Adenosine Suppresses Cholangiocarcinoma Cell Growth and Invasion in Equilibrative Nucleoside Transporters and Proline-Rich Homeodomain Protein-Dependent Pathway

Cholangiocarcinoma (CCA) is a lethal disease with increasing incidence worldwide. Previous study showed that CCA was sensitive to adenosine. Thereby, molecular mechanisms of CCA inhibition by adenosine were examined in this study. Our results showed that adenosine inhibited CCA cells via an uptake of adenosine through equilibrative nucleoside transporters (ENTs), instead of activation of adenosine receptors. The inhibition of ENTs caused the inhibitory effect of adenosine to subside, while adenosine receptor antagonists failed to do so. Intracellular adenosine level was increased after adenosine treatment. Also, a conversion of adenosine to AMP by adenosine kinase is required in this inhibition. There was no specific correlation between level of proinflammatory proteins or protein kinase CK2 and CCA responses to adenosine. Notably, the level of proline-rich homeodomain protein (PRH), a downstream target of protein kinase CK2, in adenosine-sensitive cell lines was higher than in others. This indicates the potential prognostic marker of PRH for CCA cell responses to adenosine. The role of PRH in adenosine-mediated cytotoxicity in CCA cells was further investigated, and the results demonstrated that adenosine inactivated PRH by enhancing PRH phosphorylation. In addition, PRH knockdown CCA cells were less responsive to adenosine. Accordingly, we proposed a novel adenosine-mediated cancer cell growth and invasion suppression via a receptor-independent mechanism through PRH suppression in CCA.