Iron primes 3T3-L1 adipocytes to a TLR4-mediated inflammatory response

• Low doses of soluble bivalent iron enhanced the lipopolysaccharide-induced gene expression of adipocyte inflammatory molecules.
• In particular, a dose of 40 μM FeSO4 heptahydrate primed the adipocyte inflammation response through the enhancement of TLR4 expression on the membrane.
• FeSO4 heptahydrate was able to induce gene expression of hepcidin, hemojuvelin, and ferrportin, although the priming effect on this molecules was negligible.
• FeSO4 heptahydrate was able to elicit not prime adipocyte lipolysis.

ObjectiveIron participates in several mechanisms involving inflammation and innate immunity, yet the dysregulation of its homeostasis is a major cause of metabolic syndrome. Adipocytes should play a major role in iron metabolism, as an impairment in iron turnover is closely related to insulin resistance, obesity, and type 2 diabetes. The aim of this study was to investigate the role of iron in an in vitro–inflamed adipocyte model.MethodsGene expression of tumor necrosis factor-α, interleukin-6, inflammatory chemokines (CCL3, CCL4, and CXCL12), and molecules involved in iron metabolism were evaluated in an in vitro mouse 3T3-L1 cell model. Cells underwent treatment with FeSO4 heptahydrate and lipopolysaccharide (LPS) stimulation. Toll-like receptor 4 (TLR4) membrane expression, lipid droplet immunohystochemistry, and lipolysis were also evaluated.ResultsIron sulphate heptahydrate elicited gene expression of hepcidin, hemojuvelin, and ferroportin at different time courses. Additionally, it activated lipolysis but did not trigger any adipokine gene expression. When cells treated with physiological doses of iron were also stimulated with LPS, an enhancement in the LPS-induced gene expression of cytokines and chemokines was observed. The enhancement occurred with different patterns depending on different time courses and investigated genes, showing its maximal effect for IL-6 gene expression.ConclusionsFeSO4 heptahydrate at a relatively physiological dose, induced gene expression of iron modulatory proteins and also enhanced RNA transcripts of several inflammatory cytokines and chemokines through a priming/synergistic mechanism involving membrane TLR4.