The Proteasome Inhibitor Bortezomib Induces an Inhibitory Chromatin Environment at a Distal Enhancer of the Estrogen Receptor-α Gene

Abstract

Expression of the estrogen receptor-α (ERα) gene, ESR1, is a clinical biomarker used to predict therapeutic outcome of breast cancer. Hence, there is significant interest in understanding the mechanisms regulating ESR1 gene expression. Proteasome activity is increased in cancer and we previously showed that proteasome inhibition leads to loss of ESR1 gene expression in breast cancer cells. Expression of ESR1 mRNA in breast cancer cells is controlled predominantly through a proximal promoter within ∼400 base pair (bp) of the transcription start site (TSS). Here, we show that loss of ESR1 gene expression induced by the proteasome inhibitor bortezomib is associated with inactivation of a distal enhancer located 150 kilobases (kb) from the TSS. Chromatin immunoprecipitation assays reveal several bortezomib-induced changes at the distal site including decreased occupancy of three critical transcription factors, GATA3, FOXA1, and AP2γ. Bortezomib treatment also resulted in decreased histone H3 and H4 acetylation and decreased occupancy of histone acetyltransferase, p300. These data suggest a mechanism to explain proteasome inhibitor-induced loss of ESR1 mRNA expression that highlights the importance of the chromatin environment at the −150 kb distal enhancer in regulation of basal expression of ESR1 in breast cancer cells.

The proteasome inhibitor Bortezomib (Velcade) as potential inhibitor of estrogen receptor-positive breast cancer

“…Previously, we have observed that the proteasome inhibitor Bortezomib inhibits ERα expression, a finding also reported by others, suggesting that Bortezomib treatment may overcome resistance to anti‐endocrine therapy. In this present study, we have investigated this hypothesis, and have identified multiple ways in which Bortezomib treatment can perturb key signaling pathways that mediate resistance to anti‐endocrine therapy.…”

“…As Bortezomib is an inhibitor of NF‐kB, we propose that Bortezomib may activate cell death through reduced expression of anti‐apoptotic NF‐kB target genes (directly as well as via inhibition of Akt), and through activation of Foxo3A. These observations also provide a mechanistic explanation for the down‐regulation of ERα by Bortezomib, because Foxo3A inhibits ERα expression and recruits chromatin remodeling proteins such as histone deacetylases to promoters to which it binds, which may account for the observation that Bortezomib induces an inhibitory chromatin environment within the ERα gene . Thus, we suggest that Bortezomib‐mediated inhibition of Akt and subsequent activation of Foxo3A results in suppression of ERα expression.…”

“…9,10 Signaling via ERK, MAPK, PKA and the PI3K/Akt/mTOR/p70S6K pathways also plays a key role in hormone independence, again resulting in phosphorylation of ERa that modifies its affinity for coregulators and its transcriptional activity, leading to altered sensitivity to tamoxifen. [11][12][13][14][15][16][17] Previously, we have observed that the proteasome inhibitor Bortezomib inhibits ERa expression, 18 a finding also reported by others, [19][20][21] suggesting that Bortezomib treatment may overcome resistance to anti-endocrine therapy. In this present study, we have investigated this hypothesis, and have identified multiple ways in which Bortezomib treatment can perturb key signaling pathways that mediate resistance to anti-endocrine therapy.…”

Estrogen Receptors and Ubiquitin Proteasome System: Mutual Regulation

“…et al discovered that bortezomib induced a dramatic decrease in ERα mRNA in breast cancer cells, due to direct transcriptional inhibition and loss of RNA polymerase II recruitment on the ERα gene promoter [99]. The chronic inhibition of proteasomes (for 24 h or more) leads to an almost complete loss of ERα in cell culture [106]. The loss of ERα is the result of transcriptional repression of the ESR1 gene, as evidenced by a decrease in ESR1 mRNA.…”

Endocrine Resistance in Breast Cancer: The Role of Estrogen Receptor Stability

“…Several studies have shown increased ER stability in the presence of the proteasome inhibitor, MG132, indicating that degradation by the ubiquitin-proteasome system (UPS) regulates ER stability [47,[49][50][51]. Degradation of the ER occurs predominantly through the UPS, which relies on ER ubiquitylation by ubiquitin activating enzymes (E1) and ubiquitin conjugating enzymes (E2).…”