Frontiers in Clinical Drug Research - Anti-Cancer Agents: Volume 9

By Atta-ur Rahman (Editor)

Frontiers in Clinical Drug Research - Anti-Cancer Agents is a book series intended for pharmaceutical scientists, postgraduate students and researchers seeking updated and critical information for developing clinical trials and devising research plans in anti-cancer research. Reviews in each volume are written by experts in medical oncology and clinical trials research and compile the latest information available on special topics of interest to oncology and pharmaceutical chemistry researchers. Volume 9 features reviews on these topics:

· Dietary Strategy for Cancer Therapy - Amino Acid Restrictions and Beyond
· The Revolutionary Potential of Noble Metal Nanoparticles as Anti-Cancer Agents: State-of-the-Art Applications and Future Perspectives
· Algal Polysaccharides as Promising Anticancer Agents
· Cardiotoxicity Caused by Doxorubicin and Trastuzumab: Current Understanding for Future Preventive Strategies
· Emodin: Anticancer Agent

Readership
Pharmaceutical Scientists, Medicinal Chemists, Clinical Oncologists, Researchers in Pre-clinical studies and clinical trials.

• Language: English
• Publisher: Bentham Science Publishers
• Release date: Oct 25, 2024
• ISBN: 9789815223910

Book preview

Dietary Strategy for Cancer Therapy - Amino Acid Restrictions and beyond

Shu-Ang Li¹, Jian-Sheng Kang¹, *

¹ Clinical Systems Biology Laboratories, The First Affiliated Hospital of Zhengzhou University, Zhengzhou 450052, China

Abstract

According to the World Health Organization (WHO) report, cancer is one of the leading causes of death, particularly in developing countries. The malignant proliferation and survival of cancer cells rely on the biosyntheses of proteins, nucleotides, and fatty acids. Accumulating pieces of evidence demonstrate that amino acid restrictions are valuable for cancer interventions. Meanwhile, folk remedies using dietary strategies are abused and lack solid rationale. To clarify what, why, and how the potential strategy is, here, we update and recommend a dietary strategy for cancer therapy: the intermittent dietary lysine restriction with the normal maize (lysine deficiency) as an intermittent staple food for days, weeks, or even months, will be a feasible strategy for cancer intervention. In addition, dietary and immunomodulatory supplements, such as low protein starchy foods, vegetables, fruit, and mushrooms, may serve as supplements to satisfy the daily needs of micronutrients and the plethora of dishes.

Keywords: Amino acid restriction, Arginine, Cachexia, Cancer, Glutamine, Kwashiorkor, Lysine, Tryptophan.

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* Corresponding author Jian-Sheng Kang: Clinical Systems Biology Laboratories, The First Affiliated Hospital of Zhengzhou University, Zhengzhou, 450052, China; E-mail: [e-mail]

INTRODUCTION

Cancer is a complex disease such that there are more than 100 distinct types of cancer sharing common metabolic characteristics, such as sustained proliferation and insensitivity to growth suppressors [1, 2]. The metabolisms of cancer cells are reprogrammed to maintain their proliferation and may even hijack normal cells to create a tumor microenvironment (TME) for survival and escaping the immune destruction [2]. For the sake of cancer cell growth and newly synthesized proteins as the actuators of cellular activities, protein biosynthesis consumes ~33% of total ATP consumption, which is the most energy-consumption process [3-5]. Intri-guingly, recent pieces of evidence have demonstrated that amino acid restrictions

play crucial roles in cancer intervention and therapy, including glycine restriction [6], serine starvation [7-9], leucine deprivation [10], glutamine blockade [11, 12], asparagine deficiency [13, 14], methionine limitation [15] and cysteine depletion [16]. We have recently recommended that a convenient dietary strategy for cancer intervention using the normal maize (lysine deficiency) as an intermittent staple food for days, weeks, or even months for lysine restriction, and low protein starchy foods, vegetables, and a fruit serving as complementary foods to meet daily micronutrient needs and for a rich diet [17]. Here, we update and summarize the dietary restriction of amino acids for cancer therapy in this e-book series ‘Frontiers in Clinical Drug Research - Anti-Cancer Agents’.

THE HETEROGENEITY AND HOMOGENEITY OF CANCER CELLS

According to the latest global cancer statistics in 2020, the GLOBOCAN estimates for 185 countries and 36 cancers and shows that there are an estimated 19.3 million new cancer cases and 10 million cancer deaths worldwide [18]. The top five diagnosed cancers are female breast (~2.3 million, 11.7%), lung (11.4%), colorectal (10.0%), prostate (7.3%) and stomach (5.6%) cancers. The top five leading causes of cancer death are lung (~1.8 million, 18%), colorectal (9.4%), liver (8.3%), stomach (7.7%), and female breast (6.9%) cancers [18]. These different types of tumors reflect the genetically/epigenetically diverse populations. The genetic and spatial heterogeneities of cell subpopulations are also apparent in the niche of TME. Importantly, the genetically temporal heterogeneity of cancer provides the fuel for resistance to drugs, such as tyrosine kinase inhibitors (TKIs) of EGFR [19]. The spontaneously resilient resistances in lung cancer under the pressure of TKIs strongly suggest that it is time to focus on an alternative strategy for cancer therapy beyond targeting cancer mutations.

In contrast to genetic heterogeneity, the metabolism of cancer is relatively homogenous. In the 1920s, Warburg made an important discovery (Warburg effect) that cancer cells preferred to metabolize glucose by glycolysis even in the presence of sufficient oxygen [20]. Compared to oxidative phosphorylation, glycolysis is a less efficient pathway to generate ATP with the concomitant production of lactic acid and the consequential acidification of the intracellular and extracellular microenvironment, which further leads to the deterioration of tumor [21-23]. Due to the requirement of growth of cancer cells for glucose, fluorodeoxyglucose positron emission tomography (FDG-PET) has been clinically exploited to diagnose tumors [22]. Consistently, the release and consumption patterns of NCI-60 cancer cell lines (Fig. 1) have demonstrated that glycolysis and glucose consumption are common characteristics of cancer metabolism [6].

Fig. (1))

The consumption and release profiles of 111 metabolites for the NCI-60 cancer cell lines [6] (reproduced with permission). Consumption is indicated by blue, while red color represents release.

Amino Acid Metabolism is the Leading Energy-consuming Process

The consumption and release (CORE) profiles of 111 metabolites (Fig. 1) illustrated the anabolic and catabolic features of the NCI-60 cancer cell lines [6]. The CORE profiles of cancer cells demonstrated the homogeneous demands for energy metabolism and protein synthesis, as both essential for the malignant proliferation of cancer cells. The substrates largely consumed in cancer cells include glucose and amino acids, such as leucine (L), tryptophan (W), serine (S), lysine (K), glycine (G), arginine (R), glutamine (Q), methionine (M), cysteine (C), tyrosine (Y) [6]. Meanwhile, the CORE profiles of glycine, aspartate, cytidine, uridine, and polyamines showed heterogeneous patterns in the NCI-60 cancer cell lines (Fig. 1) [6].

The CORE profiles also showed another common characteristic of the NCI-60 cancer cell lines, that is, the release of nucleotides and nucleobases [6]. Importantly, the CORE profiles suggested that cancer cells did not directly take the nucleobases and nucleotides as their substrates for RNA/DNA anabolism. The ATP consumption (approximately 25%) of RNA/DNA synthesis ranks second among the energy-consuming processes (Fig. 2B) [3, 4]. Cells can use aspartate, glutamine, serine, and glycine, as carbon and nitrogen resources for the syntheses of nucleobases (Fig. 2A) [24-26]. Hence, the amino acid metabolism for protein synthesis and RNA/DNA synthesis in cancer cells could use ~33-58% of the total energy (ATP) expenditure (Fig. 2B). Therefore, the energy expenditure of amino acid metabolism might account for the potential therapeutic effects of amino acid restrictions in cancer therapy.

Fig. (2))

Amino acid metabolism is the most ATP-consuming process and provides the carbon and nitrogen sources for protein and RNA/DNA syntheses (reused from [17]). A. Amino acids including aspartate, glutamine, serine, and glycine provide the essential carbon and nitrogen sources for the syntheses of nucleobases, such as Adenine (A), Thymine (T), Cytosine (C), Guanine (G), Uracil (U) and Hypoxanthine [26]. Those carbons and nitrogens highlighted in various colors represent their original sources, such as aspartate (brown), glutamine (blue), serine and folate cycle (purple), glycine (green), and bicarbonate radical (tan). B. The percentages of ATP-consuming processes in thymocytes [3], including protein synthesis (~33%), nucleotide synthesis (~25%), Ca-ATPase (~17%), Na/K-ATPase (~16%), and others processes (9%). Amino acid metabolism may use up to ~58% of the total ATP consumption for protein synthesis and nucleotide synthesis.

Leucine Metabolism

Since the consumptions and metabolisms of amino acids are the most crucial processes for cell growth, the most frequently used amino acid for protein synthesis might be a good candidate for dietary restriction in cancer therapy. Thus, the percentages of twenty amino acids for individual proteins in the human proteome (Uniport: UP000005640) were counted, sorted, and plotted from the largest to the smallest percentages for comparison (Fig. 3A). The results demonstrate that leucine (L) is the richest amino acid, serine (S) ranks second, and tryptophan (W) is the least used in the proteomes (Fig. 3).

Fig. (3))

A summary of amino acid abundances in humans, plants, and animals (reused from [17]). The essential and non-essential amino acids shown as their abbreviations, the essential amino acids (EAA), including Phenylalanine (F), Isoleucine (I), Lysine (K), Leucine (L), Methionine (M), Threonine (T), Valine (V), Tryptophan (W); the non-essential amino acids (NEAA) including Alanine (A), Cysteine (C), Aspartic Acid (D), Glutamic Acid (E), Glycine (G), Histidine (H), Asparagine (N), Proline (P), Glutamine (Q), Arginine (R), Serine (S), Tyrosine (Y). A. Sorted percentages of amino acids in human proteins. The percentages of twenty amino acids for all proteins in the human proteome (Uniport: UP000005640) were sorted and plotted from the largest to the smallest percentage. B. Sorted percentages of EAAs in human proteins. The arbitrary number of proteins in the human proteome is plotted with a log scale for clarity, especially EAA enrichment information. C. A 3D-bubble plotting for amino acid percentages in the proteomes of humans, plants, and animals. Human amino acid abundances are represented as the width of bubbles. D. Enriched GO-MF terms (p < 0.001) for lysine exceptional rich proteins (ERPs).

Unexpectedly, leucine ranks first since leucine is an essential amino acid (EAA). Leucine is necessary for protein synthesis and also acts as a signaling molecule activating mechanistic target of rapamycin (mTOR) signaling through sestrin-2, which acts as a cytosolic leucine sensor [30-37]. Interestingly, the circulating levels of EAAs in the blood may ideally represent a signal for food (amino acid) availability. Particularly, leucine, as one of the branched-chain amino acids (BCAAs), is not mainly catabolized in the liver where the activity of BCAA aminotransferase is low [31], so that leucine increases in circulation right after a meal [31]. Consequently, leucine acts as a nutritional signal to reduce food intake via inhibiting the expression of hypothalamic Agouti-related protein (Agrp) in a mTOR dependent manner [32, 33].

Despite leucine being the most abundant amino acid in the human proteome, leucine deprivation showed only modest effects on human breast cancer cells [10]. Leucine deprivation partially inhibited cancer proliferation by inhibiting the fatty acid synthase (FASN) expression in breast tumors [10]. On the other hand, leucine released from degraded proteins may efflux from the lysosome to meet the demand of cancer cell growth [34], so that the exceptional abundance of leucine in the human proteome might partially account for its limited effect on cell proliferation.

Tryptophan Catabolism

Tryptophan is a unique amino acid, which is the least abundant in the human proteome (Fig. 2A) and animal or plant foods (Fig. 3C). As an essential amino acid (EAA), tryptophan cannot be directly synthesized in vivo and must be acquired from food intake. This extraordinary characteristic of tryptophan gives its special roles beyond a necessity in protein synthesis. For example, the immune system could inhibit the growth of certain cancer cells by inducing tryptophan degradation and depleting nicotinamide adenine dinucleotide (NAD+) with interferon-gamma (IFN-γ) [35], which could upregulate the activity of indoleamine 2,3-dioxygenase (IDO) to facilitate tryptophan catabolism [36]. On the other hand, cancer cells could also use the same strategy to evade immune response in TME [37, 38]. Consequently, the multifaceted tryptophan metabolism (Fig. 4) might underline the off-target effects in the clinical trials of IDO inhi- bitors [39].

Among the metabolisms of amino acids, the catabolism of tryptophan might be the most sophisticated one (Fig. 4), which involved the regulation of immunity, neuronal function, and intestinal homeostasis [40]. About 95% of tryptophan is catabolized via the kynurenine pathway, where two rate-limiting enzymes are involved: indoleamine 2,3-dioxygenase (IDO) and tryptophan-2,3-dioxygenase (TDO). In addition, a small fraction of tryptophan is used beyond protein synthesis by tryptophan hydroxylase (TPH) to produce serotonin (5-hydroxytryptamine, 5-HT) and melatonin. Tryptophan and its metabolites are used and catabolized by various organs and cells to generate further bioactive metabolites, including neuroprotective kynurenic acid (KA) by astrocytes, neurotoxic quinolinic acid (QA) by microglia, neuromodulator tryptamine, immune-suppressive metabolites including 3-hydroxykynurenine (HK), 3-hydroxyanthranilic acid (HAA) and xanthurenic acid (XA) [40, 38]. Tryptophan can be used for the synthesis of nicotinamide adenine dinucleotide (NAD+) (Fig. 4) [40]. In addition, tryptophan is catabolized to generate nicotinic acid and nicotinamide (Fig. 4), two basic forms of vitamin B3, so that the deficiency of try-

ptophan is related to pellagra and pellagra-like dermatosis (Hartnup disease) [41, 42].

Fig. (4))

Tryptophan catabolism via two pathways: the kynurenine pathway and serotonin/melatonin pathway (adapted from wikipathways). Only a small portion of tryptophan is catabolized into serotonin and melatonin through TPH. The kynurenine pathway is the major pathway of tryptophan catabolism via the enzymes IDO and TDO. Kynurenine can be further catabolized into anthranilic acid (AA) through KYNU, kynurenic acid (KA) by KAT, or 3-OH kynurenine (HK) via KMO. Those catabolize highlighted in red are toxic or immune suppressive. IDO, indoleamine 2,3-dioxygenase; TDO, tryptophan-2,3-dioxygenase; TPH, tryptophan hydroxylase; AMFID, kynurenine formamidase; KYNU, kynureninase; KMO, kynurenine 3-monooxygenase; KAT, kynurenine aminotransferase; 3-HAO, 3-hydroxyanthranilate 3,4-dioxygenase; QPRT, quinolinate phosphoribosyltransferase; NAPRT, nicotinate phosphoribosyltransferase; NMNAT1/2/3, niacinamide mononucleotide adenosine transferase 1/2/3; NADSYN1, nicotinamide adenine dinucleotide synthetase 1.

Tryptophan catabolism is mainly processed in the liver by TDO [40]. Therefore, TDO knockout (Tdo-/-) mice showed increased levels of tryptophan and its catabolites (5-HT, kynurenine, and 5-hydroxyindole acetic acid) in plasma and 5-HT in the hippocampus and midbrain; moreover, Tdo-/- mice demonstrated anxiolytic modulation and the increase of adult neurogenesis [43], which was consistent to the cumulating pieces of evidence about the role of 5-HT in neurogenesis and anti-depression [44, 45]. 5-HT is also involved in food intake and mood [46], and recent advances focused on the functional modulation of the 5-HT6 receptor [47, 48]. Interestingly, both the agonists and antagonists of the 5-HT6 receptor could reduce food intake [47, 48], which might suggest that the activation curve of the 5-HT6 receptor is a bell shape and that a proper concentration of 5-HT might increase food intake through the 5-HT6 receptor. In short, tryptophan metabolism and its catabolites play active roles in many biological processes, such as proliferation, immunity, neurogenesis, anxiety, depression, and food intake.

Serine Metabolism

It is also well-known that serine is a non-essential amino acid (NEAA) and ranks second among the percentages of twenty amino acids in the human proteome (Fig. 3). The NEAA serine participates in many metabolic processes, for instance, protein, nucleotide, and glutathione syntheses, which are crucial for cell survival and proliferation [25]. Serine is an important one-carbon donor for the folate cycle (Fig. 5), which is important for nucleotide synthesis and NADPH generation for antioxidant response [25].

Fig. (5))

The one-carbon metabolism. One-carbon metabolism is comprised of coupled cycles: the folate cycle and the methionine cycle [25]. THF, methyl-THF, and formyl-THF are the key metabolites in the folate cycle. The methyl-THF can transfer its methyl to homocysteine via vitamin B12 and then turn it into methionine and THF. The formyl-THF can regenerate ATP and NADPH via MTHFD and ALDH1L enzymes, respectively. In the methionine cycle, the methionine can be converted into SAM via MAT enzyme in an ATP-dependent manner. The methyl group of SAM can be used for methylation of DNA, RNA, proteins, and lipids. Nutritional sources, including amino acids shown in green boxes, glucose, sarcosine, and choline shown in light yellow boxes, can be imported or synthesized de novo to enter one-carbon metabolism. THF, tetrahydrofolate; SAM, S-adenosylmethionine; MTHFD, methylenetetrahydrofolate dehydrogenase; ALDH1L, 10-formyl THF dehydrogenases; SHMT, serine hydroxymethyltransferase; GNMT, glycine N-methyltransferase; MTHFR, methylenetetrahydrofolate reductase; MS, methionine synthase; B12, vitamin B12; MAT, methionine adenosyltransferase.

The catabolism and anabolism of serine are increased in cancer cells (Fig. 1) [49-51]. The reinforced serine synthesis pathway (SSP) could make a significant contribution (~50%) to the anaplerosis of glutamine to α-ketoglutarate for mitochondrial tricarboxylic acid (TCA) cycle [49]. Serine starvation induced stress and promoted p53-independent and p53-dependent metabolic remodellings in cancer cells [7]. For the p53-independent metabolic remodelling, serine starvation upregulates or enhances de novo SSP and oxidative phosphorylation. For the p53-dependent metabolic remodeling, the inhibition of nucleotide synthesis was dependent on p53-p21 activation. The limited amount of de novo serine was shunted to glutathione production for cancer cells survival [7]. Consequently, serine starvation might have a potential role but is limited in the treatment of p53-deficient tumors.

Glycine Metabolism

The name glycine came from the Greek word glykys meaning sweet since glycine is sweet as glucose [52]. Glycine can be synthesized in endogenous pathways, so that glycine is an NEAA for mammals [52]. The glycine can be synthesized from serine, threonine, choline, and hydroxyproline in the liver and kidney [52]. Glycine can be catabolized through three pathways: (1) the decarboxylation and deamination by the glycine cleavage system (GCS), (2) the conversion into serine by serine hydroxymethyl-transferase (SHMT), (3) the production of glyoxylate by peroxisomal D-amino acid oxidase (DAAO) [52]. Glycine plays an essential role in the biosynthesis of glutathione, heme, creatine, nucleic acids, and uric acid [52]. Glycine could also take part in many processes, such as metabolic regulation, neurological function, and anti-oxidative reactions [52].

Although glycine might be important for rapid cancer cell proliferation by supporting de novo purine nucleotide biosynthesis [6], glycine restriction alone didn’t have the same detrimental effect on cancer cells as serine starvation. Glycine can be converted from serine by serine hydroxymethyltransferase (SHMT) in one-carbon metabolism (Fig. 5) [7, 24, 25], especially the mitochondrial glycine synthesis enzyme SHMT2 [6]. The inter-conversion between serine and glycine might account for the limited effects of glycine restriction for cancer interventions. Interestingly, the CORE profiles of the NCI-60 cancer cell lines demonstrated that glycine showed the most heterogeneous consumption/release pattern among amino acids, while serine showed relatively homogenous consumption [6]. Interestingly, beyond the potential role of glycine restriction in blocking the rapid growth of certain cancer cells, the dietary supplement of glycine was also reported to inhibit the growth of certain tumors, such as liver tumors [53] and melanoma tumors [54]. Hence, the heterogeneous anabolism and catabolism of glycine in miscellaneous cancer cells might account for its paradoxical effects.

Lysine is a Particularly Important EAA

Because there are de novo synthesis pathways for NEAA, such as serine, the restrictions of NEAA have relatively low or narrow-spectrum effectivity in cancer interventions [7]. Consequently, we focused on EAAs and plotted the percentages of EAAs in human proteins with a log scale (Fig. 3B) [17]. From this log-scaled view of EAA enrichment profiles, lysine is particularly noteworthy (Fig. 3B). The percentages of amino acids in fourteen plants and nine animals were analyzed, and the medians and standard deviations (SD) of amino acids in these proteomes were plotted. For comparison, the abundances of human amino acids are represented as the width of bubbles in Fig. (2C) [17]. The results above have demonstrated that the abundances of amino acids were consistent among plants, animals, and humans, with subtle differences. Lysine ranks in the middle level of all twenty amino acids Fig. (3C), which might lead to lysine restriction to avoid the limited inhibitory effect of leucine restriction [10], which might be compensated by the leucine re-usage from degraded proteins in cancer cells [34].

We sorted EAA exceptional rich proteins (ERPs) by the 3-sigma upper limit (median + 3x SD). For EAAs, the numbers of ERPs rang from 604 to 1298 [17]. In human proteome, the averaged percentages of lysine, valine, and leucine were 5.26% ± 3.27%, 5.88% ± 2.50% and 9.9% ± 3.71% respectively (in median ± SD, Fig. (3A). By contrast, the averaged percentages of lysine ERPs (n = 918) was 16.99% ± 3.65%, valine ERPs (n = 595) was 14.89% ± 2.62% and leucine ERPs (n = 604) was 23.28% ± 4.5%, respectively (in median ± SD, Fig. 3B). As is shown in Fig. (3), despite that the percentages of lysine ranked third after leucine and valine in the human proteome, the lysine ERPs percentage ranked second only after leucine. The averaged percentage of tryptophan ERPs (6.05% ± 1.86% in median ± SD, n = 1298) was the lowest (Fig. 3B).

Furthermore, ERPs were chosen for functional enrichment analysis with the webserver of g: Profiler [55]. Most surprisingly, there was no appropriate term to satisfy the significant threshold (p < 0.001) for leucine by gene ontology-molecular function (GO-MF), whereas lysine ERPs demonstrated many enriched GO-MF terms, including nucleic acid/chromatin/nucleosome binding, the structural constituent of ribosome, and so on (Fig. 3D). In summary, these implicated that lysine and its ERPs played important roles in cellular functions and that cell proliferation might be sensitive for lysine restriction. Indeed, lysine deprivation inhibited the proliferation of both p53-competent and p53-deficient cancer cells completely [7].

Lysine and its ERPs are not only associated with cancer cell growth but also with certain diseases. Kwashiorkor is a childhood nutritional disease associated with lysine deficiency in a normal maize diet [56]. Normal maize has higher protein levels than rice, except for the deficiencies of two EAAs, lysine and tryptophan, which caused malnutrition and imbalance of amino acids [56, 57]. Nowadays, maize is biofortified and named as quality protein maize (QPM). QPM contains an opaque-2 gene, which codes a transcriptional activator so that more lysine and tryptophan-rich proteins were expressed in QPM [57]. In 1933, Williams reported five cases about kwashiorkor; all patients were only fed with the food prepared from normal maize or cassava because of lacking breast-feeding; those children developed into the kwashiorkor