Brown adipose tissue

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Active brown adipose tissue around the chest area; the patient froze during the PET examination

The brown or plurivakuoläre adipose tissue is a special form of the adipose tissue , which cells are capable by the oxidation of fatty acids heat to produce ( thermogenesis ). This happens in numerous mitochondria , which are also responsible for the yellow-brown color of the tissue. Biochemically, the membrane protein Thermogenin decouples fatty acid oxidation from the synthesis of the energy carrier adenosine triphosphate (ATP) , so that the released energy is converted into heat.

Occurrence

Brown adipose tissue is found in all newborn mammals except pigs . Newborns are more at risk of hypothermia, because they lose more heat due to their smaller size (larger body surface compared to volume) and the mechanisms of thermoregulation (e.g. insulating white adipose tissue and shivering ) are not yet fully developed. In the human infant , brown adipose tissue is found mainly on the neck and chest.

Adults were thought to have lost active brown fat cells. Recent studies have found that "at least a few tens of percent" of adults may have active brown fat cells. In humans, the activity of the brown fat cells is strongly triggered by cold stimuli and stimulated by the sympathetic nervous system .

Rodents in particular have larger amounts of brown adipose tissue even in their adult state and, if necessary, can use catecholamines to convert white into brown adipose tissue and thus survive phases of cold well. In hibernation holding animals also larger amounts of brown fat, which serve the rapid warming of the animal in the awakenings find.

Histologically similar tissues appear in some birds , but they have no thermogenin and are not used for thermogenesis. However, some birds can generate heat in their skeletal muscles through biochemically similar mechanisms.

histology

The cells in brown adipose tissue are generally smaller than those in white adipose tissue and have many, smaller droplets of lipid. In contrast to the univacuolar cells of white adipose tissue, they are therefore referred to as plurivacuolar . In addition, they are characterized by a particular wealth of mitochondria , which are also responsible for the brown color due to their content of cytochromes .

biochemistry

The protein thermogenin , which is present in brown adipose tissue in the inner membrane of the mitochondria, acts as a decoupler by transporting protons across the membrane as a uniporter . As a result, the proton gradient built up by β-oxidation and the respiratory chain is broken down and the energy stored in it is converted into heat (thermogenesis). In addition, brown adipose tissue has a particularly high concentration of glycerokinase , so that the glycerol released during fat breakdown can be phosphorylated and also metabolized. Presumably, irisin initiates a conversion from white to brown adipose tissue.

regulation

Thermogenesis in brown adipose tissue is via the hormone norepinephrine activated, via a coupled G protein-β receptor , the adenylate cyclase activated. The cAMP formed in turn activates protein kinase A , which initiates fat breakdown via phosphorylation of lipases . In addition, brown adipose tissue is sympathetically innervated.

The activity and formation of brown adipose tissue is increased by the PGC-1alpha ( Peroxisome proliferator-activated receptor-gamma coactivator ), which in turn is released strongly after a cold stimulus .

There is evidence that the ratio between brown and white adipose tissue is influenced by microRNA 155.

The production of brown adipose tissue and its activity can be reduced or reduced by taking statins . be restricted.

literature

  • Georg Löffler, Petro E. Petrides, Peter C. Heinrich: Biochemistry and Pathobiochemistry . 8th edition. Springer, Heidelberg 2006, ISBN 3-540-32680-4 .
  • Werner A. Müller: Animal and Human Physiology . Springer, Heidelberg 1998, ISBN 3-540-63313-8 .

Individual evidence

  1. ↑ Frida Berg, Ulla Gustafson, Leif Andersson: The Uncoupling Protein 1 Gene (UCP1) Is Disrupted in the Pig Lineage: A Genetic Explanation for Poor Thermoregulation in Piglets. In: PLoS Genetics . August 18, 2006.
  2. ↑ Jan Nedergaard, Tore Bengtsson, Barbara Cannon: Unexpected evidence for active brown adipose tissue in adult humans . In: American Journal of Physiology-Endocrinology and Metabolism . tape 293 , 2007, p. E444-E452 (English).
  3. ↑ Seppo Saarelaa, Jacqueline S. KeithBot, Esa Hohtolaa, Paul Trayhurn: Is the "mammalian" brown fat-specific mitochondrial uncoupling protein present in adipose tissues of birds? In: Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology . tape 100 , no. 1 , 1991, p. 45-49 (English).
  4. ↑ Darren A. Talbot, Claude Duchamp, Benjamin Rey, Nicolas Hanuise, Jean Louis Rouanet, Brigitte Sibille, Martin D. Brand: Uncoupling protein and ATP / ADP carrier increase mitochondrial proton conductance after cold adaptation of king penguins . In: The Journal of Physiology . tape 558 , no. 1 , 2004, p. 123-135 (English).
  5. ↑ a b Huiyun Liang, Walter Ward: PGC-1alpha: a key regulator of energy metabolism. In: Advan. Physiol. Edu. 30, 2006, pp. 145–151, doi: 10.1152 / advan.00052.2006 , full text ( memento of November 23, 2010 in the Internet Archive ) (English).
  6. ↑ Yong Chen, Franziska Siegel, Stefanie Kipschull, Bodo Haas, Holger Fröhlich, Gunter Meister, Alexander Pfeifer: miR-155 regulates differentiation of brown and beige adipocytes via a bistable circuit. In: Nature Communications. 4, 2013, p. 1769. doi: 10.1038 / ncomms2742 .
  7. ↑ inhibition of mevalonates Pathway Prevents adipocyte Browning in Mice and Men by Affecting protein prenylation . In: Cell Metabolism . December 20, 2018, ISSN  1550-4131 , doi : 10.1016 / j.cmet.2018.11.017 ( sciencedirect.com [accessed December 23, 2018]).