Department of Biomedical Sciences, Cornell University, Ithaca, New York 14853, USA.
Cancer Res. 2011 Mar 15;71(6):2371-80. doi: 10.1158/0008-5472.CAN-10-2299. Epub 2011 Jan 24.
Retinoic acid (RA) is used to treat leukemia and other cancers through its ability to promote cancer cell differentiation. Strategies to enhance the anticancer effects of RA could deepen and broaden its beneficial therapeutic applications. In this study, we describe a receptor cross-talk system that addresses this issue. RA effects are mediated by RAR/RXR receptors that we show are modified by interactions with the aryl hydrocarbon receptor (AhR), a protein functioning both as a transcription factor and a ligand-dependent adaptor in an ubiquitin ligase complex. RAR/RXR and AhR pathways cross-talk at the levels of ligand-receptor and also receptor-promoter interactions. Here, we assessed the role of AhR during RA-induced differentiation and a hypothesized convergence at Oct4, a transcription factor believed to maintain stem cell characteristics. RA upregulated AhR and downregulated Oct4 during differentiation of HL-60 promyelocytic leukemia cells. AhR overexpression in stable transfectants downregulated Oct4 and also decreased ALDH1 activity, another stem cell-associated factor, enhancing RA-induced differentiation as indicated by cell differentiation markers associated with early (CD38 and CD11b) and late (neutrophilic respiratory burst) responses. AhR overexpression also increased levels of activated Raf1, which is known to help propel RA-induced differentiation. RNA interference-mediated knockdown of Oct4 enhanced RA-induced differentiation and G(0) cell-cycle arrest relative to parental cells. Consistent with the hypothesized importance of Oct4 downregulation for differentiation, parental cells rendered resistant to RA by biweekly high RA exposure displayed elevated Oct4 levels that failed to be downregulated. Together, our results suggested that therapeutic effects of RA-induced leukemia differentiation depend on AhR and its ability to downregulate the stem cell factor Oct4.
维甲酸（RA）通过促进癌细胞分化来治疗白血病和其他癌症。增强 RA 的抗癌作用的策略可以加深和扩大其有益的治疗应用。在这项研究中，我们描述了一个受体交叉对话系统，解决了这个问题。RA 效应是由 RAR/RXR 受体介导的，我们发现这些受体通过与芳香烃受体（AhR）相互作用而被修饰，AhR 既是转录因子，又是泛素连接酶复合物中的配体依赖性衔接子。RAR/RXR 和 AhR 途径在配体-受体和受体-启动子相互作用水平上相互作用。在这里，我们评估了 AhR 在 RA 诱导分化过程中的作用和在 Oct4 中的假定收敛，Oct4 被认为是维持干细胞特征的转录因子。RA 在 HL-60 早幼粒细胞白血病细胞分化过程中上调 AhR 并下调 Oct4。在稳定转染细胞中过表达 AhR 下调 Oct4 并降低 ALDH1 活性，另一种与干细胞相关的因子，如与早期（CD38 和 CD11b）和晚期（嗜中性粒细胞呼吸爆发）反应相关的细胞分化标志物所示，增强 RA 诱导的分化。AhR 过表达还增加了已知有助于推动 RA 诱导分化的活化 Raf1 的水平。与假设的 Oct4 下调对分化的重要性一致，通过每周两次高 RA 暴露使亲本细胞对 RA 产生抗性的细胞显示出升高的 Oct4 水平，无法下调。总之，我们的结果表明，RA 诱导的白血病分化的治疗效果取决于 AhR 及其下调干细胞因子 Oct4 的能力。
