Peroxisome proliferator-activated receptor gamma coactivator 1 coactivators, energy homeostasis, and metabolism

Haploinsufficiency of the retinoblastoma protein gene reduces diet-induced obesity, insulin resistance, and hepatosteatosis in mice

Brown adipose tissue activity dissipates energy as heat, and there is evidence that lack of the retinoblastoma protein (pRb) may favor the development of the brown adipocyte phenotype in adipose cells. In this work we assessed the impact of germ line haploinsufficiency of the pRb gene (Rb) on the response to high-fat diet feeding in mice. Rb(+/-) mice had body weight and adiposity indistinguishable from that of wild-type (Rb(+/+)) littermates when maintained on a standard diet, yet they gained less body weight and body fat after long-term high-fat diet feeding coupled with reduced feed efficiency and increased rectal temperature. Rb haploinsufficiency ameliorated insulin resistance and hepatosteatosis after high-fat diet in male mice, in which these disturbances were more marked than in females. Compared with wild-type littermates, Rb(+/-) mice fed a high-fat diet displayed higher expression of peroxisome proliferator-activated receptor (PPAR)gamma as well as of genes involved in mitochondrial function, cAMP sensitivity, brown adipocyte determination, and tissue vascularization in white adipose tissue depots. Furthermore, Rb(+/-) mice exhibited signs of enhanced activation of brown adipose tissue and higher expression levels of PPARalpha in liver and of PPARdelta in skeletal muscle, suggestive of an increased capability for fatty acid oxidation in these tissues. These findings support a role for pRb in modulating whole body energy metabolism and the plasticity of the adipose tissues in vivo and constitute first evidence that partial deficiency in the Rb gene protects against the development of obesity and associated metabolic disturbances.

SIRT1 controls the transcription of the peroxisome proliferator-activated receptor-gamma Co-activator-1alpha (PGC-1alpha) gene in skeletal muscle through the PGC-1alpha autoregulatory loop and interaction with MyoD

Peroxisome proliferator activated receptor-gamma co-activator-1alpha (PGC-1alpha) is a transcriptional co-activator that coordinately regulates the expression of distinct sets of metabolism-related genes in different tissues. Here we show that PGC-1alpha expression is reduced in skeletal muscles from mice lacking the sirtuin family deacetylase SIRT1. Conversely, SIRT1 activation or overexpression in differentiated C2C12 myotubes increased PGC-1alpha mRNA expression. The transcription-promoting effects of SIRT1 occurred through stimulation of PGC-1alpha promoter activity and were enhanced by co-transfection of myogenic factors, such as myocyte enhancer factor 2 (MEF2) and, especially, myogenic determining factor (MyoD). SIRT1 bound to the proximal promoter region of the PGC-1alpha gene, an interaction potentiated by MEF2C or MyoD, which also interact with this region. In the presence of MyoD, SIRT1 promoted a positive autoregulatory PGC-1alpha expression loop, such that overexpression of PGC-1alpha increased PGC-1alpha promoter activity in the presence of co-expressed MyoD and SIRT1. Chromatin immunoprecipitation showed that SIRT1 interacts with PGC-1alpha promoter and increases PGC-1alpha recruitment to its own promoter region. Immunoprecipitation assays further showed that SIRT1-PGC-1alpha interactions are enhanced by MyoD. Collectively, these data indicate that SIRT1 controls PGC-1alpha gene expression in skeletal muscle and that MyoD is a key mediator of this action. The involvement of MyoD in SIRT1-dependent PGC-1alpha expression may help to explain the ability of SIRT1 to drive muscle-specific gene expression and metabolism. Autoregulatory control of PGC-1alpha gene transcription seems to be a pivotal mechanism for conferring a transcription-activating response to SIRT1 in skeletal muscle.

PGC-1{alpha} and PGC-1{beta} regulate mitochondrial density in neurons

Recent studies indicate that regulation of cellular oxidative capacity through enhancing mitochondrial biogenesis may be beneficial for neuronal recovery and survival in human neurodegenerative disorders. The peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) has been shown to be a master regulator of mitochondrial biogenesis and cellular energy metabolism in muscle and liver. The aim of our study was to establish whether PGC-1alpha and PGC-1beta control mitochondrial density also in neurons and if these coactivators could be up-regulated by deacetylation. The results demonstrate that PGC-1alpha and PGC-1beta control mitochondrial capacity in an additive and independent manner. This effect was observed in all studied subtypes of neurons, in cortical, midbrain, and cerebellar granule neurons. We also observed that endogenous neuronal PGC-1alpha but not PGC-1beta could be activated through its repressor domain by suppressing it. Results demonstrate also that overexpression of SIRT1 deacetylase or suppression of GCN5 acetyltransferase activates transcriptional activity of PGC-1alpha in neurons and increases mitochondrial density. These effects were mediated exclusively via PGC-1alpha, since overexpression of SIRT1 or suppression of GCN5 was ineffective where PGC-1alpha was suppressed by short hairpin RNA. Moreover, the results demonstrate that overexpression of PGC-1beta or PGC-1alpha or activation of the latter by SIRT1 protected neurons from mutant alpha-synuclein- or mutant huntingtin-induced mitochondrial loss. These evidences demonstrate that activation or overexpression of the PGC-1 family of coactivators could be used to compensate for neuronal mitochondrial loss and suggest that therapeutic agents activating PGC-1 would be valuable for treating neurodegenerative diseases in which mitochondrial dysfunction and oxidative damage play an important pathogenic role.

FGF21 induces PGC-1alpha and regulates carbohydrate and fatty acid metabolism during the adaptive starvation response

The liver plays a crucial role in mobilizing energy during nutritional deprivation. During the early stages of fasting, hepatic glycogenolysis is a primary energy source. As fasting progresses and glycogen stores are depleted, hepatic gluconeogenesis and ketogenesis become major energy sources. Here, we show that fibroblast growth factor 21 (FGF21), a hormone that is induced in liver by fasting, induces hepatic expression of peroxisome proliferator-activated receptor gamma coactivator protein-1alpha (PGC-1alpha), a key transcriptional regulator of energy homeostasis, and causes corresponding increases in fatty acid oxidation, tricarboxylic acid cycle flux, and gluconeogenesis without increasing glycogenolysis. Mice lacking FGF21 fail to fully induce PGC-1alpha expression in response to a prolonged fast and have impaired gluconeogenesis and ketogenesis. These results reveal an unexpected relationship between FGF21 and PGC-1alpha and demonstrate an important role for FGF21 in coordinately regulating carbohydrate and fatty acid metabolism during the progression from fasting to starvation.

Bcl3 interacts cooperatively with peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator 1alpha to coactivate nuclear receptors estrogen-related receptor alpha and PPARalpha

Estrogen-related receptors (ERRs) play critical roles in regulation of cellular energy metabolism in response to inducible coactivators such as peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator 1alpha (PGC-1alpha). A yeast two-hybrid screen led to the identification of the cytokine-stimulated transcriptional regulator, Bcl3, as an ERRalpha coactivator. Bcl3 was shown to synergize with PGC-1alpha to coactivate ERRalpha. Chromatin immunoprecipitation studies demonstrated that ERRalpha, PGC-1alpha, and Bcl3 form a complex on an ERRalpha-responsive element within the pyruvate dehydrogenase kinase 4 gene promoter in cardiac myocytes. Mapping studies demonstrated that Bc13 interacts with PGC-1alpha and ERRalpha, allowing for interaction with both proteins. Transcriptional profiling demonstrated that Bcl3 activates genes involved in diverse pathways including a subset involved in cellular energy metabolism known to be regulated by PGC-1alpha, ERRalpha, and a second nuclear receptor, PPARalpha. Consistent with the gene expression profiling results, Bcl3 was shown to synergistically coactivate PPARalpha with PGC-1alpha in a manner similar to ERRalpha. We propose that the cooperativity between Bcl3 and PGC-1alpha may serve as a point of convergence on nuclear receptor targets to direct programs orchestrating inflammatory and energy metabolism responses in heart and other tissues.

The biology of PGC-1α and its therapeutic potential

In eukaryotes, cellular and systemic metabolism is primarily controlled by mitochondrial activity. The peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) is an important regulator of mitochondrial biogenesis and function. Furthermore, PGC-1alpha controls many of the phenotypic adaptations of oxidative tissues to external and internal perturbations. By contrast, dysregulated metabolic plasticity is involved in the etiology of numerous diseases. Accordingly, modulation of PGC-1alpha levels and activity has recently been proposed as a therapeutic option for several pathologies. However, pharmacological interventions aimed at PGC-1alpha have to overcome inherent limitations of targeting a coactivator protein. Here, I focus on the recent breakthroughs in the identification of physiological and pathophysiological contexts involving PGC-1alpha. In addition, perspectives regarding the therapeutic importance of PGC-1alpha-controlled cellular and systemic metabolism are outlined.

Elucidation of separate, but collaborative functions of the rRNA methyltransferase-related human mitochondrial transcription factors B1 and B2 in mitochondrial biogenesis reveals new insight into maternally inherited deafness

Mitochondrial biogenesis is controlled by signaling networks that relay information to and from the organelles. However, key mitochondrial factors that mediate such pathways and how they contribute to human disease are not understood fully. Here we demonstrate that the rRNA methyltransferase-related human mitochondrial transcription factors B1 and B2 are key downstream effectors of mitochondrial biogenesis that perform unique, yet cooperative functions. The primary function of h-mtTFB2 is mtDNA transcription and maintenance, which is independent of its rRNA methyltransferase activity, while that of h-mtTFB1 is mitochondrial 12S rRNA methylation needed for normal mitochondrial translation, metabolism and cell growth. Over-expression of h-mtTFB1 causes 12S rRNA hypermethylation, aberrant mitochondrial biogenesis and increased sorbitol-induced cell death. These phenotypes are recapitulated in cells harboring the pathogenic A1555G mtDNA mutation, implicating a deleterious rRNA methylation-dependent retrograde signal in maternally inherited deafness pathology and shedding significant insight into how h-mtTFB1 acts as a nuclear modifier of this disease.

IF1, the endogenous regulator of the F(1)F(o)-ATPsynthase, defines mitochondrial volume fraction in HeLa cells by regulating autophagy

The protein IF1 limits mitochondrial ATP consumption when mitochondrial respiration is impaired by inhibiting the 'reverse' activity of the F(1)F(o)-ATPsynthase. We have found that IF1 also increases F(1)F(o)-ATPsynthase activity in respiring mitochondria, promoting its dimerization and increasing the density of mitochondrial cristae. We also noted that IF1 overexpression was associated with an increase in mitochondrial volume fraction that was conversely reduced when IF1 was knocked down using small interfering RNA (siRNA). The volume change did not correlate with the level of transcription factors involved in mitochondrial biogenesis. However, autophagy was dramatically increased in the IF1siRNA treated cells (-IF1), assessed by quantifying LC3-GFP translocation to autophagosomes, whilst levels of autophagy were low in IF1 overexpressing cells (+IF1). The increase in LC3-GFP labelled autophagosomes in -IF1 cells was prevented by the superoxide dismutase mimetic, manganese (III) tetrakis (4-benzoic acid) porphyrin (MnTBAP). An increase in the basal rate of generation of reactive oxygen species (ROS) in -IF1 cells was demonstrated using the fluorescent probe dihydroethidium (DHE). Thus, IF1 appears to limit mitochondrial ROS generation, limiting autophagy which is increased by IF1 knockdown. Variations in IF1 expression level may therefore play a significant role in defining both resting rates of ROS generation and cellular mitochondrial content.

Transcriptional control of brown adipocyte development and physiological function--of mice and men

The last several years have seen an explosion of information relating to the transcriptional control of brown fat cell development. At the same time, new data have emerged that clearly demonstrate that adult humans do indeed have substantial amounts of functioning brown adipose tissue (BAT). Together, these advances are stimulating a reassessment of the role of brown adipose tissue in human physiology and pathophysiology. These data have also opened up exciting new opportunities for the development of entirely novel classes of therapeutics for metabolic diseases like obesity and type 2 diabetes.

The transcriptional cascade associated with creatine kinase down-regulation and mitochondrial biogenesis in mice sarcoma

The tissue-specific expressions of creatine kinase (CK) isoforms are regulated by the coordinated action of various transcription factors. The myogenic differentiation factor D (MyoD) family of proteins and the myocyte-specific enhancer binding factor 2 family of transcription factors are important in regulating the muscle-specific expression of cytosolic muscle-type CK (MCK) and mitochondrial CKs. As reported in some related studies, TNF-α mediated degradation of MyoD and myogenin mRNA may lead to severe muscle wasting and cachexia, which is characterized by a low transcript level of MCK and myosin heavy chain proteins. In our previous study, we reported on a complete loss of total CK activity and expression when sarcoma was induced in mouse skeletal muscle (Patra et al. FEBS J. 275 (2008) 3236–3247). This study aimed at investigating the transcriptional cascade of CK down-regulation in carcinogen-induced sarcoma in mouse muscle. Both CK deficiency and enhanced nitric oxide synthase (NOS) were known to augment mitochondrial biogenesis, so we also explored the activation of the transcriptional cascade of mitochondrial biogenesis in this cancer. We observed the activation of the TNF-α-mediated nitric oxide production pathway with NFκB activation and concomitant degradation of MyoD and myogenin mRNA. Exploration of mitochondrial biogenesis revealed high cytochrome c oxidase activity and mitochondrial DNA content in sarcoma. The PGC-related co-activator seems to have a major role in regulating mitochondrial biogenesis by upregulating nuclear respiratory factors and mitochondrial transcription factor A. From the above findings, it can be concluded that severe muscle degeneration leads to CK down-regulation in sarcoma, and that the stimulation of mitochondrial biogenesis indicated a scenario representing both CK deficiency and NOS overexpression on the one hand, and altered bioenergetic profiling on the other.

Whole body overexpression of PGC-1alpha has opposite effects on hepatic and muscle insulin sensitivity

Type 2 diabetes is characterized by fasting hyperglycemia, secondary to hepatic insulin resistance and increased glucose production. Peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) is a transcriptional coactivator that is thought to control adaptive responses to physiological stimuli. In liver, PGC-1alpha expression is induced by fasting, and this effect promotes gluconeogenesis. To examine whether PGC-1alpha is involved in the pathogenesis of hepatic insulin resistance, we generated transgenic (TG) mice with whole body overexpression of human PGC-1alpha and evaluated glucose homeostasis with a euglycemic-hyperinsulinemic clamp. PGC-1alpha was moderately (approximately 2-fold) overexpressed in liver, skeletal muscle, brain, and heart of TG mice. In liver, PGC-1alpha overexpression resulted in increased expression of hepatocyte nuclear factor-4alpha and the gluconeogenic enzymes phosphoenolpyruvate carboxykinase and glucose-6-phosphatase. PGC-1alpha overexpression caused hepatic insulin resistance, manifested by higher glucose production and diminished insulin suppression of gluconeogenesis. Paradoxically, PGC-1alpha overexpression improved muscle insulin sensitivity, as evidenced by elevated insulin-stimulated Akt phosphorylation and peripheral glucose disposal. Content of myoglobin and troponin I slow protein was increased in muscle of TG mice, indicating fiber-type switching. PGC-1alpha overexpression also led to lower reactive oxygen species production by mitochondria and reduced IKK/IkappaB signaling in muscle. Feeding a high-fat diet to TG mice eliminated the increased muscle insulin sensitivity. The dichotomous effect of PGC-1alpha overexpression in liver and muscle suggests that PGC-1alpha is a fuel gauge that couples energy demands (muscle) with the corresponding fuel supply (liver). Thus, under conditions of physiological stress (i.e., prolonged fast and exercise training), increased hepatic glucose production may help sustain glucose utilization in peripheral tissues.

PGC-1alpha/beta induced expression partially compensates for respiratory chain defects in cells from patients with mitochondrial disorders

Members of the peroxisome proliferator-activated receptor gamma coactivator (PGC) family are potent inducers of mitochondrial biogenesis. We have tested the potential effect of increased mitochondrial biogenesis in cells derived from patients harboring oxidative phosphorylation defects due to either nuclear or mitochondrial DNA mutations. We found that the PGC-1alpha and/or PGC-1beta expression improved mitochondrial respiration in cells harboring a complex III or IV deficiency as well as in transmitochondrial cybrids harboring mitochondrial encephalomyopathy lactic acidosis and stroke A3243G tRNA((Leu)UUR) gene mutation. The respiratory function improvement was found to be associated with increased levels of mitochondrial components per cell, although this increase was not homogeneous. These results reinforce the concept that increased mitochondrial biogenesis is a promising venue for the treatment of mitochondrial diseases.

Obesity increases vascular senescence and susceptibility to ischemic injury through chronic activation of Akt and mTOR

Obesity and age are important risk factors for cardiovascular disease. However, the signaling mechanism linking obesity with age-related vascular senescence is unknown. Here we show that mice fed a high-fat diet show increased vascular senescence and vascular dysfunction compared to mice fed standard chow and are more prone to peripheral and cerebral ischemia. All of these changes involve long-term activation of the protein kinase Akt. In contrast, mice with diet-induced obesity that lack Akt1 are resistant to vascular senescence. Rapamycin treatment of diet-induced obese mice or of transgenic mice with long-term activation of endothelial Akt inhibits activation of mammalian target of rapamycin (mTOR)-rictor complex 2 and Akt, prevents vascular senescence without altering body weight, and reduces the severity of limb necrosis and ischemic stroke. These findings indicate that long-term activation of Akt-mTOR signaling links diet-induced obesity with vascular senescence and cardiovascular disease.

Small molecule activators of SIRT1 replicate signaling pathways triggered by calorie restriction in vivo

Background

Calorie restriction (CR) produces a number of health benefits and ameliorates diseases of aging such as type 2 diabetes. The components of the pathways downstream of CR may provide intervention points for developing therapeutics for treating diseases of aging. The NAD⁺-dependent protein deacetylase SIRT1 has been implicated as one of the key downstream regulators of CR in yeast, rodents, and humans. Small molecule activators of SIRT1 have been identified that exhibit efficacy in animal models of diseases typically associated with aging including type 2 diabetes. To identify molecular processes induced in the liver of mice treated with two structurally distinct SIRT1 activators, SIRT501 (formulated resveratrol) and SRT1720, for three days, we utilized a systems biology approach and applied Causal Network Modeling (CNM) on gene expression data to elucidate downstream effects of SIRT1 activation.

Results

Here we demonstrate that SIRT1 activators recapitulate many of the molecular events downstream of CR in vivo, such as enhancing mitochondrial biogenesis, improving metabolic signaling pathways, and blunting pro-inflammatory pathways in mice fed a high fat, high calorie diet.

Conclusion

CNM of gene expression data from mice treated with SRT501 or SRT1720 in combination with supporting in vitro and in vivo data demonstrates that SRT501 and SRT1720 produce a signaling profile that mirrors CR, improves glucose and insulin homeostasis, and acts via SIRT1 activation in vivo. Taken together these results are encouraging regarding the use of small molecule activators of SIRT1 for therapeutic intervention into type 2 diabetes, a strategy which is currently being investigated in multiple clinical trials.

Proteomic changes associated with diabetes in the BB-DP rat

These studies were structured with the aim of utilizing emerging technologies in two-dimensional (2D) gel electrophoresis and mass spectrometry to evaluate protein expression changes associated with type 1 diabetes. We reasoned that a broad examination of diabetic tissues at the protein level might open up novel avenues of investigation of the metabolic and signaling pathways that are adversely affected in type 1 diabetes. This study compared the protein expression of the liver, heart, and skeletal muscle of diabetes-prone rats and matched control rats by semiquantitative liquid chromatography-mass spectrometry and differential in-gel 2D gel electrophoresis. Differential expression of 341 proteins in liver, 43 in heart, and 9 (2D gel only) in skeletal muscle was detected. These data were assembled into the relevant metabolic pathways affected primarily in liver. Multiple covalent modifications were also apparent in 2D gel analysis. Several new hypotheses were generated by these data, including mechanisms of net cytosolic protein oxidation, formaldehyde generation by the methionine cycle, and inhibition of carbon substrate oxidation via reduction in citrate synthase and short-chain acyl-CoA dehydrogenase.

Transcribe to survive: transcriptional control of antioxidant defense programs for neuroprotection in Parkinson's disease

Parkinson's disease (PD) is a progressive, primarily motor disorder that is characterized by loss of dopaminergic (DA) neurons within the substantia nigra (SN). Cell death in PD has been associated with impaired mitochondrial function and increased oxidative stress. Strategies to reduce the oxidative load in DA cells may be beneficial in slowing the progression of PD. The transcription factor nuclear factor-erythroid 2 (NF-E2) related factor 2 (NRF2) is emerging as a master regulator of antioxidant defense systems, which makes it an attractive target for manipulations that aim to increase cellular resistance to oxidative stress. Peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator-1 alpha (PGC1alpha) is a regulator of mitochondrial biogenesis genes that simultaneously upregulates many genes known to protect against oxidative stress. Pgc-1alpha knockout mice show enhanced susceptibility to SN neuronal loss following MPTP exposure, whilst overexpression of Pgc-1alpha appears to protect against oxidative stress in vitro. This makes PGC-1alpha a highly attractive target for neuroprotective therapies in PD. This review will explore the mechanisms behind the induction of NRF2 and PGC-1alpha in response to oxidative stress and identify common pathways that may provide targets for upregulating antioxidant defense programs.

Fibroblast growth factor-19, a novel factor that inhibits hepatic fatty acid synthesis

Previous studies have shown that administration of fibroblast growth factor-19 (FGF-19) reverses diabetes, hepatic steatosis, hyperlipidemia, and adipose accretion in animal models of obesity. To investigate the mechanism for this effect, we determined whether FGF-19 modulated hepatic fatty acid synthesis, a key process controlling glucose tolerance and triacylglycerol accumulation in liver, blood, and adipose tissue. Incubating primary hepatocyte cultures with recombinant FGF-19 suppressed the ability of insulin to stimulate fatty acid synthesis. This effect was associated with a reduction in the expression of lipogenic enzymes. FGF-19 also suppressed the insulin-induced expression of sterol regulatory element-binding protein-1c (SREBP-1c), a key transcriptional activator of lipogenic genes. FGF-19 inhibition of lipogenic enzyme expression was not mediated by alterations in the activity of the insulin signal transduction pathway or changes in the activity of ERK, p38 MAPK, and AMP-activated protein kinase (AMPK). In contrast, FGF-19 increased the activity of STAT3, an inhibitor of SREBP-1c expression and decreased the expression of peroxisome proliferator-activated receptor-gamma coactivator-1beta (PGC-1beta), an activator of SREBP-1c activity. FGF-19 also increased the expression of small heterodimer partner (SHP), a transcriptional repressor that inhibits lipogenic enzyme expression via a SREBP-1c-independent mechanism. Inhibition of SREBP-1c activity by changes in STAT3 and PGC-1beta activity and inhibition of gene transcription by an elevation in SHP expression can explain the inhibition of lipogenesis caused by FGF-19. In summary, the inhibitory effect of FGF-19 on insulin activation of hepatic fatty acid synthesis constitutes a mechanism that would explain the beneficial effect of FGF-19 on metabolic syndrome.

Gestational ethanol exposure alters the behavioral response to ethanol odor and the expression of neurotransmission genes in the olfactory bulb of adolescent rats

Fetal exposure to ethanol is highly predictive of the propensity to ingest ethanol during adolescence and in utero chemosensory plasticity has been implicated as a contributing factor in this process. Recent rodent studies have shown that fetal ethanol exposure results in a tuned unconditioned sniffing and neurophysiological olfactory response to ethanol odor in infant animals. Importantly, a significant proportion of increased ethanol avidity at this age can be attributed to the tuned behavioral response to ethanol odor. These effects are absent in adults. Using behavioral methods and comprehensive gene expression profiling to screen for robust transcriptional differences induced in the olfactory bulb, we examined whether ethanol exposure via maternal diet results in an altered responsiveness to ethanol odor that persists into late adolescence and, if so, the molecular mechanisms that may be associated with such effects. Compared to controls, fetal exposure altered: the adolescent sniffing response to ethanol odor consistent with the previously observed changes in infant animals; and the expression of genes involved in synaptic transmission and plasticity as well as neuronal development (both cell fate and axon/neurite outgrowth). These data provide evidence for a persistence of olfactory-mediated responsiveness to ethanol into the period of adolescence. Further, they provide insight into an important relationship between fetal exposure to ethanol, adolescent odor responsiveness to the drug and potential underlying molecular mechanisms for the odor-guided behavioral response.

The role of the endocannabinoid system in lipogenesis and fatty acid metabolism

Endocannabinoids (ECs) regulate energy balance by modulating hypothalamic circuits controlling food intake and energy expenditure. However, convincing evidence has accumulated indicating that the EC system is present also in peripheral tissues, in particular in adipose tissue. Fat cells produce and are targets of ECs. Glucose uptake and lipoprotein lipase (LPL) activity, lipogenesis and adipogenesis are stimulated by ECs through cannabinoid 1 (CB1) receptors. Moreover, CB1 activation leads to a decreased mitochondrial biogenesis and function through inhibition of endothelial nitric oxide synthase (eNOS). All these effects are blocked by the CB1 antagonist rimonabant, suggesting that the weight-reducing effect of CB1 blockade is due not only to the transient suppression of food intake and reduction of lipogenesis but also to an increased mitochondrial biogenesis and oxidative metabolism which counteracts the inhibitory effects of ECs, levels of which are increased in fat tissues of obese rodents and humans. This review focuses on the role of ECs in adipose tissue metabolism, adipokine production, and interactions between ECs and peroxisome proliferator-activated receptors (PPARs) during adipogenesis.

Short hairpin RNA-mediated silencing of PRC (PGC-1-related coactivator) results in a severe respiratory chain deficiency associated with the proliferation of aberrant mitochondria

PRC, a member of the PGC-1 coactivator family, is responsive to serum growth factors and up-regulated in proliferating cells. Here, we investigated its in vivo role by stably silencing PRC expression with two different short hairpin RNAs (shRNA1 and shRNA4) that were lentivirally introduced into U2OS cells. shRNA1 transductants exhibited nearly complete knockdown of PRC protein, whereas shRNA4 transductants expressed PRC protein at approximately 15% of the control level. Complete PRC silencing by shRNA1 resulted in a severe inhibition of respiratory growth; reduced expression of respiratory protein subunits from complexes I, II, III, and IV; markedly lower complex I and IV respiratory enzyme levels; and diminished mitochondrial ATP production. Surprisingly, shRNA1 transductants exhibited a striking proliferation of abnormal mitochondria that were devoid of organized cristae and displayed severe membrane abnormalities. Although shRNA4 transductants had normal respiratory subunit expression and a moderately diminished respiratory growth rate, both transductants showed markedly reduced growth on glucose accompanied by inhibition of G1/S cell cycle progression. Microarray analysis revealed striking overlaps in the genes affected by PRC silencing in the two transductants, and the functional identities of these overlapping genes were consistent with the observed mitochondrial and cell growth phenotypes. The consistency between phenotype and PRC expression levels in the two independent transductant lines argues that the defects result from PRC silencing and not from off target effects. These results support a role for PRC in the integration of pathways directing mitochondrial respiratory function and cell growth.

Nuclear control of respiratory chain expression by nuclear respiratory factors and PGC-1-related coactivator

Expression of the respiratory apparatus depends on both nuclear and mitochondrial genes. Although these genes are sequestered in distinct cellular organelles, their transcription relies on nucleus-encoded factors. Certain of these factors are directed to the mitochondria, where they sponsor the bi-directional transcription of mitochondrial DNA. Others act on nuclear genes that encode the majority of the respiratory subunits and many other gene products required for the assembly and function of the respiratory chain. The nuclear respiratory factors, NRF-1 and NRF-2, contribute to the expression of respiratory subunits and mitochondrial transcription factors and thus have been implicated in nucleo-mitochondrial interactions. In addition, coactivators of the PGC-1 family serve as mediators between the environment and the transcriptional machinery governing mitochondrial biogenesis. One family member, peroxisome proliferator-activated receptor gamma coactivator PGC-1-related coactivator (PRC), is an immediate early gene product that is rapidly induced by mitogenic signals in the absence of de novo protein synthesis. Like other PGC-1 family members, PRC binds NRF-1 and activates NRF-1 target genes. In addition, PRC complexes with NRF-2 and HCF-1 (host cell factor-1) in the activation of NRF-2-dependent promoters. HCF-1 functions in cell-cycle progression and has been identified as an NRF-2 coactivator. The association of these factors with PRC is suggestive of a role for the complex in cell growth. Finally, shRNA-mediated knock down of PRC expression results in a complex phenotype that includes the inhibition of respiratory growth on galactose and the loss of respiratory complexes. Thus, PRC may help integrate the expression of the respiratory apparatus with the cell proliferative program.

Multi-modulation of nuclear receptor coactivators through posttranslational modifications

Nuclear receptor (NR) coactivators are recruited to DNA by NRs, potentiating NR-dependent gene transcription. To obtain the complexity of NR-mediated gene regulation with a finite number of coactivators, the molecular properties of coactivators are dynamically modulated by posttranslational modifications (PTMs) in response to external stimuli. PTMs can regulate the molecular interactions of coactivators with transcription factors and other coactivators, in addition to their cellular location, protein stability, conformation and enzymatic activity. Therefore, dynamic regulation of the molecular properties of coactivators by PTMs allows for the complexity of NR-dependent gene expression and influences the regulation of NR-mediated physiological processes. This review focuses on recent progress in our understanding of how coactivator PTMs influence NR-mediated gene transcription and addresses their biological relevance.

Saturated fatty acid-mediated inflammation and insulin resistance in adipose tissue: mechanisms of action and implications

This review highlights the inflammatory and insulin-antagonizing effects of saturated fatty acids (SFA), which contribute to the development of metabolic syndrome. Mechanisms responsible for these unhealthy effects of SFA include: 1) accumulation of diacylglycerol and ceramide; 2) activation of nuclear factor-kappaB, protein kinase C-, and mitogen-activated protein kinases, and subsequent induction of inflammatory genes in white adipose tissue, immune cells, and myotubes; 3) decreased PPARgamma coactivator-1 alpha/beta activation and adiponectin production, which decreases the oxidation of glucose and fatty acids (FA); and 4) recruitment of immune cells like macrophages, neutrophils, and bone marrow-derived dendritic cells to WAT and muscle. Several studies have demonstrated potential health benefits of substituting SFA with unsaturated FA, particularly oleic acid and (n-3) FA. Thus, reducing consumption of foods rich in SFA and increasing consumption of whole grains, fruits, vegetables, lean meats and poultry, fish, low-fat dairy products, and oils containing oleic acid or (n-3) FA is likely to reduce the incidence of metabolic disease.

PGC-1alpha-mediated regulation of gene expression and metabolism: implications for nutrition and exercise prescriptions

The discovery 10 years ago of PGC-1alpha represented a major milestone towards understanding of the molecular processes regulating energy metabolism in many tissues, including skeletal muscle. PGC-1alpha orchestrates a metabolic program regulating oxidative lipid metabolism and insulin sensitivity. This is essentially the same metabolic program that is activated by exercise and down-regulated by sedentary lifestyles and high-fat diets, as well as in cases of obesity and type 2 diabetes. The present review examines the evidence in support of the key role for PGC-1alpha regulation of substrate metabolism and mitochondrial biogenesis in skeletal muscle. Surprisingly, studies with PGC-1alpha null and transgenic mice have revealed unexpected pathologies when PGC-1alpha is completely repressed (KO animals) or is massively overexpressed. In contrast, PGC-1alpha overexpression within normal physiological limits results in marked improvements in fatty acid oxidation and insulin-stimulated glucose transport. Exercise, sedentary lifestyles, and nutritional factors can regulate PGC-1alpha expression. We speculate that optimal targeting of PGC-1alpha upregulation, whether by diet, exercise, or a combination of both, could represent effective prophylactic or therapeutic means to improve insulin sensitivity. Indeed, using modern molecular tools, it may indeed be possible to prescribe optimally individualized nutrition and exercise programs.

Transcriptional co-activator peroxisome proliferator-activated receptor (PPAR)gamma co-activator-1beta is involved in the regulation of glucose-stimulated insulin secretion in INS-1E cells

Peroxisome proliferator-activated receptor-gamma co-activator-1 (PGC-1) alpha and -beta play pivotal roles in the regulation of intermediary metabolism. We have previously shown that PGC-1alpha-mediated upregulation of beta-cell sterol element binding protein (SREBP) gene expression impairs insulin secretion via increased transcription of uncoupling protein 2 (UCP2). PGC-1beta, in contrast to PGC-1alpha, directly binds to and acts as a co-activator of SREBPs and the forkhead transcription factor 2A (FOXA2) involved in pancreas development and function. To address a possible role of PGC-1beta in beta-cell function, we determined islet gene expression levels of PGC-1alpha, PGC-1beta, SREBPs, FOXA2, FOXO1, UCP2 as well as granuphilin, a critical component of the insulin secretory machinery, in Zucker diabetic fatty rats (ZDF). In comparison to controls, mRNA levels of all genes studied except for FOXA2 and FOXO1 were increased in islets of obese, fa/fa ZDF rats. The transcriptional activities of the UCP2 and granuphilin promoters were assessed in INS-1E cells in response to PGC-1beta overexpression and small interference RNA (siRNA)-mediated gene silencing. PGC-1beta as well as SREBP-1c and -2 increased transcription from the UCP2 promoter in INS-1E cells. Transient transfection of PGC-1beta-specific siRNAs significantly decreased SREBP-2-mediated transcriptional activation of the UCP2 gene. Furthermore PGC-1beta, SREBP-1c, and FOXA2 overexpression augmented granuphilin promoter activity, whereas siRNA-mediated gene knockdown of PGC-1beta reduced the effects of SREBP-1c and FOXA2 on granuphilin gene transcription and significantly increased glucose-stimulated insulin release from INS-1E cells. Our results support a role of PGC-1beta in the regulation of insulin secretion via upregulation of UCP2 and granuphilin gene expression.

Fibroblast growth factor 21 corrects obesity in mice

Fibroblast growth factor 21 (FGF21) is a metabolic regulator that provides efficient and durable glycemic and lipid control in various animal models. However, its potential to treat obesity, a major health concern affecting over 30% of the population, has not been fully explored. Here we report that systemic administration of FGF21 for 2 wk in diet-induced obese and ob/ob mice lowered their mean body weight by 20% predominantly via a reduction in adiposity. Although no decrease in total caloric intake or effect on physical activity was observed, FGF21-treated animals exhibited increased energy expenditure, fat utilization, and lipid excretion, reduced hepatosteatosis, and ameliorated glycemia. Transcriptional and blood cytokine profiling studies revealed effects consistent with the ability of FGF21 to ameliorate insulin and leptin resistance, enhance fat oxidation and suppress de novo lipogenesis in liver as well as to activate futile cycling in adipose. Overall, these data suggest that FGF21 exhibits the therapeutic characteristics necessary for an effective treatment of obesity and fatty liver disease and provides novel insights into the metabolic determinants of these activities.

The interplay of prolactin and the glucocorticoids in the regulation of beta-cell gene expression, fatty acid oxidation, and glucose-stimulated insulin secretion: implications for carbohydrate metabolism in pregnancy

Carbohydrate metabolism in pregnancy reflects the balance between counterregulatory hormones, which induce insulin resistance, and lactogenic hormones, which stimulate beta-cell proliferation and insulin production. Here we explored the interactions of prolactin (PRL) and glucocorticoids in the regulation of beta-cell gene expression, fatty acid oxidation, and glucose-stimulated insulin secretion (GSIS). In rat insulinoma cells, rat PRL caused 30-50% (P < 0.001) reductions in Forkhead box O (FoxO)-1, peroxisome proliferator activator receptor (PPAR)-gamma coactivator-1alpha (PGC-1alpha), PPARalpha, and carnitine palmitoyltransferase 1 (CPT-1) mRNAs and increased Glut-2 mRNA and GSIS; conversely, dexamethasone (DEX) up-regulated FoxO1, PGC1alpha, PPARalpha, CPT-1, and uncoupling protein 2 (UCP-2) mRNAs in insulinoma cells and inhibited GSIS. Hydrocortisone had similar effects. The effects of DEX were attenuated by coincubation of cells with PRL. In primary rat islets, PRL reduced FoxO1, PPARalpha, and CPT-1 mRNAs, whereas DEX increased FoxO1, PGC1alpha, and UCP-2 mRNAs. The effects of PRL on gene expression were mimicked by constitutive overexpression of signal transducer and activator of transcription-5b. PRL induced signal transducer and activator of transcription-5 binding to a consensus sequence in the rat FoxO1 promoter, reduced nuclear FoxO1 protein levels, and induced its phosphorylation and cytoplasmic redistribution. DEX increased beta-cell fatty acid oxidation and reduced fatty acid esterification; these effects were attenuated by PRL. Thus, lactogens and glucocorticoids have opposing effects on a number of beta-cell genes including FoxO1, PGC1alpha, PPARalpha, CPT-1, and UCP-2 and differentially regulate beta-cell Glut-2 expression, fatty acid oxidation, and GSIS. These observations suggest new mechanisms by which lactogens may preserve beta-cell mass and function and maternal glucose tolerance despite the doubling of maternal cortisol concentrations in late gestation.

ERRalpha: a metabolic function for the oldest orphan

Estrogen receptor related receptor (ERR)alpha was one of the first identified (1988) orphan nuclear receptors. Many of the orphan receptors identified after ERRalpha were deorphanized in a timely manner and appreciated as key transcriptional regulators of metabolic pathways. ERRalpha, however, remains an orphan. Nevertheless, recent studies have defined regulatory mechanisms and transcriptional targets of ERRalpha, allowing this receptor to join ranks with other nuclear receptors that control metabolism. Notably, mice lacking ERRalpha show defects when challenged with stressors that require a 'shift of gears' in energy metabolism, such as exposure to cold, cardiac overload or infection. These findings establish the importance of ERRalpha for adaptive energy metabolism, and suggest that strategies targeting ERRalpha may be useful in fighting metabolic diseases.

Diet-induced metabolic disturbances as modulators of brain homeostasis

A number of metabolic disturbances occur in response to the consumption of a high fat western diet. Such metabolic disturbances can include the progressive development of hyperglycemia, hyperinsulemia, obesity, metabolic syndrome, and diabetes. Cumulatively, diet-induced disturbances in metabolism are known to promote increased morbidity and negatively impact life expectancy through a variety of mechanisms. While the impact of metabolic disturbances on the hepatic, endocrine, and cardiovascular systems is well established there remains a noticeable void in understanding the basis by which the central nervous system (CNS) becomes altered in response to diet-induced metabolic dysfunction. In particular, it remains to be fully elucidated which established features of diet-induced pathogenesis (observed in non-CNS tissues) are recapitulated in the brain, and identification as to whether the observed changes in the brain are a direct or indirect effect of peripheral metabolic disturbances. This review will focus on each of these key issues and identify some critical experimental questions which remain to be elucidated experimentally, as well as provide an outline of our current understanding for how diet-induced alterations in metabolism may impact the brain during aging and age-related diseases of the nervous system.

A potential role for Akt/FOXO signalling in both protein loss and the impairment of muscle carbohydrate oxidation during sepsis in rodent skeletal muscle

Sepsis causes muscle atrophy and insulin resistance, but the underlying mechanisms are unclear. Therefore, the present study examined the effects of lipopolysaccharide (LPS)-induced endotoxaemia on the expression of Akt, Forkhead Box O (FOXO) and its downstream targets, to identify any associations between changes in FOXO-dependent processes influencing muscle atrophy and insulin resistance during sepsis. Chronically instrumented male Sprague-Dawley rats received a continuous intravenous infusion of LPS (15 microg kg(-1) h(-1)) or saline for 24 h at 0.4 ml h(-1). Animals were terminally anaesthetized and the extensor digitorum longus muscles from both hindlimbs were removed and snap-frozen. Measurements were made of mRNA and protein expression of selected signalling molecules associated with pathways regulating protein synthesis and degradation and carbohydrate metabolism. LPS infusion induced increases in muscle tumour necrosis factor-alpha (8.9-fold, P < 0.001) and interleukin-6 (8.4-fold, P < 0.01), paralleled by reduced insulin receptor substrate-1 mRNA expression (-0.7-fold, P < 0.01), and decreased Akt1 protein and cytosolic FOXO1 and FOXO3 phosphorylation. These changes were accompanied by significant increases in muscle atrophy F-box mRNA (5.5-fold, P < 0.001) and protein (2-fold, P < 0.05) expression, and pyruvate dehydrogenase kinase 4 mRNA (15-fold, P < 0.001) and protein (1.6-fold, P < 0.05) expression. There was a 29% reduction in the muscle protein: DNA ratio, a 56% reduction in pyruvate dehydrogenase complex (PDC) activity (P < 0.05), and increased glycogen degradation and lactate accumulation. The findings of this study suggest a potential role for Akt/FOXO in the simultaneous impairment of carbohydrate oxidation, at the level of PDC, and up-regulation of muscle protein degradation, in LPS-induced endotoxaemia.

Non-crystalline and crystalline rheumatic disorders in chronic kidney disease

Rheumatic syndromes are cause for morbidity in patients with end-stage renal disease. Recent advances in understanding the role of tissue remodeling have provided insight into the pathogenic mechanisms responsible for some of these manifestations. Here, we survey recent and clinically relevant advances in translational research that impact our understanding of rheumatic syndromes seen in patients with significant renal disease. The management of acute and chronic crystalline arthropathies in chronic kidney disease and hemodialysis patients is discussed.

Transcriptional coactivators PGC-1alpha and PGC-lbeta control overlapping programs required for perinatal maturation of the heart

Oxidative tissues such as heart undergo a dramatic perinatal mitochondrial biogenesis to meet the high-energy demands after birth. PPARgamma coactivator-1 (PGC-1) alpha and beta have been implicated in the transcriptional control of cellular energy metabolism. Mice with combined deficiency of PGC-1alpha and PGC-1beta (PGC-1alphabeta(-/-) mice) were generated to investigate the convergence of their functions in vivo. The phenotype of PGC-1beta(-/-) mice was minimal under nonstressed conditions, including normal heart function, similar to that of PGC-1alpha(-/-) mice generated previously. In striking contrast to the singly deficient PGC-1 lines, PGC-1alphabeta(-/-) mice died shortly after birth with small hearts, bradycardia, intermittent heart block, and a markedly reduced cardiac output. Cardiac-specific ablation of the PGC-1beta gene on a PGC-1alpha-deficient background phenocopied the generalized PGC-1alphabeta(-/-) mice. The hearts of the PGC-1alphabeta(-/-) mice exhibited signatures of a maturational defect including reduced growth, a late fetal arrest in mitochondrial biogenesis, and persistence of a fetal pattern of gene expression. Brown adipose tissue (BAT) of PGC-1alphabeta(-/-) mice also exhibited a severe abnormality in function and mitochondrial density. We conclude that PGC-1alpha and PGC-1beta share roles that collectively are necessary for the postnatal metabolic and functional maturation of heart and BAT.

The role of exercise and PGC1alpha in inflammation and chronic disease

Inadequate physical activity is linked to many chronic diseases. But the mechanisms that tie muscle activity to health are unclear. The transcriptional coactivator PGC1alpha has recently been shown to regulate several exercise-associated aspects of muscle function. We propose that this protein controls muscle plasticity, suppresses a broad inflammatory response and mediates the beneficial effects of exercise.

Role of the PGC-1 family in the metabolic adaptation of goldfish to diet and temperature

In mammals, the peroxisome proliferator-activated receptor (PPAR) gamma coactivator-1 (PGC-1) family members and their binding partners orchestrate remodelling in response to diverse challenges such as diet, temperature and exercise. In this study, we exposed goldfish to three temperatures (4, 20 and 35 degrees C) and to three dietary regimes (food deprivation, low fat and high fat) and examined the changes in mitochondrial enzyme activities and transcript levels for metabolic enzymes and their genetic regulators in red muscle, white muscle, heart and liver. When all tissues and conditions were pooled, there were significant correlations between the mRNA for the PGC-1 coactivators (both alpha and beta) and mitochondrial transcripts (citrate synthase), metabolic gene regulators including PPARalpha, PPARbeta and nuclear respiratory factor-1 (NRF-1). PGC-1beta was the better predictor of the NRF-1 axis, whereas PGC-1alpha was the better predictor of the PPAR axis (PPARalpha, PPARbeta, medium chain acyl CoA dehydrogenase). In contrast to these intertissue/developmental patterns, the response of individual tissues to physiological stressors displayed no correlations between mRNA for PGC-1 family members and either the NRF-1 or PPAR axes. For example, in skeletal muscles, low temperature decreased PGC-1alpha transcript levels but increased mitochondrial enzyme activities (citrate synthase and cytochrome oxidase) and transcripts for COX IV and NRF-1. These results suggest that in goldfish, as in mammals, there is a regulatory relationship between (i) NRF-1 and mitochondrial gene expression and (ii) PPARs and fatty acid oxidation gene expression. In contrast to mammals, there is a divergence in the roles of the coactivators, with PGC-1alpha linked to fatty acid oxidation through PPARalpha, and PGC-1beta with a more prominent role in mediating NRF-1-dependent control of mitochondrial gene expression, as well as distinctions between their respective roles in development and physiological responsiveness.

Activation of the PPAR/PGC-1alpha pathway prevents a bioenergetic deficit and effectively improves a mitochondrial myopathy phenotype

Neuromuscular disorders with defects in the mitochondrial ATP-generating system affect a large number of children and adults worldwide, but remain without treatment. We used a mouse model of mitochondrial myopathy, caused by a cytochrome c oxidase deficiency, to evaluate the effect of induced mitochondrial biogenesis on the course of the disease. Mitochondrial biogenesis was induced either by transgenic expression of peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator alpha (PGC-1alpha) in skeletal muscle or by administration of bezafibrate, a PPAR panagonist. Both strategies successfully stimulated residual respiratory capacity in muscle tissue. Mitochondrial proliferation resulted in an enhanced OXPHOS capacity per muscle mass. As a consequence, ATP levels were conserved resulting in a delayed onset of the myopathy and a markedly prolonged life span. Thus, induction of mitochondrial biogenesis through pharmacological or metabolic modulation of the PPAR/PGC-1alpha pathway promises to be an effective therapeutic approach for mitochondrial disorders.

Mitochondrial dysfunction in type 2 diabetes and obesity

Insulin resistance in skeletal muscle is a major hallmark of type 2 diabetes mellitus (T2D) and obesity that is characterized by impaired insulin-mediated glucose transport and glycogen synthesis and by increased intramyocellular content of lipid metabolites. Several studies have provided evidence for mitochondrial dysfunction in skeletal muscle of type 2 diabetic and prediabetic subjects, primarily due to a lower content of mitochondria (mitochondrial biogenesis) and possibly to a reduced functional capacity per mitochondrion. This article discusses the latest advances in the understanding of the molecular mechanisms underlying insulin resistance in human skeletal muscle in T2D and obesity, with a focus on possible links between insulin resistance and mitochondrial dysfunction.

Differential regulation of human ALAS1 mRNA and protein levels by heme and cobalt protoporphyrin

5-Aminolevulinic acid synthase 1 (ALAS1) is the first and rate-controlling enzyme of heme biosynthesis. This study was to determine the effects of heme and selected nonheme metalloporphyrins on human ALAS1 gene expression in hepatocytes. We found that, upon heme and cobalt protoporphyrin (CoPP) treatments, ALAS1 mRNA levels were down-regulated significantly by ca. 50% or more. Measurement of mRNA in the presence of actinomycin D showed that these down-regulations were due to the decreases in mRNA half-lives. Furthermore, the levels of mitochondrial mature ALAS1 protein were down-regulated by 60-70%, but those of the cytosolic precursor protein were up-regulated by 2-5-fold. Measurement of protein in the presence of cycloheximide (CHX) suggests that elevation of the precursor form is due to the increase in protein half-lives. These results provide novel insights into the mechanisms of heme repressional effects on ALAS1 and provide a rationale for further investigation of CoPP as a therapeutic agent for acute porphyric syndromes.

Up-regulation of mitochondrial activity and acquirement of brown adipose tissue-like property in the white adipose tissue of fsp27 deficient mice

Fsp27, a member of the Cide family proteins, was shown to localize to lipid droplet and promote lipid storage in adipocytes. We aimed to understand the biological role of Fsp27 in regulating adipose tissue differentiation, insulin sensitivity and energy balance. Fsp27^−/− mice and Fsp27/lep double deficient mice were generated and we examined the adiposity, whole body metabolism, BAT and WAT morphology, insulin sensitivity, mitochondrial activity, and gene expression changes in these mouse strains. Furthermore, we isolated mouse embryonic fibroblasts (MEFs) from wildtype and Fsp27^−/− mice, followed by their differentiation into adipocytes in vitro. We found that Fsp27 is expressed in both brown adipose tissue (BAT) and white adipose tissue (WAT) and its levels were significantly elevated in the WAT and liver of leptin-deficient ob/ob mice. Fsp27^−/− mice had increased energy expenditure, lower levels of plasma triglycerides and free fatty acids. Furthermore, Fsp27^−/−and Fsp27/lep double-deficient mice are resistant to diet-induced obesity and display increased insulin sensitivity. Moreover, white adipocytes in Fsp27^−/− mice have reduced triglycerides accumulation and smaller lipid droplets, while levels of mitochondrial proteins, mitochondrial size and activity are dramatically increased. We further demonstrated that BAT-specific genes and key metabolic controlling factors such as FoxC2, PPAR and PGC1α were all markedly upregulated. In contrast, factors inhibiting BAT differentiation such as Rb, p107 and RIP140 were down-regulated in the WAT of Fsp27^−/− mice. Remarkably, Fsp27^−/− MEFs differentiated in vitro show many brown adipocyte characteristics in the presence of the thyroid hormone triiodothyronine (T3). Our data thus suggest that Fsp27 acts as a novel regulator in vivo to control WAT identity, mitochondrial activity and insulin sensitivity.

Cannabinoid type 1 receptor blockade promotes mitochondrial biogenesis through endothelial nitric oxide synthase expression in white adipocytes

OBJECTIVE

Cannabinoid type 1 (CB1) receptor blockade decreases body weight and adiposity in obese subjects; however, the underlying mechanism is not yet fully understood. Nitric oxide (NO) produced by endothelial NO synthase (eNOS) induces mitochondrial biogenesis and function in adipocytes. This study was undertaken to test whether CB1 receptor blockade increases the espression of eNOS and mitochondrial biogenesis in white adipocytes.

RESEARCH DESIGN AND METHODS

We examined the effects on eNOS and mitochondrial biogenesis of selective pharmacological blockade of CB1 receptors by SR141716 (rimonabant) in mouse primary white adipocytes. We also examined eNOS expression and mitochondrial biogenesis in white adipose tissue (WAT) and isolated mature white adipocytes of CB1 receptor-deficient (CB1(-/-)) and chronically SR141716-treated mice on either a standard or high-fat diet.

RESULTS

SR141716 treatment increased eNOS expression in cultured white adipocytes. Moreover, SR141716 increased mitochondrial DNA amount, mRNA levels of genes involved in mitochondrial biogenesis, and mitochondrial mass and function through eNOS induction, as demonstrated by reversal of SR141716 effects by small interfering RNA-mediated decrease in eNOS. While high-fat diet-fed wild-type mice showed reduced eNOS expression and mitochondrial biogenesis in WAT and isolated mature white adipocytes, genetic CB1 receptor deletion or chronic treatment with SR141716 restored these parameters to the levels observed in wild-type mice on the standard diet, an effect linked to the prevention of adiposity and body weight increase.

CONCLUSIONS

CB1 receptor blockade increases mitochondrial biogenesis in white adipocytes by inducing the expression of eNOS. This is linked to the prevention of high-fat diet-induced fat accumulation, without concomitant changes in food intake.

PPARgamma agonists as therapeutics for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is characterized by the deposition of beta-amyloid within the brain parenchyma and is accompanied by the impairment of neuronal metabolism and function, leading to extensive neuronal loss. The disease involves the perturbation of synaptic function, energy, and lipid metabolism. The development of amyloid plaques results in the induction of a microglial-mediated inflammatory response. The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-activated transcription factor whose biological actions are to regulate glucose and lipid metabolism and suppress inflammatory gene expression. Thus, agonists of this receptor represent an attractive therapeutic target for AD. There is now an extensive body of evidence that has demonstrated the efficacy of PPARgamma agonists in ameliorating disease-related pathology and improved learning and memory in animal models of AD. Recent clinical trials of the PPARgamma agonist rosiglitazone have shown significant improvement in memory and cognition in AD patients. Thus, PPARgamma represents an important new therapeutic target in treating AD.

LPS decreases fatty acid oxidation and nuclear hormone receptors in the kidney

Inflammation produces marked changes in lipid metabolism, including increased serum fatty acids (FAs) and triglycerides (TGs), increased hepatic TG production and VLDL secretion, increased adipose tissue lipolysis, and decreased FA oxidation in liver and heart. Lipopolysaccharide (LPS) also increases TG and cholesteryl ester levels in kidneys. Here we confirm these findings and define potential mechanisms. LPS decreases renal FA oxidation by 40% and the expression of key proteins required for oxidation of FAs, including FA transport protein-2, fatty acyl-CoA synthase, carnitine palmitoyltransferase-1, medium-chain acyl-CoA dehydrogenase, and acyl-CoA oxidase. Similar decreases were observed in peroxisome proliferator-activated receptor alpha (PPARalpha)-deficient mice. LPS also caused a reduction in renal mRNA levels of PPARalpha (75% decrease), thyroid hormone receptor alpha (TRalpha) (92% decrease), and TRbeta (84% decrease), whereas PPARbeta/delta and gamma were not altered. Expression of PGC1 alpha and beta, coactivators required for PPARs and TR, was also decreased in kidneys of LPS-treated mice, as were mitochondrial genes regulated by PGC1 (Atp5g1, COX5a, Idh3a, and Ndufs8). Decreased renal FA oxidation could be a by-product of the systemic coordinated host response to increase FAs and TGs available for host defense and/or tissue repair. However, the kidney requires energy to support its transport functions, and the inability to generate energy via FA oxidation might contribute to the renal failure seen in severe sepsis.

T3-mediated expression of PGC-1alpha via a far upstream located thyroid hormone response element

Thyroid hormone (T3) has a profound influence on normal development, differentiation and metabolism. T3 induces complex gene expression patterns raises the question of how these expression patterns might be regulated. Since the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) induces very similar cellular energy metabolic pathways, we investigated the molecular mechanism of T3 regulation of PGC-1alpha. PGC-1alpha is rapidly regulated by T3, both in vivo and in cell culture. Transient transfection experiments demonstrated binding of the thyroid hormone receptor (TR) to a response element located at -4kb upstream of the transcriptional start site within the PGC-1alpha gene. Introducing of a single copy of the -4kb TRE in a heterologous promoter context is sufficient to maintain T3 responsiveness. Chromatin immunoprecipitation analysis revealed increased histone acetylation upon stimulation of T3. Finally, TR binds the -4kb TRE in electrophoretic mobility shift assays, identifying PGC-1alpha as a direct target of TR action. Since T3 directly regulates PGC-1alpha and PGC-1alpha coactivates liganded TR, we suggest an autoregulatory feed-forward loop of PGC-1alpha activation upon T3 treatment.

Mitochondria: the hub of cellular Ca2+ signaling

Mitochondria couple cellular metabolic state with Ca(2+) transport processes. They therefore control not only their own intra-organelle [Ca(2+)], but they also influence the entire cellular network of cellular Ca(2+) signaling, including the endoplasmic reticulum, the plasma membrane, and the nucleus. Through the detailed study of mitochondrial roles in Ca(2+) signaling, a remarkable picture of inter-organelle communication has emerged. We here review the ways in which this system provides integrity and flexibility for the cell to cope with the countless demands throughout its life cycle and discuss briefly the mechanisms through which it can also drive cell death.

PGC-1alpha increases skeletal muscle lactate uptake by increasing the expression of MCT1 but not MCT2 or MCT4

We examined the relationship between PGC-1alpha protein; the monocarboxylate transporters MCT1, 2, and 4; and CD147 1) among six metabolically heterogeneous rat muscles, 2) in chronically stimulated red (RTA) and white tibialis (WTA) muscles (7 days), and 3) in RTA and WTA muscles transfected with PGC-1alpha-pcDNA plasmid in vivo. Among rat hindlimb muscles, there was a strong positive association between PGC-1alpha and MCT1 and CD147, and between MCT1 and CD147. A negative association was found between PGC-1alpha and MCT4, and CD147 and MCT4, while there was no relationship between PGC-1alpha or CD147 and MCT2. Transfecting PGC-1alpha-pcDNA plasmid into muscle increased PGC-1alpha protein (RTA +23%; WTA +25%) and induced the expression of MCT1 (RTA +16%; WTA +28%), but not MCT2 and MCT4. As a result of the PGC-1alpha-induced upregulation of MCT1 and its chaperone CD147 (+29%), there was a concomitant increase in the rate of lactate uptake (+20%). In chronically stimulated muscles, the following proteins were upregulated, PGC-1alpha in RTA (+26%) and WTA (+86%), MCT1 in RTA (+61%) and WTA (+180%), and CD147 in WTA (+106%). In contrast, MCT4 protein expression was not altered in either RTA or WTA muscles, while MCT2 protein expression was reduced in both RTA (-14%) and WTA (-10%). In these studies, whether comparing oxidative capacities among muscles or increasing their oxidative capacities by PGC-1alpha transfection and chronic muscle stimulation, there was a strong relationship between the expression of PGC-1alpha and MCT1, and PGC-1alpha and CD147 proteins. Thus, MCT1 and CD147 belong to the family of metabolic genes whose expression is regulated by PGC-1alpha in skeletal muscle.

The PPAR trio: regulators of myocardial energy metabolism in health and disease

Common causes of heart failure are associated with derangements in myocardial fuel utilization. Evidence is emerging that metabolic abnormalities may contribute to the development and progression of myocardial disease. The peroxisome proliferator-activated receptor (PPAR) family of nuclear receptor transcription factors has been shown to regulate cardiac fuel metabolism at the gene expression level. The three PPAR family members (alpha, beta/delta and gamma) are uniquely suited to serve as transducers of developmental, physiological, and dietary cues that influence cardiac fatty acid and glucose metabolism. This review describes murine PPAR loss- and gain-of-function models that have shed light on the roles of these receptors in regulating myocardial metabolic pathways and have defined key links to disease states including the hypertensive and diabetic heart.

Cracking the coregulator codes

The study of the genetic code has collectively revealed that the biochemical basis of heredity is uniform for nearly all known forms of life. Genetic approaches have generated a much better appreciation and understanding of many aspects of biological processes-and in some cases provided strategies for the treatment of human diseases. Still, the enormous and undoubtedly impressive amount of information gathered on gene sequences, their myriad expression patterns and translation into proteins is insufficient to answer seemingly simpler questions such as to what sets us humans apart from much more undemanding species while sharing almost the same sets of genes. Regulation of the proteome by post-translational modifications (PTMs) is beginning to be understood as a major contributing factor to the structural and functional diversity in biology and for defining cellular mechanisms in particular. Covalent, PTMs provide an astonishingly rich and specific basis for an ultrafast regulation of cellular processes, many of which converge to transcription units to control gene expression. With this essay we intend to share with the reader the rapid growth of our knowledge of the many conjunctions that exist between PTMs and key cellular processes that have emerged by studying the nuclear receptors (NRs) and their transcriptional coregulators.