Abstract B07: Loss of APC induces a dependence on elevated eIF2B5 function in colorectal cancer

Abstract

Colorectal cancer (CRC) is the most common gastrointestinal malignancy and one of the leading causes of cancer death worldwide. Truncating mutations of the tumor suppressor APC that lead to deregulated WNT signaling are driver mutations in CRC. APC and its downstream effector, β-catenin, are regarded as non-druggable, prompting the search for novel interactions essential for survival of cells that have lost APC function. Here, we have restored doxycycline-inducible expression of full-length APC (APC wt) in SW480 cells, which carry truncated APC (APC mut). To identify genes that are essential for viability of APC mut cells, we conducted an shRNA screen targeting 5,000 genes with 25,000 shRNAs. The best scoring differential hits were multiple shRNAs that target the translation initiation factor eIF2B5, a subunit of the guanidine exchange factor (GEF) for eIF2. Validation experiments demonstrate a strong synthetic lethal effect of eIF2B5 knockdown with APC truncation in SW480 cells that can be rescued by APC induction. There is a clear survival window, which we can visualize using several shRNA sequences with different knockdown efficiencies. We confirm previous observations that loss of APC function results in an increase of global protein synthesis. Loss of APC also strongly induces expression of eIF2B5 and other subunits of eIF2B. Knockdown of eIF2B5 reduces overall protein translation. Although eIF2 function and overall ribosomal assembly are compromised upon depletion of eIF2B5, MYC protein levels are strongly upregulated, arguing that MYC can be translated in an eIF2-independent manner. Apoptosis upon eIF2B5 knockdown strictly depends on MYC upregulation. APC mut cells also display higher basal levels of the unfolded protein response (UPR) markers CHOP and ATF4, which are both further enhanced by eIF2B5 depletion. The UPR branch reflected by CHOP induction may contribute to cell death in our system. In conclusion, our data demonstrate a dual regulatory mechanism in which loss of APC induces expression of both MYC and eIF2B5. Full eIF2 activity is required to restrict the pro-apoptotic activity of MYC in APC mut cells. Thus, interfering with eIF2B5 function as GEF or GDF may be a promising treatment strategy for cells with truncated APC. Citation Format: Stefanie Peter, Friedrich-Wilhelm Uthe, Martin Eilers, Christoph-Thomas Germer, Armin Wiegering. Loss of APC induces a dependence on elevated eIF2B5 function in colorectal cancer. [abstract]. In: Proceedings of the AACR Special Conference on Translational Control of Cancer: A New Frontier in Cancer Biology and Therapy; 2016 Oct 27-30; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2017;77(6 Suppl):Abstract nr B07.