SPBP Is a Phosphoserine-Specific Repressor of Estrogen Receptor α

Abstract

Multiple signaling pathways stimulate the activity of estrogen receptor α (ERα) by direct phosphorylation within its N-terminal activation function 1 (AF1). How phosphorylation affects AF1 activity remains poorly understood. We performed a phage display screen for human proteins that are exclusively recruited to the phosphorylated form of AF1 and found the stromelysin-1 platelet-derived growth factor-responsive element-binding protein (SPBP). In a purified system, SPBP bound only the in vitro-phosphorylated form of the ERα AF1 or the phosphoserine mimic S118E, and the interaction domain could be mapped to a 42-amino-acid fragment of SPBP. In cells, SPBP preferentially interacted with liganded and phosphorylated ERα. Functionally, SPBP behaved as a repressor of activated ERα, which extends its previously demonstrated roles as a DNA binding transactivation factor and coactivator of other transcription factors. By targeting the phosphorylated form of AF1, SPBP may contribute to attenuating and fine-tuning ERα activity. A functional consequence is that SPBP inhibits the proliferation of ERα-dependent but not ERα-independent breast cancer cell lines, mirroring a reported negative correlation with the ERα status of breast tumors.

ACKNOWLEDGMENTS

We thank Jan-Åke Gustafsson, Terje Johansen, Daniel Metzger, Lorenz Poellinger, and David F. Smith for reagents. We are grateful to Olivier Donzé for guidance in the early phases of the project and to various lab members, notably Pierre-André Briand and Bruno Cenni, for contributing plasmids. We also thank Peter Dudek for critical reading of the manuscript.

This work was supported by the Swiss National Science Foundation, Krebsforschung Schweiz, the Fondation Médic, and the Canton de Genève.