A Study of Lactylation-driven METTL3-mediated RNA m6A modification promotes immunosuppression of tumor-infiltrating myeloid cells. Jia Xiong et al

[Unprocessed images of blots and gels, a single PDF is deposited as long as it is clear which figure and panel they correspond to] Tumor-infiltrating myeloid cells (TIMs) are crucial cell populations involved in tumor immune escape, and their immunosuppressive functions are regulated by multiple epigenetic mechanisms. The precise regulation mode of RNA N6-methyladenosine (m6A) modification in controlling TIMs function is still poorly understood. Our study revealed that increased expression of RNA methyltransferase methyltransferase-like 3 (METTL3) in TIMs was correlated with the poor prognosis of colon cancer patients and myeloid deficiency of METTL3 attenuated the tumor growth in mice. METTL3 mediates m6A modification of phosphokinase janus kinase 1 (Jak1) mRNA in TIMs and m6A-YTH N6-methyladenosine RNA-binding protein 1 (YTHDF1) axis enhances JAK1 protein translation efficiency in polysomes, which significantly contributes to the enhanced phosphorylation of the signal transducer and activator of transcription 3 (STAT3). Lactate accumulated in the tumor microenvironment is a key factor that induces the upregulation of METTL3 expression in TIMs via the form of histone H3K18 lactylation. Very interestingly, we also identified two lactylation modification sites in the Cys-Cys-Cys-His (CCCH) zinc finger domain of METTL3 and this type of lysine lactylation is essential for the target recognition domain of METTL3 to capture target RNA. Our results emphasize the importance of lactylation-driven METTL3-mediated RNA m6A modification for regulating the function of TIMs and provide new insights for the discovery of potential targets for cancer immunotherapy.[未处理的免疫印迹与凝胶电泳原始图像，若可明确对应具体图表及子图，则可提交单个PDF文件] 肿瘤浸润髓系细胞（Tumor-infiltrating myeloid cells, TIMs）是参与肿瘤免疫逃逸的关键细胞群，其免疫抑制功能受多种表观遗传机制调控。目前，RNA N6-甲基腺嘌呤（RNA N6-methyladenosine, m6A）修饰调控TIMs功能的具体机制仍未明确。本研究发现，TIMs中RNA甲基转移酶甲基转移酶样3（methyltransferase-like 3, METTL3）的高表达与结肠癌患者不良预后相关，且髓系细胞特异性敲除METTL3可抑制小鼠体内肿瘤生长。METTL3可介导TIMs中蛋白激酶贾纳斯激酶1（Janus kinase 1, JAK1）mRNA的m6A修饰，而m6A-YTH N6-甲基腺嘌呤RNA结合蛋白1（YTH N6-methyladenosine RNA-binding protein 1, YTHDF1）信号轴可提高多聚核糖体中JAK1蛋白的翻译效率，进而显著促进信号转导与转录激活因子3（signal transducer and activator of transcription 3, STAT3）的磷酸化。肿瘤微环境中积累的乳酸通过组蛋白H3K18乳酸化（histone H3K18 lactylation）修饰途径，是诱导TIMs中METTL3表达上调的关键因素。尤为值得关注的是，本研究还在METTL3的CCCH型锌指结构域（Cys-Cys-Cys-His, CCCH zinc finger domain）中鉴定出两个乳酸化修饰位点，该类型的赖氨酸乳酸化修饰对METTL3的靶RNA识别结构域结合靶标RNA至关重要。本研究结果凸显了乳酸化驱动的METTL3介导的RNA m6A修饰在调控TIMs功能中的重要性，为肿瘤免疫治疗潜在靶点的发掘提供了新的研究视角。