Molecular Regulation of Protein Tyrosine Kinase 6 in Prostate Cancer

Protein tyrosine kinase 6 (PTK6) is an intracellular tyrosine kinase that is oncogenic when activated at the plasma membrane. Activation of PTK6 at the plasma membrane was previously observed in Pten-deficient mouse prostate tumors. We performed in vitro, cell culture, and in vivo experiments to demonstrate that PTEN negatively regulates PTK6 activation and downstream signaling. In the PTEN-null prostate cancer cell line PC3, active PTK6, phosphorylated at tyrosine residue 342, is localized to the plasma membrane. Introduction of wild type PTEN or PTEN G129E, which has protein phosphatase but no inositol phosphatase activity, lead to PTK6 dephosphorylation at activating tyrosine residue 342 but not at inhibitory tyrosine residue 447. PTEN inhibition of PTK6 activity coincides with decreased phosphorylation of PTK6 substrates FAK and BCAR1. In constrast, catalytically dead PTEN mutants G129R and C124S have no impact on PTK6 tyrosine phosphorylation. In complementary studies, knockdown of wild type PTEN in DU145 prostate cancer cells leads to increased activation of PTK6 at the plasma membrane and enhanced downstream signaling. PTEN, but not inactive PTEN G129R, forms a complex with PTK6, which is mediated through association with the PTK6 kinase domain. Ptk6 null mice display a less severe prostate cancer phenotype following prostate-specific disruption of Pten, suggesting PTK6 is critical for tumor progression following PTEN loss of function. In human prostate tumor tissue microarrays, loss of PTEN correlates with activation of PTK6 at the membrane and poor outcome. When activated at the plasma membrane, PTK6 promotes activation of FAK, EGFR, and ERK 1/2 but this activity is reduced in the presence of the small molecule inhibitor vemurafenib. We show that PTK6-mediated cell growth, migration, and invasion are abrogated upon vemurafenib administration. Using a flank xenograft model, vemurafenib treatment reduced tumor progression of prostate cancer cells. Our data suggest that patients with PTEN loss of function and PTK6 activation may benefit from combination therapies incorporating PTK6 inhibitors.