Samant Rajeev S, Clark David W, Fillmore Rebecca A, Cicek Muzaffer, Metge Brandon J, Chandramouli Kondethimmana H, Chambers Ann F, Casey Graham, Welch Danny R, Shevde Lalita A
Mitchell Cancer Institute, University of South Alabama, Mobile, Alabama, USA.
Mol Cancer. 2007 Jan 16;6:6. doi: 10.1186/1476-4598-6-6.
Osteopontin (OPN), a secreted phosphoglycoprotein, has been strongly associated with tumor progression and aggressive cancers. MDA-MB-435 cells secrete very high levels of OPN. However metastasis-suppressed MDA-MB-435 cells, which were transfected with breast cancer metastasis suppressor 1 (BRMS1), expressed significantly less OPN. BRMS1 is a member of mSin3-HDAC transcription co-repressor complex and has been shown to suppress the metastasis of breast cancer and melanoma cells in animal models. Hence we hypothesized that BRMS1 regulates OPN expression.
The search for a BRMS1-regulated site on the OPN promoter, using luciferase reporter assays of the promoter deletions, identified a novel NF-kappaB site (OPN/NF-kappaB). Electrophoretic mobility shift assays and chromatin immunoprecipitations (ChIP) confirmed this site to be an NF-kappaB-binding site. We also show a role of HDAC3 in suppression of OPN via OPN/NF-kappaB.
Our results show that BRMS1 regulates OPN transcription by abrogating NF-kappaB activation. Thus, we identify OPN, a tumor-metastasis activator, as a crucial downstream target of BRMS1. Suppression of OPN may be one of the possible underlying mechanisms of BRMS1-dependent suppression of tumor metastasis.
骨桥蛋白（OPN）是一种分泌型磷酸糖蛋白，与肿瘤进展和侵袭性癌症密切相关。MDA-MB-435细胞分泌高水平的OPN。然而，用乳腺癌转移抑制因子1（BRMS1）转染的转移抑制型MDA-MB-435细胞表达的OPN明显减少。BRMS1是mSin3-HDAC转录共抑制复合物的成员，在动物模型中已显示可抑制乳腺癌和黑色素瘤细胞的转移。因此，我们推测BRMS1调节OPN的表达。
通过对启动子缺失进行荧光素酶报告基因检测来寻找OPN启动子上BRMS1调节的位点，确定了一个新的核因子κB位点（OPN/NF-κB）。电泳迁移率变动分析和染色质免疫沉淀（ChIP）证实该位点是一个核因子κB结合位点。我们还显示了HDAC3通过OPN/NF-κB在抑制OPN中的作用。
我们的结果表明，BRMS1通过消除核因子κB的激活来调节OPN转录。因此，我们确定肿瘤转移激活因子OPN是BRMS1的关键下游靶点。抑制OPN可能是BRMS1依赖性抑制肿瘤转移的潜在机制之一。
