The ubiquitin ligase TRIM25 targets ERG for degradation in prostate cancer

// Shan Wang 1,2 , Rahul K. Kollipara 1 , Caroline G. Humphries 1 , Shi-Hong Ma 2 , Ryan Hutchinson 2 , Rui Li 2 , Javed Siddiqui 3 , Scott A. Tomlins 3 , Ganesh V. Raj 2 and Ralf Kittler 1,4,5,6 1 Eugene McDermott Center for Human Growth and Development, University of Texas Southwestern Medical Center, Dallas, Texas, USA 2 Department of Urology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, USA 3 Department of Pathology, Michigan Center for Translational Pathology, University of Michigan, Ann Arbor, Michigan, USA 4 Simmons Comprehensive Cancer Center, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, USA 5 Department of Pharmacology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, USA 6 Green Center for Reproductive Biology Sciences, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, USA Correspondence to: Ralf Kittler, email: // Keywords : prostate cancer, transcription factor, oncogene, ubiquitination Received : August 26, 2016 Accepted : September 02, 2016 Published : September 08, 2016 Abstract Ets related gene (ERG) is a transcription factor that is overexpressed in 40% of prostate tumors due to a gene fusion between ERG and TMPRSS2 . Because ERG functions as a driver of prostate carcinogenesis, understanding the mechanisms that influence its turnover may provide new molecular handles to target the protein. Previously, we found that ERG undergoes ubiquitination and then is deubiquitinated by USP9X in prostate cancer cells to prevent its proteasomal degradation. Here, we identify Tripartite motif-containing protein 25 (TRIM25) as the E3 ubiquitin ligase that ubiquitinates the protein prior to its degradation. TRIM25 binds full-length ERG, and it also binds the N-terminally truncated variants of ERG that are expressed in tumors with TMPRSS2-ERG fusions. We demonstrate that TRIM25 polyubiquitinates ERG in vitro and that inactivation of TRIM25 resulted in reduced polyubiquitination and stabilization of ERG. TRIM25 mRNA and protein expression was increased in ERG rearrangement-positive prostate cancer specimens, and we provide evidence that ERG upregulates TRIM25 expression. Thus, overexpression of ERG in prostate cancer may cause an increase in TRIM25 activity, which is mitigated by the expression of the deubiquitinase USP9X, which is required to stabilize ERG.