Glutamine synthetase limits β-catenin–mutated liver cancer growth by maintaining nitrogen homeostasis and suppressing mTORC1
Weiwei Dai, … , Shenglan Gao, Wei-Xing Zong
Weiwei Dai, … , Shenglan Gao, Wei-Xing Zong
Published October 18, 2022
Citation Information: J Clin Invest. 2022;132(24):e161408. https://doi.org/10.1172/JCI161408.
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Glutamine synthetase limits β-catenin–mutated liver cancer growth by maintaining nitrogen homeostasis and suppressing mTORC1

Abstract

Glutamine synthetase (GS) catalyzes de novo synthesis of glutamine that facilitates cancer cell growth. In the liver, GS functions next to the urea cycle to remove ammonia waste. As a dysregulated urea cycle is implicated in cancer development, the impact of GS’s ammonia clearance function has not been explored in cancer. Here, we show that oncogenic activation of β-catenin (encoded by CTNNB1) led to a decreased urea cycle and elevated ammonia waste burden. While β-catenin induced the expression of GS, which is thought to be cancer promoting, surprisingly, genetic ablation of hepatic GS accelerated the onset of liver tumors in several mouse models that involved β-catenin activation. Mechanistically, GS ablation exacerbated hyperammonemia and facilitated the production of glutamate-derived nonessential amino acids, which subsequently stimulated mechanistic target of rapamycin complex 1 (mTORC1). Pharmacological and genetic inhibition of mTORC1 and glutamic transaminases suppressed tumorigenesis facilitated by GS ablation. While patients with hepatocellular carcinoma, especially those with CTNNB1 mutations, have an overall defective urea cycle and increased expression of GS, there exists a subset of patients with low GS expression that is associated with mTORC1 hyperactivation. Therefore, GS-mediated ammonia clearance serves as a tumor-suppressing mechanism in livers that harbor β-catenin activation mutations and a compromised urea cycle.

Authors

Weiwei Dai, Jianliang Shen, Junrong Yan, Alex J. Bott, Sara Maimouni, Heineken Q. Daguplo, Yujue Wang, Khoosheh Khayati, Jessie Yanxiang Guo, Lanjing Zhang, Yongbo Wang, Alexander Valvezan, Wen-Xing Ding, Xin Chen, Xiaoyang Su, Shenglan Gao, Wei-Xing Zong

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Figure 9

Defective nitrogen waste removal in HCC clinical samples.

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Defective nitrogen waste removal in HCC clinical samples. (A) Relative G...
(A) Relative GLUL mRNA levels in liver cancer patients with WT or mutated CTNNB1 (TCGA) were compared and are expressed as mean ± SEM. (B) Correlations between hepatic GLUL and UCE expression in patients with CTNNB1 mutations (TCGA) was calculated by Pearson’s correlation. (C) CTNNB1 mutation correlates positively with hepatic GS expression but inversely with GLUL CpG island methylation. (D) Two liver TMAs were costained for GS (green) and ARG1 (red) and quantified by ImageJ and normalized to the respective median (right panel). (E) TMAs with HCC tumors (T), adjacent normal tissue (N), or intrahepatic cholangiocarcinoma (ICC) were costained for GS (green) and p-4EBP1 T37/46 (red). (F) Ratios of p-4EBP1 T37/46 to GS intensities were plotted. Results are expressed as mean ± SEM. The cutoff value for GS-high (n = 52) versus -low (n = 372) was determined using the best separation between the 2 modes of a bimodal distribution. ****P < 0.0001 by 2-tailed t test (A, D, and F). (G) GSEA enrichment plots show that mTORC1 target genes correlate inversely with GS level in CTNNB1-mutated patients (TCGA). The cutoff value for GS-high (n = 62) versus -low (n = 34) was determined using the best separation between the 2 modes of a bimodal distribution. P value was calculated by estimated score in GSEA. (H) The TCGA data of the indicated study were stratified into the RNA-seq results obtained from WT and Glul-KO mice 2 weeks after c-Met/ΔN90-β-catenin injection (n = 4 in each group). GSEA plots show that high GS expression correlates with low recurrence. (I) Kaplan-Meier survival curves of patients with high (n = 41) versus low (n = 201) hepatic GS expression (GSE14520; P = 0.28, log rank). (J) In HCC patients harboring CTNNB1 mutations (TCGA) as shown in G, high GS expression tends to associate with better survival (P = 0.09, log rank).