After activation, immune cells engage in metabolic remodeling to support the energy and biosynthetic demands for proliferation and effector functions. Notably, boosted glucose uptake and flux through glycolysis are common metabolic features of different immune cells. In particular, targeted glycolysis has been shown to profoundly affect immune responses, including infections, inflammation, and autoimmune diseases. However, the underlying mechanisms of metabolism and immune regulation remain unclear. Previous studies have illustrated the immune-regulatory effects of endogenous metabolites generated from glycolysis, the tricarboxylic acid (TCA) cycle, and amino acid metabolism, which include lactate, itaconate, succinate. These metabolites profoundly impact immune reactions by binding to high-affinity receptors, post-translational modifications of target proteins, or unexplained mechanisms.
For example, succinate, produced by macrophages after activation with endotoxin, stabilizes the transcription factor hypoxia-inducible factor 1α (HIF-1α) and subsequently enhances IL-1β production. On the other hand, lactate, generated through aerobic glycolysis in tumors, polarizes macrophages to an anti-inflammatory phenotype that expresses genes encoding products critical for tumor growth. Methylglyoxal (MEG) is a highly reactive carbonyl species mainly generated from the glycolytic intermediates, glyceraldehyde 3-phosphate (GA3P) and dihydroxyacetone phosphate (DHAP), in a nonenzymatic manner. It has been estimated that 0.1–0.4% of the glycolytic flux results in MEG production. Thus, it is easy to understand that immune cells with higher glycolysis rate will produce lager amount of MEG. Indeed, this metabolite is upregulated in LPS-activated macrophages, which has high rate of glycolysis.
Additionally, excessive accumulation of MEG has been reported in various conditions, including diabetes, cancer, neurodegenerative and inflammatory diseases such as MS, and rheumatoid arthritis (RA). It has been proposed that MEG is a relatively low-abundance metabolite with plasma concentration of around 50–300 nM and intracellular concentration of around 1–2 μM in healthy individuals. Although the endogenous MEG is also upregulated in LPS-stimulated microglia cells, it remains to be determined whether endogenous MEG exerts similar anti-inflammatory effects. One concern is that the concentration of endogenous MEG might be far lower than exogenous MEG used in our experiments. Due to its highly reactive electrophilic nature, approximately 99% of cellular MEG is reversibly bound to biopolymers (mainly proteins) and small-metabolite thiols, with only 1% remaining unbound.
Due to its highly reactive electrophilic nature, MEG can spontaneously form covalent bonds with various biomolecules, such as nucleic acids and proteins, and affect several physiological and pathological processes. While MEG can be produced as a by-product of both amino acid and fatty acid metabolism, the glycolytic pathway represents the most important endogenous source of MEG. Thus, it is easy to interpreted that immune cells with higher glycolysis rate will produce lager amount of MEG. Indeed, accumulation of MEG might be a common feature of activated immune cells with enhanced glycolysis, like LPS-activated microglia, macrophages and activated CD4+ and CD8+ lymphocytes. Additionally, increased MEG levels have been observed in the serum of patients with RA and they were also positively correlated with the disease activity.
Extensive studies have revealed the immune regulation of metabolites generated from glycolysis pathway, TCA cycle and amino acid metabolism. Itaconate is one of the best examples of metabolic reprogramming which connects cell metabolism, oxidative and electrophilic stress responses and immune responses. Upon inflammatory macrophage activation, itaconate is synthesized by diverting aconitate away from the TCA cycle, which in turn modulated macrophage metabolism and restrains macrophages inflammatory response. The uncovered mechanisms of itaconate action include transcriptional regulation of ATF3 transcription factor and protein modification of KEAP1, inflammasome, protein kinase JAK1 and the chromatin regulator TET2. Interestingly, as a by-product of glycolytic pathway, MEG shares several similarities with itaconate.
Firstly, both of them are generated during the activation of LPS-stimulated macrophage. Secondly, both can activate the NRF2 pathway and upregulate antioxidant proteins. Along with itaconate, post-translational modifications of proteins by MEG may be the primary mechanism through which it regulates several cellular and biological processes. Investigations carried out by Gilligan et al., for example, demonstrated that MEG-derived arginine modifications are abundant in histones, and their levels are comparable to those of other canonical lysine or arginine modifications. In addition, 28 site-specific histone modifications were identified. Thus, these modifications were proposed to disrupt basal canonical histone modifications by acetylation, phosphorylation and ubiquitylation. These phenomena have implications for carcinogenesis and aging. In fact, histone acetylation is a continuous mechanism that the cell uses to regulate gene expression.
So much so that histone deacetylase (HDACs) inhibitors are used in the study of chromatin biology as well as anti-cancer drugs. Histone phosphorylation is a much more specific phenomenon, as it usually occurs when there is a response to chromatin lesions. But since these inevitably lead to cellular aging (at best) or to malignant transformation (at worst), the “glyoxylation” of histones can also be a modification that MEG adopts to exert its indirect mutagenic effects. Glyoxylation may be a mechanism by which MEG effects the aging and deterioration of stem cell reserves through the influence of histone methylation. While DNA methylation has direct effects on the maintenance of the cellular differentiated state, histone methylation is an important process for an analogous but related phenomenon at another level: the maintenance of heterochromatin.
The histones of the terminal chromatin which will never be reopened during cellular life, have histones whose lysines in some places are modified with three methyl groups (tri-methylation). These residues will be linked to some proteins called “readers”, which work like permanent caps that promote chromatine condensation in a virtually inaccessible status. This mechanism will grant either the genomic and, particularly, the telomeric stability in order to positively affect the process of aging. Indeed, some heterochromatin-dependent proteins (e.g. HP-1 gamma) can be found also at the telomere level. It is curious that an old anticancer drug called MGBG (methylglioxal-bis-guanyl-hydrazone), works as a polyamine antagonist and an inhibitor of the enzyme SAM decarboxylase. This rate-limiting enzyme uses the cofactor SAM, which is primarily a methyl-group donor affecting several metabolic pathways.
Polyamines also are connected to cellular proliferation or differentiation; their heir catabolism,however, produces aldehydes or aminoaldehydes. These may affect thiol-dependent enzymes and main intracellular free antioxidants like glutathione (GSH). MEG is also one of the carbonylated metabolites that are produced by aberrant glucose metabolism in diabetes. Through its conjugation with proteins it can give rise to the formation of advanced glycated compounds (collectively known as AGEs), which contribute to the appearance of neuropathy, nephropathy and other complications in this condition. Paradoxically, MEG has very recently been reported to inactivate the NLRP3 inflammasome by direct covalent modification. Together with its ability to deactivate another inflammatory transcription factor such as NF-kB, and activate one that is anti-inflammatory, namely Nrf-2, this makes this molecule a double-edged sword that can act as “friend and foe” depending on the biological contexts.
- Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
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