Abstract 952: The SCF F box protein, Skp2, is a key component of an E3 ubiqutin ligase that governs estrogen receptor ≤ stability

Abstract

Abstract Breast carcinoma is the most common cancer among women in developed countries, and about 70% of these tumors express estrogen receptor ≤ (ERα). ERα is a transcription factor and master regulator of estrogen stimulated proliferation and its expression indicates potential for response to estrogenic stimulation. A majority of ERα positive breast cancers initially respond well to selective estrogen receptor modulators (such as tamoxifen) or to aromatase inhibitors. Understanding the regulation of ERα levels and its role in transcription of estrogen driven genes is thus highy germane to breast cancer therapy. There is evidence that for many transcription factors, activation is linked to factor degradation. Moreover, estrogen stimulation is known to activate ERα proteolysis. Given the importance of estrogen as a driver of breast cancer progression, we have investigated mechanisms governing ERα proteolysis. S phase kinase-associated protein 2 (Skp2) is an F box component of a multi-protein SCF ubiquitin ligase that acts on multiple substrates. Levels of Skp2 have been reported to be inversely related with the levels of ERα in breast cancer. Here we report that SCFSkp2, comprised of Skp2, in association with Skp1, Cul1 and Rbx1, is an E3 ubiquitin ligase for ERα. Ectopic expression of dominant negative Cul1 increases ERα levels by impeding ERα degradation in breast cancer cells. Knockdown of Skp2 impairs estrogen-triggered ERα proteolysis, while ectopic Skp2 expression decreased ERα stability. We show that Skp2, Skp1, Rbx1 and Cul1 co-precipitate with cellular ERα and the formation of this ERα/SCFSkp2 complex is cell cycle regulated and parallels CDK2 activation. We also show ER is an in vitro substrate that is ubiquitylated and degraded by SCFSkp2. The involvement of CDK2-dependent ERα phosphorylation in estrogen activated ERα/SCFSkp2 binding, ERα proteolysis and the functional consequences of this on ERα transcriptional activity are under investigation. These data suggest that Skp2 plays an important role in the regulation of ERα stability, and potentially in the biologic action of this key steroid hormone receptor in breast cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 952. doi:1538-7445.AM2012-952