Abstract 1659: Targeting SIAH2-dependent proteolysis downstream of the HER2/EGFR/RAS signaling pathway to inhibit tumorigenesis and metastasis of highly invasive human breast cancer

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL Breast cancer is the most common form of cancer diagnosed and the second leading cause of cancer death among women in the United States. Cancer metastases are the major cause of morbidity and mortality among breast cancer patients. The central importance of the HER2/RAS signaling pathway has been well established in promoting tumor growth, invasion and metastasis of human breast cancer. Even though oncogenic K-RAS mutations are rare in breast cancer, activation of the growth-promoting ERBB/RAS pathway has been consistently documented in high-grade breast tumors. As such, novel approaches to inhibit activated HER2/RAS signals constitute important measures to block tumor growth and metastasis in mammary tumors. In this study, instead of targeting an upstream signaling component such as HER2/EGFR/RAS, we targeted the most downstream signaling module identified in the ERBB/RAS pathway – the SIAH proteolytic machinery. SIAH is an evolutionarily conserved RING E3 ligase that is a critical “gatekeeper” required for RAS signal transduction in human cancer cells. SIAH is specifically expressed in tumor cells in 120 human breast cancer patients examined so far. The increased SIAH expression is correlated with increased grades and aggressiveness of human breast cancer. We asked whether inhibiting the gatekeeper function of SIAH would be effective in blocking mammary tumorigenesis and metastasis in human breast cancer. Our results indicated that SIAH2-deficiency successfully abolished tumor growth of three breast cancer cell lines (MDA-MB-231, MDA-MB-435 and MDA-MB-468) in soft agar assays as well as in athymic nude mice. Importantly, by inhibiting SIAH2 function in highly aggressive human breast cancer cells, we are able to completely block tumorigenesis and metastasis of two highly invasive and metastatic human breast cancer cell lines (MDA-MB-231 and MDA-MB-435). These results suggest that SIAH2 may be an attractive and new drug target for novel breast cancer therapy. Developing anti-SIAH molecules will aid in expanding our arsenal of effective anticancer therapies. By attacking the growth-promoting HER2/EGFR/RAS pathway at multiple signaling modules simultaneously – a multi-pronged synergistic inhibition at upstream (HER2/EGFR membrane receptors), midstream (RAS/RAF/MEK/mTOR) and downstream (SIAH E3 ligase) signaling modules in the HER2/EGFR/RAS pathway in parallel, we may be in a position to halt the genesis, progression and metastasis of human breast cancer. Thus, SIAH-dependent proteolysis may be an Achilles heel for breast cancer. SIAH inhibition may represent an innovative and logical way to inhibit HER2/EGFR/K-RAS activation, alleviate tumor burdens, halt cancer metastasis, and improve the efficacy for the future treatment 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 1659. doi:10.1158/1538-7445.AM2011-1659