Exercise, Mitohormesis, and Mitochondrial ORF of the 12S rRNA Type-C (MOTS-c).

The expansion of subcutaneous (SC) white adipose tissue (WAT) has beneficial effects on metabolic health. Our previous work showed an increased number of bone morphogenetic protein 4 (BMP4)-activated beige adipocytes in SC WAT, indicating a potential role of BMP4 in adipocyte recruitment. It was also demonstrated that BMP4 committed multipotent mesodermal C3H10T1/2 stem cells to the adipocyte lineage ex vivo However, the mechanism by which BMP4 regulates adipogenesis in vivo has not been clarified. In this study, we found that BMP4 stimulated de novo adipogenesis in SC WAT concomitant with enhanced blood vessel formation, thus promoting adipose tissue angiogenesis. Platelet-derived growth factor receptor-β-positive (PDGFRβ(+)) multipotent stem cells within the neoangiogenic vessels were found to be adipocyte progenitors. Moreover, BMP4 downregulated PDGFRβ by stimulating the lysosome-dependent degradation, which efficiently initiated adipogenic differentiation. These results suggest how BMP4 regulates adipocyte recruitment in SC WAT, and thus promote its beneficial metabolic effects.

Angiotensin (Ang)-(1-7) is a beneficial hormone of the renin-angiotensin system that is emerging as a promising target for obesity. We previously showed that chronic Ang-(1-7) treatment attenuates high fat diet (HFD)-induced weight gain in mice by increasing markers of thermogenesis in subcutaneous white adipose tissue (“browning”) to increase energy expenditure. In this study, we tested the hypothesis that Ang-(1-7) could acutely increase adipose thermogenesis in mice. To test this, adult male C57BL/6J mice were placed on a chow diet or 60% HFD for 12 weeks and then received a single subcutaneous injection of Ang-(1-7) (2mg/kg) or saline. This study included 4 groups of mice: chow+saline (n=9), chow+Ang-(1-7) (n=15), HFD+saline (n=8), HFD+Ang-(1-7) (n=10). Core temperature was measured at baseline and at 6 hours after injection. Subcutaneous white and brown adipose tissues were collected 6 hours post-injection. Gene expression of the thermogenic marker uncoupling protein 1 (UCP1) was measured in tissues by quantitative real-time PCR and quantified with 2-ΔΔCT methods. Acutely, Ang-(1-7) did not alter body mass in chow or HFD mice ( P= 0.992). There were 5 chow fed mice (33%) and 2 HFD mice (20%) that did not respond to Ang-(1-7) injection. In the chow diet responders, Ang-(1-7) increased core temperature (36.7±0.2 vs 35.9±0.2°C saline; P =0.012) and UCP1 gene expression in subcutaneous white adipose tissue (14.3±5.8 vs 1.9±0.6 saline; P =0.050), with no effect in brown adipose tissue (1.7±0.6 vs 1.0±0.2 saline, P =0.284). In the HFD responders, core temperature was not altered by Ang-(1-7) (37.8±0.1 vs 37.8±0.1 saline, P =0.689). There was a trend towards decreased UCP1 expression in white adipose of HFD-saline mice compared to chow-saline mice ( P =0.196). Ang-(1-7) significantly increased UCP1 expression in subcutaneous white adipose of HFD responders (6.2±3.1 vs 0.9±0.2 saline, P =0.036), with no effect in brown adipose tissue (1.2±0.1 vs saline: 1.7±0.4 saline, P =0.402). These data provide further evidence that targeting Ang-(1-7) may be a promising strategy to increase white adipose thermogenesis, an effect that could serve to enhance energy expenditure and restore energy balance in obesity.

Recent evidence supports the role of menthol, a TRPM8 agonist, in enhanced energy expenditure, thermogenesis and BAT-like activity in classical WAT depots in a TRPM8 dependent and independent manner. The present study was designed to analyse whether oral and topical administration of menthol is bioavailable at subcutaneous adipose tissue and is sufficient to directlyinduce desired energy expenditure effects. GC-FID was performed to study menthol bioavailability in serum and subcutaneous white adipose tissue following oral and topical administration. Further, 3T3L1 adipocytes were treated with bioavailable menthol doses and different parameters (lipid accumulation, “browning/brite” and energy expenditure gene expression, metal analysis, mitochondrial complex’s gene expression) were studied. No difference was observed in serum levels but significant difference was seen in the menthol concentration on subcutaneous adipose tissues after oral and topical application. Menthol administration at bioavailable doses significantly increased “browning/brite” and energy expenditure phenotype, enhanced mitochondrial activity related gene expression, increased metal concentration during adipogenesis but did not alter the lipid accumulation as well as acute experiments were performed with lower dose of menthol on mature adipocytes In conclusion, the present study provides evidence that bioavailable menthol after single oral and topical administration is sufficient to induce “brite” phenotype in subcutaneous adipose tissue However, critical dose characterization for its clinical utility is required.

The role of mitochondria in white adipose tissue has traditionally received little attention. This historic neglect is based on the assumption that the white adipocyte's poor mitochondrial equipment, being confined to the small cellular space adjacent to the fat droplet, could have little relevance to white adipose cell function. In contrast, the brown adipocyte, which specializes in thermogenesis, has a robust content of mitochondria; thus the role of mitochondrial function in brown adipose cells has been recognized as a worthwhile topic of study. Brown fat has been traditionally considered to play only a minor role in adult humans, but recent data from positron emission tomography and other tools of nuclear medicine have evidenced the presence of substantial amounts of brown adipose tissue in adult humans ((1),(2)). Hence, the role of brown adipose tissue in adult human metabolic pathologies, such as obesity, has not been elucidated but the discovery of previously uncovered brown adipose tissue in adult humans is expected to stimulate further research on the role of brown fat in adult human energy balance. On the other hand, in recent years, accumulating evidence from a number of research areas, including studies on obesity, type 2 diabetes and the lipodystrophies, has highlighted the previously unrecognized relevance of mitochondrial function for white adipose tissue biology and systemic metabolic regulation. Mitochondria in mammalian cells, including adipocytes, have a unique feature—the mitochondrial genome—that distinguishes them from other cellular organelles. This distinct 16-kb circular DNA-based genetic system encodes mitochondrial components that are important for mitochondrial function. In mammals, mitochondrial DNA (mtDNA) encodes proteins of the respiratory chain/oxidative phosphorylation system (OXPHOS) as well as other components of the mitochondrial translation system (i.e., mitochondrial transfer RNAs and ribosomal RNAs) ((3)). Coordinate regulation of mtDNA- and nuclear genome–mediated gene expression is required for the correct synthesis of a functional mitochondrial OXPHOS ((4)). In addition to its genetic variability in humans, mtDNA is particularly susceptible to somatic mutations that can affect mtDNA-dependent gene expression and mitochondrial function. This is likely due to the direct exposure of mtDNA to locally produced oxygen superoxides arising from the respiratory chain as well as to poor performance of repair systems in mtDNA replication ((5)). Tissues from patients bearing mtDNA mutations show a mixture of mutated and wild-type forms of mtDNA at a given proportion (heteroplasmy) and the percentage of mutated mtDNA is usually associated with the severity of the pathology symptoms ((6)). Moreover, the mtDNA gene expression machinery is distinct from that which determines expression from nuclear genes. Pathogenic processes may alter components of the mtDNA-specific expression machinery, which is comprised mainly of nuclear-encoded proteins that are transported to the mitochondria after cytosolic synthesis ((7)). These pathologies may lead to alterations in overall mitochondrial function, altering energy balance and disturbing the respiratory chain, and leading to enhanced oxygen superoxide production or promotion of mitochondrially driven apoptosis—even in the absence of mtDNA mutations. Current research coming from the fields of genetics, endocrinology, and pharmacology indicate that mtDNA biology may be relevant to white adipose tissue function, and suggest that altered mtDNA in white fat may have local and systemic consequences. However, we are far from having developed a full mechanistic understanding of how mtDNA alterations cause these effects. The present review is intended to provide the first summary of our current knowledge of mtDNA biology in white adipose tissue and how it might be relevant to human pathologies, from obesity to lipodystrophy, that are related to energy balance. Human mtDNA was first sequenced in 1981. Since that time, extensive knowledge of the genetics and physiological processes that rely on mtDNA gene expression in human cells has become available ((3),(4)). Mitochondrial DNA in humans encodes two ribosomal RNAs, components of mitochondrial ribosomes; the 22 mitochondrial transfer RNAs; and 13 messenger RNAs that are translated into protein components of distinct complexes of the mitochondrial respiratory chain and OXPHOS. Thus, the energy-producing machinery in mammalian mitochondria relies on respiratory complexes built from nuclear and mtDNA-encoded polypeptides, and thus constitutes a unique cellular system that results from the coordinate expression of two distinct genomes. However, despite the fact that mtDNA encodes structural components of the OXPHOS system, most of the protein components of the mtDNA replication and expression machinery are encoded by the nuclear genome. The replication of mtDNA depends on mitochondrial DNA polymerase-γ, whereas mitochondrial RNA polymerase is responsible for mtDNA-driven transcription. Proteins, such as TFAM, TFB1M, and TFB2M, are involved in the control of these processes through interaction with the regulatory D-loop region of mtDNA. As shown in Figure 1, the control of mtDNA expression appears to be directed by nuclear gene–encoded transcription factors (e.g., nuclear respiratory factors-1 and -2), nuclear hormone receptors (e.g., thyroid hormone receptor, estrogen-related receptor-α), transcriptional coactivators (e.g., PGC-1s), and corepressors (e.g., RIP140). These factors control the expression of nuclear genes that encode structural components of the OXPHOS system and of the nuclear-encoded components of mtDNA replication and transcription (TFAM, TFB1M, and TFB2M) ((8)). The amounts of mtDNA in the mitochondria and in the whole cell, and the extent of mtDNA-dependent protein expression are therefore under complex control of nuclear-encoded components of the machinery. In contrast with nuclear gene expression, the expression of mtDNA genes can not only be controlled at the level of transcription but also at the level of mtDNA replication. Thus, mtDNA gene dosage may affect the extent of mtDNA-encoded gene expression, with consequences for mitochondrial function. Several physiological adaptive processes, such as chronic exercise training effects on muscle, mediate enhanced mtDNA expression by increasing mtDNA replication and thereby increasing mtDNA gene dosage ((9)). It is not known whether this mechanism of regulation of mtDNA expression through changes in mtDNA levels occurs in adipose tissue in response to physiological adaptations, although it takes place in association with brown and white adipocyte differentiation processes in vitro ((10),(11)), in pathological conditions, and under several pharmacological treatments (see below). Schematic representation of the factors involved in the control of mitochondrial DNA (mtDNA) expression. DNA-Polγ, DNA polymerase γ; ERRα, estrogen-related receptor-α; mtRPOL, mitochondrial RNA polymerase; TFAM, mitochondrial transcription factor A; TFB1M and TFB2M, mitochondrial transcription factors B1 and B2; NRF-1 and NRF-2, nuclear respiratory factors-1 and -2; OXPHOS, respiratory chain/oxidative phosphorylation system; PGC-1; peroxisome proliferator–related receptors coactivators; RIP140, receptor interacting protein-140. Until recently, studies on mtDNA biology in adipose tissue have been limited to the analysis of mitochondrial biogenesis in brown adipose tissue—the adipose tissue that specializes in thermogenesis. In contrast to white adipocytes, brown adipocytes possess large amounts of oxidative active mitochondria. During brown adipose tissue development and differentiation, mitochondrial biogenesis is enhanced and is accompanied by increased levels of mtDNA and expression of the mtDNA-encoded components of the OXPHOS system ((10)). The transcriptional activators, PGC-1α and PGC-1β, play an important role in this process by coordinately enhancing the expression of nuclear factors that stimulate mtDNA replication and transcription, and regulating the expression of specific thermogenic genes ((12)). Recent studies have highlighted the importance of mitochondrial biogenesis in white adipose tissue and the potential for mitochondrial alterations to disturb white adipocyte development and function. Studies by Corvera and collaborators have shown that (i) mitochondrial biogenesis is directly associated with white adipocyte differentiation; (ii) obese ob/ob mice displayed impaired mitochondrial mass and function in white fat; and (iii) thiazolidinediones, peroxisome proliferator–activated receptor-γ activators that favor adipocyte differentiation, ameliorated these alterations ((13),(14)). It has been also shown that white adipocyte differentiation is associated with increases in the relative abundance of mtDNA, and upregulation of components of the mtDNA replication and transcription machinery, such as TFAM ((11)), and components of deoxynucleotide metabolism required for mtDNA replication ((15)). Agents that promote white adipocytes differentiation in vitro, such as glitazones, also increase mtDNA levels in human adipocytes in vitro ((16)). Studies of potential alterations in white adipose tissue mtDNA as they relate to obesity have focused on two facets: changes in mtDNA levels that underlie obese phenotypes; and the occurrence of mutated, polymorphic, forms of mtDNA that are specifically associated with obesity. In experimental models of obesity, such as ob/ob or db/db mice, abnormally low levels of mtDNA have been reported ((17),(18)). As noted above for in vitro studies, treatment of obese mice with glitazones increases mtDNA levels in white fat ((18)). A study in which mice were treated with a diet enriched in polyunsaturated fatty acids identified mtDNA-encoded transcripts and proteins among the most upregulated genes in white adipose tissue; this upregulation was associated with an enhancement of fatty acid oxidation in white adipose tissue ((19)). In humans, the scenario appears to be more complex. It has been reported that mtDNA levels in adipose tissue are lowered in type 2 diabetic patients ((20)), and studies by Arner and collaborators have confirmed that mtDNA levels are not associated with obesity per se, but rather with type 2 diabetes phenotypes ((21)). Moreover, mtDNA levels were found to be strongly related to lipogenesis in white adipose tissue ((22)), rather than to BMI. The mechanism by which mtDNA copy number in white adipose tissue could affect lipogenesis rate remains to be established, but it stands in contrast to the expected relationship between mtDNA level variations and energy expenditure and fat oxidation. Moreover, in humans, as in rodents, pioglitazone treatment causes an increase in mtDNA levels in white adipose tissue of type 2 diabetes patients ((20)) but not in nondiabetic obese individuals ((23)). Probably, one of the more outstanding pieces of evidence in support of the potential relationship between mtDNA levels in white adipose tissue and human obesity comes from a recent study of a series of rare monozygotic twins discordant for obesity ((24)). Sequence analyses of mtDNA in subcutaneous adipose tissue revealed no aberrant heteroplasmy between the co-twins. However, mtDNA copy number was reduced by 47% in the obese co-twin's fat. These findings highlight the potential role of mtDNA levels in white adipose tissue mass. Further research will be needed to clarify the role of mtDNA levels in influencing biological processes in white adipose tissue beyond the expected consequences on the extent of metabolic energy consumption through oxidative pathways. However, recent data indicate that mitochondrial oxidative capacity may not be a major factor determining events such the extent of free fatty acid release by white adipose cells ((25)). Another important line of research is the analysis of mtDNA mutations for which there is no direct evidence for pathogenic consequences, but may constitute mtDNA polymorphic forms related to obesity. Several reports indicate such association, but no clear-cut conclusions have been reached to date. The single-nucleotide polymorphism, 15497 G/A, which leads to a glycine-to-serine amino acid replacement at residue 251 (Gly251Ser) in the mtDNA-encoded cytochrome b, has been associated with obesity in a Japanese population ((26)). In fact, individuals bearing the A allele presented an increased waist-to-hip ratio and other obesity-related variables including elevated triglyceride levels relative to individuals bearing the G allele. However, the functional consequences of this polymorphism for adipose tissue biology, and whether they affect primarily white adipose tissue function or overall energy balance, and thus adipose tissue expansion, are unknown. Studies of white populations found that this mutation contributes to severe obesity only in rare instances ((27)), and similarly, a polymorphism in the mtDNA D-loop noncoding region was not found to be associated with obesity ((28)). Recently, single-nucleotide polymorphisms in the mtDNA-encoded reduced nicotinamide adenine dinucleotide dehydrogenase subunit I gene and in the 12S ribosomal RNA gene in mtDNA have also been associated with obesity in a Japanese population ((29)) whereas a common mtDNA variant has been associated with low BMI in white women ((30)). Since the 1980s, deletions and point mutations in mtDNA have been known to cause human diseases. The common pathogenic features of diseases attributed to genetic mtDNA alterations involve defects in neural and muscular tissues, but not adipose tissues. Nevertheless, there is a remarkable exception: mutations in the mtDNA-encoded tRNA-Lys cause lipomatosis. Most patients bearing pathogenic mutations in the mtDNA-encoded tRNA-Lys (A8344G, T8356C, G8361A, G8363A) show the myoclonic epilepsy associated with ragged-red fibers syndrome, characterized by neuropathy and myopathy symptoms. However, other patients show an abnormal enlargement of adipose tissue in the dorsal area, similar to the Madelung syndrome ((31),(32),(33),(34)). The lipomatous tissues in the dorso-cervical area of these patients do not show changes in mtDNA levels or in the expression of mtDNA genes. Nuclear DNA–encoded components of the mitochondria are similarly unchanged, despite altered expression of master regulators of adipogenesis. A distorted pattern of brown vs. white adipocyte gene expression, characterized by the expression of the brown adipose tissue–specific uncoupling protein-1 gene but without the acquisition of a complete brown fat phenotype, occurs in the lipomatous tissue ((35),(36)). How such a specific mutation in an mtDNA gene causes adipose tissue hypertrophy remains a mystery. However, it highlights the importance of mtDNA for the normal development of adipose tissue. Certain pathogenic mutations in mtDNA are strongly associated with diabetes caused by progressive insulinopenia, particularly the A3243G mutation in the mtDNA-encoded tRNA-Leu gene. This mutation has been associated with a low BMI ((37),(38)), and lipomatosis has been reported in some patients, despite good glycemic control ((39),(40)). However, it is not known to what extent the adipose disturbances are caused by direct alterations of mtDNA function in white adipose tissue, or are caused indirectly by insulinopenia. Finally, one additional piece of evidence that support the involvement of mtDNA-related pathologies in altering white adipose tissue is provided by mitochondrial diseases caused by mutations in nuclear genes involved in the mtDNA replication/transcription machinery. Obesity has been reported to be a feature of Finnish patients with ataxia due to mutations in mitochondrial DNA polymerase-γ ((41)). In a parallel experimental model, transgenic mice bearing a proof-reading-deficient version of the mitochondrial DNA polymerase-γ showed reduced subcutaneous white adipose tissue in association with the accumulation of somatic mtDNA mutations during ageing ((42)). Research interest in the role of mtDNA in adipose tissue biology has received a dramatic boost in the last few years from studies of human immunodeficiency virus (HIV) lipodystrophy. This pathology occurs in a substantial subset of HIV-infected patients under antiretroviral treatment; its main features include lipoatrophy of subcutaneous fat, visceral obesity and, occasionally, lipomatosis. A major hypothesis proposed to account for the etiopathogenesis of this lipodystrophy is based on the action of the nucleoside-analog inhibitors of the viral reverse transcriptase, a class of antiretroviral drugs used in HIV-1 infected patients. A side effect of these drugs is inhibition of DNA polymerase-γ, the enzyme responsible for mtDNA replication. Accordingly, antiretroviral-treated patients show abnormal reductions (depletion) in mtDNA abundance ((43)). A systematic analysis of published reports based on cross-sectional studies indicates that most such studies consistently identify an association between mtDNA depletion in subcutaneous adipose tissue and lipoatrophy ((44)). However, other tissues besides white adipose tissue also exhibit a reduction in mtDNA abundance; thus, if mtDNA depletion is the cause of the syndrome, it remains unknown why it so specifically affects adipose tissue. Perhaps the positive association between mtDNA levels and lipogenesis reported elsewhere ((22)) may underlie the association between mtDNA depletion in adipose tissue and lowered fat mass accretion in subcutaneous white adipose tissue from patients. The reduction in mtDNA levels occurs in conjunction with impaired expression of genes related to adipocyte differentiation (i.e., peroxisome proliferator–activated receptor-γ), lipid accretion (e.g., lipoprotein lipase, GLUT4), and adipokines (i.e., adiponectin) ((45),(46),(47),(48)). In fact, thiazolidinedione treatment–mediated activation of peroxisome proliferator–activated receptor-γ in humans leads to increased body weight through the upregulation of genes facilitating adipocyte lipid storage ((49)) and, therefore, the impaired peroxisome proliferator–activated receptor-γ expression in adipose tissue from patients may contribute to lipoatrophy. However, studies in which antiretroviral treatment of HIV-1 patients with lipoatrophy has been interrupted ((50)) or shifted to drugs with a lesser effect on DNA polymerase-γ have indicated a substantial reversion of mtDNA depletion in white adipose tissue but a milder amelioration of lipoatrophy ((51),(52),(53),(54)). Thus, cause-and-effect relationships remain unclear. Exposure of brown or white adipocytes in culture to nucleoside-analog reverse transcriptase inhibitors leads to a reduction in mtDNA as well as to an impairment in adipocyte differentiation ((55),(56),(57)). However, other antiretroviral drugs that do not inhibit DNA polymerase-γ nor cause changes in mtDNA levels, such as protease inhibitors, are also powerful blockers of white adipocyte differentiation in vitro ((58)). Ultimately, the potential impact of mtDNA depletion on cell and tissue function can be expected to depend on the extent to which the depletion reduces the levels of mtDNA-encoded gene products. Although mtDNA depletion is usually associated with a reduction in mtDNA-encoded transcripts ((47)), mtDNA-encoded protein levels may be unaltered ((48),(59)), suggesting the action of compensatory mechanisms that upregulate the synthesis of mtDNA-encoded proteins. It is worth noting that mitochondrial mass, as measured directly through mitochondrial protein quantification after mitochondria isolation ((45),(59)) or indirectly by assessment of citrate synthase activity ((48)), may even be increased in white adipose tissue from patients with lowered mtDNA levels. This indicates that, at least in pathogenic conditions, mtDNA levels may be dissociated from mitochondrial mass in adipose tissue. However, the enhancement of mitochondrial biogenesis in mtDNA-depleted adipose tissue does not necessarily lead to enhancement of mitochondria functional activity. The scenario may be similar to the proliferation of abnormal mitochondria occurring in skeletal muscle from patients with mtDNA pathogenic mutations and leading to the appearance of the ragged-red structures in muscle fibers from these patients ((60)). The consequences on bioenergetic mitochondrial function of the enhanced mitochondrial mass observed in mtDNA-depleted white adipose tissue from patients with lipodystrophy have not been extensively studied, but a recent report indicates no increase in cytochrome c oxidase activity in association with the increased mitochondrial mass ((48)). On the other hand, even if mtDNA depletion causes some impairment in respiratory chain, it has been proposed that its consequences may result not only from lowered ATP production but also from a reduced synthesis of uridine. An active respiratory chain is required for the function of dihydroorotate dehydrogenase, a key enzyme in the de novo synthesis of all intracellular pyrimidines. Treatment of white adipocytes with uridine ameliorates the disturbances elicited by drugs that cause mtDNA depletion, thus suggesting that unaltered mtDNA levels are required for correct rates of uridine synthesis and that uridine homeostasis is closely related to the maintenance of white adipocyte differentiation and function ((56)). It is likely that a reduction in the levels of mtDNA to a certain extent is capable of maintaining normal function; at less than permissive levels, a threshold is crossed below which white adipocyte homeostatic mechanisms cannot ensure unaltered levels of mtDNA-encoded gene products. Currently, a precise understanding of these mtDNA expression regulatory mechanisms, which are likely to involve the mtDNA transcription and translation machinery, is lacking. This is a promising area of research for pharmacological or nutritional intervention designed to enhance the rates of mtDNA gene expression, and thereby ameliorate the pathogenic consequences of suprathreshold mtDNA depletion in white adipose tissue. Most studies have focused on relating mtDNA depletion in fat to white adipose tissue atrophy in HIV-1 patients. This reflects the ease of access to the atrophic subcutaneous adipose tissue for biopsies and subsequent analysis. Other signs of HIV lipodystrophy, such as visceral obesity or lipomatosis are not associated with any loss of adipose tissue mass. A study of biopsies from “buffalo hump,” an enlargement of the adipose mass in the dorso-cervical area that occurs in some HIV-1 patients with lipodystrophy, revealed a depletion of mtDNA similar to that found in subcutaneous lipoatrophic adipose tissue ((47)). Recently, the availability of samples from HIV-1 patient visceral adipose tissue, a site prone to enlargement in patients with HIV lipodystrophy, revealed also a similar reduction in mtDNA levels with respect to subcutaneous white fat ((61)). These observations question the presumption that abnormally low mtDNA levels could influence the outcome—atrophy vs. hypertrophy—for adipose tissue in a simple manner. On the other hand, some reports indicate that the first signs of mtDNA depletion in white adipose tissue occur in the absence of treatment, simply due to HIV infection ((62)) although most reports attribute mtDNA depletion exclusively to antiretroviral treatment ((63)). In experimental rodent models in which there is no HIV-1 infection, it has been very difficult to induce mtDNA depletion and lipodystrophy, even using large doses of antiretroviral drugs ((64)), suggesting that HIV-1 events contribute to the full of the mtDNA depletion Recently, two models of mtDNA have been through or of forms of the gene for mitochondrial the enzyme responsible for for DNA synthesis the mitochondria In there was a depletion of mtDNA levels in white and brown adipose tissue. In addition to a pathology of human in patients with adipose tissues were also strongly and mice showed signs of lipodystrophy including loss of subcutaneous white fat and abnormal brown adipose tissue Another recent report mtDNA depletion in fat in a a of the nuclear estrogen-related This receptor appears to play a major role in the expression of nuclear genes involved in mitochondrial among these are related to the mtDNA replication machinery. show amounts of white adipose tissue, abnormal accumulation of brown fat, and to obesity Although no data are available for white fat, the brown fat from these mice also a reduction of mtDNA levels and of mtDNA-encoded transcripts This summary of the available data evidence for a role for mtDNA in white adipose tissue biology, and indicates that changes in mtDNA levels and expression of mtDNA-encoded genes in white adipose tissue may have important consequences in pathogenic conditions, including obesity. However, we are far from having a full understanding of the mechanistic that relate mtDNA biology and white adipose tissue The relationship between mtDNA and adipose tissue mass is not and our present knowledge does not a mechanistic that our current knowledge of the role of mtDNA in cellular with the of white adipose tissue disturbances associated with mtDNA Studies of genetic mtDNA diseases in several tissues and have provided evidence to support the relevance of threshold effects that relate mtDNA altered levels and function with cellular and there are that such threshold effects may be do not what threshold level of white adipocyte mtDNA is with the of white adipose cell However, certain observations suggest that changes in mtDNA levels are not involved in determining intracellular oxidative in white glitazones increase mtDNA levels in white adipocytes, with increased differentiation and enhancement of fat in adipocytes, and mtDNA levels are associated with lipogenesis in Thus, the impact of in white adipocyte mtDNA not be as the simple of the potential changes in mitochondrial and cellular ATP synthesis elicited by altered mtDNA function. In this a recent study has reported that reductions in mtDNA levels and respiratory chain activity in adipocytes, elicited by of TFAM, caused impaired and through unknown mechanisms It is likely that the mtDNA levels or mtDNA expression in the white adipocyte mitochondria may influence nuclear gene expression through (e.g., through intracellular oxygen or changes in and thus could affect overall adipocyte in a complex manner. Moreover, the consequences of in mtDNA levels and expression of mtDNA-encoded genes on the expression of nuclear genes in the white adipocyte may involve the synthesis and release of adipokines and therefore may have systemic consequences that underlie the of In evidence a role for mtDNA in white adipose tissue and indicates that disturbances in mtDNA function may be involved in adipose tissue pathologies, including obesity. Further research will be required to the precise mechanisms by which mtDNA functional changes result in of adipose tissue mass, and identify pharmacological nutritional for mtDNA that This was by de and de de The no of

Antisense reduction of 11β-hydroxysteroid dehydrogenase type 1 enhances energy expenditure and insulin sensitivity independent of food intake in C57BL/6J mice on a Western-type diet

Low-protein/high-carbohydrate diet induces AMPK-dependent canonical and non-canonical thermogenesis in subcutaneous adipose tissue

LSD1 mediates microbial metabolite butyrate-induced thermogenesis in brown and white adipose tissue

Aims/hypothesisObesity and insulin resistance may be associated with elevated plasma concentration of branched-chain amino acids (BCAAs) and impaired BCAA metabolism. However, it is unknown whether the insulin-sensitising effect of long-term exercise can be explained by concomitant change in BCAAs and their metabolism.MethodsWe included 26 sedentary overweight and normal-weight middle-aged men from the MyoGlu clinical trial, with or without dysglycaemia, for 12 weeks of supervised intensive exercise intervention, including two endurance and two resistance sessions weekly. Insulin sensitivity was measured as the glucose infusion rate (GIR) from a hyperinsulinaemic−euglycaemic clamp. In addition, maximum oxygen uptake, upper and lower body strength and adipose tissue depots (using MRI and spectroscopy) were measured, and subcutaneous white adipose tissue (ScWAT) and skeletal muscle (SkM) biopsies were harvested both before and after the 12 week intervention. In the present study we have measured plasma BCAAs and related metabolites using CG-MS/MS and HPLC-MS/MS, and performed global mRNA-sequencing pathway analysis on ScWAT and SkM.ResultsIn MyoGlu, men with dysglycaemia displayed lower GIR, more fat mass and higher liver fat content than normoglycaemic men at baseline, and 12 weeks of exercise increased GIR, improved body composition and reduced liver fat content similarly for both groups. In our current study we observed higher plasma concentrations of BCAAs (14.4%, p = 0.01) and related metabolites, such as 3-hydroxyisobutyrate (19.4%, p = 0.034) in dysglycaemic vs normoglycaemic men at baseline. Baseline plasma BCAA levels correlated negatively to the change in GIR (ρ = −0.41, p = 0.037) and dot{V}{mathrm{O}}_{2max } (ρ = −0.47, p = 0.015) after 12 weeks of exercise and positively to amounts of intraperitoneal fat (ρ = 0.40, p = 0.044) and liver fat (ρ = 0.58, p = 0.01). However, circulating BCAAs and related metabolites did not respond to 12 weeks of exercise, with the exception of isoleucine, which increased in normoglycaemic men (10 μmol/l, p = 0.01). Pathway analyses of mRNA-sequencing data implied reduced BCAA catabolism in both SkM and ScWAT in men with dysglycaemia compared with men with normoglycaemia at baseline. Gene expression levels related to BCAA metabolism correlated positively with GIR and markers of mitochondrial content in both SkM and ScWAT, and negatively with fat mass generally, and particularly with intraperitoneal fat mass. mRNA-sequencing pathway analysis also implied increased BCAA metabolism after 12 weeks of exercise in both groups and in both tissues, including enhanced expression of the gene encoding branched-chain α-ketoacid dehydrogenase (BCKDH) and reduced expression of the BCKDH phosphatase in both groups and tissues. Gene expression of SLC25A44, which encodes a mitochondrial BCAA transporter, was increased in SkM in both groups, and gene expression of BCKDK, which encodes BCKDH kinase, was reduced in ScWAT in dysglycaemic men. Mediation analyses indicated a pronounced effect of enhanced SkM (~53%, p = 0.022), and a moderate effect of enhanced ScWAT (~18%, p = 0.018) BCAA metabolism on improved insulin sensitivity after 12 weeks of exercise, based on mRNA sequencing. In comparison, plasma concentration of BCAAs did not mediate any effect in this regard.Conclusion/interpretationPlasma BCAA concentration was largely unresponsive to long-term exercise and unrelated to exercise-induced insulin sensitivity. On the other hand, the insulin-sensitising effect of long-term exercise in men may be explained by enhanced SkM and, to a lesser degree, also by enhanced ScWAT BCAA catabolism.Graphical abstract

Phosphodiesterase type 10A (PDE10A) is highly enriched in striatum and is under evaluation as a drug target for several psychiatric/neurodegenerative diseases. Preclinical studies implicate PDE10A in the regulation of energy homeostasis, but the mechanisms remain unclear. By utilizing small‐animal PET/MRI and the novel radioligand [18F]‐AQ28A, we found marked levels of PDE10A in interscapular brown adipose tissue (BAT) of mice. Pharmacological inactivation of PDE10A with the highly selective inhibitor MP‐10 recruited BAT and potentiated thermogenesis in vivo. In diet‐induced obese mice, chronic administration of MP‐10 caused weight loss associated with increased energy expenditure, browning of white adipose tissue, and improved insulin sensitivity. Analysis of human PET data further revealed marked levels of PDE10A in the supraclavicular region where brown/beige adipocytes are clustered in adults. Finally, the inhibition of PDE10A with MP‐10 stimulated thermogenic gene expression in human brown adipocytes and induced browning of human white adipocytes. Collectively, our findings highlight a novel thermoregulatory role for PDE10A in mouse and human adipocytes and promote PDE10A inhibitors as promising candidates for the treatment of obesity and diabetes.

We have investigated whether GH treatment influences the expression of UCP1, 2 and 3 mRNA in a KK-Ay obese mouse model. KK-Ay mice (n = 10) and C57Bl/6J control mice (n = 10) were injected subcutaneously with human GH (1.0 mg/kg/day and 3.5 mg/kg/day) for 10 days, and compared with mice injected with physical saline. The KK-Ay obese mice weighed significantly less (p < 0.01 : 1.0 mg/kg/day, p < 0.05 : 3.5 mg/kg/day) and had smaller inguinal subcutaneous and perimetric white adipose tissue (WAT) pads (p < 0.05 : 3.5 mg/kg/day), but increased skeletal muscle weight (p < 0.05). The brown adipose tissue (BAT) weight did not change significantly. Not only plasma free fatty acid and glucose levels but also plasma insulin levels decreased. The reduced HOMA-IR (homeostasis model assessment-insulin resistance) values suggested that insulin resistance was improved by GH treatment. UCP1 mRNA levels increased after the 3.5 mg GH treatment by 2.8-fold (p < 0.01 vs. saline controls) and 2.0-fold (p < 0.05 vs. 1 mg GH treatment) in BAT, and by 6.0-fold in subcutaneous WAT (p < 0.05 vs. controls). UCP2 mRNA levels increased 2.2-fold (p < 0.05 vs. control) and 2.1-fold (p < 0.05 vs. 1 mg GH treatment) in BAT, and 2.0-fold (p < 0.05 vs. controls) in skeletal muscle. One mg GH administration also stimulated UCP1 mRNA expression by 2.5-fold (p < 0.05 vs. controls) and UCP3 mRNA expression by 2.8-fold (p < 0.05 vs. controls) in the muscle. On the other hand, lean mice showed no significant difference in body composition or plasma parameters. UCP1, 2 and 3 mRNA expression in lean mice did not show any significant change after treatment with GH. We conclude that GH treatment increased mRNA levels for not only UCP1, but also UCP 2 and 3 in BAT, WAT and muscle in a KK-Ay obese mouse model. These findings suggest that GH-induced thermogenesis may contribute to the reduction in WAT and energy expenditure.

Targeted lipidomics and transcriptomics profiling reveal the heterogeneity of visceral and subcutaneous white adipose tissue

Introdution and Aims: The myokine irisin is critical to modulating adipocytes thermogenesis and influence whole-body metabolism. However, whether there is difference in the effects of irisin on adipocytes derived from different depots remains unknown, and the receptor of irisin on adipocytes is still unclear. In this study, we determine the browning effect of irisin on adipocytes of subcutaneous and visceral human adipose tissue and explore the possibility that integrin αV was the receptor of irisin on human adipocytes. Methods: Human adipose-derived stem cells were isolated from human subcutaneous and visceral white adipose tissues and induced to differentiate into mature adipocytes, and the expression of UCP1 and thermogenic genes in mature adipocytes were examined with or without irisin treatment and compared between groups of different adiposity and different spots. Immunoprecipitation analysis was used to detect the interaction between irisin and integrin αV on adipocytes, and the protein expression of integrin αV in adipocytes was also compared between groups of different adiposity and anatomic position. Results: Irisin treatment could increase the expression level of beige adipocyte marker protein UCP1 and specific thermogenic genes in mature adipocytes derived from subcutaneous white adipose tissue but not in visceral adipose tissue. The results of immunoprecipitation showed that irisin could be attached to integrin αV on mature adipocytes, and there was no significant difference in the gene and protein expression of integrin αV in adipocytes, either derived from subcutaneous and visceral adipose tissue, or derived from obese and normal-weight individuals. Conclusion: The results of the present study indicated that irisin contributed to the transformation of mature white adipocytes to beige adipocytes in human subcutaneous adipose tissue but not in visceral adipose tissue. Integrin αV may mediate the browning effects of irisin on human mature adipocytes, which could provide the potential therapeutic targets for obesity and metabolic syndrome by promoting human brown adipose tissue activity.

Objective To explore the effect and mechanism of omega 3-polyunsaturated fatty acid(ω3-PUFA) dietary intervention on mitochondrial function of white adipose tissue in adult rats with postnatal early overfeeding. Methods An overfed animal model by adjusting litter size was developed for the study of neonatal overfeeding. The litter size was adjusted to 3 male rats per litter(small litter, SL group) and 10 pups per litter(normal litter, NL group). After weaning(week 3), the pups were fed standard chow or ω3-PUFA diet(SL-FO) until postnatal weeks 13. Food intake, body weight, and rectal temperature of rats were measured regularly, and energy metabolism of animals was monitored in week 13. During week 3 and 13, subcutaneous adipose tissue was collected. Inguinal preadipocytes of mice were isolated and induced to differentiate, and 50 μmol/L eicosapentaenoicacid(EPA) was administered for 48 h at the late stage of differentiation. The mRNA and protein expression levels of mitochondrial related genes, mitochondrial copy number, and oxygen consumption rate of adipocytes were detected in adipose tissue and adipocytes. Results By the 3rd week, the body weight, food intake, and fat cell area in SL group were higher than those in NL group while the body temperature was lower until to 13 weeks. By the 13th week, the O2 consumption, CO2 output, and heat production of rats in SL group were lower than those in NL group. Meanwhile, the expressions of mitochondrial function related genes such as uncoupling protein 1(UCP1), carnitine palmitoyltransferase 1(CPT1), SIRT1, and mitochondrial biosynthesis regulatory gene peroxisome proliferator-activated receptor coativator-1 (PGC1α) in adipose tissue by the 3rd and 13th week were significantly reduced(P<0.05). After weaning, ω3-PUFA diet significantly reduced weight gain in SL rats, increased UCP1 protein expression, restored energy metabolism level and mitochondrial function related gene expression. In vitro intervention of EPA increased the mitochondrial copy number, the mRNA and protein expression levels of mitochondrial biosynthesis and functional genes, as well as the mitochondrial basic oxygen consumption rate(P<0.05). Conclusion ω3-PUFA improves postnatal overfeeding-induced impairment of the mitochondrial function and biosynthesis of subcutaneous white adipose tissue in rats, which may be an important mechanism for fish oil diet to inhibit the early over-nutrition program and restore the thermogenic metabolism. Key words: Omega 3-PUFA; Postnatal overfeeding; White fat; Mitochondria

Given that increased thermogenesis in white adipose tissue, also known as browning, promotes energy expenditure, significant efforts have been invested to determine the molecular factors involved in this process. Here we show that HOXC10, a homeobox domain-containing transcription factor expressed in subcutaneous white adipose tissue, is a suppressor of genes involved in browning white adipose tissue. Ectopic expression of HOXC10 in adipocytes suppresses brown fat genes, whereas the depletion of HOXC10 in adipocytes and myoblasts increases the expression of brown fat genes. The protein level of HOXC10 inversely correlates with brown fat genes in subcutaneous white adipose tissue of cold-exposed mice. Expression of HOXC10 in mice suppresses cold-induced browning in subcutaneous white adipose tissue and abolishes the beneficial effect of cold exposure on glucose clearance. HOXC10 exerts its effect, at least in part, by suppressing PRDM16 expression. The results support that HOXC10 is a key negative regulator of the process of browning in white adipose tissue.

Cardiotrophin-1 Is a Key Regulator of Glucose and Lipid Metabolism