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.

Introduction: Cronkhite-Canada syndrome (CCS) is a rare non-inherited disease characterized by extensive gastrointestinal (GI) polyposis and ectodermal dysplasia. So far, most of CCS related literatures are published as single case report or reviewed with limited case numbers. Our study was to update the clinical and endoscopic characteristics of Chinese CCS patients. Methods: This retrospective study was conducted in 103 Chinese CCS patients (102 cases from literatures and 1 case from our department). Their clinical and endoscopic data were collected, and statistical analyses were performed. Results: 1) In Chinese population, people aged 50-70 years (62.62%) had a high incidence of CCS, and the ratio of male-to-female was 2.68:1. 2) The diverse range of GI manifestations were observed in all the patients, and almost all the patients had at least one symptom of ectodermal dysplasias. 3) All CCS patients presented multiple polyps in the GI tract except esophagus, and the size and appearance of polyps were diverse. Congestion, edema and erosion were very common on the surface of polyps (96.83%) and the surrounding mucosa (85.71%) . 4) The common pathological features of polyps were hyperplastic polyps (49.25%) and tubular adenomatous polyps (44.78%). There is 5.97% cancer reported. Conclusions: middle-aged and elderly people are the high-risk group; various GI symptoms are observed in Chinese patients; the typical endoscopic finding is multiple small sessile polyps; these GI polyps has a chance of malignant potential. Long-term endoscopic surveillance and follow-up are recommended for the Chinese CCS patients. Key words: Cronkhite-Canada syndrome, Clinical characteristics, Endoscopy

Background and aims Mucosal‐associated invariant T (MAIT) cells are innate‐like lymphocytes that display a critical role in various liver diseases. However, the role of MAIT cells in cholestatic liver fibrogenesis remains obscure. Our study aims to assess the contribution of MAIT cells and underlying mechanisms during this process. Methods Cholestatic murine models using MAIT cell‐deficient (MR1−/−) and wild‐type (WT) mice were established by feeding a 0.1% 3,5‐diethoxycarbonyl‐1,4‐dihydrocollidine (DDC)‐enriched diet or bile duct ligation (BDL). Liver samples were collected to determine the severity of fibrosis. Lymphocytes of the liver were isolated for analysing the phenotype and function of MAIT cells. Cell co‐culture experiments were performed to investigate the cross‐talk between MAIT and NK cells. Results Liver MAIT cells were more activated with increased cytokines in cholestatic mice models than in control mice, although their frequency was decreased. MAIT cell deficiency led to severe liver inflammation and fibrosis with more activated HSCs in cholestatic mice. In addition, MR1−/− mice had an increased frequency of NK cells with higher expression of stimulatory receptors relative to WT mice. Paradoxically, activated MAIT cells significantly promoted the anti‐fibrotic ability of NK cells by enhancing their cytotoxicity against HSCs in co‐culture experiments. Importantly, this effect depended on direct cell–cell contact and TNF‐α produced by MAIT cells. Conclusion Our findings indicate that MAIT cells ameliorate cholestatic liver fibrosis by enhancing the cytotoxicity of NK cells against HSCs. An in‐depth understanding of the MAIT cell‐mediated regulatory effect will provide more valuable immunotherapy strategies to treat liver fibrosis.