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

Abstract

Many transcription factors undergo transcription-coupled proteolysis. Although ligand binding activates ubiquitin proteolysis of estrogen receptor α (ERα), mechanisms governing this and its relationship to transcriptional activation were unclear. Data presented link cross talk between the Src kinase and liganded ERα with ERα activation and its ubiquitylation. Liganded ERα rapidly activates and recruits Src, which phosphorylates ERα at tyrosine 537 (Y537). This enhances ERα binding to the ubiquitin ligase/ERα coactivator, E6-associated protein (E6-AP), stimulating ERα ubiquitylation, target gene activation, and ultimately ERα loss. ERα phosphorylation by Src promotes ERα ubiquitylation by E6-AP and proteasomal degradation in vitro. Src inhibition impairs estrogen (E2)-activated ERα:E6-AP binding, reducing ERα degradation. ERα-Y537F shows little E2-stimulated degradation and activates native ERα target genes poorly. Src activation enhances ERα and E6-AP binding and their occupancy at ERα target gene promoters to enhance transcription. Thus, ERαY537 phosphorylation drives ERα:E6-AP binding to at least a subset of target promoters, linking transcriptional activation to ERα degradation and providing a novel mechanism to fine tune ERα action. The observation that ERα transcriptional activity can be briskly maintained in a context of reduced ERα levels raises the possibility that hormonally sensitive tissues may not always show robust ERα protein levels.

The Dual Estrogen ReceptorαInhibitory Effects of the Tissue-Selective Estrogen Complex for Endometrial and Breast Safety

“…TSEC significantly reduced CE-mediated enhancement of ERa activity in endometrial and breast cancer cells by degrading ERa protein, in contrast with the E2-induced interaction of ERa with E6-AP for the ERa activation (Sun et al, 2012). However, TSEC caused a differential interaction of ERa with FBXO45 for the suppression of ERa activity.…”

“…However, each hormone-induced ERa degradation was accompanied by an opposite effect on ERa activity: TSEC-induced ERa degradation was associated with inhibition of ERa activity, whereas CE-mediated ERa destabilization was correlated with enhancement of ERa activity. Previous studies also revealed that enhanced estradiol-induced ERa activation is associated with ERa degradation (Wijayaratne and McDonnell, 2001;Callige and Richard-Foy, 2006), and a specific E3 ubiquitin ligase, such as E6-AP, might be involved in this process (Li et al, 2006;Sun et al, 2012). However, the molecular mechanism underlying the functional correlation between estrogen-induced ERa degradation and ERa activation has not been fully elucidated.…”

Protein-Tyrosine Phosphatase H1 Increases Breast Cancer Sensitivity to Antiestrogens by Dephosphorylating Estrogen Receptor at Tyr537

“…For ER/ Y537 dephosphorylation in cells, MCF-7 cells were cultured for 24 hours in medium without serum and phenol red, which were then pulsed with estrogen (100 nmol/L for 30 minutes), and lysates were then subjected to immunoprecipitation with p-ER/Y537 antibody. Precipitates were subjected to Western blot analysis with ER or p-Tyr antibody (14).…”

“…Recent studies further showed that Src-induced ER/Y537 phosphorylation is essential for ER nuclear export (13) and for E6AP-triggered ER degradation (14). Furthermore, increased levels of p-ER/ Y537 expression in clinical breast cancer are associated with a poor therapeutic response to TAM (15) and the Tyr537Asn mutation was detected in metastatic breast cancer (16).…”

LYN-activating mutations mediate antiestrogen resistance in estrogen receptor–positive breast cancer

“…Overexpression of LYN, as measured by immunohistochemistry (IHC), is associated with an epithelial-to-mesenchymal transition and correlates with a shorter overall survival in breast cancer (3). SRC engages in bidirectional crosstalk with the estrogen receptor α (ERα) (4), in which its kinase phosphorylates ERα at Y537 (5), resulting in an enhancement of ER transcriptional activity (6).…”