Ligand-specific regulation of proteasome-mediated proteolysis of estrogen receptor-α

Abstract

Ligand-specific regulation of proteasome-mediated proteolysis of estrogen receptor-␣. Am J Physiol Endocrinol Metab 282: E891-E898, 2002. First published December 18, 2001 10.1152/ajpendo.00353.2001.-Proteasome-mediated proteolysis modulates the cellular concentration of estrogen receptor-␣ (ER␣) and is induced by treatment of cells with 17␤-estradiol. Herein, we show that multiple receptor agonists, including 17␣-estradiol and estriol as well as the antagonist ICI-182780, stimulate proteasome-dependent proteolysis of ER␣ in a process that requires ligand binding to the receptor. Proteolysis of receptor depends on ligand concentration, and there exists a direct correlation between ligand-binding affinity and the half-maximal dose of ligand required to stimulate receptor degradation. Furthermore, introduction of a point mutation into the receptor ligand-binding pocket yields a stable receptor resistant to proteolysis. Interestingly, although all ligands stimulate receptor degradation, the extent to which overall ER levels are affected varies with each ligand and is not related to ligand-binding affinity or activation of transcription. These results demonstrate ligand-specific regulation of ER␣ proteolysis, and they introduce the concept that cellular receptor concentration is governed not only at the level of induction of proteolysis but also by the efficiency with which the receptor is degraded. steroid; nuclear receptor; antagonist; estriol; pituitary CELLULAR ESTROGEN RECEPTOR LEVELS are dynamic and are particularly sensitive to changes in circulating levels of 17␤-estradiol. It has been demonstrated through a number of studies that the decline in estrogen receptor-␣ (ER␣) upon exposure to 17␤-estradiol results from a combination of mechanisms that control both receptor synthesis and degradation through transcriptional, posttranscriptional, and posttranslational mechanisms (20,27,32,33,35,36). The most rapid of these regulatory mechanisms is the direct loss of ER␣ protein brought about by the induction of proteasome-mediated proteolysis (1,21,28).Regulated proteolysis by proteasomes accounts for the turnover of most short-lived proteins, including many nuclear receptors (9,16,23,29,41,43,45). Through a series of three enzymatic reactions, ubiquitin moieties are attached to a protein substrate, which targets it to the 26S proteasome. The molecular events that direct ER␣ into this pathway have not been clearly established. However, earlier studies that examine changes in receptor-binding capacity have shown that receptor levels can be controlled by both receptor agonists and antagonists, suggesting the possibility that receptor occupation by ligand may provide specificity (3,4,15,18,22,33).In our original report of ER␣ protein regulation by proteolysis, we utilized a pituitary lactotrope model system, the PR1 cell line. The lactotrope cell population of the anterior pituitary is a major target of estrogen action. Animals that lack ER␣ show a decrease in lactotrope cell density and prolactin expression (37). In ...

Estradiol Binding to Maxi-K Channels Induces Their Down-regulation via Proteasomal Degradation

“…4G). Estradiol-mediated down-regulation of maxi-K channel membrane expression may indicate that proteasome-dependent proteolytic degradation may be responsible, since this mechanism is involved in down-regulation of the ligand-activated ER␣ (24). According to this hypothesis, inhibition of the proteasome complex would stabilize the maxi-K channel protein on the cell membrane.…”

“…Ligand binding is required for the estradiol receptor to be targeted to, and degraded by, proteasomes (24,27). It has also been reported that the estrogen sensitivity of the maxi-K channel is conferred by its accessory ␤1-subunit (14,15).…”

Differentiation of murine embryonic stem cells induces progesterone receptor gene expression

“…The expression of ERα was higher in the E 2 and OHT treated cells, and these ligands have been reported to stabilize ERα protein in other systems [45]. We saw a dramatic loss of ERα protein upon ICI treatment, and this ligand has been reported to promote rapid degradation of the receptor in other systems [46,47]. A non-specific protein band (NS) was used as a loading control for mES cell samples.…”

Ubiquitin and breast cancer

“…Treatment of ER a-transfected HeLa cells with the proteasome inhibitor, MG132, stabilized ER a levels but impaired ER a-mediated transcription (Lonard et al, 2000). Mutations in helix 12 of ER a, the critical core of the AF-2 function of the receptor that interacts with transcriptional coactivators such as SRC-1, abolished ligand-mediated degradation and transactivation of the receptor (Lonard et al, 2000;Preisler-Mashek et al, 2002). Interestingly, some receptor antagonists including tomoxifen, which is an antiestrogen used in breast cancer therapy, stimulate proteasome-dependent proteolysis of ER a (Preisler-Mashek et al, 2002, Pearce et al, 2003.…”

“…Mutations in helix 12 of ER a, the critical core of the AF-2 function of the receptor that interacts with transcriptional coactivators such as SRC-1, abolished ligand-mediated degradation and transactivation of the receptor (Lonard et al, 2000;Preisler-Mashek et al, 2002). Interestingly, some receptor antagonists including tomoxifen, which is an antiestrogen used in breast cancer therapy, stimulate proteasome-dependent proteolysis of ER a (Preisler-Mashek et al, 2002, Pearce et al, 2003. It is likely that the antiestrogen effect of such ligand antagonists is, at least in part, mediated by the modification of ubiquitin-proteasome-dependent degradation of ER a.…”