Brown and beige fat are specialized adipose tissues found in almost all mammals that can increase energy expenditure and produce heat. Cold exposure and b3-adrenergic stimulation has been extensively shown to activate brown adipose tissue (BAT) in rodents, which promotes uncoupled respiration of glucose and lipid substrates via uncoupling protein 1 (UCP1). Prolonged stimulation can induce white adipose browning, which leads to the emergence of thermogenic cells within white fat depots, called beige adipocytes. The beige adipocyte possesses a unique molecular signature, yet shares several characteristics of brown adipocytes, including high mitochondrial content. When activated, beige fat can be induced to initiate a thermogenic transcriptional program similar to that of BAT. Recent human studies have identified brown and/or beige fat in the supraclavicular region using various radiation imaging modalities. This remarkable discovery has reinvigorated scientific interest in adipose browning and brown/beige fat activation as possible therapeutic targets for obesity. Like in rodents, several groups have previously tested the potential impact of cold exposure and b3-adrenergic agonism on BAT-mediated thermogenesis in humans. However, even though these approaches were shown to significantly increase energy expenditure and promote weight loss in obese individuals, they are not ideal clinical interventions. Cold exposure is uncomfortable and requires prolonged treatment, while b3-adrenergic agonists may lead to many adverse effects like cardiovascular problems. This thesis will evaluate the therapeutic potential and clinical relevance of alternative anti-obesity approaches that target adipose browning and beige adipose activation.

Obesity is a disorder of energy imbalance in which energy intake exceeds energy expenditure (EEX). Brown adipose tissue (BAT) is unique in that it can increase whole body EEX when it is adrenergically activated. The thermogenic capacity of BAT is mediated by mitochondrial uncoupling through the activation of Uncoupling Protein 1 which uncouple respiration from ATP production. Mitochondria is a dynamic organelle that undergo continuous cycles of fusion and fission. Alteration in mitochondrial dynamics correlates with changes in energy efficiency in different cell types; however, its role in regulating EEX in BAT has not been investigated. Here we describe that mitochondrial dynamics is a physiological regulator of adrenergically-induced changes in EEX in BAT. Norepinephrine (NE) induces mitochondrial fragmentation in brown adipocytes (BA) though posttranslational modifications - phosphorylation and proteolytic cleavage -of mitochondrial dynamic proteins. NE-induced EEX is reduced in fission-deficient brown adipocytes while forced mitochondrial fragmentation increases the respiration in response to exogenous free fatty acids (FFAs) indicating increase in EEX. We further investigated whether forced mitochondrial fragmentation in BAT could be utilized as an approach to increase whole body EEX is response to FFA in vivo. We found that a mouse model with forced mitochondrial fragmentation in BAT (BAT-Mitofusin2-KO) gained less body weight and less fat mass and remained more glucose tolerant and insulin sensitive under high fat diet (HFD) compared to the wild type. Additionally, FFA oxidation was enhanced in BAT-Mitofusin2-KO mice indicated by lower respiratory exchange ratio. We also found that subcellular heterogeneity in dynamics leads to the generation of subpopulations of mitochondria with diverse bioenergetics characteristics within the same cell. We described that a subpopulation of mitochondria surrounding the lipid droplet in BA had higher ATP synthesis capacity, supported by higher ATP synthase protein expression and elongated morphology. We suggest that this subpopulation of mitochondria is responsible for addressing the ATP demand of the BA when it is not activated. In conclusion, changes to mitochondrial dynamics are required for BAT thermogenic activity and for the control of EEX efficiency from sub-cellular to the whole body level. Additionally, mitochondrial dynamics in BAT can regulate fatty acid oxidation. / 2018-06-15T00:00:00Z

Due to the high childhood obesity rates within the United States, it is necessary to develop efficacious strategies to combat childhood obesity. To explore whether early intervention can produce lasting metabolic improvements, we used a mouse model of genetically-induced hypothalamic leptin resistance (LeprNkx2.1knockout, hereby known as KO) that exhibits early-onset hyperphagia and obesity. We found that KO mice exhibit reduced capacity of the brown adipose tissue (as seen by disorganized mitochondrial structure). Brown adipose tissue capacity can be restored by paired-feeding in the peri-weaning period, leading to persistent improvements in later adiposity even after restriction ends. These studies lead us to investigate the maturation process of brown adipose tissue in the peri-weaning period. We found that brown adipose tissue expansion between 2 to 3 weeks of age is accompanied by a reduced thermogenic capacity in control mice, as determined by protein levels of uncoupling protein 1 and disorganization of the mitochondrial cristae. Thermogenic function was restored by 5 weeks of age, as demonstrated by a peak of uncoupling protein 1, in control mice but not KO mice. Paired-feeding of KO mice in the peri-weaning period rescued this peak at 5 weeks of age. These studies elucidate a critical period when brown adipose tissue expansion is followed by activation. The magnitude of brown adipose tissue activation at this time might be predictive of future obesity and metabolic rate, highlighting a potential therapeutic time window in which to intervene in pediatric obesity.

Uncoupling protein, UCP, present in the inner mitochondrial membrane of brown adipose tissue (BAT) contributes to adaptive thermogenesis. UCP functions as a proton pore and can dissipate the proton electrochemical gradient established by the respiratory chain during fuel oxidation, and thus generates heat without producing ATP. However, the brown adipose tissue thermogenesis is not likely to be a major mechanism in controlling energy expenditure for humans because adults have only residual amounts of the tissue. Two new members of the UCP family have been identified based on their high sequence homology to UCP in BAT and named UCP2 and UCP3. The original UCP was renamed UCP1. At the amino acid level, human UCP2 and UCP3 are 59% and 57% identical to UCP1, respectively. In contrast to UCP1, UCP2 is expressed in many tissues such as brown adipose tissue, white adipose tissue, muscle, spleen and macrophages. UCP3 is expressed preferentially in skeletal muscle in humans, and brown adipose tissue and skeletal muscle in rodents. Since their identification many functional studies, including transgenic animals and ectopic expression of UCP2 or UCP3 in yeast, showed uncoupling activity of UCP2 and UCP3. A number of studies have been done that show increased expression of UCP2 and UCP3 by fasting, high-fat diets and suckling of newborn mice. A common characteristic of these circumstances is an associated increase in plasma free fatty acid levels. This study aimed to investigate effects of fatty acids, peroxisome proliferator-activated receptors (PPARs) and other transcription factors on UCP2 and UCP3 gene expression and to explore the molecular mechanism of their regulation through analysis of the promoter of the UCP2 and UCP3 genes. The 3.1 kb and 3.2 kb 5�-flanking regions of the mouse UCP2 and UCP3 genes, respectively, were cloned and used to construct promoter reporter gene (firefly luciferase) plasmids. The cloned region of the UCP2 and UCP3 genes contained putative binding motifs for several transcription factors, including PPAR, myogenin, and MyoD. Luciferase assays of both constructs showed basal promoter activity with 20~190-fold induction for the UCP2 promoter and 1.3~23-fold induction for the UCP3 promoter in several transfected cell lines, including 3T3-L1, C2C12, L6, COS7 and HepG2. Oleic acid (0.3 mM) up-regulated endogenous UCP2 mRNA by 2.3-fold in 3T3-L1 preadipocytes but not in C2C12 myotubes, and UCP3 mRNA by 2.5-fold in C2C12 myotubes. Responsiveness of the cloned promoter to oleic acid reflected the tissue-specific responsiveness of their endogenous genes but with less fold induction, 1.4-fold for UCP2 promoter in 3T3-L1 preadipocytes and 1.5-fold for UCP3 promoter in C2C12 myotubes. Forced expression of PPAR isotypes (PPARα, PPAR[delta] and PPARγ) showed tissue and isotype-specific activation of the UCP2 promoter. UCP2 promoter activity was induced by 2-fold by PPARγ in 3T3-L1 and by 2.8-fold by PPAR[delta] in C2C12. Treatment of oleic acid (0.3 mM) brought about further induction of the UCP2 promoter activity only in 3T3-L1. In contrast, all three isotypes induced activation of the UCP3 promoter in 3T3-L1, C2C12 and HepG2 cells. Treatment with oleic acid (0.3 mM) or isotype-specific agonist (10 [mu]M) resulted in further increased activity of the UCP3 promoter in 3T3-L1 and HepG2 cells. In particular, rosiglitazone (10 [mu]M) induced a 41-fold increase in UCP3 promoter activity in PPARγ transfected HepG2 cells, and this induction returned to basal level by treatment with bisphenol A diglycidyl ether (BADGE) (50 [mu]M), an antagonist for PPARγ. In addition, UCP3 promoter activity increased up to 20-fold 4 days after induction of C2C12 myoblasts differentiation, whereas UCP2 promoter activity increased only up to 2-fold. Forced expression of myogenin and MyoD in C2C12 myoblasts to mimic differentiation, induced UCP3 promoter activity in an additive manner, consistent with UCP3 being regulated by muscle differentiation. In the present study, it has been shown that UCP2 and UCP3 genes are regulated differently by fatty acids. The tissue-type dependence in regulation of endogenous UCP2 and UCP3 paralleled the cell type-specific effect of oleic acid on the promoter-reporter constructs, suggesting that fatty acid effects are at the transcriptional level. UCP2 and UCP3 promoters showed differences in their response to PPARs. Mediation of the fatty acid effect through PPARs has been also demonstrated, but direct binding of PPARs and particular regulatory motifs on the cloned promoter region have not yet been investigated.

Caveolae are 50-100 nm invaginations in the plasma membrane. Caveolae and the protein caveolin-1 (Cav1) have been shown to be important in many signaling pathways in different cell types; however, in some cell types caveolae and Cav1 do not seem to affect the investigated signaling pathways. In my thesis, I have investigated the role of caveolin-1 (Cav1) in metabolism and b3-adrenergic, LPA-, EGF- and PDGF-receptor signaling in brown adipocytes. Brown adipose tissue is responsible for nonshivering thermogenesis. Recent studies have shown that not only infants but also adult man can have brown adipose tissue and that the presence is negatively correlated with both obesity and age. By understanding how signaling for proliferation and differentiation in brown adipocytes is regulated, it could be possible in the future to activate brown adipose tissue to combat obesity and the metabolic syndrome. In brown adipocytes, both epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) were able to induce proliferation, which was dependent on Erk1/2 activation. However, EGF and PDGF utilized different pathways to activate Erk1/2, with EGF signaling partially occurring via a Src-pathway (not involving PI3K/PKC) and PDGF via a PI3K/PKC/Src-pathway. Furthermore, LPA receptors were able to activate Erk1/2 via two pathways, one Gi/PKC/Src-pathway and one PI3K-pathway. For these receptors, Cav1-ablation did not affect the agonist-induced Erk1/2 activation. Cav1 was, however, required for proper b3-adrenergic receptor (b3-AR) signaling to cAMP and for adenylyl cyclase activity. In Cav1-ablated mice, the adrenergic receptors are desensitized. However, this desensitization could be overcome physiologically, and the Cav1-ablated mice were therefore able to survive in prolonged cold by nonshivering thermogenesis. In conclusion, ablation of Cav1 affected certain signaling pathways in brown adipocytes, while other pathways were not affected or could be physiologically rescued. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript.

This thesis is focused on the role of mitochondria in pancreatic β-cells and brown adipose tissue (BAT). Two main aspects of mitochondria were explored; mitochondrial functional efficiency and the interrelationship between mitochondrial shape and function. Mitochondria in β-cells were found to exhibit heterogeneity in mitochondrial membrane potential. This functional diversity decreased when cells were challenged with glucose stimuli, suggesting that at higher fuel levels low-activity mitochondria are recruited into a pool of high-activity mitochondria. Glucolipotoxic conditions increased the functional diversity suggesting that this may be of importance for diabetes pathophysiology. To examine mitochondrial efficiency in intact islets a high throughput islet respirometry method was developed. Due to increased uncoupling, islets from a diabetic animal model exhibit lower respiratory efficiency. Glucose, free fatty acids and amino acids all decreased respiratory efficiency. A large portion of the respiratory efficiency was mediated by reactive oxygen species and the adenine nucleotide translocase. In β-cells mitochondria were found to undergo cycles of fusion and fission. During glucolipotoxicity mitochondria fragmented and lost their fusion ability. Knock down of the fission protein Fis1 rescued the β-cells from glucolipotoxic induced cell death. BAT mitochondria also showed fusion and fission. The mitochondrial dynamics proteins Mfn2 and Drp1 were shown to strongly affect BAT mitochondrial morphology. In response to a combination of adrenergic and free fatty acid stimuli mitochondria drastically changed from long filamentous structures to fragmented spheres. Inhibiting fission by the negative form of Drp1 decreased BAT response to adrenergic stimuli by half. In conclusion, mitochondrial efficiency may be of importance for normal as well as compromised β-cell and islet function. Mitochondrial morphology appears critical for mitochondrial function in β-cells and BAT. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

No description available.

No description available.

There are two distinct types of adipose tissue which have different functions within the body, white (WAT) and brown (BAT). Browning of WAT occurs with increases in the WAT sympathetic nervous system (SNS) drive. In this regard we previously reported that melatonin (MEL) stimulation of MEL receptor 1A (MEL1a) within the SNS outflow to the WAT might be implicated in a naturally-occurring reversal of obesity (by ~30% of total body fat). Therefore, in this study we tested the hypothesis that MEL causes browning of WAT through the stimulation of SNS drive to WAT. This was done by comparing specific browning and lipolytic markers in WAT following 10 weeks of MEL treatment, short day housing (SD), and long day housing with saline injections (LD+VEH). Browning effects of a 5 day treatment of a β3-adrenergeric (β3 AR), CL 316, 243, were also measured. We found that CL 316, 243, MEL treatment, and SD housing had increased expressions of browning markers within WAT and lipolytic activity in MEL treated animals was increased in specific WAT.

Uncoupling protein, UCP, present in the inner mitochondrial membrane of brown adipose tissue (BAT) contributes to adaptive thermogenesis. UCP functions as a proton pore and can dissipate the proton electrochemical gradient established by the respiratory chain during fuel oxidation, and thus generates heat without producing ATP. However, the brown adipose tissue thermogenesis is not likely to be a major mechanism in controlling energy expenditure for humans because adults have only residual amounts of the tissue. Two new members of the UCP family have been identified based on their high sequence homology to UCP in BAT and named UCP2 and UCP3. The original UCP was renamed UCP1. At the amino acid level, human UCP2 and UCP3 are 59% and 57% identical to UCP1, respectively. In contrast to UCP1, UCP2 is expressed in many tissues such as brown adipose tissue, white adipose tissue, muscle, spleen and macrophages. UCP3 is expressed preferentially in skeletal muscle in humans, and brown adipose tissue and skeletal muscle in rodents. Since their identification many functional studies, including transgenic animals and ectopic expression of UCP2 or UCP3 in yeast, showed uncoupling activity of UCP2 and UCP3. A number of studies have been done that show increased expression of UCP2 and UCP3 by fasting, high-fat diets and suckling of newborn mice. A common characteristic of these circumstances is an associated increase in plasma free fatty acid levels. This study aimed to investigate effects of fatty acids, peroxisome proliferator-activated receptors (PPARs) and other transcription factors on UCP2 and UCP3 gene expression and to explore the molecular mechanism of their regulation through analysis of the promoter of the UCP2 and UCP3 genes. The 3.1 kb and 3.2 kb 5�-flanking regions of the mouse UCP2 and UCP3 genes, respectively, were cloned and used to construct promoter reporter gene (firefly luciferase) plasmids. The cloned region of the UCP2 and UCP3 genes contained putative binding motifs for several transcription factors, including PPAR, myogenin, and MyoD. Luciferase assays of both constructs showed basal promoter activity with 20~190-fold induction for the UCP2 promoter and 1.3~23-fold induction for the UCP3 promoter in several transfected cell lines, including 3T3-L1, C2C12, L6, COS7 and HepG2. Oleic acid (0.3 mM) up-regulated endogenous UCP2 mRNA by 2.3-fold in 3T3-L1 preadipocytes but not in C2C12 myotubes, and UCP3 mRNA by 2.5-fold in C2C12 myotubes. Responsiveness of the cloned promoter to oleic acid reflected the tissue-specific responsiveness of their endogenous genes but with less fold induction, 1.4-fold for UCP2 promoter in 3T3-L1 preadipocytes and 1.5-fold for UCP3 promoter in C2C12 myotubes. Forced expression of PPAR isotypes (PPARα, PPAR[delta] and PPARγ) showed tissue and isotype-specific activation of the UCP2 promoter. UCP2 promoter activity was induced by 2-fold by PPARγ in 3T3-L1 and by 2.8-fold by PPAR[delta] in C2C12. Treatment of oleic acid (0.3 mM) brought about further induction of the UCP2 promoter activity only in 3T3-L1. In contrast, all three isotypes induced activation of the UCP3 promoter in 3T3-L1, C2C12 and HepG2 cells. Treatment with oleic acid (0.3 mM) or isotype-specific agonist (10 [mu]M) resulted in further increased activity of the UCP3 promoter in 3T3-L1 and HepG2 cells. In particular, rosiglitazone (10 [mu]M) induced a 41-fold increase in UCP3 promoter activity in PPARγ transfected HepG2 cells, and this induction returned to basal level by treatment with bisphenol A diglycidyl ether (BADGE) (50 [mu]M), an antagonist for PPARγ. In addition, UCP3 promoter activity increased up to 20-fold 4 days after induction of C2C12 myoblasts differentiation, whereas UCP2 promoter activity increased only up to 2-fold. Forced expression of myogenin and MyoD in C2C12 myoblasts to mimic differentiation, induced UCP3 promoter activity in an additive manner, consistent with UCP3 being regulated by muscle differentiation. In the present study, it has been shown that UCP2 and UCP3 genes are regulated differently by fatty acids. The tissue-type dependence in regulation of endogenous UCP2 and UCP3 paralleled the cell type-specific effect of oleic acid on the promoter-reporter constructs, suggesting that fatty acid effects are at the transcriptional level. UCP2 and UCP3 promoters showed differences in their response to PPARs. Mediation of the fatty acid effect through PPARs has been also demonstrated, but direct binding of PPARs and particular regulatory motifs on the cloned promoter region have not yet been investigated.