key: cord-0960645-kqsxtuzi
authors: González-Bosch, Carmen; Boorman, Emily; Zunszain, Patricia A.; Mann, Giovanni E.
title: Short-chain fatty acids as modulators of redox signaling in health and disease
date: 2021-10-14
journal: Redox Biol
DOI: 10.1016/j.redox.2021.102165
sha: 661fb96dbbceeb872c8304f18914744cefcb1f3c
doc_id: 960645
cord_uid: kqsxtuzi

Short-chain fatty acids (SCFAs), produced by colonic bacteria and obtained from the diet, have been linked to beneficial effects on human health associated with their metabolic and signaling properties. Their physiological functions are related to their aliphatic tail length and dependent on the activation of specific membrane receptors. In this review, we focus on the mechanisms underlying SCFAs mediated protection against oxidative and mitochondrial stress and their role in regulating metabolic pathways in specific tissues. We critically evaluate the evidence for their cytoprotective roles in suppressing inflammation and carcinogenesis and the consequences of aging. The ability of these natural compounds to induce signaling pathways, involving nuclear erythroid 2-related factor 2 (Nrf2), contributes to the maintenance of redox homeostasis under physiological conditions. SCFAs may thus serve as nutritional and therapeutic agents in healthy aging and in vascular and other diseases such as diabetes, neuropathologies and cancer.

The key roles played by the major short-chain fatty acids (SCFAs) acetate, propionate and butyrate (commonly named as their anions) as mediators of the beneficial effects of dietary fibre and gut microbiota in human health has led to renewed interest in their mechanisms of action [1] . Here, we review in detail the in vitro and in vivo evidence supporting a role of SCFAs in regulating redox homeostasis, crosstalk between the redox sensitive Kelch-like ECH-associated protein 1 (Keap1)-Nrf2 signaling pathway and metabolism of free fatty acids (FFAs), and the novel roles that specific SCFAs play in activating Nrf2-regulated gene transcription (see Section 4 and Tables 3 and 4). We also address the beneficial effects of specific SCFAs on redox homeostasis under physiological and pathological conditions, and critically evaluate the potential of these biomolecules as nutritional and therapeutic targets in diseases such as diabetes, neuropathology and cancer.

Short-chain fatty acids, mainly produced by colonic bacteria, have been linked with benefits for human health due to their metabolic and signaling properties [2] . The specific physiological functions of this type of fatty acid are related to their aliphatic tail length. FFAs are classified based on their carbon chain length, with SCFAs having <6 carbon atoms, MCFAs 6-12 carbons atoms and long-chain fatty acids (LCFAs) more than 12 carbon atoms (see Table 1 ).

FFAs are versatile molecules involved in many physiological functions in mammals. In addition to their well characterized structural and metabolic roles, recent evidence highlights their involvement in cell signaling in a wide range of physiological and pathological conditions [3, 4] . Evidence for a role of SCFAs in organs outside the digestive system stems from the fact that numerous transmembrane proteins, receptors and transporters, that specifically bind SCFAs and other monocarboxylic acids, are expressed in a large variety of cell types, including neurons [5, 6] . For example, butyrate and other fermentation or diet derived SCFAs is poorly understood, they may be critical in defining physiologically relevant roles of specific SCFAS during immune and inflammatory responses [17] . This is particularly important for clinical translation of findings in animal models, which often utilize oral SCFAs supplementation or high dietary fibre supplementation to induce changes in SCFAs production [18] .

Notably, mother's milk constitutes an important source of SCFAs for new-born mammals in the form of triglycerides and phospholipids, while animal milk and milk products constitute the main dietary source of SCFAs, mainly butyrate, in adult humans [19] . SCFAs can also be Table 1 Nomenclature and structure of SCFAs.

formed in the mammalian liver through the peroxisomal β-oxidation of long-chain fatty acids (LCFAs) [20] . A feature of SCFAs that distinguishes them from LCFAs is that they are rapidly absorbed in the intestine due to their water solubility and transported via the hepatic portal bloodstream to the liver, where they are readily metabolized instead of being stored as fat [9, 19] . SCFAs modulate tissue metabolism of carbohydrates and lipids by inhibiting glycolysis and stimulating lipogenesis and gluconeogenesis [21] . However, the traditional view that FFAs serve as metabolic substrates only during carbohydrate restriction has been revised based on observations that ketone bodies play pivotal roles as signaling mediators, drivers of protein post-translational modification and modulators of inflammation and oxidative stress when carbohydrates are abundant [22] . The antioxidant and oxidative stress-mitigating roles of ketone bodies have been widely described in the context of neuroprotection, cardio-protection and cancer [23] and will not be reviewed further.

The regulatory functions of SCFAs depend on specific receptors expressed in different cell types, as well as their developmental stage and differentiation process [5, 6] . In vitro and in vivo studies have shown that the physiological functions of free fatty acid receptors (FFARs) contribute not only to the regulation of metabolic energy but also affect the immune system [24] . These transmembrane receptors are members of G protein-coupled receptors (GPR) that detect extracellular molecules and then activate intracellular signal transduction pathways to promote cellular responses [25] .

FFARs are activated by free fatty acids with different carbon chain lengths and, under physiological conditions, several FFAs can activate the same receptor, whereas one FFA can activate several FFARs. In fact, SCFAs are ligands of FFAR2/GPR43 and FFAR3/GPR41, but the former is preferentially activated by C3:0-C6:0 and the latter by C2:0-C4:0. Notably, long-chain fatty acids activate both FFAR1/GPR40 and FFAR4 (GPR120), whereas medium chain fatty acids (C6:0-C12:0) can also activate FFAR1/GPR40 5 . In addition, several FFAs are able to activate specific receptors, such as propionic (C3:0) (Olfr78), butyric (C4:0) (GPR109A), and capric (C10:0) and lauric (C12:0) acids (GPR84) (see Table 2 and Fig. 2 ). Kimura et al. (2020) have recently described FFARs in detail, and we refer readers to their comprehensive and elegant review [5] .

Depending on the type of α subunit of the heterotrimeric proteins associated with the FFARs, the reported physiological function of FFARs mainly involves activation of intracellular calcium (Ca 2+ ), cyclic adenosine monophosphate (cAMP) or extracellular-signal-regulated kinase 1/2 (ERK1/2) signaling, via G protein (Gq or Gi/o)-dependent or G protein-independent pathways [25, 26] . Furthermore, there is evidence supporting functional redundancy of FFARs that might contribute to maintenance of fatty acid homeostasis under different physiological conditions [5] .

The health benefits of redox signaling have been defined as "oxidative eustress", whereas the deleterious outcomes in pathophysiology and disease due to higher levels of reactive oxygen species (ROS) is referred to as "oxidative distress" [27] . Under physiological conditions, different ROS play key roles in redox signaling via different post-translational modifications, therefore controlling specific ROS-mediated signaling pathways offers a strategy for refining future redox medicine [28, 29] .

The evidence that living organisms have developed mechanisms for the advantageous use of free radicals revealed the biological relevance of redox regulation in health and disease [30] . One of the main sources of cellular ROS are mitochondria which constitute a metabolic hub facilitating crosstalk between the metabolic state of the cell with relevant signaling pathways, including those regulating immune responses [31] . Notably, mitochondrial ROS serve as an alarm of extracellular environment changes that in small amounts can promote adaptation to the stressor but in larger quantities produce cell damage and subsequent cell death [32] . Therefore, the original concept of oxidative stress is linked to cellular energy balance, has led to characterization of compartmentation of redox processes and the spatiotemporal organization of hydrogen peroxide metabolism and its relationship to bioenergetics [33] . Discovery of the role of Keap1-Nrf2 system as the major Note + and ++ denote low and high affinity, respectively [5, 6] .

regulator of redox homeostasis has informed new approaches to improve human health and combat diseases [34] .

Several health benefits of SCFAs and other food derived non-nutrient molecules are related to their ability to modulate gene expression and thereby influence intracellular signaling pathways [35] . Plant-derived compounds can activate signaling pathways involved in the maintenance of cellular redox homeostasis [36, 37] . One of the best characterized targets of pharmacological and/or dietary interventions is nuclear erythroid 2-related factor 2 (Nrf2), a master regulator of cellular antioxidant defenses controlling more than 200 genes [34, [38] [39] [40] [41] [42] [43] .

Under quiescent conditions, Nrf2 is sequestered by its cytosolic repressor Keap1 (Kelch-like ECH-associated protein 1), promoting rapid proteasomal degradation via ubiquitination, whereas oxidative and electrophilic stress promote nuclear accumulation of de novo synthesized Nrf2, which together with small Maf proteins binds to the antioxidant response element (ARE) in the promoter of target genes to upregulate phase II and antioxidant enzymes to counteract oxidative stress [38, [40] [41] [42] 44] . Thus, current research in this field is focused on identifying biomolecules with significant nutrigenomic potential and efficacy as activators of Nrf2 [45, 46] . Among them, broccoli-derived sulforaphane (SFN) is the most potent inducer of Nrf2-targeted cytoprotective genes [39, 47, 48] . As discussed in this section, specific SCFAs such as butyrate and propionate are activators of the Keap1-Nrf2 defence pathway [49, 50] and lipid metabolism is regulated by Nrf2 Table 3 SCFAs mediated activation of the Keap1-Nrf2 defense pathway protects against oxidative stress in vitro. 

Abbreviations: AMPK: AMP-activated protein kinase; BBB: blood-brain barrier; BMECs: brain microvascular endothelial cells; Upward arrow (↑) indicates an increase and downward arrow (↓) a decrease in respective measured outcomes. [51, 52] .

Despite the regulatory functions reported for the main gut microbiome fermentation products acetate, propionate and butyrate, their role in influencing cellular redox homeostasis is currently not well understood [53] . Nrf2 is a major regulator of cellular metabolism in normal, stressed and cancer cells directing the transcription of the metabolic processes [52] . There is increasing evidence that Nrf2 participates in hepatic fatty acid metabolism, suppressing fatty acid synthesis and desaturation [54, 55] . Notably, Notch signaling, which enables regulation and control of development, differentiation, and homeostasis through cell-cell communication has been identified as a novel regulator of fatty acid transport across the endothelium [56] , and the ROS-Nrf2-Notch pathway seems key for cellular homeostasis [57] .

SCFAs have been screened for protection against oxidative damage, and the molecular mechanisms underlying this effect and the physiological significance are beginning to be elucidated [58] . In this section, we focus on their role in regulating the Keap1-Nrf2 pathway. Of relevance, targeting the Keap1-Nrf2 signaling pathway using pharmacological and/or dietary interventions has been shown to protect against oxidative stress induced vascular damage in ischemic stroke, gestational diabetes and atherosclerosis as well as other chronic diseases [38, [59] [60] [61] [62] [63] .

In Tables 3 and 4 , we summarize selected cell culture and animal studies in vivo supporting an antioxidant role for specific SCFAs as modulators of Nrf2 redox signaling, with the majority of studies focused on butyrate. In both type of studies, epigenetic regulation involving histone deacetylases (HDACs) and/or Nrf2 nuclear translocation induced by butyrate are the main reported mechanisms of action.

Notably, studies in Nrf2 knockout mice with induced diabetes showed that butyrate mediates protection against endothelial dysfunction by P300 mediated activation of Nrf2 via inhibition of HDAC while, under the same conditions, SFN facilitates Nrf2 nuclear translocation [64] . Recent evidence in bovine epithelial cells implicated GPR109A inactivation of AMPK signaling by butyrate to promote Nrf2 nuclear accumulation [50] . In this case, butyrate also induced epigenetic modification on the Nrf2 promoter associated with synergistic antioxidant effects. Interestingly, in rat intestinal epithelial cells butyrate, in addition to enhancing antioxidant activities via Nrf2, decreased mRNA and protein levels of the tumor suppressor p53 [65] , supporting crosstalk between p53 and Nrf2 in the regulation of cellular redox homeostasis [66] [67] [68] .

Relatively few studies investigating the antioxidant roles of SCFAs have focused on propionate, despite its similar plasma concentration and receptor affinity [69] . Hoyles et al. (2018) demonstrated the expression of the receptor FFAR3 in human brain endothelium and determined that a physiologically relevant propionate concentration (1 μM) protected the blood-brain barrier (BBB) against oxidative stress via Nrf2 signaling [49] . Notably, acetate, propionate and butyrate can cross BBB, affecting BBB integrity and transport rates by reducing permeability and regulating the expression of tight junction proteins [70] . A study using a germ-free mouse model suggested that the microbiota and its metabolites (especially butyrate) can upregulate tight junctional protein expression in the BBB, thereby regulating the interaction between the periphery and the central nervous system (CNS) [71] . This beneficial effect of specific SCFAs on the BBB may affect the transport of molecules such as docosahexaenoic acid (DHA), an omega-3 fatty acid highly enriched in brain, that is essential for normal brain growth and cognitive function and inhibits inflammation in endothelial cells to reduce cardiovascular risk [72, 73] . DHA cannot be synthesized de novo in the brain and must be supplied from the blood via specific transporters like the major facilitator super family domain containing 2a (Mfsd2a) and/or through passive diffusion across the endothelial membrane [74, 75] .

DHA in the plasma is found esterified with phospholipids and other lipids and its uptake into the brain seems to involve endothelial lipase [76] . The significant enrichment of DHA within the brain must be regulated by a number of additional pathways associated with the activation and metabolism of DHA. Once in the brain, DHA is esterified into membrane phospholipids, being released and converted to bioactive mediators during neurotransmission and following brain injury [77] . The antioxidant properties of DHA and its metabolites, resolvins, neuroprotectins and 4-hydroxy-2E-hexenal, are mediated by Nrf2 activation and involve upregulation of heme oxygenase 1 (HO-1) to protect the brain against ischemic damage [78] [79] [80] . Post-stroke administration of DHA is effective in reducing brain injury [81] [82] [83] and improving sensorimotor function [84, 85] . Notably, stroke lowers blood SCFA concentrations, and SCFA supplementation has been reported to reduce damage following post-stroke reperfusion [86] [87] [88] [89] . As SCFAs improve functional outcomes after stroke, further research focused on their link with neuroprotective functions of Nrf2 and DHA is warranted.

The experimental data summarized in Tables 3 and 4 provide convincing evidence that SCFAs, in particular butyrate but also propionate, by direct or indirect mechanisms, activate the Keap1-Nrf2 signaling pathway to maintain redox homeostasis under physiological conditions (see Fig. 3 ).

Considering that butyrate is a fatty acid oxidized in mitochondria and acts as a signal transduction molecule via FFAR2, FFAR3 and GPR109A, it is reasonable to postulate its involvement in energy and redox homeostasis via the Nrf2 pathway.

Thus, specific SCFAs produced by microbial fermentation or provided in the diet, contribute to host redox homeostasis via epigenetic regulation and/or promoting Nrf2 nuclear translocation, highlighting an interesting link between the microbiota, redox signaling and host metabolism. 

SCFAs play an important role in beneficial effects of dietary fibre and gut microbiota, through regulation of inflammation in the gut and other organs [90] . In fact, SCFAs may suppress inflammation by reducing migration and proliferation of immune cells, reducing cytokine levels and inducing apoptosis, but marked changes in SCFAs concentrations in blood or tissues can also cause immunological and metabolic imbalances [17] . Therefore, appropriate concentrations of SCFAs are needed to maintain normal metabolism and in the prevention and treatment of disease [24] . Table 5 includes selected experiments showing that treatment with SCFAs can activate cellular antioxidant mechanisms and downregulate pro-inflammatory mediators.

SCFAs are generally considered to have beneficial effects in cardiovascular disease, and studies in different animal models report inhibitory effects of butyrate on NLRP3 inflammasome formation associated with activation of antioxidant signaling pathways [91, 92] . Notably, treatment of endothelial cells with low concentrations of SCFAs does not disrupt barrier integrity under inflammatory conditions and moreover increases mitochondrial respiratory capacity [93, 94] . In addition, butyrate plays an important role in the assembly of tight junctions in intestinal and vascular endothelial cells by inducing cyclo-oxygenase (COX), lipoxygenase (LOX) and reducing inducible NO synthase (iNOS) [95, 96] . These findings suggest that supplementation of SCFAs, especially butyrate, may serve as therapeutic nutrients for inflammatory diseases due to their contribution to redox homeostasis.

The anti-inflammatory effects of butyrate and other SCFAs are partly achieved through HDAC inhibition [97, 98] . Notably, butyrate modulates immune responses in intestinal macrophages via the reduction of NO due to inhibition of iNOS and decreased activation of nuclear NF-kB [99] . In rats, butyrate inhibition of HDAC leads to downregulation of secondary response genes like Nos2 and Il6 [100] . Thus, butyrate may behave as a microbial signal to maintain host immunity [101] . It is important to note that the beneficial effects of SCFAs-mediated HDAC inhibition may depend not only on the concentration of SCFAs but also on the target tissue or cell type [102] .

The demonstrated redox signaling roles of specific lipids underpins the importance of emerging metabolomics and redox lipidomics for diagnosis and therapeutics [103, 104] . In addition, systems-level models that permit interpretation of big data, particularly those that account for multiple interactions between metabolic intermediates, are necessary to understand the scope, scale, and complexity of effectors and targets of redox metabolism [29, 105] .

In recent years, both human and animal experiments have revealed that gut microflora dysbiosis can accelerate the progression of cardiovascular diseases [106] , diabetes [107] , and neurodegenerative diseases [108] , with SCFAs playing a key role. Gut dysbiosis and increased gut permeability are associated with heightened levels of oxidative stress [109] . Although our understanding of gut microbiota-host interactions has advanced significantly, many questions remain concerning the mechanistic links between the gut microbiome and host diseases, in particular those related with redox dysregulation [58] .

In Table 6 , we have selected reports that illustrate the effect of specific SCFAs on redox homeostasis in different pathological conditions. For example, mice receiving dietary butyrate supplementation exhibit reduced oxidative stress, attenuated endothelial dysfunction, and decreased aortic atherosclerotic lesions [18] . In rat aortic cells, butyrate and acetate, at pharmacological concentrations (5-10 mM), also improve endothelial dysfunction induced by angiotensin II by stimulating NO production and decreasing NADPH oxidase and mitochondrial ROS generation [110] .

Butyrate and its synthetic derivative, N-(1-carbamoyl-2-phenylethyl) butyramide (FBA), protect mice against insulin resistance and liver steatosis by acting on hepatic mitochondria, reducing lipid accumulation and oxidative stress [111] . In a recent study in mice with type 2 diabetes mellitus, dysbiosis was associated with a reduction in butyrate-forming bacteria coupled to a decrease in cecal and fecal butyrate content leading to increased activity of the colon HDAC3 [112] . Butyrate treatment also reduced inflammatory markers and ROS production, a mechanism of action previously discussed in Section 5. Acetate has been reported to improve the viability of islets and the mouse insulinoma cell line MIN6 subjected to oxidative stress by enhancing metabolism of ROS, whilst butyrate promotes "oxidative eustress" by inhibiting FFAR2 and NO generation [113] . These findings suggest that SCFAs play an essential role in supporting β-cell metabolism and promoting survival under stressful conditions contributing to reduce diseases such as diabetes. Interestingly, acetate, propionate and butyrate recapitulate chromatin modification states and transcriptional responses associated with gut microbiota in multiple murine tissues [114] . Notably, accumulating evidence indicates that the beneficial effect of these SCFAs in obesity and diabetes are related to their influences on host epigenetics [115] . In fact, butyrate and propionate protect against diet-induced obesity in mice by regulating gut hormones via FFAR3-independent mechanisms [116] .

Oxidative stress is also linked to the etiology of many neurodegenerative diseases such as Alzheimer's disease (AD), Amyotrophic lateral sclerosis, Friedreich's ataxia, Huntington's disease, Multiple sclerosis, and Parkinson's disease [117] . Interestingly, there are data linking regulation of redox homeostasis by specific SCFAs with the development of neurodegenerative diseases, constituting an interesting target for pharmacological interventions in psychiatric disorders [118] . In fact, while many studies have shown that saturated long chain fatty acids (C:20-C:26) increase the risk of AD by promoting amyloid-β peptide (Aβ) generated oxidative stress [119] , SFCAs can activate Nrf2 and thereby prevent Aβ accumulation [120] . Recent studies have reported a link between butyrate and Nrf2 in protection against high cholesterol-induced neuronal amyloidogenesis in mice [121] . Obesity increases Aβ accumulation in the brain and reduces butyrate-producing bacteria, and notably butyrate treatment has been reported to decrease expression levels of beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) and Aβ accumulation. In this study, butyrate taken up by cells via a sodium-coupled monocarboxylate transporter 1 (SMCT1), prevented excessive ROS generation by inhibiting NOX2 and upregulating SOD1, following activation of the cyclin-dependent kinase inhibitor 1 (p21)/Nrf2 pathway through acetylation of Sp1. Notably, in mouse microglia, DHA suppresses Aβ-induced ROS production by upregulating the Nrf2/HO-1 pathway [122] . These results highlight that SCFAs link the microbiota with the maintenance of host redox homeostasis through Nrf2 signaling via mechanisms that may involve omega-3 fatty acids, as discussed in Section 4. The experimental data reviewed in this and other sections highlight redox signaling roles of butyrate and other SCFAs and their potential as therapeutic nutrients alone or in combination with other treatments.

Human trials suggest that many biological effects may be mediated by SCFAs (see Table 7) , and, as shown, promising in vitro and animal studies have been published, albeit they cannot easily be extrapolated to humans [123] . Although the interplay of SCFAs with the gut microbiome and the associated immune system as well as the role of SCFAs in the gut-brain connection has been established [24, 124] , a better understanding of the mechanism of action of FFAs will facilitate translation for control and prevention of metabolic diseases.

The health benefits of specific FFAs discussed in this review implicate plant-derived compounds in the maintenance of mammalian cellular redox homeostasis [125] . The fact that specific FFAs act as Nrf2 regulators suggests that plant-derived compounds may serve as natural redox regulators [126] . The FFA hexanoic acid (C6:0) is particularly interesting, because it is a natural priming agent that protects plants against a wide range of pathogens inducing defense mechanisms and controlling plant redox homeostasis [127] . This FFA was considered a SCFA in previous classifications, and in fact acts as ligand of the SCFA receptor FFAR3 and the MCFA receptor FFAR1 in mammalian cells [5] . There are reports demonstrating that treatment of endothelial cells with hexanoic acid improves energy production and attenuates pro-inflammatory cytokine generation [94] , with no effects on endothelial barrier integrity [93] . Moreover, hexanoic acid has anti-proliferative and anti-inflammatory properties in mammalian cells [128, 129] .

Of relevance, hexanoic acid protects tomato plants against infection by the necrotrophic pathogen Botrytis cinerea by regulating GSH levels and potentiating redox-sensitive genes encoding GSTs, peroxiredoxins and glutathione reductase amongst others [130] . Notably, the activation of redox-related genes by this priming agent closely resembles that induced by sulforaphane (SFN) in the preconditioning of mammalian cells [131, 132] . In plants, SFN is a secondary metabolite, contributing to the hypersensitive response as well as priming defense genes to protect against biotic stresses [133] . In Arabidopsis plants, treatment with SFN protects against Hyaloperonospora arabidopsidis inducing histone epigenetic modifications in the promoter of defense genes [134] . Therefore, the mechanisms of action of the FFA hexanoic acid and SFN in priming Table 6 Protective effects of SCFAs in animal models of cardiovascular diseases and diabetes. Acute rectal administration of SCFAs modulates whole-body substrate and energy metabolism, with an increase in fasting fat oxidation and resting energy expenditure [123] Abbreviations: NaAc: sodium acetate; NaB: sodium butyrate; NaP: sodium propionate; SCFAs: short chain fatty acids.

C. González-Bosch et al.

plants resemble those reported for butyrate and SFN in the preconditioning of mammalian cells, supporting conservation of redox-activated pathways.

In this review, we discussed the role of SCFAs in regulating redox homeostasis mainly via targeting Keap1-Nrf2 signaling, as illustrated in Fig. 4 . These compounds, especially butyrate, exhibit anti-inflammatory and anti-proliferative properties related to their redox signaling activity. SCFAs can inhibit HDAC activities, contributing to the epigenetic regulation of genes, including Nrf2. Among the natural compounds that can modulate the Keap1-Nrf2 pathway, SCFAs offer an advantage as their activity depends on specific receptors and transporters [5] , potentially avoiding widespread activation of Nrf2 with undesirable effects on the redox status of healthy versus cancerous tissues [135] .

Accelerated biological aging is a feature of age-related morbidities, which share common features, including low-grade persistent inflammation, diminished Nrf2 activity, a depleted metabolic capability and a low diversity gut microbiome [136] . In support of the beneficial effect of the microbiome metabolites in aging, recent multi-omics approaches in mice revealed a pattern of shared pathways of improved health and lifespan that included SCFAs and essential polyunsaturated fatty acid (PUFA) metabolism [137] . Notably, SCFA concentrations are likely less than optimal in older adults, possibly due to insufficient daily dietary fibre intake and to a lower capacity to produce butyrate in the elderly gut microbiome [138] . High fibre supplementation increases butyrate levels that attenuate pro-inflammatory cytokine expression in microglia in aged mice [139] . Long-term treatment with butyrate reduced muscle atrophy in mice during aging, leading to reduced fat mass, improved glucose metabolism, and increased enzymes involved in oxidative metabolism, mainly catalase, in old mice [140] . Interestingly, the beneficial effects exerted by butyrate on muscle mass during aging are consistent with inhibition of HDAC3, supporting that inhibitors of specific HDACs could be used to treat age-related metabolic disease and sarcopenia [141] . In fact, an optimal status of relevant nutrients to effectively reduce inflammation and oxidative stress, strengthens the immune system, is important for human health, and particularly relevant at this time when the world is facing the coronavirus-disease 2019 (COVID-19) crisis [142] .

The majority of studies in vitro have screened 'modulators' of redox homeostasis in cells cultured under hyperoxic conditions in incubators gassed with 5% CO 2 and room air (21%, 20.9 kPa O 2 ). Under these conditions, cells are exposed to elevated O 2 levels never encountered in vivo, and as consequence Nrf2-regulated antioxidant defences are upregulated to reduce oxidative stress [143] [144] [145] . To our knowledge, there are no reported studies of the effects of SCFAs under physiologically relevant O 2 levels and thus such studies are necessary to recapitulate SCFAs mediated redox signaling in vivo. Based on emerging data, the use of specific SCFAs as nutritional and therapeutic agents in inflammation, cancer and aging warrants further investigation. The design of improved experimental approaches to explore the mechanisms of production and action of SCFAs in cellular redox signaling under physiological conditions will underpin strategies for developing personalized nutrition and therapeutics for redox medicine. 

Authors declare no conflicts of interest.

Impact of diet-modulated butyrate production on intestinal barrier function and inflammation

Nutritional signaling via free fatty acid receptors

Redox signaling in the gastrointestinal tract, Free Radic

Redox-dependent antiinflammatory signaling actions of unsaturated fatty acids

Free fatty acid receptors in health and disease

Free fatty acid (FFA) and hydroxy carboxylic acid (HCA) receptors

Potential beneficial effects of butyrate in intestinal and extraintestinal diseases

From gut microbiota dysfunction to obesity: could short-chain fatty acids stop this dangerous course?

Short-and medium-chain fatty acids in energy metabolism: the cellular perspective

Production rates and metabolism of short-chain fatty acids in the colon and whole body using stable isotopes

Short chain fatty acids in human large intestine, portal, hepatic and venous blood

Formation of short chain fatty acids by the gut microbiota and their impact on human metabolism

Review article: the role of butyrate on colonic function

Application of liquid chromatography-mass spectrometry to measure the concentrations and study the synthesis of short chain fatty acids following stable isotope infusions

Simultaneous measurement of plasma concentrations and 13C-enrichment of short-chain fatty acids, lactic acid and ketone bodies by gas chromatography coupled to mass spectrometry

High-resolution nuclear magnetic resonance spectroscopic study of metabolites in the cerebrospinal fluid of patients with cervical myelopathy and lumbar radiculopathy

The role of the microbial metabolites including tryptophan catabolites and short chain fatty acids in the pathophysiology of immuneinflammatory and neuroimmune disease

Oral butyrate reduces oxidative stress in atherosclerotic lesion sites by a mechanism involving NADPH oxidase down-regulation in endothelial cells

Medium chain triglycerides and structured lipids

The emerging role of acyl-CoA thioesterases and acyltransferases in regulating peroxisomal lipid metabolism

Fuel metabolism in starvation

Anticatabolic effects of ketone bodies in skeletal muscle

Suppression of oxidative stress by β-hydroxybutyrate, an endogenous histone deacetylase inhibitor

Impacts of microbiome metabolites on immune regulation and autoimmunity

G-protein-coupled receptors and signaling networks: emerging paradigms

Metabolite-sensing receptors GPR43 and GPR109A facilitate dietary fibre-induced gut homeostasis through regulation of the inflammasome

Findings in redox biology: from H2O2 to oxidative stress

Reactive oxygen species (ROS) as pleiotropic physiological signalling agents

The reactive species interactome: evolutionary emergence, biological significance, and opportunities for redox metabolomics and personalized medicine

Free radicals in the physiological control of cell function

Reactive oxygen species and mitochondria: a nexus of cellular homeostasis

Physiological roles of mitochondrial reactive oxygen species

Oxidative stress: a concept in redox biology and medicine

The KEAP1-NRF2 system: a thiolbased sensor-effector apparatus for maintaining redox homeostasis

Sulforaphane and other nutrigenomic Nrf2 activators: can the clinician's expectation Be matched by the reality? Oxidative medicine and cellular longevity

Dietary phytochemicals in colorectal cancer prevention and treatment: a focus on the molecular mechanisms involved

Oxidative stress and dietary phytochemicals: role in cancer chemoprevention and treatment

Introduction to special issue on 'Nrf2 regulated redox signaling and metabolism in physiology and medicine, Free Radic

Therapeutic targeting of the NRF2 and KEAP1 partnership in chronic diseases

Transcription factor Nrf2 coordinately regulates a group of oxidative stress-inducible genes in macrophages

Molecular basis of the Keap1-Nrf2 system, Free Radic

Mechanisms of activation of the transcription factor Nrf2 by redox stressors, nutrient cues, and energy status and the pathways through which it attenuates degenerative disease, Free Radic

Keap1 regulates both cytoplasmic-nuclear shuttling and degradation of Nrf2 in response to electrophiles

Keap1 represses nuclear activation of antioxidant responsive elements by Nrf2 through binding to the amino-terminal Neh2 domain

Activation of Nrf2 by natural bioactive compounds: a promising approach for stroke?

The role of natural products in revealing NRF2 function

Isothiocyanate from broccoli, sulforaphane, and its properties

Glucosinolates and isothiocyanates in health and disease

Microbiome-host systems interactions: protective effects of propionate upon the blood-brain barrier

Butyrate alleviates oxidative stress by regulating NRF2 nuclear accumulation and H3K9/14 acetylation via GPR109A in bovine mammary C. González-Bosch et al. epithelial cells and mammary glands, Free Radic

The dual role of Nrf2 in nonalcoholic fatty liver disease: regulation of antioxidant defenses and hepatic lipid metabolism

Dissecting the crosstalk between NRF2 signaling and metabolic processes in cancer

The Nrf2 regulatory network provides an interface between redox and intermediary metabolism

Nrf2 affects the efficiency of mitochondrial fatty acid oxidation

Keap1/Nrf2 pathway activation leads to a repressed hepatic gluconeogenic and lipogenic program in mice on a high-fat diet

Inhibition of endothelial Notch signaling impairs fatty acid transport and leads to metabolic and vascular remodeling of the adult Heart

Crosstalk between Nrf2 and Notch signaling, Free Radic

Redox relationships in gut-microbiome interactions, Free Radic

Sulforaphane preconditioning of the Nrf2/HO-1 defense pathway protects the cerebral vasculature against blood-brain barrier disruption and neurological deficits in stroke, Free Radic

Temporal and spatial distribution of Nrf2 in rat brain following stroke: quantification of nuclear to cytoplasmic Nrf2 content using a novel immunohistochemical technique

Gestational diabetes mellitus impairs Nrf2-mediated adaptive antioxidant defenses and redox signaling in fetal endothelial cells in utero

Targeting the KEAP1-NRF2 system to prevent kidney disease progression

Transcription factor NRF2 as a therapeutic target for chronic diseases: a systems medicine approach

Sodium butyrate attenuates diabetes-induced aortic endothelial dysfunction via P300-mediated transcriptional activation of Nrf2, Free Radic

The enhancement of phase 2 enzyme activities by sodium butyrate in normal intestinal epithelial cells is associated with Nrf2 and p53

Non-esterified fatty acids activate the ROS-p38-p53/Nrf2 signaling pathway to induce bovine hepatocyte apoptosis in vitro

p53 suppresses the Nrf2-dependent transcription of antioxidant response genes

Non-esterified fatty acid-induced reactive oxygen species mediated granulosa cells apoptosis is regulated by nrf2/p53 signaling pathway

The neuropharmacology of butyrate: the bread and butter of the microbiota-gut-brain axis?

Gut reactions: how the blood-brain barrier connects the microbiome and the brain

The gut microbiota influences blood-brain barrier permeability in mice

Docosahexaenoic acid regulates vascular endothelial cell function and prevents cardiovascular disease

Effects of DHA on hippocampal autophagy and lysosome function after traumatic brain injury

Mfsd2a is a transporter for the essential omega-3 fatty acid docosahexaenoic acid

Polyunsaturated fatty acids and their metabolites in brain function and disease

Phospholipid and fatty acid specificity of endothelial lipase: potential role of the enzyme in the delivery of docosahexaenoic acid (DHA) to tissues

Brain docosahexaenoic acid uptake and metabolism

Docosahexaenoic acid induces expression of heme oxygenase-1 and NAD(P)H:quinone oxidoreductase through activation of Nrf2 in human mammary epithelial cells

Therapeutic effects of specialized pro-resolving lipids mediators on cardiac fibrosis via NRF2 activation

Omega-3 fatty acids protect the brain against ischemic injury by activating Nrf2 and upregulating heme oxygenase 1

Dietary docosahexaenoic acid inhibits neurodegeneration and prevents stroke

Docosanoids promote neurogenesis and angiogenesis, bloodbrain barrier integrity, penumbra protection, and neurobehavioral recovery after experimental ischemic stroke

The role of resolvins: EPA and DHA derivatives can Be useful in the prevention and treatment of ischemic stroke

Low brain DHA content worsens sensorimotor outcomes after TBI and decreases TBI-induced Timp1 expression in juvenile rats

DHA but not EPA emulsions preserve neurological and mitochondrial function after brain hypoxia-ischemia in neonatal mice

Transplantation of fecal microbiota rich in short chain fatty acids and butyric acid treat cerebral ischemic stroke by regulating gut microbiota

Gut microbiota-derived short-chain fatty acids promote poststroke recovery in aged mice

Short-chain fatty acids improve poststroke recovery via immunological mechanisms

Dysbiosis of gut microbiota and short-chain fatty acids in acute ischemic stroke and the subsequent risk for poor functional outcomes

Role of short-chain fatty acids in colonic inflammation, carcinogenesis, and mucosal protection and healing

Sodium butyrate alleviates adipocyte inflammation by inhibiting NLRP3 pathway

Differential effects of short chain fatty acids on endothelial Nlrp3 inflammasome activation and neointima formation: antioxidant action of butyrate

Selective disruption of endothelial barrier function in culture by pure fatty acids and fatty acids derived from animal and plant fats

Impact of short-and medium-chain fatty acids on mitochondrial function in severe inflammation

Short-chain fatty acids and trichostatin A alter tight junction permeability in human umbilical vein endothelial cells

Short-chain fatty acids alter tight junction permeability in intestinal monolayer cells via lipoxygenase activation

Short-chain fatty acids: bacterial messengers modulating the immunometabolism of T cells

Connections between metabolism and epigenetics in programming cellular differentiation

Essential role of the JAK/ STAT1 signaling pathway in the expression of inducible nitric-oxide synthase in intestinal epithelial cells and its regulation by butyrate

The microbial metabolite butyrate regulates intestinal macrophage function via histone deacetylase inhibition

The short chain fatty acid butyrate imprints an antimicrobial program in macrophages

Sodium propionate and sodium butyrate effects on histone deacetylase (HDAC) activity, histone acetylation, and inflammatory gene expression in bovine mammary epithelial cells

Computational solutions in redox lipidomics -current strategies and future perspectives, Free Radic

Redox lipidomics to better understand brain aging and function, Free Radic

Systems biology approaches to redox metabolism in stress and disease states

The gut microbiota as a novel regulator of cardiovascular function and disease

Impact of the gut microbiota on the development of obesity and type 2 diabetes mellitus

Microbiota-gut-brain Axis: new therapeutic opportunities

Mechanisms and consequences of intestinal dysbiosis

Protective effects of short-chain fatty acids on endothelial dysfunction induced by angiotensin II

Butyrate regulates liver mitochondrial function, efficiency, and dynamics in insulin-resistant obese mice

Butyrate modulates diabetes-linked gut dysbiosis: epigenetic and mechanistic modifications

Acetate and butyrate improve beta-cell metabolism and mitochondrial respiration under oxidative stress

Diet-microbiota interactions mediate global epigenetic programming in multiple host tissues

The epigenetic connection between the gut microbiome in obesity and diabetes

Butyrate and propionate protect against diet-induced obesity and regulate gut hormones via free fatty acid receptor 3-independent mechanisms

Oxidative stress and mitochondrial dysfunction-linked neurodegenerative disorders

Oxidation-reduction mechanisms in psychiatric disorders: a novel target for pharmacological intervention

Effects of chain length of saturated fatty acids on Aβ generation in SH-SY5Y cells

Diet and Alzheimer's dementianutritional approach to modulate inflammation

Sodium butyrate inhibits high cholesterol-induced neuronal amyloidogenesis by modulating NRF2 stabilization-mediated ROS levels: involvement of NOX2 and SOD1

Effects of docosahexaenoic acid and its peroxidation product on amyloid-β peptide-stimulated microglia

Intestinal short chain fatty acids and their link with diet and human health

The microbiota-gut-brain Axis

Dietary fat, the gut microbiota, and metabolic health -a systematic review conducted within the MyNewGut project

Priming plant resistance by activation of redox-sensitive genes, Free Radic

Priming of plant resistance by natural compounds. Hexanoic acid as a model

In vitro characterization of gut microbiota-derived bacterial strains with neuroprotective properties, Front

Anticarcinogenic properties of medium chain fatty acids on human colorectal, skin and breast cancer cells in vitro

Hexanoic acid protects tomato plants against Botrytis cinerea by priming defence responses and reducing oxidative stress

Nrf2-mediated redox signalling in vascular health and disease, Free Radic

Concerted redox modulation by sulforaphane alleviates diabetes and cardiometabolic syndrome, Free Radic

Involvement of the electrophilic isothiocyanate sulforaphane in Arabidopsis local defense responses

Sulforaphane modifies histone H3, unpacks chromatin, and primes defense

Modulating NRF2 in disease: timing is everything

The gut microbiome, aging, and longevity: a systematic review

Untangling determinants of enhanced health and lifespan through a multi-omics approach in mice

Quantification of butyryl CoA:acetate CoA-transferase genes reveals different butyrate production capacity in individuals according to diet and age

Butyrate and dietary soluble fiber improve neuroinflammation associated with aging in mice

The histone deacetylase inhibitor butyrate improves metabolism and reduces muscle atrophy during aging

Emerging roles for histone deacetylases in agerelated muscle atrophy

COVID-19 and cardiovascular injury: a role for oxidative stress and antioxidant treatment?

Bach1 differentially regulates distinct Nrf2-dependent genes in human venous and coronary artery endothelial cells adapted to physiological oxygen levels, Free Radic

Defining physiological normoxia for improved translation of cell physiology to animal models and humans

Nrf2-regulated redox signaling in brain endothelial cells adapted to physiological oxygen levels: consequences for sulforaphane mediated protection against hypoxia-reoxygenation

The authors acknowledge the support of Heart Research UK