Metabolic Heterogeneity, Plasticity, and Adaptation to "Glutamine Addiction" in Cancer Cells: The Role of Glutaminase and the GTωA [Glutamine Transaminase-ω-Amidase (Glutaminase II)] Pathway

Many cancers utilize l-glutamine as a major energy source. Often cited in the literature as "l-glutamine addiction", this well-characterized pathway involves hydrolysis of l-glutamine by a glutaminase to l-glutamate, followed by oxidative deamination, or transamination, to α-ketoglutarate, which enters the tricarboxylic acid cycle. However, mammalian tissues/cancers possess a rarely mentioned, alternative pathway (the glutaminase II pathway): l-glutamine is transaminated to α-ketoglutaramate (KGM), followed by ω-amidase (ωA)-catalyzed hydrolysis of KGM to α-ketoglutarate. The name glutaminase II may be confused with the glutaminase 2 (GLS2) isozyme. Thus, we recently renamed the glutaminase II pathway the "glutamine transaminase-ω-amidase (GTωA)" pathway. Herein, we summarize the metabolic importance of the GTωA pathway, including its role in closing the methionine salvage pathway, and as a source of anaplerotic α-ketoglutarate. An advantage of the GTωA pathway is that there is no net change in redox status, permitting α-ketoglutarate production during hypoxia, diminishing cellular energy demands. We suggest that the ability to coordinate control of both pathways bestows a metabolic advantage to cancer cells. Finally, we discuss possible benefits of GTωA pathway inhibitors, not only as aids to studying the normal biological roles of the pathway but also as possible useful anticancer agents.

Non-alcoholic fatty liver disease-associated DNA methylation and gene expression alterations in the livers of Collaborative Cross mice fed an obesogenic high-fat and high-sucrose diet

Non-alcoholic fatty liver disease (NAFLD) is a highly prevalent chronic liver disease, and patient susceptibility to its onset and progression is influenced by several factors. In this study, we investigated whether altered hepatic DNA methylation in liver tissue correlates with the degree of severity of NAFLD-like liver injury induced by a high-fat and high-sucrose (HF/HS) diet in Collaborative Cross (CC) mice. Using genome-wide targeted bisulphite DNA methylation next-generation sequencing, we found that mice with different non-alcoholic fatty liver (NAFL) phenotypes could be distinguished by changes in hepatic DNA methylation profiles. Specifically, NAFL-prone male CC042 mice exhibited more prominent DNA methylation changes compared with male CC011 mice and female CC011 and CC042 mice that developed only a mild NAFL phenotype. Moreover, these mouse strains demonstrated different patterns of DNA methylation. While the HF/HS diet induced both DNA hypomethylation and DNA hypermethylation changes in all the mouse strains, the NAFL-prone male CC042 mice demonstrated a global predominance of DNA hypermethylation, whereas a more pronounced DNA hypomethylation pattern developed in the mild-NAFL phenotypic mice. In a targeted analysis of selected genes that contain differentially methylated regions (DMRs), we identified NAFL phenotype-associated differences in DNA methylation and gene expression of the Apoa4, Gls2, and Apom genes in severe NAFL-prone mice but not in mice with mild NAFL phenotypes. These changes in the expression of Apoa4 and Gls2 coincided with similar findings in a human in vitro cell model of diet-induced steatosis and in patients with NAFL. These results suggest that changes in the expression and DNA methylation status of these three genes may serve as a set of predictive markers for the development of NAFLD.

The Expression of Glutaminases and their Association with Clinicopathological Parameters in the Head and Neck Cancers

BACKGROUND

The increased glutamine metabolism is a characteristic feature of cancer cells. The interconversion between glutamine and glutamate is catalyzed by two glutaminase isoforms, GLS1 and GLS2, which appear to have different roles in different types of cancer. We investigated for the first time the protein expression of GLS1 and GLS2, and their correlation with advanced clinicopathological parameters in head and neck cancers.

METHOD

Consecutive slides from a tissue microarray comprised of 80 samples ranging from normal to metastatic, were stained immunohistochemically for GLS1, GLS2, HIF-1α or CD147. Following analysis by two expert pathologists we carried out statistical analysis of the scores.

RESULTS

GLS1 and GLS2 are upregulated at protein level in head and neck tumours compared to normal tissues and this increased expression correlated positively (GLS1) and negatively (GLS2) with tumor grade, indicating a shift of expression between GLS enzyme isoforms based on tumor differentiation. Increased expression of GLS1 was associated with high CD147 expression; and elevated GLS2 expression was associated with both high CD147 and high HIF-1α expressions. The correlation of the GLS1 and GLS2 with HIF-1α or CD147 was strongly associated with more advanced clinicopathological parameters.

CONCLUSION

The increased expression of GLS1 and GLS2 may be explored as a new treatment for head and neck cancers.

miR-190a-5p regulates cardiomyocytes response to ferroptosis via directly targeting GLS2

Ferroptosis is a novel identified form of regulated cell death that has been implied in the pathology of myocardial infarction (MI). However, the regulation mechanisms of ferroptosis in cardiomyocyte are still elusive. MiRNAs are a group of small non-coding RNAs that play crucial roles in various biological activities. Till now, little is known about the role of miRNA in the ferroptosis of cardiomyocytes. In the current study, we found that miR-190a-5p negatively regulate ferroptosis via directly targeting GLS2 in rat cardiomyocyte H9c2 cells. Forced expression of miR-190a-5p inhibited GLS2, resulting in downregulation of ROS, MDA and Fe ²⁺ accumulation. Meanwhile, inhibition of miR-190a-5p caused upregulation of GLS2, resulting in opposite effects which could be blocked by GLS2 inhibitor compound 968. In summary, our findings suggest that miR-190a-5p plays an essential role in regulation of ferroptosis of cardiomyocytes and suggest a potential therapeutic target for MI.

METTL3 Promotes Esophageal Squamous Cell Carcinoma Metastasis Through Enhancing GLS2 Expression

Recent studies have identified pleiotropic roles of methyltransferase-like 3 (METTL3) in tumor progression. However, the roles of METTL3 in esophageal squamous cell carcinoma (ESCC) are still unclear. Here, we investigated the function and mechanism of METTL3 in ESCC tumorigenesis. We reported that higher METTL3 expression was found in ESCC tissues and was markedly associated with depth of invasion and poor prognosis. Loss- and gain-of function studies showed that METTL3 promoted the migration and invasion of ESCC cells in vitro. Integrated methylated RNA immunoprecipitation sequencing (MeRIP-Seq) and RNA sequencing (RNA-Seq) analysis first demonstrated that glutaminase 2 (GLS2) was regulated by METTL3 via m6A modification. Our findings identified METTL3/GLS2 signaling as a potential therapeutic target in antimetastatic strategies against ESCC.

Glutaminases regulate glutathione and oxidative stress in cancer

Targeted therapies against cancer have improved both survival and quality of life of patients. However, metabolic rewiring evokes cellular mechanisms that reduce therapeutic mightiness. Resistant cells generate more glutathione, elicit nuclear factor erythroid 2-related factor 2 (NRF2) activation, and overexpress many anti-oxidative genes such as superoxide dismutase, catalase, glutathione peroxidase, and thioredoxin reductase, providing stronger antioxidant capacity to survive in a more oxidative environment due to the sharp rise in oxidative metabolism and reactive oxygen species generation. These changes dramatically alter tumour microenvironment and cellular metabolism itself. A rational design of therapeutic combination strategies is needed to flatten cellular homeostasis and accomplish a drop in cancer development. Context-dependent glutaminase isoenzymes show oncogenic and tumour suppressor properties, being mainly associated to MYC and p53, respectively. Glutaminases catalyze glutaminolysis in mitochondria, regulating oxidative phosphorylation, redox status and cell metabolism for tumour growth. In addition, the substrate and product of glutaminase reaction, glutamine and glutamate, respectively, can work as signalling molecules moderating redox and bioenergetic pathways in cancer. Novel synergistic approaches combining glutaminase inhibition and redox-dependent modulation are described in this review. Pharmacological or genetic glutaminase regulation along with oxidative chemotherapy can help to improve the design of combination strategies that escalate the rate of therapeutic success in cancer patients.

Targeting Hepatic Glutaminase 1 Ameliorates Non-alcoholic Steatohepatitis by Restoring Very-Low-Density Lipoprotein Triglyceride Assembly

Non-alcoholic steatohepatitis (NASH) is characterized by the accumulation of hepatic fat in an inflammatory/fibrotic background. Herein, we show that the hepatic high-activity glutaminase 1 isoform (GLS1) is overexpressed in NASH. Importantly, GLS1 inhibition reduces lipid content in choline and/or methionine deprivation-induced steatotic mouse primary hepatocytes, in human hepatocyte cell lines, and in NASH mouse livers. We suggest that under these circumstances, defective glutamine fueling of anaplerotic mitochondrial metabolism and concomitant reduction of oxidative stress promotes a reprogramming of serine metabolism, wherein serine is shifted from the generation of the antioxidant glutathione and channeled to provide one-carbon units to regenerate the methionine cycle. The restored methionine cycle can induce phosphatidylcholine synthesis from the phosphatidylethanolamine N-methyltransferase-mediated and CDP-choline pathways as well as by base-exchange reactions between phospholipids, thereby restoring hepatic phosphatidylcholine content and very-low-density lipoprotein export. Overall, we provide evidence that hepatic GLS1 targeting is a valuable therapeutic approach in NASH.

The Epithelial to Mesenchymal Transition Promotes Glutamine Independence by Suppressing GLS2 Expression

Identifying bioenergetics that facilitate the epithelial to mesenchymal transition (EMT) in breast cancer cells may uncover targets to treat incurable metastatic disease. Metastasis is the number one cause of cancer-related deaths; therefore, it is urgent to identify new treatment strategies to prevent the initiation of metastasis. To characterize the bioenergetics of EMT, we compared metabolic activities and gene expression in cells induced to differentiate into the mesenchymal state with their epithelial counterparts. We found that levels of GLS2, which encodes a glutaminase, are inversely associated with EMT. GLS2 down-regulation was correlated with reduced mitochondrial activity and glutamine independence even in low-glucose conditions. Restoration of GLS2 expression in GLS2-negative breast cancer cells rescued mitochondrial activity, enhanced glutamine utilization, and inhibited stem-cell properties. Additionally, inhibition of expression of the transcription factor FOXC2, a critical regulator of EMT in GLS2-negative cells, restored GLS2 expression and glutamine utilization. Furthermore, in breast cancer patients, high GLS2 expression is associated with improved survival. These findings suggest that epithelial cancer cells rely on glutamine and that cells induced to undergo EMT become glutamine independent. Moreover, the inhibition of EMT leads to a GLS2-directed metabolic shift in mesenchymal cancer cells, which may make these cells susceptible to chemotherapies.

Multiomics Analysis Reveals that GLS and GLS2 Differentially Modulate the Clinical Outcomes of Cancer

Kidney-type glutaminase (GLS) and liver-type glutaminase (GLS2) are dysregulated in many cancers, making them appealing targets for cancer therapy. However, their use as prognostic biomarkers is controversial and remains an active area of cancer research. Here, we performed a systematic multiomic analysis to determine whether glutaminases function as prognostic biomarkers in human cancers. Glutaminase expression and methylation status were assessed and their prominent functional protein partners and correlated genes were identified using various web-based bioinformatics tools. The cross-cancer relationship of glutaminases with mutations and copy number alterations was also investigated. Gene ontology (GO) and pathway analysis were performed to assess the integrated effect of glutaminases and their correlated genes on various cancers. Subsequently, the prognostic roles of GLS and GLS2 in human cancers were mined using univariate and multivariate survival analyses. GLS was frequently over-expressed in breast, esophagus, head-and-neck, and blood cancers, and was associated with a poor prognosis, whereas GLS2 overexpression implied poor overall survival in colon, blood, ovarian, and thymoma cancers. BothGLS and GLS2 play oncogenic and anti-oncogenic roles depending on the type of cancer. The varying prognostic characteristics of glutaminases suggest that GLS and GLS2 expression differentially modulate the clinical outcomes of cancers.

Glutaminase 2 is a novel negative regulator of small GTPase Rac1 and mediates p53 function in suppressing metastasis

Glutaminase (GLS) isoenzymes GLS1 and GLS2 are key enzymes for glutamine metabolism. Interestingly, GLS1 and GLS2 display contrasting functions in tumorigenesis with elusive mechanism; GLS1 promotes tumorigenesis, whereas GLS2 exhibits a tumor-suppressive function. In this study, we found that GLS2 but not GLS1 binds to small GTPase Rac1 and inhibits its interaction with Rac1 activators guanine-nucleotide exchange factors, which in turn inhibits Rac1 to suppress cancer metastasis. This function of GLS2 is independent of GLS2 glutaminase activity. Furthermore, decreased GLS2 expression is associated with enhanced metastasis in human cancer. As a p53 target, GLS2 mediates p53's function in metastasis suppression through inhibiting Rac1. In summary, our results reveal that GLS2 is a novel negative regulator of Rac1, and uncover a novel function and mechanism whereby GLS2 suppresses metastasis. Our results also elucidate a novel mechanism that contributes to the contrasting functions of GLS1 and GLS2 in tumorigenesis.

How Does p73 Cause Neuronal Defects?

The p53-family member, p73, plays a key role in the development of the central nervous system (CNS), in senescence, and in tumor formation. The role of p73 in neuronal differentiation is complex and involves several downstream pathways. Indeed, in the last few years, we have learnt that TAp73 directly or indirectly regulates several genes involved in neural biology. In particular, TAp73 is involved in the maintenance of neural stem/progenitor cell self-renewal and differentiation throughout the regulation of SOX-2, Hey-2, TRIM32 and Notch. In addition, TAp73 is also implicated in the regulation of the differentiation and function of postmitotic neurons by regulating the expression of p75NTR and GLS2 (glutamine metabolism). Further still, the regulation of miR-34a by TAp73 indicates that microRNAs can also participate in this multifunctional role of p73 in adult brain physiology. However, contradictory results still exist in the relationship between p73 and brain disorders, and this remains an important area for further investigation.

Downregulation of GLS2 in glioblastoma cells is related to DNA hypermethylation but not to the p53 status

Human phosphate-activated glutaminase (GA) is encoded by two genes: GLS and GLS2. Glioblastomas (GB) usually lack GLS2 transcripts, and their reintroduction inhibits GB growth. The GLS2 gene in peripheral tumors may be i) methylation- controlled and ii) a target of tumor suppressor p53 often mutated in gliomas. Here we assessed the relation of GLS2 downregulation in GB to its methylation and TP53 status. DNA demethylation with 5-aza-2'-deoxycytidine restored GLS2 mRNA and protein content in human GB cell lines with both mutated (T98G) and wild-type (U87MG) p53 and reduced the methylation of CpG1 (promoter region island), and CpG2 (first intron island) in both cell lines. In cell lines and clinical GB samples alike, methylated CpG islands were detected both in the GLS2 promoter (as reported earlier) and in the first intron of this gene. CpG methylation of either island was absent in GLS2-expressing non-tumoros brain tissues. Screening for mutation in the exons 5-8 of TP53 revealed a point mutation in only one out of seven GB examined. In conclusion, aberrant methylation of CpG islands, appear to contribute to silencing of GLS2 in GB by a mechanism bypassing TP53 mutations. © 2015 Wiley Periodicals, Inc.

Opposing roles of glutaminase isoforms in determining glioblastoma cell phenotype

Glutamine (Gln) and glutamate (Glu) play pivotal roles in the malignant phenotype of brain tumors via multiple mechanisms. Glutaminase (GA, EC 3.5.1.2) metabolizes Gln to Glu and ammonia. Human GA isoforms are encoded by two genes: GLS gene codes for kidney-type isoforms, KGA and GAC, whereas GLS2 codes for liver-type isoforms, GAB and LGA. The expression pattern of both genes in different neoplastic cell lines and tissues implicated that the kidney-type isoforms are associated with cell proliferation, while the liver-type isoforms dominate in, and contribute to the phenotype of quiescent cells. GLS gene has been demonstrated to be regulated by oncogene c-Myc, whereas GLS2 gene was identified as a target gene of p53 tumor suppressor. In glioblastomas (GBM, WHO grade IV), the most aggressive brain tumors, high levels of GLS and only traces or lack of GLS2 transcripts were found. Ectopic overexpression of GLS2 in human glioblastoma T98G cells decreased their proliferation and migration and sensitized them to the alkylating agents often used in the chemotherapy of gliomas. GLS silencing reduced proliferation of glioblastoma T98G cells and strengthen the antiproliferative effect evoked by previous GLS2 overexpression.

GLS2 is transcriptionally regulated by p73 and contributes to neuronal differentiation

The amino acid Glutamine is converted into Glutamate by a deamidation reaction catalyzed by the enzyme Glutaminase (GLS). Two isoforms of this enzyme have been described, and the GLS2 isoform is regulated by the tumor suppressor gene p53. Here, we show that the p53 family member TAp73 also drives the expression of GLS2. Specifically, we demonstrate that TAp73 regulates GLS2 during retinoic acid-induced terminal neuronal differentiation of neuroblastoma cells, and overexpression or inhibition of GLS2 modulates neuronal differentiation and intracellular levels of ATP. Moreover, inhibition of GLS activity, by removing Glutamine from the growth medium, impairs in vitro differentiation of cortical neurons. Finally, expression of GLS2 increases during mouse cerebellar development. Although, p73 is dispensable for the in vivo expression of GLS2, TAp73 loss affects GABA and Glutamate levels in cortical neurons. Together, these findings suggest a role for GLS2 acting, at least in part, downstream of p73 in neuronal differentiation and highlight a possible role of p73 in regulating neurotransmitter synthesis.