Regulation of Estrogen Receptor α N-Terminus Conformation and Function by Peptidyl Prolyl Isomerase Pin1

Abstract

Estrogen receptor alpha (ERα), a key driver of growth in the majority of breast cancers, contains an unstructured transactivation domain (AF1) in its N terminus that is a convergence point for growth factor and hormonal activation. This domain is controlled by phosphorylation, but how phosphorylation impacts AF1 structure and function is unclear. We found that serine 118 (S118) phosphorylation of the ERα AF1 region in response to estrogen (agonist), tamoxifen (antagonist), and growth factors results in recruitment of the peptidyl prolyl cis/trans isomerase Pin1. Phosphorylation of S118 is critical for Pin1 binding, and mutation of S118 to alanine prevents this association. Importantly, Pin1 isomerizes the serine118-proline119 bond from a cis to trans isomer, with a concomitant increase in AF1 transcriptional activity. Pin1 overexpression promotes ligand-independent and tamoxifen-inducible activity of ERα and growth of tamoxifen-resistant breast cancer cells. Pin1 expression correlates with proliferation in ERα-positive rat mammary tumors. These results establish phosphorylation-coupled proline isomerization as a mechanism modulating AF1 functional activity and provide insight into the role of a conformational switch in the functional regulation of the intrinsically disordered transactivation domain of ERα.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://dx.doi.org/10.1128/MCB.06073-11.

ACKNOWLEDGMENTS

We thank the McArdle Laboratories for Cancer Research for support of this project. We also thank the UW Carbone Comprehensive Cancer Center (UWCCC) for use of its shared services to complete this research. Protein samples used in our NMR studies were produced by the Center for Eukaryotic Structural Genomics (CESG) with support from NIH grant U54 GM074901 (to J.L.M.). NMR data were collected at the National Magnetic Resonance Facility (Madison), which is supported by NIH grants P41 RR02301 and P41 GM66326 (to J.L.M.). This work was also supported by NIH grants CA159578 (to E.T.A.), T32 GM08688 (to P.R.), and R01 GM56230 (to K.P.L.) and in part by NIH/NCI P30 CA014520-UWCCC support. In vivo studies were supported by the U.S. Army Medical Research and Materiel Command under W81XWH-07-1-0502 (to S.Z.H.) and by NIH Breast Cancer and the Environment Research Centers grant U01 ES/CA012800 (to S.Z.H.).

We thank the CESG staff members who contributed to preparing protein samples used in ou-NMR studies. We thank Zafar Nawaz, Jing Zhang, Benita Katzenellenbogen, Edwin Sanchez, Yongna Xing, and Wei Xu for appropriate expression plasmids and purified protein and Shigeki Miyamoto and Wei Xu for careful reading of the manuscript.