bims-mimbat Biomed News
on Mitochondrial metabolism in brown adipose tissue
Issue of 2023‒05‒21
twelve papers selected by
José Carlos de Lima-Júnior
Washington University

  1. Trends Cell Biol. 2023 May 13. pii: S0962-8924(23)00081-8. [Epub ahead of print]
      Extracellular purinergic molecules act as signaling molecules that bind to cellular receptors and regulate signaling pathways. Growing evidence suggests that purines regulate adipocyte function and whole-body metabolism. Here, we focus on one specific purine: inosine. Brown adipocytes, which are important regulators of whole-body energy expenditure (EE), release inosine when they are stressed or become apoptotic. Unexpectedly, inosine activates EE in neighboring brown adipocytes and enhances differentiation of brown preadipocytes. Increasing extracellular inosine, either directly by increasing inosine intake or indirectly via pharmacological inhibition of cellular inosine transporters, increases whole-body EE and counteracts obesity. Thus, inosine and other closely related purines might be a novel approach to tackle obesity and associated metabolic disorders by enhancing EE.
    Keywords:  brown adipocytes; diabetes; inosine; metabolism; purines; thermogenic fat
  2. Front Endocrinol (Lausanne). 2023 ;14 1176733
      Introduction: Brown adipocytes produce heat through non shivering thermogenesis (NST). To adapt to temperature cues, they possess a remarkably dynamic metabolism and undergo substantial cellular remodeling. The proteasome plays a central role in proteostasis and adaptive proteasome activity is required for sustained NST. Proteasome activators (PAs) are a class of proteasome regulators but the role of PAs in brown adipocytes is unknown. Here, we studied the roles of PA28α (encoded by Psme1) and PA200 (encoded by Psme4) in brown adipocyte differentiation and function.Methods: We measured gene expression in mouse brown adipose tissue. In cultured brown adipocytes, we silenced Psme1 and/or Psme4 expression through siRNA transfection. We then assessed impact on the ubiquitin proteasome system, brown adipocyte differentiation and function.
    Results: We found that Psme1 and Psme4 are expressed in brown adipocytes in vivo and in vitro. Through silencing of Psme1 and/or Psme4 expression in cultured brown adipocytes, we found that loss of PAs did not impair proteasome assembly or activity, and that PAs were not required for proteostasis in this model. Loss of Psme1 and/or Psme4 did not impair brown adipocyte development or activation, suggesting that PAs are neither required for brown adipogenesis nor NST.
    Discussion: In summary, we found no role for Psme1 and Psme4 in brown adipocyte proteostasis, differentiation, or function. These findings contribute to our basic understanding of proteasome biology and the roles of proteasome activators in brown adipocytes.
    Keywords:  BAT; PA200; PA28αβ; Psme1; Psme4; brown adipocytes; proteostasis; ubiquitin-proteasome-system
  3. bioRxiv. 2023 May 04. pii: 2023.05.04.539294. [Epub ahead of print]
      Mitochondria-rich brown adipocytes dissipate cellular fuel as heat by thermogenic energy expenditure (TEE). Prolonged nutrient excess or cold exposure impair TEE and contribute to the pathogenesis of obesity, but the mechanisms remain incompletely understood. Here we report that stress-induced proton leak into the matrix interface of mitochondrial innermembrane (IM) mobilizes a group of proteins from IM into matrix, which in turn alter mitochondrial bioenergetics. We further determine a smaller subset that correlates with obesity in human subcutaneous adipose tissue. We go on to show that the top factor on this short list, acyl-CoA thioesterase 9 (ACOT9), migrates from the IM into the matrix upon stress where it enzymatically deactivates and prevents the utilization of acetyl-CoA in TEE. The loss of ACOT9 protects mice against the complications of obesity by maintaining unobstructed TEE. Overall, our results introduce aberrant protein translocation as a strategy to identify pathogenic factors.One-Sentence Summary: Thermogenic stress impairs mitochondrial energy utilization by forcing translocation of IM-bound proteins into the matrix.
  4. Nat Commun. 2023 May 13. 14(1): 2754
      Active thermogenesis in the brown adipose tissue (BAT) facilitating the utilization of lipids and glucose is critical for maintaining body temperature and reducing metabolic diseases, whereas inactive BAT accumulates lipids in brown adipocytes (BAs), leading to BAT whitening. Although cellular crosstalk between endothelial cells (ECs) and adipocytes is essential for the transport and utilization of fatty acid in BAs, the angiocrine roles of ECs mediating this crosstalk remain poorly understood. Using single-nucleus RNA sequencing and knock-out male mice, we demonstrate that stem cell factor (SCF) derived from ECs upregulates gene expressions and protein levels of the enzymes for de novo lipogenesis, and promotes lipid accumulation by activating c-Kit in BAs. In the early phase of lipid accumulation induced by denervation or thermoneutrality, transiently expressed c-Kit on BAs increases the protein levels of the lipogenic enzymes via PI3K and AKT signaling. EC-specific SCF deletion and BA-specific c-Kit deletion attenuate the induction of the lipogenic enzymes and suppress the enlargement of lipid droplets in BAs after denervation or thermoneutrality in male mice. These data provide insight into SCF/c-Kit signaling as a regulator that promotes lipid accumulation through the increase of lipogenic enzymes in BAT when thermogenesis is inhibited.
  5. J Lipid Res. 2023 May 10. pii: S0022-2275(23)00059-7. [Epub ahead of print] 100386
      Levels of circulating FABP4 protein are strongly associated with obesity and metabolic disease in both mice and humans, and secretion is stimulated by ß-adrenergic stimulation both in vivo and in vitro. Previously, lipolysis-induced FABP4 secretion was found to be significantly reduced upon pharmacological inhibition of adipose triglyceride lipase (ATGL), and was absent from adipose tissue explants from mice specifically lacking ATGL in their adipocytes (ATGLAdpKO). Here we find that upon activation of ß-adrenergic receptors in vivo, ATGLAdpKO mice unexpectedly exhibited significantly higher levels of circulating FABP4 as compared to ATGLfl/fl controls, despite no corresponding induction of lipolysis. We generated an additional model with adipocyte-specific deletion of both FABP4 and ATGL (ATGL/FABP4AdpKO) to evaluate the cellular source of this circulating FABP4. In these animals, there was no evidence of lipolysis-induced FABP4 secretion, indicating that the source of elevated FABP4 levels in ATGLAdpKO mice was indeed from the adipocytes. ATGLAdpKO mice exhibited significantly elevated corticosterone levels, which positively correlated with plasma FABP4 levels. Pharmacological inhibition of sympathetic signaling during lipolysis using hexamethonium, or housing mice at thermoneutrality to chronically reduce sympathetic tone significantly reduced FABP4 secretion in ATGLAdpKO mice compared to controls. Therefore, activity of a key enzymatic step of lipolysis mediated by ATGL, per se, is not required for in vivo stimulation of FABP4 secretion from adipocytes, which can be induced through sympathetic signaling.
    Keywords:  Adipocytes; Adipose Triglyceride Lipase (ATGL); Adipose tissue; Fatty Acid Binding Proteins; Fatty Acid Transport; Lipolysis; Sympathetic Tone
  6. Sci Rep. 2023 Apr 25. 13(1): 6738
      Respiratory complex I is a major cellular energy transducer located in the inner mitochondrial membrane. Its inhibition by rotenone, a natural isoflavonoid, has been used for centuries by indigenous peoples to aid in fishing and, more recently, as a broad-spectrum pesticide or even a possible anticancer therapeutic. Unraveling the molecular mechanism of rotenone action will help to design tuned derivatives and to understand the still mysterious catalytic mechanism of complex I. Although composed of five fused rings, rotenone is a flexible molecule and populates two conformers, bent and straight. Here, a rotenone derivative locked in the straight form was synthesized and found to inhibit complex I with 600-fold less potency than natural rotenone. Large-scale molecular dynamics and free energy simulations of the pathway for ligand binding to complex I show that rotenone is more stable in the bent conformer, either free in the membrane or bound to the redox active site in the substrate-binding Q-channel. However, the straight conformer is necessary for passage from the membrane through the narrow entrance of the channel. The less potent inhibition of the synthesized derivative is therefore due to its lack of internal flexibility, and interconversion between bent and straight forms is required to enable efficient kinetics and high stability for rotenone binding. The ligand also induces reconfiguration of protein loops and side-chains inside the Q-channel similar to structural changes that occur in the open to closed conformational transition of complex I. Detailed understanding of ligand flexibility and interactions that determine rotenone binding may now be exploited to tune the properties of synthetic derivatives for specific applications.
  7. Mitochondrion. 2023 May 16. pii: S1567-7249(23)00037-5. [Epub ahead of print]
      The antifungal activity of the drug micafungin, a cyclic lipopeptide that interacts with membrane proteins, may involve inhibition of fungal mitochondria. In humans, mitochondria are spared by the inability of micafungin to cross the cytoplasmic membrane. Using isolated mitochondria, we find that micafungin initiates the uptake of salts, causing rapid swelling and rupture of mitochondria with release of cytochrome c. The inner membrane anion channel (IMAC) is altered by micafungin to transfer both cations and anions. We propose that binding of anionic micafungin to IMAC attracts cations into the ion pore for the rapid transfer of ion pairs.
    Keywords:  Anion transport; Cyclic lipopeptides; Inner mitochondrial anion channel; Ion channel; Micafungin; Mitochondrial respiratory chain complex; Mitochondrial transport
  8. bioRxiv. 2023 May 06. pii: 2023.05.05.539595. [Epub ahead of print]
      The mitochondrial phospholipid cardiolipin (CL) promotes bioenergetics via oxidative phosphorylation (OXPHOS). Three tightly bound CLs are evolutionarily conserved in the ADP/ATP carrier (AAC in yeast; adenine nucleotide translocator, ANT in mammals) which resides in the inner mitochondrial membrane and exchanges ADP and ATP to enable OXPHOS. Here, we investigated the role of these buried CLs in the carrier using yeast Aac2 as a model. We introduced negatively charged mutations into each CL-binding site of Aac2 to disrupt the CL interactions via electrostatic repulsion. While all mutations disturbing the CL-protein interaction destabilized Aac2 monomeric structure, transport activity was impaired in a pocket-specific manner. Finally, we determined that a disease-associated missense mutation in one CL-binding site in ANT1 compromised its structure and transport activity, resulting in OXPHOS defects. Our findings highlight the conserved significance of CL in AAC/ANT structure and function, directly tied to specific lipid-protein interactions.
  9. Int J Mol Sci. 2023 May 01. pii: 8123. [Epub ahead of print]24(9):
      The uncoupling protein UCP2 is a mitochondrial carrier for which transport activity remains controversial. The physiological contexts in which UCP2 is expressed have led to the assumption that, like UCP1, it uncouples oxidative phosphorylation and thereby reduces the generation of reactive oxygen species. Other reports have involved UCP2 in the Warburg effect, and results showing that UCP2 catalyzes the export of matrix C4 metabolites to facilitate glutamine utilization suggest that the carrier could be involved in the metabolic adaptations required for cell proliferation. We have examined the role of UCP2 in the energy metabolism of the lung adenocarcinoma cell line A549 and show that UCP2 silencing decreased the basal rate of respiration, although this inhibition was not compensated by an increase in glycolysis. Silencing did not lead to either changes in proton leakage, as determined by the rate of respiration in the absence of ATP synthesis, or changes in the rate of formation of reactive oxygen species. The decrease in energy metabolism did not alter the cellular energy charge. The decreased cell proliferation observed in UCP2-silenced cells would explain the reduced cellular ATP demand. We conclude that UCP2 does not operate as an uncoupling protein, whereas our results are consistent with its activity as a C4-metabolite carrier involved in the metabolic adaptations of proliferating cells.
    Keywords:  UCP2; Warburg; cancer; proliferation; reactive oxygen species; uncoupling
  10. Biochim Biophys Acta Biomembr. 2023 May 16. pii: S0005-2736(23)00057-3. [Epub ahead of print] 184175
      Mitochondria, composed of two membranes, play a key role in energy production in eukaryotic cells. The main function of the inner membrane is oxidative phosphorylation, while the mitochondrial outer membrane (MOM) seems to control the energy flux and exchange of various charged metabolites between mitochondria and the cytosol. Metabolites cross MOM via the various isoforms of voltage-dependent anion channel (VDAC). In turn, VDACs interact with some enzymes, other proteins and molecules, including drugs. This work aimed to analyze various literature experimental data related to targeting mitochondrial VDACs and VDAC-kinase complexes on the basis of the hypothesis of generation of the outer membrane potential (OMP) and OMP-dependent reprogramming of cell energy metabolism. Our previous model of the VDAC-hexokinase-linked generation of OMP was further complemented in this study with an additional regulation of the MOM permeability by the OMP-dependent docking of cytosolic proteins like tubulin to VDACs. Computational analysis of the model suggests that OMP changes might be involved in the mechanisms of apoptosis promotion through the so-called transient hyperpolarization of mitochondria. The high concordance of the performed computational estimations with many published experimental data allows concluding that OMP generation under physiological conditions is highly probable and VDAC might function as an OMP-dependent gatekeeper of mitochondria, controlling cell life and death. The proposed model of OMP generation allows understanding in more detail the mechanisms of cancer death resistance and anticancer action of various drugs and treatments influencing VDAC voltage-gating properties, VDAC content, mitochondrial hexokinase activity and VDAC-kinase interactions in MOM.
    Keywords:  Diseases; Membrane potential; Mitochondria; Mitochondrial outer membrane; VDAC
  11. Nat Commun. 2023 May 15. 14(1): 2783
      Cardiolipin is a hallmark phospholipid of mitochondrial membranes. Despite established significance of cardiolipin in supporting respiratory supercomplex organization, a mechanistic understanding of this lipid-protein interaction is still lacking. To address the essential role of cardiolipin in supercomplex organization, we report cryo-EM structures of a wild type supercomplex (IV1III2IV1) and a supercomplex (III2IV1) isolated from a cardiolipin-lacking Saccharomyces cerevisiae mutant at 3.2-Å and 3.3-Å resolution, respectively, and demonstrate that phosphatidylglycerol in III2IV1 occupies similar positions as cardiolipin in IV1III2IV1. Lipid-protein interactions within these complexes differ, which conceivably underlies the reduced level of IV1III2IV1 and high levels of III2IV1 and free III2 and IV in mutant mitochondria. Here we show that anionic phospholipids interact with positive amino acids and appear to nucleate a phospholipid domain at the interface between the individual complexes, which dampen charge repulsion and further stabilize interaction, respectively, between individual complexes.
  12. Mol Cell. 2023 May 06. pii: S1097-2765(23)00316-7. [Epub ahead of print]
      Mitophagy plays an important role in mitochondrial homeostasis by selective degradation of mitochondria. During mitophagy, mitochondria should be fragmented to allow engulfment within autophagosomes, whose capacity is exceeded by the typical mitochondria mass. However, the known mitochondrial fission factors, dynamin-related proteins Dnm1 in yeasts and DNM1L/Drp1 in mammals, are dispensable for mitophagy. Here, we identify Atg44 as a mitochondrial fission factor that is essential for mitophagy in yeasts, and we therefore term Atg44 and its orthologous proteins mitofissin. In mitofissin-deficient cells, a part of the mitochondria is recognized by the mitophagy machinery as cargo but cannot be enwrapped by the autophagosome precursor, the phagophore, due to a lack of mitochondrial fission. Furthermore, we show that mitofissin directly binds to lipid membranes and brings about lipid membrane fragility to facilitate membrane fission. Taken together, we propose that mitofissin acts directly on lipid membranes to drive mitochondrial fission required for mitophagy.
    Keywords:  Atg44; autophagy; crystal structure analysis; dynamin-related protein; high-speed atomic force microscopy; mitochondria; mitochondrial fission; mitofissin; mitophagy; yeast