Regulation of β-catenin by t-DARPP in upper gastrointestinal cancer cells

Background: Truncated dopamine and cyclic-AMP-regulated phosphoprotein (t-DARPP) is frequently overexpressed in gastrointestinal malignancies. In this study, we examined the role of t-DARPP in regulating beta-catenin. Results: The pTopFlash construct that contains multiple TCF/LEF-binding sites was used as a measure of beta-catenin/TCF transcription activity. Gastric (AGS, MKN28) and esophageal (FLO-1) adenocarcinoma cancer cell lines that lack t-DARPP expression were utilized to establish stable and transient in vitro expression models of t-DARPP. The expression of t-DARPP led to a significant induction of the pTOP reporter activity, indicative of activation of beta-catenin/TCF nuclear signaling. Immunofluorescence assays supported this finding and showed accumulation and nuclear translocation of beta-catenin in cells expressing t-DARPP. These cells had a significant increase in their proliferative capacity and demonstrated up-regulation of two transcription targets of beta-catenin/TCF: Cyclin D1 and c-MYC. Because phosphorylated GSK-3 beta is inactive and loses its ability to phosphorylate beta-catenin and target it towards degradation by the proteasome, we next examined the levels of phospho-GSK-3 beta. These results demonstrated an increase in phospho-GSK-3 beta and phospho-AKT. The knockdown of endogenous t-DARPP in MKN45 cancer cells demonstrated a reversal of the signaling events. To examine whether t-DARPP mediated GSK-3 beta phosphorylation in an AKT-dependent manner, we used a pharmacologic inhibitor of PI3K/AKT, LY294002, in cancer cells expressing t-DARPP. This treatment abolished the phosphorylation of AKT and GSK-3 beta leading to a reduction in beta-catenin, Cyclin D1, and c-MYC protein levels. Conclusions: Our findings demonstrate, for the first time, that t-DARPP regulates beta-catenin/TCF activity, thereby implicating a novel oncogenic signaling in upper gastrointestinal cancers.