ERα Phosphorylation at Y537 by Src Triggers E6-AP-ERα Binding, ERα Ubiquitylation, Promoter Occupancy, and Target Gene Expression

Many transcription factors undergo transcription-coupled proteolysis. Although ligand binding activates ubiquitin proteolysis of estrogen receptor alpha (ER alpha), mechanisms governing this and its relationship to transcriptional activation were unclear. Data presented link cross talk between the Src kinase and liganded ER alpha with ER alpha activation and its ubiquitylation. Liganded ER alpha rapidly activates and recruits Src, which phosphorylates ER alpha at tyrosine 537 (Y537). This enhances ER alpha binding to the ubiquitin ligase/ER alpha coactivator, E6-associated protein (E6-AP), stimulating ER alpha ubiquitylation, target gene activation, and ultimately ER alpha loss. ER alpha phosphorylation by Src promotes ER alpha ubiquitylation by E6-AP and proteasomal degradation in vitro. Src inhibition impairs estrogen (E2)-activated ER alpha:E6-AP binding, reducing ER alpha degradation. ER alpha-Y537F shows little E2-stimulated degradation and activates native ER alpha target genes poorly. Src activation enhances ER alpha and E6-AP binding and their occupancy at ER alpha target gene promoters to enhance transcription. Thus, ER alpha Y537 phosphorylation drives ER alpha:E6-AP binding to at least a subset of target promoters, linking transcriptional activation to ER alpha degradation and providing a novel mechanism to fine tune ER alpha action. The observation that ER alpha transcriptional activity can be briskly maintained in a context of reduced ER alpha levels raises the possibility that hormonally sensitive tissues may not always show robust ER alpha protein levels. (Molecular Endocrinology 26: 1567-1577, 2012)