Abstract C26: Oncogenic mechanism of soluble keratin 17 offers potential therapeutic vulnerability in pancreatic cancer

Clinical Need and Objectives: Our previous reports suggest that keratin 17 (K17), an intermediate filament (IF), is an oncoprotein and biomarker of the most aggressive and treatment-resistant form of pancreatic ductal adenocarcinoma (PDAC). Importantly, K17 transitions from its “insoluble” IF form to a “soluble” form that can regulate cell cycle progression and gene expression, though the mechanisms that mediate K17 solubilization are yet unknown. Determining these mechanisms may uncover new keratin 17-specific PDAC therapeutic vulnerabilities. Methods and Results: We found that in human and murine PDAC models, the soluble fraction of K17 exists in the full-length form (flK17) and as a caspase-cleaved peptide (ccK17), each with potential roles as biomarkers and/or functional drivers of PDAC aggression. In a PDAC tumor mouse model and human samples, we show, independent of total K17 protein amount, that the percent of soluble and ccK17 in tumor cells is negatively prognostic. We sequenced both the soluble and cytoskeletal insoluble forms of K17 from primary PDACs using liquid chromatography mass spectrometry (LC-MS) and identified that K17 is phosphorylated on several N-terminal serine residues, some of which identify a semiconserved IF motif that may regulate caspase cleavage and solubilization. Serine phosphatase inhibition in vitro demonstrated that phosphorylation is necessary to trigger K17 solubilization followed by nuclear translocation. Mutagenesis of N-terminal serine sites demonstrated that phosphorylation of three serine residues is sufficient to solubilize K17 from its filamentous state, and that phosphorylation of K17 by a PKC-RSK signaling pathway is likely to induce solubilization. In vitro and in vivo experiments are ongoing to address the functional role of this mechanism in regulating PDAC pathogenesis. Conclusion: Phosphorylation is an important regulator of K17 dynamics in PDAC, promoting solubilization and, in some cases, caspase cleavage. Our results suggest that soluble K17 may have a functional role in promoting tumor aggression. Inhibiting RSK or PKC kinases that phosphorylate K17 could impact tumor growth by blocking K17 solubilization/cleavage, and this line of research could accelerate the development of more effective treatments, coupled to soluble K17 testing as a predictive biomarker for PDAC. Citation Format: Ryan R. Kawalerski, Lucia Roa-Pena, Luke A. Torre-Healy, Taryn Boyle, Cindy V. Leiton, Natasha T. Snider, Kenneth R. Shroyer, Luisa F. Escobar-Hoyos. Oncogenic mechanism of soluble keratin 17 offers potential therapeutic vulnerability in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr C26.