Estrogen Receptor-Dependent Proteasomal Degradation of the Glucocorticoid Receptor Is Coupled to an Increase in Mdm2 Protein Expression

Abstract

Glucocorticoids and estrogens regulate a number of vital physiological processes. We developed a model breast cancer cell line, MCF-7 M, to examine potential mechanisms by which the ligand-bound estrogen receptor (ER) regulates glucocorticoid receptor (GR)-mediated transcription. MCF-7 cells, which endogenously express ER␣, were stably transfected with mouse mammary tumor virus promoter-luciferase (MMTV-LUC) reporter and GR expression constructs. Our results demonstrate that treatment with estrogen agonists (17␤-estradiol [E2], diethylstilbestrol, genistein), but not antagonists (tamoxifen or raloxifene), for 48 h inhibits GR-mediated MMTV-LUC transcription and chromatin remodeling. Furthermore, estrogen agonists inhibit glucocorticoid induction of p21 mRNA and protein levels, suggesting that the repressive effect applies to other GR-regulated genes and proteins in MCF-7 cells. Importantly, GR transcriptional activity is compromised because treatment with estrogen agonists down regulates GR protein levels. The protein synthesis inhibitor cycloheximide and the proteasome inhibitor MG132 block E2-mediated decrease in GR protein levels, suggesting that estrogen agonists down regulate the GR via the proteasomal degradation pathway. In support of this, we demonstrate that E2-mediated GR degradation is coupled to an increase in p53 and its key regulator protein Mdm2 (murine double minute 2), an E3 ubiquitin ligase shown to target the GR for degradation. Using the chromatin immunoprecipitation assay, we demonstrate an E2-dependent recruitment of ER␣ to the Mdm2 promoter, suggesting a role of ER in the regulation of Mdm2 protein expression and hence the enhanced GR degradation in the presence of estrogen agonists. Our study shows that cross talk between the GR and ER involves multiple signaling pathways, indicative of the mechanistic diversity within steroid receptor-regulated transcription.Physiological and therapeutic activities of glucocorticoids and estrogens are mediated by the glucocorticoid receptor (GR) and estrogen receptor (ER), respectively. As mediators of glucocorticoid and estrogenic hormones, the GR and ER play a critical role in a diverse array of physiological processes, including metabolism, immunity, cell growth and proliferation, reproduction, and development (12, 58). Both GR and ER exert important actions in tissues other than their primary target tissues. In tissues that express both receptors, glucocorticoids often oppose the actions of estrogens. For example, in the mammary gland, glucocorticoids exert antiproliferative effects, whereas estrogens promote cell growth and proliferation (69, 79). In contrast, in bone, glucocorticoids induce bone resorption (57), whereas estrogens inhibit this action (23). Although glucocorticoids and estrogens act within the same cellular context in these biological processes, little is known about the cross talk between the GR and ER signaling pathways.The GR and ER are both members of the steroid hormone receptor superfamily of nuclear receptors that includes the...

The P53 pathway: what questions remain to be explored?

“…The MDM-2 gene, can under certain circumstances be regulated by the estrogen receptor. 74 The IGF-1 pathway (which interfaces with the p53 pathway via the activation of MDM-2 by AKT-1) is also influenced by sexual hormones and sensory inputs. 39,40 The SIR-2 or SIRT-1 gene product, an NAD-dependent histone deacetylase, acts to remove acetyl groups from both the forkhead and the p53 proteins and can modulate the activity of the IGF-1 pathway.…”

A single nucleotide polymorphism in the p53 pathway interacts with gender, environmental stresses and tumor genetics to influence cancer in humans

“…Subsequently, the expression of ER-a was shown to induce transcription of MDM2. The ERdependent increase of MDM2 transcription was shown to be mediated, at least in part, through the region of the MDM2 promoter where the SNP309 locus resides (Okumura et al, 2002;Kinyamu and Archer, 2003;Phelps et al, 2003). In fact, the ER was shown to bind to this region of the promoter in vivo (Kinyamu and Archer, 2003).…”

“…The ERdependent increase of MDM2 transcription was shown to be mediated, at least in part, through the region of the MDM2 promoter where the SNP309 locus resides (Okumura et al, 2002;Kinyamu and Archer, 2003;Phelps et al, 2003). In fact, the ER was shown to bind to this region of the promoter in vivo (Kinyamu and Archer, 2003). Given that ER binds to the same region of the MDM2 promoter that harbors the SNP309 locus, and that the G-allele of SNP309 was shown to alter the affinity of a well-characterized co-transcriptional activator for multiple hormone receptors, including ER, namely Sp1 (Khan et al, 2003;Petz et al, 2004;Stoner et al, 2004), a recent report reasoned that the estrogensignaling pathway could affect how the SNP309 locus influences MDM2 transcription, and, therefore, tumor formation in humans (Bond et al, 2006a).…”

Progressive and Inhibitory Cell Cycle Proteins Act Simultaneously to Regulate Neurotrophin-mediated Proliferation and Maturation of Neuronal Precursors

“…[57][58][59] In addition, several recent reports have linked inhibition of protein synthesis with reductions in protein degradation. [60][61][62][63][64] To determine whether this was the case in our system, we examined the effects of two inhibitors of the 26S proteosome, Z-Leu-Leu-Leu-H 1 and Z-Leu-Leu-Nva-H 2, on cyclin D1 and p21 levels in response to BDNF, NGF, CNP, or CNP with neurotrophin (Fig. 8A, B, C and D).…”