Oncogenic Mutations in Armadillo Repeats 5 and 6 of β-Catenin Reduce Binding to APC, Increasing Signaling and Transcription of Target Genes.

BACKGROUND &

aimsThe β-catenin signaling pathway is one of the most commonly deregulated pathways in cancer cells. Amino acid substitutions within armadillo repeats 5 and 6 (K335, W383, and N387) of β-catenin are found in several tumor types, including liver tumors. We investigated the mechanisms by which these substitutions increase signaling and the effects on liver carcinogenesis in mice.

methodsPlasmids encoding tagged full-length β-catenin (CTNNB1) or β-catenin with the K335I or N387K substitutions, along with MET, were injected into tails of FVB/N mice. Tumor growth was monitored, and livers were collected and analyzed by histology, immunohistochemistry, and quantitative reverse-transcription polymerase chain reaction. Tagged full-length and mutant forms of β-catenin were expressed in HEK293, HCT116, and SNU449 cells, which were analyzed by immunoblots and immunoprecipitation. A panel of β-catenin variants and cell lines with knock-in mutations were analyzed for differences in N-terminal phosphorylation, half-life, and association with other proteins in the signaling pathway.

resultsMice injected with plasmids encoding K335I or N387K β-catenin and MET developed larger, more advanced tumors than mice injected with plasmids encoding WT β-catenin and MET. K335I and N387K β-catenin bound APC with lower affinity than WT β-catenin but still interacted with scaffold protein AXIN1 and in the nucleus with TCF7L2. This interaction resulted in increased transcription of genes regulated by β-catenin. Studies of protein structures supported the observed changes in relative binding affinities.

conclusionExpression of β-catenin with mutations in armadillo repeats 5 and 6, along with MET, promotes formation of liver tumors in mice. In contrast to N-terminal mutations in β-catenin that directly impair its phosphorylation by GSK3 or binding to BTRC, the K335I or N387K substitutions increase signaling via reduced binding to APC. However, these mutant forms of β-catenin still interact with the TCF family of transcription factors in the nucleus. These findings show how these amino acid substitutions increase β-catenin signaling in cancer cells.