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

ABSTRACT: 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.

ORGANISM(S): Homo sapiens

PROVIDER: GSE207435 | GEO | 2022/07/11

REPOSITORIES: GEO

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Project description:Protein methylation has been implicated in many important biological contexts including signaling, metabolism, and transcriptional control. Despite the importance of this post- translational modification, the global analysis of protein methylation by mass spectrometry-based proteomics has not been extensively studied due to the lack of robust, well-characterized techniques for methyl-peptidemethyl peptide enrichment. Here, to better investigate protein methylation, we optimized and compared two methods for methyl-peptidemethyl peptide enrichment: immunoaffinity purification (IAP) and high pH strong cation exchange (SCX). Comparison of these methods revealed that they are largely orthogonal for monomethyl arginine (MMA), suggesting that the usage of both techniques is required to provide a global view of protein methylation. Using both IAP and SCX, we investigated changes in protein methylation downstream of protein arginine methyltransferase 1 (PRMT1). Together, these techniques allowed us to quantify over ~1,000 arginine methylation sites on 407 proteins. Of these methylation sites, PRMT1 knockdown resulted in significant changes to 110 arginine methylation sites on 59 proteins. In contrast, zero lysine methylation sites were significantly changed upon PRMT1 knockdown. In PRMT1 knockdown cells, 24 MMA sites exhibited both significant downregulation (ie, putative PRMT1-mediated MMA targets) and 63 significant upregulation (ie, putative PRMT1-mediated ADMA targets). PRMT1 knockdown induced significant changes in asymmetric dimethyl arginine (ADMA), consistent with being PRMT1 targets. Additionally, We also observed that PRMT1 knockdown induced significant increases in symmetric dimethyl arginine (SDMA) methylation, suggesting that loss of PRMT1 activity allows scavenging of PRMT1 substrates by Type II PRMTs. Analysis of the subcellular localization and protein function of the significantly changing methyl-proteins revealed that the majority of PRMT1 substrates are localized to the nucleus and involved in RNA and DNA binding. Taken together, our results suggest that deep protein methylation profiling by mass spectrometry requires orthogonal enrichment techniques, identify novel PRMT1 methylation targets, and highlight in order to understand the dynamic interplay between methyltransferases in mammalian cells.

Project description:D-3-Phosphoglycerate dehydrogenase (Phgdh; EC 1.1.1.95) is a necessary enzyme for de novo L-serine biosynthesis via the phosphorylated pathway. We demonstrated previously that Phgdh is expressed exclusively by neuroepithelium and radial glia in developing mouse brain and later mainly by astrocytes. Mutations in the human PHGDH gene cause serine deficiency disorders (SDD) associated with severe neurological symptoms such as congenital microcephaly, psychomotor retardation, and intractable seizures. We recently demonstrated that genetically engineered mice, in which the gene for Phgdh has been disrupted, have significantly decreased levels of serine and glycine, and exhibit malformation of brain such as microcephaly. The Phgdh null (KO) embryos exhibit lethal phenotype after gestational day 14, indicating that the phosphorylated pathway is essential for embryogenesis, especially for brain development. It is worth noting that the Phgdh knockout (KO) embryos primarily displayed microcephaly, which is the most conspicuous phenotype of patients with SDD. Thus, Phgdh KO mice are a useful animal model for studying the effect of diminished L-serine levels on development of the central nervous system and other organs. To better understand the mechanism underlying the molecular pathogenesis of SDD, we sought to examine whether gene expression is altered in the Phgdh KO mouse model. We identify genes that have altered expression in the head of the Phgdh KO embryos using the GeneChip array. Some of the genes identified by this method belong in functional categories that are relevant to the biochemical and morphological aberrations of the Phgdh deletion. Experiment Overall Design: Total RNA samples were prepared from head tissues from 2 embryos of Phgdh knockout and littermate wild-type controls. Experiment Overall Design: RNA of 4 biological replicates was hybridized to Affymetrix Mouse Genome 430 2.0 arrays. Five microgram total RNA was labelled according to the ENZO-protocol, fragmented and hybridized according to Affymetrix's protocols.

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

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