidative phosphorylation and thereby reduces the generation of reactive oxygen species. Other reports have involved UCP2 in the Warburg effect, and results showing that UCP2 catalyzes the export of matrix C4 metabolites to facilitate glutamine utilization suggest that the carrier could be involved in the metabolic adaptations required for cell proliferation. We have examined the role of UCP2 in the energy metabolism of the lung adenocarcinoma cell line A549 and show that UCP2 silencing decreased the basal rate of respiration, although this inhibition was not compensated by an increase in glycolysis. Silencing did not lead to either changes in proton leakage, as determined by the rate of respiration in the absence of ATP synthesis, or changes in the rate of formation of reactive oxygen species. The decrease in energy metabolism did not alter the cellular energy charge. The decreased cell proliferation observed in UCP2-silenced cells would explain the reduced cellular ATP demand. We conclude that UCP2 does not operate as an uncoupling protein, whereas our results are consistent with its activity as a C4-metabolite carrier involved in the metabolic adaptations of proliferating cells.

Fatty Liver Disease, Metabolism and Alcohol Interplay: A Comprehensive Review

Non-alcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease worldwide, and its incidence has been increasing in recent years because of the high prevalence of obesity and metabolic syndrome in the Western population. Alcohol-related liver disease (ArLD) is the most common cause of cirrhosis and constitutes the leading cause of cirrhosis-related deaths worldwide. Both NAFLD and ArLD constitute well-known causes of liver damage, with some similarities in their pathophysiology. For this reason, they can lead to the progression of liver disease, being responsible for a high proportion of liver-related events and liver-related deaths. Whether ArLD impacts the prognosis and progression of liver damage in patients with NAFLD is still a matter of debate. Nowadays, the synergistic deleterious effect of obesity and diabetes is clearly established in patients with ArLD and heavy alcohol consumption. However, it is still unknown whether low to moderate amounts of alcohol are good or bad for liver health. The measurement and identification of the possible synergistic deleterious effect of alcohol consumption in the assessment of patients with NAFLD is crucial for clinicians, since early intervention, advising abstinence and controlling cardiovascular risk factors would improve the prognosis of patients with both comorbidities. This article seeks to perform a comprehensive review of the pathophysiology of both disorders and measure the impact of alcohol consumption in patients with NAFLD.

Mitochondrial Uncoupling Proteins (UCPs) as Key Modulators of ROS Homeostasis: A Crosstalk between Diabesity and Male Infertility?

Uncoupling proteins (UCPs) are transmembrane proteins members of the mitochondrial anion transporter family present in the mitochondrial inner membrane. Currently, six homologs have been identified (UCP1-6) in mammals, with ubiquitous tissue distribution and multiple physiological functions. UCPs are regulators of key events for cellular bioenergetic metabolism, such as membrane potential, metabolic efficiency, and energy dissipation also functioning as pivotal modulators of ROS production and general cellular redox state. UCPs can act as proton channels, leading to proton re-entry the mitochondrial matrix from the intermembrane space and thus collapsing the proton gradient and decreasing the membrane potential. Each homolog exhibits its specific functions, from thermogenesis to regulation of ROS production. The expression and function of UCPs are intimately linked to diabesity, with their dysregulation/dysfunction not only associated to diabesity onset, but also by exacerbating oxidative stress-related damage. Male infertility is one of the most overlooked diabesity-related comorbidities, where high oxidative stress takes a major role. In this review, we discuss in detail the expression and function of the different UCP homologs. In addition, the role of UCPs as key regulators of ROS production and redox homeostasis, as well as their influence on the pathophysiology of diabesity and potential role on diabesity-induced male infertility is debated.

Human intestinal lipid storage through sequential meals reveals faster dinner appearance is associated with hyperlipidemia

Background

It is increasingly recognized that intestinal cells can store lipids after a meal, yet the effect of this phenomenon on lipid absorption patterns in insulin resistance remains unknown.

Methods

The kinetics of meal fat appearance were measured in insulin-sensitive (IS, n = 8) and insulin-resistant (IR, n = 8) subjects after sequential, isotopically labeled lunch and dinner meals. Plasma dynamics on triacylglycerol-rich (TAG-rich) lipoproteins and plasma hormones were analyzed using a nonlinear, non–steady state kinetic model.

Results

At the onset of dinner, IS subjects showed an abrupt plasma appearance of lunch lipid consistent with the “second-meal effect,” followed by slower appearance of dinner fat in plasma, resulting in reduced accumulation of dinner TAG of 48% compared with lunch. By contrast, IR subjects exhibited faster meal TAG appearance rates after both lunch and dinner. This effect of lower enterocyte storage between meals was associated with greater nocturnal and next-morning hyperlipidemia. The biochemical data and the kinetic analysis of second-meal effect dynamics are consistent with rapid secretion of stored TAG bypassing lipolysis and resynthesis. In addition, the data are consistent with a role for the diurnal pattern of plasma leptin in regulating the processing of dietary lipid.

Conclusion

These data support the concept that intestinal lipid storage may be physiologically beneficial in IS subjects.

Trial registration

ClinicalTrials.gov NCT02020343.

Funding

This study was supported by a grant from the American Diabetes Association (grant 1-13-TS-12).

Potential Role of Leptin in Cardiac Steatosis Induced by Highly Saturated Fat Intake during Adolescence

SCOPE

To identify the age-dependent effect of diets containing elevated amounts of either saturated or unsaturated fatty acids on cardiac steatosis in mice.

METHODS AND RESULTS

Five- and eight-week-old C57BL/6J mice cohorts are given free access to either a saturated or an unsaturated fatty-acid-enriched diet during 8 weeks. Body weight (BW) and food intake are monitored during this period. Cardiac lipid content, carnitine palmitoyltransferase-I (CPT-I) activity, and the amount of uncoupling proteins 2 and 3 (UCP2 and UCP3) are analyzed and correlated with blood leptin concentration. Leptin and PPARγ gene expression is quantified in white adipose tissue (WAT). Both diets have a similar effect on food intake, BW, and adiposity, independently of the age. Nevertheless, cardiac steatosis is specifically identified in adolescent mice consuming the saturated diet. These animals also display lower activity of cardiac CPT-I, a down-regulation of cardiac UCP2, together with lower concentration of plasma leptin. Accordingly, leptin gene expression is reduced in the visceral WAT.

CONCLUSION

Consumption of diets containing elevated amounts of saturated fat during adolescence and early adult life promotes cardiac steatosis in mice. An insufficient endocrine activity of WAT, in terms of leptin production, may account for such an effect.

How important is tryptophan in human health?

Tryptophan (Trp) is an amino acid and an essential component of the human diet. It plays a crucial role in many metabolic functions. Clinicians can use Trp levels in the course of diagnosing various metabolic disorders and the symptoms associated with those diseases. Furthermore, supplementation with this amino acid is considered in the treatment of depression and sleep disorders, mainly due to the Trp relationship with the synthesis of serotonin (5-HT) and melatonin. It is also used in helping to resolve cognitive disorders, anxiety, or neurodegenerative diseases. Reduced secretion of serotonin is associated with autism spectrum disorder, obesity, anorexia and bulimia nervosa, and other diseases presenting peripherals symptoms. The literature strongly suggests that Trp has a significant role in the correct functionality of the brain-gut axis and immunology. This information leads to the consideration of Trp as an essential dietary component due to its role in the serotonin pathway. A reduced availability of Trp in diet and nutraceutical supplementation should be considered with greater concern than one might expect. This paper constitutes a review of the more salient aspects gleaned from the current knowledge base about the role of Trp in diseases, associated nutritional disorders, and food science, in general.

Triglycerides Revisited to the Serial

This review discusses the role of triglycerides (TGs) in the normal cardiovascular system as well as in the development and clinical manifestation of cardiovascular diseases. Regulation of TGs at the enzymatic and genetic level, in addition to their possible relevance as preclinical and clinical biomarkers, is discussed, culminating with a description of available and emerging treatments. Due to the high complexity of the subject and the vast amount of material in the literature, the objective of this review was not to exhaust the subject, but rather to compile the information to facilitate and improve the understanding of those interested in this topic. The main publications on the topic were sought out, especially those from the last 5 years. The data in the literature still give reason to believe that there is room for doubt regarding the use of TG as disease biomarkers; however, there is increasing evidence for the role of hypertriglyceridemia on the atherosclerotic inflammatory process, cardiovascular outcomes, and mortality.

Role of adipocytes in hypertension

Although it has known for some time that obesity is associated with salt sensitivity and hypertension, recent data suggests that the adipocyte may actually be the proximate cause of this physiological changes. In the following review, the data demonstrating this association as well as the potentially operative pathophysiological mechanisms are reviewed and discussed.

Effects of UCP4 on the Proliferation and Apoptosis of Chondrocytes: Its Possible Involvement and Regulation in Osteoarthritis

Reactive oxygen species (ROS)-induced chondrocytes apoptosis plays a key role in osteoarthritis (OA) pathogenesis. Uncoupling protein 4 (UCP4) can protect cells against oxidative stress via reducing ROS production and cell apoptosis. Here, silencing of UCP4 in primary chondrocytes significantly inhibited cell survival, but induced ROS production and cell apoptosis. UCP4 mRNA of cartilage tissues was decreased in osteoarthritis patients, which was negatively correlated with synovial fluid (SF) leptin concentration. Moreover, leptin treatment (5, 10 and 20 ng/ml) of primary cultured chondrocytes significantly decreased mRNA and protein levels of UCP4, but increased ROS production and cell apoptosis in a dose-dependent manner. The effects of leptin treatment (20 ng/ml) on chondrocytes was partially reversed by ectopic expression of UCP4. More importantly, intraarticularly injection of UCP4 adenovirus remarkably alleviate OA progression and cell apoptosis in a rat OA model induced by anterior cruciate ligament transection (ACLT). In conclusion, UCP4, whose expression was suppressed by leptin, may be involved in the ROS production and apoptosis of chondrocytes, thus contributing to the OA pathogenesis.

Identifying clues to molecular etiology of multiple sclerosis in South Indian patients

BACKGROUND

Environmental risk factors have a dominant role in the pathogenesis of multiple sclerosis (MS). Unhealthy lifestyle can predispose people to autoimmune diseases. MS was a rare disease in Kerala, but now, we notice frequent cases of MS at the city neurology clinic. Changing lifestyle and associated changes in the level of proinflammatory biomolecules like: leptin, soluble leptin receptor (SLR) and free fatty acids (FA) could be contributing to rise in MS incidence.

OBJECTIVE

To identify variations in the levels of bio-molecules: leptin, SLR and FA, between MS patients and matched healthy control.

METHOD

Leptin and SLR levels in the blood serum, were estimated using ELISA, while total FA levels, were estimated using an enzyme based calorimetric assay.

RESULT

Mean serum FA levels in MS patients (31.39 ± 4.83 nmole/100 μl) were 2.7 fold higher than controls (11.54 ± 2.66 nmoles/100 μl) at more than 99% CI. The differences in mean leptin and SLR levels were not statistically significant.

CONCLUSION

MS patients had high level of total FA in their blood. High FA in blood may have a role in MS pathogenesis. More in-depth study is required to understand the precise mechanism by which FA rise in MS blood sample can contribute to pathogenesis.

Role of leptin in body temperature regulation and lipid metabolism following splenectomy

OBJECTIVE

The physiological changes in serum triglycerides and body temperature that are induced by splenectomy are poorly understood. Therefore, the aim of this study was to investigate parameters related to lipid and glucose metabolism, as well as thermoregulation, in splenectomized mice.

DESIGN AND METHODS

Splenectomized and sham-operated WT mice (C57Bl/6) and ob/ob mice were randomly divided and treated with a standard or high fat diet, and several metabolic parameters and the body temperature were investigated.

RESULTS

Splenectomy induced a significant increase in triglyceride levels regardless of the diet. It was found that the splenectomized WT mice showed greater serum leptin and insulin levels compared with the sham-operated mice. Additionally, the body temperatures of the splenectomized WT mice were greater than the body temperatures of the control animals regardless of diet; this result too was observed without any significant change in the temperature of the splenectomized ob/ob animals.

CONCLUSION

The results suggest that splenectomy interferes with serum triglyceride metabolism and body temperature regardless of the fat content in the diet and that leptin is involved in the regulation of body temperature related to splenectomy.

FABP4 reversed the regulation of leptin on mitochondrial fatty acid oxidation in mice adipocytes

Fatty acid binding protein 4 (FABP4), plays key role in fatty acid transportation and oxidation, and increases with leptin synergistically during adipose inflammation process. However, the regulation mechanism between FABP4 and leptin on mitochondrial fatty acid oxidation remains unclear. In this study, we found that FABP4 reduced the expression of leptin, CPT-1 and AOX1 in mice adipocytes. Conversely, FABP4 was down-regulated in a time-dependent manner by leptin treatment. Additionally, forced expression of FABP4 attenuated the expression of PGC1-α, UCP2, CPT-1, AOX1 and COX2 compared with leptin incubation. Moreover, mitochondrial membrane potential, fatty acid oxidation enzyme medium-chain acyl-CoA dehydrogenase (MCAD), long-chain acyl-CoA dehydrogenase (LCAD) and Cyt C levels were reduced in response to the overexpression of FABP4. These reductions correspond well with the reduced release of free fatty acid and the inactivation of mitochondrial complexes I and III by FABP4 overexpression. Furthermore, addition of the Akt/mTOR pathway-specific inhibitor (MK2206) blocked the mitochondrial fatty acid oxidation and respiration factors, whereas interference of FABP4 overcame these effects. Taken together, FABP4 could reverse the activation of the leptin-induced mitochondrial fatty acid oxidation, and the inhibition of Akt/mTOR signal pathway played a key role in this process.

Obesity-induced changes in kidney mitochondria and endoplasmic reticulum in the presence or absence of leptin

We investigated obesity-induced changes in kidney lipid accumulation, mitochondrial function, and endoplasmic reticulum (ER) stress in the absence of hypertension, and the potential role of leptin in modulating these changes. We compared two normotensive genetic mouse models of obesity, leptin-deficient ob/ob mice and hyperleptinemic melanocortin-4 receptor-deficient mice (LoxTB MC4R-/-), with their respective lean controls. Compared with controls, ob/ob and LoxTB MC4R-/- mice exhibit significant albuminuria, increased creatinine clearance, and high renal triglyceride content. Renal ATP levels were decreased in both obesity models, and mitochondria isolated from both models showed alterations that would lower mitochondrial ATP production. Mitochondria from hyperleptinemic LoxTB MC4R-/- mice kidneys respired NADH-generating substrates (including palmitate) at lower rates due to an apparent decrease in complex I activity, and these mitochondria showed oxidative damage. Kidney mitochondria of leptin-deficient ob/ob mice showed normal rates of respiration with no evidence of oxidative damage, but electron transfer was partially uncoupled from ATP synthesis. A fourfold induction of C/EBP homologous protein (CHOP) expression indicated induction of ER stress in kidneys of hyperleptinemic LoxTB MC4R-/- mice. In contrast, ER stress was not induced in kidneys of leptin-deficient ob/ob mice. Our findings show that obesity, in the absence of hypertension, is associated with renal dysfunction in mice but not with major renal injury. Alterations to mitochondria that lower cellular ATP levels may be involved in obesity-induced renal injury. The type and severity of mitochondrial and ER dysfunction differs depending upon the presence or absence of leptin.

PPARα-Deficient ob/ob Obese Mice Become More Obese and Manifest Severe Hepatic Steatosis Due to Decreased Fatty Acid Oxidation

Obesity poses an increased risk of developing metabolic syndrome and closely associated nonalcoholic fatty liver disease, including liver cancer. Satiety hormone leptin-deficient (ob/ob) mice, considered paradigmatic of nutritional obesity, develop hepatic steatosis but are less prone to developing liver tumors. Sustained activation of peroxisome proliferator-activated receptor α (PPARα) in ob/ob mouse liver increases fatty acid oxidation (FAO), which contributes to attenuation of obesity but enhances liver cancer risk. To further evaluate the role of PPARα-regulated hepatic FAO and energy burning in the progression of fatty liver disease, we generated PPARα-deficient ob/ob (PPARα(Δ)ob/ob) mice. These mice become strikingly more obese compared to ob/ob littermates, with increased white and brown adipose tissue content and severe hepatic steatosis. Hepatic steatosis becomes more severe in fasted PPARα(Δ)ob/ob mice as they fail to up-regulate FAO systems. PPARα(Δ)ob/ob mice also do not respond to peroxisome proliferative and mitogenic effects of PPARα agonist Wy-14,643. Although PPARα(Δ)ob/ob mice are severely obese, there was no significant increase in liver tumor incidence, even when maintained on a diet containing Wy-14,643. We conclude that sustained PPARα activation-related increase in FAO in fatty livers of obese ob/ob mice increases liver cancer risk, whereas deletion of PPARα in ob/ob mice aggravates obesity and hepatic steatosis. However, it does not lead to liver tumor development because of reduction in FAO and energy burning.

Social overcrowding as a chronic stress model that increases adiposity in mice

Stress is a widely recognized risk factor for psychiatric and metabolic disorders. A number of animal models utilizing various stressors have been developed to facilitate our understanding in the pathophysiology of stress-related dysfunctions. The most commonly used chronic stress paradigms include the unpredictable chronic mild stress paradigm, the social defeat paradigm and the social deprivation paradigm. Here we assess the potential of social crowding as an alternative chronic stress model to study the effects on affective behaviors and metabolic disturbances. Ten-week-old male C57BL/6 mice were housed in groups of four (control) or eight (social crowding; SC) in standard cage for 9 weeks. Exploration, anxiety- and depressive-like behaviors were assessed in the open field test, the elevated T-maze, the novelty-suppressed feeding test and the forced swim test. SC mice exhibited a modest anxiety-like phenotype without change in depressive-like behaviors. Nine weeks of social crowding did not affect the body weight, but robustly increased adiposity as determined by increased mass of fat depots. Consistent with the increased fat content, serum leptin was markedly elevated in the SC mice. Specific changes in gene expression were also observed in the hypothalamus and the white adipose tissue following SC housing. Our study demonstrates the potential of social crowding as an alternative model for the study of stress-related metabolic and behavioral dysfunctions.

Impaired cytosolic NADH shuttling and elevated UCP3 contribute to inefficient citric acid cycle flux support of postischemic cardiac work in diabetic hearts

Diabetic hearts are subject to more extensive ischemia/reperfusion (ISC/REP) damage. This study examined the efficiency of citric acid cycle (CAC) flux and the transfer of cytosolic reducing equivalents into the mitochondria for oxidative support of cardiac work following ISC/REP in hearts of c57bl/6 (NORM) and type 2 diabetic, db/db mouse hearts. Flux through the CAC and malate-aspartate shuttle (MA) were monitored via dynamic (13)C NMR of isolated hearts perfused with (13)C palmitate+glucose. MA flux was lower in db/db than NORM. Oxoglutarate malate carrier (OMC) was elevated in the db/db heart, suggesting a compensatory response to low NADHc. Baseline CAC flux per unit work (rate-pressure-product, RPP) was similar between NORM and db/db, but ISC/REP reduced the efficiency of CAC flux/RPP by 20% in db/db. ISC/REP also increased UCP3 transcription, indicating potential for greater uncoupling. Therefore, ISC/REP induces inefficient carbon utilization through the CAC in hearts of diabetic mice due to the combined inefficiencies in NADHc transfer per OMC content and increased uncoupling via UCP3. Ischemia and reperfusion exacerbated pre-existing mitochondrial defects and metabolic limitations in the cytosol of diabetic hearts. These limitations and defects render diabetic hearts more susceptible to inefficient carbon fuel utilization for oxidative energy metabolism.

Counterregulation of insulin by leptin as key component of autonomic regulation of body weight

A re-examination of the mechanism controlling eating, locomotion, and metabolism prompts formulation of a new explanatory model containing five features: a coordinating joint role of the (1) autonomic nervous system (ANS); (2) the suprachiasmatic (SCN) master clock in counterbalancing parasympathetic digestive and absorptive functions and feeding with sympathetic locomotor and thermogenic energy expenditure within a circadian framework; (3) interaction of the ANS/SCN command with brain substrates of reward encompassing dopaminergic projections to ventral striatum and limbic and cortical forebrain. These drive the nonhomeostatic feeding and locomotor motivated behaviors in interaction with circulating ghrelin and lateral hypothalamic neurons signaling through melanin concentrating hormone and orexin-hypocretin peptides; (4) counterregulation of insulin by leptin of both gastric and adipose tissue origin through: potentiation by leptin of cholecystokinin-mediated satiation, inhibition of insulin secretion, suppression of insulin lipogenesis by leptin lipolysis, and modulation of peripheral tissue and brain sensitivity to insulin action. Thus weight-loss induced hypoleptimia raises insulin sensitivity and promotes its parasympathetic anabolic actions while obesity-induced hyperleptinemia supresses insulin lipogenic action; and (5) inhibition by leptin of bone mineral accrual suggesting that leptin may contribute to the maintenance of stability of skeletal, lean-body, as well as adipose tissue masses.

Adipokines and insulin action: A sensitive issue

Obesity is a major public health concern and a strong risk factor for insulin resistance, type 2 diabetes mellitus (T2DM), and cardiovascular disease. The last two decades have seen a reconsideration of the role of white adipose tissue (WAT) in whole body metabolism and insulin action. Adipose tissue-derived cytokines and hormones, or adipokines, are likely mediators of metabolic function and dysfunction. While several adipokines have been associated with obese and insulin-resistant phenotypes, a select group has been linked with insulin sensitivity, namely leptin, adiponectin, and more recently, adipolin. What is known about these insulin-sensitizing molecules and their effects in healthy and insulin resistant states is the subject of this review. There remains a significant amount of research to do to fully elucidate the mechanisms of action of these adipokines for development of therapeutics in metabolic disease.

Heterozygosity for leptin receptor (fa) accelerates hepatic triglyceride accumulation without hyperphagia in Zucker rats

SUMMARY

Leptin, ob gene product, and its receptors are involved in the regulation of peripheral lipid and glucose metabolism. The present study sought to clarify the functional role of peripheral leptin receptors in hepatic lipid metabolism through analysis of Zucker rats (fa/fa, +/fa), as complete or partial leptin receptor insufficiency models, respectively. In Zucker fa/fa rats, calorie intake, body weight, liver weight, hepatic triglyceride content and serum insulin, triglycerides, FFA, and leptin were elevated compared to lean littermates (+/+ rats). In contrast, Zucker +/fa rats showed no remarkable changes in calorie intake, body weight and serum FFA compared with +/+ rats. Nevertheless, hepatic triglyceride content, liver weight and other serum parameters such as insulin, triglyceride and leptin were higher than in +/+ rats. In the representation of fatty acids component in the liver, there were no changes in +/fa rats relative to +/+ rats. Thus, in Zucker +/fa rats, fatty liver may develop in the absence of hyperphagia, obesity or changes in hepatic fatty acid metabolism. These results indicate that partial insufficiency of leptin receptor rather than changes in serum insulin, triglyceride and leptin may contribute to the increase in hepatic triglyceride content observed in +/fa rats.:

Direct and indirect effects of leptin on adipocyte metabolism

Leptin is hypothesized to function as a negative feedback signal in the regulation of energy balance. It is produced primarily by adipose tissue and circulating concentrations correlate with the size of body fat stores. Administration of exogenous leptin to normal weight, leptin responsive animals inhibits food intake and reduces the size of body fat stores whereas mice that are deficient in either leptin or functional leptin receptors are hyperphagic and obese, consistent with a role for leptin in the control of body weight. This review discusses the effect of leptin on adipocyte metabolism. Because adipocytes express leptin receptors there is the potential for leptin to influence adipocyte metabolism directly. Adipocytes also are insulin responsive and receive sympathetic innervation, therefore leptin can also modify adipocyte metabolism indirectly. Studies published to date suggest that direct activation of adipocyte leptin receptors has little effect on cell metabolism in vivo, but that leptin modifies adipocyte sensitivity to insulin to inhibit lipid accumulation. In vivo administration of leptin leads to a suppression of lipogenesis, an increase in triglyceride hydrolysis and an increase in fatty acid and glucose oxidation. Activation of central leptin receptors also contributes to the development of a catabolic state in adipocytes, but this may vary between different fat depots. Leptin reduces the size of white fat depots by inhibiting cell proliferation both through induction of inhibitory circulating factors and by contributing to sympathetic tone which suppresses adipocyte proliferation. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Mitochondrial hormesis in pancreatic β cells: does uncoupling protein 2 play a role?

In pancreatic β cells, mitochondrial metabolism translates glucose sensing into signals regulating insulin secretion. Chronic exposure of β cells to excessive nutrients, namely, glucolipotoxicity, impairs β-cell function. This is associated with elevated ROS production from overstimulated mitochondria. Mitochondria are not only the major source of cellular ROS, they are also the primary target of ROS attacks. The mitochondrial uncoupling protein UCP2, even though its uncoupling properties are debated, has been associated with protective functions against ROS toxicity. Hormesis, an adaptive response to cellular stresses, might contribute to the protection against β-cell death, possibly limiting the development of type 2 diabetes. Mitochondrial hormesis, or mitohormesis, is a defense mechanism observed in ROS-induced stress-responses by mitochondria. In β cells, mitochondrial damages induced by sublethal exogenous H₂O₂ can induce secondary repair and defense mechanisms. In this context, UCP2 is a marker of mitohormesis, being upregulated following stress conditions. When overexpressed in nonstressed naïve cells, UCP2 confers resistance to oxidative stress. Whether treatment with mitohormetic inducers is sufficient to restore or ameliorate secretory function of β cells remains to be determined.

Triglyceride-lowering Effect of Respiratory Uncoupling in White Adipose Tissue

OBJECTIVE

Hypolipidemic drugs such as bezafibrate and thiazolidinediones are known to induce the expression of mitochondrial uncoupling proteins (UCPs) in white adipose tissue. To analyze the potential triglyceride (TG)-lowering effect of respiratory uncoupling in white fat, we evaluated systemic lipid metabolism in aP2-Ucp1 transgenic mice with ectopic expression of UCP1 in adipose tissue.

RESEARCH METHODS AND PROCEDURES

Hemizygous and homozygous transgenic mice and their nontransgenic littermates were fed chow or a high-fat diet for up to 3 months. Total TGs, nonesterified fatty acids, and the composition of plasma lipoproteins were analyzed. Hepatic TG production was measured in mice injected with Triton WR1339. Uptake and the use of fatty acids were estimated by measuring adipose tissue lipoprotein lipase activity and fatty acid oxidation, respectively. Adipose tissue gene expression was assessed by quantitative reverse transcriptase-polymerase chain reaction.

RESULTS

Transgene dosage and the high-fat diet interacted to markedly reduce plasma TGs. This was reflected by decreased concentrations of very-low-density lipoprotein particles in the transgenic mice. Despite normal hepatic TG secretion, the activity of lipoprotein lipase in epididymal fat was enhanced by the high-fat diet in the transgenic mice in a setting of decreased re-esterification and increased in situ fatty acid oxidation.

DISCUSSION

Respiratory uncoupling in white fat may lower plasma lipids by enhancing their in situ clearance and catabolism.

Behavioral and physiological effects of photoperiod-induced migratory state and leptin on Zonotrichia albicollis: II. Effects on fatty acid metabolism

The migratory flights of birds are fuelled largely by fatty acids. Fatty acid transporters, including FAT/CD36, FABPpm and H-FABP, and enzymes involved in fatty acid oxidation (CPT, CS, HOAD) are seasonally up-regulated in flight muscle to meet the demands of this intense aerobic exercise. The mechanisms that control these biochemical changes in response to migration are mostly unknown. We studied the effects of a photoperiod-induced migratory state and a 7 day treatment with murine leptin (1 μg/g body mass, twice per day) on fatty acid metabolism in captive white-throated sparrows. Sparrows that were exposed to a long-day migratory photoperiod increased flight muscle FAT/CD36 and H-FABP mRNA by 154% and 589%, respectively, and had 32% higher H-FABP protein than birds kept on a short-day photoperiod that mimicked wintering conditions. Migrants increased activities of flight muscle CPT, CS and HOAD by 57%, 23% and 74%, respectively, and decreased LDH activity by 31%, reflecting an increase in aerobic relative to anaerobic capacity. The expression of fatty acid transporters and the activities of metabolic enzymes in cardiac muscle were unaffected by migratory state. Leptin had no effect on transport proteins or enzymes in either skeletal or cardiac muscle suggesting that other signaling pathways control fatty acid metabolism during migration. These data indicate that photoperiod alone is sufficient to prime flight muscles for migratory flights by promoting enhanced protein-mediated fatty acid transport and oxidation. However, the endocrine controls and other factors underlying these changes remain to be thoroughly investigated.

Caloric restriction in leptin deficiency does not correct myocardial steatosis: failure to normalize PPAR{alpha}/PGC1{alpha} and thermogenic glycerolipid/fatty acid cycling

OBJECTIVE

Evidence supports an antilipotoxic role for leptin in preventing inappropriate peripheral tissue lipid deposition. Obese, leptin-deficient mice develop left ventricular (LV) hypertrophy and myocardial steatosis with increased apoptosis and decreased longevity. Here we investigated the cardiac effects of caloric restriction versus leptin repletion in obese leptin-deficient (ob/ob) mice.

METHODS

Echocardiography was performed on 7 mo old C57BL/6 wild-type mice (WT) and ob/ob mice fed ad libitum, leptin-repleted (LR-ob/ob), or calorie-restricted (CR-ob/ob) for 4 wk. Ventricular tissue was examined by electron microscopy (EM), triglyceride (TAG) content, oil red O staining, mitochondrial coupling assay, and microarray expression profiling.

RESULTS

LR and CR-ob/ob mice showed decreased body and heart weight, and LV wall thickness compared with ad libitum ob/ob mice. LV fractional shortening was decreased in ad libitum ob/ob mice, but restored to WT in LR and CR groups. However, myocardial lipid content by EM and TAG analysis revealed persistent cardiac steatosis in the CR-ob/ob group. Although CR restored mitochondrial coupling to WT levels, PPARα was suppressed and genes associated with oxidative stress and cell death were upregulated in CR-ob/ob animals. In contrast, LR eliminated cardiac steatosis, normalized mitochondrial coupling, and restored PGC1α and PPARα expression, while inducing core genes involved in glycerolipid/free fatty acid (GL/FFA) cycling, a thermogenic pathway that can reduce intracellular lipids.

CONCLUSIONS

Thus, CR in the absence of leptin fails to normalize cardiac steatosis. GL/FFA cycling may be, at least in part, leptin-dependent and a key pathway that protects the heart from lipid accumulation.

Leptin receptor signaling supports cancer cell metabolism through suppression of mitochondrial respiration in vivo

Obesity represents a risk factor for certain types of cancer. Leptin, a hormone predominantly produced by adipocytes, is elevated in the obese state. In the context of breast cancer, leptin derived from local adipocytes is present at high concentrations within the mammary gland. A direct physiological role of peripheral leptin action in the tumor microenvironment in vivo has not yet been examined. Here, we report that mice deficient in the peripheral leptin receptor, while harboring an intact central leptin signaling pathway, develop a fully mature ductal epithelium, a phenomenon not observed in db/db mice to date. In the context of the MMTV-PyMT mammary tumor model, the lack of peripheral leptin receptors attenuated tumor progression and metastasis through a reduction of the ERK1/2 and Jak2/STAT3 pathways. These are tumor cell-autonomous properties, independent of the metabolic state of the host. In the absence of leptin receptor signaling, the metabolic phenotype is less reliant on aerobic glycolysis and displays an enhanced capacity for β-oxidation, in contrast to nontransformed cells. Leptin receptor-free tumor cells display reduced STAT3 tyrosine phosphorylation on residue Y705 but have increased serine phosphorylation on residue S727, consistent with preserved mitochondrial function in the absence of the leptin receptor. Therefore, local leptin action within the mammary gland is a critical mediator, linking obesity and dysfunctional adipose tissue with aggressive tumor growth.

Evolution of inflammation in nonalcoholic fatty liver disease: the multiple parallel hits hypothesis

Whereas in most cases a fatty liver remains free of inflammation, 10%-20% of patients who have fatty liver develop inflammation and fibrosis (nonalcoholic steatohepatitis [NASH]). Inflammation may precede steatosis in certain instances. Therefore, NASH could reflect a disease where inflammation is followed by steatosis. In contrast, NASH subsequent to simple steatosis may be the consequence of a failure of antilipotoxic protection. In both situations, many parallel hits derived from the gut and/or the adipose tissue may promote liver inflammation. Endoplasmic reticulum stress and related signaling networks, (adipo)cytokines, and innate immunity are emerging as central pathways that regulate key features of NASH.

Hepatic steatosis, low-grade chronic inflammation and hormone/growth factor/adipokine imbalance

Non-alcoholic fatty liver disease (NAFLD), a further expression of metabolic syndrome, strictly linked to obesity and diabetes mellitus, is characterized by insulin resistance (IR), elevated serum levels of free fatty acids and fatty infiltration of the liver, which is known as hepatic steatosis. Hepatocyte apoptosis is a key feature of this disease and correlates with its severity. Free-fatty-acid-induced toxicity represents one of mechanisms for the pathogenesis of NAFLD and hormones, growth factors and adipokines influence also play a key role. This review highlights the various pathways that contribute to the development of hepatic steatosis. Circulating concentrations of inflammatory cytokines are reckoned to be the most important factor in causing and maintaining IR. Low-grade chronic inflammation is fundamental in the progression of NAFLD toward higher risk cirrhotic states.

Histamine regulation in glucose and lipid metabolism via histamine receptors: model for nonalcoholic steatohepatitis in mice

Histamine has been proposed to be an important regulator of energy intake and expenditure. The aim of this study was to evaluate histamine regulation of glucose and lipid metabolism and development of nonalcoholic steatohepatitis (NASH) with a hyperlipidemic diet. Histamine regulation of glucose and lipid metabolism, adipocytokine production, and development of hyperlipidemia-induced hepatic injury were studied in histamine H1 (H1R(-/-)) and H2 (H2R(-/-)) receptor knockout and wild-type mice. H1R(-/-) mice showed mildly increased insulin resistance. In contrast, H2R(-/-) mice manifested profound insulin resistance and glucose intolerance. High-fat/high-cholesterol feeding enhanced insulin resistance and glucose intolerance. Studies with two-deoxy-2-[(18)F]-fluoro-d-glucose and positron emission tomography showed a brain glucose allocation in H1R(-/-) mice. In addition, severe NASH with hypoadiponectinemia as well as hepatic triglyceride and free cholesterol accumulation and increased blood hepatic enzymes were observed in H2R(-/-) mice. H1R(-/-) mice showed an obese phenotype with visceral adiposity, hyperleptinemia, and less severe hepatic steatosis and inflammation with increased hepatic triglyceride. These data suggest that H1R and H2R signaling may regulate glucose and lipid metabolism and development of hyperlipidemia-induced NASH.

Utility of transplantation in studying adipocyte biogenesis and function

Adipose tissue plays important roles in the regulation of energy homeostasis and metabolism. Two features distinguish adipose tissue from other organs--the ability to greatly expand its mass, via increases in cell size and/or number, and the wide anatomical distribution. While adipose tissue function is greatly affected by adipocyte size and anatomic location, regulations of adipocyte size, number, and body fat distribution are poorly understood. Transplantation of either mature adipose tissue or adipocyte progenitor cells has been used in studying adipocyte function and biogenesis. In this review, we will attempt to summarize methodological considerations for transplantation, including selections of donor material, transplantation site and the length of transplantation study, as well as effects of these factors and vascularization and innervation on the function of transplants. Specific studies are also reviewed to illustrate the utility of adipose tissue transplants in studying adipose tissue function and biogenesis. The focus is on studies in three areas: (1) use of transplants in demonstrating adipose tissue function, such as effects of adipose tissue transplants on metabolism and energy homeostasis of the recipient animals and depot-specific differences in adipose tissue function; (2) use of transplantation to dissect direct or cell-autonomous from indirect or non-cell-autonomous effects of leptin signaling and sex on adipocyte size; (3) use of transplantation in the identification of adipocyte progenitor cells and lineage analysis. Finally, future applications of transplantation in studying depot-specific adipocyte biogenesis, and genetic and hormonal effects of sex and age on adipocyte biogenesis and function are discussed.

Role of uncoupling proteins in cancer

Uncoupling proteins (UCPs) are a family of inner mitochondrial membrane proteins whose function is to allow the re-entry of protons to the mitochondrial matrix, by dissipating the proton gradient and, subsequently, decreasing membrane potential and production of reactive oxygen species (ROS). Due to their pivotal role in the intersection between energy efficiency and oxidative stress, UCPs are being investigated for a potential role in cancer. In this review we compile the latest evidence showing a link between uncoupling and the carcinogenic process, paying special attention to their involvement in cancer initiation, progression and drug chemoresistance.

The silencing mediator of retinoid and thyroid hormone receptors (SMRT) regulates adipose tissue accumulation and adipocyte insulin sensitivity in vivo

The silencing mediator of retinoid and thyroid hormone receptors (SMRT) serves as a corepressor for nuclear receptors and other factors. Recent evidence suggests that SMRT is an important regulator of metabolism, but its role in adipocyte function in vivo remains unclear. We generated heterozygous SMRT knock-out (SMRT(+/-)) mice to investigate the function of SMRT in the adipocyte and the regulation of adipocyte insulin sensitivity. We show that SMRT(+/-) mice are normal weight on a regular diet, but develop increased adiposity on a high-fat diet (HFD). The mechanisms underlying this phenotype are complex, but appear to be due to a combination of an increased number of smaller subcutaneous adipocytes as well as decreased leptin expression, resulting in greater caloric intake. In addition, adipogenesis of mouse embryonic fibroblasts (MEFs) derived from these mice was increased. However, adipocyte insulin sensitivity, measured by insulin-induced Akt phosphorylation and insulin-mediated suppression of lipolysis, was enhanced in SMRT(+/-) adipocytes. These finding suggest that SMRT regulates leptin expression and limits the ability of fat mass to expand with increased caloric intake, but that SMRT also negatively regulates adipocyte insulin sensitivity.

Regulation of lipid metabolism by energy availability: a role for the central nervous system

The central nervous system (CNS) is crucial in the regulation of energy homeostasis. Many neuroanatomical studies have shown that the white adipose tissue (WAT) is innervated by the sympathetic nervous system, which plays a critical role in adipocyte lipid metabolism. Therefore, there are currently numerous reports indicating that signals from the CNS control the amount of fat by modulating the storage or oxidation of fatty acids. Importantly, some CNS pathways regulate adipocyte metabolism independently of food intake, suggesting that some signals possess alternative mechanisms to regulate energy homeostasis. In this review, we mainly focus on how neuronal circuits within the hypothalamus, such as leptin- ghrelin-and resistin-responsive neurons, as well as melanocortins, neuropeptide Y, and the cannabinoid system exert their actions on lipid metabolism in peripheral tissues such as WAT, liver or muscle. Dissecting the complicated interactions between peripheral signals and neuronal circuits regulating lipid metabolism might open new avenues for the development of new therapies preventing and treating obesity and its associated cardiometabolic sequelae.

Nutritional and hormonal regulation of uncoupling protein 2

Uncoupling proteins (UCPs) belong to a family of mitochondrial carrier proteins that are present in the mitochondrial inner membrane. Genetic and experimental studies have shown that UCP dysfunction can be involved in metabolic disorders and in obesity. Uncoupling protein-1 (UCP1; also known as thermogenin) was identified in 1988 and found to be highly expressed in brown adipose tissue. UCP1 allows the leak of protons in respiring mitochondria, dissipating the energy as heat; the enzyme has an important role in nonshivering heat production induced by cold exposure or food intake. In 1997, two homologs of UCP1 were identified and named UCP2 and UCP3. These novel proteins also lower mitochondrial membrane potential, but whether they can dissipate metabolic energy as heat as efficiently as UCP1 is open to dispute. Even after a decade of study, the physiological roles of these novel proteins have still not been completely elucidated. This review aims to shed light on the nutritional and hormonal regulation of UCP2 and on its physiological roles.

An intrinsic gut leptin-melanocortin pathway modulates intestinal microsomal triglyceride transfer protein and lipid absorption

Fat is delivered to tissues by apoB-containing lipoproteins synthesized in the liver and intestine with the help of an intracellular chaperone, microsomal triglyceride transfer protein (MTP). Leptin, a hormone secreted by adipose tissue, acts in the brain and on peripheral tissues to regulate fat storage and metabolism. Our aim was to identify the role of leptin signaling in MTP regulation and lipid absorption using several mouse models deficient in leptin receptor (LEPR) signaling and downstream effectors. Mice with spontaneous LEPR B mutations or targeted ablation of LEPR B in proopiomelanocortin (POMC) or agouti gene related peptide (AGRP) expressing cells had increased triglyceride in plasma, liver, and intestine. Furthermore, melanocortin 4 receptor (MC4R) knockout mice expressed a similar triglyceride phenotype, suggesting that leptin might regulate intestinal MTP expression through the melanocortin pathway. Mechanistic studies revealed that the accumulation of triglyceride in the intestine might be secondary to decreased expression of MTP and lipid absorption in these mice. Surgical and chemical blockade of vagal efferent outflow to the intestine in wild-type mice failed to alter the triglyceride phenotype, demonstrating that central neural control mechanisms were likely not involved in the observed regulation of intestinal MTP. Instead, we found that enterocytes express LEPR, POMC, AGRP, and MC4R. We propose that a peripheral, local gut signaling mechanism involving LEPR B and MC4R regulates intestinal MTP and controls intestinal lipid absorption.

Diabetes and apoptosis: lipotoxicity

Obesity is an established risk factor in the pathogenesis of insulin resistance, type 2 diabetes mellitus and cardiovascular disease; all components that are part of the metabolic syndrome. Traditionally, insulin resistance has been defined in a glucocentric perspective. However, elevated systemic levels of fatty acids are now considered significant contributors towards the pathophysiological aspects associated with the syndrome. An overaccumulation of unoxidized long-chain fatty acids can saturate the storage capacity of adipose tissue, resulting in a lipid 'spill over' to non-adipose tissues, such as the liver, muscle, heart, and pancreatic-islets. Under these circumstances, such ectopic lipid deposition can have deleterious effects. The excess lipids are driven into alternative non-oxidative pathways, which result in the formation of reactive lipid moieties that promote metabolically relevant cellular dysfunction (lipotoxicity) and programmed cell-death (lipoapoptosis). Here, we focus on how both of these processes affect metabolically significant cell-types and highlight how lipotoxicity and sequential lipoapoptosis are as major mediators of insulin resistance, diabetes and cardiovascular disease.

Uncoupling protein-2 negatively regulates polymorphonuclear leukocytes chemotaxis via modulating [Ca2+] influx

OBJECTIVE

Previous studies demonstrated that uncoupling protein 2 (UCP2) plays a negative role in modulating leukocyte inflammatory responses. The mechanism underneath the role of UCP2 in modulating leukocyte inflammatory responses, however, is incompletely understood. Here, we investigated the effect of UCP2 in polymorphonuclear leukocyte (PMN) chemotaxis.

METHODS AND RESULTS

First, we assessed PMN chemotaxis in zymosan-induced murine peritonitis and found that UCP2(-/-) mice had significantly more migrated PMN in peritoneal lavage compared to their wild-type littermates. In vitro transmigration assays using isolated PMN also showed that PMN from UCP2(-/-) mice migrated faster than those from wild-type mice in response to N-formyl-methionyl-leucyl-phenylalanine (fMLP). Second, in supporting an inhibitory role of UCP2 in PMN transmigration, migrated PMN had a decreased UCP2 expression compared to nonmigrated PMN. In contrast, in streptozotocin-induced diabetic mice in which UCP2 expression was enhanced, PMN chemotaxis was reduced. Third, comparing to UCP2(+/+) PMN, UCP2(-/-) PMN had a stronger upregulation of fMLP-induced surface CD11b/CD18 and CD11a/CD18. Finally, UCP2(-/-) PMN showed a quicker and larger fMLP-triggered intracellular calcium mobilization compared to UCP2(+/+) PMN.

CONCLUSIONS

Our study demonstrates that UCP2 serves as a brake in controlling PMN chemotaxis and that the effect of UCP2 on PMN chemotaxis may be through modulating calcium influx.

Lipid homeostasis, lipotoxicity and the metabolic syndrome

In the 20th century industrialized nations have become afflicted with an unprecedented pandemic of increased adiposity. In the United States, the epicenter of the epidemic, over 2/3 of the population, is overweight and 1 of every 6 Americans carries the diagnosis of metabolic syndrome. Although genes determine susceptibility to environmental factors, the epidemic is clearly due to increased consumption of calorie-dense, highly lipogenic foods, coupled with a marked decrease in physical exertion resulting from modern technologies. If this lifestyle continues, morbid consequences are virtually inevitable. They include type II diabetes and a cluster of disorders known as "the metabolic syndrome" usually appearing in middle age. The morbid consequences of the chronic caloric surplus are buffered before middle age by the partitioning of these calories as fat in the adipocyte compartment which is specifically designed to store triglycerides. Leptin has been proposed as the major hormonal regulator of the partitioning of surplus calories. However, multiple factors can determine the storage capacity of the fat tissue and when it is exceeded ectopic lipid deposition begins. The organs affected in metabolic syndrome include skeletal muscle, liver, heart and pancreas, which are now known to contain abnormal levels of triglycerides. While neutral fat is probably harmless, it is an index of ectopic lipid overload. Fatty acid derivatives can interfere with the function of the cell and ultimately lead to its demise through lipoapoptosis, the consequences of which are gradual organ failure.

Fat and water 1H MRI to investigate effects of leptin in obese mice

Leptin is known to be associated with regulation of body weight and fat content. The effects of exogenous leptin on abdominal visceral (VS) and subcutaneous (SC) fat volume and hepatic fat-to-water ratio in leptin-deficient obese mice were investigated by (1)H magnetic resonance imaging (MRI). Chemical shift-selected fat and water (1)H MRI of control and leptin-treated mice were obtained 1 day before treatment and after 7 days of treatment (0.3 mg/kg/day). Hepatic fat-to-water ratio and VS fat volume decreased significantly with treatment, whereas SC fat volume did not change. Noninvasive measurement of fat and water content in different body regions using MRI should prove useful for evaluating new drugs for the treatment of obesity and other metabolic disorders.

Isomer-specific effects of CLA on gene expression in human adipose tissue depending on PPARgamma2 P12A polymorphism: a double blind, randomized, controlled cross-over study

BACKGROUND

Peroxisome proliferator-activated receptor (PPAR)gamma is a key regulator in adipose tissue. The rare variant Pro12Ala of PPARgamma2 is associated with a decreased risk of insulin resistance. Being dietary PPARgamma ligands, conjugated linoleic acids (CLAs) received considerable attention because of their effects on body composition, cancer, atherosclerosis, diabetes, obesity and inflammation, although some effects were only demonstrated in animal trials and the results in human studies were not always consistent. In the present study effects of CLA supplementation on genome wide gene expression in adipose tissue biopsies from 11 Ala12Ala and 23 Pro12Pro men were investigated. Subjects underwent four intervention periods (4 wk) in a randomized double blind cross-over design receiving 4.25 g/d of either cis-9, trans-11 CLA, trans-10,cis-12 CLA, 1:1 mixture of both isomers or a reference linoleic acid oil preparation. After each intervention biopsies were taken, whole genome expression microarrays were applied, and genes of interest were verified by realtime PCR.

RESULTS

The following genes of lipid metabolism were regulated by CLA: LDLR, FASN, SCD, FADS1 and UCP2 were induced, while ABCA1, CD36 and CA3 were repressed. Transcription factors PPARgamma, NFAT5, CREB5 and EBF1, the adipokine NAMPT, members of the insulin signaling cascade SORBS1 and IGF1 and IL6ST were repressed, while the adipokine THBS1 and GLUT4 involved in insulin signaling were induced. Compared to trans-10,cis-12 CLA and the CLA mixture the cis-9, trans-11 CLA isomer exerted weaker effects. Only CD36 (-1.2 fold) and THBS1 (1.5 fold) were regulated. The CLA effect on expression of PPARgamma and leptin genes depends on the PPARgamma2 genotype.

CONCLUSION

The data suggest that the isomer specific influence of CLA on glucose and lipid metabolism is genotype dependent and at least in part mediated by PPARgamma.

TRIAL REGISTRATION

http://www.controlled-trials.com: ISRCTN91188075.

Impact of plasmatic lipids in glycemic control and its influence in the cardiometabolic risk in morbidly obese subjects

OBJECTIVES: To evaluate whether biochemical parameters are associated with a good glycemic control and to identify the occurrence of cardiometabolic risk variables. MATERIAL AND METHODS: One hundred forty Brazilians were evaluated. The subjects were characterized with regard to glycemic control as good, fair and poor and were divided into tertiles by TG and HbA1c. We use the ROC curve to determine which variables were predicted of poor glycemic control and the factor analyses to identify the domains that segregated among the risk variables. RESULTS: Fasting glucose and insulin levels, TG level, VLDL-C and HOMA-IR increased significantly across HbA1c tertiles. The best marker for identification of poor glycemic control was triglycerides. The presence of cardiometabolic abnormalities did not alter the glycemic control, but HOMA-IR was significantly higher in subjects with abnormalities. CONCLUSION: The use of TG levels offers a reasonable degree of clinical utility. In morbidly obese subjects insulin resistance is associated with individual cardiometabolic factors.

Adipokines as regulators of muscle metabolism and insulin sensitivity

Skeletal muscle is the largest tissue responsible for the insulin-stimulated disposal of glucose. However, identifying the link between excess body fat and impaired insulin sensitivity in skeletal muscle has been difficult. Several adipose-derived cytokines (adipokines) have been implicated in the impairment of insulin sensitivity, while adipokines such as leptin and adiponectin exert an insulin-sensitizing effect. Leptin and adiponectin have each been shown to increase fatty acid (FA) oxidation and decrease triglyceride storage in muscle, which may explain, in part, the insulin-sensitizing effect of these cytokines. Recent evidence strongly implicates an increased localization of the FA transporters to the plasma membrane (PM) as an important factor in the accumulation of intramuscular lipids with high-fat diets and obesity. Perhaps surprisingly, relatively little attention has been paid to the ability of insulin-sensitizing compounds, such as leptin and adiponectin, to decrease the abundance of FA transporters in the PM, thereby decreasing lipid accumulation. In the case of both adipokines, there is also evidence that a resistance to their ability to stimulate FA oxidation in skeletal muscle develops during obesity. One of our recent studies indicates that this development can be very rapid (i.e., within days), and precedes the increase in lipid uptake and accumulation that leads to insulin resistance. It is noteworthy that leptin resistance can be modulated by both diet and training in rodents. Further studies examining the underlying mechanisms of the development of leptin and adiponectin resistance are warranted.

Preadipocyte transplantation: an in vivo study of direct leptin signaling on adipocyte morphogenesis and cell size

Leptin has profound effects on adipose tissue metabolism. However, it remains unclear whether direct leptin signaling in adipocytes is involved. We addressed this question by transplanting inguinal adipose tissue stromal vascular cells (SVCs) from 4- to 5-wk-old wild-type (WT) and leptin receptor-deficient [Lepr(db/db) (db)] mice to inguinal and sternal subcutaneous sites in Ncr nude mice. Both WT and db SVCs gave rise to mature adipocytes with normal morphologies 3 mo after the transplantation. The average adipocyte size (microm(2)/cell) was not significantly different between WT and db transplants at either the inguinal (1,630 +/- 103 vs. 1,491 +/- 74) or the sternal site (1,788 +/- 107 vs. 1,596 +/- 92). Expression levels of beta(3)-adrenergic receptor, a major mediator of lipid mobilization, were indistinguishable between WT and db transplants and similar to those of the hosts. Additionally, adipocyte sizes of inguinal transplants and endogenous inguinal adipose tissues were closely correlated (beta = 0.76, P < 0.001), suggesting that the metabolic milieu of host mice has significant effects on adipocyte size of the transplants. Contrary to the indifference to donor's Lepr genotype, adipocyte size of the transplants was significantly affected by the donor's sex in a leptin receptor-dependent manner. In WT transplants, female SVCs gave rise to smaller adipocytes than male SVCs (1,358 +/- 127 vs. 2,133 +/- 171, P < 0.05). However, this sex difference was not significant in db transplants (1,537 +/- 121 vs. 1,655 +/- 140, P = 0.22). These data suggest that: 1) long-form receptor-mediated direct leptin signaling has no significant cell-autonomous effect on adipocyte differentiation and metabolism in adult mice, 2) sex may affect adipocyte metabolism via genetic and/or epigenetic programming, and 3) leptin may potentiate sexual dimorphism in adipocyte metabolism.

Chronic leptin treatment stimulates lipid oxidation in immortalized and primary mouse skeletal muscle cells

Leptin administration enhances lipid oxidation in skeletal muscle. Nevertheless, direct and chronic effect of leptin has not been well characterized. Here, we measured the effect of leptin on skeletal muscles and their signaling pathways using differentiated C(2)C(12) myotubes and primary myotube cultures. Differentiated myotubes expressed both the short and long forms of leptin receptors. Leptin increased lipid oxidation in myotubes in a concentration- and time-dependent manner, with significant induction of lipid oxidation occurring after 6 h. Actinomycin D completely blocked leptin-induced lipid oxidation. Leptin significantly increased phosphorylation of JAK2 and STAT3 in myotubes, and leptin-induced lipid oxidation was abolished by treatment with a JAK2 inhibitor or STAT3 siRNA. We then used mouse myotubes to measure these effects under physiological conditions. Leptin increased lipid oxidation, which again was blocked by a JAK2 inhibitor and STAT3 siRNA. These results suggest that the JAK2/STAT3 signaling pathway may underlie the chronic effects of leptin on lipid oxidation in skeletal muscles.

The effect of amino-acid infusion during off-pump coronary arterial bypass surgery on thermogenic and hormonal regulation

PURPOSE

Amino-acid (AA) infusions promote thermogenesis and prevent perioperative hypothermia, but the mechanism of action is unknown. We sought to verify the hypothesis that AA infusions stimulate the release of metabolic hormones during surgery and increase energy expenditure, resulting in thermogenesis.

METHODS

Twenty-four patients were randomly assigned to receive AA (4 kJ x kg(-1) x h(-1)) or saline, which was infused for 2 h during off-pump coronary artery bypass surgery (OPCABS). Arterial adrenaline, thyroid hormone, insulin, and leptin levels were determined at five defined times during surgery. Oxygen consumption was measured 3 h after the start of infusion.

RESULTS

AA infusion maintained the body core temperature during OPCABS. This effect was accompanied by an increase in oxygen consumption, which depended on increased heart rate. AA infusion prominently stimulated the secretion of insulin and leptin; the insulin level increased rapidly within 2 h after the start of infusion, whereas leptin levels increased gradually over a 6-h period after the start of infusion.

CONCLUSION

AA infusion significantly increased body core temperature and oxygen consumption during surgery. Given the release of insulin and leptin in response to AA infusion, it is likely that these hormonal signaling pathways may, in part, have contributed to the thermogenic response that occurred during the surgery.

Different physiological roles of serum leptin in the regulation of energy intake and thermogenesis between pregnancy and lactation in primiparous Brandt's voles (Lasiopodomys brandtii)

Reproduction, especially lactation, is associated with major metabolic adaptive changes. In this study, we investigated the metabolic changes and the roles of leptin during different periods of reproduction in primiparous Brandt's voles (Lasiopodomys brandtii). Energy intake, thermogenic capacity and serum leptin levels were examined in non-reproductive, mid pregnant, late pregnant, early lactating and peak lactating voles. Voles increased body mass by nearly 70% during late pregnancy compared to the non-breeding controls. The increase in body mass was mainly due to the increase in body fat mass which increased by 56%, and the growth of the reproductive tissues and digestive organs. Lactating voles decreased body fat by nearly 27% at peak lactation compared to the controls, and 53% compared to late pregnant voles. At the same time they increased food intake significantly. Uncoupling protein 1 (UCP1) content in brown adipose tissue (BAT) decreased significantly at peak lactation. Serum leptin increased significantly in the mid pregnancy, at a time when there was no increase in body fat, and remained at this high level in late pregnancy. Leptin levels decreased after parturition and reached a nadir at peak lactation. Serum leptin was negatively correlated with energy intake during lactation, but not during pregnancy. These data suggest that Brandt's voles adjust energy intake, thermogenic capacity and body reserves to match the high energy demands for reproduction. Hyperleptinemia, without decreased energy intake suggests a state of leptin resistance during pregnancy, and hypoleptinemia during lactation might act as a signal to stimulate energy intake.

Mice deficient in mitochondrial glycerol-3-phosphate acyltransferase-1 have diminished myocardial triacylglycerol accumulation during lipogenic diet and altered phospholipid fatty acid composition

Glycerol-3-phosphate acyltransferase-1 (GPAT1), which is located on the outer mitochondrial membrane comprises up to 30% of total GPAT activity in the heart. It is one of at least four mammalian GPAT isoforms known to catalyze the initial, committed, and rate-limiting step of glycerolipid synthesis. Because excess triacylglycerol (TAG) accumulates in cardiomyocytes in obesity and type 2 diabetes, we determined whether lack of GPAT1 would alter the synthesis of heart TAG and phospholipids after a 2-week high-sucrose diet or a 3-month high-fat diet. Even in the absence of hypertriglyceridemia, TAG increased 2-fold with both diets in hearts from wildtype mice. In contrast, hearts from Gpat1(-/-) mice contained 20-80% less TAG than the wildtype controls. In addition, hearts from Gpat1(-/-) mice fed the high-sucrose diet incorporate 60% less [(14)C]palmitate into heart TAG as compared to wildtype mice. Because GPAT1 prefers 16:0-CoA to other long-chain acyl-CoA substrates, we determined the fatty acid composition of heart phospholipids. Compared to wildtype littermate controls, hearts from Gpat1(-/-)(-/-) mice contained a lower amount of 16:0 in phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine/phosphatidylinositol and significantly more C20:4n6. Phosphatidylcholine and phosphatidylethanolamine from Gpat1(-/-)(-/-) hearts also contained higher amounts of 18:0 and 18:1. Although at least three other GPAT isoforms are expressed in the heart, our data suggest that GPAT1 contributes significantly to cardiomyocyte TAG synthesis during lipogenic or high-fat diets and influences the incorporation of 20:4n6 into heart phospholipids.