PHGDH Arginine Methylation by PRMT1 Promotes Serine Synthesis and Represents a Therapeutic Vulnerability in Hepatocellular Carcinoma. PHGDH Arginine Methylation by PRMT1 Promotes Serine Synthesis and Represents a Therapeutic Vulnerability in Hepatocellular Carcinoma

Serine synthesis is crucial for tumor growth and survival, but its regulatory mechanism in cancer remains elusive. Here, using integrative metabolomics and transcriptomics analyses, we show a heterogeneity between metabolite and transcript profiles. Specifically, the level of serine in HCC tissues is increased, whereas the expression of phosphoglycerate dehydrogenase (PHGDH), the first rate-limiting enzyme in serine biosynthesis pathway, is dramatically downregulated. Interestingly, the increased serine level is obtained by enhanced PHGDH catalytic activity due to protein arginine methyltransferase 1 (PRMT1)-mediated methylation of PHGDH at arginine 236. PRMT1-mediated PHGDH methylation and activation potentiates serine synthesis, ameliorates oxidative stress, and promotes HCC growth in vitro and in vivo. FBXO7, an E3 ubiquitin ligase which is downregulated in human HCC tissues, ubiquitinates and downregulates PRMT1 to suppress PHGDH methylation and serine synthesis, leading to the inhibition of HCC cell growth. Furthermore, PRMT1-mediated PHGDH methylation correlates with PHGDH hyperactivation and serine accumulation in human HCC tissues, and is predictive of poor prognosis of HCC patients. Notably, blocking PHGDH methylation with a TAT-tagged non-methylated peptide inhibits serine synthesis and restrains HCC growth, both in an HCC patient-derived xenograft (PDX) model and subcutaneous HCC cell-derived xenograft model. Overall, our findings reveal a novel regulatory mechanism of PHGDH activity and serine synthesis, and suggest PHGDH methylation as a potential therapeutic vulnerability in HCC. Overall design: Total RNA from 27 human HCC samples and paired normal liver tissues was extracted and subjected to RNA sequencing with Illumina NovaSeq 6000 by Novogene Inc. (Tianjin, China).

丝氨酸合成对于肿瘤的生长与存活至关重要，但其在癌症中的调控机制仍未明确。本研究通过整合代谢组学与转录组学分析，揭示了代谢物谱与转录本谱之间的异质性。具体而言，肝细胞癌（hepatocellular carcinoma, HCC）组织中的丝氨酸水平显著升高，而丝氨酸生物合成通路的首个限速酶——磷酸甘油酸脱氢酶（phosphoglycerate dehydrogenase, PHGDH）的表达却被显著下调。有趣的是，这种丝氨酸水平的升高源于PHGDH催化活性的增强，而该活性增强是由蛋白质精氨酸甲基转移酶1（protein arginine methyltransferase 1, PRMT1）介导PHGDH在精氨酸236位点的甲基化所导致。

PRMT1介导的PHGDH甲基化与激活可促进丝氨酸合成、缓解氧化应激，并在体内外促进肝细胞癌的生长。FBXO7是一种在人肝细胞癌组织中表达下调的E3泛素连接酶（E3 ubiquitin ligase），它通过泛素化并下调PRMT1，抑制PHGDH甲基化与丝氨酸合成进程，最终阻碍肝癌细胞的生长。进一步研究表明，PRMT1介导的PHGDH甲基化与人类肝细胞癌组织中PHGDH的过度激活及丝氨酸积累密切相关，且可预测肝细胞癌患者的不良预后。值得注意的是，使用携带TAT标签的非甲基化肽阻断PHGDH甲基化，无论是在患者来源异种移植（patient-derived xenograft, PDX）模型还是皮下肝癌细胞来源的异种移植模型中，均能抑制丝氨酸合成并延缓肝细胞癌的生长。

综上，本研究揭示了PHGDH活性与丝氨酸合成的全新调控机制，并提出PHGDH甲基化可作为肝细胞癌潜在的治疗干预靶点。

整体实验设计：提取27例人肝细胞癌组织及其配对正常肝组织的总RNA，交由中国天津诺禾致源有限公司（Novogene Inc.）使用Illumina NovaSeq 6000平台进行RNA测序。