Understanding the metabolic factors that contribute to energy metabolism (EM) is critical for the development of new treatments for obesity and related diseases. Mitochondrial oxidative phosphorylation is not perfectly coupled to ATP synthesis, and the process of proton-leak plays a crucial role. Proton-leak accounts for a significant part of the resting metabolic rate (RMR) and therefore enhancement of this process represents a potential target for obesity treatment. Since their discovery, uncoupling proteins have stimulated great interest due to their involvement in mitochondrial-inducible proton-leak. Despite the widely accepted uncoupling/thermogenic effect of uncoupling protein one (UCP1), which was the first in this family to be discovered, the reactions catalyzed by its homolog UCP3 and the physiological role remain under debate. This review provides an overview of the role played by UCP1 and UCP3 in mitochondrial uncoupling/functionality as well as EM and suggests that they are a potential therapeutic target for treating obesity and its related diseases such as type II diabetes mellitus.

Familial hemophagocytic lymphohistiocytosis (FHLH) is a rare, rapidly progressive disorder of early childhood characterized by uncontrolled activation of T cells and macrophages. Although perforin gene mutations have been described in a proportion of patients with FHLH, the genotype/ phenotype correlation is still limited. Only a few patients with late onset clinical manifestations have been reported. The biochemical and immunologic alterations in the asymptomatic phase are not well known. We report on a family in which 2 fraternal twins both homozygous for a perforin mutation previously described as causative of the disease, markedly differed in phenotypic expression of FHLH. The twins also had a second novel heterozygous mutation. Natural killer (NK) activity was severely impaired in the patient and was normal in the asymptomatic fraternal twin. Our report highlights that FHLH may present after a long diseasefree interval during which biochemical or immunologic alterations may be not evident, thus implying a role for interfering factors. (Blood. 2004;103:4610-4612)

SummaryGenetic alterations of the FOXN1 transcription factor, selectively expressed in thymic epithelia and skin, are responsible in both mice and humans for the Nude/SCID phenotype. The first described human FOXN1 mutation was a C792T transition in exon 5 resulting in the nonsense mutation R255X, and was detected in two probands originated from a small community in southern Italy. In this community, four additional children affected with congenital alopecia died in early childhood because of severe infections. In this study, we report on the screening for this mutation in 30% of the village population. This analysis led us to identify 55 heterozygous carriers (6.52%) of the R255X mutation out of 843 inhabitants screened. A genealogical study revealed that these subjects, belonging to 39 families, were linked in an extended 7-generational pedigree comprising 483 individuals. Through the archival database a single ancestral couple, born at the beginning of the 19th century, was identified. To confirm the ancestral origin of the mutation we genotyped two microsatellite markers, D17S2187 and D17S1880, flanking the FOXN1 gene on chromosome 17. The three haplotypes identified, 3/R255X/3, 3/R255X/2 and 3/R255X/1, are consistent with a single ancestral origin for the mutation R255X.

,5-Diiodo-L-thyronine rapidly enhances mitochondrial fatty acid oxidation rate and thermogenesis in rat skeletal muscle: AMP-activated protein kinase involvement. Am J Physiol Endocrinol Metab 296: E497-E502, 2009. First published December 30, 2008 doi:10.1152/ajpendo.90642.2008.-Triiodothyronine regulates energy metabolism and thermogenesis. Among triiodothyronine derivatives, 3,5-diiodo-L-thyronine (T2) has been shown to exert marked effects on energy metabolism by acting mainly at the mitochondrial level. Here we investigated the capacity of T2 to affect both skeletal muscle mitochondrial substrate oxidation and thermogenesis within 1 h after its injection into hypothyroid rats. Administration of T2 induced an increase in mitochondrial oxidation when palmitoyl-CoA (ϩ104%), palmitoylcarnitine (ϩ80%), or succinate (ϩ30%) was used as substrate, but it had no effect when pyruvate was used. T2 was able to 1) activate the AMPK-ACC-malonyl-CoA metabolic signaling pathway known to direct lipid partitioning toward oxidation and 2) increase the importing of fatty acids into the mitochondrion. These results suggest that T2 stimulates mitochondrial fatty acid oxidation by activating several metabolic pathways, such as the fatty acid import/␤-oxidation cycle/ FADH2-linked respiratory pathways, where fatty acids are imported. T 2 also enhanced skeletal muscle mitochondrial thermogenesis by activating pathways involved in the dissipation of the proton-motive force not associated with ATP synthesis ("proton leak"), the effect being dependent on the presence of free fatty acids inside mitochondria. We conclude that skeletal muscle is a target for T2, and we propose that, by activating processes able to enhance mitochondrial fatty acid oxidation and thermogenesis, T2 could play a role in protecting skeletal muscle against excessive intramyocellular lipid storage, possibly allowing it to avoid functional disorders. adenosine 5Ј-monophosphate; thyroid hormone; mitochondria AMONG THE ENDOCRINE FACTORS able to regulate substrate metabolism and thermogenesis, thyroid hormones (THs) play important roles. 3,5,3Ј-Triiodothyronine (T 3 ) exerts a plethora of effects, including upregulation of peripheral and hepatic glucose uptake, cholesterol reduction, loss of body weight and adiposity, cardiac acceleration, and increases in metabolic rate (28,32). In adults, T 3 regulates energy metabolism by increasing respiration and energy expenditure and by lowering metabolic efficiency (for review, see Ref. 11). Because of this, T 3 was tested in the past as an antiobesity and hypolipidemic agent. However, due to its undesirable side effects, particularly within the cardiovascular system, its use was not continued (19). The development of TH derivatives that, while retaining lipid-lowering and antiobesity efficacy, are devoid of cardiovascular side effects would represent a potentially valuable therapeutic tool for the reduction of some important risk factors. Many laboratories have demonstrated metabolic effects of 3,5-diiodothyronine (T 2 ; a...

Although the literature contains many studies on the function of UCP3, its role is still being debated. It has been hypothesized that UCP3 may mediate lipid hydroperoxide (LOOH) translocation across the mitochondrial inner membrane (MIM), thus protecting the mitochondrial matrix from this very aggressive molecule. However, no experiments on mitochondria have provided evidence in support of this hypothesis. Here, using mitochondria isolated from UCP3-null mice and their wild-type littermates, we demonstrate the following. (i) In the absence of free fatty acids, proton conductance did not differ between wild-type and UCP3-null mitochondria. Addition of arachidonic acid (AA) to such mitochondria induced an increase in proton conductance, with wild-type mitochondria showing greater enhancement. In wild-type mitochondria, the uncoupling effect of AA was significantly reduced both when the release of O 2 . in the matrix was inhibited and when the formation of LOOH was inhibited. In UCP3-null mitochondria, however, the uncoupling effect of AA was independent of the above mechanisms. (ii) In the presence of AA, wild-type mitochondria released significantly more LOOH compared with UCP3-null mitochondria. This difference was abolished both when UCP3 was inhibited by GDP and under a condition in which there was reduced LOOH formation on the matrix side of the MIM. These data demonstrate that UCP3 is involved both in mediating the translocation of LOOH across the MIM and in LOOH-dependent mitochondrial uncoupling.Uncoupling proteins (UCPs) 3 are homologous proteins belonging to a subfamily of mitochondrial anion carriers. Although both the function and mechanism of action of the first cloned UCP (UCP1) are quite well established, those of the UCP1 homologs are still far from clear. Because of its homology to UCP1 and its prevalent skeletal muscle expression, UCP3 has been hypothesized to be a thermogenic protein. However, the literature contains conflicting results, and an increased expression of UCP3 has not always been associated with mitochondrial uncoupling. This may suggest that uncoupling is not its primary function but rather a consequence of its real function (for reviews, see Refs.

Chronic overnutrition and modern lifestyles are causing a worldwide epidemic of obesity and associated comorbidities, which is creating a demand to identify underlying biological mechanisms and to devise effective treatments. In rats receiving a high-fat diet (HFD), we analyzed the effects of a 4-wk administration of a novel functional analog of iodothyronines, TRC150094 (TRC). HFD-TRC rats exhibited increased energy expenditure (+24% vs. HFD rats; P<0.05) and body weight (BW) gain comparable to that of standard chow-fed (N) rats [N, HFD, and HFD-TRC rats, +97 g, +140 g (P<0.05 vs. N), and +98 g (P<0.05 vs. HFD)]. HFD-TRC rats had significantly less visceral adipose tissue (vs. HFD rats) and exhibited altered metabolism in two major tissues that are very active metabolically. In liver, mitochondrial fatty acid import and oxidation were increased (+56 and +32%, respectively; P<0.05 vs. HFD rats), and consequently the hepatic triglyceride content was lower (-35%; P<0.05 vs. HFD rats). These effects were independent of the AMP-activated protein kinase-acetyl CoA-carboxylase-malonyl CoA pathway but involved sirtuin 1 activation. In skeletal muscle, TRC induced a fiber shift toward the oxidative type in tibialis anterior muscle, increasing its capacity to oxidize fatty acids. HFD-TRC rats had lower (vs. HFD rats) plasma cholesterol and triglyceride concentrations. If reproduced in humans, these results will open interesting possibilities regarding the counteraction of metabolic dysfunction associated with ectopic/visceral fat accumulation.

Controversy exists on whether uncoupling protein 3 (UCP3) positively or negatively influences insulin sensitivity in vivo, and the underlying signaling pathways have been scarcely studied. We studied how a progressive reduction in UCP3 expression (using UCP3 +/+, UCP3 +/-, and UCP3 -/- mice) modulates insulin sensitivity and related metabolic parameters. In order to further validate our observations, we also studied animals in which insulin resistance was induced by administration of a high-fat diet (HFD). In UCP3 +/- and UCP3 -/- mice, gastrocnemius muscle Akt/protein kinase B (Akt/PKB) (serine 473) and AMP-activated protein kinase (AMPK) (threonine 171) phosphorylation, and glucose transporter 4 (GLUT4) membrane levels were reduced compared to UCP3 +/+ mice. The HOMA-IR index (insulin resistance parameter) was increased both in the UCP3 +/- and UCP3 -/- mice. In these mice, insulin administration normalized Akt/PKB phosphorylation between genotypes while AMPK phosphorylation was further reduced, and sarcolemmal GLUT4 levels were induced but did not reach control levels. Furthermore, non-insulin-stimulated muscle fatty acid oxidation and the expression of several involved genes both in muscle and in liver were reduced. HFD administration induced insulin resistance in UCP3 +/+ mice and the aforementioned parameters resulted similar to those of chow-fed UCP3 +/- and UCP3 -/- mice. In conclusion, high-fat-diet-induced insulin resistance in wild-type mice mimics that of chow-fed UCP3 +/- and UCP3 -/- mice showing that progressive reduction of UCP3 levels results in insulin resistance. This is accompanied by decreased fatty acid oxidation and a less intense Akt/PKB and AMPK signaling.

3,5-diiodo-l-thyronine (T2), a thyroid hormone derivative, is capable of increasing energy expenditure, as well as preventing high fat diet-induced overweight and related metabolic dysfunction. Most studies to date on T2 have been carried out on liver and skeletal muscle. Considering the role of brown adipose tissue (BAT) in energy and metabolic homeostasis, we explored whether T2 could activate BAT thermogenesis. Using euthyroid, hypothyroid, and T2-treated hypothyroid rats (all maintained at thermoneutrality) in morphological and functional studies, we found that hypothyroidism suppresses the maximal oxidative capacity of BAT and thermogenesis, as revealed by reduced mitochondrial content and respiration, enlarged cells and lipid droplets, and increased number of unilocular cells within the tissue. In vivo administration of T2 to hypothyroid rats activated BAT thermogenesis and increased the sympathetic innervation and vascularization of tissue. Likewise, T2 increased BAT oxidative capacity in vitro when added to BAT homogenates from hypothyroid rats. In vivo administration of T2 to hypothyroid rats enhanced mitochondrial respiration. Moreover, UCP1 seems to be a molecular determinant underlying the effect of T2 on mitochondrial thermogenesis. In fact, inhibition of mitochondrial respiration by GDP and its reactivation by fatty acids were greater in mitochondria from T2-treated hypothyroid rats than untreated hypothyroid rats. In vivo administration of T2 led to an increase in PGC-1α protein levels in nuclei (transient) and mitochondria (longer lasting), suggesting a coordinate effect of T2 in these organelles that ultimately promotes net activation of mitochondrial biogenesis and BAT thermogenesis. The effect of T2 on PGC-1α is similar to that elicited by triiodothyronine. As a whole, the data reported here indicate T2 is a thyroid hormone derivative able to activate BAT thermogenesis.