Metabolomics in oncology

Abstract Background Oncogenic transformation alters intracellular metabolism and contributes to the growth of malignant cells. Metabolomics, or the study of small molecules, can reveal insight about cancer progression that other biomarker studies cannot. Number of metabolites involved in this process have been in spotlight for cancer detection, monitoring, and therapy. Recent Findings In this review, the “Metabolomics” is defined in terms of current technology having both clinical and translational applications. Researchers have shown metabolomics can be used to discern metabolic indicators non‐invasively using different analytical methods like positron emission tomography, magnetic resonance spectroscopic imaging etc. Metabolomic profiling is a powerful and technically feasible way to track changes in tumor metabolism and gauge treatment response across time. Recent studies have shown metabolomics can also predict individual metabolic changes in response to cancer treatment, measure medication efficacy, and monitor drug resistance. Its significance in cancer development and treatment is summarized in this review. Conclusion Although in infancy, metabolomics can be used to identify treatment options and/or predict responsiveness to cancer treatments. Technical challenges like database management, cost and methodical knowhow still persist. Overcoming these challenges in near further can help in designing new treatment régimes with increased sensitivity and specificity.

life. 5,6 Metabolites can be evaluated in several body fluids such as blood, plasma and urine and therefore represent a potential noninvasive tool for the management of cancer patients, eventually providing a novel set of diagnostic biomarkers for tumor status and progression. Moreover, its study can support the management of the anticancer treatment response at an individualized level and also predict failures. 7 Biochemical processes and metabolic pathways can be described in details using metabolomics rather than standard clinical laboratory procedures. 8 In metabolomics, metabolites such as monosaccharides, amino acids, small lipids, co-factors, vitamin B complexes, energy cycle intermediates, nucleotides, exogenous xenobiotics and more are measured and studied comprehensively. 9,10 Biomass, energy and redox balance all play a key role in cell metabolism, which is essential for life.
There have been numerous studies demonstrating the importance of metabolic reprogramming in a variety of disorders, including cancer, diabetes, cardiovascular disease, and neurological disease. 11,12 In order to uncover the underlying causes of disease and devise new treatments, understanding metabolism is a necessity.
The objective of this article is to give a general overview on the current and future prospects on metabolomics and its role in cancer detection, monitoring and treatment. Here, we discuss recent developments in metabolomics and then emphasize on the clinical applications of metabolomics.

| REPROGRAMMING OF CANCER CELL METABOLISM
Metabolic reprogramming helps cancer cells to survive and proliferate during the course of cancer development. The enhanced growth and proliferation in malignant cells require an increased amount of energy in the form of ATP and other co-factors. This increased demand for resources is fulfilled through alteration of flux via multiple metabolic pathways. Altered Glycolysis and glucose metabolism (Warburg effect) is the most well-known and studied pathways in cancer metabolism. Over time multiple other pathways have been found to be altered in cancer cells for example lipid metabolism pathway, glutamine metabolism pathway, amino acid metabolism, citric acid cycle, fatty acid oxidation, one-carbon metabolism etc. 13 The reprogramming in theses pathways is complex and involves multiple factors. Also depending on the cancer type the reprogramming occurs in various degrees and in contexts with the microenvironmental conditions, providing required plasticity to cancer cell.
In this review we have tried to differentiate the metabolic state of cancer cells depending on the stages of cancer progression. For cancer cells to proliferate, invade, and metastasize they need to acquire a different metabolic state. It can broadly be classified into 3 stages. (A) Tumor micro-environments are typically acidic and hypoxic with a distinct nutrient composition compared to normal tissues, forcing cancer cells to adapt to such conditions in order to survive.
(B) During invasion, for survival in blood vessels, cancer cells must reprogram their metabolic state, allowing for anchorage-independent growth. (C) Lastly, when cancer cells colonize other organs, they must adapt to a completely new metabolic environments compared to primary sites in order to grow. 14,15 Understanding the mechanisms underlying this metabolic reprogramming can help in identification of new therapeutic targets for cancer.

| Metabolic reprogramming and tumor microenvironment
Tumor microenvironments have altered metabolic mechanism compared to normal tissues. This metabolism is influenced by number of intrinsic and extrinsic factors ( Figure 1). The classic example of cellintrinsic factor is altered glycolysis (also known as Warburg effect), 16 a fast glycolysis event due to the necessity of malignant proliferation.
Lack of oxygen in the local tumor microenvironment is frequently caused by tumor cells high proliferative capacity and high energy requirement. However, despite the fact that glycolysis does not give as much energy as aerobic respiration, it is 100 times faster and produces the amino acids and pentose phosphates required by rapidly proliferating cancer cells. 17,18 Similarly, another most frequently altered signaling pathway in human cancer is phosphatidylinositol-3-kinase (PI3K)/Akt signaling pathway 19 which along with mTOR (mammalian target of rapamycin) controls the uptake of glucose, lipids, nucleotides and amino acids. 20 29,30 Other extrinsic factors for example hypoxia and acidification also play a critical role in metabolic reprogramming 31,32 (Figure 1). Hypoxic microenvironments lead to upregulation and stabilization of hypoxia- inducible factors (HIFs), which are known to regulate expression of several genes (for example SNAIL, ZEB1, TWIST, matrix metalloproteinases, lactate dehydrogenase A and pyruvate dehydrogenase kinase 1 [33][34][35][36][37][38] ) that contribute to cancer progression, including many involved in cell survival, angiogenesis, glycolysis, cancer invasion, and metastasis. Understanding these metabolic switches and their role in metabolic reprogramming can provide critical insights in designing effective treatment regimens.

| Metabolic reprograming for anchorageindependent growth
For cancer cells to metastasise anchorage independent growth is a requirement, wherein cancer cells must detach from extra cellular matrix, enter blood/lymphatic vessel and survive in anchorageindependent manner. Interestingly a very small portion of circulating cancer cells are capable of doing this. 39,40 Primarily because anchorage independent growth requires metabolic reprogramming, which induce oxidative stress on the cells. 41 A classic example of this metabolic reprogramming is reductive carboxylation of glutamine that reduces excessive reactive oxygen species (ROS) in mitochondria. 42 This process converts glutamine derived α-ketoglutarate (αKG) to citrate through cytosolic IDH1 enzyme. Another enzyme fatty acid synthase (FASN) is essential in maintaining the IDH1 activity 44 43 Similarly, this reductive carboxylation pathway was also found to be essential in maintaining cell proliferation under hypoxic conditions in renal cell carcinoma (RCC). 44,45 Also, pentose phosphate pathway upregulation 46 is observed in multiple cancer types and is associated with anchorage independent growth, invasion and metastasis 47 mainly observed in KRAS-induced anchorageindependent growth. 48 Amino acid metabolism also play an important role in Anchorage-Independent cell survival, it works by altering sphingolipid diversity through deregulation of serine, alanine, and pyruvate. 49 In conclusion decrypting this metabolic network involved in anchorage-independent growth can provide helpful insights in designing therapeutic strategies to prevent cancer metastasis.

| METABOLIC REPROGRAMMING TO FORM METASTATIC TUMOR
One of the leading causes of death in cancer patients is metastasis.
After extravasation, in order to survive cancer cells must reprogramme their metabolic status as per the new site which is distinct from that of the primary site. Thus, to adapt to this new microenvironment multiple enzymes and pathways are deregulated. For example, in metastatic breast cancer increased proline catabolism is observed via proline dehydrogenase (PRODH) upregulation compared to primary breast cancer cases, 50 Similarly asparagine is known to increase the metastatic and invasive capabilities in breast cancer cells. It works through upregulation of asparagine synthetase (ASNS) an enzyme responsible to synthesize asparagine from aspartate. 51

| METABOLITE-BASED BIOMARKERS FOR DIAGNOSIS, PROGNOSIS AND PERSONALIZED CANCER TREATMENT
It is projected by 2030 around 17 million people would die per year from cancer. 125 Discovery of sensitive biomarkers for cancer using a tailored strategy is now a focus in cancer research and can be utilized as a detection tool for therapeutic targeting of metabolic enzymes.
Early intervention in cancer treatment could lead to better outcomes if these tactics are successfully applied. In future, diagnostic and prognostic biomarkers of disease will play an important role in individualized treatment and precision medicine. Analyzing metabolic phenotypes will allow for the categorization of patients by their metabolic profiles.
For example, larkin et al., 126 recently showed metabolomic biomarkers in blood samples can be used to identify cancer with nonspecific symptoms. 126     resonant frequency of that atomic nucleus. Thus, providing information about the molecular structure, motion, and chemical environment of the molecules. Hydrogen is most commonly targeted nucleus in biological samples. 147 NMR spectroscopy can be used to identify and quantify a wide range of metabolites, including small molecules, lipids, and amino acids, in cancer cells and tissues. One of the advantages of using NMR for metabolomics in cancer is its ability to provide metabolic fingerprints that can be used to distinguish between normal and cancerous tissue. 148 NMR spectroscopy can also be used to monitor changes in metabolite levels during cancer progression and treatment. It has also been used to study a variety of cancer types, including colon, 149   Metabolomics can make cancer precision medicine more feasible.

| Metabolic markers in the progression of cancer
Prior to moving into in vivo testing, in-silico models can be employed to understand the effects of medicines on metabolic characteristics.
Metabolomics has opened new avenues for cancer research and is already influencing cancer diagnosis and treatment in number of different ways. 161

| CONCLUSION
Although in infancy metabolomics can make a substantial impact on personalized cancer medicine. Using metabolomics, many cancer phenotypes can be accurately described with individualized metabolic markers.
This can be used to identified treatment options and/or predict responsiveness to treatments. Also, it will be vital to combine the outcomes of metabolomic assessments with other omics techniques to characterized the entire spectrum of the malignant phenotypes. Technical challenges like database management, cost and methodical knowhow still persist.
Overcoming these challenges in near further can help in designing new treatment régimes with increased sensitivity and specificity.

ETHICAL STATEMENT
Ethical Approval and Consent to participate is not required for such type of studies. Also, the review has been done in accordance to ethical guidelines and that is has been performed in a responsible way, with no misconduct.

CONFLICT OF INTEREST STATEMENT
The authors have stated explicitly that there are no conflicts of interest in connection with this article.

DATA AVAILABILITY STATEMENT
Data sharing is not applicable to this article as no new data were created or analyzed in this study.