Interferon-γ down-regulates adenosine 2b receptor-mediated signaling and short circuit current in the intestinal epithelia by inhibiting the expression of adenylate cyclase

Abstract

Adenosine is an endogenous signaling molecule that is highly up-regulated in inflammatory states. Adenosine acts through the A2b receptor, a G protein-coupled receptor that couples positively to Gαs and activates adenylate cyclase. This leads to cAMP-mediated electrogenic chloride secretion in intestinal epithelia. To better understand the regulation of the A2b receptor in intestinal epithelia, we studied the effects of interferon-γ (IFN-γ), a potent immunomodulatory cytokine, in the T84 cell line. Pretreatment of cells with 500 units/ml IFN-γ for 12 h inhibited an adenosine-induced short circuit current (Isc) without affecting the transepithelial resistance. Under these conditions, IFN-γ did not inhibit the protein expression or membrane recruitment of the A2b receptor, shown to be essential for its function. Interestingly, IFN-γ inhibited cAMP levels as well as its downstream signaling pathway as shown by the inhibition of adenosine-induced phosphorylation of cAMP response element-binding protein and protein kinase A activity. Similar studies with forskolin, a direct activator of adenylate cyclase, also demonstrated inhibition of cAMP and its downstream response by IFN-γ. However, IFN-γ did not affect secretory responses to the calcium-dependent secretagogue carbachol or cAMP analog 8-bromo-cAMP, indicating that normal secretory responses to adequate second messengers in IFN-γ-treated cells are achievable. Moreover, IFN-γ inhibited the expression of adenylate cyclase isoforms 5 and 7. In conclusion, we demonstrate that IFN-γ down-regulates adenosine-mediated signaling possibly through the direct inhibition of adenylate cyclase expression. We propose that IFN-γ may acutely affect global cAMP-mediated responses in the intestinal epithelia, thereby decreasing secretory responses, which may consequently aggravate inflammatory processes.