Natasa Petrovic

Chronic peroxisome proliferator-activated receptor gamma (PPARgamma) activation of epididymally derived white adipocyte cultures reveals a population of thermogenically competent, UCP1-containing adipocytes molecularly distinct from classic brown adipocytes.

The recent insight that brown adipocytes and muscle cells share a common origin and in this respect are distinct from white adipocytes has spurred questions concerning the origin and molecular characteristics of the UCP1-expressing cells observed in classic white adipose tissue depots under certain physiological or pharmacological conditions. Examining precursors from the purest white adipose tissue depot (epididymal), we report here that chronic treatment with the peroxisome proliferator-activated receptor γ agonist rosiglitazone promotes not only the expression of PGC-1α and mitochondriogenesis in these cells but also a norepinephrine-augmentable UCP1 gene expression in a significant subset of the cells, providing these cells with a genuine thermogenic capacity. However, although functional thermogenic genes are expressed, the cells are devoid of transcripts for the novel transcription factors now associated with classic brown adipocytes (Zic1, Lhx8, Meox2, and characteristically PRDM16) or for myocyte-associated genes (myogenin and myomirs (muscle-specific microRNAs)) and retain white fat characteristics such as Hoxc9 expression. Co-culture experiments verify that the UCP1-expressing cells are not proliferating classic brown adipocytes (adipomyocytes), and these cells therefore constitute a subset of adipocytes (“brite” adipocytes) with a developmental origin and molecular characteristics distinguishing them as a separate class of cells.

Myogenic gene expression signature establishes that brown and white adipocytes originate from distinct cell lineages

Attainment of a brown adipocyte cell phenotype in white adipocytes, with their abundant mitochondria and increased energy expenditure potential, is a legitimate strategy for combating obesity. The unique transcriptional regulators of the primary brown adipocyte phenotype are unknown, limiting our ability to promote brown adipogenesis over white. In the present work, we used microarray analysis strategies to study primary preadipocytes, and we made the striking discovery that brown preadipocytes demonstrate a myogenic transcriptional signature, whereas both brown and white primary preadipocytes demonstrate signatures distinct from those found in immortalized adipogenic models. We found a plausible SIRT1-related transcriptional signature during brown adipocyte differentiation that may contribute to silencing the myogenic signature. In contrast to brown preadipocytes or skeletal muscle cells, white preadipocytes express Tcf21, a transcription factor that has been shown to suppress myogenesis and nuclear receptor activity. In addition, we identified a number of developmental genes that are differentially expressed between brown and white preadipocytes and that have recently been implicated in human obesity. The interlinkage between the myocyte and the brown preadipocyte confirms the distinct origin for brown versus white adipose tissue and also represents a plausible explanation as to why brown adipocytes ultimately specialize in lipid catabolism rather than storage, much like oxidative skeletal muscle tissue.

Retinoblastoma protein functions as a molecular switch determining white versus brown adipocyte differentiation

Adipocyte precursor cells give raise to two major cell populations with different physiological roles: white and brown adipocytes. Here we demonstrate that the retinoblastoma protein (pRB) regulates white vs. brown adipocyte differentiation. Functional inactivation of pRB in wild-type mouse embryo fibroblasts (MEFs) and white preadipocytes by expression of simian virus 40 large T antigen results in the expression of the brown fat-specific uncoupling protein 1 (UCP-1) in the adipose state. Retinoblastoma gene-deficient (Rb–/–) MEFs and stem cells, but not the corresponding wild-type cells, differentiate into adipocytes with a gene expression pattern and mitochondria content resembling brown adipose tissue. pRB-deficient MEFs exhibit an increased expression of the Forkhead transcription factor Foxc2 and its target gene cAMP-dependent protein kinase regulatory subunit RIα, resulting in increased cAMP sensitivity. Suppression of cAMP-dependent protein kinase activity in Rb–/–MEFs blocked the brown adipocyte-like gene expression pattern without affecting differentiation per se. Immunohistochemical studies revealed that pRB is present in the nuclei of white but not brown adipocyte precursor cells at a developmental stage where both cell types begin to accumulate lipid and brown adipocytes express UCP-1. Furthermore, pRB rapidly undergoes phosphorylation upon cold-induced neodifferentiation and up-regulation of UCP-1 expression in brown adipose tissue. Finally, down-regulation of pRB expression accompanies transdifferentiation of white into brown adipocytes in response to β3-adrenergic receptor agonist treatment. We propose that pRB acts as a molecular switch determining white vs. brown adipogenesis, suggesting a previously uncharacterized function of this key cell cycle regulator in adipocyte lineage commitment and differentiation.

Ablation of PGC-1beta results in defective mitochondrial activity, thermogenesis, hepatic function, and cardiac performance.

The transcriptional coactivator peroxisome proliferator-activated receptor-gamma coactivator-1β (PGC-1β) has been implicated in important metabolic processes. A mouse lacking PGC-1β (PGC1βKO) was generated and phenotyped using physiological, molecular, and bioinformatic approaches. PGC1βKO mice are generally viable and metabolically healthy. Using systems biology, we identified a general defect in the expression of genes involved in mitochondrial function and, specifically, the electron transport chain. This defect correlated with reduced mitochondrial volume fraction in soleus muscle and heart, but not brown adipose tissue (BAT). Under ambient temperature conditions, PGC-1β ablation was partially compensated by up-regulation of PGC-1α in BAT and white adipose tissue (WAT) that lead to increased thermogenesis, reduced body weight, and reduced fat mass. Despite their decreased fat mass, PGC1βKO mice had hypertrophic adipocytes in WAT. The thermogenic role of PGC-1β was identified in thermoneutral and cold-adapted conditions by inadequate responses to norepinephrine injection. Furthermore, PGC1βKO hearts showed a blunted chronotropic response to dobutamine stimulation, and isolated soleus muscle fibres from PGC1βKO mice have impaired mitochondrial function. Lack of PGC-1β also impaired hepatic lipid metabolism in response to acute high fat dietary loads, resulting in hepatic steatosis and reduced lipoprotein-associated triglyceride and cholesterol content. Altogether, our data suggest that PGC-1β plays a general role in controlling basal mitochondrial function and also participates in tissue-specific adaptive responses during metabolic stress.

Hypoxia-Independent Angiogenesis in Adipose Tissues during Cold Acclimation

The molecular mechanisms of angiogenesis in relation to adipose tissue metabolism remain poorly understood. Here, we show that exposure of mice to cold led to activation of angiogenesis in both white and brown adipose tissues. In the inguinal depot, cold exposure resulted in elevated expression levels of brown-fat-associated proteins, including uncoupling protein-1 (UCP1) and PGC-1alpha. Proangiogenic factors such as VEGF were upregulated, and endogenous angiogenesis inhibitors, including thrombospondin, were downregulated. In wild-type mice, the adipose tissues became hypoxic during cold exposure; in UCP1(-/-) mice, hypoxia did not occur, but, remarkably, the augmented angiogenesis was unaltered and was thus hypoxia independent. Intriguingly, VEGFR2 blockage abolished the cold-induced angiogenesis and significantly impaired nonshivering thermogenesis capacity. Unexpectedly, VEGFR1 blockage resulted in the opposite effects: increased adipose vascularity and nonshivering thermogenesis capacity. Our findings have conceptual implications concerning application of angiogenesis modulators for treatment of obesity and metabolic disorders.

Dysregulation of mitochondrial dynamics and the muscle transcriptome in ICU patients suffering from sepsis induced multiple organ failure.

Background Septic patients treated in the intensive care unit (ICU) often develop multiple organ failure including persistent skeletal muscle dysfunction which results in the patients protracted recovery process. We have demonstrated that muscle mitochondrial enzyme activities are impaired in septic ICU patients impairing cellular energy balance, which will interfere with muscle function and metabolism. Here we use detailed phenotyping and genomics to elucidate mechanisms leading to these impairments and the molecular consequences. Methodology/Principal Findings Utilising biopsy material from seventeen patients and ten age-matched controls we demonstrate that neither mitochondrial in vivo protein synthesis nor expression of mitochondrial genes are compromised. Indeed, there was partial activation of the mitochondrial biogenesis pathway involving NRF2α/GABP and its target genes TFAM, TFB1M and TFB2M yet clearly this failed to maintain mitochondrial function. We therefore utilised transcript profiling and pathway analysis of ICU patient skeletal muscle to generate insight into the molecular defects driving loss of muscle function and metabolic homeostasis. Gene ontology analysis of Affymetrix analysis demonstrated substantial loss of muscle specific genes, a global oxidative stress response related to most probably cytokine signalling, altered insulin related signalling and a substantial overlap between patients and muscle wasting/inflammatory animal models. MicroRNA 21 processing appeared defective suggesting that post-transcriptional protein synthesis regulation is altered by disruption of tissue microRNA expression. Finally, we were able to demonstrate that the phenotype of skeletal muscle in ICU patients is not merely one of inactivity, it appears to be an actively remodelling tissue, influenced by several mediators, all of which may be open to manipulation with the aim to improve clinical outcome. Conclusions/Significance This first combined protein and transcriptome based analysis of human skeletal muscle obtained from septic patients demonstrated that losses of mitochondria and muscle mass are accompanied by sustained protein synthesis (anabolic process) while dysregulation of transcription programmes appears to fail to compensate for increased damage and proteolysis. Our analysis identified both validated and novel clinically tractable targets to manipulate these failing processes and pursuit of these could lead to new potential treatments.

Thermogenically competent nonadrenergic recruitment in brown preadipocytes by a PPARγ agonist

Most physiologically induced examples of recruitment of brown adipose tissue (BAT) occur as a consequence of chronic sympathetic stimulation (norepinephrine release within the tissue). However, in some physiological contexts (e.g., prenatal and prehibernation recruitment), this pathway is functionally contraindicated. Thus a nonsympathetically mediated mechanism of BAT recruitment must exist. Here we have tested whether a PPARgamma activation pathway could competently recruit BAT, independently of sympathetic stimulation. We continuously treated primary cultures of mouse brown (pre)adipocytes with the potent peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist rosiglitazone. In rosiglitazone-treated cultures, morphological signs of adipose differentiation and expression levels of the general adipogenic marker aP2 were manifested much earlier than in control cultures. Importantly, in the presence of the PPARgamma agonist the brown adipocyte phenotype was significantly enhanced: UCP1 was expressed even in the absence of norepinephrine, and PPARalpha expression and norepinephrine-induced PGC-1alpha mRNA levels were significantly increased. However, the augmented levels of PPARalpha could not explain the brown-fat promoting effect of rosiglitazone, as this effect was still evident in PPARalpha-null cells. In continuously rosiglitazone-treated brown adipocytes, mitochondriogenesis, an essential part of BAT recruitment, was significantly enhanced. Most importantly, these mitochondria were capable of thermogenesis, as rosiglitazone-treated brown adipocytes responded to the addition of norepinephrine with a large increase in oxygen consumption. This thermogenic response was not observable in rosiglitazone-treated brown adipocytes originating from UCP1-ablated mice; hence, it was UCP1 dependent. Thus the PPARgamma pathway represents an alternative, potent, and fully competent mechanism for BAT recruitment, which may be the cellular explanation for the enigmatic recruitment in prehibernation and prenatal states.

SOD2 overexpression: enhanced mitochondrial tolerance but absence of effect on UCP activity

We have created P1 artificial chromosome transgenic mice expressing the human mitochondrial superoxide dismutase 2 (SOD2) and thus generated mice with a physiologically controlled augmentation of SOD2 expression leading to increased SOD2 enzyme activities and lowered superoxide levels. In the transgenic mice, effects on mitochondrial function such as enhanced oxidative capacity and greater resistance against inducers of mitochondrial permeability were observed. Superoxide in the mitochondrial matrix has been proposed to activate uncoupling proteins (UCPs), thus providing a feedback mechanism that will lower respiratory chain superoxide production by increasing a proton leak across the inner mitochondrial membrane. However, UCP1 and UCP3 activities and mitochondrial ATP production rates were not altered in isolated mitochondria from SOD2 transgenic mice, despite lowered superoxide levels. Globally, the transgenic mice displayed normal resting metabolic rates, indicating an absence of effect on any UCP activities, and normal oxygen consumption responses after norepinephrine injection. These results strongly suggest that endogenously generated matrix superoxide does not regulate UCP activity and in vivo energy expenditure.

The human PINK1 locus is regulated in vivo by a non-coding natural antisense RNA during modulation of mitochondrial function

BackgroundMutations in the PTEN induced putative kinase 1 (PINK1) are implicated in early-onset Parkinsons disease. PINK1 is expressed abundantly in mitochondria rich tissues, such as skeletal muscle, where it plays a critical role determining mitochondrial structural integrity in Drosophila.ResultsHerein we characterize a novel splice variant of PINK1 (svPINK1) that is homologous to the C-terminus regulatory domain of the protein kinase. Naturally occurring non-coding antisense provides sophisticated mechanisms for diversifying genomes and we describe a human specific non-coding antisense expressed at the PINK1 locus (naPINK1). We further demonstrate that PINK1 varies in vivo when human skeletal muscle mitochondrial content is enhanced, supporting the idea that PINK1 has a physiological role in mitochondrion. The observation of concordant regulation of svPINK1 and naPINK1 during in vivo mitochondrial biogenesis was confirmed using RNAi, where selective targeting of naPINK1 results in loss of the PINK1 splice variant in neuronal cell lines.ConclusionOur data presents the first direct observation that a mammalian non-coding antisense molecule can positively influence the abundance of a cis-transcribed mRNA under physiological abundance conditions. While our analysis implies a possible human specific and dsRNA-mediated mechanism for stabilizing the expression of svPINK1, it also points to a broader genomic strategy for regulating a human disease locus and increases the complexity through which alterations in the regulation of the PINK1 locus could occur.

Hormone‐induced mitochondrial fission is utilized by brown adipocytes as an amplification pathway for energy expenditure

Adrenergic stimulation of brown adipocytes (BA) induces mitochondrial uncoupling, thereby increasing energy expenditure by shifting nutrient oxidation towards thermogenesis. Here we describe that mitochondrial dynamics is a physiological regulator of adrenergically‐induced changes in energy expenditure. The sympathetic neurotransmitter Norepinephrine (NE) induced complete and rapid mitochondrial fragmentation in BA, characterized by Drp1 phosphorylation and Opa1 cleavage. Mechanistically, NE‐mediated Drp1 phosphorylation was dependent on Protein Kinase‐A (PKA) activity, whereas Opa1 cleavage required mitochondrial depolarization mediated by FFAs released as a result of lipolysis. This change in mitochondrial architecture was observed both in primary cultures and brown adipose tissue from cold‐exposed mice. Mitochondrial uncoupling induced by NE in brown adipocytes was reduced by inhibition of mitochondrial fission through transient Drp1 DN overexpression. Furthermore, forced mitochondrial fragmentation in BA through Mfn2 knock down increased the capacity of exogenous FFAs to increase energy expenditure. These results suggest that, in addition to its ability to stimulate lipolysis, NE induces energy expenditure in BA by promoting mitochondrial fragmentation. Together these data reveal that adrenergically‐induced changes to mitochondrial dynamics are required for BA thermogenic activation and for the control of energy expenditure.