Gastrin stabilizes β-catenin protein levels in colorectal cancer

Abstract

Introduction and Purpose: In addition to its recograzed role in the physiological regulation of acid secretion, another biological property attributed to gastrin (G) is its trophic effect on gastrointestinal (G1) mucosa. Numerous studies have demonstrated that G stimulates not only the growth of normal GI epithelial cells, but also malignancies of colorectal (CRC), gastric, and pancreatic etiology. However, the precise mechanisms mediating these trophic effects are unclear. Recent studies have indicated that G is a target of [3-catenin dependent T-cell factor (TCF) transcription. [3-catenin, which serves as a critical co-activator of the Wnt signaling pathway, has previously been shown to activate TCFAymphocyte enhancer binding factor (TCF/LEF) mediated transcription of oncogenes in CRC cells. Methods and Results: To determine the effect of G on various Wnt signaling components, we initially investigated the effect of G on [3-catenin, the necessary co-factor for TCF/LEF dependent transcription. Western analysis of MC-26 cells, a mouse CRC cell line that possesses G receptors, demonstrated induction of [3-cateinn (3-fold at 20 nM), cyclin D1 (2-fold at 50 nM), and COX-2 (2-fold at 10 nM) expression within a period of 4 h in the presence of amidated G-17. In response to the co-incubation of G-17 and L365,260, a G-specific receptor antagonist, the upregufation of [3-eatenin was attenuated. In addition, G-17 augmented LEF1 dependent transcription by -80% at 50 nM. Not only did G upregnlate LEF-1 dependent transcription and one of its target proteins, cyclin D1, but it also enhanced promoter activity of a full-length (-1745) cyclin D1 promoter compared to the minimal promoter (-66). Furthermore, half-life analysis of endogenous [3-catenin suggested that the addition of G in the presence of cycloheximide (CHX) prolonged the protein levels of [3-catenin (T~/~ > 6 h) when compared to CHX only (T~/2 -3 h), suggesting that stabilizes [3-catenin protein. Interestingly, CK2 kinase activity, which has been previously reported to phosphorylate [3eatenin and stabilize its protein levels, was enhanced nearly 2-fold at 20 nM when compared to untreated samples. Summary: The results of these studies suggest that G may promote proliferation in CRC, at least in part, through the stabilization of [3-catenin, possibly via the upregulation of CK2 activity. Conclusion: Our findings are consistent with a novel mechanism that links this important regulatory peptide with factors that enhance oncogenic 13-catenin protein and its central role in tumorigenesis.