mRNA expression of the long and short forms of uncoupling protein-3 in obese and lean humans

Metabolic signatures of renal cell carcinoma

Clear cell renal cell carcinoma (ccRCC) is characterized by the constitutive up-regulation of the hypoxia inducible factor-1. One of its target enzymes, pyruvate dehydrogenase (PDH) kinase 1 (PDHK1) showed increased protein expression in tumor as compared to patient-matched normal tissues. PDHK1 phosphorylated and inhibited PDH whose enzymatic activity was severely diminished, depriving the TCA cycle of acetylCoA. We and others have shown a decrease in the protein expressions of all respiratory complexes alluding to a compromise in oxidative phosphorylation (OXPHOS). On the contrary, we found that key parameters of OXPHOS, namely ATP biosynthesis and membrane potential were consistently measurable in mitochondria isolated from ccRCC tumor tissues. Interestingly, an endogenous mitochondrial membrane potential (MMP) was evident when ADP was added to mitochondria isolated from ccRCC but not in normal tissues. In addition, the MMP elicited in the presence of ADP by respiratory substrates namely malate/glutamate, succinate, α-ketoglutarate and isocitrate was invariably higher in ccRCC. Two additional hallmarks of ccRCC include a loss of uncoupling protein (UCP)-2 and an increase in UCP-3. Based on our data, we proposed that inhibition of UCP3 by ADP could contribute to the endogenous MMP observed in ccRCC and other cancer cells.

Differential regulation of peroxisome proliferator-activated receptor (PPAR)-alpha1 and truncated PPARalpha2 as an adaptive response to fasting in the control of hepatic peroxisomal fatty acid beta-oxidation in the hibernating mammal

Seasonal obesity and fasting-associated hibernation are the two major metabolic events governing hepatic lipid metabolism in hibernating mammals. In this process, however, the role of the nuclear receptor known as peroxisome proliferator-activated receptor (PPAR)-alpha has not been elucidated yet. Here we show, as in human, that jerboa (Jaculus orientalis) liver expresses both active wild-type PPARalpha (PPARalpha1wt) and truncated PPARalpha forms and that the PPARalpha1wt to truncated PPARalpha2 ratio, which indicates the availability of active PPARalpha1wt, is differentially regulated during fasting-associated hibernation. Functional activation of hepatic jerboa PPARalpha, during prehibernating and hibernating states, was demonstrated by the induction of its target genes, which encode peroxisomal proteins such as acyl-CoA oxidase 1, peroxisomal membrane protein 70, and catalase, accompanied by a concomitant induction of PPARalpha thermogenic coactivator PPARgamma coactivator-1alpha. Interestingly, sustained activation of PPARalpha by its hypolipidemic ligand, ciprofibrate, abrogates the adaptive fasting response of PPARalpha during prehibernation and overinduces its target genes, disrupting the prehibernation fattening process. In striking contrast, during fasting-associated hibernation, jerboas exhibit preferential up-regulation of hepatic peroxisomal fatty acid oxidation instead of the mitochondrial pathway, which is down-regulated. Taken together, our results strongly suggest that PPARalpha is subject to a hibernation-dependent splicing regulation in response to feeding-fasting conditions, which defines the activity of PPARalpha and the activation of its target genes during hibernation bouts of jerboas.

Splicing factor ASF/SF2 and transcription factor PPAR-gamma cooperate to directly regulate transcription of uncoupling protein-3

The different isoforms of the uncoupling protein-3 (UCP3) are expressed in skeletal muscle and are up-regulated by splicing factors. Here, we report that UCP3 alternative splicing (alternative polyadenylation) is regulated by cooperation between the splicing factor ASF/SF2 and the transcription factor PPAR-gamma. We found that ASF/SF2 activates formation of long-form UCP3 (UCP3(L)) by inhibiting a cleavage and polyadenylation signal (AATAAA) located in its final intron that prematurely terminates message elongation. PPAR-gamma activates this process by directly interacting with ASF/SF2, providing the first example of a direct link between a transcription factor and alternative splicing. Activation of ASF/SF2 promotes formation of UCP3(L), whereas loss of ASF/SF2 decreases production of both UCP3(L) and short-form UCP3 (UCP3(S)). We suggest that the relative abundance of ASF/SF2 and PPAR-gamma determines the ratio of UCP3 isoforms.

How frequent is altered gene expression among susceptibility genes to human complex disorders?

It is regularly thought that human complex disorder susceptibility genes show differences in gene expression between normal and pathologic tissues. Thus, differences of transcript amounts could be indicative of complex disorder susceptibility loci and, therefore, be used for the discovery or the validation of human susceptibility genes to complex disorders/traits. Whether human complex disorder susceptibility genes effectively display differences in transcript amounts was tested by meta-analysis of the published literature comparing transcript amounts of well-validated human susceptibility genes to complex traits/disorders. A total of 94 gene-disease associations, which were studied in at least three independent studies and showed strong evidence of positive association, were analyzed. For 23 out of these 94 well-validated gene-disease associations, 120 gene expression studies comparing normal and pathologic human tissues were found. For 60 out of these 120 gene expression studies, the difference of level expression between normal and pathologic human tissues was statistically significant. This result was highly significant, as only 6 significant results were expected randomly under the null hypothesis (P < 10(-112)). A large excess of replication studies were also found, which were in agreement with the original report (P = 6 x 10(-4)). However, the overall level of expression change between normal and pathologic human tissues was relatively moderate, because only 36 (60%) and 19 (31.6%) out of the 62 statistically significant gene expression studies reached 2- or 3-fold changes in expression level, respectively. The present meta-analysis confirms statistical differences of expression levels between normal and pathologic human tissues for human susceptibility genes to complex traits/disorders. However, the levels of differences in transcript amounts appear to be relatively weak. These findings rationalize the use of gene expression for the discovery/validation of human susceptibility genes, but the weak differences of expression typically found should be taken into account for the design of such studies.

Increased adiposity and reduced adipose tissue mRNA expression of uncoupling protein-2 in first-degree relatives of type 2 diabetic patients: evidence for insulin stimulation of UCP-2 and UCP-3 gene expression in adipose tissue

The mitochondrial uncoupling proteins (UCP-2 and UCP-3), which have been suggested to be involved in the development of obesity by controlling the energy expenditure (EE), were studied in 22 healthy first-degree relatives (FDRs) of patients with type 2 diabetes and 13 body mass index (BMI)- and age-matched healthy control subjects. Abdominal subcutaneous adipose tissue biopsies were obtained before and after 150-min hyperinsulinaemic clamp (average serum insulin 250 pM). Basal adipose tissue UCP-2 mRNA levels in the FDR group were significantly lower than that in the control group. After the hyperinsulinaemic clamp, adipose tissue UCP-2 mRNA levels were increased by 32% in the control group (p < 0.05) and 32% in the FDR group (p < 0.05). The basal adipose tissue UCP-3 mRNA level was similar in the two groups and increased in both the groups during hyperinsulinaemia (p < 0.001). Dual energy X-ray absorptiometry showed that despite similar BMI the FDR group had significantly higher fat mass (FM) per cent compared to that of the control group (p < 0.01). The UCP-2 mRNA expression was inversely correlated with the amount of adipose tissue (r = -0.53, p < 0.001), and multiple regression analysis revealed that only the amount of FM was independently correlated with basal UCP-2 mRNA levels, whereas age, gender nor family history of type 2 diabetes contributed independently to the variation in UCP-2 mRNA levels. No differences in EE were observed between the two groups, and no association between EE and UCP mRNA expression was found. In conclusion, we have demonstrated that adipose tissue UCP-2 and UCP-3 mRNA levels are significantly increased during a 150-min hyperinsulinaemic clamp. The UCP-2 mRNA levels were expressed at a significantly lower level FDR to type 2 diabetes compared to control subjects. However, in multiple regression analysis controlling for amount of adipose tissue, the difference between the two groups disappeared. Thus, only the amount of adipose tissue contributed independently to the variation in UCP-2 mRNA expression.

COPD Results in a Reduction in UCP3 Long mRNA and UCP3 Protein Content in Types I and IIa Skeletal Muscle Fibers

PURPOSE

Findings recently have shown coupling protein-3 (UCP3) content to be decreased in the skeletal muscle of patients with chronic obstructive pulmonary disease (COPD). Uncoupling protein-3 mRNA exists as two isoforms: long (UCP3L) and short (UCP3S). The UCP3 protein is expressed the least in oxidative and the most in glycolytic muscle fibers. Levels of UCP3 have been associated positively with intramyocellular triglyceride (IMTG) contents in conditions of altered fatty acid metabolism. As a source for muscle free fatty acid metabolism, IMTG is decreased in COPD. The current study completely characterized all the parameters of UCP3 expression (ie, UCP3L and UCP3S mRNA expression in whole muscle samples) and UCP3 protein content as well as IMTG content in the different fiber types in patients with COPD and healthy control subjects.

METHODS

Using real-time polymerase chain reaction, UCP3 gene expression was quantified. Skeletal muscle fiber type and UCP3 protein and IMTG content were measured using immunofluorescence and Oil red oil staining, respectively.

RESULTS

The findings showed that UCP3L mRNA expression was 44% lower (P < .005) in the patients with COPD than in the control subjects, whereas the UCP3S mRNA content was similar in the two groups. As compared with control subjects, UCP3 protein content was decreased by 89% and 83% and the IMTG content by 64% and 54%, respectively, in types I and IIa fibers (P < .0167) of patients with COPD, whereas they were unchanged in IIx fibers.

CONCLUSIONS

The reduced UCP3 and IMTG content in the more oxidative fibers may be linked to the altered muscle fatty acid metabolism associated with COPD. Further studies are required to determine the exact role and clinical relevance of the reduced UCP3 content in patients with COPD.

Decreased uncoupling protein expression and intramyocytic triglyceride depletion in formerly obese subjects

OBJECTIVE

To examine the muscular uncoupling protein expression 2 (UCP2) and UCP3 gene expression in morbid obese subjects before and after bariatric surgery [bilio-pancreatic diversion (BPD)].

RESEARCH METHODS AND PROCEDURES

Eleven obese subjects (BMI = 49 +/- 2 kg/m(2)) were studied before BPD and 24 months after BPD. Skeletal muscle UCP2 and UCP3 mRNA was measured using reverse transcriptase-competitive polymerase chain reaction and UCP3 protein by Western blotting. Intramyocytic triglycerides were quantified by high-performance liquid chromatography. Twenty-four-hour energy expenditure and respiratory quotient (RQ) were measured in a respiratory chamber.

RESULTS

After BPD, the average weight loss was approximately 38%. Nonprotein RQ was increased in the postobese subjects (0.73 +/- 0.00 vs. 0.83 +/- 0.02, p < 0.001). The intramyocytic triglyceride level dropped (3.66 +/- 0.16 to 1.60 +/- 0.29 mg/100 mg of fresh tissue, p < 0.0001) after BPD. Expression of UCP2 and UCP3 mRNA was significantly reduced (from 35.9 +/- 6.1% to 18.6 +/- 4.5% of cyclophilin, p = 0.02; from 60.2 +/- 14.0% to 33.4 +/- 8.5%, p = 0.03; respectively). UCP3 protein content was also significantly reduced (272.19 +/- 84.13 vs. 175.78 +/- 60.31, AU, p = 0.04). A multiple regression analysis (R(2) = 0.90) showed that IMTG levels (p = 0.007) represented the most powerful independent variable for predicting UCP3 variation.

DISCUSSION

The strong correlation of UCP expression and decrease in IMTG levels suggests that triglyceride content plays an even more important role in the regulation of UCP gene expression than the circulating levels of free fatty acids or the achieved degree of weight loss.

Relationship between plasma free fatty acids and uncoupling protein-3 gene expression in skeletal muscle of obese subjects: in vitro evidence of a causal link

OBJECTIVE

To investigate whether skeletal muscle uncoupling protein-2 (UCP2) and uncoupling protein-3 (UCP3) gene expression is altered in massive obesity and whether it correlates with in vivo insulin sensitivity and with metabolic and hormonal status.

DESIGN

Quantification of UCP2 and UCP3 gene expression in skeletal muscle of obese and lean subjects displaying different degrees of insulin sensitivity.

PATIENTS

Fourteen obese and 10 age- and sex-matched healthy control subjects with a mean body mass index (BMI) of 43.6 +/- 1.4 and 22.8 +/- 1.8 (+/- SEM), respectively.

MEASUREMENTS

Insulin sensitivity by glucose clamp, body composition by bio-impedance, fasting plasma glucose, insulin, leptin and free fatty acids (FFA). Skeletal muscle UCP2 and UCP3 mRNA levels by quantitative reverse transcription polymerase chain reaction (RT-PCR).

RESULTS

No significant differences in UCP2 or UCP3 mRNA levels were found between obese and control subjects. No significant correlation was observed, in both groups, between UCP2 or UCP3 mRNA levels and both anthropometrical and metabolic parameters. In contrast, a highly significant correlation was observed between skeletal muscle UCP3, but not UCP2, mRNA levels and plasma FFA in the obese, but not in the lean, group. Furthermore, exposure of human myocytes to FFA for 24 h strongly induced both UCP3 and peroxisome proliferator-activated receptor-gamma (PPARgamma) but not UCP2 gene expression.

CONCLUSIONS

FFA levels correlate strongly with skeletal muscle UCP3 mRNA levels in obese, but not in lean, subjects; in addition, in human myocytes, high FFA concentrations promote UCP3 expression. Our studies therefore provide evidence that supports a role for increased plasma FFA concentrations in the regulation of human skeletal muscle UCP3 gene expression.

The effect of mild cold exposure on UCP3 mRNA expression and UCP3 protein content in humans

OBJECTIVE

In rodents, adaptive thermogenesis in response to cold exposure and high-fat feeding is accomplished by the activation of the brown adipose tissue specific mitochondrial uncoupling protein, UCP1. The recently discovered human uncoupling protein 3 is a possible candidate for adaptive thermogenesis in humans. In the present study we examined the effect of mild cold exposure on the mRNA and protein expression of UCP3.

SUBJECTS

Ten healthy male volunteers (age 24.4 +/- 1.6 y; height 1.83 +/- 0.02 m; weight 77.3 +/- 3.0 kg; percentage body fat 19 +/- 2).

DESIGN

Subjects stayed twice in the respiration chamber for 60 h (20.00-8.00 h); once at 22 degrees C (72 degrees F), and once at 16 degrees C (61 degrees F). After leaving the respiration chamber, muscle biopsies were taken and RT-competitive-PCR and Western blotting was used to measure UCP3 mRNA and protein expression respectively.

RESULTS

Twenty-four-hour energy expenditure was significantly increased at 16 degrees C compared to 22 degrees C (P<0.05). At 16 degrees C, UCP3T (4.6 +/- 1.0 vs 7.7 +/- 1.5 amol/microg RNA, P=0.07), UCP3L (2.0 +/- 0.5 vs 3.5 +/- 0.9 amol/microg RNA, P=0.1) and UCP3S (2.6 +/- 0.6 vs 4.2 +/- 0.7 amol/microg RNA, P=0.07) mRNA expression tended to be lower compared with at 22 degrees C, whereas UCP3 protein content was, on average, not different. However, the individual differences in UCP3 protein content (16-22 degrees C) correlated positively with the differences in 24 h energy expenditure (r=0.86, P<0.05).

CONCLUSION

The present study suggests that UCP3 protein content is related to energy metabolism in humans and might help in the metabolic adaptation to cold exposure. However, the down-regulation of UCP3 mRNA with mild cold exposure suggests that prolonged cold exposure will lead to lower UCP3 protein content. What the function of such down-regulation of UCP3 could be is presently unknown.

UCP5/BMCP1 transcript isoforms in human skeletal muscle: relationship of the short-insert isoform with lipid oxidation and resting metabolic rates

Uncoupling protein 5 (UCP5) or brain mitochondrial carrier protein-1 (BMCP1) enhances mitochondrial proton leak in vitro and its hepatic and brain expression profiles are modulated by diet and cold exposure in mice. Alternative splicing generates three isoforms: a long form (UCP5L), a short form (UCP5S), and a short form with a 31 amino acid insert (UCP5SI). We investigated the relationship between skeletal muscle UCP5 expression and in vivo energy metabolism in 36 non-diabetic Pima Indians. We determined the expression levels of total UCP5 (UCP5T), and the isoforms UCP5L, UCP5S, and UCP5SI (66.8, 32.5, and 0.8% of UCP5T, respectively). None correlated with body weight or percent body fat. The transcript level of UCP5SI, but not the others, was positively correlated with resting metabolic rate (r=0.38, P=0.02, adjusted for age, sex, fat mass, and fat-free mass) and lipid oxidation rate (adjusted for age, sex, and percent body fat) during a euglycemic clamp with infusion of insulin at a physiologic concentration (r=0.42, P=0.01).

Effect of acute exercise on uncoupling protein 3 is a fat metabolism-mediated effect

Human and rodent uncoupling protein (UCP)3 mRNA is upregulated after acute exercise. Moreover, exercise increases plasma levels of free fatty acid (FFA), which are also known to upregulate UCP3. We investigated whether the upregulation of UCP3 after exercise is an effect of exercise per se or an effect of FFA levels or substrate oxidation. Seven healthy untrained men [age: 22.7 +/- 0.6 yr; body mass index: 23.8 +/- 1.0 kg/m(2); maximal O2 uptake (VO2 max): 3,852 +/- 211 ml/min] exercised at 50% VO2 max for 2 h and then rested for 4 h. Muscle biopsies and blood samples were taken before and immediately after 2 h of exercise and 1 and 4 h in the postexercise period. To modulate plasma FFA levels and fat/glucose oxidation, the experiment was performed two times, one time with glucose ingestion and one time while fasting. UCP3 mRNA and UCP3 protein were determined by RT-competitive PCR and Western blot. In the fasted state, plasma FFA levels significantly increased (P < 0.0001) during exercise (293 +/- 25 vs. 1,050 +/- 127 micromol/l), whereas they were unchanged after glucose ingestion (335 +/- 54 vs. 392 +/- 74 micromol/l). Also, fat oxidation was higher after fasting (P < 0.05), whereas glucose oxidation was higher after glucose ingestion (P < 0.05). In the fasted state, UCP3L mRNA expression was increased significantly (P < 0.05) 4 h after exercise (4.6 +/- 1.2 vs. 9.6 +/- 3.3 amol/microg RNA). This increase in UCP3L mRNA expression was prevented by glucose ingestion. Acute exercise had no effect on UCP3 protein levels. In conclusion, we found that acute exercise had no direct effect on UCP3 mRNA expression. Abolishing the commonly observed increase in plasma FFA levels and/or fatty acid oxidation during and after exercise prevents the upregulation of UCP3 after acute exercise. Therefore, the previously observed increase in UCP3 expression appears to be an effect of prolonged elevation of plasma FFA levels and/or increased fatty acid oxidation.

The regulation of uncoupling protein-2 gene expression by omega-6 polyunsaturated fatty acids in human skeletal muscle cells involves multiple pathways, including the nuclear receptor peroxisome proliferator-activated receptor beta

Fatty acids have been postulated to regulate uncoupling protein (UCP) gene expression in skeletal muscle in vivo. We have identified, at least in part, the mechanism by which polyunsaturated fatty acids increase UCP-2 expression in primary culture of human muscle cells. omega-6 fatty acids and arachidonic acid induced a 3-fold rise in UCP-2 mRNA levels possibly through transcriptional activation. This effect was prevented by indomethacin and mimicked by prostaglandin (PG) E(2) and carbaprostacyclin PGI(2), consistent with a cyclooxygenase-mediated process. Incubation of myotubes for 6 h with 100 micrometer arachidonic acid resulted in a 150-fold increase in PGE(2) and a 15-fold increase in PGI(2) in the culture medium. Consistent with a role of cAMP and protein kinase A, both prostaglandins induced a marked accumulation of cAMP in human myotubes, and forskolin reproduced the effect of arachidonic acid on UCP-2 mRNA expression. Inhibition of protein kinase A with H-89 suppressed the effect of PGE(2), whereas cPGI(2) and arachidonic acid were still able to increase ucp-2 gene expression, suggesting additional mechanisms. We found, however, that the MAP kinase pathway was not involved. Prostaglandins, particularly PGI(2), are potent activators of the peroxisome proliferator-activated receptors. A specific agonist of peroxisome proliferator-activated receptor (PPAR) beta (L165041) increased UCP-2 mRNA levels in myotubes, whereas activation of PPARalpha or PPARgamma was ineffective. These results suggest thus that ucp-2 gene expression is regulated by omega-6 fatty acids in human muscle cells through mechanisms involving at least protein kinase A and the nuclear receptor PPARbeta.

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.

Impact of endotoxin on UCP homolog mRNA abundance, thermoregulation, and mitochondrial proton leak kinetics

Linking tissue uncoupling protein (UCP) homolog abundance with functional metabolic outcomes and with expression of putative genetic regulators promises to better clarify UCP homolog physiological function. A murine endotoxemia model characterized by marked alterations in thermoregulation was employed to examine the association between heat production, UCP homolog expression, and mitochondrial proton leak ("uncoupling"). After intraperitoneal lipopolysaccharide (LPS, approximately 6 mg/kg) injection, colonic temperature (T(c)) in adult female C57BL6/J mice dropped to a nadir of approximately 30 degrees C by 8 h, preceded by a four- to fivefold drop in liver UCP2 and UCP5/brain mitochondrial carrier protein 1 mRNA levels, with no change in their hindlimb skeletal muscle (SKM) expression. SKM UCP3 mRNA rose fivefold during development of hypothermia and was correlated with an LPS-induced increase in plasma free fatty acid concentration. UCP2 and UCP5 transcripts recovered about three- to sixfold in both tissues starting at 6-8 h, preceding a recovery of T(c) between 16 and 24 h. SKM UCP3 followed an opposite pattern. Such results are not consistent with an important influence of UCP3 in driving heat production but do not preclude a role for UCP2 or UCP5 in this process. The transcription coactivator PGC-1 displayed a transient LPS-evoked rise (threefold) or drop (two- to fivefold) in SKM and liver expression, respectively. No differences between control and LPS-treated mouse liver or SKM in vitro mitochondrial proton leak were evident at time points corresponding to large differences in UCP homolog expression.

First evidence of uncoupling protein-2 (UCP-2) and -3 (UCP-3) gene expression in piglet skeletal muscle and adipose tissue

Uncoupling proteins (UCPs) facilitate proton transport inside the mitochondria and decrease the proton gradient, leading to heat production. Until now, the presence of UCP1 or other UCP homologs had not been detected in tissues of pig, a species where evidence for the presence of brown adipose tissue has only been provided in 2-3 month old animals. In the light of the improving knowledge on the UCPs family, we decided to examine both UCP2 and UCP3 mRNA expression in piglet skeletal muscle and adipose tissue. Using RT-PCR we have successfully cloned a partial UCP2 sequence and a complete UCP3 cDNA. UCP3's open reading frame (936bp) shares 90, 89 and 85% similarity with bovine, human and rat UCP3 nucleotide sequences, respectively. In 3-5 day old piglets, these genes are expressed in adipose tissue and in both longissimus thoracis (LT) and rhomboïdeus (RH) muscles, without any effect of muscle metabolic type. This is in good agreement with the measurement of the same membrane potential in mitochondria isolated from both types of muscles. In triiodothyronine-treated piglets, UCP3 mRNA is more expressed in LT than in RH muscle. These genes may be involved in the control of the energy metabolism of the piglet.

Uncoupling protein homologs: emerging views of physiological function

The widespread occurrence of excess weight and related diseases demands that efforts be made to understand energy expenditure from the gene to the whole animal. For some time, it has been understood that mitochondrial oxidation of fuels generates an electrochemical gradient via outward pumping of protons by the electron transport chain. ATP production via F(1)F(0) ATP synthase is then facilitated by the inward flux of protons down the gradient. There is a growing appreciation that a significant portion of the metabolic rate of endotherms is attributable to counteracting "proton leak" (uncoupling), wherein a flux of protons down the electrochemical gradient generates heat independently of ATP production. Proton leak is especially apparent in thermogenic brown adipose tissue, which expresses a tissue-specific uncoupling protein (UCP1). The recent discovery of widely expressed putative UCP1 homologs [UCP2, UCP3, UCP4, UCP5/brain mitochondrial carrier protein-1 (BMCP1)] raised the possibility that innate proton leak and metabolic rate are regulated by UCP1-like proteins. On the basis of current published data, one may not exclude the possibility that UCP homologs influence metabolic rate.

Alternative mitochondrial functions in cell physiopathology: beyond ATP production

It is well known that mitochondria are the main site for ATP generation within most tissues. However, mitochondria also participate in a surprising number of alternative activities, including intracellular Ca2+ regulation, thermogenesis and the control of apoptosis. In addition, mitochondria are the main cellular generators of reactive oxygen species, and may trigger necrotic cell death under conditions of oxidative stress. This review concentrates on these alternative mitochondrial functions, and their role in cell physiopathology.

Properties of the human long and short isoforms of the uncoupling protein-3 expressed in yeast cells

Two splice variants of the human uncoupling protein-3 (UCP3L and UCP3S) are highly expressed in skeletal muscle. The properties of UCP3L and S have been compared to those of UCP1 in a heterologous yeast expression system under the control of the galactose promoter. Both UCP3 isoforms were found to strongly impair the coupling efficiency of respiring cells thus resulting in increased thermogenesis. The uncoupling properties of both UCP3L and S could be clearly demonstrated also in isolated yeast mitochondria both in terms of coupled respiration and in the capacity to polarize the inner membrane in conditions of limited substrate availability. Contrary to what was observed with mitochondria containing UCP1, millimolar GDP and ATP had little if any effect on the uncoupling activity of UCP3. A very marked uncoupling of whole cells and isolated mitochondria was observed at very low expression levels of UCP3S indicating that the short isoform is more active than the long one.

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

Skeletal muscle uncoupling protein 2 and 3 (UCP-2 and UCP-3) mRNA levels are increased during calorie restriction in lean and nondiabetic obese subjects. In this work, we have investigated the effect of a 5-day hypocaloric diet (1,045 kJ/day) on UCP-2 and UCP-3 gene expression in the skeletal muscle of type-2 diabetic obese patients. Before the diet, UCP-2 and UCP-3 mRNA levels were more abundant in diabetic than in nondiabetic subjects. The long (UCP-3(L)) and short (UCP-3(S)) forms of UCP-3 transcripts were expressed at similar levels in nondiabetic subjects, but UCP-3(S) transcripts were twofold more abundant than UCP-3(L) transcripts in the muscle of diabetic patients. Calorie restriction induced a two- to threefold increase in UCP-2 and UCP-3 mRNA levels in nondiabetic patients. No change was observed in type-2 diabetic patients. Variations in plasma nonesterified fatty acid level were positively correlated with changes in skeletal muscle UCP-3(L) (r = 0.6, P < 0.05) and adipose tissue hormone-sensitive lipase (r = 0.9, P < 0.001) mRNA levels. Lack of increase in plasma nonesterified fatty acid level and in hormone-sensitive lipase upregulation in diabetic patients during the diet strengthens the hypothesis that fatty acids are associated with the upregulation of uncoupling proteins during calorie restriction.

Assessment of uncoupling activity of the human uncoupling protein 3 short form and three mutants of the uncoupling protein gene using a yeast heterologous expression system

The human uncoupling protein 3 gene generates two mRNA transcripts, uncoupling protein 3L and uncoupling protein 3S, which are predicted to encode long and short forms of the uncoupling protein 3 protein, respectively. While uncoupling protein 3L is similar in length to the other known uncoupling proteins, uncoupling protein 3S lacks the last 37 C-terminal residues. A splice site mutation in the human uncoupling protein 3 gene, resulting in the exclusive expression of uncoupling protein 3S, and a number of point mutations in the uncoupling protein 3 gene have been described. This study compares the biochemical activity of uncoupling protein 3S as well as three mutants of the uncoupling protein 3 gene (V9M, V102I, R282C) with that of uncoupling protein 3L utilizing a yeast expression system. All proteins were expressed at similar levels and had qualitatively similar effects on parameters related to the uncoupling function. Both uncoupling protein 3S and uncoupling protein 3L decreased the yeast growth rate by 35 and 52%, increased the whole yeast basal O2 consumption by 26 and 48%, respectively, and decreased the mitochondrial membrane potential as measured in whole yeast by uptake of the fluorescent potential-sensitive dye 3'3-dihexyloxacarbocyanine iodide. In isolated mitochondria, uncoupling protein 3S and uncoupling protein 3L caused a similar (33 and 35%, respectively) increase in state 4 respiration, which was relatively small compared to uncoupling protein 1 (102% increase). A truncated version of uncoupling protein 3S, lacking the last three C-terminal residues, Tyr, Lys and Gly, that are part of a carrier motif that is highly conserved among all mitochondrial carriers, had a greatly reduced uncoupling activity. The two naturally occurring uncoupling protein 3 mutants, V9M and V102I, were similar to uncoupling protein 3L with respect to effects on the yeast growth and whole yeast O2 consumption. The R282C mutant had a reduced effect compared to uncoupling protein 3L. In summary, uncoupling protein 3S and the three mutants of uncoupling protein 3 appear to be functional proteins with biochemical activities similar to uncoupling protein 3L, although uncoupling protein 3S and the R282C mutant have a modestly reduced function.

Effects of obesity and stable weight reduction on UCP2 and UCP3 gene expression in humans

OBJECTIVES

The molecular determinants of energy expenditure are presently unknown. Recently, two uncoupling protein homologues, UCP2 and UCP3, have been identified. UCP2 is expressed widely, and UCP3 is expressed abundantly in skeletal muscle. Both could be important regulators of energy balance. In this paper, we investigated whether altered UCP2 and UCP3 mRNA levels are associated with obesity or weight reduction.

RESEARCH METHODS AND PROCEDURES

UCP2, UCP3 long and short mRNA levels were examined in skeletal muscle and in white adipose tissue of lean, obese, and weight-reduced individuals by RNase protection assay.

RESULTS

Expression of UCP2, UCP3S, and UCP3L mRNA in skeletal muscle was similar in lean individuals and in individuals with obesity at stable weight. In contrast, UCP3L and UCP3S mRNAs were decreased by 38% (p<0.0059) and 48% (p<0.0047), respectively, in 20% weight-reduced patients with obesity at stable weight. In contrast, UCP2 mRNA levels were increased by 30% in skeletal muscle of 20% weight-reduced subjects with obesity. In a different set of patients, mostly lean, UCP3L mRNA in skeletal muscle was decreased by 28% (p = 0.0425) after 10% weight reduction at stable weight. Expression of UCP2 mRNA in subcutaneous adipose tissue was similar in lean individuals and in individuals with obesity, and was increased by 58% during active weight loss.

DISCUSSION

Stabilization at reduced body weight in humans is associated with a decrease in UCP3 mRNA in muscle. It is possible that reduced UCP3 expression could contribute to decreased energy expenditure in weight-stable, weight-reduced individuals.