title: Novel regulatory mechanisms for generation of the soluble leptin receptor: implications for leptin action.
abstract: BACKGROUND
  The adipokine leptin realizes signal transduction via four different membrane-anchored leptin receptor (Ob-R) isoforms in humans. However, the amount of functionally active Ob-R is affected by constitutive shedding of the extracellular domain via a so far unknown mechanism. The product of the cleavage process the so-called soluble leptin receptor (sOb-R) is the main binding protein for leptin in human blood and modulates its bioavailability. sOb-R levels are differentially regulated in metabolic disorders like type 1 diabetes mellitus or obesity and can, therefore, enhance or reduce leptin sensitivity.
  
  
  RESULTS
  To describe mechanisms of Ob-R cleavage and to investigate the functional significance of differential sOb-R levels we established a model of HEK293 cells transiently transfected with different human Ob-R isoforms. Using siRNA knockdown experiments we identified ADAM10 (A Disintegrin And Metalloproteinase 10) as a major protease for constitutive and activated Ob-R cleavage. Additionally, the induction of lipotoxicity and apoptosis led to enhanced shedding shown by increased levels of the soluble leptin receptor (sOb-R) in cell supernatants. Conversely, high leptin concentrations and ER stress reduced sOb-R levels. Decreased amounts of sOb-R due to ER stress were accompanied by impaired leptin signaling and reduced leptin binding.
  
  
  CONCLUSIONS
  Lipotoxicity and apoptosis increased Ob-R cleavage via ADAM10-dependent mechanisms. In contrast high leptin levels and ER stress led to reduced sOb-R levels. While increased sOb-R concentrations seem to directly block leptin action, reduced amounts of sOb-R may reflect decreased membrane expression of Ob-R. These findings could explain changes of leptin sensitivity which are associated with variations of serum sOb-R levels in metabolic diseases.
authors.affiliation: Institute of Laboratory Medicine, Clinical Chemistry and Molecular Diagnostics, University of Leipzig, Leipzig, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Schaab
authors.forename: Henriette
authors.initials: H
authors.lastname: Kausch
authors.forename: Juergen
authors.initials: J
authors.lastname: Klammt
authors.forename: Marcin
authors.initials: M
authors.lastname: Nowicki
authors.forename: Ulf
authors.initials: U
authors.lastname: Anderegg
authors.forename: Rolf
authors.initials: R
authors.lastname: Gebhardt
authors.forename: Stefan
authors.initials: S
authors.lastname: Rose-John
authors.forename: Juergen
authors.initials: J
authors.lastname: Scheller
authors.forename: Joachim
authors.initials: J
authors.lastname: Thiery
authors.forename: Juergen
authors.initials: J
authors.lastname: Kratzsch
mesh_terms: D051722:ADAM Proteins / Q000235:genetics / Q000378:metabolism*; D000072197:ADAM10 Protein; D000072198:ADAM17 Protein; D053829:Amyloid Precursor Protein Secretases / Q000235:genetics / Q000378:metabolism*; D017209:Apoptosis; D020169:Caspases / Q000378:metabolism; D059865:Endoplasmic Reticulum Stress; D005786:Gene Expression Regulation; D057809:HEK293 Cells; D006801:Humans; D020738:Leptin / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D019308:Palmitic Acid / Q000378:metabolism; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D011493:Protein Kinase C / Q000378:metabolism; D034741:RNA, Small Interfering / Q000235:genetics; D054411:Receptors, Leptin / Q000235:genetics / Q000378:metabolism*; D050796:STAT3 Transcription Factor / Q000378:metabolism; D012995:Solubility; D014162:Transfection
chemical_list: D020738:Leptin; D008565:Membrane Proteins; D020033:Protein Isoforms; D034741:RNA, Small Interfering; D054411:Receptors, Leptin; D050796:STAT3 Transcription Factor; D019308:Palmitic Acid; D011493:Protein Kinase C; D053829:Amyloid Precursor Protein Secretases; D020169:Caspases; D051722:ADAM Proteins; D000072197:ADAM10 Protein; C491236:ADAM10 protein, human; D000072198:ADAM17 Protein