Abstract A13: A positive feedback loop for transcriptional regulation of beta-catenin that favors sustained colorectal cancer cell invasion
Authors: Ehrmann Franziska , Frank Linda , Schirmacher Peter , Brand Karsten , Bandapalli Obul R. 
Published: October 2015

Wnt/beta-catenin signaling is a branch of a functional network that is involved in a broad range of biological systems and deregulation of components involved in Wnt/beta-catenin signaling has been implicated in a wide spectrum of cancers. We have shown earlier that beta-catenin, is up regulated in the invasion front compared to the inner parts of the tumor in a murine model of colorectal liver metastasis and clinical specimens. This activity is due to the fact that beta-catenin acts as co-transcription factor of its own promoter contributing to the dynamic changes of beta-catenin levels upon the confrontation of tumor cells with the host microenvironment. To examine the putative role of the host cells for transcriptional activation of beta-catenin and its promoter, co-culture experiments and application of recombinant proteins or small molecule inhibitors were performed.

To find out which cells play a role in activation of the beta-catenin promoter both hepatocytes and hepatic stellate cells were co-cultured with a colorectal tumor cell line LS174T that was stably transfected with a SEAP reporter gene driven by a fragment of the beta-catenin promoter. We could demonstrate that co-culture of LS174T cells with the hepatocytes (AML12) results in an increase of the SEAP reporter indicating an activation of the beta-catenin promoter. In contrast, co-culturing of the LS174T with hepatic stellate cells (LX-2) caused a decrease of reporter gene expression.

Treatment of LS174T beta-catenin cells with FCS showed a 5-fold up-regulation of the beta-catenin promoter as compared to serum-free conditions concluding that fetal calf serum contains factors that activate the beta-catenin promoter. Application of different cytokines and growth factors that are involved in tumor formation and metastasis did not lead to any alteration of the reporter gene expression.

Upon application of several small molecule inhibitors, Wortmannin and LY492004 that inhibits the PI3 kinase showed a pronounced up-regulation of the beta-catenin promoter in a concentration dependent manner. PI3K is known to activate Akt that in turn stabilizes beta-catenin and translocates to the nucleus. Inhibition of the above described PI3K-akt- beta-catenin pathway by application of PI3K inhibitors leads to the activation of the beta-catenin-promoter indicating the presence of a feed-back loop that leads to increased beta-catenin transcription in response to decreased beta-catenin protein levels that in turn results in sustained up regulation of the invasiveness of CRC cells. These results warrant for the development of small molecule inhibitors targeting beta-catenin rather than the PI3K, Wnt/beta-catenin signaling pathways.

Citation Format: Franziska Ehrmann, Linda Frank, Peter Schirmacher, Karsten Brand, Obul R. Bandapalli. A positive feedback loop for transcriptional regulation of beta-catenin that favors sustained colorectal cancer cell invasion. [abstract]. In: Proceedings of the AACR Special Conference: Targeting the PI3K-mTOR Network in Cancer; Sep 14-17, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(7 Suppl):Abstract nr A13.