Greenebaum Cancer Center, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
Breast Cancer Res Treat. 2011 Feb;126(1):27-36. doi: 10.1007/s10549-010-0873-4. Epub 2010 Apr 9.
The ErbB2/3 heterodimer plays a critical role in breast cancer progression and in the development of endocrine resistance. EBP1, an ErbB3 binding protein, inhibits HRG-stimulated breast cancer growth, decreases ErbB2 protein levels and contributes to tamoxifen sensitivity. We report here that ectopic expression of EBP1 in Estrogen Receptor (ER) positive breast cancers that express ErbB2 at both high and low levels decreased ErbB2 protein levels. ErbB2 protein expression was also increased in mammary glands of Ebp1 knock out mice. To define the mechanism of ErbB2 down regulation, we examined the effects of EBP1 on ErbB2 mRNA levels, transcription of the ErbB2 gene and ErbB2 protein stability. We found that ectopic expression of EBP1 decreased steady state levels of endogenous ErbB2 mRNA in all cell lines tested. EBP1 overexpression decreased the activity of an ErbB2 promoter reporter in cells which overxpress ErbB2. However, reporter activity was unchanged or increased in cells which express low endogenous levels of ErbB2. We also found that ectopic expression of EBP1 accelerated ErbB2 protein degradation and enhanced ErbB2 ubiquitination in cells which express both low and high levels of ErbB2. Treatment with proteasome inhibitors prevented this decrease in ErbB2 protein levels. Ablation of EBP1 expression led to tamoxifen resistance that was abrogated by inhibition of ErbB2 activity. These results suggest that EBP1 inhibits expression of ErbB2 protein levels by multiple mechanisms and that EBP1's effects on tamoxifen sensitivity are mediated in part by its ability to modulate ErbB2 levels.
ErbB2/3 异二聚体在乳腺癌的进展和内分泌抵抗的发展中起着关键作用。EBP1 是一种 ErbB3 结合蛋白，它抑制 HRG 刺激的乳腺癌生长，降低 ErbB2 蛋白水平，并有助于他莫昔芬的敏感性。我们在这里报告，在高表达和低表达 ErbB2 的雌激素受体（ER）阳性乳腺癌中异位表达 EBP1，可降低 ErbB2 蛋白水平。Ebp1 敲除小鼠的乳腺中 ErbB2 蛋白表达也增加。为了确定 ErbB2 下调的机制，我们研究了 EBP1 对 ErbB2 mRNA 水平、ErbB2 基因转录和 ErbB2 蛋白稳定性的影响。我们发现，在所有测试的细胞系中，异位表达 EBP1 降低了内源性 ErbB2 mRNA 的稳定水平。EBP1 过表达降低了过表达 ErbB2 的细胞中 ErbB2 启动子报告基因的活性。然而，在表达低内源性 ErbB2 水平的细胞中，报告基因活性不变或增加。我们还发现，在表达低水平和高水平 ErbB2 的细胞中，异位表达 EBP1 加速了 ErbB2 蛋白降解并增强了 ErbB2 的泛素化。用蛋白酶体抑制剂处理可防止 ErbB2 蛋白水平的降低。EBP1 表达的缺失导致他莫昔芬耐药，而 ErbB2 活性的抑制可消除这种耐药性。这些结果表明，EBP1 通过多种机制抑制 ErbB2 蛋白水平的表达，EBP1 对他莫昔芬敏感性的影响部分是通过其调节 ErbB2 水平的能力介导的。
