Probing partial/biased agonism at the G protein‐coupled A2B adenosine receptor (662.11)

Abstract

The A2B adenosine receptor (AR) is involved in many important functions and couples to several G proteins, but the endogenous effects have not been well documented partly due to the lack of selective agonists at this subtype until recently. In the present study, we probed the potential partial and/or biased agonism at the A2B AR in several types of cells, including HEK293 cells and T24 bladder cancer cells, using a series of A2B agonists of adenosine analogs substituted at 5’, N6 and C2 positions, including NECA, MRS5911 and MRS3997, and the non‐nucleoside A2B agonist, BAY60‐6583. All A2B agonists tested induced accumulation of cyclic AMP fully or partially in all cells tested, with NECA being the most efficacious. However, only 5’‐substituted analogs are full agonists in calcium mobilization and ERK1/2 phosphorylation. Over‐expression of the A2B AR in HEK293 cells increased the agonist potency and efficacy in inducing the Gs‐mediated accumulation of cyclic AMP, but not the ability to induce calcium mobilization by the C2‐substituted adenosine derivatives or non‐adenosine A2B agonists. Results from an arrestin translocation assay showed that the 5’‐substituted NECA is more efficacious than adenosine derivatives substituted at C2 and N6 positions and non‐adenosine agonists. BAY actually did not induce arrestin translocation in this assay. The data suggest that 5’‐substituted adenosine analogs are balanced agonists of different pathways, whereas the C2‐ and N6‐substituted adenosine analogs and the class of non‐nucleoside A2B agonists are Gs‐biased agonists. The data also suggest that BAY and some N6‐ and C2‐substituted adenosine analogs may be full agonist, partial agonist or antagonist under various conditions. Indeed, BAY behaves as an A2B antagonist to induce insulin release in MIN6 pancreatic β cells. Thus, differences in efficacy and effector pathways might explain some of the contradictory results reported for A2B AR activation.Grant Funding Source: Supported by NIDDK Intramural Research, NIH