Abstract P1-01-02: Targeting PTK6 to treat mesenchymal triple negative breast cancer

Background/Rational: Patients with triple negative breast cancers (TNBC) have limited therapeutic options beyond conventional chemotherapy. Unfortunately, high-risk for metastatic recurrence and chemotherapy resistant diseases cause the worst 5-year survival rate in patients with TNBC, which have been significant clinical challenges. Novel therapeutic targets or strategies to combat metastasis and chemotherapy resistance are necessary to improve quality of life and outcomes for patients with high risk TNBC. Epithelial-to-mesenchymal transition (EMT) and anoikis resistance are processes recognized as contributing to enhanced metastatic potential and treatment resistance. A subset of TNBC exhibits mesenchymal gene signatures and phenotypes that may be associated with high metastatic recurrence, chemotherapy resistance and immunosuppression. In a functional genomic screen, we identified several candidates as novel regulators of EMT and anoikis sensitivity of TNBC cells. We have focused on roles of one highly validated candidate, protein tyrosine kinase 6 (PTK6) on EMT, anoikis resistance and metastatic capacity in TNBC. Methods: We analyzed expression of PTK6 and mesenchymal markers in patient triple negative tumors by immunohistochemistry. In breast epithelial and TNBC cell lines, the levels of PTK6 were genetically modulated, and determined effects on growth, migration and EMT. In vivo mouse models were used to show effects of PTK6 inhibition on metastatic capacity of TNBC cells. We have also validated effects of PTK6 specific small molecule inhibitor on TNBC growth and metastases. In order to dissect specific mechanisms by which PTK6 inhibition regulates TNBC mesenchymal phenotypes, we used a siRNA library screening and identified novel E3 ligases that may be responsible for PTK6 inhibition-induced EMT regulation. Results: Overexpression of PTK6 in MCF10A cells is sufficient to promote an EMT; promotes migration, suppresses epithelial markers (E-cadherin/claudin-1) and increases mesenchymal markers (N-cadherin and fibronectin). In contrast, PTK6 inhibition either PTK6 shRNAs or treatment with a specific kinase inhibitor enhances E-cadherin expression and suppresses migration, anoikis resistance and lung colonization of TNBC cells. PTK6-dependent E-cadherin regulation is specifically dependent on levels of SNAIL, a transcriptional repressor that is associated with poor TNBC patient prognosis. SNAIL down-regulation by PTK6 inhibition is directly responsible for the modulation of anoikis sensitivity, which is in turn causally linked to lung colonization potential. PTK6 inhibition promotes the proteasome-dependent degradation of SNAIL via a novel mechanism independent of GSK3β/β-TRCP pathway or Fbox E3ligases (FBXO5, FBXO11, FBXL14) that are known to regulate SNAIL ubiquitination. Using a siRNA library screening approach, we identified novel E3 ligase candidates that may be responsible for SNAIL ubiquitination and degradation downstream of PTK6 inhibition. Conclusion/Future direction: PTK6 is a representative novel regulator of EMT and anoikis resistance that can be targeted to prevent metastases of TNBC. Modulation of mesenchymal phenotypes of TNBC cells may be able to regulate chemotherapy resistance and/or immunosuppressive microenvironment.

Citation Format: Ito K, Park SH, Nayak A, Byerly J, Irie HY. Targeting PTK6 to treat mesenchymal triple negative breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-01-02.

Abstract P2-06-07: PRKCQ regulates taxol sensitivity of triple negative breast cancer cells via IL-6/Stat3 signaling

Background/Rationale. While some patients with triple negative breast cancer achieve long-term remission with chemotherapy, many have cancers that are chemotherapy resistant. The lack of targeted therapies for this subtype also makes some triple negative cancers difficult to treat and control. PRKCQ, a member of the novel protein kinase C family, is preferentially expressed in triple negative breast cancers compared to ER+/Luminal cancers. We previously reported that PRKCQ expression drives growth-factor independent growth, anoikis resistance and migration of breast epithelial cells. In addition, PRKCQ is required for in vivo growth of triple negative breast cancers tumor xenografts. We sought to determine if PRKCQ expression modulates sensitivity of triple negative breast cancer cells to standard of care chemotherapy and whether PRKCQ inhibition could be a strategy to induce death of chemotherapy-resistant triple negative breast cancer cells. Methods. We determined the effects of modulating PRKCQ expression, using PRKCQ cDNA or shRNA vectors, on Doxorubicin or Taxol treatment-induced effects on triple negative breast cancer cells, including those that are relatively chemotherapy resistant at baseline. We determined the mechanisms by which PRKCQ expression regulates sensitivity to Taxol. Results. Increased PRKCQ expression in MCF-10A breast epithelial cells suppresses the apoptosis-inducing effects of Doxorubicin or Taxol treatment. PRKCQ-induced Taxol resistance is dependent on PRKCQ kinase activity. PRKCQ-expressing MCF-10A cells secrete enhanced levels of IL-6, leading to the autocrine activation of Stat3; IL-6/Stat3 activation is necessary for PRKCQ-induced resistance to Taxol. Finally, downregulation of PRKCQ sensitized MDA-231-Luc cells to Taxol treatment and induced apoptosis of these cells which are relatively resistant to Taxol at baseline. Conclusions. PRKCQ regulates sensitivity to standard of care chemotherapies used in the treatment of triple negative breast cancer. IL-6/Stat3 signaling induced by PRKCQ kinase activity is responsible for resistance to the effects of Taxol treatment. Targeting PRKCQ therefore could be an attractive strategy to overcome chemotherapy resistance of a subset of triple negative breast cancers.

Citation Format: Irie HY, Byerly J. PRKCQ regulates taxol sensitivity of triple negative breast cancer cells via IL-6/Stat3 signaling. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P2-06-07.