Oncogenic mechanism of soluble Keratin 17 offers potential therapeutic vulnerability in pancreatic cancer

CLINICAL NEED & OBJECTIVES Our group found that Keratin 17 (K17), an intermediate filament (IF), is a tumor cell-specific marker of aggressive pancreatic ductal adenocarcinoma (PDAC), and that the detection of K17 by immunohistochemistry level is a more accurate prognostic indicator than transcriptomic subtyping. We determined that K17 can act as an oncoprotein after transitioning from its “insoluble” cytoskeleton form to a “soluble” form that can regulate cell cycle progression. In patients, mice, and PDAC cell lines, the soluble fraction of K17 exists as full length (flK17) and caspase-cleaved peptide (ccK17), each with potentially unique tumorigenic capacity. Literature suggests that post-translational modifications (PTMs) are necessary to regulate these and other IF dynamics, though the mechanisms behind K17 solubilization in PDAC are still unclear. Thus, our aims are to 1) determine the mechanisms regulating K17 dynamics, 2) demonstrate inhibited cell cycle progression and tumor growth following disruption of these mechanisms, and 3) verify soluble K17 as a robust negative prognostic biomarker of PDAC. METHODS & RESULTS In a PDAC tumor mouse model, we show, independent of total K17 protein amount, that the percent of soluble ccK17 in tumor cells is negatively prognostic. Work is ongoing to validate this finding in human PDAC samples. 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. Specifically, we reproducibly identified phosphorylation of a semi-conserved IF motif that may promote caspase cleavage and solubilization. Ser/Thr phosphatase inhibition in vitro demonstrated that phosphorylation is necessary to trigger K17 solubilization followed by nuclear translocation. Mutagenesis of serine sites is ongoing to determine the specific motifs regulating K17. Using an unbiased kinase inhibitor screen and follow-up pharmacologic analyses, we identified that polo-like kinase 1 (PLK1) likely phosphorylates K17. Further studies are underway to validate K17 as a key mediator of PLK1 inhibitor sensitivity in PDAC. 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 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. Support or Funding Information This work was partially supported by the 2018 Pancreatic Cancer Action Network Translational Research Grant (K.R.S.). This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.