*SCFAs, Inflammation and Oxidative Stress*

SCFAs produced by the intestinal microbiome are able to act on inflammation and oxidative stress through complex mechanisms of regulation, and, moreover, they also regulate the immune response. SCFAs suppress inflammation in many organs by reducing the migration and proliferation of immune cells and cytokine levels and by inducing apoptosis [122]. Through the inhibition of HDAC, they influence the inhibition of the nuclear factor, NF-κB, and the transcription of genes that code for pro-inflammatory cytokines. Furthermore, they are also able to reduce the inflammatory response through the reduction in neutrophil recruitment, with increased levels of TGF-β and IL-10 and reduced levels of IL-6, IL-1β, NO, and TNF-α. At the same time, SCFAs promote the production of T cells that release IL-10 and T-reg to prevent inflammatory responses and act on DCs to limit the expression of T cell activating molecules, resulting in the generation of tolerogenic rather than inflammatory T cells, thus reducing inflammatory responses. SCFAs can also modulate the immune response due to a direct effect on T cells, binding to GPR41, GPR43, and GPR109A receptors and activating Olfr78 receptor signalling to regulate T lymphocyte function by increasing the generation of Th1 and Th17 cells to improve immunity (Figure 1). Butyrate, for example, has shown both an inhibitory effect on the formation of NLRP3 inflammasomes and an improvement in tight junction function in intestinal and vascular endothelial cells [123,124]. Moreover, butyrate, through HDAC inhibition, was able to modulate the immune response by reducing iNOS levels and NF-κB activation [125].

**Figure 1.** Overview of the effect of the SCFAs on inflammation. In the intestinal lumen, SCFAs induce the secretion of IL-18, MUC2 and antimicrobial peptides from intestinal epithelial cells, induce IgA secretion from B lymphocytes and regulate tight junction expression. SCFAs bind to GPR41, GPR43, GPR109A receptors and activate Olfr78 receptor signalling to regulate T cell function increasing the generation of Th1 and Th17 cells and promoting the production of T cells that release IL-10 and T regs. SCFAs act on DCs to limit the expression of T cells activating molecules, resulting in the generation of tolerogenic T cells rather than inflammatory T-cells. SCFAs also reduce neutrophil recruitment, with increased levels of TGF-β, IL-10 and decreased levels of IL-6, IL-1β, NO, and TNF-α. Instead, through HDAC inhibition, they influence the inhibition of nuclear factor NF-κB, to inhibit inflammation. Abbreviations: DCs, Dendritic Cells; SCFAs, Short-chain Fatty Acids; GPCRs, G-Protein-coupled Receptors; HDAC, Histone Decetylase; NO, Nitric Oxide; TGF-β, Transforming Growth Factor beta; TNF-α, Tumor Necrosis Factor alpha; IL, Interleukin; Mucin, MUC2; NF-κB, Nuclear Factor Kappa-light-chain-enhancer of Activated B cells.

Furthermore, numerous studies have shown that SCFAs, particularly butyrate and propionate, were also able to modulate the Keap1-Nrf2-dependent cellular signalling pathway to maintain redox homeostasis through both direct and indirect mechanisms (Figure 2; [125–130]). Butyrate, through the recognition of the GPR109A receptor, induces the activation of the nuclear factor Nrf2, which encodes antioxidant enzymes for the inactivation of ROS [108]. Furthermore, butyrate has a synergistic action on the activation of Nrf2 because, by spreading in the cell lumen, it inactivates HDAC and consequently increases the production of histone H3K9ac thus inducing an epigenetic modification on the Nrf2 promoter, as demonstrated through various studies [125,126,131–133]. Acetate, propionate, and butyrate can synergistically activate the translocation of Nrf2 through the recognition of GPR41 and GPR43 receptors [134–136].

**Figure 2.** Direct and indirect mechanism of SCFAs on Nrf2 activation for modulation of oxidative stress. Binding of SCFAs to GPRC receptors induces direct activation of the nuclear factor Nrf2. Butyrate, on the other hand, also has a synergistic effect on Nrf2 activation because it diffuses into the cell lumen and, through HDAC inhibition, increases the production of histone H3K9ac, thus inducing an epigenetic modification on the Nrf2 promoter, indirectly activating Nrf2-dependent gene translocation and transcription. Abbreviations: AMPK, Activated Protein Kinase; HDAC, Histone Deacetylase; Nrf2, Nuclear Erythroid-Related Factor 2; ARE, Antioxidant Response element; HO-1, Heme Oxygenase-1; NQO1, NAD(P)H Quinone Dehydrogenase-1; NF-κB, Nuclear Factor Kappa-light-chain-enhancer of Activated B cells; SOD1, Superoxide Dismutase 1; GST, Glutathione S-transferase.
