1. ¹Laboratory of Experimental Oncology and Radiobiology, University of Amsterdam, The Netherlands
2. ²Department of Oncology, University of Amsterdam, The Netherlands
3. ³Laboratory of Experimental Internal Medicine, University of Amsterdam, The Netherlands
4. ⁴Department of Gastroenterology, University of Amsterdam, The Netherlands
5. ⁵Department of Cell Biology, University of Groningen, The Netherlands

Correspondence: Dr JCH Hardwick, Department of Gastroenterology, C2-112, Academic Medical Center, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands. E-mail: j.c.hardwick@amc.uva.nl

Received 27 September 2005; Revised 21 March 2006; Accepted 4 April 2006; Published online 31 July 2006.

Abstract

Nonsteroidal anti-inflammatory drugs show chemopreventive efficacy in colon cancer, but the mechanism behind this remains unclear. Elucidating this mechanism is seen as vital to the development of new chemopreventive agents. We studied the effects of aspirin on the oncogenic Wnt/-catenin pathway activity in colorectal cancer cell lines and observed that aspirin dose-dependently decreased the activity of this pathway, as judged by TCF-driven luciferase activity, reduced Wnt target gene expression and increased phosphorylation of -catenin by immunoblotting. Furthermore, the ubiquitination and cytoplasmic levels of -catenin were assessed by immunoblotting, and also the localization of -catenin was shown by green fluorescent protein-tagged -catenin and time-lapse fluorescent imaging. Importantly, aspirin treatment caused increased phosphorylation of protein phosphatase 2A (PP2A), an event associated with inhibition of PP2A enzymatic activity, which was confirmed by a reduction in enzymatic PP2A activity. Moreover, this inhibition of PP2A enzymatic activity was essential for the effects of aspirin on the Wnt/-catenin pathway as shown by transient transfection with PP2A constructs. The findings in this article provide a molecular explanation for the efficacy of aspirin in chemoprevention of colorectal cancer and shows biochemical evidence that PP2A is an important regulator of Wnt/-catenin pathway activity in these cells.