Triple-negative breast cancers (TNBCs) are highly aggressive cancers that lack targeted therapies. However, EGFR is frequently activated in a subset of TNBCs and represents a viable clinical target. Because the endocytic adaptor protein Endophilin A2 (SH3GL1/Endo II) has been implicated in EGFR internalization, we investigated Endo II expression and function in human TNBCs. Endo II expression was high in several TNBC cells compared with normal breast epithelial cells. Stable knockdown (KD) of Endo II was achieved in two TNBC cell lines, and although cell viability was unaffected, defects in receptor-mediated endocytosis were observed. EGFR signaling to Erk and Akt kinases was impaired in Endo II KD cells, and this correlated with reduced rates of EGFR internalization and cell motility. Endo II KD cells also displayed defects in three dimensional (3D) cell invasion, and this correlated with impaired extracellular matrix degradation and internalization of MT1-MMP. Endo II silencing also caused a significant reduction in TNBC tumor growth and lung metastasis in mammary orthotopic tumor xenograft assays. In human breast tumor specimens, Endo II expression was highest in TNBC tumors compared with other subtypes, and at the level of gene expression, high Endo II was associated with reduced relapse-free survival in patients with basal-like breast cancers. Together, these results identify a positive role for Endo II in TNBC tumor metastasis and a potential link with poor prognosis.Implications: Endophilin A2 and related adaptor proteins represent important signaling hubs to target in metastatic cancers. Mol Cancer Res; 13(6); 1044-55. Ó2015 AACR.

BackgroundHuman epidermal growth factor receptor-2 (HER2) is amplified and a clinical target in a subset of human breast cancers with high rates of metastasis. Targeted therapies involving the antibody trastuzumab and trastuzumab-emtansine (T-DM1) have greatly improved outcomes for HER2-positive (HER2+) breast cancer patients. However, resistance to these targeted therapies can develop and limit their efficacy. Here, we test the involvement of the endocytic adaptor protein endophilin A2 (Endo II) in HER2+ breast cancer models, and their responses to treatments with trastuzumab and T-DM1.MethodsEndo II expression in human breast tumors and lymph node metastases were analyzed by immunohistochemistry. Stable silencing of Endo II was achieved in HER2+ cancer cell lines (SK-BR-3 and HCC1954) to test Endo II effects on HER2 levels, localization and signaling, cell motility and tumor metastasis. The effects of Endo II silencing on the responses of HER2+ cancer cells to trastuzumab or T-DM1 treatments were tested using real-time cell motility and cytotoxicity assays.ResultsHigh Endo II protein expression was detected in HER2-positive tumors, and was linked to worse overall survival in node-positive HER2+ breast cancers at the mRNA level. Stable silencing of Endo II in HER2+ cell lines led to elevated levels of HER2 on the cell surface, impaired epidermal growth factor-induced HER2 internalization, and reduced signaling to downstream effector kinases Akt and Erk. Endo II silencing also led to decreased migration and invasion of HER2+ cancer cells in vitro, and impaired lung seeding following tail vein injection in mice. In addition, Endo II silencing also impaired HER2 internalization in response to Trastuzumab, and led to reduced cytotoxicity response in HER2+ cancer cells treated with T-DM1.ConclusionsOur study provides novel evidence of Endo II function in HER2+ cancer cell motility and trafficking of HER2 that relates to effective treatments with trastuzumab or T-DM1. Thus, differential expression of Endo II may relate to sensitivity or resistance to trastuzumab-based therapies for HER2+ cancers.Electronic supplementary materialThe online version of this article (doi:10.1186/s13058-017-0900-z) contains supplementary material, which is available to authorized users.

Signaling via epidermal growth factor receptor (EGFR) and Src kinase pathways promote triple-negative breast cancer (TNBC) cell invasion and tumor metastasis. Here, we address the role of Cdc42-interacting protein-4 (CIP4) in TNBC metastasis in vivo, and profile CIP4 expression in human breast cancer patients. In human TNBC cells, CIP4 knock-down (KD) led to less sustained activation of Erk kinase and impaired cell motility compared to control cells. This correlated with significant defects in 3D invasion of surrounding extracellular matrix by CIP4 KD TNBC cells when grown as spheroid colonies. In mammary orthotopic xenograft assays using both human TNBC cells (MDA-MB-231, HCC 1806) and rat MTLn3 cells, CIP4 silencing had no overt effect on tumor growth, but significantly reduced the incidence of lung metastases in each tumor model. In human invasive breast cancers, high CIP4 levels was significantly associated with high tumor stage, TNBC and HER2 subtypes, and risk of progression to metastatic disease. Together, these results implicate CIP4 in promoting metastasis in TNBCs.

<p>Supplementary Figures S1-7. Supplementary Figure S1. Endo II mRNA expression profiling in breast cancer cell lines arranged by molecular subtypes. Supplementary Figure S2. Endo II silencing causes defects in EGFR internalization. Supplementary Figure S3. Endo II silencing impairs ECM degradation in HCC 1806 cells. Supplementary Figure S4. Endo II promotes HCC 1806 tumor growth and metastasis in mice. Supplementary Figure S5. Assessing tumor mass and incidence of metastases in TNBC models. Supplementary Figure S6. Endo II promotes lung seeding efficiency of TNBC cells in mice. Supplementary Figure S7. Testing the specificity of Endo II IHC staining.</p>

Breast cancers in the human epidermal growth factor receptor 2 (HER2) and triple-negative breast cancer (TNBC) subtypes have high rates of tumor metastasis. Tumor metastasis is driven by formation of invadopodia that degrade extracellular matrix (ECM) in basement membranes to spread locally and colonize distant sites. Endophilin A2 (Endo II) is an adaptor protein that coordinates internalization and trafficking of receptors that drive invadopodia formation and breast cancer metastasis. We have recently identified Endo II as a positive regulator of human TNBC metastasis, invadopodia formation, epidermal growth factor receptor (EGFR) internalization and signaling. Here, we extend these studies of Endo II to HER2 breast cancers, and define molecular mechanisms that link Endo II to invadopodia formation and cell invasion. Analysis of Endo II expression in human invasive ductal carcinomas revealed significantly high expression of Endo II in HER2 primary tumors, and significantly higher expression in paired lymph node metastases compared to other subtypes. We also observed that high levels of Endo II mRNA was associated with reduced rates of relapse free survival in patients with lymph node positive, estrogen receptor negative tumors. This correlates with our findings that Endo II promotes invadopodia formation and breast cancer cell invasion. Mechanistically, these defects may be due to reduced EGFR signaling and lowered expression of epithelial-mesenchymal transition (EMT) signature genes observed in Endo II knock-down (KD) cells. We are currently validating these results in mammary orthotopic tumor xenograft models of HER2 and TNBC cell models with stable Endo II KD. Together, these findings identify high levels of Endo II in high risk breast cancer subtypes, with high expression of EMT genes, and a potential link between Endo II levels and poor prognosis in breast cancer patients. Citation Format: Tomas Baldassarre, Kathleen Watt, Peter Truesdell, Mark Schneider, Sandip Sengupta, Andrew WB Craig. Defining molecular mechanisms linking Endophilin A2 to metastasis in human breast cancer models. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2260. doi:10.1158/1538-7445.AM2015-2260