Small molecule inhibitors reveal PTK6 kinase is not an oncogenic driver in breast cancers

Abstract

Protein tyrosine kinase 6 (PTK6, or BRK) is aberrantly expressed in breast cancers, and emerging as an oncogene that promotes tumor cell proliferation, migration and evasion. Both kinase-dependent and -independent functions of PTK6 in driving tumor growth have been described, therefore targeting PTK6 kinase activity by small molecule inhibitors as a therapeutic approach to treat cancers remains to be validated. In this study, we identified novel, potent and selective PTK6 kinase inhibitors as a means to investigate the role of PTK6 kinase activity in breast tumorigenesis. We report here the crystal structures of apo-PTK6 and inhibitor-bound PTK6 complexes, providing the structural basis for small molecule interaction with PTK6. The kinase inhibitors moderately suppress tumor cell growth in 2D and 3D cell cultures. However, the tumor cell growth inhibition shows neither correlation with the PTK6 kinase activity inhibition, nor the total or activated PTK6 protein levels in tumor cells, suggesting that the tumor cell growth is independent of PTK6 kinase activity. Furthermore, in engineered breast tumor cells overexpressing PTK6, the inhibition of PTK6 kinase activity does not parallel the inhibition of tumor cell growth with a >500-fold shift in compound potencies (IC50 values). Overall, these findings suggest that the kinase activity of PTK6 does not play a significant role in tumorigenesis, thus providing important evidence against PTK6 kinase as a potential therapeutic target for breast cancer treatment.

Prognostic biomarker identification and tumor classification in breast cancer patients by methylation and transcriptome analysis

“…However, another study suggested that the BC cell growth was independent of PTK6 kinase activity. The tumor cell growth inhibition showed no correlation with PTK6 kinase activity inhibition, nor with total or activated PTK6 protein levels [ 28 ].…”

Putting the BRK on breast cancer: From molecular target to therapeutics

“…Treatment with BRK kinase inhibitor PF-6689840 and structural analogue PF-6737007 that lacks BRK activity yielded similarly poor cell growth inhibition of MDA-MB-231 cells 85 . Interestingly, after rigorous experimentation, Qiu et al concluded that the kinase specific inhibitors did not yield any anticancer efficacy 85 . This suggests that the kinase independent functions of BRK may play an important role in oncogenesis.…”

“…Importantly, cellular activity showed that 21a and 21d were effective in attenuating the phosphorylation of SAM68, a substrate of BRK 84 . Unfortunately, this compound was revisited in 2018, and a kinase panel screening revealed that the analogues unselectively target 6% of the kinases in the panel of 320 kinases 85 .…”

“…Finally, in 2018, PF-6683324 and PF-6689840 were designed specifically to bind to unphosphorylated BRK 85 . Based on a kinase panel screening, these compounds appear to be selective toward BRK.…”

Multi-omics data integration reveals correlated regulatory features of triple negative breast cancer

“…Recently, we reported that breast tumor kinase (BRK), promotes metastatic potential by phosphorylating SMAD4, the major component of TGFb/SMAD signaling in mammary epithelial cells. 76 However, the inhibition of the kinase activity of BRK with small molecules did not inhibit cancer cell growth 77 suggesting functional redundancy and kinase-independent roles of BRK in breast cancer. In this study, we also observed that besides ptk6/BRK (Supplemental files 2 and 4, ESI †), nonreceptor protein tyrosine kinase LYN is hyperphosphorylated (Fig.…”