Abstract 4695: Apoptosis inhibitor ARC as a novel mediator of breast tumorigenesis, metastasis and chemoresistance

Apoptosis is critical for normal development and post-natal tissue homeostasis. ARC (Apoptosis Repressor with CARD), is an inhibitor of apoptosis with the unusual property of antagonizing both death receptor and mitochondrial apoptosis pathways. Under normal conditions, ARC is expressed in cardiac and skeletal myocytes and neurons. Suppression of cell death is critical for carcinogenesis and chemoresistance and we previously reported that ARC expression is induced in a wide variety of primary human epithelial cancers, including breast. Herein we present the first data demonstrating critical roles for ARC in several aspects of breast carcinogenesis. To test the hypothesis that ARC plays an important role in the pathogenesis of breast cancer in vivo, ARC-/- mice were bred onto the polyoma middle T-antigen (PyMT) transgenic mouse model of breast cancer. PyMT expression activates many oncogenic pathways common to human breast cancer including the induction of ARC in early and late-stage tumors and invading and metastatic cells. PyMT mice lacking ARC exhibit a 34% decrease in primary tumor burden compared to PyMT mice with ARC, although tumor onset and multiplicity remains unchanged. Unexpectedly, the reduced tumor burden can be accounted for by a novel function of ARC to promote proliferation, while baseline apoptosis remained comparable. The absence of ARC also results in a 44% reduction in the number of lung metastases independent of primary tumor burden. In vivo invasion assays and blood burden intravasation assays demonstrate that PyMT mice lacking ARC have a significant reduction in invading and circulating tumor cells when compared to those with ARC. Conversely, overexpression of ARC in a PyMT-derived metastatic cell line caused a significant increase in basement membrane invasion in vitro and lung metastases when injected into tail veins of mice. To test the function of ARC in a humanized model of breast cancer, ARC was knocked-down in the MDA-MB-231-derived LM2 human metastatic breast cancer cell line which was subsequently transplanted into mammary glands of SCID mice. Diminished ARC expression resulted in a 50% reduction in xenograft tumor volume and a drastic decline in stromal invasion in vivo. We also examined if the presence of ARC in invasive and metastatic cells contributes to chemoresistance. Xenograft LM2 mammary tumors deficient in ARC were sensitized to apoptosis induced by doxorubicin while the control tumors remained resistant. Moreover, deletion of ARC sensitized the invading cell population collected from primary PyMT breast tumors to tamoxifen-induced killing. This study demonstrates that ARC promotes primary tumor growth, improves the efficiency of metastasis at several stages and renders invasive breast cancer cells resistant to chemotherapies. These data implicate ARC as a novel mediator in the pathogenesis of breast cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4695. doi:10.1158/1538-7445.AM2011-4695