McMaster Stem Cell and Cancer Research Institute, McMaster University, Hamilton, Ontario, Canada.
Oncogene. 2012 Jan 12;31(2):187-99. doi: 10.1038/onc.2011.232. Epub 2011 Jun 20.
Bmi1 is a key stem cell regulatory gene implicated in the pathogenesis of many aggressive cancers, including medulloblastoma. Overexpression of Bmi1 promotes cell proliferation and is required for hedgehog (Hh) pathway-driven tumorigenesis. This study aimed to determine if Sonic hedgehog (Shh) modulates the key stem cell regulatory gene Bmi1 in childhood medulloblastoma brain tumor-initiating cells (BTICs). Although current literature suggests that there is a correlation between Shh pathway genes and Bmi1 expression, it is unclear whether there is indeed a direct regulatory mechanism. To address whether Shh induces expression of Bmi1, stem cell-enriched populations from medulloblastoma cell lines and primary samples were treated with Shh ligand and KAAD-cyclopamine (Shh antagonist). Our data indicate that Bmi1 expression positively correlates with increasing Shh ligand concentrations. Chromatin immunoprecipitation reveals that Gli1 preferentially binds to the Bmi1 promoter, and Bmi1 transcript levels are increased and decreased by Gli1 overexpression and downregulation, respectively. Knockdown experiments of Bmi1 in vitro and in vivo demonstrate that Hh signaling not only drives Bmi1 expression, but a feedback mechanism exists wherein downstream effectors of Bmi1 may, in turn, activate Hh pathway genes. These findings implicate Bmi1 and Hh as mutually indispensable pathways in medulloblastoma BTIC maintenance. Recent molecular characterization of medulloblastoma also reveals that Bmi1 is overexpressed across all subgroups of medulloblastoma, particularly in the most aggressive subtypes. Lastly, despite recent identification of BTIC markers, the molecular characterization of these cell populations remains unclear. In this work, we propose that the BTIC marker CD133 may segregate a cell population with a Hh-receptor phenotype, thus demonstrating a cell-cell interaction between the CD133+ Hh receptor cells and the CD133- Hh-secreting cells.
BMI1 是一种关键的干细胞调节基因，与许多侵袭性癌症的发病机制有关，包括髓母细胞瘤。BMI1 的过表达促进细胞增殖，是 Hedgehog（Hh）通路驱动肿瘤发生所必需的。本研究旨在确定 Sonic Hedgehog（Shh）是否在儿童髓母细胞瘤脑肿瘤起始细胞（BTIC）中调节关键的干细胞调节基因 BMI1。尽管目前的文献表明 Shh 通路基因与 BMI1 表达之间存在相关性，但尚不清楚是否存在直接的调节机制。为了确定 Shh 是否诱导 BMI1 的表达，我们用 Shh 配体和 KAAD-环巴胺（Shh 拮抗剂）处理来自髓母细胞瘤细胞系和原代样本的富含干细胞的群体。我们的数据表明，BMI1 表达与 Shh 配体浓度的增加呈正相关。染色质免疫沉淀揭示 Gli1 优先结合到 BMI1 启动子上，Gli1 的过表达和下调分别导致 BMI1 转录本水平的增加和减少。体外和体内的 BMI1 敲低实验表明，Hh 信号不仅驱动 BMI1 的表达，而且存在一个反馈机制，即 BMI1 的下游效应物反过来可能激活 Hh 通路基因。这些发现表明 BMI1 和 Hh 在髓母细胞瘤 BTIC 维持中是相互不可或缺的通路。最近对髓母细胞瘤的分子特征分析也表明，BMI1 在髓母细胞瘤的所有亚组中均过度表达，特别是在最具侵袭性的亚型中。最后，尽管最近确定了 BTIC 标记物，但这些细胞群体的分子特征仍然不清楚。在这项工作中，我们提出 BTIC 标记物 CD133 可能分离出具有 Hh 受体表型的细胞群体，从而证明了 CD133+ Hh 受体细胞和 CD133- Hh 分泌细胞之间存在细胞-细胞相互作用。
