巨噬细胞移动抑制因子（MIF）的互变异构酶和氧化还原酶活性对胰腺癌生长和转移的影响

Pancreatic cancer (PC) is one of the most common cancers worldwide and a leading cause of cancer-related death. Discovering novel targets is a key for its therapy. Recent studies have demonstrated that macrophage migration inhibitory factor (MIF) is highly expressed in PC cells and plays a key role in cancer growth and metastasis through at least two pathways: 1) direct involvement in PC cell proliferation and metastasis, and 2) promotion of pre-metastatic niche formation in the liver as a component of PC-derived exosomes which facilitate hepatic stellate cells to produce fibronectin that is critical for the recruitment of bone marrow-derived cells. In addition, host MIF appears to play a role in cancer growth and metastasis by reprogramming the tumor-associated macrophages to the immunosuppressive M2 phenotype. Thus, targeted inhibition of MIF may not only directly suppress tumor growth and metastasis, but also modulate immune cells, rendering them more tumor-suppressive. MIF possesses two enzymatic activities, the tautomerase activity determined by the first Pro residue (Pro-1) in the N-terminal domain and the thio-protein oxidoreductase activity determined by the Cys residue in the central domain of the protein (Cys-60). However, which enzymatic activity of MIF is involved in its cancer-promoting functions remains unclear. As one aim of this study, we will generate PC cell lines carrying the tautomerase-null mutant (MIF-P1G) and the oxidoreductase-null mutant (MIF-C60S), respectively, and will determine which enzymatic activity is critical for MIF’s cancer-promoting functions. The second aim of this study is to determine which enzymatic activity of the host MIF is critical for its involvement in cancer growth and metastasis by using MIF-P1G and MIF-C60S knockin mice. These studies will pave the way for the development of novel therapeutic approaches against PC if the expected results are obtained.

胰腺癌恶性度很高，开发新型药物靶点是攻克胰腺癌的关键。近年研究表明巨噬细胞移动抑制因子（MIF）在胰腺癌高表达，直接促进肿瘤生长侵袭；作为外泌体的重要成分，被肝脏枯否细胞摄取后促进星形细胞产生纤连蛋白，形成促进肿瘤转移的微环境。宿主细胞表达的MIF通过使肿瘤相关巨噬细胞极化成M2型，促进肿瘤生长。因此抑制MIF可能不仅抑制肿瘤生长转移，还能改善免疫细胞功能。MIF有互变异构酶和氧化还原酶两种活性，但何种酶活性决定其参与胰腺癌生长转移尚不清楚。本课题目标之一是通过构建互变异构酶和氧化还原酶活性缺失的基因敲入胰腺癌细胞系，阐明MIF何种酶活性决定其对胰腺癌细胞增殖转移的促进作用；目标之二是通过构建互变异构酶和氧化还原酶活性缺失的基因敲入小鼠，并构建动物模型，阐明MIF的何种酶活性决定宿主细胞MIF对胰腺癌生长转移的促进作用。如果达到预期目标，将为开发针对MIF的新型小分子靶向药物奠定坚实基础。

我们之前的研究表明巨噬细胞移动抑制因子（MIF）在胰腺癌高表达，促进肿瘤生长侵袭，但MIF及其受体在胰腺癌发展中的作用机制尚不完全清楚。在本课题中，我们发现胰腺癌外泌体可以诱导髓源性抑制细胞（MDSC）的形成，敲除外泌体中的MIF无法诱导单核系MDSC（mMDSC）形成，但对粒系MDSC（gMDSC）没有影响。MIF受体CD74主要由mMDSC而非gMDSC表达。CD74敲除可阻止脾脏和肿瘤组织中mMDSC形成并提高CD8+T细胞数量，抑制胰腺癌生长。在LysMCreCd74f/f而非Ly6GCreCd74f/f小鼠中，肿瘤生长受到抑制，且其脾脏和肿瘤组织中的mMDSC降低，CD8+T细胞增加。MIF互变异构酶抑制剂4-IPP抑制了肿瘤外泌体介导的mMDSC形成，并可阻止野生型荷瘤小鼠胰腺癌生长，使肿瘤组织中的mMDSC明显减少，而CD8+T细胞增加，表明MIF互变异构酶活性在这一过程中发挥了重要作用。同时我们还发现肿瘤组织和细胞的m6A去甲基化酶FTO、甲基转移酶METTL3和致癌基因MYC表达上调，FTO通过降低MYC的m6A甲基化来稳定MYC mRNA，从而增强MYC表达，促进肿瘤发展。而METTL3可以增强MYC m6A甲基化，进而提高MYC的翻译水平，促进肿瘤发展。我们分析了相关数据库，结果显示胰腺癌组织中MIF、FTO、MYC表达显著升高，且MIF与FTO、FTO与MYC、MIF与MYC间均存在正相关性，同时METTL3表达亦升高，MIF与METTL3、METTL3与MYC间亦存在正相关性。因此，MIF可能通过FTO/m6A/MYC和METTL3/m6A/MYC通路促进胰腺癌发展。上述结果表明抑制MIF-CD74轴是治疗胰腺癌的潜在策略。