• tumour suppressor;
  • Bcr;
  • Bcr–Abl;
  • β-catenin;
  • Myc;
  • STI-571;
  • Gleevec

The Wnt signalling pathway can activate transcription of genes such as c-myc through β-catenin. Here, we describe the protein breakpoint cluster region, Bcr, as a negative regulator of this pathway. Bcr can form a complex with β-catenin and negatively regulate expression of c-Myc. Knockdown of Bcr by short interfering RNA relieves the block and activates expression of c-Myc. Expression of Bcr in the human colon carcinoma cell line HCT116, which has a high level of endogenous β-catenin, leads to reduced c-Myc expression. The negative effect is exerted by the amino terminus of Bcr, which does not harbour the kinase domain. Bcr-Abl, the oncogene protein expressed in chronic myelogenous leukaemia (CML), does not bind to β-catenin. It phosphorylates Bcr in the first exon sequence on tyrosines, which abrogates the binding of Bcr to β-catenin. The inhibitor of the Bcr–Abl tyrosine kinase, STI-571 or Gleevec, a drug against CML, reverses this effect. Our data contribute to the understanding of Bcr as a tumour suppressor in the Wnt signalling pathway, as well as in CML.