Abstract B40: SIAH-dependent SMAD2/3/4 ubiquitination and degradation in mediating the K-RAS-TGFβ antagonism during tumorigenesis and metastasis in human pancreatic cancer cells.

The pivotal roles of the K-RAS and TGFβ signaling pathways have been well characterized in a number of normal and pathological conditions including human pancreatic cancer. Hyperactivation of the ERBB/KRAS pathway is known to drive neoplastic transformation, tumorigenesis and metastasis in a large percentage of human malignancies while the growth inhibitory actions of TGFβ are critical in preventing tumor initiation and progression. A better understanding of the context-dependent antagonistic interaction and cross-talk between the K-RAS and TGFβ signaling pathways that control tumorigenesis and metastasis should give rise to novel therapeutic interventions. Finding novel measures that inhibit hyperactivated 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” in the RAS signal transduction pathway. We have demonstrated that proper SIAH function is required for K-RAS signal transduction, and SIAH2 inhibition can successfully abolish both tumorigenesis and metastasis of human pancreatic cancer cells in athymic nude mice. These observations highlight the SIAH2 importance in K-RAS-dependent tumorigenesis and metastasis in pancreatic cancer. In this study, we reported that SMAD2/3/4 – the essential downstream mediators of TGFβ signaling, are bona fide new SIAH2 substrates. SIAH2 physically interacts with, ubiquitinates and degrades SMAD2/3/4 (not SMAD1 – the downstream mediator of the BMP signaling pathway). The identification of the SIAH2-mediated SMAD2/3/4 modulation/degradation may provide an additional molecular mechanism to explain the well documented ERBB/K-RAS and TGFβ antagonism described in pancreatic tumorigenesis and metastasis. The discovery of the SIAH2-SMAD2/3/4 antagonism downstream of K-RAS and TGFβ signaling pathways would provide a novel and logical molecular mechanism to explain the growth inhibitory action of the TGFβ pathway and the growth-promoting activity of the K-RAS pathway. Since the substrate specificity of ubiquitin-mediated proteolysis is primarily determined by the E3 ligase, our results provide new insights into the role of altered proteolysis in mediating the K-RAS-TGFβ antagonism and cross-talk observed in the genesis, progression and metastasis of human pancreatic cancers. Through our work, SIAH has begun to emerge as a logical and potent anti-K-RAS and anticancer drug target in human pancreatic cancer. Citation Format: Amy H. Tang, Yang Liao, Maryanne Edens, Ralf Janknecht, Edward B. Leof. SIAH-dependent SMAD2/3/4 ubiquitination and degradation in mediating the K-RAS-TGFβ antagonism during tumorigenesis and metastasis in human pancreatic cancer cells. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Progress and Challenges; Jun 18-21, 2012; Lake Tahoe, NV. Philadelphia (PA): AACR; Cancer Res 2012;72(12 Suppl):Abstract nr B40.