This is an open access article under the terms of the Creat ive Commo ns Attri butio n-NonCo mmerc ial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. AbstractResistance to chemotherapy is a major challenge for the treatment of patients with colorectal cancer (CRC). Previous studies have found that microRNAs (miRNAs) play key roles in drug resistance; however, the role of miRNA-373-3p (miR-375-3p) in CRC remains unclear. The current study aimed to explore the potential function of miR-375-3p in 5-fluorouracil (5-FU) resistance. MicroRNA-375-3p was found to be widely downregulated in human CRC cell lines and tissues and to promote the sensitivity of CRC cells to 5-FU by inducing colon cancer cell apoptosis and cycle arrest and by inhibiting cell growth, migration, and invasion in vitro. Thymidylate synthase (TYMS) was found to be a direct target of miR-375-3p, and TYMS knockdown exerted similar effects as miR-375-3p overexpression on the CRC cellular response to 5-FU. Lipidcoated calcium carbonate nanoparticles (NPs) were designed to cotransport 5-FU and miR-375-3p into cells efficiently and rapidly and to release the drugs in a weakly acidic tumor microenvironment. The therapeutic effect of combined miR-375 + 5-FU/NPs was significantly higher than that of the individual treatments in mouse s.c. xenografts derived from HCT116 cells. Our results suggest that restoring miR-375-3p levels could be a future novel therapeutic strategy to enhance chemosensitivity to 5-FU. K E Y W O R D S5-fluorouracil, chemosensitivity, colorectal cancer, miR-375-3p, nanoparticles | INTRODUC TI ONAlthough considerable progress has been made in the treatment of CRC in recent years, CRC accounts for approximately 13% of all tumors and is the second leading cause of tumor-related death in developed countries. 1-3 Fluorouracil-based chemotherapy has served as the first-line standard of care and most common regimen for CRC over the past 50 years. 4,5 However, patient resistance to 5-FU is a major obstacle to effective therapy. Therefore, efforts to clarify the molecular mechanism underlying 5-FU resistance and to identify new

Ferroptosis, a type of cell death triggered by excessive accumulation of iron-dependent lipid peroxidation, possesses an excellent potential in cancer treatment. However, many colorectal cancer (CRC) cell lines are resistant to ferroptosis induced by erastin and RSL3, the classical ferroptotic inducers. Moreover, the underlying mechanism of resistance remains poorly elucidated. This study sought to discover the major factor contributing to ferroptosis resistance in CRC. The study findings will help design strategies for triggering ferroptosis for application in individualized tumor therapy. Here, we show that tetrahydrobiopterin (BH4) determines the sensitivity of CRC cells to ferroptosis induced by erastin. GTP cyclohydrolase-1 (GCH1) is the first rate-limiting enzyme of BH4. Genetic or pharmacological inhibition of GCH1 decreased BH4 and assisted erastin in cell death induction, lipid peroxidation enhancement, and ferrous iron accumulation. BH4 supplementation completely inhibited ferroptotic features resulting from GCH1 knockdown. Unexpectedly, GCH1 knockdown failed to enhance RSL3-induced cell death in CRC. Mechanistically, GCH1 knockdown drastically activated ferritinophagy during erastin treatment rather than RSL3 treatment. Administration of an autophagy inhibitor reversed erastin resistance in GCH1-knockdown cells. GCH1 inhibitor and erastin co-treatment in vivo synergistically inhibited tumor growth in CRC. Overall, our results identified GCH1/BH4 metabolism as a burgeoning ferroptosis defense mechanism in CRC. Inhibiting GCH1/BH4 metabolism promoted erastin-induced ferroptosis by activating ferritinophagy, suggesting that combining GCH1 inhibitors with erastin in the treatment of CRC is a novel therapeutic strategy.

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