Indole-3-carbinol and its N-alkoxy derivatives preferentially target ERα-positive breast cancer cells

Indole-3-carbinol (I3C) is a natural anti-carcinogenic compound found at high concentrations in Brassica vegetables. I3C was recently reported to inhibit neutrophil elastase (NE) activity, while consequently limiting the proteolytic processing of full length cyclin E into pro-tumorigenic low molecular weight cyclin E (LMW-E). In this study, we hypothesized that inhibition of NE activity and resultant LMW-E generation is critical to the anti-tumor effects of I3C. LMW-E was predominately expressed by ERα-negative breast cancer cell lines. However, ERα-positive cell lines demonstrated the greatest sensitivity to the anti-tumor effects of I3C and its more potent N-alkoxy derivatives. We found that I3C was incapable of inhibiting NE activity or the generation of LMW-E. Therefore, this pathway did not contribute to the anti-tumor activity of I3C. Gene expression analyzes identified ligand-activated aryl hydrocarbon receptor (AhR), which mediated sensitivity to the anti-tumor effects of I3C in ERα-positive MCF-7 cells. In this model system, the reactive oxygen species (ROS)-induced upregulation of ATF-3 and pro-apoptotic BH3-only proteins (e.g. NOXA) contributed to the sensitivity of ERα-positive breast cancer cells to the anti-tumor effects of I3C. Overexpression of ERα in MDA-MB-231 cells, which normally lack ERα expression, increased sensitivity to the anti-tumor effects of I3C, demonstrating a direct role for ERα in mediating the sensitivity of breast cancer cell lines to I3C. Our results suggest that ERα signaling amplified the pro-apoptotic effect of I3C-induced AhR signaling in luminal breast cancer cell lines, which was mediated in part through oxidative stress induced upregulation of ATF-3 and downstream BH3-only proteins.

吲哚-3-羧酸（I3C）是一种天然的抗肿瘤化合物，广泛存在于十字花科蔬菜中。近期研究表明，I3C能够抑制中性粒细胞弹性蛋白酶（NE）的活性，进而限制全长细胞周期蛋白E向促肿瘤低分子量细胞周期蛋白E（LMW-E）的蛋白水解加工。本研究中，我们假设NE活性的抑制及随之产生的LMW-E生成对于I3C的抗肿瘤效应至关重要。LMW-E主要由ERα阴性乳腺癌细胞系表达。然而，ERα阳性细胞系对I3C及其更强效的N-烷氧基衍生物的抗肿瘤效应最为敏感。我们发现I3C无法抑制NE活性或LMW-E的生成，因此，此途径并未对I3C的抗肿瘤活性做出贡献。基因表达分析鉴定出配体激活的芳烃受体（AhR），该受体介导了ERα阳性MCF-7细胞对I3C抗肿瘤效应的敏感性。在此模型系统中，活性氧（ROS）诱导的ATF-3和促凋亡BH3-only蛋白（例如NOXA）的上调，共同导致了ERα阳性乳腺癌细胞对I3C抗肿瘤效应的敏感性。在通常缺乏ERα表达的MDA-MB-231细胞中过表达ERα，增加了其对I3C抗肿瘤效应的敏感性，这表明ERα在调节乳腺癌细胞系对I3C的敏感性中发挥了直接作用。我们的结果表明，ERα信号放大了I3C诱导的AhR信号在管腔型乳腺癌细胞系中的促凋亡效应，部分是通过氧化应激诱导的ATF-3的上调和下游BH3-only蛋白的下游调节实现的。