Joan Villarroya

An endocrine role for brown adipose tissue

White adipose tissue is recognized as both a site of energy storage and an endocrine organ that produces a myriad of endocrine factors called adipokines. Brown adipose tissue (BAT) is the main site of nonshivering thermogenesis in mammals. The amount and activity of brown adipocytes are associated with protection against obesity and associated metabolic alterations. These effects of BAT are traditionally attributed to its capacity for the oxidation of fatty acids and glucose to sustain thermogenesis. However, recent data suggest that the beneficial effects of BAT could involve a previously unrecognized endocrine role through the release of endocrine factors. Several signaling molecules with endocrine properties have been found to be released by brown fat, especially under conditions of thermogenic activation. Moreover, experimental BAT transplantation has been shown to improve glucose tolerance and insulin sensitivity mainly by influencing hepatic and cardiac function. It has been proposed that these effects are due to the release of endocrine factors by brown fat, such as insulin-like growth factor I, interleukin-6, or fibroblast growth factor-21. Further research is needed to determine whether brown fat plays an endocrine role and, if so, to comprehensively identify which endocrine factors are released by BAT. Such research may reveal novel clues for the observed association between brown adipocyte activity and a healthy metabolic profile, and it could also enlarge a current view of potential therapeutic tools for obesity and associated metabolic diseases.

Brown adipose tissue as a secretory organ

Brown adipose tissue (BAT) is the main site of adaptive thermogenesis and experimental studies have associated BAT activity with protection against obesity and metabolic diseases, such as type 2 diabetes mellitus and dyslipidaemia. Active BAT is present in adult humans and its activity is impaired in patients with obesity. The ability of BAT to protect against chronic metabolic disease has traditionally been attributed to its capacity to utilize glucose and lipids for thermogenesis. However, BAT might also have a secretory role, which could contribute to the systemic consequences of BAT activity. Several BAT-derived molecules that act in a paracrine or autocrine manner have been identified. Most of these factors promote hypertrophy and hyperplasia of BAT, vascularization, innervation and blood flow, processes that are all associated with BAT recruitment when thermogenic activity is enhanced. Additionally, BAT can release regulatory molecules that act on other tissues and organs. This secretory capacity of BAT is thought to be involved in the beneficial effects of BAT transplantation in rodents. Fibroblast growth factor 21, IL-6 and neuregulin 4 are among the first BAT-derived endocrine factors to be identified. In this Review, we discuss the current understanding of the regulatory molecules (the so-called brown adipokines or batokines) that are released by BAT that influence systemic metabolism and convey the beneficial metabolic effects of BAT activation. The identification of such adipokines might also direct drug discovery approaches for managing obesity and its associated chronic metabolic diseases.

FGF21 expression and release in muscle cells: involvement of MyoD and regulation by mitochondria-driven signalling

Although the liver is generally considered the main site of production of FGF21 (fibroblast growth factor-21), high FGF21 levels have been found to be associated with neuromuscular mitochondrial genetic diseases, and there are indications that the muscle may be a relevant site of FGF21 production under conditions of muscular mitochondrial stress. In the present study, we found that expression and release of FGF21 was associated with myogenic differentiation, and we identified MyoD as a major controller of FGF21 gene transcription. Mimicking mitochondrial dysfunction using respiratory chain/oxidative phosphorylation inhibitors resulted in enhanced expression and release of FGF21 by muscle cells. The increased production of reactive oxygen species, subsequent induction of p38 MAPK (mitogen-activated protein kinase) and activation of an ATF2 (activating transcription factor 2)-binding site at the proximal promoter region of the FGF21 gene was found to be a major mechanism linking mitochondrial dysfunction with enhanced FGF21 gene transcription in myogenic cells. The myogenic factor MyoD was required for the induction of FGF21 gene transcription by mitochondrial dysfunction, thus explaining the preferential response of muscle cells to mitochondrial dysfunction-induced FGF21 expression and secretion. FGF21 release by muscle cells in response to mitochondrial alterations may represent a physiological mechanism by which the sensing of internal energetic status by muscles results in the release of FGF21 to favour systemic metabolic adaptations.

Mitochondrial DNA: an up-and-coming actor in white adipose tissue pathophysiology.

IntroductIon: MItochondrIal dna and adIpose cells, What’s up? 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 mtDNAand 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.

Hypothalamic AMPK-ER Stress-JNK1 Axis Mediates the Central Actions of Thyroid Hormones on Energy Balance

Summary Thyroid hormones (THs) act in the brain to modulate energy balance. We show that central triiodothyronine (T3) regulates de novo lipogenesis in liver and lipid oxidation in brown adipose tissue (BAT) through the parasympathetic (PSNS) and sympathetic nervous system (SNS), respectively. Central T3 promotes hepatic lipogenesis with parallel stimulation of the thermogenic program in BAT. The action of T3 depends on AMP-activated protein kinase (AMPK)-induced regulation of two signaling pathways in the ventromedial nucleus of the hypothalamus (VMH): decreased ceramide-induced endoplasmic reticulum (ER) stress, which promotes BAT thermogenesis, and increased c-Jun N-terminal kinase (JNK) activation, which controls hepatic lipid metabolism. Of note, ablation of AMPKα1 in steroidogenic factor 1 (SF1) neurons of the VMH fully recapitulated the effect of central T3, pointing to this population in mediating the effect of central THs on metabolism. Overall, these findings uncover the underlying pathways through which central T3 modulates peripheral metabolism.

HIV-1-Infected Long-Term Non-Progressors have Milder Mitochondrial Impairment and Lower Mitochondrially-Driven Apoptosis in Peripheral Blood Mononuclear Cells than Typical Progressors

Mitochondrial parameters in peripheral blood mononuclear cells (PBMC) and their relationship with mitochondrially-driven PBMC apoptosis were investigated in a group of HIV-1-infected long-term nonprogressors (LTNP) and compared with untreated asymptomatic HIV-1 infected typical progressors (TP) and uninfected healthy controls (HC). Twenty-six LTNP, 27 TP and 31 HC were evaluated. Studies were performed in PBMCs. Mitochondrial DNA content (mtDNA) was assessed by quantitative real-time PCR. Activities of mitochondrial respiratory chain complexes (MRC) II, III and IV were determined by spectrophotometry. Caspase-3 activity was assessed by fluorimetry, and caspase-9 activation and Bcl-2 levels were assessed by immunoblotting. mtDNA abundance (p<0.05), MRC complex II (p<0.001), complex III (p<0.01) and complex IV (p=0.01) were lower in the TP group than in the HC group. In the LTNP group these parameters were similar to those of the HC group except for complex II, which was decreased (p<0.01). The PBMC of TP showed the highest overall apoptotic activation, since their caspase-3 activity was greater than that of HC (p<0.05) and LTNP. In the case of LTNP, however, the difference was non-significant. Caspase-9 and the caspase-9/Bcl-2 ratio were both over-expressed in TP compared to HC (p<0.01) and LTNP (p<0.05). Both of these measurements indicate that mitochondrially-driven apoptosis in TP is greater than in LTNP and HC. A relationship between mitochondrial damage and apoptotic activation was found in TP. Mitochondrial damage is associated with increased PBMC apoptosis in patients with active HIV-1 replication (TP). These abnormalities are slight or not present in LTNP.

Epidemiología e impacto de la otitis media aguda en la Comunidad Valenciana

Objetivo Estimar el impacto (incidencia, tratamiento y complicaciones) de la otitis media aguda (OMA) y la otitis media serosa (OMS) en los ninos menores de 5 anos de la Comunidad Valenciana (Espana) Sujetos y metodos Cohorte retrospectiva de 1.399 ninos seguidos durante los primeros 5 anos de vida. Diecisiete pediatras revisaron las historias clinicas de sus pacientes nacidos en 1995 y 1996, controlados desde su nacimiento hasta los 5 anos. Se obtuvo de cada nino el numero de episodios de otitis, tratamiento, complicaciones e intervenciones quirurgicas requeridas Resultados Hubo 2.961 episodios de OMA en los primeros 5 anos de vida (2,23 casos/nino), 476 casos (16,1 %) antes del ano y 1.346 entre el primer y segundo ano (45,5 %). Al tercer ano, el 59,8 % habian presentado al menos un episodio. La mayoria de casos (80,9 %) se diagnosticaron en atencion primaria, con 1,81 visitas/episodio de media para su seguimiento. El 94,5% de los episodios fueron tratados con antibiotico (amoxicilina-acido clavulanico 38,8 %, cefuroxima- axetilo, 14,3%; claritromicina, 8,2%, y amoxicilina 5,9 %). El 8,5 % requirio cambio a un segundo antibiotico. Presentaron al menos un episodio de OMS 217 ninos (15,2 %). Necesitaron implantacion de tubos de ventilacion 26 pacientes (1,8 %, intervalo de confianza del 95 % (IC 95 %), 1,2–2,7). Presentaron hipoacusia secundaria 24 ninos (1,7 %). Hubo un caso de meningitis y dos de otorrea cronica. No hubo mastoiditis Conclusiones La incidencia de OMA en la Comunidad Valenciana es de 40.014 episodios/100.000 ninos menores de 5 anos/ano (IC 95 %, 39.700–40.300). Supone un gran impacto sanitario por el elevado numero de visitas, uso antibiotico, cirugia asociada y necesidad de rehabilitacion auditiva

Histone macroH2A1.2 promotes metabolic health and leanness by inhibiting adipogenesis

BackgroundObesity has tremendous impact on the health systems. Its epigenetic bases are unclear. MacroH2A1 is a variant of histone H2A, present in two alternatively exon-spliced isoforms macroH2A1.1 and macroH2A1.2, regulating cell plasticity and proliferation, during pluripotency and tumorigenesis. Their role in adipose tissue plasticity is unknown.ResultsHere, we show evidence that macroH2A1.1 protein levels in the visceral adipose tissue of obese humans positively correlate with BMI, while macroH2A1.2 is nearly absent. We thus introduced a constitutive GFP-tagged transgene for macroH2A1.2 in mice, and we characterized their metabolic health upon being fed a standard chow diet or a high fat diet. Despite unchanged food intake, these mice exhibit lower adipose mass and improved glucose metabolism both under a chow and an obesogenic diet. In the latter regimen, transgenic mice display smaller pancreatic islets and significantly less inflammation. MacroH2A1.2 overexpression in the mouse adipose tissue induced dramatic changes in the transcript levels of key adipogenic genes; genomic analyses comparing pre-adipocytes to mature adipocytes uncovered only minor changes in macroH2A1.2 genomic distribution upon adipogenic differentiation and suggested differential cooperation with transcription factors. MacroH2A1.2 overexpression markedly inhibited adipogenesis, while overexpression of macroH2A1.1 had opposite effects.ConclusionsMacroH2A1.2 is an unprecedented chromatin component powerfully promoting metabolic health by modulating anti-adipogenic transcriptional networks in the differentiating adipose tissue. Strategies aiming at enhancing macroH2A1.2 expression might counteract excessive adiposity in humans.

Differentially altered molecular signature of visceral adipose tissue in HIV-1-associated lipodystrophy.

Objective:Lipodystrophy in HIV-1–infected antiretroviral-treated patients is often associated with opposite alterations in adipose tissue depots as follows: lipoatrophy of subcutaneous adipose tissue (SAT) versus lipohypertrophy of visceral adipose tissue (VAT). We determined the specific molecular alterations in VAT relative to SAT in patients. Design:We analyzed the expression of marker genes of mitochondrial function, adipogenesis, and inflammation in a unique collection of 8 biopsies of omental VAT from HIV-1–infected antiretroviral-treated patients with lipodystrophy. For comparison, we analyzed SAT from 10 patients, and SAT and VAT from 10 noninfected individuals. Methods:Quantitative real-time polymerase chain reaction of mitochondrial DNA and gene transcripts; immunoblot and multiplex for quantification of specific proteins. Results:Similar mitochondrial DNA depletion and abnormal increases in mitochondrial protein levels were found in VAT and SAT from patients. Transcript levels of adipogenesis and metabolism marker genes were unaltered in VAT but were decreased in SAT. Tumor necrosis factor &agr; and CD68 were similarly induced in both adipose depots from patients, but other markers of inflammation-related pathways showed distinct alterations as follows: interleukin 18 and interleukin 1 receptor antagonist were induced only in SAT, whereas interleukin 6, interleukin 8, and monocyte chemoattractant protein 1 expression was reduced in VAT but not in SAT. Conclusions:Mitochondrial alterations are similar in VAT and SAT from patients, whereas adipogenic gene expression is decreased in SAT but unaltered in VAT, highlighting the relevance of adipogenic processes in the differential alterations of fat depots. Specific disturbances in inflammatory status in VAT relative to SAT are present. Milder induction of proinflammatory signaling in VAT could be involved in preventing fat wasting in this depot.

Selective muscle fiber loss and molecular compensation in mitochondrial myopathy due to TK2 deficiency

A 12-year-old patient with mitochondrial DNA (mtDNA) depletion syndrome due to TK2 gene mutations has been evaluated serially over the last 10 years. We observed progressive muscle atrophy with selective loss of type 2 muscle fibers and, despite severe depletion of mtDNA, normal activities of respiratory chain (RC) complexes and levels of COX II mitochondrial protein in the remaining muscle fibers. These results indicate that compensatory mechanisms account for the slow progression of the disease. Identification of factors that ameliorate mtDNA depletion may reveal new therapeutic targets for these devastating disorders.