Inflammation-driven NFκB signaling represses Ferroportin transcription in macrophages via HDAC 1 and 3

Anemia of Inflammation is a prevalent co-morbidity in patients with chronic inflammatory disorders. Inflammation causes hypoferremia and iron-restricted erythropoiesis by limiting Ferroportin (FPN)-mediated iron export from macrophages that recycle senescent erythrocytes. Macrophage cell surface expression of FPN is reduced by hepcidin-induced degradation and/or by repression of FPN (Slc40a1) transcription via cytokine and Toll-like receptor (TLR) stimulation. While the mechanisms underlying hepcidin-mediated control of FPN have been extensively studied, those inhibiting Slc40a1 mRNA expression remain unknown. We applied targeted RNA interference and pharmacological screens in macrophages stimulated with the TLR2/6 ligand FSL1 and identified critical signalling regulators of Slc40a1 mRNA repression downstream of TLRs and NFкB signaling. Interestingly, the NFкB regulatory hub is equally relevant for Slc40a1 mRNA repression driven by the TLR4 ligand LPS, the cytokine TNFβ/LTA and heat-killed bacteria. Mechanistically, macrophage stimulation with heat-killed Staphylococcus aureus recruits the Histone deacetylases (HDAC) 1 and 3 to the antioxidant response element (ARE) located in the Slc40a1 promoter. Accordingly, pre-treatment with a pan-HDAC inhibitor abrogates Slc40a1 mRNA repression in response to inflammatory cues, suggesting that HDACs act downstream of NFкB to repress Slc40a1 transcription. Consistently, recruitment of HDAC 1 and 3 to the Slc40a1 ARE following stimulation with heat-killed Staphylococcus aureus is dependent on NFκB signaling. These results support a model in which the ARE integrates the transcriptional responses of Slc40a1 triggered by signals from redox, metabolic and inflammatory pathways. This work identifies the long-sought mechanism of Slc40a1 transcriptional downregulation upon inflammation, paving the way for therapeutic interventions at this critical juncture.