Modification of ER alpha by UFM1 Increases Its Stability and Transactivity for Breast Cancer Development

The post-translational modification (e.g., phosphorylation) of estrogen receptor alpha (ER alpha) plays a role in controlling the expression and subcellular localization of ER alpha as well as its sensitivity to hormone response. Here, we show that ER alpha is also modified by UFM1 and this modification (ufmylation) plays a crucial role in promoting the stability and transactivity of ER alpha, which in turn promotes breast cancer development. The elevation of ufmylation via the knockdown of UFSP2 (the UFM1-deconjugating enzyme in humans) dramatically increases ER alpha stability by inhibiting ubiquitination. In contrast, ER alpha stability is decreased by the prevention of ufmylation via the silencing of UBA5 (the UFM1-activating E1 enzyme). Lys171 and Lys180 of ER alpha were identified as the major UFM1 acceptor sites, and the replacement of both Lys residues by Arg (2KR mutation) markedly reduced ER alpha stability. Moreover, the 2KR mutation abrogated the 17 beta-estradiol-induced transactivity of ER alpha and the expression of its downstream target genes, including pS2, cyclin D1, and c-Myc: this indicates that ER alpha ufmylation is required for its transactivation function. In addition, the 2KR mutation prevented anchorage-independent colony formation by MCF7 cells. Most notably, the expression of UFM1 and its conjugating machinery (i.e., UBA5, UFC1, UFL1, and UFBP1) were dramatically upregulated in ER alpha-positive breast cancer cell lines and tissues. Collectively, these findings implicate a critical role attributed to ER alpha ufmylation in breast cancer development by ameliorating its stability and transactivity.