Beta-catenin cleavage enhances transcriptional activation.

ABSTRACT

Nuclear activation of Wnt/ß-catenin signaling is required for cell proliferation in inflammation and cancer. Studies from our group indicate that ß-catenin activation in colitis and colorectal cancer (CRC) correlates with increased nuclear levels of ß-catenin phosphorylated at serine 552 (pß-Cat552). Biochemical analysis of nuclear extracts from cancer biopsies revealed the existence of low molecular weight (LMW) pß-Cat552, increased to the exclusion of full size (FS) forms of ß-catenin. LMW ß-catenin lacks both termini, leaving residues in the armadillo repeat intact. Further experiments showed that TCF4 predominantly binds LMW pß-Cat552 in the nucleus of inflamed and cancerous cells. Nuclear chromatin bound localization of LMW pß-Cat552 was blocked in cells by inhibition of proteasomal chymotrypsin-like activity but not by other protease inhibitors. K48 polyubiquitinated FS and LMW ß-catenin were increased by treatment with bortezomib. Overexpressed in vitro double truncated ß-catenin increased transcriptional activity, cell proliferation and growth of tumor xenografts compared to FS ß-catenin. Serine 552-> alanin substitution abrogated K48 polyubiquitination,  ß-catenin nuclear translocation and tumor xenograft growth. These data suggest that a novel proteasome-dependent posttranslational modification of ß-catenin enhances transcriptional activation. Discovery of this pathway may be helpful in the development of diagnostic and therapeutic tools in colitis and cancer.