Abstract 2335: Oncogenic K-RAS activation promotes cell motility, invasion, tumor growth and cancer metastasis via its most downstream signaling module, the SIAH2-dependent proteolytic machinery, in the highly aggressive and metastatic human cancer cells

Hyperactivation of EGFR/HER2/K-RAS is often associated with reduced cell/focal adhesion, altered cell junctions, increased cell motility, and enhanced invasion/metastasis in a large percentage of human cancers. Finding novel measures that inhibit activated ERBB/K-RAS signals and, thus, reverse tumorigenesis and metastasis remains an important goal in cancer biology. Instead of targeting an upstream component in the EGFR/HER2/K-RAS pathway, we have targeted the most downstream signaling module, called the SIAH-dependent proteolytic machinery. SIAH/SINA are evolutionarily conserved RING E3 ligases that act as an essential “gatekeeper” signaling module in the RAS signal transduction pathway. We have demonstrated that proper SIAH function is required for K-RAS signal transduction. SIAH is a novel and tumor-specific biomarker for proliferating cells in human cancer. Furthermore, we have shown that SIAH2 inhibition can successfully block both tumorigenesis and metastasis in eleven of the most aggressive human cancer cell lines known in athymic nude mice. These observations highlight the importance of SIAH2 in K-RAS-dependent tumorigenesis and metastasis in human cancer. To elucidate the molecular mechanism of SIAH2 action in human cancer cells, a proteomic approach was taken to identify novel SIAH2-interacting proteins in three highly aggressive human cancer cell lines carrying K-RAS mutations (MiaPaCa, A549 and MDA-MB-435). From these studies, three focal adhesion LIM domain proteins (LPXN, TRIP6 and FHL2) were found to interact with SIAH2 endogenously in these human cancer cells. Here, we provide extensive biochemical evidence demonstrating that SIAH2 interacts with, ubiquitinates and degrades TRIP6/FHL2/LPXN in a dose-dependent fashion, both exogenously and endogenously. This study clearly demonstrates that LPXN, TRIP6 and FHL2 are bona fide SIAH2 substrates in vivo. Ectopic expression of LPXN/TRIP6/FHL2 successfully rescued the defects in cell motility and tumor growth observed in these SIAH2-deficient human cancer cells. These focal adhesion LIM domain proteins may, therefore, represent a novel class of SIAH2 substrates (logical RAS targets) that function downstream of SIAH2 and modulate focal adhesion, increased cell motility, invasion, tumorigenesis and metastasis in response to active ERBB/K-RAS signals. The identification of SIAH2 as an important regulator of focal adhesion and cell motility downstream of K-RAS pathway may help to explain the well-documented changes in cell adhesion, aggressive tumor growth, invasion and metastasis in response to K-RAS pathway activation in the most lethal forms of human cancers. 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 2335. doi:10.1158/1538-7445.AM2011-2335