The Role of Iron in Intestinal Mucus: Perspectives from Both the Host and Gut Microbiota

Abstract

Although research on the role of iron in host immunity has a history spanning decades, it is only relatively recently that attention has been directed toward the biological effects of iron on the intestinal mucus layer, prompted by an evolving understanding of the role of this material in immune defense. The mucus layer, secreted by intestinal goblet cells, covers the intestinal epithelium, and given its unique location, interactions between the host and gut microbiota, as well as among constituent microbiota, occur frequently within the mucus layer. Iron, as an essential nutrient for the vast majority of life forms, regulates immune responses from both the host and microbial perspectives. In this review, we summarize the iron metabolism of both the host and gut microbiota and describe how iron contributes to intestinal mucosal homeostasis via the intestinal mucus layer with respect to both host and constituent gut microbiota. The findings described herein offer a new perspective on iron-mediated intestinal mucosal barrier function.

Keywords: goblet cells; gut microbiota; intestinal inflammation; iron; mucus.

FIGURE 1

Iron metabolism. Nonheme iron is reduced to ferrous ions by Dcytb and subsequently enters enterocytes via DMT1. Cells other than enterocytes obtain iron mainly through the activity of TfR1. The ferrous ions in cells are primarily stored by ferritin and used in a range of metabolic process or transported by FPN. The degradation of FPN is regulated by hepcidin and RNF217. By sensing a low-iron status in enterocytes, PHD stabilizes the expression of HIF-2α, which promotes the expression of Dcytb, DMT1, and FPN to enhance the absorption and utilization of iron. Under conditions of iron deficiency, IRP1/2 binds to the 3ʹ UTR of IRE on TfR1 and DMT1, promoting their expression. Simultaneously, IRP1/2 binds to the 5ʹ UTR of IRE on ferritin and FPN, thereby inhibiting their expression. In iron deficiency, the expression of IRP1/2 is activated. However, duodenal epithelial cells and erythroid precursor cells can circumvent IRP-mediated repression of FPN under iron-deficient conditions by expressing FPN1B, which lacks the 5ʹ IRE. Iron ions circulating within the blood are derived not only from the absorption of iron by epithelial cells in the intestinal lumen but also from the phagocytosis of senescent erythrocytes by macrophages. Hepcidin, partially regulated by cytokines and iron, is derived mainly from hepatocytes. Figure created with BioRender.com. BMP, bone morphogenetic protein; BS, binding site; C/EBP, CCAAT enhancer-binding protein; CER, ceruloplasmin; Dcytb, duodenal cytochrome B reductase; DMT1, divalent cation transporter 1; FPN, ferroportin; HEPH, hephaestin; HIF, hypoxia-inducible factor; IRE, iron-responsive element; IRP, iron regulatory protein; JAK, Janus kinase; PHD, prolyl hydroxylase domain; RNF217, ring finger protein 217; SMAD, mothers against decapentaplegic homolog; STAT, signal transducer and activator of transcription; Tf, transferrin; TfR, transferrin receptor; UTR, untranslated region.

FIGURE 2

The role of iron in intestinal microbes. (A) Gut bacteria acquire iron through Feo, siderophores, or binding to heme. (B) Although the host can block bacterial access to iron by producing LCN2, CP, and lactoferrin, certain bacteria survive by adopting novel strategies for acquiring iron. (C) Clostridioides difficile uses LCN2 to produce ferrosomes as a means of storing iron, thereby mitigating the damage caused by iron overload and iron deficiency. Similarly, Salmonella typhimurium gains access to iron by producing salmochelin that contributes to the evasion LCN2 interception. In addition, S. typhimurium ensures the adequacy of its iron sources via intracellular and systemic coregulation of FPN following host invasion. Figure created with BioRender.com. CP, calprotectin; FPN, ferroportin; JAK, Janus kinase; LCN2, lipocalin-2; NRF2, nuclear factor erythroid-derived 2-related factor 2; SpvB, Salmonella plasmid virulence B; STAT, signal transducer and activator of transcription; TREM1, triggering receptor expressed on myeloid cells 1.

FIGURE 3

The effects of gut microbiota and metabolites on the mucus layer. (A) Excess iron promotes the growth of pathogenic bacteria. Enzymes produced by these pathogenic bacteria degrade the mucus layer. Additionally, interactions among certain pathogenic bacteria can facilitate the formation of biofilms that are more effective in colonizing the mucus layer. Collectively, these strategies contribute to the pathogenicity of these bacteria. (B) Moderate iron supplementation enhances the production of γ-glutamylcysteine, acetate, and GABA, and improves goblet cell function to promote MUC2 production by upregulating the abundance of Bifidobacterium dentium. Additionally, it may promote increases in the abundance of Akkermansia muciniphila, thereby promoting MUC2 production by upregulating the abundance of Bacteroides vulgatus SNUG 40005. However, when present in excess, iron inhibits increases in the abundance of A. muciniphila. (C) Moderate iron supplementation enhances the production of SCFAs, among which, propionate and butyrate promote the production of IL22 by stimulating ILC3 or CD4+ T cells, thereby enhancing mucus barrier function. Additionally, SCFAs contribute to inhibiting the virulence factors of S. typhimurium. Iron deficiency, however, suppresses the production of SCFAs, thereby accelerating consumption of the mucus layer by Mucinophilus. Figure created with BioRender.com. A1PK1, alpha kinase 1; ADP-H, ADP-heptose; AhR, aryl hydrocarbon receptor ; AKT, protein kinase B; EHEC, enterohaemorrhagic E. coli; ER, endoplasmic reticulum; ERK, extracellular signal-regulated kinase; GABA, γ-aminobutyric acid; GlcNAc, N-acetylglucosamine; GPR, G-protein coupled receptor; GSH, glutathione; HIF, hypoxia-inducible factor; ILC3, group 3 innate lymphoid cell; mTOR, mammalian target of rapamycin; MUC2, mucin 2; SCFA, short-chain fatty acid; SPI-1, Salmonella pathogenicity island-1; St6gal1, ST6 N-acetylgalactosaminide α-2,6-sialyltransferase; STAT, signal transducer and activator of transcription; StcE, secreted protease the of C1-esterase inhibitor; TIFA, TNF receptor associated factor-interacting protein with forkhead-associated domain.

FIGURE 4

The effects of host iron levels and metabolism on the mucus layer. (A) Systemic downregulation of hepcidin due to iron deficiency may have a positive effect on mucosal barrier function by promoting the upregulation of IL6 and IL13. However, the upregulation of local hepcidin expression due to inflammation may enhance intestinal mucosal barrier function via microbial interactions. Sufficient iron under normal conditions and low-iron levels associated with inflammation could protect the integrity of the intestinal barrier by promoting the production of IL22. (B and C) Both iron deficiency and excess impair the mucus layer by inducing ER or oxidative stress. Figure created with BioRender.com. AIM2, absent in melanoma 2; BP, binding protein; CAT, catalase; ER, endoplasmic reticulum; FPN, ferroportin; GXP, glutathione peroxidase; IRE, iron-responsive element; IRP, iron regulatory protein; mtROS, mitochondrial reactive oxygen species; MUC2, mucin 2; ROS, reactive oxygen species; SOD, superoxide dismutase; STAT, signal transducer and activator of transcription; TLR, toll-like receptor.

FIGURE 5

The effects of mucin2 on iron metabolism. Muc2^−/−-induced inflammation in mice leads to an increase in hemolysis and elevated levels of serum iron, mediated by increases in erythrocyte membrane fragility and attenuation of the ability of macrophages to phagocytose senescent erythrocytes. This occurs independent of genotype. Figure created with BioRender.com. MUC2, mucin 2; MUFA, monounsaturated fatty acid; SCD, stearoyl-CoA desaturase.