Protein tyrosine kinase 7 (PTK7) promotes metastasis in hepatocellular carcinoma under SOX9 regulation and TGFβ signalling

Abstract

Metastasis has posed serious clinical challenges to cancer treatment. It was estimated that up to 90% of cancer-related deaths were contributed by metastases (Lambert et al., 2017). Metastasis encompasses a transition of tumour from a proliferative state to an invasive one with acquired therapeutic resistances (Lambert et al., 2017). Despite our increasing understanding of the mechanism and molecular drivers of metastasis, therapeutically impairing these drivers is not desirable due to their fundamental roles in homeostasis and plasticity in pathological settings (Pastushenko & Blanpain, 2019). Alternatively, kinases, which are easily detectable and druggable, serve as a reservoir for novel targets specific for metastasis. By profiling over 500 kinases, I have identified protein tyrosine kinase-like 7 (PTK7) as the most significantly upregulated kinase in advanced and metastatic HCC. The clinical relevance of PTK7 in HCC was further validated through bioinformatics and immunohistochemistry (IHC) on patient tissue microarrays. Functionally, I was able to demonstrate a positive correlation between metastatic behaviours and the level of PTK7 expression through CRISPR-Cas9 knockout, lentiviral-based overexpression and fluorescence-activated cell sorting of human and murine HCC cells, followed by in vivo lung metastasis models. Coupling Gene Set Enrichment Analyses on RNA-seq and TCGA data, PTK7 was found to significantly correlate with pro-metastatic pathways such as epithelial-mesenchymal transition (EMT) and TGFβ signalling. Core components of EMT were found to be upregulated upon PTK7 overexpression. SOX9 was found to interact with PTK7 promoter region and to be an important transcriptional activator, as validated by luciferase reporter and knockdown assays. Data also suggested a more global regulation of the SOX9-PTK7 axis by TGFβ signalling activity. PTK7 appeared to be a particularly favourable candidate in the fight against HCC metastasis due to its secretory nature as a soluble marker and the recent development of an PTK7-specific antibody-drug conjugate (Damelin et al., 2017).