Protein tyrosine kinase 6 protects cells from anoikis by directly phosphorylating focal adhesion kinase and activating AKT

Abstract

Protein tyrosine kinase 6 (PTK6) is a non-receptor tyrosine kinase expressed in epithelial cancers. Disruption of Ptk6 decreases AOM-induced colon tumorigenesis in mice by preventing STAT3 activation. Relocalization of PTK6 in prostate cancers contributes to increased growth. Although not expressed in normal breast or ovary, PTK6 promotes anchorage-independent survival of breast and ovarian tumor cells. We identified several potential PTK6 substrates in the human SW620 colon cancer cell line using mass spectrometry, including FAK (focal adhesion kinase). We show that FAK is a direct substrate of PTK6 in vitro and in vivo. Expression of membrane targeted active PTK6 (Palm-PTK6-YF) induces constitutive activation of FAK and cell morphology changes, which are independent of SRC family kinases in Src−/−, Yes−/−, Fyn−/− (SYF) mouse embryonic fibroblasts (MEFs). Palm-PTK6-YF expressing SYF cells are transformed and overcome contact inhibition, form colonies in transformation assays, proliferate in suspension, and form tumors in a xenograft model. Expression of FAK and Palm-PTK6-YF in Fak−/− MEFs synergistically activates AKT and protects cells against anoikis. However, expression of Palm-PTK6-YF in Akt1/2−/− MEFs fails to protect cells from anoikis, indicating AKT is critical in PTK6 and FAK mediated survival signaling. In a conditional Pten knockout murine prostate cancer model, we identify prostate epithelial cells with enhanced activation of endogenous PTK6 and FAK at the plasma membrane. Knockdown of PTK6 in the PC3 human prostate cancer cell line disrupts FAK and AKT activation and promotes anoikis, which can be rescued by exogenous expression of FAK. Our data reveal important roles for a PTK6-FAK-AKT signaling axis in promoting anchorage-independent cell survival.

HIF-1α-independent hypoxia-induced rapid PTK6 stabilization is associated with increased motility and invasion

“…11,14 Radiation treatment has also been reported to lead to the induction of PTK6 in both mouse intestine epithelial cells and human colorectal cancer cells, however, little is known about the mechanisms that regulate the de novo expression of PTK6 in tumors. 15,16 The importance of the microenvironment, particularly hypoxia, for tumor establishment and metastasis is well characterized. 17 Tumor hypoxia arises as a consequence of high metabolic demand for oxygen caused by rapid tumor growth and the inefficiency of the tumor vasculature.…”

PTK6 (protein tyrosine kinase 6)

“…Targeting PTK6 to plasma membrane enhanced proliferation, survival, migration and anchorageindependent growth in HEK293 cells, while nuclear targeting inhibited the invasive phenotype (Kim and Lee, 2009). Targeting active PTK6 to the plasma membrane in SYF (deficient for SRC, YES, and FYN) mouse embryonic fibroblasts was sufficient to transform these cells (Zheng et al, 2013a). In prostate cancer cells, targeting PTK6 to the nucleus inhibits proliferation while cytoplasmic PTK6 promotes proliferation .…”

“…In prostate cancer cells, targeting PTK6 to the nucleus inhibits proliferation while cytoplasmic PTK6 promotes proliferation . Overexpression of membrane-targeted active PTK6 in prostate cancer cells drives epithelialmesenchymal transition (Zheng et al, 2012) and anchorage-independent survival (Zheng et al, 2013a) as well as in vivo xenograft metastasis (Zheng et al, 2013b). Recently, PTK6 was detected in nuclei of normal mammary gland epithelium (Peng et al, 2014), and overexpression of PTK6 promotes oncogenic signaling and invasive phenotypes in breast cancer cells (Harvey and Crompton, 2003;Xiang et al, 2008;Irie et al, 2010).…”

“…PTK6 has also been shown to promote breast cancer cell migration via phosphorylation of KAP3A (Lukong and Richard, 2008) and Dok1 (Miah et al, 2014). Membrane-targeted active PTK6 in prostate cancer cell lines phosphorylates prooncogenic substrates BCAR1 (Zheng et al, 2012) and FAK (Zheng et al, 2013a). PTK6 phosphorylates and activates Signal Transducers and Activators of Transcription STAT3 (Liu et al, 2006) and STAT5a (Weaver and Silva, 2007) as well as the related scaffolding protein STAP2 (Mitchell et al, 2000).…”

“…In the cytoplasm, PTK6 may phosphorylate AKT on tyrosine residues to promote activation and downstream signaling . Targeting active PTK6 to the plasma membrane in prostate cancer cells drives epithelialmesenchymal transition via activation of BCAR1 (Zheng et al, 2012) and anchorage-independent survival via activation of FAK (Zheng et al, 2013a). RSK-mediated expression of PTK6 may contribute to metastasis of prostate cancer cells (Yu et al, 2014).…”