Contributions of studies on uncoupling proteins to research on metabolic diseases

The coupling of O2 consumption to ADP phosphorylation in mitochondria is partial. This is particularly obvious in brown adipocyte mitochondria which use a regulated uncoupling mechanism generating heat production from substrate oxidation, and catalysing thermogenesis in rodents or infants in response to cold, and arousing hibernators. In the case of brown adipose tissue, the uncoupling mechanism is related to a specific protein in the inner mitochondrial membrane referred to as UCP1. Although the biological importance of UCP1 in human adults is not demonstrated, genetic analysis of various human cohorts suggested a participation of UCP1 to control of fat content and body weight. Very recently, the cloning of UCP2 and UCP3, two homologues of UCP1, has renewed the field of research on the importance of respiration control in metabolic processes and metabolic diseases. UCP2 is widely expressed in organs, whereas UCP3 is mainly present in muscles. These proteins may explain why the coupling of respiration to ADP phosphorylation is less than perfect. Their biological importance should be studied. They also represent new putative targets for drugs against metabolic diseases such as obesity.

An uncoupling protein 2 gene variant is associated with a raised body mass index but not Type II diabetes

AIMS/HYPOTHESIS

Linkage between markers close to the uncoupling protein 2 and 3 genes (11q13) and resting metabolic rate and a pre-diabetic phenotype have been found. The syntenic region in mouse has been found to be linked to quantitative traits associated with obesity and diabetes. UCP2 and UCP3 could therefore have an important role in body weight regulation and susceptibility to diabetes. We investigated a recently identified variant of the UCP2 gene in exon 8 as a marker for glucose and weight homeostasis.

METHODS

Length variation of the UCP2 exon 8 variant was studied by the polymerase chain reaction and agarose gel electrophoresis. Sequence variants of the UCP3 gene were sought by semi-automated DNA sequencing.

RESULTS

In 453 South Indian subjects, we found an association in women between the UCP2 exon variant and body mass index (p = 0.018). These findings were replicated in a separate group of South Indian subjects (n = 143, p < 0.001) irrespective of sex. Although no association was found between the UCP2 exon 8 variant and overt obesity in British subjects, the UCP2 genotype of obese women (n = 83) correlated with fasting serum leptin concentration (p = 0.006) in the presence of extreme obesity. These observations could not be explained by tight linkage disequilibrium with a coding region variant in the region of the UCP3 gene of biological significance. Lastly, no association was found between UCP2 and Type II (non-insulin-dependent) diabetes using either a family based design (85 families) or case control study (normal glucose tolerance n = 335, impaired glucose tolerance n = 42, Type II diabetes n = 76).

CONCLUSION/INTERPRETATION

We have described a UCP2 gene exon 8 variant that may affect susceptibility to weight gain by influencing regulation of leptin.

Functional organization of the human uncoupling protein-2 gene, and juxtaposition to the uncoupling protein-3 gene

Human and mouse UCP2 genes were cloned and sequenced. Transcriptional start sites were identified using primer extension analysis. The transcription unit of UCP2 gene is made of 2 untranslated exons followed by 6 exons encoding UCP2. In vitro translation analysis demonstrated that an open-reading-frame for a putative peptide of 36 residues present in exon 2 did not prevent UCP2 translation and confirmed that the initiation site of translation was in exon 3 as predicted from sequencing data. Short (bp -125 to +93) and long (bp -1383 and +93) CAT-constructs containing DNA upstream of the transcriptional start site of the human gene were made and transfected in adipocytes or HeLa cells allowing characterization of a potent promoter. Analysis of several genomic clones encompassing UCP2 and/or UCP3 genes demonstrated that the 2 genes are adjacent, the human UCP2 gene being located 7 kb downstream of the UCP3 gene.

Association between uncoupling protein polymorphisms (UCP2-UCP3) and energy metabolism/obesity in Pima indians

The UCP2-UCP3 gene cluster maps to chromosome 11q13 in humans, and polymorphisms in these genes may contribute to obesity through effects on energy metabolism. DNA sequencing of UCP2 and UCP3 revealed three polymorphisms informative for association studies: an Ala-->Val substitution in exon 4 of UCP2, a 45 bp insertion/deletion in the 3'-untranslated region of exon 8 of UCP2 and a C-->T silent polymorphism in exon 3 of UCP3. Initially, 82 young (mean age = 30 +/- 7 years), unrelated, full-blooded, non-diabetic Pima Indians were typed for these polymorphisms by direct sequencing. The three sites were in linkage disequilibrium ( P < 0.00001). The UCP2 variants were associated with metabolic rate during sleep (exon 4, P = 0.007; exon 8, P = 0.016) and over 24 h (exon 8, P = 0.038). Heterozygotes for UCP2 variants had higher metabolic rates than homozygotes. The UCP3 variant was not significantly associated with metabolic rate or obesity. In a further 790 full-blooded Pima Indians, there was no significant association between the insertion/deletion polymorphism and body mass index (BMI). However, when only individuals >45 years of age were considered, heterozygotes (subjects with the highest sleeping metabolic rate) had the lowest BMI (P = 0.04). The location of the insertion/deletion polymorphism suggested a role in mRNA stability; however, it appeared to have no effect on skeletal muscle UCP2 mRNA levels in a subset of 23 randomly chosen Pima Indians. In conclusion, these results suggest a contribution from UCP2 (or UCP3) to variation in metabolic rate in young Pima Indians which may contribute to overall body fat content later in life.

Kupffer cells are a dominant site of uncoupling protein 2 expression in rat liver

The mechanisms underlying thermogenesis in liver are not well understood. They may involve proteins related to the mitochondrial uncoupling protein (UCP1) of brown adipocytes. In this paper, it is demonstrated that UCP1 is not expressed in any liver cell type of rat while UCP2, a recently cloned homologue of UCP1, is expressed at a very high level in Kupffer cells but not in hepatocytes. This high level of expression of UCP2 in Kupffer cells allowed cross immunoreactivity with antibodies directed against UCP1. This cross reactivity was confirmed by the detection of UCP2 with anti-UCP1 antibody, in western blotting analysis of transfected yeasts expressing rat UCP2. The high level expression of UCP2 in Kupffer cells suggests a particular function of UCP2 in macrophages.

In vivo resistance of lipolysis to epinephrine. A new feature of childhood onset obesity

A decreased mobilization of triglycerides may contribute to fat accumulation in adipocytes, leading to obesity. However, this hypothesis remains to be proven. In this study, epinephrine-induced lipid mobilization was investigated in vivo in nine markedly obese children (160+/-5% ideal body weight) aged 12.1+/-0.1 yr during the dynamic phase of fat deposition, compared with six age-matched nonobese children. As an in vivo index of lipolysis, we measured glycerol flux using a nonradioactive tracer dilution approach, and plasma free fatty acid concentrations. In the basal state, the obese children had a 30% lower rate of glycerol release per unit fat mass than the lean children. To study the regulation of lipolysis, epinephrine was infused stepwise at fixed doses of 0.75 and then 1. 50 microg/min in both groups. In lean children, glycerol flux and plasma free fatty acid increased to an average of 249-246% of basal values, respectively, in response to a mean plasma epinephrine of 396+/-41 pg/ml. The corresponding increase was only 55-72% in the obese children, although their mean plasma epinephrine reached 606+/-68 pg/ml. All obese and nonobese children, except an Arg64Trp heterozygote, were homozygotes for Trp at position 64 of their beta3-adrenoreceptor. The resistance of lipolysis to epinephrine showed no relationship with the Arg64 polymorphism of the beta3-adrenoreceptor gene. In summary, in vivo lipolysis, which mostly reflects the mobilization of lipid stores from subcutaneous adipose tissue, shows a decreased sensitivity to epinephrine in childhood onset obesity. Since our study was carried out in obese children during the dynamic phase of fat accumulation, the observed resistance to catecholamines might possibly be causative rather than the result of obesity.

Organisation and promoter activity of the retinoic-acid-induced-heparin-binding (RIHB) gene

The heparin-binding protein, RIHB, is encoded by a gene that is expressed temporarily during the early period of chicken embryogenesis. We have now isolated genomic clones of the chicken RIHB gene, determined its transcription initiation sites, characterized its promoter region and established the functional activity of the RIHB gene promoter. The 5' flanking region and the beginning of the first exon revealed a structural organisation characteristic of housekeeping and growth-control-related genes. It lacked canonical TATA or CAAT boxes but contained several GC boxes. The structure of the RIHB gene is compared with those of the human and mouse midgestation kidney (MK) genes. In the coding exons, the similarities between the three genes are striking. In contrast, the structure and location of the first non-coding exon is different. Analysis of the promoter activity suggests an indirect regulation of transcription by retinoic acid and demonstrates a high degree of complexity of the regulation of the RIHB gene.