Cytokine-mediated regulation of iron transport in human monocytic cells

Abstract

Under chronic inflammatory conditions cytokines induce a diversion of iron traffic, leading to hypoferremia and retention of the metal within the reticuloendothelial system. However, the regulatory pathways underlying these disturbances of iron homeostasis are poorly understood. We investigated transferrin receptor (TfR)-dependent and -independent iron transport mechanisms in cytokine-stimulated human monocytic cell lines THP-1 and U937. Combined treatment of cells with interferon-␥ (IFN-␥) and lipopolysaccharide (LPS) reduced TfR mRNA levels, surface expression, and iron uptake, and these effects were reversed by interleukin-10 (IL-10), thus stimulating TfR-mediated iron acquisition. IFN-␥ and LPS dosedependently increased the cellular expression of divalent metal transporter-1, a transmembrane transporter of ferrous iron, and stimulated the uptake of nontransferrin bound iron (NTBI) into cells. At the same time, IFN-␥ and LPS downregulated the expression of ferroportin mRNA, a putative iron exporter, and decreased iron release from monocytes. Preincubation with IL-10 partly counteracted these effects. Our results demonstrate that the proinflammatory stimuli IFN-␥ and LPS increase the uptake of NTBI via stimulation of divalent metal transporter-1 expression and cause retention of the metal within monocytes by down-regulating ferroportin synthesis. Opposite, the anti-inflammatory cytokine IL-10 stimulates TfR-mediated iron uptake into activated monocytes. The regulation of iron transport by cytokines is a key mechanism in the pathogenesis of anemia of chronic disease and a promising target for therapeutic intervention. IntroductionIron metabolism and immunity are closely interconnected. 1,2 This is due, on the one hand, to divergent regulatory effects of the metal on immune cell proliferation 3 and on the effectiveness of cellular immune effector pathways. [4][5][6] On the other hand, cytokines derived from T cells and monocytes regulate cellular iron homeostasis by affecting the expression of proteins involved in the uptake and storage of the metal. Proinflammatory cytokines such as tumor necrosis factor-␣ (TNF-␣), interleukin-1 (IL-1), or interleukin-6 (IL-6) directly stimulate the transcription and translation of the major iron storage protein ferritin, the latter by interfering with a so-called acute phase box. 7,8 Moreover, cytokines influence the posttranscriptional control of iron homeostasis by modulating the binding affinity of iron regulatory proteins (IRP-1 and IRP-2) to specific RNA stem loop structures, termed iron responsive elements (IREs), which are found within the 5Ј untranslated region of ferritin mRNA and within the 3Ј untranslated region of transferrin receptor (TfR) mRNA, the surface protein involved in the uptake of transferrinbound iron. Activation of IRP binding to IREs is controlled by intracellular iron availability, with IRP binding affinity being high when intracellular iron concentrations are low, thus resulting in stabilization of TfR mRNA while ferritin translation is reduced (fo...

Differential regulation of iron homeostasis during human macrophage polarized activation

“…2D). In line with previous results obtained in mouse spleen macrophages [12], human monocytic cell lines [19] and human monocyte-derived macrophages [14], LPS/IFN-g stimulation of human macrophages led to much lower Fpn expression levels than in unpolarized M0 cells; however, M2 polarization repressed Fpn less, and thus there was a tenfold difference in Fpn mRNA expression between M1 and M2 macrophages (Fig. 3A).…”

“…However, inflammation may increase the uptake of other forms of iron. Enhanced divalent metal transporter1-mediated acquisition of non-transferrin-bound iron has been described in cytokine-stimulated human monocytic cell lines [19] and the increased erythrophagocytosis reported in TNF-a-stimulated macrophages could increase the acquisition of heme iron [21].…”

“…In particular, the role of transferrin receptor 1 (TfR1), which mediates transferrinbound iron uptake [1][2][3], is not fully understood. Studies of murine and human reticuloendothelial cells [8,[17][18][19] have shown that, despite an early NF-kB and HIF-1-dependent transcriptional upregulation [20], TfR1 expression is post-transcriptionally downmodulated by exposure to inflammatory stimuli for 10-24 h, an effect that would impair a major iron uptake mechanism. However, inflammation may increase the uptake of other forms of iron.…”

“…However, inflammation may increase the uptake of other forms of iron. Enhanced divalent metal transporter1-mediated acquisition of non-transferrin-bound iron has been described in cytokine-stimulated human monocytic cell lines [19] and the increased erythrophagocytosis reported in TNF-a-stimulated macrophages could increase the acquisition of heme iron [21].Most of the information available concerning the molecular regulation of macrophage iron metabolism during inflammation has come from studies in which macrophage cell lines or freshly isolated monocytes/macrophages have been challenged with LPS/IFN-g, a model system that mimicks exposure to micro-organisms and/or Th1 pro-inflammatory cytokines (see [6] for a recent review). However, cells of the monocyte-macrophage lineage are characterized by marked phenotypical and functional heterogeneity [22][23][24].…”

From phagocyte diversity and activation to probiotics: Back to Metchnikoff

“…Iron metabolism, as an example, is regulated at multiple steps differently affected by polarizing cytokines, with M1 cells being characterized by increased iron uptake and intracellular retention of the metal, and M2 cells releasing iron in the extracellular milieu. The cytokine regulation of iron metabolism in macrophages is a crucial bacteriostatic mechanism and a key element in the pathogenesis of anemia of chronic disease [25].…”