With the nearly 100% mutation frequency of KRAS in pancreatic ductal adenocarcinoma (PDAC), the development of therapeutic strategies to target KRAS is a high priority for the pancreatic cancer field. In the current study, we aimed to identify signaling changes caused by the acute suppression of mutant KRAS in PDAC cell lines. Strikingly, acute suppression of mutant KRAS in PDAC cell lines caused potent and rapid proteasome-dependent degradation of MYC protein. Ablation of MYC also suppressed PDAC growth both in vitro and in vivo, indicating a critical driver role for MYC in KRAS-dependent PDAC maintenance. A mechanism by which RAS effector signaling regulates MYC protein stability has been described, however we determined that this mechanism cannot fully account for how endogenous mutant KRAS stabilizes MYC protein in PDAC cells. We verified a role for the Raf-MEK-ERK but not the PI3K-AKT-GSK3β effector signaling pathway. Unexpectedly, we also excluded a role for MYC protein phosphorylation at MYC residue T58, and determined that ubiquitin ligases other than FBW7 are involved. These findings prompted us to search for additional KRAS-dependent protein kinases that facilitate MYC protein stability. We applied multiple screening strategies. First, we developed a novel fluorescence-based system to monitor real-time MYC protein degradation in PDAC cells and we adapted this system for use in a high-throughput flow-cytometry based assay. Second, we applied a mass spectrometry-based approach to profile the human kinome for KRAS-dependent changes in protein kinases. Finally, we applied gain-of-function (Cancer Toolkit) and loss-of-function (CRISPR/Cas9) genetic screens to identify signaling regulators of MYC protein stability. The results of these screens will be presented.

Citation Format: Angelina V. Vaseva, Devon R. Blake, Salma H. Azam, Karim T. Gilbert, Chad V. Pecot, Kenneth H. Pearce, Laura E. Herring, Lee M. Graves, Peter J. Houghton, Channing J. Der. Regulation of MYC protein stability by mutant KRAS in pancreatic ductal adenocarcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4458. doi:10.1158/1538-7445.AM2017-4458