Uncoupling protein-3 gene expression: reduced skeletal muscle mRNA in obese humans during pronounced weight loss

Abstract

Obesity, the most common nutritional problem in Western societies, is caused by a sustained imbalance of food intake and energy expenditure. A long-term reduction in energy dissipation could therefore be a predisposing factor for this disorder. Indeed, a decrease in the resting metabolic rate is a risk factor for obesity [1,2] and genetic factors appreciably contribute to the magnitude of the basal metabolic rate [3,4]. Brown adipose tissue (BAT) is specialized for thermogenesis, a chief determinant of energy expenditure [5]. Central to BAT-function is the uncoupling protein-1 (UCP1) which stimulates heat production by uncoupling substrate oxidation from ATP synthesis [6]. Although the importance of BAT as a regulator of body fat stores has been established in rodents [7,8], its role in human obesity is not clear because adult humans possess very little BAT [9]. Notwithstanding this qualification, considerably less mRNA Diabetologia (1999) Abstract Aims: Uncoupling protein-3 is a member of a protein family that serves to dissipate energy in the form of heat thereby modulating energy expenditure. Alternative processing of uncoupling protein-3 transcripts results in two mRNA species that encode a large and small protein, perhaps differing in functional activity. Since obesity is associated with disrupted energy homeostasis, we measured muscle mRNA expression in morbidly obese and lean subjects. Methods:The two uncoupling protein-3 mRNA species were quantified in muscle tissue using an RNase protection assay. Gene locus effects on mRNA expression were studied by quantitative allele-specific primer extension. Results: In both obese and lean subjects, the mRNA species encoding the small protein isoform was twice as abundant as the mRNA species encoding the large protein isoform. Neither the total uncoupling protein-3 mRNA expression nor the molar abundance ratios of the two mRNA species differed between obese and lean male or female subjects. Women who had lost 37 ± 22 kg of weight in response to dietary restriction and continued a hypocaloric diet displayed lower mRNA than obese (p < 0.005) or lean women (p < 0.05). Primer extension assays in lean and obese subjects showed similar allelic mRNA abundance in all but one subject studied. Conclusion: Muscle expression of the two uncoupling protein-3 mRNA species is similar in obese and lean people. In obese patients, prolonged hypocaloric diet downregulates uncoupling protein-3 mRNA expression in muscle and can thereby enhance its energy efficiency. Sequence substitutions at the gene locus may only be minor determinants of mRNA expression in muscle tissue. [Diabetologia (1999)

The Uncoupling Protein-3 Gene Is Transcribed from Tissue-specific Promoters in Humans but Not in Rodents

“…This result was verified by sequencing of mouse and rat intron 6 lacking a poly(A) signal (not shown) and is consistent with a previous report (17). In humans, UCP3 L /UCP3 S transcript abundance ratios differed between skeletal muscle and adipose tissue obtained from the same individuals in that molar abundance ratios of UCP3 L and UCP3 S transcripts averaged ϳ1.0 (range 0.9 -1.2) in samples representing adipose tissue deposits, whereas UCP3 S was twice as abundant as UCP3 L in the skeletal muscle samples as has been reported previously (29,50). Recent in vivo studies in humans demonstrated distinct responses of UCP3 L and UCP3 S mRNA levels to triglyceride and glycerol infusions (50), and studies in type 2 diabetics showed that UCP3 L more closely paralleled changes in free fatty acid levels than UCP3 S (25).…”

“…Ribonuclease (RNase) Protection Assays-The UCP3 cDNA probe spanning exons 6 and 7 for the simultaneous quantification of human UCP3 L and UCP3 S transcripts was prepared as described (29). A mouse and rat UCP3 probe that was homologous to the respective human region was constructed using MR808F and MR1167R as primers (Table I).…”

“…These results that supported a role of UCP3 L in fuel metabolism are at variance with findings of another study (28) that found no abnormalities in the respiratory quotient of affected individuals. Finally, two independent studies reported down-regulation of UCP3 mRNA expression in muscle of weight-reduced subjects (29,30), an observation that could help to explain the reduced energy expenditure that has been reported after weight reduction (31).…”

“…To determine whether tissue-specific transcript initiation is maintained over a wide range of expression levels, human muscle tissues from obese and weight-reduced obese subjects displaying up to 10-fold differences in UCP3 mRNA levels were used (29). In all samples studied, Ͼ98% of the transcripts initiated at SM-1 with the remaining transcripts initiating at SM-2.…”

The uncoupling protein homologues: UCP1, UCP2, UCP3, StUCP and AtUCP

“…Samec et al [225] measured a marked increased in UCP2 and UCP3 mRNA levels in the gastrocnemius muscle of rats put on a restricted diet and then fed on a high-fat diet. During the analysis of UCP2 and UCP3 mRNA levels in obese individuals after stable weight reduction or during prolonged weight loss, expression of the UCP3 mRNA level decreased, suggesting that reduced UCP3 expression could contribute to decreased energy expenditure in weight-stable, weight-reduced individuals [226,227].…”