Absence Of Uncoupling Protein 1 Research Articles

Prolonged FGF21 treatment increases energy expenditure and induces weight loss in obese mice independently of UCP1 and adrenergic signaling

Fibroblast growth factor 21 (FGF21) reduces body weight, which was attributed to induced energy expenditure (EE). Conflicting data have been published on the role of uncoupling protein 1 (UCP1) in this effect. Therefore, we aimed to revisit the thermoregulatory effects of FGF21 and their implications for body weight regulation.We found that an 8-day treatment with FGF21 lowers body weight to similar extent in both wildtype (WT) and UCP1-deficient (KO) mice fed high-fat diet. In WT mice, this effect is solely due to increased EE, associated with a strong activation of UCP1 and with excess heat dissipated through the tail. This thermogenesis takes place in the interscapular region and can be attenuated by a β-adrenergic inhibitor propranolol. In KO mice, FGF21-induced weight loss correlates with a modest increase in EE, which is independent of adrenergic signaling, and with a reduced energy intake. Interestingly, the gene expression profile of interscapular brown adipose tissue (but not subcutaneous white adipose tissue) of KO mice is massively affected by FGF21, as shown by increased expression of genes encoding triacylglycerol/free fatty acid cycle enzymes.Thus, FGF21 elicits central thermogenic and pyretic effects followed by a concomitant increase in EE and body temperature, respectively. The associated weight loss is strongly dependent on UCP1-based thermogenesis. However, in the absence of UCP1, alternative mechanisms of energy dissipation may contribute, possibly based on futile triacylglycerol/free fatty acid cycling in brown adipose tissue and reduced food intake.

1379-P: Combination of FGF21 and ß3-Adrenergic Treatment in Obese Mice

An induction of lipolysis by β3-adrenergic agonist CL316,243 (CL) increases plasma free fatty acid (FFA) level resulting later in ectopic lipid accumulation in liver. The peak of FFA stimulates insulin secretion and causes acute drop of blood glucose (BG) in chow-fed mice. This glucose-lowering effect is attenuated in insulin-resistant dietary obese (DIO) mice. Fibroblast growth factor 21 (FGF21) treatment increases insulin sensitivity while inhibiting lipolysis. Both FGF21 and CL induce uncoupling protein 1 (UCP1) in brown adipose tissue, increasing capacity for combustion of glucose. Thus, the FGF21 treatment has a potential to modulate the glucoregulatory action of CL. Here we aim to characterize the effect of the CL+FGF21 combination and to investigate the role of UCP1 in this effect. DIO C57Bl/6 mice were housed at 30 °C in order to minimize basal adrenergic tone. Animals were treated 2x daily with 65 μg FGF21 or its vehicle for 8 days. This treatment lowered insulin (increased insulin sensitivity) . Subsequently, the mice were injected with 35 μg CL or with saline. CL increased FFA and insulin in control mice, but did not affect BG (probably because of impaired insulin sensitivity) . The day after CL injection, 4-fold rise in ectopic lipid content in liver was observed in comparison to saline-treated group. In FGF21-treated mice, the CL-induced rise of NEFA and insulin was only transient, but sufficient to cause a drop of BG. The hepatic lipid accumulation after CL was completely preveted by FGF21. The same experimental set-up was repeated in DIO UCP1+/+ and -/- mice. Although the BG-lowering effect of the CL+FGF21combination was observed in both genotypes, the effect was significantly stronger in UCP1+/+. Thus, FGF21 sensitizes DIO mice to the BG-lowering effect of adrenergic stimulation while it prevents ectopic lipid accummulation. The drop of BG is observed even in the absence of UCP1, but UCP1 activity is required to reach the maximal BG-lowering effect of the combination. Disclosure P.Zouhar: None. Funding Czech Science Foundation (19-05356Y)

Endogenous FGF21-signaling controls paradoxical obesity resistance of UCP1-deficient mice

Uncoupling protein 1 (UCP1) executes thermogenesis in brown adipose tissue, which is a major focus of human obesity research. Although the UCP1-knockout (UCP1 KO) mouse represents the most frequently applied animal model to judge the anti-obesity effects of UCP1, the assessment is confounded by unknown anti-obesity factors causing paradoxical obesity resistance below thermoneutral temperatures. Here we identify the enigmatic factor as endogenous FGF21, which is primarily mediating obesity resistance. The generation of UCP1/FGF21 double-knockout mice (dKO) fully reverses obesity resistance. Within mild differences in energy metabolism, urine metabolomics uncover increased secretion of acyl-carnitines in UCP1 KOs, suggesting metabolic reprogramming. Strikingly, transcriptomics of metabolically important organs reveal enhanced lipid and oxidative metabolism in specifically white adipose tissue that is fully reversed in dKO mice. Collectively, this study characterizes the effects of endogenous FGF21 that acts as master regulator to protect from diet-induced obesity in the absence of UCP1.

The anti-obesity effect of FGF19 does not require UCP1-dependent thermogenesis

ObjectiveFibroblast growth factor 19 (FGF19) is a postprandial hormone which plays diverse roles in the regulation of bile acid, glucose, and lipid metabolism. Administration of FGF19 to obese/diabetic mice lowers body weight, improves insulin sensitivity, and enhances glycemic control. The primary target organ of FGF19 is the liver, where it regulates bile acid homeostasis in response to nutrient absorption. In contrast, the broader pharmacologic actions of FGF19 are proposed to be driven, in part, by the recruitment of the thermogenic protein uncoupling protein 1 (UCP1) in white and brown adipose tissue. However, the precise contribution of UCP1-dependent thermogenesis to the therapeutic actions of FGF19 has not been critically evaluated. MethodsUsing WT and germline UCP1 knockout mice, the primary objective of the current investigation was to determine the in vivo pharmacology of FGF19, focusing on its thermogenic and anti-obesity activity. ResultsWe report that FGF19 induced mRNA expression of UCP1 in adipose tissue and show that this effect is required for FGF19 to increase caloric expenditure. However, we demonstrate that neither UCP1 induction nor an elevation in caloric expenditure are necessary for FGF19 to induce weight loss in obese mice. In contrast, the anti-obesity action of FGF19 appeared to be associated with its known physiological role. In mice treated with FGF19, there was a significant reduction in the mRNA expression of genes associated with hepatic bile acid synthesis enzymes, lowered levels of hepatic bile acid species, and a significant increase in fecal energy content, all indicative of reduced lipid absorption in animals treated with FGF19. ConclusionTaken together, we report that the anti-obesity effect of FGF19 occurs in the absence of UCP1. Our data suggest that the primary way in which exogenous FGF19 lowers body weight in mice may be through the inhibition of bile acid synthesis and subsequently a reduction of dietary lipid absorption.

Mitochondrial Patch Clamp of Beige Adipocytes Reveals UCP1-Positive and UCP1-Negative Cells Both Exhibiting Futile Creatine Cycling

Summary Cold and other environmental factors induce browning of white fat depots—development of beige adipocytes with morphological and functional resemblance to brown fat. Similar to brown fat, beige adipocytes are assumed to express mitochondrial uncoupling protein 1 (UCP1) and are thermogenic due to the UCP1-mediated H + leak across the inner mitochondrial membrane. However, this assumption has never been tested directly. Herein we patch clamped the inner mitochondrial membrane of beige and brown fat to provide a direct comparison of their thermogenic H + leak ( I H ). All inguinal beige adipocytes had robust UCP1-dependent I H comparable to brown fat, but it was about three times less sensitive to purine nucleotide inhibition. Strikingly, only ∼15% of epididymal beige adipocytes had I H , while in the rest UCP1-dependent I H was undetectable. Despite the absence of UCP1 in the majority of epididymal beige adipocytes, these cells employ prominent creatine cycling as a UCP1-independent thermogenic mechanism.

UCP1-independent signaling involving SERCA2b-mediated calcium cycling regulates beige fat thermogenesis and systemic glucose homeostasis.

Uncoupling Protein 1 (UCP1) plays a central role in non-shivering thermogenesis in brown fat; however, its role in beige fat remains unclear. Here we report a robust UCP1-independent thermogenic mechanism in beige fat that involves enhanced ATP-dependent Ca2+ cycling by sarco/endoplasmic reticulum Ca2+-ATPase2b (SERCA2b) and ryanodine receptor 2 (RyR2). Inhibition of SERCA2b impairs UCP1-independent beige fat thermogenesis in humans and mice, as well as in pigs, a species that lacks a functional UCP1 protein. Conversely, enhanced Ca2+ cycling by the activation of α1/β3-adrenergic receptors or the SERCA2b-RyR2 pathway stimulates UCP1-independent thermogenesis. In the absence of UCP1, beige fat dynamically expends glucose through enhanced glycolysis, tricarboxylic acid metabolism, and pyruvate dehydrogenase activity for ATP-dependent thermogenesis by the SERCA2b pathway; beige fat thereby functions as a “glucose-sink” and improves glucose tolerance independent of body-weight loss. Our study uncovers a non-canonical thermogenic mechanism by which beige fat controls whole-body energy homeostasis through Ca2+ cycling.

Mitochondrial Patch Clamp of Beige Adipocytes Reveals UCP1-Positive and UCP1-Negative Cells Both Exhibiting Futile Creatine Cycling

Cold and other environmental factors induce "browning" of white fat depots-development of beige adipocytes with morphological and functional resemblance to brown fat. Similar to brown fat, beige adipocytes are assumed to express mitochondrial uncoupling protein 1 (UCP1) and are thermogenic due to the UCP1-mediated H+ leak across the inner mitochondrial membrane. However, this assumption has never been tested directly. Herein we patch clamped the inner mitochondrial membrane of beige and brown fat to provide a direct comparison of their thermogenic H+ leak (IH). All inguinal beige adipocytes had robust UCP1-dependent IH comparable to brown fat, but it was about three times less sensitive to purine nucleotide inhibition. Strikingly, only∼15% of epididymal beige adipocytes had IH, while in the rest UCP1-dependent IH was undetectable. Despite the absence of UCP1 in the majority of epididymal beige adipocytes, these cells employ prominent creatine cycling as a UCP1-independent thermogenic mechanism.

When pigs fly, UCP1 makes heat.

Brown and beige adipose tissue may represent important therapeutic targets for the treatment of diabetes and obesity as these organs dissipate nutrient energy as heat through the thermogenic uncoupling protein 1 (UCP1). While mice are commonly used to mimic the potential effects of brown/beige adipose tissue that may act in human metabolism, new animal models are edging into the market for translational medicine. Pigs reflect human metabolism better than mice in multiple parameters such as obesity-induced hyperglycemia, cholesterol profiles and energy metabolism. Recently, it was reported that energy expenditure and body temperature in pigs is induced by the hormone leptin, and that leptin's action is mediated by UCP1 in adipose tissue. Given the tremendous importance of identifying molecular mechanisms for targeting therapeutics, we critically examine the evidence supporting the presence of UCP1 in pigs and conclude that methodological shortcomings prevent an unequivocal claim for the presence of UCP1 in pigs. Despite this, we believe that leptin's effects on energy expenditure in pigs are potentially more transformative to human medicine in the absence of UCP1, as adult and obese humans possess only minor amounts of UCP1. In general, we propose that the biology of new animal models requires attention to comparative studies with humans given the increasing amount of genomic information for various animal species.

Native UCP1 Displays Simple Competitive Kinetics between the Regulators Purine Nucleotides and Fatty Acids

Elucidation of the regulation of uncoupling protein 1 (UCP1) activity in its native environment, i.e. the inner membrane of brown-fat mitochondria, has been hampered by the presence of UCP1-independent, quantitatively unresolved effects of investigated regulators on the brown-fat mitochondria themselves. Here we have utilized the availability of UCP1-ablated mice to dissect UCP1-dependent and UCP1-independent effects of regulators. Using a complex-I-linked substrate (pyruvate), we found that UCP1 can mediate a 4-fold increase in thermogenesis when stimulated with the classical positive regulator fatty acids (oleate). After demonstrating that the fatty acids act in their free form, we found that UCP1 increased fatty acid sensitivity approximately 30-fold (as compared with the 1.5-fold increase reported earlier based on nominal fatty acid values). By identifying the UCP1-mediated fraction of the response, we could conclude that the interaction between purine nucleotides (GDP) and fatty acids (oleate) unexpectedly displayed simple competitive kinetics. In GDP-inhibited mitochondria, oleate apparently acted as an activator. However, only a model in which UCP1 is inherently active (i.e."activating" fatty acids cannot be included in the model), where GDP functions as an inhibitor with a K(m) of 0.05 mm, and where oleate functions as a competitive antagonist for the GDP effect (with a K(i) of 5 nm) can fit all of the experimental data. We conclude that, when examined in its native environment, UCP1 functions as a proton (equivalent) carrier in the absence of exogenous or endogenous fatty acids.

Non-shivering thermogenesis in a carnivorous marsupial, Sarcophilus harrisii, in the absence of UCP1

The presence of, and the physiological mechanisms behind, non-shivering thermogenesis (NST) were investigated in the Tasmanian Devil ( Sarcophilus harrisii, Boitard 1841). This was performed by measuring metabolic rate in response to cold exposure with and without an injection of norepinephrine. We also attempted to identify the presence of uncoupling protein 1 (UCP1) and its involvement (if any) in NST in S. harrisii, prior to and after cold exposure. UCP1 is a specialized protein channel found within the mitochondria of brown adipose tissue that is known to be involved in eutherian NST. Increase in resting oxygen consumption (VO 2), were measured under constant conditions at 20°C after various stimuli; long-term cold exposure (14 days at 2–3°C), and with or without subsequent norepinephrine injection was used to indicate the presence of NST. Concurrent with the functional studies, molecular level studies including Western blots and RT-PCR were used to identify the expression of UCP1. We found that injection of norepinephrine significantly increased NST after cold exposure in S. harrisii. The post-acclimation resting (VO 2) increased by 11%, whereas the combination of cold exposure and injection of NE elicited an approximately 30% increase in metabolic rate. However, expression of UCP1 in S. harrisii was not identified by the molecular techniques employed, in either the pre- or post-cold-acclimated tissues. These data suggest that S. harrisii shows NST ability and that it is accomplished in the absence of UCP1.