Establishment and characterization of new breast cancer cell lines from A∆16HER2 transgenic mouse: A promising model to test innovative therapies

The human epidermal growth factor receptor 2 (HER2) is a validated therapeutic target in breast cancer therapy. However, increasing evidence points to a major role for the Δ16HER2 splice variant, which has been identified as a clinically important and tumor-specific HER2 molecular alteration promoting aggressive metastatic breast cancer and conferring resistance to anti-HER2 therapies. Δ16HER2 has an increased transforming potency compared to wild type HER2 receptor, since it promotes constitutive dimerization of the receptor and activation of Src-mediated oncogenic signaling pathways. Consistently, mice transgenic for the human Δ16HER2 isoform (Δ16HER2 mice) develop invasive mammary carcinomas with early onset and 100% penetrance. From these mice, an epithelial cancer cell line (CAM6 cells) was established and characterized. When injected in syngeneic mice, CAM6 cells give rise to tumours growing with a short latency time and able to metastasize to lung. Δ16HER2 mice and CAM6 cells represent, respectively, in vivo and in vitro models to investigate Δ16HER2 behavior in terms of oncogenicity and response to therapies. In this study, once confirmed the cooperation between Δ16HER2 and Src tyrosine kinase to promote breast tumorigenesis, we evaluated the ability of Saracatinib, a potent Src inhibitor, to suppress Δ16HER2-driven breast carcinogenesis. Saracatinib treatment resulted in a strong reduction of CAM6 cell migration and matrigel invasion, in a dose-dependent inhibition of CAM6 cell viability and in the induction of CAM6 cell death by apoptosis. In vivo experiments demonstrated that oral administration of Saracatinib reduced the development and growth of CAM6 transplantable tumors in FVB syngeneic mice and autochthonous mammary carcinogenesis in Δ16HER2 mice. Western blotting analysis revealed that suppression of Δ16HER2-positive breast cancer growth by Saracatinib was correlated exclusively with the inhibition Src activation, while the phosphorylation level of Δ16HER2 resulted unchanged. These results provided the rationale for targeting Src in Δ16HER2-positive breast cancer.