bims-mimbat Biomed News
on Mitochondrial metabolism in brown adipose tissue
Issue of 2022‒10‒30
seven papers selected by
José Carlos de Lima-Júnior
Washington University

  1. Trends Cell Biol. 2022 Oct 19. pii: S0962-8924(22)00230-6. [Epub ahead of print]
      Brown adipose tissue (BAT) controls mammalian core body temperature by non-shivering thermogenesis. BAT is extraordinarily rich in mitochondria, which have the peculiarity of generating heat by uncoupled respiration. Since the mitochondrial activity of BAT is subject to cycles of activation and deactivation in response to environmental temperature changes, an integrated mitochondrial quality control (MQC) system is of fundamental importance to ensure BAT physiology. Here, we provide an overview of the conventional and alternative mechanisms through which thermogenic adipocytes selectively remove damaged parts of mitochondria and how macrophages participate in the MQC system by removing extracellular mitochondrial waste to maintain the thermogenic function of BAT.
    Keywords:  adipocytes; extracellular vesicles; immune cells; metabolism; mitochondrial transfer; thermogenesis
  2. J Biol Chem. 2022 Oct 19. pii: S0021-9258(22)01062-6. [Epub ahead of print] 102619
      Thermoregulation is a process by which core body temperature is maintained in mammals. Males typically have a lower body temperature than females. However, the effects of androgens, sex hormones that show higher levels in males, on adrenergic receptor-mediated thermogenesis remains unclear. Here, we demonstrate that androgen-androgen receptor (AR) signaling suppresses the β-adrenergic agonist-induced rise of core body temperature using castrated and AR knockout (ARKO) male mice. Furthermore, in vitro mechanistic studies show that activated AR inhibits cAMP response element (CRE)-mediated transcription by suppressing cAMP response element-binding protein (CREB) phosphorylation. The elevation of body temperature induced by the β-adrenergic agonist CL316243 was higher in ARKO and castrated mice than in control mice. Similarly, CL316243 induced a greater increase in Uncoupling protein 1 (Ucp1) expression and CREB phosphorylation in the brown adipose tissue (BAT) of ARKO mice than in that of controls. We determined that activation of AR by dihydrotestosterone suppressed β3-agonist- or forskolin-induced CRE-mediated transcription, which was prevented by AR antagonist bicalutamide. AR activation also suppressed CREB phosphorylation induced by forskolin. Moreover, we found AR nuclear localization, but not transcriptional activity, was necessary for the suppression of CRE-mediated transcription. Finally, modified mammalian two-hybrid and immunoprecipitation analyses suggest nuclear AR and CREB form a protein complex both in the presence and absence of dihydrotestosterone and forskolin. These results suggest androgen-AR signaling suppresses β-adrenoceptor-induced UCP1-mediated BAT thermogenesis by suppressing CREB phosphorylation, presumably owing to a protein complex with AR and CREB. This mechanism explains sex differences in body temperature, at least partially.
    Keywords:  adipose tissue metabolism; adrenergic receptor; androgen receptor; cAMP response element‐binding protein (CREB); core body temperature; gene knockout; norepinephrine; sex differences; testosterone; uncoupling protein
  3. Biochim Biophys Acta Bioenerg. 2022 Oct 19. pii: S0005-2728(22)00400-5. [Epub ahead of print]1864(1): 148930
      At low inner mitochondrial membrane potential (ΔΨ) oxaloacetate (OAA) accumulates in the organelles concurrently with decreased complex II-energized respiration. This is consistent with ΔΨ-dependent OAA inhibition of succinate dehydrogenase. To assess the metabolic importance of this process, we tested the hypothesis that perturbing metabolic clearance of OAA in complex II-energized mitochondria would alter O2 flux and, further, that this would occur in both ΔΨ and tissue-dependent fashion. We carried out respiratory and metabolite studies in skeletal muscle and interscapular brown adipose tissue (IBAT) directed at the effect of OAA transamination to aspartate (catalyzed by the mitochondrial form of glutamic-oxaloacetic transaminase, Got2) on complex II-energized respiration. Addition of low amounts of glutamate to succinate-energized mitochondria at low ΔΨ increased complex II (succinate)-energized respiration in muscle but had little effect in IBAT mitochondria. The transaminase inhibitor, aminooxyacetic acid, increased OAA concentrations and impaired succinate-energized respiration in muscle but not IBAT mitochondria at low but not high ΔΨ. Immunoblotting revealed that Got2 expression was far greater in muscle than IBAT mitochondria. Because we incidentally observed metabolism of OAA to pyruvate in IBAT mitochondria, more so than in muscle mitochondria, we also examined the expression of mitochondrial oxaloacetate decarboxylase (ODX). ODX was detected only in IBAT mitochondria. In summary, at low but not high ΔΨ, mitochondrial transamination clears OAA preventing loss of complex II respiration: a process far more active in muscle than IBAT mitochondria. We also provide evidence that OAA decarboxylation clears OAA to pyruvate in IBAT mitochondria.
    Keywords:  Brown adipose tissue; Mitochondria; Mitochondrial complex II; Muscle; Oxaloacetate; Succinate dehydrogenase
  4. Commun Biol. 2022 Oct 26. 5(1): 1131
      Voltage-clamp fluorometry (VCF) supplies information about the conformational changes of voltage-gated proteins. Changes in the fluorescence intensity of the dye attached to a part of the protein that undergoes a conformational rearrangement upon the alteration of the membrane potential by electrodes constitute the signal. The VCF signal is generated by quenching and dequenching of the fluorescence as the dye traverses various local environments. Here we studied the VCF signal generation, using the Hv1 voltage-gated proton channel as a tool, which shares a similar voltage-sensor structure with voltage-gated ion channels but lacks an ion-conducting pore. Using mutagenesis and lipids added to the extracellular solution we found that the signal is generated by the combined effects of lipids during movement of the dye relative to the plane of the membrane and by quenching amino acids. Our 3-state model recapitulates the VCF signals of the various mutants and is compatible with the accepted model of two major voltage-sensor movements.
  5. J Vis Exp. 2022 Sep 28.
      Measuring brown adipose tissue (BAT) activity by positron emission tomography computed tomography (PET-CT) via the accumulation of 18F-fluorodeoxyglucose (FDG) after a meal or in obese or diabetic patients fails as the method of choice. The main reason is that 18F-FDG competes with the postprandial high glucose plasma concentration for the same glucose transporter on the membrane of BAT cells. In addition, BAT uses fatty acids as a source of energy as well, which is not visible with PET-CT and could be changed along with glucose concentration in obese and diabetic patients. Therefore, to estimate the physiological importance of BAT in animals and humans, a new infrared thermography method used in recent publications is applied. After overnight fasting, BAT activity was measured by infrared thermography before and after a meal in human volunteers and female wild-type mice. The camera software calculates the object's temperature using distance from the object, skin emissivity, reflected room temperature, air temperature, and relative humidity. In mice, the shaved area above the BAT was a region of interest for which average and maximal temperatures were measured. The phase of the estrous cycle in female mice was determined after an experiment by vaginal smears stained with cresyl violet (0.1%) stain solution. In healthy volunteers, two skin areas of the neck were selected: the supraclavicular area (above the collarbone, where BAT cells are present) and the interclavicular area (between the collarbones, where there is no BAT tissue detected). BAT activity is determined by the subtraction of those two values. Also, the average and maximal temperatures of skin areas could be determined in animals and human participants. Changes in BAT activity after a meal measured by infrared thermography, a non-invasive and more sensitive method, were shown to be sex, age, and phase of the estrous cycle dependent in laboratory animals. As part of diet-induced thermogenesis, BAT activation in humans was also proven to be sex, age, and body mass index dependent. Further determining the pathophysiological changes in BAT activity after a meal will be of great importance for participants with high glucose plasma concentrations (obesity and diabetes mellitus type 2), as well as in different laboratory animals (knock-out mice). This method is also a variable tool for determining possible activating drugs that could rejuvenate BAT activity.
  6. Proc Natl Acad Sci U S A. 2022 Nov;119(44): e2206649119
      Conformational changes in voltage-sensing domains (VSDs) are driven by the transmembrane electric field acting on the protein charges. Yet, the overall energetics and detailed mechanism of this process are not fully understood. Here, we determined free energy and displacement charge landscapes as well as the major conformations visited during a complete functional gating cycle in the isolated VSD of the phosphatase Ci-VSP (Ci-VSD) comprising four transmembrane helices (segments S1 to S4). Molecular dynamics simulations highlight the extent of S4 movements. In addition to the crystallographically determined activated "Up" and resting "Down" states, the simulations predict two Ci-VSD conformations: a deeper resting state ("down-minus") and an extended activated ("up-plus") state. These additional conformations were experimentally probed via systematic cysteine mutagenesis with metal-ion bridges and the engineering of proton conducting mutants at hyperpolarizing voltages. The present results show that these four states are visited sequentially in a stepwise manner during voltage activation, each step translocating one arginine or the equivalent of ∼1 e0 across the membrane electric field, yielding a transfer of ∼3 e0 charges in total for the complete process.
    Keywords:  Ci-VSP; energy landscape; gating currents; voltage dependence
  7. J Lipid Res. 2022 Oct 20. pii: S0022-2275(22)00138-9. [Epub ahead of print] 100305
      Hormone-sensitive lipase (HSL) plays a crucial role in intracellular lipolysis, and loss of HSL leads to diacylglycerol (DAG) accumulation, reduced fatty acid mobilization, and impaired peroxisome proliferator-activated receptor (PPAR) γ signaling. HSL knock-out mice exhibit adipose tissue inflammation, but the underlying mechanisms are still not clear. Here we investigated if and to what extent HSL loss contributes to endoplasmic reticulum (ER) stress and adipose tissue inflammation in HSL knock-out mice. Further, we were interested in how impaired PPARγ signaling affects the development of inflammation in epididymal white adipose tissue (eWAT) and inguinal white adipose tissue (iWAT) of HSL knock-out mice, and if DAG and ceramide (Cer) accumulation contribute to adipose tissue inflammation and ER stress. Ultrastructural analysis showed a markedly dilated ER in both eWAT and iWAT upon loss of HSL. In addition, HSL knock-out mice exhibited macrophage infiltration and increased F4/80 mRNA expression, a marker of macrophage activation, in eWAT, but not in iWAT. We show treatment with rosiglitazone, a PPARγ agonist, attenuated macrophage infiltration and ameliorated inflammation of eWAT, but expression of ER stress markers remained unchanged, as did DAG and Cer levels in eWAT. Taken together, we show HSL loss promoted ER stress in both eWAT and iWAT of HSL knock-out mice, but inflammation and macrophage infiltration occurred mainly in eWAT. Also, PPARγ activation reversed inflammation, but not ER stress and DAG accumulation. These data indicate that neither reduction of DAG levels nor ER stress contribute to the reversal of eWAT inflammation in HSL knock-out mice.
    Keywords:  adipocytes; adipose tissue; dysfunctional adipocytes; electron microscopy; fatty acid; fatty acid metabolism; inflammation; lipase; lipolysis; lipotoxicity