Differential Regulation of Estrogen-Inducible Proteolysis and Transcription by the Estrogen Receptor α N Terminus

Abstract

The ubiquitin-proteasome pathway has emerged as an important regulatory mechanism governing the activity of several transcription factors. While estrogen receptor ␣ (ER␣) is also subjected to rapid ubiquitinproteasome degradation, the relationship between proteolysis and transcriptional regulation is incompletely understood. Based on studies primarily focusing on the C-terminal ligand-binding and AF-2 transactivation domains, an assembly of an active transcriptional complex has been proposed to signal ER␣ proteolysis that is in turn necessary for its transcriptional activity. Here, we investigated the role of other regions of ER␣ and identified S118 within the N-terminal AF-1 transactivation domain as an additional element for regulating estrogen-induced ubiquitination and degradation of ER␣. Significantly, different S118 mutants revealed that degradation and transcriptional activity of ER␣ are mechanistically separable functions of ER␣. We find that proteolysis of ER␣ correlates with the ability of ER␣ mutants to recruit specific ubiquitin ligases regardless of the recruitment of other transcription-related factors to endogenous model target genes. Thus, our findings indicate that the AF-1 domain performs a previously unrecognized and important role in controlling ligandinduced receptor degradation which permits the uncoupling of estrogen-regulated ER␣ proteolysis and transcription.The ubiquitin-proteasome pathway contributes to the control of transcription through the ubiquitination and regulated degradation of multiple components of the transcriptional machinery (10, 34). Among these components is a large list of transactivators whose activity can be related to their proteolytic degradation. For many, particularly those that possess acidic activation domains, such as VP16 and c-myc, sequence elements essential for proteasome-mediated proteolysis reside within transactivation domains (33,43). Several members of the nuclear receptor superfamily are substrates for the ubiquitin-proteasome pathway (11,19,21,24,26,29,37,50,52,55), the first identified being estrogen receptor ␣ (ER␣) (1, 13, 36). ER␣ possesses two transactivation domains, AF-1 and AF-2, which reside in the N terminus and C terminus of the receptor, respectively. These activation domains are bridged by a conserved DNA binding domain and a hinge region responsible for receptor nuclear localization.The transcriptional activity of AF-2 is strictly ligand dependent, but the AF-1 is not; thus, AF-2 received much attention for analysis of the relationship between estrogen-stimulated proteolysis and transcription. AF-2 is highly structured, consisting of 12 ␣-helices that adopt an active conformation upon agonist binding, which exposes a hydrophobic surface where coactivator proteins bind (6). It has been shown that mutations of residues critical for AF-2-mediated transactivation disrupt proteolysis (27, 53). E6-AP, a ubiquitin ligase (35), and the TRIP1/Rpt6/SUG1 (28, 42) subunit of the 19S regulatory cap of the proteasome bind to ER␣ through the coactiva...

Isoform-Specific Regulation of a Steroid Hormone Nuclear Receptor by an E3 Ubiquitin Ligase inDrosophila melanogaster

“…This interpretation is akin to the case of the vertebrate estrogen receptor a, ERa, where the two processes, ubiquitin/proteasome degradation of the protein and transcriptional activation of the gene, are independent and mediated through distinct structural motifs (Tateishi et al 2004;Valley et al 2005).…”

Effects of Inhibiting the Proteasomal Degradation of Estrogen Receptor .ALPHA. on Estrogen Receptor .ALPHA. Activation under Hypoxic Conditions

“…Our data from this hypoxic system provide additional evidence that ER down-regulation may be essential to ERa transactivation, as other researchers have proposed. 10,24) A deeper understanding of the hypoxia-induced ER pathway will facilitate the elucidation of ERa transcriptional activation. …”

p38γ Mitogen-activated Protein Kinase (MAPK) Confers Breast Cancer Hormone Sensitivity by Switching Estrogen Receptor (ER) Signaling from Classical to Nonclassical Pathway via Stimulating ER Phosphorylation and c-Jun Transcription

“…This phosphorylation can occur in response to estrogens and SERMs (11,12) and is required for ER regulating gene expression (13). Increased levels of p-ER/Ser-118 in primary breast cancer correlate with a better clinical response to TAM therapy (14), indicating that p-ER/Ser-118 is a determinant factor in breast cancer hormone sensitivity.…”

“…Several MAPKs including ERK1/2 (40) and ERK7 (41) can decrease ER levels, but the underlying mechanisms have not been determined. Mutation of the Ser-118 to S118A protects ER from estrogen-induced proteolysis through proteasomal pathways (12). We explored whether the p38␥ phosphorylation of ER at Ser-118 promotes ER degradation.…”