|Ph.D Student||Thomas Jane Joy|
|Subject||The Role of RNF4 in Activation of Oncogenic Pathways by|
Selective Protein Stabilization
|Department||Department of Medicine||Supervisor||Professor Amir )Oryan )Orian|
|Full Thesis text|
Ubiquitylation regulates signaling pathways critical for cancer development, and in many cases targets proteins for degradation. We report here that RNF4, a ubiquitin ligase, is involved in stabilization, rather than degradation of selected short-lived oncogenic transcription factors, including β-catenin, c-Myc, c-Jun, and the Notch intracellular-domain (N-ICD) protein. RNF4 enhances the transcriptional activity of these factors, as well as Wnt- and Notch-dependent gene expression. While RNF4 is a SUMO-targeted ubiquitin ligase (STUbL), protein stabilization requires the substrate’s phosphorylation rather than SUMOylation. These phosphorylated oncoproteins are recognized by binding to RNF4’s Arginine Rich Motif (ARM) domain. Stabilization also involves generation of unusual poly-ubiquitin chains, and docking of RNF4 to chromatin. RNF4 generates atypical polyubiquitin chains with internal ubiquitin linkages via K11 and K33, in combination with specific E2 ubiquitin conjugating enzyme. Also preventing the association of RNF4 with nucleosomes, by a point mutation K179D in RNF4, can prevent ubiquitylation, stabilization and enhancement of transcription of oncoproteins by RNF4. Biologically, RNF4 enhances the tumorous phenotype, and is essential for cancer cell survival. RNF4 serves as a positive agonist of Wnt and Notch oncogenic pathways where c-Myc is the main target and RNF4 is a direct c-Myc target. Stabilization of β-catenin, c-Myc, c-Jun, and NICD by RNF4 makes RNF4 a potential target in cancers where the degradation of these oncoproteins is compromised. RNF4 also potentiates tumorous phenotypes of colon and breast cancer cells. High levels of RNF4 mRNA correlate with poor survival of a subgroup of breast cancer patients, and RNF4 protein levels are elevated in 30% of biopsies of human colon adenocarcinomas. Thus, RNF4 fits into non oncogenic addiction (NOA) class of genes; that are crucial for cancer cell survival, enhance oncogene activation but are not oncogenic on their own. In a nutshell, RNF4-dependent ubiquitylation translates transient phosphorylation signal(s) into long-term protein stabilization, resulting in enhanced oncoprotein activation, potentiating tumorigenesis.