Abstract

Adenosine is a potent inhibitor of inflammatory processes, and the A_2A adenosine receptor (A_2AAR) plays a key nonredundant role as a suppresser of inflammatory responses in vivo. In this study, we demonstrate that increasing A_2AAR gene expression suppressed multiple inflammatory responses in both human umbilical vein endothelial cells (HUVECs) and rat C6 glioma cells in vitro. In particular, the induction of the adhesion molecule E-selectin by either tumor necrosis factor α (TNFα) or Escherichia coli lipopolysaccharide (LPS) was reduced by more than 70% in HUVECs, whereas inducible nitric-oxide synthase (iNOS) induction was abolished in C6 cells after exposure to interferon-γ in combination with LPS and TNFα, suggesting that the receptor inhibited a common step in the induction of each of these pro-inflammatory genes. Consistent with this hypothesis, A_2AAR expression inhibited the activation of NF-κB, a key transcription factor whose proper function was essential for optimal iNOS and E-selectin induction. However, although NF-κB binding to target DNA was severely compromised in both cell types, the mechanisms by which this occurred were distinct. In C6 cells, A_2AAR expression blocked IκBα degradation by inhibiting stimulus-induced phosphorylation, whereas in HUVECs, A_2AAR expression inhibited NF-κB translocation to the nucleus independently of any effect on IκBα degradation. Together, these observations suggest that A_2AAR-mediated inhibition NF-κB activation is a critical aspect of its anti-inflammatory signaling properties and that the molecular basis of this inhibition varies in a cell type-specific manner.