Coordinate regulation of estrogen receptor β degradation by Mdm2 and CREB-binding protein in response to growth signals

Abstract

The biological actions of estrogen are mediated via estrogen receptors ERα and ERβ. Yet, other cellular signaling events that also impact ER functions have an important role in breast carcinogenesis. Here, we show that activation of ErbB2/ErbB3 tyrosine kinase receptors with growth factor heregulin-β prompts ERβ degradation by the 26S proteasome, a mechanism that requires the coactivator cAMP response element-binding (CREB)-binding protein (CBP). We found that CBP promotes ERβ ubiquitination and degradation through enhancement of the PI3-K/Akt pathway by heregulin-β, an effect potentiated by a negatively charged hinge region of ERβ. Activated Akt triggered the recruitment of E3 ubiquitin ligase Mdm2 to ERβ, which was further stabilized by CBP, resulting in ERβ poly-ubiquitination. Mutation of CBP Thr-1872 or Mdm2 Ser-186/188 Akt sites resulted in a dissociation of the ERβ-CBP-Mdm2 complex and reduced ERβ turnover. We found that the decrease in ERβ induced by heregulin-β was associated with reduced target gene promoter occupancy and enhanced proliferation of breast cancer cells. However, knockdown of Mdm2 restored endogenous ERβ levels resulting in reduction of breast cancer cell growth. These studies identify a tripartite Akt-regulated phosphorylation mechanism that functions to hamper normal ERβ activity and turnover through the concerted actions of CBP and Mdm2 in response to growth factor signaling pathways in breast cancer cells.

MDM2, MDMX and p53 in oncogenesis and cancer therapy

“…Intriguingly, MDM2 can regulate gene expression and DNA repair by interacting with chromatin or chromatin-associated factors. For example, MDM2 associates with and ubiquitylates oestrogen receptor and androgen receptor 136–140 . This leads to changes in the expression of hormone-responsive genes and enhances cell proliferation in some contexts 137,141 .…”

Estrogen Receptor β (ERβ1) Transactivation Is Differentially Modulated by the Transcriptional Coregulator Tip60 in a cis-Acting Element-dependent Manner

“…ER␣ also binds to PGC-1 at its hinge domain in a ligand-independent manner (52). Although the hinge domain of ERs is not as well characterized, it has been shown to affect protein degradation and activity of ER␤1 (53,54), ER␣ tethered-mediated AP-1 transactivation (55), and the functional synergy between AF-1 and AF-2 of ERs (56). Because AF-1 and AF-2 domains are responsible for E 2 -independent and E 2 -dependent activation of the transactivation of ERs (50), we speculate that the atypical interaction interface between ER␤1 and Tip60 at the hinge domain may contribute to the unique regulation of ER␤1 activity by Tip60.…”

Ubiquitylation of nuclear receptors: new linkages and therapeutic implications

“…Mutations of Ser 210 and Ser 790, or Ser 515, to alanine prevents recruitment of Mdm2 and ubiquitylation of AR. Like AR, Mdm2 is also recruited to ERα and ERβ complexes when the corresponding ER is phosphorylated (Valley et al 2005; Picard et al 2008; Sanchez et al 2013). However, degradation of ERα upon Mdm2 over-expression provides an example wherein specific NR protein complexes are also a requirement for this response, in this case, a complex with p53 (Duong et al 2007).…”

“…Similarly, GR degradation following dexamethasone treatment involves the formation of a GR complex containing p53 and Hdm2 (Sengupta & Wasylyk 2001). In the case of ERβ, Mdm2 works in concert with a different coregulator, CREB-Binding Protein (CBP), to form a complex that results in ubiquitylation and ultimate degradation of ERβ (Sanchez et al 2013). Interestingly, unlike ERβ, the Mdm2-CBP complex was unable to target ERα for degradation.…”