Up-regulation of vascular endothelial growth factor expression by adenosine through adenosine A2 receptors in the rat tongue treated with endotoxin

The main focus of the present investigation is to evaluate a differential effect of adenosine on the up-regulation of vascular endothelial growth factor (VEGF) expression through adenosine A2 receptors in the rat tongue treated with endotoxin (lipopolysaccharide: LPS). Angiogenesis in the rat tongue treated with LPS/incomplete Freund's adjuvant (IFA) or endotoxin/IFA/adenosine A2 receptor (A2R) antagonists was examined using immunohistochemistry for LYVE-1, ED1, ED2, OX6, langerin and VEGF, and real-time polymerase chain reaction (PCR) for VEGF. The distributional density of both blood vessels and OX6+ cells was significantly increased at day 8 after injection of LPS/IFA. The immunoreactive products of VEGF were intensely labelled in the cytoplasm of various antigen presenting cells (APCs) including dendritic cells (DCs) with double-immunofluorescence technique. Increase in VEGF mRNA expression level, the occupancy ratio of blood vessels, and the number of ED1+, ED2+, OX6+, and langerin+ cells was inhibited in the injured tongue of rats as a consequence of the treatment with A2R antagonists. The present results indicate that the LPS-induced adenosine might promote angiogenesis by the up-regulation of VEGF expression in macrophages/DCs through A2 receptors. This suggests that the synergistic interaction between toll-like receptor (TLR) and A2 receptor signalling observed in vivo plays an important role in oral mucosal wound healing.