Abstract B007: c-Src and PRC2 activity in ErbB2-driven mammary tumorigenesis and acquired resistance to ErbB2-targeted therapy

Abstract Elevated expression and activity of the non-receptor tyrosine kinase c-Src are associated with a poor prognosis in breast cancer (1). In this context c-Src, which has not been found to be mutated in breast cancer, is thought to be activated primarily through oncogenic signaling by upstream receptors. This frequently occurs in tumors driven by the overexpression of ErbB2 (2), a receptor tyrosine kinase (RTK) amplified in approximately 20-30% of all breast cancer cases (3). To investigate the mechanisms by which c-Src co-operates with activated RTK signaling to promote mammary tumorigenesis, we have developed transgenic mouse models allowing conditional c-Src ablation specifically in the mammary epithelium. In contrast to germline knockout of c-Src (4), mammary epithelial-specific deletion of c-Src does not affect ductal outgrowth. However, mammary epithelial c-Src knockout significantly delays tumorigenesis driven by ErbB2 or PyV mT, a viral oncogene that acts as an RTK mimic. The tumors that eventually develop exhibit an inflammatory and necrotic phenotype with severely impaired growth compared to wild-type controls. This phenotype correlates with alterations in the expression and activity of key metabolic regulators and with activation of the retinoblastoma tumor suppressor pathway. The expression of a known target of the Rb-E2F1 pathway, the lysine methyltransferase EZH2 (5), is also dependent on c-Src activity in transgenic mouse tumours and in human ErbB2-amplified breast cancer cell lines. A component of the Polycomb repressor complex 2 (PRC2) that di/tri-methylates histone H3 on lysine 27 (H3K27me2/me3), EZH2 is also a crucial promoter of ErbB2-driven tumour progression in vivo. Previous studies of cell line models have demonstrated that activation of Src family kinases (SFKs) is associated with the acquisition of resistance to targeted therapies directed against ErbB2 (6, 7). We demonstrate here that, in addition to elevated activity of SFKs, ErbB2+ breast cancer cell lines with acquired resistance to the ErbB2 tyrosine kinase inhibitor Lapatinib also overexpress EZH2 and have significantly increased H3K27me3 compared to parental cells. Chemical inhibition or RNAi-mediated silencing of either EZH2 or c-Src significantly attenuates the growth of Lapatinib-resistant cells, indicating a possible reversal of the drug-resistant phenotype. Overall, our data indicate that control of PRC2 expression and activity by c-Src may be an important factor in the progression of ErbB2-driven breast cancer and the acquisition of resistance to ErbB2-targeted therapies. 1. Elsberger B, Fullerton R, Zino S, Jordan F, Mitchell TJ, Brunton VG, et al. Breast cancer patients' clinical outcome measures are associated with Src kinase family member expression. Br J Cancer.103:899-909. 2. Wilson GR, Cramer A, Welman A, Knox F, Swindell R, Kawakatsu H, et al. Activated c-SRC in ductal carcinoma in situ correlates with high tumour grade, high proliferation and HER2 positivity. Br J Cancer. 2006;95:1410-4. 3. Slamon DJ, Clark GM, Wong SG, Levin WJ, Ullrich A, McGuire WL. Human breast cancer: correlation of relapse and survival with amplification of the HER-2/neu oncogene. Science. 1987;235:177-82. 4. Kim H, Laing M, Muller W. c-Src-null mice exhibit defects in normal mammary gland development and ERalpha signaling. Oncogene. 2005;24:5629-36. 5. Bracken AP, Pasini D, Capra M, Prosperini E, Colli E, Helin K. EZH2 is downstream of the pRB-E2F pathway, essential for proliferation and amplified in cancer. Embo J. 2003;22:5323-35. 6. Rexer BN, Ham AJL, Rinehart C, Hill S, de Matos Granja-Ingram N, Gonzalez-Angulo AM, et al. Phosphoproteomic mass spectrometry profiling links Src family kinases to escape from HER2 tyrosine kinase inhibition. Oncogene. 2011. 7. Zhang S, Huang WC, Li P, Guo H, Poh SB, Brady SW, et al. Combating trastuzumab resistance by targeting SRC, a common node downstream of multiple resistance pathways. Nat Med. 2011;17:461-9. Citation Format: Harvey W. Smith, Alison Hirukawa, Virginie Sanguin-Gendreau, Dongmei Zuo, William J. Muller. c-Src and PRC2 activity in ErbB2-driven mammary tumorigenesis and acquired resistance to ErbB2-targeted therapy. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr B007.

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