Lack of skeletal muscle uncoupling protein 2 and 3 mRNA induction during fasting in type-2 diabetic subjects

Skeletal Muscle Uncoupling Proteins in Mice Models of Obesity

Obesity and accompanying type 2 diabetes are among major and increasing worldwide problems that occur fundamentally due to excessive energy intake during its expenditure. Endotherms continuously consume a certain amount of energy to maintain core body temperature via thermogenic processes, mainly in brown adipose tissue and skeletal muscle. Skeletal muscle glucose utilization and heat production are significant and directly linked to body glucose homeostasis at rest, and especially during physical activity. However, this glucose balance is impaired in diabetic and obese states in humans and mice, and manifests as glucose resistance and altered muscle cell metabolism. Uncoupling proteins have a significant role in converting electrochemical energy into thermal energy without ATP generation. Different homologs of uncoupling proteins were identified, and their roles were linked to antioxidative activity and boosting glucose and lipid metabolism. From this perspective, uncoupling proteins were studied in correlation to the pathogenesis of diabetes and obesity and their possible treatments. Mice were extensively used as model organisms to study the physiology and pathophysiology of energy homeostasis. However, we should be aware of interstrain differences in mice models of obesity regarding thermogenesis and insulin resistance in skeletal muscles. Therefore, in this review, we gathered up-to-date knowledge on skeletal muscle uncoupling proteins and their effect on insulin sensitivity in mouse models of obesity and diabetes.

Transcription of the human uncoupling protein 3 gene is governed by a complex interplay between the promoter and intronic sequences

AIMS/HYPOTHESIS

Uncoupling protein (UCP) 3 is an inner mitochondrial membrane transporter mainly produced in skeletal muscle in humans. UCP3 plays a role in fatty acid metabolism and energy homeostasis and modulates insulin sensitivity. In humans, UCP3 content is higher in fast-twitch glycolytic muscle than in slow-twitch oxidative muscle and is dysregulated in type 2 diabetes. Here, we studied the molecular mechanisms determining human UCP3 levels in skeletal muscle and their regulation by fasting in transgenic mice.

METHODS

We produced a series of transgenic lines with constructs bearing different putative regulatory regions of the human UCP3 gene, including promoter and intron sequences. UCP3 mRNA and reporter gene expression and activity were measured in different skeletal muscles and tissues.

RESULTS

The profile of expression and the response to fasting and thyroid hormone of human UCP3 mRNA in transgenic mice with 16 kb of the human UCP3 gene were similar to that of the endogenous human gene. Various parts of the UCP3 promoter did not confer expression in transgenic lines. Inclusion of intron 1 resulted in an expression profile in skeletal muscle that was identical to that of human UCP3 mRNA. Further dissection of intron 1 revealed that distinct regions were involved in skeletal muscle expression, distribution among fibre types and response to fasting.

CONCLUSIONS/INTERPRETATION

The control of human UCP3 transcription in skeletal muscle is not solely conferred by the promoter, but depends on several cis-acting elements in intron 1, suggesting a complex interplay between the promoter and intronic sequences.

Genetic variants in the UCP2-UCP3 gene cluster and risk of diabetes in the Women's Health Initiative Observational Study

OBJECTIVE

Mitochondrial uncoupling proteins (UCPs) are involved in body weight regulation and glucose homeostasis. Genetic variants in the UCP2-UCP3 gene cluster, located on chromosome 11q13, may play a significant role in the development of type 2 diabetes.

RESEARCH DESIGN AND METHODS

We conducted a comprehensive assessment of common single nucleotide polymorphisms (SNPs) at the 70-kb UCP2-UCP3 gene cluster in relation to type 2 diabetes risk in a prospective, case-control study nested in the Women's Health Initiative Observational Study, an ethnically diverse cohort of postmenopausal women including Caucasian, African, Hispanic, and Asian American subjects. We genotyped 14 tag SNPs in 1,584 incident type 2 diabetes case and 2,198 control subjects matched by age, ethnicity, clinical center, time of blood draw, and length of follow-up.

RESULTS

We identified a haplotype set (rs591758-rs668514- rs647126-rs1800006, spanning the UCP2-UCP3 intergenic and UCP3 regions) as significantly associated with greater type 2 diabetes risk (nominal P = 0.0011, permutation P = 0.046) in Caucasian women, especially among overweight Caucasians (BMI >25 kg/m(2)) (nominal P = 0.0006, permutation P = 0.032). Compared with the most common haplotype (h1010 as the referent), haplotype h0001 (19.5% in control subjects) had odds ratios of 2.0 (95% CI 1.13-3.37) in Caucasians and 3.8 (1.44-9.93) in Caucasian overweight women. Similar haplotype-type 2 diabetes association was also observed among Hispanic women who were overweight.

CONCLUSIONS

These findings suggest a role of UCP2-UCP3 gene cluster haplotypes in diabetes; in particular, the effects of the high-risk haplotypes were more apparent in overweight Caucasian women. These data warrant further confirmation in future prospective and experimental studies.

Reduced skeletal muscle uncoupling protein-3 content in prediabetic subjects and type 2 diabetic patients: restoration by rosiglitazone treatment

CONTEXT

The mitochondrial uncoupling protein-3 (UCP3) has been implicated in the protection of the mitochondrial matrix against lipid-induced mitochondrial damage. Recent evidence points toward mitochondrial aberrations as a major contributor to the development of insulin resistance and diabetes, and UCP3 is reduced in diabetes.

OBJECTIVE

We compared skeletal muscle UCP3 protein levels in prediabetic subjects [i.e. impaired glucose tolerance (IGT)], diabetic patients, and healthy controls and examined whether rosiglitazone treatment was able to restore UCP3. PATIENTS, DESIGN, INTERVENTION: Ten middle-aged obese men with type 2 diabetes mellitus [age, 61.4 +/- 3.1 yr; body mass index (BMI), 29.8 +/- 2.9 kg/m(2)], nine IGT subjects (age, 59.0 +/- 6.6 yr; BMI, 29.7 +/- 3.0 kg/m(2)), and 10 age- and BMI-matched healthy controls (age, 57.3 +/- 7.4 yr; BMI, 30.1 +/- 3.9 kg/m(2)) participated in this study. After baseline comparisons, diabetic patients received rosiglitazone (2 x 4 mg/d) for 8 wk.

MAIN OUTCOME MEASURES

Muscle biopsies were sampled to determine UCP3 and mitochondrial protein (complex I-V) content.

RESULTS

UCP3 protein content was significantly lower in prediabetic IGT subjects and in diabetic patients compared with healthy controls (39.0 +/- 28.5, 47.2 +/- 24.7, and 72.0 +/- 23.7 arbitrary units, respectively; P < 0.05), whereas the levels of the mitochondrial protein complex I-V were similar between groups. Rosiglitazone treatment for 8 wk significantly increased insulin sensitivity and muscle UCP3 content (from 53.2 +/- 29.9 to 66.3 +/- 30.9 arbitrary units; P < 0.05).

CONCLUSION

We show that UCP3 protein content is reduced in prediabetic subjects and type 2 diabetic patients. Eight weeks of rosiglitazone treatment restores skeletal muscle UCP3 protein in diabetic patients.

Adipose tissue gene expression in obese subjects during low-fat and high-fat hypocaloric diets

AIMS/HYPOTHESIS

Adaptation to energy restriction is associated with changes in gene expression in adipose tissue. However, it is unknown to what extent these changes are dependent on the energy restriction as such or on the macronutrient composition of the diet.

METHODS

We determined the levels of transcripts for 38 genes that are expressed in adipose tissue and encode transcription factors, enzymes, transporters and receptors known to play critical roles in the regulation of adipogenesis, mitochondrial respiration, and lipid and carbohydrate metabolism. Two groups of 25 obese subjects following 10-week hypocaloric diet programmes with either 20-25 or 40-45% of total energy derived from fat were investigated. Levels of mRNA were measured by performing real-time RT-PCR on subcutaneous fat samples obtained from the subjects before and after the diets.

RESULTS

The two groups of subjects lost 7 kg over the duration of the diets. Ten genes were regulated by energy restriction; however, none of the genes showed a significantly different response to the diets. Levels of peroxisome proliferator-activated receptor gamma co-activator 1alpha mRNA were increased, while the expression of the genes encoding leptin, osteonectin, phosphodiesterase 3B, hormone-sensitive lipase, receptor A for natriuretic peptide, fatty acid translocase, lipoprotein lipase, uncoupling protein 2 and peroxisome proliferator-activated receptor gamma was decreased. Clustering analysis revealed new potential coregulation of genes. For example, the expression of the genes encoding the adiponectin receptors may be regulated by liver X receptor alpha.

CONCLUSIONS/INTERPRETATION

In accordance with the comparable loss of fat mass produced by the two diets, this study shows that energy restriction and/or weight loss rather than the ratio of fat: carbohydrate in a low-energy diet is of importance in modifying the expression of genes in the human adipose tissue.

In vivo regulation of SREBP-1c in skeletal muscle: effects of nutritional status, glucose, insulin, and leptin

Sterol regulatory element binding protein-1c (SREBP-1c), a transcription factor that is important for mediating insulin effects on metabolic gene expression in liver during the fasted-to-fed transition, is also expressed in skeletal muscle. However, the regulation and role of SREBP-1c in skeletal muscle are poorly understood. The present study compared the effects of nutritional status, physiological hyperinsulinemic clamps, and adenovirus-mediated hyperleptinemia (HLEP) in rats on expression of SREBP-1c and other metabolic genes in skeletal muscle. Three- and 6-h refeeding of 18-h-fasted animals increased levels of SREBP-1c mRNA and the SREBP-1 protein (full length and mature) in gastrocnemius muscle (P < 0.05). Fatty acid synthase (FAS) and hexokinase II (HKII) mRNA levels were also increased by refeeding, and uncoupling protein 3 (UCP3) mRNA level was decreased (all P < 0.05). Surprisingly, 3-h hyperinsulinemic clamps did not increase gastrocnemius muscle SREBP-1c and FAS mRNA levels or SREBP-1 protein levels but did increase HKII mRNA levels and decrease UCP3 mRNA levels (P < 0.05). HLEP reduced refeeding-induced increases of SREBP-1c and FAS mRNA levels but did not reduce the level of SREBP-1 protein. We conclude that 1) skeletal muscle SREBP-1c gene expression is regulated by nutritional status in a fashion similar to that observed in liver and adipose tissue, 2) physiological hyperinsulinemia is not sufficient to imitate the effects of refeeding on SREBP-1c gene expression, and 3) leptin suppresses refeeding effects on SREBP-1c mRNA levels.

Lifestyle changes and lipid metabolism gene expression and protein content in skeletal muscle of subjects with impaired glucose tolerance

AIMS/HYPOTHESIS

Skeletal muscle of pre-diabetic patients is characterised by a diminished capacity to handle fatty acids. A diminished content of several enzymes involved in fatty-acid transport and oxidation have been suggested to underlie these defects. The aim of this study was to investigate whether the combination of dietary advice, increased physical activity and weight loss improves lipid metabolic gene and protein expression in skeletal muscle of subjects with impaired glucose tolerance.

METHODS

Before and after 1 year of a lifestyle-intervention programme, expression of several genes and proteins involved in lipid metabolism were measured in vastus lateralis muscle biopsies from subjects in the intervention ( n=7) and control group ( n=6).

RESULTS

After 1 year the intervention group had an improved glycaemic control and reduced body fat compared to the control group. Significant differences were observed for acetyl CoA-carboxylase 2 and uncoupling protein 2 expression (ACC2: -16.8+/-12.4% vs +51.5+/-32.3% for the intervention and control group respectively; p<0.05) (UCP2: -26.9+/-10.3% vs +10.5+/-6.2% for the intervention and control group respectively; p<0.05). Change in 3-hydroxyacyl-CoA dehydrogenase protein content tended to be different between groups (+3.2+/-1.1 vs -0.9+/-1.9 U/mg.ww for the intervention and control group, p=0.07).

CONCLUSIONS/INTERPRETATION

Lifestyle changes leading to an improved glycaemic control and reduced adiposity, resulted in a down-regulation of ACC-2 and UCP2 expression and in an increase in HAD protein content, reflecting a better capacity to utilise fatty acids.

UCP2 and UCP3 in muscle controlling body metabolism

The uncoupling protein-1 (UCP1) homologues UCP2 and UCP3 are able to uncouple ATP production from mitochondrial respiration, thereby dissipating energy as heat and affecting energy metabolism efficiency. In contrast to UCP1, which plays an important role in adaptive thermogenesis, UCP2 and UCP3 do not have a primary role in the regulation of energy metabolism. UCP2, which is expressed in a wide variety of tissues, including white adipose tissue,skeletal muscle and tissues of the immune system, has been suggested to affect the production of reactive oxygen species. UCP2 has also been suggested to regulate the [ATP]/[ADP] ratio and was recently shown to influence insulin secretion in the β-cells of the pancreas. UCP3, in contrast, is expressed predominantly in skeletal muscle and has been associated with whole-body energy metabolism. However, the primary function of UCP3 is not the regulation of energy metabolism. For example, fasting, a condition attenuating energy expenditure, upregulates UCP3 expression. Moreover, UCP3-knockout mice have a normal metabolic rate. The exact function of UCP3 therefore remains to be elucidated, but putative roles for UCP3 include involvement in the regulation of ROS, in mitochondrial fatty acid transport and in the regulation of glucose metabolism in skeletal muscle. Whatever the primary function of these novel uncoupling proteins, a secondary effect via uncoupling might allow them to influence (but not to regulate) energy metabolism, which would be consistent with the observations from linkage and association studies. Therefore, UCP2 and UCP3 remain interesting targets for pharmacological upregulation in the treatment of obesity and diabetes.

Uncoupling protein 3 content is decreased in skeletal muscle of patients with type 2 diabetes

Recently, a role for uncoupling protein-3 (UCP3) in carbohydrate metabolism and in type 2 diabetes has been suggested. Mice overexpressing UCP3 in skeletal muscle showed reduced fasting plasma glucose levels, improved glucose tolerance after an oral glucose load, and reduced fasting plasma insulin levels. However, data regarding the expression of UCP3 in patients with type 2 diabetes is inconsistent, and so far, there have been no reports of UCP3 protein content. Here we compared, for the first time, the protein levels of UCP3 in vastus lateralis muscle in 14 male type 2 diabetic patients (age 49.8 +/- 2.1 years; BMI 27.2 +/- 1.2 kg/m(2); mean +/- SE) with 16 male control subjects (age 48.0 +/- 1.9 years; BMI 23.4 +/- 0.6 kg/m(2)). We found that UCP3 protein levels were twice as low in patients with type 2 diabetes compared with control subjects (117 +/- 16 vs. 58 +/- 12 AU; P = 0.007). There was no correlation between UCP3 content and BMI. In conclusion, UCP3 content is lower in type 2 diabetic patients compared with healthy control subjects. These results are consistent with a role for UCP3 in glucose homeostasis and suggest a role for UCP3 in type 2 diabetes.

Triiodothyronine-mediated up-regulation of UCP2 and UCP3 mRNA expression in human skeletal muscle without coordinated induction of mitochondrial respiratory chain genes

Triiodothyronine (T3) increases mitochondrial respiration and promotes the uncoupling between oxygen consumption and ATP synthesis. T3 effect is mediated partly through transcriptional control of genes encoding mitochondrial proteins. We determined the effect of T3 on mRNA levels of uncoupling proteins (UCP) and proteins involved in the biogenesis of the respiratory chain in human skeletal muscle and on UCP2 mRNA expression in adipose tissue. Ten young, healthy males received 75 to 100 5g of T3 per day for 14 days. The increase in plasma-free T3 levels was associated with an increase of resting metabolic rate and a decrease of respiratory quotient. In skeletal muscle, treatment with T3 induced a twofold increase of both UCP2 and UCP3 mRNA levels (p c oxidase subunits 2 and 4, nuclear respiratory factor 1, mitochondrial transcription factor A, and the co-activator PGC1 did not change during the treatment. In adipose tissue, UCP2 mRNA levels increased threefold. The direct effect of T3 on skeletal muscle an d adipose tissue UCP2 and UCP3 mRNA expression was demonstrated in vitro in human primary cultures. Our data show that T3 induces UCP2 and UCP3 mRNA expression in humans. In skeletal muscle, UCP regulation by T3 is not associated with the transcriptional regulation of respiratory chain proteins.