Interferon‐γ and lipopolysaccharide regulate the expression of Nramp2 and increase the uptake of iron from low relative molecular mass complexes by macrophages

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Abstract
The natural resistance associated macrophage protein 2 (Nramp2) is a transporter that is involved in iron (Fe) uptake from transferrin (Tf) and low molecular mass Fe complexes. Here we describe the effect of the inflammatory mediators interferon-γ (IFN-γ) and lipopolysaccharide (LPS) on the expression of Nramp2 mRNA and Fe uptake by cells of the macrophage lineage. After incubation of the RAW264.7 macrophage cell line with LPS there was a sevenfold increase in the expression of the 2.3 kb Nramp2 mRNA transcript when compared with the control, but little effect on the Nramp2 3.1 kb transcript. These results indicate differential regulation of the two transcripts. Treatment with LPS resulted in an increase in 59Fe uptake from 59Fe–nitrilotriacetic acid, while transferrin receptor (TfR) mRNA levels and 59Fe uptake from 59Fe–Tf were decreased. Paradoxically, at the same time, an increase in iron regulatory protein (IRP)1 RNA-binding activity was observed. Incubation with IFN-γ (50 U·mL−1) resulted in a marked decrease in TfR mRNA levels but had no effect on Nramp2 mRNA expression. Exposure of RAW264.7 cells to both IFN-γ and LPS resulted in a fourfold increase in the Nramp2 2.3-kb transcript and a four to fivefold decrease in the 3.1-kb transcript when compared with the control. Furthermore, there was a decrease in TfR mRNA levels despite an increase in IRP1 RNA-binding activity and a marked increase in inducible nitric oxide synthase mRNA expression. Hence, TfR and Nramp2 mRNA expression did not appear to be regulated in a concerted manner. Similar responses to those found above for RAW264.7 cells were also observed in the J774 macrophage cell line and also for primary cultures of mouse peritoneal macrophages. These results are of interest as the TfR and Nramp2 are thought to act together during Fe uptake from Tf. This is the first report to demonstrate regulation of the Nramp2 mRNA transcripts by inflammatory mediators.