Department of Molecular Oncology, Göttingen Center for Molecular Biosciences, Department of Medical Statistics, University Medical Center Göttingen, Germany.
Cancer Res. 2011 Sep 1;71(17):5739-53. doi: 10.1158/0008-5472.CAN-11-1896. Epub 2011 Aug 23.
The estrogen receptor-α (ERα) determines the phenotype of breast cancers where it serves as a positive prognostic indicator. ERα 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 ERα 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 ERα recruitment or assembly of its transcriptional activation complex on ERα 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 ERα-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 ERα 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.
雌激素受体-α（ERα）决定了乳腺癌的表型，它是一个阳性预后指标。ERα 是乳腺癌治疗的一个既定靶点，但靶向其功能的策略仍然是为了应对治疗抵抗并进一步改善治疗效果。最近的研究结果表明，蛋白酶体抑制可以调节雌激素诱导的转录，但是 ERα 功能如何被调节还不清楚。在这项研究中，我们研究了蛋白酶体抑制剂硼替佐米对 MCF7 人乳腺癌细胞中雌激素调节转录的全转录组效应，并表明硼替佐米导致雌激素诱导基因表达的特异性整体下降。这种效应是特异性的，因为糖皮质激素受体诱导的基因表达不受硼替佐米的影响。令人惊讶的是，我们没有观察到 ERα 募集或其转录激活复合物在 ERα 靶基因上的组装发生变化。相反，我们发现蛋白酶体抑制导致组蛋白 H2B 单泛素化（H2Bub1）的整体减少，导致雌激素靶基因上的转录延伸缺陷，并导致整体染色质动力学降低。在证实这一联系的功能意义时，我们表明，通过 RNA 干扰介导的 H2B 泛素连接酶 RNF40 的敲低降低了 ERα 诱导的基因转录。令人惊讶的是，RNF40 的敲低也支持了雌激素非依赖性细胞增殖和细胞存活信号通路的激活。最重要的是，我们发现 H2Bub1 水平在肿瘤进展过程中下降。H2Bub1 在正常乳腺上皮和良性乳腺肿瘤中丰富，但在大多数恶性和转移性乳腺癌中不存在。总之，我们的研究结果表明，硼替佐米治疗如何通过降低 H2Bub1 依赖泛素的下游功能来削弱 ERα 活性。在支持 H2Bub1 在乳腺癌中作为肿瘤抑制因子的作用时，我们的研究结果为基于 H2Bub1 的治疗提供了合理的依据，以用于未来乳腺癌的管理。
