5-fluorouracil (5-FU) is widely used in chemotherapy for colorectal cancer (CRC), but a high rate of chemoresistance reduces its effectiveness in clinical treatment. We found remarkably decreased expression of forkhead box 3 (FoxO3) protein, a tumor inhibitor, in 5-FU-resistant SW620 and HCT-8 (SW620/5-FU and HCT-8/5-FU) cells. Moreover, FoxO3 overexpression sensitized SW620/5-FU and HCT-8/5-FU cells to 5-FU. Mechanistically, FoxO3 inhibited the nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway by directly binding to Keap1 promoter. Thioredoxin reductase 1 (TR1), a pivotal target gene of Nrf2, was observed to promote 5-FU resistance by reducing intracellular ROS levels. Clinical data also revealed that significant upregulation of TR1 was associated with poor outcome in CRC patients. Auranofin (AUR), a FoxO3 agonist and TR1 inhibitor, enhanced the sensitivity of HCT-8/5-FU and SW620/5-FU cells to 5-FU in vitro and in vivo. Taken together, our results suggest that FoxO3 could reverse 5-FU resistance in CRC via inhibiting the Nrf2/TR1 signaling pathway, and increasing the level of intracellular reactive oxygen species. Chemotherapeutic agents targeting FoxO3 and/or TR1, including AUR, might be promising adjuvant sensitizers to reverse chemoresistance in 5-FU-resistant CRC.

中文翻译：

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FoxO3通过抑制Nrf2 / TR1信号通路逆转人结肠直肠癌细胞中的5-氟尿嘧啶耐药性。

5-氟尿嘧啶（5-FU）已广泛用于结直肠癌（CRC）的化学疗法中，但是较高的化学抗药性降低了其在临床治疗中的有效性。我们发现在5-FU耐药的SW620和HCT-8（SW620 / 5-FU和HCT-8 / 5-FU）细胞中，前叉3（FoxO3）蛋白（一种肿瘤抑制剂）的表达显着降低。此外，FoxO3过表达使SW620 / 5-FU和HCT-8 / 5-FU细胞对5-FU敏感。从机制上讲，FoxO3通过直接与Keap1启动子结合来抑制核因子红系2相关因子2（Nrf2）信号传导途径。硫氧还蛋白还原酶1（TR1），Nrf2的关键目标基因，通过降低细胞内ROS水平促进5-FU耐药。临床数据还显示，CRC患者TR1的显着上调与不良预后相关。金诺芬（AUR），FoxO3激动剂和TR1抑制剂可在体外和体内增强HCT-8 / 5-FU和SW620 / 5-FU细胞对5-FU的敏感性。两者合计，我们的结果表明FoxO3可以通过抑制Nrf2 / TR1信号通路并增加细胞内活性氧的水平来逆转CRC对5-FU的耐药性。针对FoxO3和/或TR1的化学治疗剂（包括AUR）可能是有希望的佐剂敏化剂，可逆转对5-FU耐药的CRC的化学耐药性。