title: Priming of signal transducer and activator of transcription proteins for cytokine-triggered polyubiquitylation and degradation by the A 2A adenosine receptor.
abstract: Here we demonstrate that overexpression of the human A(2A) adenosine receptor (A(2A)AR) in vascular endothelial cells confers an ability of interferon-alpha and a soluble IL-6 receptor/IL-6 (sIL-6R alpha/IL-6) trans-signaling complex to trigger the down-regulation of signal transducer and activator of transcription (STAT) proteins. It is noteworthy that STAT down-regulation could be reversed by coincubation with A(2A)AR-selective inverse agonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM241385) but not adenosine deaminase, suggesting that constitutive activation of the receptor was responsible for the effect. Moreover, STAT down-regulation was selectively abolished by proteasome inhibitor N-benzoyloxycarbonyl (Z)-Leu-Leu-leucinal (MG132), whereas lysosome inhibitor chloroquine was without effect. Down-regulation required Janus kinase (JAK) activity and a Tyr705-->Phe-mutated STAT3 was resistant to the phenomenon, suggesting that JAK-mediated phosphorylation of this residue is required. Consistent with this hypothesis, treatment of A(2A)AR-overexpressing cells with sIL-6R alpha/IL-6 triggered the accumulation of polyubiquitylated wild-type but not Tyr705-->Phe-mutated STAT3. Support for a functional role of this process was provided by the observation that A(2A)AR overexpression attenuated the JAK/STAT-dependent up-regulation of vascular endothelial growth factor receptor-2 by sIL-6R alpha/IL-6. Consistent with a role for endogenous A(2A)ARs in regulating STAT protein levels, prolonged exposure of endogenous A(2A)ARs in endothelial cells with ZM241385 in vitro triggered the up-regulation of STAT3, whereas deletion of the A(2A)AR in vivo potentiated STAT1 expression and phosphorylation. Together, these experiments support a model whereby the A(2A)AR can prime JAK-phosphorylated STATs for polyubiquitylation and proteasomal degradation and represents a new mechanism by which an anti-inflammatory seven-transmembrane receptor can negatively regulate JAK/STAT signaling.
authors.affiliation: Faculty of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, UK.
authors.forename: Mohammed M A
authors.initials: MM
authors.lastname: Safhi
authors.forename: Claire
authors.initials: C
authors.lastname: Rutherford
authors.forename: Catherine
authors.initials: C
authors.lastname: Ledent
authors.forename: William A
authors.initials: WA
authors.lastname: Sands
authors.forename: Timothy M
authors.initials: TM
authors.lastname: Palmer
mesh_terms: D000818:Animals; D015153:Blotting, Western; D002478:Cells, Cultured; D016207:Cytokines / Q000378:metabolism*; D015536:Down-Regulation; D006801:Humans; D006868:Hydrolysis; D047468:Immunoprecipitation; D051379:Mice; D018345:Mice, Knockout; D010766:Phosphorylation; D043683:Receptors, Adenosine A2 / Q000378:metabolism*; D050791:STAT Transcription Factors / Q000502:physiology*; D014227:Triazines / Q000494:pharmacology; D014230:Triazoles / Q000494:pharmacology; D054875:Ubiquitination
chemical_list: D016207:Cytokines; D043683:Receptors, Adenosine A2; D050791:STAT Transcription Factors; D014227:Triazines; D014230:Triazoles; C097270:ZM 241385