Biphosphorylated Src At Y416/527 Is Active In Triple-Negative Breast Cancer Cell Lines And A Subset Of Patient Samples.

Abstract Targeted therapeutics are needed for triple-negative breast cancer (TNBC). In this study, we investigated the activation of Src family of cytoplasmic tyrosine kinases (SFKs) and two SFK substrates: CUB-domain containing protein 1 (CDCP1) and protein kinase C delta (PKCd) in 56 formalin-fixed, paraffin-embedded (FFPE) TNBCs. Expression of SFK phosphorylated at Y416 (SFK_pY416+) in tumor cells was strongly associated with phosphorylation of CDCP1 and PKCd (CDCP1_ pY743+ and PKCd_pY311+), as assessed by immunohistochemistry, indicating increased SFK activity in situ. To enable biochemical analysis, protein extraction from FFPE tissue was optimized. Cleaved CDCP1 isoform (70 kDa) was expressed to a varying degree in all samples but only phosphorylated in TNBC tumor cells that expressed SFK_pY416. Interestingly, active SFK was found to be biphosphorylated (SFK_pY416+/pY527+). Biphosphorylated active SFK was observed more frequently in forkhead box protein A1 (FOXA1)-TNBCs. In addition, in SFK_pY416-samples, FOXA1+ TNBC tended to be SFK_pY527+ (classic inactive SFK), and FOXA1- TNBC tended to be SFK_pY527-(SFK poised for activation). Strong SFK_pY416 staining was also observed in tumor-infiltrating lymphocytes in a subset of TNBCs with high tumor-infiltrating lymphocyte content. This report will facilitate protein biochemical analysis of FFPE tumor samples and justifies the development of therapies targeting the SFK/CDCP1/PKCd pathway for TNBC treatment. Citation Format: Olga V. Razorenova, Luke Nelson, Heather Wright, Nguyen Dinh, Kevin Nguyen, Scott Heinemann. Biphosphorylated Src at Y416/527 is active in triple-negative breast cancer cell lines and a subset of patient samples [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB202.