Proteasome-Mediated Proteolysis of Estrogen Receptor: A Novel Component in Autologous Down-Regulation

Abstract

Regulation of estrogen receptor (ER) concentration is a key component in limiting estrogen responsiveness in target cells. Yet the mechanisms governing ER concentration in the lactotrope cells of the anterior pituitary, a major site of estrogen action, are undetermined. In this study, we used a lactotrope cell line, PR1, to explore regulation of ER protein by estrogen. Estrogen treatment resulted in an approximate 60% decrease in ER steady state protein levels. Suprisingly, the decline in ER protein was apparent within 1 h of estrogen treatment and occurred in the absence of protein synthesis and transcription. Direct regulation of ER protein was further confirmed by pulse chase analysis, which showed that ER protein half-life was shortened from greater than 3 h to 1 h in the presence of estrogen. The estrogen-induced degradation of ER protein could be prevented by pretreatment with peptide aldehyde inhibitors of proteasome protease whereas inhibitors of calpain and lysosomal proteases were ineffective. Inhibition of proteasome activity maintained ER protein at a level equivalent to control cells not stimulated with estrogen but increased estrogen-binding activity by 1.75-fold. Proteolytic regulation of ER by the proteasome is not limited to pituitary lactotrope cells but is also operational in MCF-7 breast cancer cells, suggesting that this may be a common regulatory pathway used by estrogen. These studies describe a nongenomic action of estrogen that involves nuclear ER: rapid proteolysis of ER protein via a proteasome-mediated pathway.

The Aryl Hydrocarbon Receptor Mediates Degradation of Estrogen Receptor α through Activation of Proteasomes

“…Therefore, this study used E-responsive breast cancer cell lines which express functional AhR and ER␣ as models for studying the mechanisms of AhR-dependent degradation of ER␣ through activation of proteasomes. Proteasome-dependent degradation of ER␣ is dependent on multiple factors, including ligand structure, cell context, and the form of ER␣ expressed (i.e., wild type, chimeric, or mutant) (2,11,29,52,54). For example, one study reported that ligand-mediated downregulation of ER␣ is linked to coactivator recruitment and coordinate degradation of both receptor and coactivator proteins in HeLa cells transfected with ER␣ or mutant ER␣ expression plasmids (29).…”

Calmodulin, a regulatory partner of the estrogen receptor alpha in breast cancer cells

“…Studies conducted in various laboratories have demonstrated the implication of the ubiquitin-proteasome system (UPS) in the elimination of the mature receptor which has achieved transactivation and has become functionally obsolete (Alarid et al, 1999;El Khissiin, Leclercq, 1999;Laïos et al, 2005;Nawaz et al, 1999a;Wijayaratne, McDonnell, 2001). Natural and pharmacological ligands either accelerate (estrogens, pure antiestrogens) or reduce (partial antiestrogens) the proteasomal degradation of ERα (Laïos et al, 2005;Read et al, 1989;Wijayaratne, McDonnell, 2001) by selecting UPS with high or low degradation rate.…”

Effect of nuclear export inhibition on estrogen receptor regulation in breast cancer cells

“…MCF-7, IBEP-2), the steady-state level of the receptor is mostly determined by modulation of its degradation rate , Laïos et al 2003. As shown by studies employing specific inhibitors (El Khissiin & Leclercq 1999;Alarid et al 1999Alarid et al , 2006Lonard et al 2000;Journé et al 2004;Laïos et al 2005), ER breakdown involves the ubiquitinproteasome pathway. Modalities of ER degradation can, however, vary according to the receptor conformation, which is itself determined by the ligand-binding status and/or interactions with partner proteins , Tateishi et al 2004.…”