MCC inhibits beta-catenin transcriptional activity by sequestering DBC1 in the cytoplasm

摘要

The mutated in colorectal cancer (MCC) is a multifunctional gene showing loss of expression in colorectal and liver cancers. MCC mutations can drive colon carcinogenesis in the mouse and in vitro experiments suggest that loss of MCC function promotes cancer through several important cellular pathways. In particular, the MCC protein is known to regulate beta-catenin (beta-cat) signaling, but the mechanism is poorly understood. Here we show that the beta-cat repressor function of MCC is strongly impaired by the presence of a disease-associated mutation. We also identify deleted in breast cancer 1 (DBC1) as a new MCC interacting partner and regulator of beta-cat signaling. RNA interference experiments show that DBC1 promotes beta-cat transcrip-tionat activity and that the presence of DBC1 is required for MCC-mediated beta-cat repression. In contrast to all other DBCl interacting partners, MCC does not interact through the DBCl Leucine Zipper domain but with a glutamic-acid rich region located between the Nudix and EF-hand domains. Furthermore, MCC overexpression relocalizes DBCl from the nucleus to the cytoplasm and reduces beta-cat K49 acetylation. Treatment of cells with the SIRT1 inhibitor Nicotinamide reverses MCC-induced deacetylation of beta-cat K49. These data suggest that the cytoplasmic MCC-DBCl interaction sequesters DBCl away from the nucleus, thereby removing a brake on DBCl nuclear targets, such as SIRT1. This study provides new mechanistic insights into the DBC1-MCC axis as a new APC independent beta-cat inhibitory pathway.