Glutamine synthetase: a tumor suppressor in hepatocellular carcinoma?

GS has been shown to enhance cell survival, growth, and repair in breast, glioblastoma, sarcoma, and pancreatic cancers by promoting de novo glutamine synthesis and subsequent nucleotide biosynthesis as well as optimal mitochondrial bioenergetics both in vitro and in vivo ( Altman et al., 2016 ; Bott et al., 2019a ) . Genetic ablation of GS suppresses the development of PDAC in the Kras/p53/Pdx1-Cre mouse model ( Bott et al., 2019b ) . In the liver, GS-mediated Gln synthesis is an important mechanism for ammonia detoxification. GS is mainly expressed in the perivenous area and removes ammonia that has escaped the urea cycle, which takes place in the periportal and midzonal areas. In liver malignancy, GS is highly expressed, most likely as a transcriptional target of the oncogenic β-catenin, the most prevalent oncogenic event in the liver. Indeed, a strong positive correlation between β-catenin activation and GS expression exists in patients with hepatocellular carcinoma ( HCC ) , the most common type of liver cancer. However, opposite to the commonly recognized pro-tumorigenic role of GS, we unveiled a tumor suppressing role of hepatic GS in β-catenin-mutated liver cancer in a recent study ( Dai et al., 2022 ) . Using the Glul flox/flox mice ( He et al., 2010 ) , GS was specifically ablated in the liver by crossing with the albumin-Cre mice. HCC was induced in two well-characterized by introducing combinations of oncogenes ( c-Met/ N90β-catenin and Yap/ N90β-catenin ) via the sleeping beauty transposon and hydrodynamic tail vein injection system, and in two carcinogen-driven HCC models ( diethylnitrosamine/high fat diet and diethynitrosamine/phenobarbital ) that also involve β-catenin activation. In all four models, GS expression was induced by the oncogenic events. Surprisingly, unlike in the PDAC, hepatic GS ablation accelerated the onset of liver tumors. Furthermore, injection of adenoviral Cre via the tail vein to knock out GS one week after oncogene injection also enhanced tumor growth, similar to the embryonic GS ablation, indicating that hepatic GS deletion mainly facilitates the progression rather than the initiation of HCC. To find out what might cause the faster tumor growth upon GS ablation, RNA sequencing analysis using early liver tissues ( harvested 2–3 weeks after oncogene expression ) was performed. The results indicated that GS ablation led to increased mTORC1 signaling in response to oncogenic β-catenin activation ( c-Met/ N90β-catenin ) . Elevated mTORC1 activation by hepatic GS ablation was further confirmed by immunoblotting [p-S6 ( S235/235, S240/244 ) and p-4EBP1 ( T37/46, S65 ) ] and immunohistochemistry ( p-S6 S235/235 and p-4EBP1 T37/46 ) . Importantly, administration of the mTORC1 inhibitor rapamycin led to a Glutamine ( Gln ) is the most abundant free amino acid in the circulation. In addition to being a proteinogenic amino acid, it has multifaceted functions by serving as a nitrogen and carbon donor for the biosynthesis of essential metabolites, including nucleotides, hexosamines, and fatty acids, contributing to energy production via fueling the tricarboxylic acid cycle, regulating the redox balance, and stimulating mechanistic target of rapamycin complex 1 ( mTORC1 ) ( Altman et al., 2016 ; Bott et al., 2019a ) . Cancer cells have a high demand for Gln to support their malignant growth, which can be met by an increased expression of Gln transporter to facilitate its uptake from the tumor microenvironment ( Nicklin et al., 2009 ) . In a situation where extracellular supply of Gln is limited, such as in poorly vascularized cancers including breast cancer and pancreatic ductal adenocarcinoma ( PDAC ) , cell autonomous Gln synthesis kicks in. In mammals, Gln can be synthesized from ammonium and glutamate ( Glu ) , catalyzed by glutamine synthetase ( GS ) , also named glutamate ammonium ligase ( GLUL ) . In light of the increased demand of Gln in cancer cells,