Sumoylation of the estrogen receptor α hinge region regulates its transcriptional activity

The steroid hormone 17 beta-estradiol ( estrogen) plays a significant role in the normal physiology of the mammary gland and breast cancer development primarily through binding to its receptor, the estrogen receptor alpha (ER alpha). ER alpha is a nuclear transcription factor undergoing different types of post-translational modifications, i.e. phosphorylation, acetylation, and ubiquitination, which regulate its transcriptional activation and/or stability. Here we identify ER alpha as a new target for small ubiquitin-like modifier (SUMO)-1 modification in intact cells and in vitro. Moreover, ER alpha sumoylation occurs strictly in the presence of hormone. SUMO-1 appears to regulate ER alpha-dependent transcription. Using a series of mutants, we demonstrated that ER alpha is sumoylated at conserved lysine residues within the hinge region. Mutations that prevented SUMO modification impaired ER alpha-induced transcription without influencing ER alpha cellular localization. In addition to identifying protein inhibitor of activated signal transducer and activator of transcription (PIAS)1 and PIAS3 as E3 ligases for ER alpha, we also found that PIAS1 and PIAS3, as well as Ubc9, modulated ER alpha-dependent transcription independently from their SUMO-1 conjugation activity. These findings identify sumoylation as a new mechanism modulating ER alpha-dependent cellular response and provide a link between the SUMO and estrogen pathways.