The resurgence of A2B adenosine receptor signaling

Abstract

Since its discovery as a low-affinity adenosine receptor (AR), the A2B receptor (A2BAR), has proven enigmatic in its function. The previous discovery of the A2AAR, which shares many similarities with the A2BAR but demonstrates significantly greater affinity for its endogenous ligand, led to the original perception that the A2BAR was not of substantial physiologic relevance. In addition, lack of specific pharmacological agents targeting the A2BAR made its initial characterization challenging. However, the importance of this receptor was reconsidered when it was observed that the A2BAR is highly transcriptionally regulated by factors implicated in inflammatory hypoxia. Moreover, the notion that during ischemia or inflammation extracellular adenosine is dramatically elevated to levels sufficient for A2BAR activation, indicated that A2BAR signaling may be important to dampen inflammation particularly during tissue hypoxia. In addition, the recent advent of techniques for murine genetic manipulation along with development of pharmacological agents with enhanced A2BAR specificity has provided invaluable tools for focused studies on the explicit role of A2BAR signaling in different disease models. Currently, studies performed with combined genetic and pharmacological approaches have demonstrated that A2BAR signaling plays a tissue protective role in many models of acute diseases e.g. myocardial ischemia, or acute lung injury. These studies indicate that the A2BAR is expressed on a wide variety of cell types and exerts tissue/cell specific effects. This is an important consideration for future studies where tissue or cell type specific targeting of the A2BAR may be used as therapeutic approach.

Fig. 1

A2BAR receptor signal transduction. Coupling of the A2BAR to Gs proteins results in cAMP upregulation and downstream PKA activation. Signaling through the Gq receptor leads to elevations in intracellular Ca²⁺ levels and an increase in PKC.

Fig. 2

Putative transcription factor binding sites in the human A2BAR promoter. 1 (TGAAGTG) is the proposed binding site for Nkx2.5, 2 (GCGTG), 3 (CACGC) and 4 (CACGT) are HIF-binding sites. 4 has been confirmed as an HRE bound by HIF-1 in hypoxia. 5 depicts the CRE binding site (CGTCA).

Fig. 3

A2BAR involvement in inflammatory disease.