Metabolic changes associated with methionine stress sensitivity in MDA-MB-468 breast cancer cells

Nutrient control of splice site selection contributes to methionine addiction of cancer

OBJECTIVE

Many cancer cells depend on exogenous methionine for proliferation, whereas non-tumorigenic cells can divide in media supplemented with the metabolic precursor homocysteine. This phenomenon is known as methionine dependence of cancer or methionine addiction. The underlying mechanisms driving this cancer-specific metabolic addiction are poorly understood. Here we find that methionine dependence is associated with severe dysregulation of pre-mRNA splicing.

METHODS

We used triple-negative breast cancer cells and their methionine-independent derivatives R8 to compare RNA expression profiles in methionine and homocysteine growth media. The data set was also analyzed for alternative splicing.

RESULTS

When tumorigenic cells were cultured in homocysteine medium, cancer cells failed to efficiently methylate the spliceosomal snRNP component SmD1, which resulted in reduced binding to the Survival-of-Motor-Neuron protein SMN leading to aberrant splicing. These effects were specific for cancer cells as neither Sm protein methylation nor splicing fidelity was affected when non-tumorigenic cells were cultured in homocysteine medium. Sm protein methylation is catalyzed by Protein Arginine Methyl Transferase 5 (Prmt5). Reducing methionine concentrations in the culture medium sensitized cancer cells to Prmt5 inhibition supporting a mechanistic link between methionine dependence of cancer and splicing.

CONCLUSIONS

Our results link nutritional demands to splicing changes and thereby provide a link between the cancer-specific metabolic phenomenon, described as methionine addiction over 40 years ago, with a defined cellular pathway that contributes to cancer cell proliferation.

Multimodal Phasor Approach to study breast cancer cells invasion in 3D spheroid model

We implemented a multimodal set of functional imaging techniques optimized for deep-tissue imaging to investigate how cancer cells invade surrounding tissues and how their physiological properties change in the process. As a model for cancer invasion of the extracellular matrix, we created 3D spheroids from triple-negative breast cancer cells (MDA-MB-231) and non-tumorigenic breast epithelial cells (MCF-10A). We analyzed multiple hallmarks of cancer within the same spheroid by combining a number of imaging techniques, such as metabolic imaging of NADH by Fluorescence Lifetime Imaging Microscopy (NADH-FLIM), hyperspectral imaging of a solvatochromic lipophilic dye (Nile Red) and extracellular matrix imaging by Second Harmonic Generation (SHG). We included phasor-based bioimage analysis of spheroids at three different time points, tracking both morphological and biological properties, including cellular metabolism, fatty acids storage, and collagen organization. Employing this multimodal deep-imaging framework, we observed and quantified cancer cell plasticity in response to changes in the environment composition.

Deprivation of methionine inhibits osteosarcoma growth and metastasis via C1orf112-mediated regulation of mitochondrial functions

Osteosarcoma is a malignant bone tumor that primarily inflicts the youth. It often metastasizes to the lungs after chemotherapy failure, which eventually shortens patients' lives. Thus, there is a dire clinical need to develop a novel therapy to tackle osteosarcoma metastasis. Methionine dependence is a special metabolic characteristic of most malignant tumor cells that may offer a target pathway for such therapy. Herein, we demonstrated that methionine deficiency restricted the growth and metastasis of cultured human osteosarcoma cells. A genetically engineered Salmonella, SGN1, capable of overexpressing an L-methioninase and hydrolyzing methionine led to significant reduction of methionine and S-adenosyl-methionine (SAM) specifically in tumor tissues, drastically restricted the growth and metastasis in subcutaneous xenograft, orthotopic, and tail vein-injected metastatic models, and prolonged the survival of the model animals. SGN1 also sharply suppressed the growth of patient-derived organoid and xenograft. Methionine restriction in the osteosarcoma cells initiated severe mitochondrial dysfunction, as evident in the dysregulated gene expression of respiratory chains, increased mitochondrial ROS generation, reduced ATP production, decreased basal and maximum respiration, and damaged mitochondrial membrane potential. Transcriptomic and molecular analysis revealed the reduction of C1orf112 expression as a primary mechanism underlies methionine deprivation-initiated suppression on the growth and metastasis as well as mitochondrial functions. Collectively, our findings unraveled a molecular linkage between methionine restriction, mitochondrial function, and osteosarcoma growth and metastasis. A pharmacological agent, such as SGN1, that can achieve tumor specific deprivation of methionine may represent a promising modality against the metastasis of osteosarcoma and potentially other types of sarcomas as well.

Rescue of Methionine Dependence by Cobalamin in a Human Colorectal Cancer Cell Line

Methionine dependence is a characteristic of most cancer cells where they are unable to proliferate when the essential amino acid methionine is replaced with its precursor homocysteine in the growing media. Normal cells, on the other hand, thrive under these conditions and are referred to as methionine-independent. The reaction that adds a methyl group from 5-methyltetrahydrofolate to homocysteine to regenerate methionine is catalyzed by the enzyme methionine synthase with the cofactor cobalamin (vitamin B12). However, decades of research have shown that methionine dependence in cancer is not due to a defect in the activity of methionine synthase. Cobalamin metabolism has been tied to the dependent phenotype in rare cell lines. We have identified a human colorectal cancer cell line in which the cells regain the ability to proliferation in methionine-free, L-homocystine-supplemented media when cyanocobalamin is supplemented at a level of 1 µg/mL. In human SW48 cells, methionine replacement with L-homocystine does not induce any measurable increase in apoptosis or reactive oxygen species production in this cell line. Rather, proliferation is halted, then restored in the presence of cyanocobalamin. Our data show that supplementation with cyanocobalamin prevents the activation of the integrated stress response (ISR) in methionine-deprived media in this cell line. The ISR-associated cell cycle arrest, characteristic of methionine-dependence in cancer, is also prevented, leading to the continuation of proliferation in methionine-deprived SW48 cells with cobalamin. Our results highlight differences between cancer cell lines in the response to cobalamin supplementation in the context of methionine dependence.

The Epigenetic Landscape of Breast Cancer, Metabolism, and Obesity

Obesity is a risk factor for developing breast cancer, and significantly increases mortality rates in patients diagnosed with this disease. Drivers of this unfortunate relationships are multifactorial, with obesity-induced changes in the epigenetic state of breast cancer cells being identified as a critical mechanism that impact survival, metastasis, and therapeutic responses. Recent studies have investigated the epigenetic landscape of breast cancer to elucidate the molecular interplay between the breast tissue epigenome and its cellular microenvironment. This chapter highlights studies that demonstrates the impact of obesity on the epigenome and metabolome of breast cancer cells. Furthermore, we discuss how obesity impacts the efficacy of chemotherapy and epigenetic targeting drugs, including the emergence of drug-resistance clonal populations. Delineating the relationships between the obesity and epigenetic changes in breast cancer cells will help identify therapeutic strategies which could improve survival outcomes in the rapidly growing number of patients with obesity and cancer.

The Role of Amino Acids in the Diagnosis, Risk Assessment, and Treatment of Breast Cancer: A Review

This review summarizes the role of amino acids in the diagnosis, risk assessment, imaging, and treatment of breast cancer. It was shown that the content of individual amino acids changes in breast cancer by an average of 10-15% compared with healthy controls. For some amino acids (Thr, Arg, Met, and Ser), an increase in concentration is more often observed in breast cancer, and for others, a decrease is observed (Asp, Pro, Trp, and His). The accuracy of diagnostics using individual amino acids is low and increases when a number of amino acids are combined with each other or with other metabolites. Gln/Glu, Asp, Arg, Leu/Ile, Lys, and Orn have the greatest significance in assessing the risk of breast cancer. The variability in the amino acid composition of biological fluids was shown to depend on the breast cancer phenotype, as well as the age, race, and menopausal status of patients. In general, the analysis of changes in the amino acid metabolism in breast cancer is a promising strategy not only for diagnosis, but also for developing new therapeutic agents, monitoring the treatment process, correcting complications after treatment, and evaluating survival rates.

Denoising Autoencoder Normalization for Large-Scale Untargeted Metabolomics by Gas Chromatography-Mass Spectrometry

Large-scale metabolomics assays are widely used in epidemiology for biomarker discovery and risk assessments. However, systematic errors introduced by instrumental signal drifting pose a big challenge in large-scale assays, especially for derivatization-based gas chromatography-mass spectrometry (GC-MS). Here, we compare the results of different normalization methods for a study with more than 4000 human plasma samples involved in a type 2 diabetes cohort study, in addition to 413 pooled quality control (QC) samples, 413 commercial pooled plasma samples, and a set of 25 stable isotope-labeled internal standards used for every sample. Data acquisition was conducted across 1.2 years, including seven column changes. In total, 413 pooled QC (training) and 413 BioIVT samples (validation) were used for normalization comparisons. Surprisingly, neither internal standards nor sum-based normalizations yielded median precision of less than 30% across all 563 metabolite annotations. While the machine-learning-based SERRF algorithm gave 19% median precision based on the pooled quality control samples, external cross-validation with BioIVT plasma pools yielded a median 34% relative standard deviation (RSD). We developed a new method: systematic error reduction by denoising autoencoder (SERDA). SERDA lowered the median standard deviations of the training QC samples down to 16% RSD, yielding an overall error of 19% RSD when applied to the independent BioIVT validation QC samples. This is the largest study on GC-MS metabolomics ever reported, demonstrating that technical errors can be normalized and handled effectively for this assay. SERDA was further validated on two additional large-scale GC-MS-based human plasma metabolomics studies, confirming the superior performance of SERDA over SERRF or sum normalizations.

Signaling pathways in cancer metabolism: mechanisms and therapeutic targets

A wide spectrum of metabolites (mainly, the three major nutrients and their derivatives) can be sensed by specific sensors, then trigger a series of signal transduction pathways and affect the expression levels of genes in epigenetics, which is called metabolite sensing. Life body regulates metabolism, immunity, and inflammation by metabolite sensing, coordinating the pathophysiology of the host to achieve balance with the external environment. Metabolic reprogramming in cancers cause different phenotypic characteristics of cancer cell from normal cell, including cell proliferation, migration, invasion, angiogenesis, etc. Metabolic disorders in cancer cells further create a microenvironment including many kinds of oncometabolites that are conducive to the growth of cancer, thus forming a vicious circle. At the same time, exogenous metabolites can also affect the biological behavior of tumors. Here, we discuss the metabolite sensing mechanisms of the three major nutrients and their derivatives, as well as their abnormalities in the development of various cancers, and discuss the potential therapeutic targets based on metabolite-sensing signaling pathways to prevent the progression of cancer.

Acute stress reduces population-level metabolic and proteomic variation

BACKGROUND

Variation in omics data due to intrinsic biological stochasticity is often viewed as a challenging and undesirable feature of complex systems analyses. In fact, numerous statistical methods are utilized to minimize the variation among biological replicates.

RESULTS

We demonstrate that the common statistics relative standard deviation (RSD) and coefficient of variation (CV), which are often used for quality control or part of a larger pipeline in omics analyses, can also be used as a metric of a physiological stress response. Using an approach we term Replicate Variation Analysis (RVA), we demonstrate that acute physiological stress leads to feature-wide canalization of CV profiles of metabolomes and proteomes across biological replicates. Canalization is the repression of variation between replicates, which increases phenotypic similarity. Multiple in-house mass spectrometry omics datasets in addition to publicly available data were analyzed to assess changes in CV profiles in plants, animals, and microorganisms. In addition, proteomics data sets were evaluated utilizing RVA to identify functionality of reduced CV proteins.

CONCLUSIONS

RVA provides a foundation for understanding omics level shifts that occur in response to cellular stress. This approach to data analysis helps characterize stress response and recovery, and could be deployed to detect populations under stress, monitor health status, and conduct environmental monitoring.

Oncogenes and Methionine Addiction of Cancer: Role of c-MYC

BACKGROUND/AIM

Methionine addiction is a general and fundamental hallmark of cancer cells, termed the Hoffman effect. Previously Vanhamme and Szpirer showed that methionine addiction could be induced by transfection of the activated HRAS1 gene to a normal cell line. In the present study, we investigated the role of the c-MYC oncogene in methionine addiction of cancer, by comparison of c-Myc expression and malignancy of methionine-addicted osteosarcoma cells and rare methionine-independent revertants, derived from the methionine-addicted cells.

MATERIALS AND METHODS

Methionine-independent revertant 143B osteosarcoma cells (143B-R) were derived from methionine-addicted parental 143B osteosarcoma cells (143B-P), by continuous culture in medium depleted of methionine by recombinant methioninase. To compare in vitro malignancy of methionine-addicted parental cells and methionine-independent revertant cells, the following experiments were performed: for 143B-P and 143B-R cells, cell proliferation capacity was measured with a cell-counting assay, and colony-formation capacity was determined on plastic and in soft agar, all in methionine-containing Dulbecco's Modified Eagle's Medium (DMEM). Tumor growth was measured in orthotopic xenograft nude-mouse models, to compare in vivo malignancy of 143B-P and 143B-R cells. c-MYC expression was examined with western immunoblotting and compared in 143B-P and 143B-R cells.

RESULTS

143B-R cells had reduced cell proliferation capacity, compared to 143B-P cells, in methionine-containing medium (p=0.003). 143B-R cells had reduced colony formation capacity on plastic (p=0.003) and in soft agar, compared to 143B-P cells in methionine-containing medium. 143B-R cells had reduced tumor growth in orthotopic xenograft nude-mouse models, compared to 143B-P cells, (p=0.002). These results demonstrate that 143B-R methionine-independent revertant cells lost malignancy. Expression of c-MYC was reduced in 143B-R methionine-independent revertant osteosarcoma cells, compared to 143B-P cells, (p=0.0007).

CONCLUSION

The present study demonstrated that c-MYC expression is linked to malignancy and methionine addiction of cancer cells. The present study on c-MYC, and the previous study on HRAS1, suggest that oncogenes may play a role in methionine addiction, which is a hallmark of all cancers, as well as in malignancy.

Simultaneous SGLT2 inhibition and caloric restriction improves insulin resistance and kidney function in OLETF rats

SGLT2 inhibitors (SGLT2i) are emerging as a novel therapy for type 2 diabetes due to their effective hypoglycemic and potential cardio- and nephroprotective effects, while caloric restriction (CR) is a common behavioral modification to improve adiposity and insulin resistance. Therefore, both interventions simultaneously may potentially further improve metabolic syndrome by enhancing carbohydrate metabolism. To test this hypothesis, cohorts of 10-week old, male Long Evans Tokushima Otsuka (LETO) and Otsuka Long Evans Tokushima Fatty (OLETF) rats were treated with SGLT2i (10 mg luseoglifozin/kg/day x 4 wks) (OLETF only) and/or 30% CR (2 wks at 12 weeks of age). CR maintained body mass in both strains while SGLT2i alone did not have any effect on body mass. Simultaneous treatments decreased SBP in OLETF vs SGLT2i alone, decreased insulin resistance index (IRI), and increased creatinine clearance vs OLETF ad lib. Conversely, CR decreased albuminuria independent of SGLT2i. In conclusion, SGLT2i treatment by itself did not elicit significant improvements in insulin resistance, kidney function or blood pressure. However, when combined with CR, these changes where more profound than with CR alone without inducing chronic hypoglycemia.

One-Carbon and Polyamine Metabolism as Cancer Therapy Targets

Cancer metabolic reprogramming is essential for maintaining cancer cell survival and rapid replication. A common target of this metabolic reprogramming is one-carbon metabolism which is notable for its function in DNA synthesis, protein and DNA methylation, and antioxidant production. Polyamines are a key output of one-carbon metabolism with widespread effects on gene expression and signaling. As a result of these functions, one-carbon and polyamine metabolism have recently drawn a lot of interest for their part in cancer malignancy. Therapeutic inhibitors that target one-carbon and polyamine metabolism have thus been trialed as anticancer medications. The significance and future possibilities of one-carbon and polyamine metabolism as a target in cancer therapy are discussed in this review.

Reversion of methionine addiction of osteosarcoma cells to methionine independence results in loss of malignancy, modulation of the epithelial-mesenchymal phenotype and alteration of histone-H3 lysine-methylation

Methionine addiction, a fundamental and general hallmark of cancer, known as the Hoffman Effect, is due to altered use of methionine for increased and aberrant transmethylation reactions. However, the linkage of methionine addiction and malignancy of cancer cells is incompletely understood. An isogenic pair of methionine-addicted parental osteosarcoma cells and their rare methionine-independent revertant cells enabled us to compare them for malignancy, their epithelial-mesenchymal phenotype, and pattern of histone-H3 lysine-methylation. Methionine-independent revertant 143B osteosarcoma cells (143B-R) were selected from methionine-addicted parental cells (143B-P) by their chronic growth in low-methionine culture medium for 4 passages, which was depleted of methionine by recombinant methioninase (rMETase). Cell-migration capacity was compared with a wound-healing assay and invasion capability was compared with a transwell assay in 143B-P and 143B-R cells in vitro. Tumor growth and metastatic potential were compared after orthotopic cell-injection into the tibia bone of nude mice in vivo. Epithelial-mesenchymal phenotypic expression and the status of H3 lysine-methylation were determined with western immunoblotting. 143B-P cells had an IC50 of 0.20 U/ml and 143B-R cells had an IC50 of 0.68 U/ml for treatment with rMETase, demonstrating that 143B-R cells had regained the ability to grow in low methionine conditions. 143B-R cells had reduced cell migration and invasion capability in vitro, formed much smaller tumors than 143B-P cells and lost metastatic potential in vivo, indicating loss of malignancy in 143B-R cells. 143B-R cells showed gain of the epithelial marker, ZO-1 and loss of mesenchymal markers, vimentin, Snail, and Slug and, an increase of histone H3K9me3 and H3K27me3 methylation and a decrease of H3K4me3, H3K36me3, and H3K79me3 methylation, along with their loss of malignancy. These results suggest that shifting the balance in histone methylases might be a way to decrease the malignant potential of cells. The present results demonstrate the rationale to target methionine addiction for improved sarcoma therapy.

Targeting the methionine addiction of cancer

Methionine is the initiator amino acid for protein synthesis, the methyl source for most nucleotide, chromatin, and protein methylation, and the carbon backbone for various aspects of the cellular antioxidant response and nucleotide biosynthesis. Methionine is provided in the diet and serum methionine levels fluctuate based on dietary methionine content. Within the cell, methionine is recycled from homocysteine via the methionine cycle, which is linked to nutrient status via one-carbon metabolism. Unlike normal cells, many cancer cells, both in vitro and in vivo, show high methionine cycle activity and are dependent on exogenous methionine for continued growth. However, the molecular mechanisms underlying the methionine dependence of diverse malignancies are poorly understood. Methionine deprivation initiates widespread metabolic alterations in cancer cells that enable them to survive despite limited methionine availability, and these adaptive alterations can be specifically targeted to enhance the activity of methionine deprivation, a strategy we have termed "metabolic priming". Chemotherapy-resistant cell populations such as cancer stem cells, which drive treatment-resistance, are also sensitive to methionine deprivation, suggesting dietary methionine restriction may inhibit metastasis and recurrence. Several clinical trials in cancer are investigating methionine restriction in combination with other agents. This review will explore new insights into the mechanisms of methionine dependence in cancer and therapeutic efforts to translate these insights into enhanced clinical activity of methionine restriction in cancer.

Loss of MAT2A compromises methionine metabolism and represents a vulnerability in H3K27M mutant glioma by modulating the epigenome

Diffuse midline gliomas (DMGs) bearing driver mutations of histone 3 lysine 27 (H3K27M) are incurable brain tumors with unique epigenomes. Here, we generated a syngeneic H3K27M mouse model to study the amino acid metabolic dependencies of these tumors. H3K27M mutant cells were highly dependent on methionine. Interrogating the methionine cycle dependency through a short-interfering RNA screen identified the enzyme methionine adenosyltransferase 2A (MAT2A) as a critical vulnerability in these tumors. This vulnerability was not mediated through the canonical mechanism of MTAP deletion; instead, DMG cells have lower levels of MAT2A protein, which is mediated by negative feedback induced by the metabolite decarboxylated S-adenosyl methionine. Depletion of residual MAT2A induces global depletion of H3K36me3, a chromatin mark of transcriptional elongation perturbing oncogenic and developmental transcriptional programs. Moreover, methionine-restricted diets extended survival in multiple models of DMG in vivo. Collectively, our results suggest that MAT2A presents an exploitable therapeutic vulnerability in H3K27M gliomas.

Linkage of methionine addiction, histone lysine hypermethylation, and malignancy

Methionine addiction, found in all types of cancer investigated, is because of the overuse of methionine by cancer cells for excess transmethylation reactions. In the present study, we compared the histone H3 lysine-methylation status and degree of malignancy between methionine-addicted cancer cells and their isogenic methionine-independent revertants, selected by their growth in low concentration of methionine. The methionine-independent revertans can grow on low levels of methionine or independently of exogenous methionine using methionine precursors, as do normal cells. In the methionine-independent revertants, the excess levels of trimethylated histone H3 lysine marks found in the methionine-addicted parental cancer cells were reduced or lost, and their tumorigenicity and experimental metastatic potential in nude mice were also highly reduced. Methionine addiction of cancer is linked with malignancy and hypermethylation of histone H3 lysines. The results of the present study thus provide a unique framework to further understand a fundamental basis of malignancy.

•

Methionine(MET)-independent revertants were selected from MET-addicted cancer cells

•

MET-independent revertants had greatly reduced malignancy

•

MET-independent revertants have lost or reduced methylation of H3 lysine marks

•

MET addiction, malignancy, and hypermethylated H3 lysine marks are linked

Imaging Sub-Cellular Methionine and Insulin Interplay in Triple Negative Breast Cancer Lipid Droplet Metabolism

Triple negative breast cancer (TNBC) is a particularly aggressive cancer subtype that is difficult to diagnose due to its discriminating epidemiology and obscure metabolome. For the first time, 3D spatial and chemometric analyses uncover the unique lipid metabolome of TNBC under the tandem modulation of two key metabolites - insulin and methionine - using non-invasive optical techniques. By conjugating heavy water (D2O) probed Raman scattering with label-free two-photon fluorescence (TPF) microscopy, we observed altered de novo lipogenesis, 3D lipid droplet morphology, and lipid peroxidation under various methionine and insulin concentrations. Quantitative interrogation of both spatial and chemometric lipid metabolism under tandem metabolite modulation confirms significant interaction of insulin and methionine, which may prove to be critical therapeutic targets, and proposes a powerful optical imaging platform with subcellular resolution for metabolic and cancer research.

Mass recovery following caloric restriction reverses lipolysis and proteolysis, but not gluconeogenesis, in insulin resistant OLETF rats

Caloric restriction (CR) is one of the most important behavioral interventions to reduce excessive abdominal adiposity, which is a risk factor for the development of insulin resistance. Previous metabolomics studies have characterized substrate metabolism during healthy conditions; however, the effects of CR and subsequent mass recovery on shifts in substrate metabolism during insulin resistance (IR) have not been widely investigated. To assess the effects of acute CR and the subsequent mass recovery on shifts in substrate metabolism, a cohort of 15-week old Long Evans Tokushima Otsuka (LETO) and Otsuka Long Evans Tokushima Fatty (OLETF) rats were calorie restricted (CR: 50% × 10 days) with or without partial body mass recovery (PR; 73% x 7 days), along with their respective ad libitum controls. End-of-study plasma samples were analyzed for primary carbon metabolites by gas chromatography (GC) time-of-flight (TOF) mass spectrometry (MS) data acquisition. Data analysis included PCA, Pearson correlation vs previously reported variables (adipose and body masses, and insulin resistance index, IRI), and metabolomics maps (MetaMapp) generated for the most significant group comparisons. All treatments elicited a significant group differentiation in at least one principal component. CR improved TCA cycle in OLETF, and increased lipolysis and proteolysis. These changes were reversed after PR except for gluconeogenesis. Plasma lipid concentrations were inversely correlated to IRI in LETO, but not OLETF. These shifts in substrate metabolism suggest that the CR-induced decreases in adipose may not be sufficient to more permanently alter substrate metabolism to improve IR status during metabolic syndrome.

The Interleukin-6/Signal Transducer and Activator of Transcription-3/Cystathionine γ-Lyase Axis Deciphers the Transformation Between the Sensitive and Resistant Phenotypes of Breast Cancer Cells

Drug resistance of cancer cells is associated with redox homeostasis. The mechanism of acquired resistance of cancer cells to antitumor drugs is not well understood. Our previous studies revealed that drug resistance and highly expressed P-glycoprotein (P-gp) of MCF-7 breast cancer cells was dependent on intracellular redox homeostasis and declined capacity for scavenging reactive oxygen species (ROS). Recently, we observed that, unlike nontumorigenic cells MCF-10A, three tumorigenic breast cancer cells (MCF-7S, BT474, MDA-MB-231) reprogrammed their metabolism, highly expressed cystathionine-γ-lyase (CTH), and acquired a particular specialty to use methionine (Met) to synthesize glutathione (GSH) through the transsulfuration pathway. Interestingly, doxorubicin (adriamycin) further reprogrammed metabolism of MCF-7 cells sensitive to adriamycin (MCF-7S) and induced them to be another MCF-7 cell line resistant to adriamycin (MCF-7R) with dramatically downregulated CTH. The two MCF-7 cell lines showed distinctly different phenotypes in terms of intracellular GSH, ROS levels, expression and activity of P-gp and CTH, and drug resistance. We showed that CTH modulation or the methionine supply brought about the interconversion between MCF-7S and MCF-7R. Methionine deprivation or CTH silencing induced a resistant MCF-7R and lowered paclitaxel activity, yet methionine supplementation or CTH overexpression reversed the above effects, induced a sensitive phenotype of MCF-7S, and significantly increased the cytotoxicity of paclitaxel both in vitro and in vivo. Interleukin-6 (IL-6)/signal transducer and activator of transcription-3 (STAT3) initiated CTH expression and activity, and the effect on the resistant phenotype was exclusively dependent on CTH and ROS. This study suggests that the IL-6/STAT3/CTH axis plays a key role in the transformation between sensitive and resistant MCF-7 cells. SIGNIFICANCE STATEMENT: Cystathionine γ-lyase (CTH) plays a key role in transformation between the sensitive and resistant phenotypes of MCF-7 cells and is dependent on the interleukin-6 (IL-6)/signal transducer and activator of transcription-3 (STAT3) signaling axis. Modulation of the transsulfuration pathway on CTH or IL-6/STAT3 or methionine supplementation is beneficial for reversing the resistance of MCF-7 cells, which indicates a clinical translation potential.

Aberrant Metabolism as Inductor of Epigenetic Changes in Breast Cancer: Therapeutic Opportunities

Aberrant metabolism is arising interest in the scientific community not only because of the role it plays in the development and establishment of the tumor mass but also the possibility of drug poisoning of key enzymes overexpressed in tumor cells. Moreover, tumor metabolism provides key molecules to maintain the epigenetic changes that are also an undisputed characteristic of each tumor type. This metabolic change includes the Warburg effect and alterations in key pathways involved in glutaminolysis, pentose phosphate, and unsaturated fatty acid biosynthesis. Modifications in all these pathways have consequences that impact genetics and epigenetics processes such as DNA methylation patterns, histone post-translational modifications, triggering oncogenes activation, and loss in tumor suppressor gene expression to lead the tumor establishment. In this review, we describe the metabolic rearrangement and its association with epigenetic regulation in breast cancer, as well as its implication in biological processes involved in cancer progression. A better understanding of these processes could help to find new targets for the diagnosis, prognosis, and treatment of this human health problem.

Methylosystem for Cancer Sieging Strategy

As cancer is a genetic disease, methylation defines a biologically malignant phenotype of cancer in the association of one-carbon metabolism-dependent S-adenosylmethionine (SAM) as a methyl donor in each cell. Methylated substances are involved in intracellular metabolism, but via intercellular communication, some of these can also be secreted to affect other substances. Although metabolic analysis at the single-cell level remains challenging, studying the "methylosystem" (i.e., the intercellular and intracellular communications of upstream regulatory factors and/or downstream effectors that affect the epigenetic mechanism involving the transfer of a methyl group from SAM onto the specific positions of nucleotides or other metabolites in the tumor microenvironment) and tracking these metabolic products are important research tasks for understanding spatial heterogeneity. Here, we discuss and highlight the involvement of RNA and nicotinamide, recently emerged targets, in SAM-producing one-carbon metabolism in cancer cells, cancer-associated fibroblasts, and immune cells. Their significance and implications will contribute to the discovery of efficient methods for the diagnosis of and therapeutic approaches to human cancer.

Integration of Epigenetic Mechanisms into Non-Genotoxic Carcinogenicity Hazard Assessment: Focus on DNA Methylation and Histone Modifications

Epigenetics involves a series of mechanisms that entail histone and DNA covalent modifications and non-coding RNAs, and that collectively contribute to programing cell functions and differentiation. Epigenetic anomalies and DNA mutations are co-drivers of cellular dysfunctions, including carcinogenesis. Alterations of the epigenetic system occur in cancers whether the initial carcinogenic events are from genotoxic (GTxC) or non-genotoxic (NGTxC) carcinogens. NGTxC are not inherently DNA reactive, they do not have a unifying mode of action and as yet there are no regulatory test guidelines addressing mechanisms of NGTxC. To fil this gap, the Test Guideline Programme of the Organisation for Economic Cooperation and Development is developing a framework for an integrated approach for the testing and assessment (IATA) of NGTxC and is considering assays that address key events of cancer hallmarks. Here, with the intent of better understanding the applicability of epigenetic assays in chemical carcinogenicity assessment, we focus on DNA methylation and histone modifications and review: (1) epigenetic mechanisms contributing to carcinogenesis, (2) epigenetic mechanisms altered following exposure to arsenic, nickel, or phenobarbital in order to identify common carcinogen-specific mechanisms, (3) characteristics of a series of epigenetic assay types, and (4) epigenetic assay validation needs in the context of chemical hazard assessment. As a key component of numerous NGTxC mechanisms of action, epigenetic assays included in IATA assay combinations can contribute to improved chemical carcinogen identification for the better protection of public health.

Impact of the Pd2Spm (Spermine) Complex on the Metabolism of Triple-Negative Breast Cancer Tumors of a Xenograft Mouse Model

The interest in palladium(II) compounds as potential new anticancer drugs has increased in recent years, due to their high toxicity and acquired resistance to platinum(II)-derived agents, namely cisplatin. In fact, palladium complexes with biogenic polyamines (e.g., spermine, Pd₂Spm) have been known to display favorable antineoplastic properties against distinct human breast cancer cell lines. This study describes the in vivo response of triple-negative breast cancer (TNBC) tumors to the Pd₂Spm complex or to cisplatin (reference drug), compared to tumors in vehicle-treated mice. Both polar and lipophilic extracts of tumors, excised from a MDA-MB-231 cell-derived xenograft mouse model, were characterized through nuclear magnetic resonance (NMR) metabolomics. Interestingly, the results show that polar and lipophilic metabolomes clearly exhibit distinct responses for each drug, with polar metabolites showing a stronger impact of the Pd(II)-complex compared to cisplatin, whereas neither drug was observed to significantly affect tumor lipophilic metabolism. Compared to cisplatin, exposure to Pd₂Spm triggered a higher number of, and more marked, variations in some amino acids, nucleotides and derivatives, membrane precursors (choline and phosphoethanolamine), dimethylamine, fumarate and guanidine acetate, a signature that may be relatable to the cytotoxicity and/or mechanism of action of the palladium complex. Putative explanatory biochemical hypotheses are advanced on the role of the new Pd₂Spm complex in TNBC metabolism.

Radix Tetrastigma Extracts Enhance the Chemosensitivity in Triple-Negative Breast Cancer Via Inhibiting PI3K/Akt/mTOR-Mediated Autophagy

OBJECTIVE

Drug resistance in tumors is one of the major factors that leads to chemotherapy failure. This study aims to investigate the effect of Radix Tetrastigma extracts (RTEs) on Taxol-induced autophagy and the chemosensitivity against drug resistance in triple-negative breast cancer (TNBC).

METHODS

Taxol-resistant MDA-MB-468 (MDA-MB-468/Taxol) cells were induced and treated with RTEs and/or Taxol. Mice were subcutaneously inoculated with MDA-MB- 468/Taxol cells to establish xenograft models. The associated protein levels were measured by western blotting. Flow cytometry, CCK-8 and EdU assay were performed to detect cell apoptosis, viability, and proliferation, respectively.

RESULTS

In MDA-MB-468/Taxol cells, RTEs & Taxol treatment increased cell apoptosis, reduced cell viability and proliferation, up-regulated anti-autophagy marker LC3I/LC3II ratio, and enhanced mTOR level. With RTEs & Taxol treatment, mTOR silencing downregulated LC3I/LC3II ratio, increased cell viability and proliferation, and reduced cell apoptosis, while mTOR overexpression showed the opposite results. PI3K inhibitor reduced AKT and mTOR levels, and the effects on cell activities were similar to the results of mTOR silencing. After RTEs & Taxol injection, xenograft tumor was smaller, and AKT, mTOR, LC3I/LC3II ratio and apoptotic marker cleaved caspase-3 were increased.

CONCLUSION

RTEs enhanced the chemosensitivity of resistant TNBC cells to Taxol through inhibiting PI3K/Akt/mTOR-mediated autophagy.

MICRO

RTEs exerted anti-tumor effects in various cancers, and this study determined its role in TNBC. Taxol-resistant MDA-MB-468 cells were induced and xenograft models were established. We found that RTEs inhibited autophagy of MDA-MB-468/Taxol cells and reduced tumor growth. Inhibition of PI3K/Akt/mTOR pathway promoted autophagy of MDA-MB-468/Taxol cells. We may provide a new potential strategy for TNBC treatment.

Metabolic Response of Triple-Negative Breast Cancer to Folate Restriction

Background: Triple-negative breast cancers (TNBCs), accounting for approximately 15% of breast cancers, lack targeted therapy. A hallmark of cancer is metabolic reprogramming, with one-carbon metabolism essential to many processes altered in tumor cells, including nucleotide biosynthesis and antioxidant defenses. We reported that folate deficiency via folic acid (FA) withdrawal in several TNBC cell lines results in heterogenous effects on cell growth, metabolic reprogramming, and mitochondrial impairment. To elucidate underlying drivers of TNBC sensitivity to folate stress, we characterized in vivo and in vitro responses to FA restriction in two TNBC models differing in metastatic potential and innate mitochondrial dysfunction. Methods: Metastatic MDA-MB-231 cells (high mitochondrial dysfunction) and nonmetastatic M-Wnt cells (low mitochondrial dysfunction) were orthotopically injected into mice fed diets with either 2 ppm FA (control), 0 ppm FA, or 12 ppm FA (supplementation; in MDA-MB-231 only). Tumor growth, metabolomics, and metabolic gene expression were assessed. MDA-MB-231 and M-Wnt cells were also grown in media with 0 or 2.2 µM FA; metabolic alterations were assessed by extracellular flux analysis, flow cytometry, and qPCR. Results: Relative to control, dietary FA restriction decreased MDA-MB-231 tumor weight and volume, while FA supplementation minimally increased MDA-MB-231 tumor weight. Metabolic studies in vivo and in vitro using MDA-MB-231 cells showed FA restriction remodeled one-carbon metabolism, nucleotide biosynthesis, and glucose metabolism. In contrast to findings in the MDA-MB-231 model, FA restriction in the M-Wnt model, relative to control, led to accelerated tumor growth, minimal metabolic changes, and modest mitochondrial dysfunction. Increased mitochondrial dysfunction in M-Wnt cells, induced via chloramphenicol, significantly enhanced responsiveness to the cytotoxic effects of FA restriction. Conclusions: Given the lack of targeted treatment options for TNBC, uncovering metabolic vulnerabilities that can be exploited as therapeutic targets is an important goal. Our findings suggest that a major driver of TNBC sensitivity to folate restriction is a high innate level of mitochondrial dysfunction, which can increase dependence on one-carbon metabolism. Thus, folate deprivation or antifolate therapy for TNBCs with metabolic inflexibility due to their elevated levels of mitochondrial dysfunction may represent a novel precision-medicine strategy.

Mechanisms of Metabolic Reprogramming in Cancer Cells Supporting Enhanced Growth and Proliferation

Cancer cells alter metabolic processes to sustain their characteristic uncontrolled growth and proliferation. These metabolic alterations include (1) a shift from oxidative phosphorylation to aerobic glycolysis to support the increased need for ATP, (2) increased glutaminolysis for NADPH regeneration, (3) altered flux through the pentose phosphate pathway and the tricarboxylic acid cycle for macromolecule generation, (4) increased lipid uptake, lipogenesis, and cholesterol synthesis, (5) upregulation of one-carbon metabolism for the production of ATP, NADH/NADPH, nucleotides, and glutathione, (6) altered amino acid metabolism, (7) metabolism-based regulation of apoptosis, and (8) the utilization of alternative substrates, such as lactate and acetate. Altered metabolic flux in cancer is controlled by tumor-host cell interactions, key oncogenes, tumor suppressors, and other regulatory molecules, including non-coding RNAs. Changes to metabolic pathways in cancer are dynamic, exhibit plasticity, and are often dependent on the type of tumor and the tumor microenvironment, leading in a shift of thought from the Warburg Effect and the “reverse Warburg Effect” to metabolic plasticity. Understanding the complex nature of altered flux through these multiple pathways in cancer cells can support the development of new therapies.

Conjugates of methionine γ-lyase with polysialic acid: Two approaches to antitumor therapy

The methionine dependence is a well known phenomenon in metabolism of cancer cells. Methionine γ-lyase (EC 4.4.1.11, MGL) catalyzes the γ-elimination reaction of L-methionine and thus could effectively inhibit the growth of malignant cells. Recently we have demonstrated that the mutant form of the enzyme C115H MGL can be used as a component of the pharmacological pair enzyme/S-(allyl/alkyl)-L-cysteine sulfoxides to yield thiosulfinates in situ. Thiosulfinates were shown to be toxic to various cancer cell lines. Therefore the application of the enzyme in enzyme pro-drug therapy may be promising. The conjugates of MGL and C115H MGL with polysialic acid were obtained and their kinetic and pharmacokinetic parameters were determined. The formation of polysialic shell around the enzyme was confirmed by atomic force microscopy. The half-life of conjugated enzymes increased 3-6 times compared to the native enzyme. The cytotoxic effect of conjugated MGL against methionine dependent cancer cell lines was increased two times compared to the values for the native enzymes. The anticancer efficiency of thiosulfinates produced by pharmacological pair C115H MGL/S-(allyl/alkyl)-L-cysteine sulfoxides was demonstrated in vitro. The results indicate that the conjugates of MGL with polysialic acid could be new antitumor drugs.

Lipid remodeling in response to methionine stress in MDA-MBA-468 triple-negative breast cancer cells

Methionine (Met) is an essential amino acid and critical precursor to the cellular methyl donor S-adenosylmethionine. Unlike nontransformed cells, cancer cells have a unique metabolic requirement for Met and are unable to proliferate in growth media where Met is replaced with its metabolic precursor, homocysteine. This metabolic vulnerability is common among cancer cells regardless of tissue origin and is known as "methionine dependence", "methionine stress sensitivity", or the Hoffman effect. The response of lipids to Met stress, however, is not well-understood. Using mass spectroscopy, label-free vibrational microscopy, and next-generation sequencing, we characterize the response of lipids to Met stress in the triple-negative breast cancer cell line MDA-MB-468 and its Met stress insensitive derivative, MDA-MB-468res-R8. Lipidome analysis identified an immediate, global decrease in lipid abundances with the exception of triglycerides and an increase in lipid droplets in response to Met stress specifically in MDA-MB-468 cells. Furthermore, specific gene expression changes were observed as a secondary response to Met stress in MDA-MB-468, resulting in a downregulation of fatty acid metabolic genes and an upregulation of genes in the unfolded protein response pathway. We conclude that the extensive changes in lipid abundance during Met stress is a direct consequence of the modified metabolic profile previously described in Met stress-sensitive cells. The changes in lipid abundance likely results in changes in membrane composition inducing the unfolded protein response we observe.

From Embryo to Adult: One Carbon Metabolism in Stem Cells

Stem cells are undifferentiated cells with self-renewal property and varying differentiation potential that allow the regeneration of tissue cells of an organism throughout adult life beginning from embryonic development. Through the asymmetric cell divisions, each stem cell replicates itself and produces an offspring identical with the mother cell, and a daughter cell that possesses the characteristics of a progenitor cell and commits to a specific lineage to differentiate into tissue cells to maintain homeostasis. To maintain a pool of stem cells to ensure tissue regeneration and homeostasis, it is important to regulate the metabolic functioning of stem cells, progenitor cells and adult tissue stem cells that will meet their internal and external needs. Upon fertilization, the zygote transforms metabolic reprogramming while implantation, embryonic development, organogenesis processes and after birth through adult life. Metabolism in stem cells is a concept that is relatively new to be enlightened. There are no adequate and comprehensive in vitro studies on the comparative analysis of the effects of one-carbon (1-C) metabolism on fetal and adult stem cells compared to embryonic and cancer stem cells' studies that have been reported recently. Since 1-C metabolism is linking parental environmental/ dietary factors and fetal development, investigating the epigenetic, genetic, metabolic and developmental effects on adult period is necessary. Several mutations and abnormalities in 1-C metabolism have been noted in disease changing from diabetes, cancer, pregnancy-related outcomes such as pre-eclampsia, spontaneous abortion, placental abruption, premature delivery, and cardiovascular diseases. In this review, the effects of 1-C metabolism, mainly the methionine and folate metabolism, in stem cells that exist in different developmental stages will be discussed.

Sensing and Signaling of Methionine Metabolism

Availability of the amino acid methionine shows remarkable effects on the physiology of individual cells and whole organisms. For example, most cancer cells, but not normal cells, are hyper dependent on high flux through metabolic pathways connected to methionine, and diets restricted for methionine increase healthy lifespan in model organisms. Methionine's impact on physiology goes beyond its role in initiation of translation and incorporation in proteins. Many of its metabolites have a major influence on cellular functions including epigenetic regulation, maintenance of redox balance, polyamine synthesis, and phospholipid homeostasis. As a central component of such essential pathways, cells require mechanisms to sense methionine availability. When methionine levels are low, cellular response programs induce transcriptional and signaling states to remodel metabolic programs and maintain methionine metabolism. In addition, an evolutionary conserved cell cycle arrest is induced to ensure cellular and genomic integrity during methionine starvation conditions. Methionine and its metabolites are critical for cell growth, proliferation, and development in all organisms. However, mechanisms of methionine perception are diverse. Here we review current knowledge about mechanisms of methionine sensing in yeast and mammalian cells, and will discuss the impact of methionine imbalance on cancer and aging.

Exploiting the metabolic dependencies of the broad amino acid transporter SLC6A14

Tumor cells typically enhance their metabolic capacity to sustain their higher rate of growth and proliferation. One way to elevate the nutrient intake into cancer cells is to increase the expression of genes encoding amino acid transporters, which may represent targetable vulnerabilities. Here, we study the regulation and function of the broad amino acid transporter SLC6A14 in combination with metabolic stress, providing insights into an uncharacterized aspect of the transporter activity. We analyze the pattern of transcriptional changes in a panel of breast cancer cell lines upon metabolic stress and found that SLC6A14 expression levels are increased in the absence of methionine. Methionine deprivation, which can be achieved via modulation of dietary methionine intake in tumor cells, in turn leads to a heightened activation of the AMP-activated kinase (AMPK) in SLC6A14-deficient cells. While SLC6A14 genetic deficiency does not have a major impact on cell proliferation, combined depletion of AMPK and SLC6A14 leads to an increase in apoptosis upon methionine starvation, suggesting that combined targeting of SLC6A14 and AMPK can be exploited as a therapeutic approach to starve tumor cells.

In Silico Prediction of Metabolic Fluxes in Cancer Cells with Altered S-adenosylmethionine Decarboxylase Activity

This paper investigates the redistribution of metabolic fluxes in the cell with altered activity of S-adenosylmethionine decarboxylase (SAMdc, EC: 4.1.1.50), the key enzyme of the polyamine cycle and the common target for antitumor therapy. To address these goals, a stoichiometric metabolic model was developed that includes five metabolic pathways: polyamine, methionine, methionine salvage cycles, folic acid cycle, and the pathway of glutathione and taurine synthesis. The model is based on 51 reactions involving 57 metabolites, 31 of which are internal metabolites. All calculations were performed using the method of Flux Balance Analysis. The outcome indicates that the inactivation of SAMdc results in a significant increase in fluxes through the methionine, the taurine and glutathione synthesis, and the folate cycles. Therefore, when using therapeutic agents inactivating SAMdc, it is necessary to consider the possibility of cellular tumor metabolism reprogramming. S-adenosylmethionine affects serine methylation and activates serine-dependent de novo ATP synthesis. Methionine-depleted cell becomes methionine-dependent, searching for new sources of methionine. Inactivation of SAMdc enhances the transformation of S-adenosylmethionine to homocysteine and then to methionine. It also intensifies the transsulfuration process activating the synthesis of glutathione and taurine.

Association between Metabolites and the Risk of Lung Cancer: A Systematic Literature Review and Meta-Analysis of Observational Studies

Globally, lung cancer is the most prevalent cancer type. However, screening and early detection is challenging. Previous studies have identified metabolites as promising lung cancer biomarkers. This systematic literature review and meta-analysis aimed to identify metabolites associated with lung cancer risk in observational studies. The literature search was performed in PubMed and EMBASE databases, up to 31 December 2019, for observational studies on the association between metabolites and lung cancer risk. Heterogeneity was assessed using the I2 statistic and Cochran's Q test. Meta-analyses were performed using either a fixed-effects or random-effects model, depending on study heterogeneity. Fifty-three studies with 297 metabolites were included. Most identified metabolites (252 metabolites) were reported in individual studies. Meta-analyses were conducted on 45 metabolites. Five metabolites (cotinine, creatinine riboside, N-acetylneuraminic acid, proline and r-1,t-2,3,c-4-tetrahydroxy-1,2,3,4-tetrahydrophenanthrene) and five metabolite groups (total 3-hydroxycotinine, total cotinine, total nicotine, total 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (sum of concentrations of the metabolite and its glucuronides), and total nicotine equivalent (sum of total 3-hydroxycotinine, total cotinine and total nicotine)) were associated with higher lung cancer risk, while three others (folate, methionine and tryptophan) were associated with lower lung cancer risk. Significant heterogeneity was detected across most studies. These significant metabolites should be further evaluated as potential biomarkers for lung cancer.

Methionine Restriction Regulates Cognitive Function in High-Fat Diet-Fed Mice: Roles of Diurnal Rhythms of SCFAs Producing- and Inflammation-Related Microbes

SCOPE

Methionine restriction (MR) is known to potently alleviate inflammation and improve gut microbiome in obese mice. The gut microbiome exhibits diurnal rhythmicity in composition and function, and this, in turn, drives oscillations in host metabolism. High-fat diet (HFD) strongly altered microbiome diurnal rhythmicity, however, the role of microbiome diurnal rhythmicity in mediating the improvement effects of MR on obesity-related metabolic disorders remains unclear.

METHODS AND RESULTS

10-week-old male C57BL/6J mice are fed a low-fat diet or HFD for 4 weeks, followed with a full diet (0.86% methionine, w/w) or a methionine-restricted diet (0.17% methionine, w/w) for 8 weeks. Analyzing microbiome diurnal rhythmicity at six time points, the results show that HFD disrupts the cyclical fluctuations of the gut microbiome in mice. MR partially restores these cyclical fluctuations, which lead to time-specifically enhance the abundance of short-chain fatty acids producing bacteria, increases the acetate and butyric, and dampens the oscillation of inflammation-related Desulfovibrionales and Staphylococcaceae over the course of 1 day. Notably, MR, which protects against systemic inflammation, influences brain function and synaptic plasticity.

CONCLUSION

MR could serve as a potential nutritional intervention for attenuating obesity-induced cognitive impairments by balancing the circadian rhythm in microbiome-gut-brain homeostasis.

Evaluation of MDA-MB-468 Cell Culture Media Analysis in Predicting Triple-Negative Breast Cancer Patient Sera Metabolic Profiles

Triple-negative breast cancer (TNBC) is characterized by limited survival, poor prognosis, and high recurrence. Understanding the metabolic adaptations of TNBC could help reveal improved treatment regiments. Here we performed a comprehensive ¹H NMR metabolic characterization of the MDA-MB-468 cell line, a commonly used model of TNBC, followed by an analysis of serum samples obtained from TNBC patients and healthy controls. MDA-MB-468 cells were cultured, and changes in the metabolic composition of the medium were monitored for 72 h. Based on time courses, metabolites were categorized as being consumed, being produced, or showing a mixed behavior. When comparing TNBC and control samples (HC), and by using multivariate and univariate analyses, we identified nine metabolites with differing profiles). The serum of TNBC patients was characterized by higher levels of glucose, glutamine, citrate, and acetoacetate and by lower levels of lactate, alanine, tyrosine, glutamate, and acetone. A comparative analysis between MDA-MB-468 cell culture media and TNBC patients' serum identified a potential systemic response to the carcinogenesis-associated processes, highlighting that MDA-MB-468 cells footprint does not reflect metabolic changes observed in studied TNBC serum fingerprint.

Methionine Dependence of Cancer

Tumorigenesis is accompanied by the reprogramming of cellular metabolism. The shift from oxidative phosphorylation to predominantly glycolytic pathways to support rapid growth is well known and is often referred to as the Warburg effect. However, other metabolic changes and acquired needs that distinguish cancer cells from normal cells have also been discovered. The dependence of cancer cells on exogenous methionine is one of them and is known as methionine dependence or the Hoffman effect. This phenomenon describes the inability of cancer cells to proliferate when methionine is replaced with its metabolic precursor, homocysteine, while proliferation of non-tumor cells is unaffected by these conditions. Surprisingly, cancer cells can readily synthesize methionine from homocysteine, so their dependency on exogenous methionine reflects a general need for altered metabolic flux through pathways linked to methionine. In this review, an overview of the field will be provided and recent discoveries will be discussed.

Metabolic reprogramming in triple-negative breast cancer

Since triple-negative breast cancer (TNBC) was first defined over a decade ago, increasing studies have focused on its genetic and molecular characteristics. Patients diagnosed with TNBC, compared to those diagnosed with other breast cancer subtypes, have relatively poor outcomes due to high tumor aggressiveness and lack of targeted treatment. Metabolic reprogramming, an emerging hallmark of cancer, is hijacked by TNBC to fulfill bioenergetic and biosynthetic demands; maintain the redox balance; and further promote oncogenic signaling, cell proliferation, and metastasis. Understanding the mechanisms of metabolic remodeling may guide the design of metabolic strategies for the effective intervention of TNBC. Here, we review the metabolic reprogramming of glycolysis, oxidative phosphorylation, amino acid metabolism, lipid metabolism, and other branched pathways in TNBC and explore opportunities for new biomarkers, imaging modalities, and metabolically targeted therapies.

Amino acids in cancer

Over 90 years ago, Otto Warburg's seminal discovery of aerobic glycolysis established metabolic reprogramming as one of the first distinguishing characteristics of cancer¹. The field of cancer metabolism subsequently revealed additional metabolic alterations in cancer by focusing on central carbon metabolism, including the citric acid cycle and pentose phosphate pathway. Recent reports have, however, uncovered substantial non-carbon metabolism contributions to cancer cell viability and growth. Amino acids, nutrients vital to the survival of all cell types, experience reprogrammed metabolism in cancer. This review outlines the diverse roles of amino acids within the tumor and in the tumor microenvironment. Beyond their role in biosynthesis, they serve as energy sources and help maintain redox balance. In addition, amino acid derivatives contribute to epigenetic regulation and immune responses linked to tumorigenesis and metastasis. Furthermore, in discussing the transporters and transaminases that mediate amino acid uptake and synthesis, we identify potential metabolic liabilities as targets for therapeutic intervention.

Is the Hoffman Effect for Methionine Overuse Analogous to the Warburg Effect for Glucose Overuse in Cancer?

The general cancer-specific metabolic defect of methionine (MET) dependence is due to MET overuse for aberrant transmethylation reactions. The excess use of MET for aberrant transmethylation reactions apparently diverts methyl groups from DNA. The resulting global DNA hypomethylation is also a general phenomenon in cancer and leads to unstable genomes and aneuploid karyotypes. The excessive and aberrant use of MET in cancer is readily observed in [¹¹C]-MET-PET imaging, where high uptake of [¹¹C]-MET results in a very strong and selective tumor signal compared to normal tissue background for brain cancer and possibly other cancers. [¹¹C]-MET is superior to [¹⁸C]-fluorodeoxyglucose (FDG) for PET imaging, suggesting that MET overuse in cancer ("Hoffman effect") is greater than glucose overuse in cancer ("Warburg effect").

The Non-Essential Amino Acid Cysteine Becomes Essential for Tumor Proliferation and Survival

The non-essential amino acid cysteine is used within cells for multiple processes that rely on the chemistry of its thiol group. Under physiological conditions, many non-transformed tissues rely on glutathione, circulating cysteine, and the de novo cysteine synthesis (transsulfuration) pathway as sources of intracellular cysteine to support cellular processes. In contrast, many cancers require exogeneous cystine for proliferation and viability. Herein, we review how the cystine transporter, xCT, and exogenous cystine fuel cancer cell proliferation and the mechanisms that regulate xCT expression and activity. Further, we discuss the potential contribution of additional sources of cysteine to the cysteine pool and what is known about the essentiality of these processes in cancer cells. Finally, we discuss whether cyst(e)ine dependency and associated metabolic alterations represent therapeutically targetable metabolic vulnerabilities.

Mapping Metabolic Events in the Cancer Cell Cycle Reveals Arginine Catabolism in the Committed SG2M Phase

Alterations in cell-cycle regulation and cellular metabolism are associated with cancer transformation, and enzymes active in the committed cell-cycle phase may represent vulnerabilities of cancer cells. Here, we map metabolic events in the G1 and SG2M phases by combining cell sorting with mass spectrometry-based isotope tracing, revealing hundreds of cell-cycle-associated metabolites. In particular, arginine uptake and ornithine synthesis are active during SG2M in transformed but not in normal cells, with the mitochondrial arginase 2 (ARG2) enzyme as a potential mechanism. While cancer cells exclusively use ARG2, normal epithelial cells synthesize ornithine via ornithine aminotransferase (OAT). Knockdown of ARG2 markedly reduces cancer cell growth and causes G2M arrest, while not inducing compensation via OAT. In human tumors, ARG2 is highly expressed in specific tumor types, including basal-like breast tumors. This study sheds light on the interplay between metabolism and cell cycle and identifies ARG2 as a potential metabolic target.

Toward a better understanding of folate metabolism in health and disease

Folate metabolism is crucial for many biochemical processes, including purine and thymidine monophosphate (dTMP) biosynthesis, mitochondrial protein translation, and methionine regeneration. These biochemical processes in turn support critical cellular functions such as cell proliferation, mitochondrial respiration, and epigenetic regulation. Not surprisingly, abnormal folate metabolism has been causally linked with a myriad of diseases. In this review, we provide a historical perspective, delve into folate chemistry that is often overlooked, and point out various missing links and underdeveloped areas in folate metabolism for future exploration.

Isolation and Characterization of Methionine-Independent Clones from Methionine-Dependent Cancer Cells

Unlike normal cells, transformed cells are unable to grow when methionine in the growth media is restricted. Reversion to methionine independence is a rare event in transformed and malignant cells. Methionine-independent revertants provide an excellent system to identify metabolic signatures and molecular characteristics associated with methionine dependency of transformed cells. Revertants maintain the genetic background and general growth behavior of the parental cell line, except that they proliferate under methionine restriction such as in methionine-free media supplemented with homocysteine. Here we describe a general approach to generate methionine-independent revertants using the example of the triple-negative breast cancer cell line MDA-MB-468. To validate and characterize reversion we describe assays to evaluate cell proliferation and anchorage-independent growth in soft agar.

Metabolomics and 16S rRNA sequencing of human colorectal cancers and adjacent mucosa

Colorectal cancer (CRC) is ranked the third most common cancer in human worldwide. However, the exact mechanisms of CRC are not well established. Furthermore, there may be differences between mechanisms of CRC in the Asian and in the Western populations. In the present study, we utilized a liquid chromatography-mass spectrometry (LC-MS) metabolomic approach supported by the 16S rRNA next-generation sequencing to investigate the functional and taxonomical differences between paired tumor and unaffected (normal) surgical biopsy tissues from 17 Malaysian patients. Metabolomic differences associated with steroid biosynthesis, terpenoid biosynthesis and bile metabolism could be attributed to microbiome differences between normal and tumor sites. The relative abundances of Anaerotruncus, Intestinimonas and Oscillibacter displayed significant relationships with both steroid biosynthesis and terpenoid and triterpenoid biosynthesis pathways. Metabolites involved in serotonergic synapse/ tryptophan metabolism (Serotonin and 5-Hydroxy-3-indoleacetic acid [5-HIAA]) were only detected in normal tissue samples. On the other hand, S-Adenosyl-L-homocysteine (SAH), a metabolite involves in methionine metabolism and methylation, was frequently increased in tumor relative to normal tissues. In conclusion, this study suggests that local microbiome dysbiosis may contribute to functional changes at the cancer sites. Results from the current study also contributed to the list of metabolites that are found to differ between normal and tumor sites in CRC and supported our quest for understanding the mechanisms of carcinogenesis.

Metabolic Regulation of Methionine Restriction in Diabetes

Although the effects of dietary methionine restriction have been investigated in the physiology of aging and diseases related to oxidative stress, the relationship between methionine restriction (MR) and the development of metabolic disorders has not been explored extensively. This review summarizes studies of the possible involvement of dietary methionine restriction in improving insulin resistance, glucose homeostasis, oxidative stress, lipid metabolism, the pentose phosphate pathway (PPP), and inflammation, with an emphasis on the fibroblast growth factor 21 and protein phosphatase 2A signals and autophagy in diabetes. Diets deficient in methionine may be a useful nutritional strategy in patients with diabetes.

Amino Acids in the Development of Prodrugs

Although drugs currently used for the various types of diseases (e.g., antiparasitic, antiviral, antibacterial, etc.) are effective, they present several undesirable pharmacological and pharmaceutical properties. Most of the drugs have low bioavailability, lack of sensitivity, and do not target only the damaged cells, thus also affecting normal cells. Moreover, there is the risk of developing resistance against drugs upon chronic treatment. Consequently, their potential clinical applications might be limited and therefore, it is mandatory to find strategies that improve those properties of therapeutic agents. The development of prodrugs using amino acids as moieties has resulted in improvements in several properties, namely increased bioavailability, decreased toxicity of the parent drug, accurate delivery to target tissues or organs, and prevention of fast metabolism. Herein, we provide an overview of models currently in use of prodrug design with amino acids. Furthermore, we review the challenges related to the permeability of poorly absorbed drugs and transport and deliver on target organs.