Abstract LB-059: Opposing roles for protein tyrosine kinase 6 (PTK6) in colon cancer

Abstract

Abstract Background: Protein Tyrosine Kinase 6 (PTK6, also called BRK) is a non-receptor tyrosine kinase expressed in differentiated epithelial cells of the skin, gastrointestinal tract, and prostate. Disruption of the Ptk6 gene led to impaired intestinal differentiation and increased intestinal proliferation in mice. However, PTK6 also has a tumor-promoting role in the colon, as disruption of the Ptk6 gene impaired carcinogen-induced STAT3 activation and tumorigenesis in mice. PTK6 also promotes survival of human colon cancer cell lines following DNA damaging treatments including γ-irradiation and chemotherapeutic drugs via STAT3 activation. Results: We examined and manipulated colon cancer cell lines to understand these two seemingly contradictory roles for PTK6 in the colon. The human colon adenocarcinoma cell lines SW480 and SW620 represent a primary colon tumor and a metastasis from the same patient, respectively. PTK6 is highly expressed in SW480 cells and significantly reduced in SW620 cells. Active PTK6, phosphorylated on tyrosine residue 342, is not detectable in either cell line. Stable knockdown of PTK6 in SW480 cells results in an epithelial-to-mesenchymal transition (EMT), evidenced by decreased E-cadherin expression and increased ZEB-1 and Vimentin; as well as increased migration, wound-healing, and anchorage-independent growth of cells in culture. Additionally, knockdown of PTK6 led to increased in vivo tumorigenesis of xenograft cells in nude mice. Ectopic expression of wild-type PTK6 in SW620 cells drives the reverse process (MET), demonstrating a rescue of the EMT phenotype. When a constitutively active PTK6 construct is expressed in SW620 cells, it promotes activation of cytoplasmic oncogenic targets and downstream effectors, STAT3, FAK, BCAR1 and ERK5. Analysis of patient tissues demonstrates a downregulation of mRNA and protein levels, as well as a change in protein localization in tumor versus normal tissues; while PTK6 is detectable in the nuclei of differentiating normal cells it is excluded from nuclei in colon tumors. Conclusions: Our studies indicate that PTK6 promotes the epithelial phenotype to antagonize the EMT in a kinase-independent manner and a kinase switch activates PTK6 to promote oncogenic signaling. PTK6 protein localization may function to regulate its specific role. Further examination of the complex role of this kinase could allow for a more nuanced understanding of colon cancer initiation and progression for the development of novel treatments. Citation Format: Priya S. Mathur, Jessica J. Gierut, Rosa M. Xicola, Xavier Llor, Angela L. Tyner. Opposing roles for protein tyrosine kinase 6 (PTK6) in colon cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-059. doi:10.1158/1538-7445.AM2015-LB-059