Studies on the role of ASC1 ufmylation in ERα transactivation and breast cancer development

Abstract

UFM1 (Ubiquitin-fold modifier 1) is a recently identified ubiquitin-like protein (Ubl) based on the similarity of its tertiary structure to ubiquitin (Ub). Similar to Ub and other Ubls, UFM1 is conjugated to cellular proteins via its C-terminal Gly residue by sequential action of the UFM1-activating enzyme UBA5, the UFM1-conjugating enzyme UFC1, and the UFM1 ligase UFL1. UFM1 conjugation is reversible and the release of free UFM1 from target proteins is carried out by two UFM1-specific proteases, UfSP1 and UfSP2. These UfSPs are also responsible for the generation of mature form of UFM1 from its precursors. The UFM1 conjugation and deconjugation systems are well conserved from C. elegans to human (but not in yeast or prokaryotes), implicating their critical role in multicellular organisms. However, target proteins for UFM1 modification and the role of this conjugation system remain totally unknown. By using mass spectrometry, I identified the activating signal cointegrator 1 (ASC1: also called as thyroid hormone receptor interactor 4, TRIP4) as a candidate target protein for UFM1 modification (ufmylation). ASC1 serves as a transcriptional coactivator of various nuclear receptors, including RARα, steroid receptors, and NF-κB. It forms a complex with other transcriptional coactivators, such as p300 and SRC1, and promotes the transcriptional activity of nuclear receptors. Under overexpression conditions, ASC1 was poly-ufmylated by the UFM1 system (i.e., UFM1, UBA5, UFC1, and UFL1) in an ATP-dependent manner and this modification could be dramatically stimulated by C20orf116, which had previously been identified as a target substrate for ufmylation. This stimulatory effect required ufmylation of C20orf116, suggesting that C20orf116 serves not only as a target protein but also as a component of the UFM1 system. Since C20orf116 has also been called as UFBP1 based on its ability to bind UFM1, henceforth I recurred C20orf116 to as UFBP1. ASC1 was indeed found to be conjugated by UFM1 and this modification (ufmylation) was crucial for ERα transactivation and breast tumorigenesis. Without 17 beta-estradiol, the UFM1-specific protease UfSP2 bound to and deufmylated ASC1. In its presence, however, ERα displaced UfSP2 for binding to and ufmylation of ASC1. Poly-UFM1 chains conjugated to ASC1 served as a scaffold that recruits p300 and SRC1 to the promoters of ERα downstream genes, including pS2, CYCLIN D, and c-MYC, for their transcription. ASC1 overexpression or UfSP2 knockdown promoted anchorage-independent cell growth and tumor formation in vivo, whereas overexpression of ufmylation-deficient ASC1 mutant or knockdown of the UFM1-activating enzyme UBA5 prevented them. Furthermore, the expression of UFM1-conjugating machinery, including UBA5, was dramatically up-regulated in ERα-positive human breast tumors. These findings establish the role of ASC1 ufmylation in promotion of ERα transactivation and breast cancer development.