Estrogen-Dependent Gene Transcription in Human Breast Cancer Cells Relies upon Proteasome-Dependent Monoubiquitination of Histone H2B

Abstract

The estrogen receptor-a (ERa) determines the phenotype of breast cancers where it serves as a positive prognostic indicator. ERa is a well-established target for breast cancer therapy, but strategies to target its function remain of interest to address therapeutic resistance and further improve treatment. Recent findings indicate that proteasome inhibition can regulate estrogen-induced transcription, but how ERa function might be regulated was uncertain. In this study, we investigated the transcriptome-wide effects of the proteasome inhibitor bortezomib on estrogen-regulated transcription in MCF7 human breast cancer cells and showed that bortezomib caused a specific global decrease in estrogen-induced gene expression. This effect was specific because gene expression induced by the glucocorticoid receptor was unaffected by bortezomib. Surprisingly, we observed no changes in ERa recruitment or assembly of its transcriptional activation complex on ERa target genes. Instead, we found that proteasome inhibition caused a global decrease in histone H2B monoubiquitination (H2Bub1), leading to transcriptional elongation defects on estrogen target genes and to decreased chromatin dynamics overall. In confirming the functional significance of this link, we showed that RNA interference-mediated knockdown of the H2B ubiquitin ligase RNF40 decreased ERa-induced gene transcription. Surprisingly, RNF40 knockdown also supported estrogen-independent cell proliferation and activation of cell survival signaling pathways. Most importantly, we found that H2Bub1 levels decrease during tumor progression. H2Bub1 was abundant in normal mammary epithelium and benign breast tumors but absent in most malignant and metastatic breast cancers. Taken together, our findings show how ERa activity is blunted by bortezomib treatment as a result of reducing the downstream ubiquitin-dependent function of H2Bub1. In supporting a tumor suppressor role for H2Bub1 in breast cancer, our findings offer a rational basis to pursue H2Bub1-based therapies for future management of breast cancer. Cancer Res; 71(17); 5739-53. Ó2011 AACR.

Ctr9, a key subunit of PAFc, affects global estrogen signaling and drives ERα-positive breast tumorigenesis

“…The fact that Ctr9-dependent expression of genes is involved in actin organization is consistent with a recent report that a PAF1-ERK5-ERα complex plays a role in actin reorganization via regulation of a transcriptional program of ERα in breast cancer (Madak-Erdogan et al 2014). PAFc has been previously shown to facilitate transcription elongation and transcription-coupled histone modifications (Tomson and Arndt 2013), including H2Bub1, which is required for ERα-regulated gene transcription (Prenzel et al 2011). Consistent with these reports, we demonstrated that Ctr9 promotes E2-stimulated transcription initiation by stimulating recruitment of ERα and RNAPII to target promoters and promoting downstream H3K27ac.…”

Histone H2B monoubiquitination: roles to play in human malignancy

“…Notably, CDC73 was shown to be WT in benign parathyroid tumours where H2Bub1 levels were maintained (Hahn et al 2012). Furthermore, in breast cancer, basal levels of H2Bub1 were found to be unchanged when comparing normal mammary epithelium with benign breast tumours; however, H2Bub1 levels in malignant and metastatic breast cancer cells were found to be significantly reduced (Prenzel et al 2011). The mechanism of H2Bub1 loss in malignant and metastatic breast cancer, lung cancer and colorectal cancer cells remains to be elucidated.…”

“…Identification of H2Bub1 as a key transcriptional regulator raised the likelihood that it may be altered in cancer development and the possibility that H2Bub1 itself may have tumour-suppressive roles (Espinosa 2008). Loss of global H2Bub1 detected by immunohistochemical staining has been reported for a number of cancers, including breast (Prenzel et al 2011), colorectal (Urasaki et al 2012), lung (Urasaki et al 2012) and parathyroid (Hahn et al 2012). In the case of parathyroid cancer, a mechanistic explanation for the loss of H2Bub1 is provided by the frequent occurrence of mutations in the tumour suppressor CDC73, leading to the disruption of the RNA polymerase II-associated factor 1 (PAF1) transcriptional complex that is important for the regulation of H2Bub1 (Hahn et al 2012).…”

“…H2Bub1 also has roles in the maintenance of stem cell multipotency (Karpiuk et al 2012), regulation of DNA replication (Trujillo & Osley 2012) and maintenance of centromeric chromatin, revealing significant roles in genome stability (Sadeghi et al 2014). Recently, H2Bub1 has been shown to be lost in advanced cancers (Prenzel et al 2011, Hahn et al 2012, Urasaki et al 2012. This review draws together discoveries related to H2Bub1, including those in the area of H2Bub1-associated ubiquitin ligases and deubiquitinases (DUBs) that may offer new opportunities for the design of epigenomic-based cancer therapeutics.…”

Histone H2B Monoubiquitination Mediated by HISTONE MONOUBIQUITINATION1 and HISTONE MONOUBIQUITINATION2 Is Involved in Anther Development by Regulating Tapetum Degradation-Related Genes in Rice

“…Depletion of RNF20 results in a decrease in p53 expression and an increase in cell migration and tumorigenesis as well as an increase in expression of c-myc, a proto-oncogene . A tumorsuppressive role for RNF40 mediated through H2Bub1 has also been demonstrated in breast cancer cells (Prenzel et al, 2011).…”