Inhibition of RAS-Mediated Transformation and Tumorigenesis by Targeting the Downstream E3 Ubiquitin Ligase Seven in Absentia Homologue

Abstract

Constitutively active RAS small GTPases promote the genesis of human cancers. An important goal in cancer biology is to identify means of countervailing activated RAS signaling to reverse malignant transformation. Oncogenic K-RAS mutations are found in virtually all pancreatic adenocarcinomas, making the RAS pathway an ideal target for therapeutic intervention. How to best contravene hyperactivated RAS signaling has remained elusive in human pancreatic cancers. Guided by the Drosophila studies, we reasoned that a downstream mediator of RAS signals might be a suitable anti-RAS target. The E3 ubiquitin ligase seven in absentia (SINA) is an essential downstream component of the Drosophila RAS signal transduction pathway. Thus, we determined the roles of the conserved human homologues of SINA, SIAHs, in mammalian RAS signaling and RAS-mediated tumorigenesis. We report that similar to its Drosophila counterpart, human SIAH is also required for oncogenic RAS signaling in pancreatic cancer. Inhibiting SIAH-dependent proteolysis blocked RAS-mediated focus formation in fibroblasts and abolished the tumor growth of human pancreatic cancer cells in soft agar as well as in athymic nude mice. Given the high level of conservation of RAS and SIAH function, our study provides useful insights into altered proteolysis in the RAS pathway in tumor initiation, progression, and oncogenesis. By targeting SIAH, we have found a novel means to contravene oncogenic RAS signaling and block RAS-mediated transformation/tumorigenesis. Thus, SIAH may offer a novel therapeutic target to halt tumor growth and ameliorate RAS-mediated pancreatic cancer.

Vascular Normalization by Loss of Siah2 Results in Increased Chemotherapeutic Efficacy

“…[45][46][47] and biomarker expression (37), the PyMT-driven breast cancer model can assist in the future to further define the various roles of Siah2 in breast cancer progression. Our findings, that the loss of Siah2 results in increased tumor perfusion through vascular normalization, are in accordance with previous work from our group and others showing that blockade of Siah substrate binding (14,15,18) and dominant negative Siah proteins (16,17) have tumor inhibiting properties in various cancer models. Siah inhibitors can be expected to inhibit several protumoral responses, in addition to the hypoxic response pathway (48), including Ras (16,17,49), estrogen receptors (50,51), and DNA damage signaling networks (13,52).…”

“…Previously, we described that the loss of Siah2 reduces the occurrence of aggressive neuroendocrine tumors and metastasis of prostate adenocarcinoma (14) and Siah2 is required for melanoma metastasis through Hif-1 (15). Furthermore, a dominant negative version of Siah blocks oncogenic Ras signaling, thereby reducing pancreatic and lung tumor growth (16,17). We have previously described that inhibition of Siah2 by using a high affinity blocking peptide that binds to its substrate recognition domain can reduce growth and vascular development of tumors in a syngeneic and orthotopic breast cancer mouse model (18).…”

Expression of seven-in-absentia homologue 1 and hypoxia-inducible factor 1 alpha: Novel prognostic factors of nasopharyngeal carcinoma

“…Upon these findings, we speculate that downstream factors of Siah1 other than HIF1α may affect tumour progression. Siah was initially identified as a tumour suppressor gene [12], although recent studies have shown that knock-down of Siah by shRNA could significantly impede lung and pancreatic tumour growth in vitro and in vivo, indicating that expression of Siah itself promotes tumour growth [9,13,23]. Another study in breast cancer also showed that increased Siah expression was related to the aggressiveness of breast cancers [24].…”

Novel Function of E3 Ubiquitin Ligase Siah2 to Regulate ROS Metabolism

“…The function of Siah in cancer progression was first investigated in pancreatic cancer. In human pancreatic cancer cells, overexpression of both mutated Siah1 and Siah2 was found to cause growth arrest [28] . In addition, overexpression of Siah1 arrested the growth of hepatoma cells [29] .…”