New Insights into the Pivotal Role of Iron/Heme Metabolism in TLR4/NF-κB Signaling-Mediated Inflammatory Responses in Human Monocytes

Abstract

Iron metabolism and heme biosynthesis are essential processes in cells during the energy cycle. Alteration in these processes could create an inflammatory condition, which results in tumorigenesis. Studies are conducted on the exact role of iron/heme metabolism in induced inflammatory conditions. This study used lipopolysaccharide (LPS)- or high-glucose-induced inflammation conditions in THP-1 cells to study how iron/heme metabolism participates in inflammatory responses. Here, we used iron and heme assays for measuring total iron and heme. We also used flow cytometry and Western blotting to analyze molecular responses. Our results demonstrated that adding LPS or high-glucose induced iron formation and heme synthesis and elevated the expression levels of proteins responsible for iron metabolism and heme synthesis. We then found that further addition of heme or 5-aminolevulinic acid (ALA) increased heme biosynthesis and promoted inflammatory responses by upregulating TLR4/NF-κB and inflammatory cytokine expressions. We also demonstrated the inhibition of heme synthesis using succinylacetone (SA). Moreover, N-MMP inhibited LPS- or high-glucose-induced inflammatory responses by inhibiting TLR4/NF-κB signaling. Hence, iron/heme metabolism checkpoints could be considered a target for treating inflammatory conditions.

Gestational diabetes is driven by microbiota-induced inflammation months before diagnosis

“…There is evidence that mevalonate can have negative implications on host inflammation—its presence reduces effects of statins in decreasing IL-6 and IL-8,23 and a kinase deficiency, which increases free mevalonate, leads to autoinflammation 49. The heme biosynthesis pathway was previously associated with elevated IL-650 51 and with type 2 diabetes 52. We do note, however, that further research is needed to understand if the bacteria themselves, their excreted metabolites or some other factors control this phenotype.…”

Insights into the molecular basis and mechanism of heme‐triggered TLR4 signalling: The role of heme‐binding motifs in TLR4 and MD2

“…At the peptide level, the heme-binding affinity of this motif is rather moderate but might be increased for the protein due to supportive hydrophobic interactions with the lipid bilayer, as observed in this study. Based on these results it is conceivable that TLR4 could also convey heme step by step from the horseshoe-shaped domains towards the lipid bilayer, which will not only allow heme bound at this site to exert its oxidative damage towards the membrane but also suggests TLR4 as a potential heme importer facilitating an assisted transfer of heme through the membrane as described for bacterial ABC transporters [e.g., [70,71]], which is further substantiated by the recent finding of intracellular heme binding to TLR4 [72].…”

Hemin and iron increase synthesis and trigger export of xanthine oxidoreductase from hepatocytes to the circulation

“…In the case of free Fe, direct activation of TLR4 has not been shown. However, intracellular processing of excess Fe into hemin has been shown to activate TLR4, potentially making this pathway operable [ 59 ]. Likewise, Fe-induced oxidants or lipid peroxidation may lead to NF-κB and Sp1 actiavtion.…”