Hypoxia-inducible adenosine A2B receptor modulates proliferation of colon carcinoma cells

Extracellular adenosine regulates a wide variety of physiological processes by interacting with 4 adenosine receptor subtypes: A1, A2A, A2B, and A3. However, little is known of their pathophysiological roles in human cancers. In this study, we examined the expression pattern of adenosine receptors in various colorectal tissues and human colon carcinoma cell lines and investigated the biologic functions regarding colon carcinogenesis. Using reverse transcriptase polymerase chain reaction and Western blotting, we found that adenosine receptor A2B (ADORA2B) was consistently up-regulated in colorectal carcinoma tissues and colon cancer cell lines compared with normal colorectal mucosa. In immunohistochemistry, we observed diffuse immunopositivity of ADORA2B in 67% of colorectal adenocarcinomas (39/58), 17% of tubular adenomas (5/30), and 0% of normal colon glands (0/62). During a hypoxic state, there was also a significant induction of ADORA2B expression in the messenger RNA level at 8 hours of incubation and in the protein level at 24 hours of incubation in colon carcinoma cell lines. To examine the function of ADORA2B, we applied an ADORA2B-selective antagonist (MRS1754) to the colon carcinoma cells, which significantly inhibited cell growth in a dose-dependent manner as demonstrated with a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide cell proliferation assay. In conclusions, ADORA2B was overexpressed in colorectal carcinomas grown under a hypoxic state, presumably promoting cancer cell growth. Our data suggest that this adenosine receptor is a potential therapeutic target for colorectal cancer.