Oncogenic mutations in intestinal adenomas regulate Bim-mediated apoptosis induced by TGF-β

In the majority of microsatellite-stable colorectal cancers (CRCs), an initiating mutation occurs in the adenomatous polyposis coli (APC) or beta-catenin gene, activating the beta-catenin/TCF pathway. The progression of resulting adenomas is associated with oncogenic activation of KRas and inactivation of the p53 and TGF-beta/Smad functions. Most established CRC cell lines contain mutations in the TGF-beta/Smad pathway, but little is known about the function of TGF-beta in the early phases of intestinal tumorigenesis. We used mouse and human ex vivo 3D intestinal organoid cultures and in vivo mouse models to study the effect of TGF-beta on the Lgr5(+) intestinal stem cells and their progeny in intestinal adenomas. We found that the TGF-beta-induced apoptosis in Apc-mutant organoids, including the Lgr5(+) stem cells, was mediated by up-regulation of the BH3-only proapoptotic protein Bcl-2-like protein 11 (Bim). BH3-mimetic compounds recapitulated the effect of Bim not only in the adenomas but also in human CRC organoids that had lost responsiveness to TGF-beta-induced apoptosis. However, wild-type intestinal crypts were markedly less sensitive to TGF-beta than Apc-mutant adenomas, whereas the KRas oncogene increased resistance to TGF-beta via the activation of the Erk1/2 kinase pathway, leading to Bim down-regulation. Our studies identify Bim as a critical mediator of TGF-beta-induced apoptosis in intestinal adenomas and show that the common progression mutations modify Bim levels and sensitivity to TGF-beta during intestinal adenoma development.