代谢重编程在肿瘤的发生和发展中起着越来越重要的作用，是肿瘤的主要特征之一。肿瘤细胞通过代谢重编程改变自身的代谢途径来满足快速生长过程中所需要的能量和原材料。比如，有名的瓦博格效应（Warburg effect）就是肿瘤代谢重编程的一种结果。肿瘤细胞可以通过多种调控途径进行代谢重编程，目前，越来越多的研究表明肿瘤抑癌因子p53对于肿瘤代谢重编程中扮演着极其重要的角色。一碳代谢通路对于肿瘤的生长是必不可少的，它能提供一碳单位用于肿瘤细胞增殖过程中所需要的核苷酸合成，甲基化反应，同时维持细胞中NADPH的稳态。但是对于肿瘤细胞调控一碳代谢通路的分子机制以及是否参与其它生物学功能仍然是不清楚的。在本课题中，我们首先发现在一碳代谢各个代谢酶中，线粒体亚甲基四氢叶酸脱氢酶MTHFD2在肿瘤中上调最明显。以此为出发点，我们进一步表明p53能够在转录水平直接抑制MTHFD2基因表达，并进而抑制一碳代谢通路和嘌呤核苷酸合成。值得注意的是，敲低MTHFD2会选择性的抑制p53缺陷型肿瘤的增殖和存活，对于p53野生型却没有太大影响。对其分子机制的探究中，我们发现了一个MTHFD2本身代谢作用之外的功能：通过结合PARP3增强肿瘤细胞内非同源性末端接合 (c-NHEJ) 修复基因组DNA损伤的能力。MTHFD2的敲低使得p53缺陷型肿瘤细胞基因组双链损伤以及对化疗药物Doxorubicin的敏感性大大提高。在p53野生型肿瘤细胞中，敲低MTHFD2会抑制formate的产生并导致AICAR在细胞中累积，进而激活了AICAR介导的AMPK-p53-p21通路，维持基因组稳定性和细胞存活。综上所述，本文首次发现p53通过抑制MTHFD2的转录来调控一碳代谢通路，并发现MTHFD2会选择性影响p53缺陷型肿瘤的增殖和存活。我们还发现MTHFD2能调控PARP3介导的c-NHEJ修复通路活性影响基因组稳定性，这能特异性增强p53缺陷型肿瘤对化疗药物Doxorubicin的抵抗力。我们的研究揭示了MTHFD2不依赖本身代谢作用的新功能，并将一碳代谢和DNA修复联系到了一起，为治疗p53缺陷型肿瘤提供了新的药物靶点。
Metabolic reprogramming plays an important role in tumor occurrence and development, and emerges as one of hallmarks of tumors. Tumor cells remodel their metabolic pathways to meet their high demands of energy and materials for biogenesis. Tumor cells can regulate metabolic reprogramming through a variety of pathways. At present, increasing studies have shown that tumor suppressor p53 plays an essential role in tumor metabolic reprogramming.One carbon metabolism provides one carbon unit for nucleotide synthesis, methylation reaction and maintenance of NADPH homeostasis to support tumor cell growth. However, the mechanism of how tumor cells regulate one carbon metabolism and whether they are involved in other biological functions remain poorly understood.In this study, we found that mitochondrial methylenetetrahydrofolate dehydrogenase MTHFD2 is transcriptionally downregulated by p53, and contributes to p53-mediated suppression of one carbon metabolism and de novo purine synthesis. It should be noted that MTHFD2 depletion can selectively inhibit the proliferation and survival of p53 deficient tumors. Unexpectedly we identified a metabolism-independent function of MTHFD2 in non-homologous end joining （NHEJ） in response to genome DNA damage. MTHFD2 binds to PARP3 and hence accelerates PARP3 mediated NHEJ repair pathway. MTHFD2 depletion strongly restrains p53 deficient tumor cells proliferation and sensitizes them to the chemotherapeutic agent doxorubicin. Interestingly, these effects are overwhelmed by AICAR-mediated p53/p21 activation induced by decreased formate production due to one carbon metabolism inhibition in p53 wild-type tumor cells.In conclusion, we first found that p53 regulates one carbon metabolism by inhibiting MTHFD2 transcription, and MTHFD2 depletion selectively impairs the proliferation and survival of p53 deficient tumors. We also found that MTHFD2 can promote PARP3 mediated NHEJ repair activity and maintain genomic stability, resulted in increasing resistance of p53 deficient tumors to doxorubicin treatment. Our findings highlight a metabolism-independent role for MTHFD2 which connects one carbon metabolism to DNA repair, indicating a potential drug target for treatment of p53 deficient tumors.
