The metabolomics deals with the analysis of small molecules known as metabolites (less than 1.5kDa) that are the end products of the genomic, transcriptomic, proteomic or environmental changes affecting the cell or biological system (Wishart et al., 2007). It is possible to obtain a relatively full picture of the state of a given cell or tissue by analysing its endogenous and exogenous metabolites (Johnson et al., 2016). The metabolomics reflects the influence of external factors and phenotype of the cell that cannot be effectively assessed in genomics or proteomics investigations (Schmidt et al., 2004).
The metabolomic reprogramming of cancer cells is a well-known phenomenon (Jaroch et al., 2021). It is well known that the cancer cells maintain the cellular proliferation and survival by exhibiting changes in the metabolism pertaining to the enhanced levels of glucose and energy. Numerous reports have suggested that amino acids, nucleotides and lipids significantly assists in the cancer cell metabolism. Therefore, the present study attempts to sketch the major dysregulated metabolic pathways that supports cancer initiation and progression, including glycolysis and Warburg effect, metabolic symbiosis, metabolism of glutamine, serine, methionine and proline, involvement of arginine and ornithine in linking tricarboxylic acid and urea cycles, and lipid and nucleotide synthesis pathways. Recent findings suggested that altered fatty acid oxidation (FAO) is also an important marker for initiation of glioma and development (Kant et al., 2020; Juraszek et al., 2021). Melone et al., (2018) suggested that carnitine shuttle system regulates the metabolic pathway of fatty acid oxidation, it consists of enzymes and protein transporters that are responsible for transporting of fatty acids through the mitochondrial membrane. Previous investigations have reported the levels of carnitine in other malignant neoplasms such as glioma, hepatocellular carcinoma, breast cancer, and prostate cancer, exhibit increased concentrations in tumour tissues when compared to the control samples
Prior studies have examined levels of carnitine (CPT-1, CPT-2, CACT) metabolite in a variety of malignant neoplasms, including glioma, hepatocellular carcinoma, breast cancer, and prostate cancer, with findings showing higher concentrations in malignant tissues compared to histologically healthy samples (Lu et al., 2019; Yu et al., 2020; Zoni et al.,2019). In the present study (Fig. 1), it was found there was significant increase in the mRNA levels of carnitine palmitoyltransferase 2 in high grades of glioma when compared to the lower grades and the results of our study is in agreement with the reports of Bogusiewicz et al., (2021) and Melone et al’s (2018)., suggesting that higher carnitine content in higher-grade tumors was observed when compared to the lower grades of glioma. Thus, suggesting that higher levels of carnitine in high grades could be related to increased metabolism of tumors when compared to those of low grades. The shuttling of the carnitine system exhibits a vital role not only in cancer plasticity but also it enhances the metabolic demands of the proliferating cancerous cells to be attained, even in adverse state. The monoglyceride lipase (MGLL) catabolizes the final step in neutral lipolysis to produce fatty acid and glycerol and it can act as a significant contributor towards cancer aggressiveness (Nomura et al., 2010 & 2011). Thus, it could be inferred that monoacylglycerols-free fatty acid pathway feeding into a diverse lipid network enriched in protumorigenic signaling molecules eventually promoting migration, survival, and in vivo tumor growth. Subsequently, aggressive cancer cells could pair lipogenesis with high lipolytic activity to generate an array of protumorigenic signals could thereby supporting their malignant behavior. Our observation of biopsies of higher grades exhibited increase in the mRNA levels of MGLL provide a striking example of the co-opting of an enzyme by cancer cells to serve a distinct metabolic purpose that supports their pathogenic behavioral progress in higher grades.
The glucose transporter 3 (GLUT 3) in the brain is known as the neuronal glucose transporter and increase in the GLUT3 levels was found to be highly metaboloically plastic in higher grades of glioma called brain tumor initiating cells, which promotes the survival in restricted glucose (Libby et al., 2020; Flavahan et al., 2013). This brain tumor initiating cells were reported to be more invasive than more differentiated cells within high grade tumors (Ruiz-Ontanon et al., 2013; Ortensi et al., 2013). Boral et al., (2017) demonstrated that GLUT3 was increased in circulating tumor cells have higher propensity to target the brain and GLUT 3 was essential for their survival within the brain (Kuo et al., 2019). In the present study, it could be evident that higher grades of glioma exhibited significant increase in the levels of GLUT3 when compared to low grades. The data suggest that glycolytic shift or elevated glucose uptake mediated by GLUT3 could significantly increase the circulating tumor cell survival, which in turn promotes tumor metastasis. The present study coincides with evidences by Puchalski et al (2018) suggesting that high GLUT3 expression positively correlated with an increased incidence of metastasis in breast and head and neck cancers.
Phosphofructokinase-1 (PFK-1) has been widely recognized to catalyze the second rate-limiting procedure of glycolysis, transforming fructose-6-phosphate (F-6-P) to fructose-1, 6-biphosphate (F-1, 6-BP) mediated by Mg2 + and ATP (Rhodes et al., 2004). The glycolytic flux of tumor cells could be modulated by PFK-1, usually activated in a significant number of tumor types, thus affecting cellular growth, invasiveness, and survival (Yalcin et al., 2009; Bartrons et al., 2007). In our present study, increased mRNA levels of phosphofructokinase in higher grades of glioma suggested that enhanced glycolytic flux could be regulated by PFK-1, which could significantly enrich the tumor microenvironment thereby aggravating the aggressive behaviour in higher grades.
The main hallmark of cancer is alteration in the metabolism for the promotion of efficient tumor growth and acquired resistance to therapy (Hanahan et al., 2011). The glioma cells exhibit abnormal energy metabolism, including alterations of glucose, amino acid, and fatty acid metabolism (Vander et al., 2009). Our previous study exhibited that altered metabolic pathways including choline, taurine, hypotaurine, or glutamate/glutamine correlate with different WHO grades in diffuse glioma (Jayalakshmi et al., 2020). In the present study, glutamate, choline and tyrosine metabolites were found to be significantly altered in higher grades of glioma. Our study coincides with the reports of Bianchi, et al. (2004) where increased levels of choline in higher grades of glioma have been reported to be the most reliable indicator for detecting the malignancy in human tumors by proton magnetic resonance spectroscopy (MRS). Bianchi, et al. (2004) suggested that higher levels of choline reflect an increase in the metabolites that are precursors of the membrane phospholipids that are vital for the support of neoplastic proliferation and increase in the tumor cellular turnover. Earlier in the brain, Tedeschi et al. (1997) reported an increase in choline levels was found to parallel clinical deterioration in human samples.
Metabolic reprogramming is one of the hallmarks of cancer (Faubert et al., 2020; Leone & Powell, 2020; Hoxhaj &Manning, 2020). As an abnormal amino acid metabolism has potential impact on the metabolic control and regulation of cancer microenvironment (Wu et al., 2021). We explored the dysregulation of amino acid metabolism in tumor microenvironment not only facilitate the survival and proliferation of cancer cells but also the non-essential amino acids aid in the production of protein biosynthesis, conversion to glucose, lipids, and precursors of nitrogen-containing metabolites, such as purines and pyrimidines for nucleic acid synthesis by acting as building blocks to the cancer cells. Our NMR analysis had shown the altered levels of pyruvate, citrate, aspartate, N-acetylaspartate (carbohydrate metabolism), choline, myoinositol (lipid metabolism), glutamate, GABA, tyrosine, lysine (amino acid metabolism) that might affect the metabolism represented in the parenthesis.
Glutamatergic signaling is essential for the maintenance, proliferation and invasive behavior of higher grades of glioma. Many investigations have reported that the capability of increased paracrine glutamate signaling between neurons and glioblastoma promotes cell proliferation, migration and tumor establishment (Sontheimer et al., 2008; Corsi et al., 2019). Maier et al., (2021) suggested that there is a strong relationship between glutamate release and stressors to the glioblastoma, validated using a spatial multi-omics approach. They also found that any alterations in the uptake/release of glutamate results in significant changes in the glioblastoma behavior, playing a vital role in proliferation and invasive behavior. Recently, Subramani et al., (2020) investigated the non-invasive detection of glutamate by magnetic resonance spectroscopy, which can serve as a metabolic imaging biomarker in response to temozolomide treatment to IDH1 mutant glioma. In concordance with the reports, we also register the metabolic glutamate in higher grades of glioma. In concordance to the present study, Yamashita et al. (2020) performed comprehensive metabolic analyses using high-grade glioma tissues and glioblastoma patient–derived sphere culture models and reported that tyrosine metabolism could be a novel target in high grades of glioma. Thus, dysregulation in the metabolomic profile could contribute to the aggressiveness of malignancy in higher grades of glioma.
The MetaboAnalyst 4.0 software was used to identify pathways that could be altered in the glioma. From the Fig. 6, the scatterplot of joint metabolic pathway analysis of the metabolites and genes from KEGG database, top six metabolically altered pathways have been identified as follows: central carbon metabolism in cancer, alanine, aspartate and glutamate metabolism, ABC Transporter, aminoacyl-tRNA biosynthesis and also the process of protein digestion and adsorption.