Abstract 5068: Identification of RASSF1A as a novel RhoA antagonist: direct interaction with and Smurf1-mediated ubiquitination of RhoA

Abstract

Abstract RASSF1A is a tumor suppressor which plays a critical role in the regulation of various aspects of cellular functions, including cell proliferation, apoptosis, and motility. RASSF1A is frequently inactivated in various types of human cancers by transcriptional silencing of the gene due to aberrant promoter hypermethylation. However, molecular mechanisms underlying its tumor suppressive functions were largely undefined. We characterized RASSF1A regulation of a small GTPase RhoA and its implication in tumor cell migration and invasion. Effect of RASSF1A expression on RhoA activity was examined by immunoblot, immunopricipitation, and ubiquitinylation assays, and RASSF1A modulation of RhoA-mediated cell migration and invasion was determined using wound healing and in vitro invasion assay. We identified that RASSF1A inhibits serum- or LPA-induced RhoA activation in both cancer cells and noncancerous cells. SiRNA-mediated knockdown of RASSF1A increases RhoA activity and RhoA-mediated tumor cell migration. RASSF1A promotes the protesomal degradation of GTP-bound RhoA via Smurf1-mediated ubiquitinylation, but does not affect the activity and stability of Rac1 and CDC42. Immunofluoroscence and immunopricipitation assays revealed that RASSF1A colocalizes and interacts with RhoA and Smurf1. The N-terminal region of RASSF1A interacts with Smurf1, and its C-terminal residues (amino acids 266-272) are critical for the interaction with RhoA. The mutant RASSF1A (delNC-RASSF1A) having a sequence alteration in the Smurf1 and RhoA binding region fails to inhibit the RhoA-mediated tumor cell migration and invasion. Consistent with theses results, loss or abnormal reduction of RASSF1A expression exhibits a tight correlation with RhoA activation in both cancer cell lines and primary tumors of various tissues origins. Our study demonstrates first that RASSF1A regulates cell motility through the proteosomal degradation of activated RhoA through Smurf1-mediated ubiquitinylation, indicating that genetic and epigenetic inactivation of RASSF1A during tumorigenic process increases the invasive and metastatic potential of tumor cells by deregulation of RhoA. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5068.