Iron Loading Increases Ferroportin Heterogeneous Nuclear RNA and mRNA Levels in Murine J774 Macrophages

The transmembrane protein ferroportin is highly expressed in tissue macrophages, where it mediates iron export into the bloodstream. Although ferroportin expression can be controlled post-transcriptionally through a 5' iron-responsive element in its mRNA, various studies have documented increased ferroportin mRNA levels in response to iron, suggesting transcriptional regulation. We studied the effect of iron loading on levels of macrophage ferroportin mRNA, as well as heterogeneous nuclear RNA (hnRNA), the immediate product of ferroportin gene transcription. J774 cells, a mouse macrophage cell line, were incubated for 0, 3, 6, 9, 12, and 24 h in medium supplemented or not with 200 mumol/L iron. Quantitative RT-PCR was used to measure steady-state levels of ferroportin mRNA and hnRNA. Ferroportin mRNA levels increased by 12 h after iron treatment, reaching 6 times the control levels after 24 h. Changes in ferroportin mRNA levels were paralleled by similar changes in the levels of ferroportin hnRNA. Time course studies of ferroportin mRNA and hnRNA abundance after incubating cells with the transcriptional inhibitor actinomycin D revealed that ferroportin mRNA has a half-life of approximately 4 h and that iron loading does not stabilize ferroportin mRNA or hnRNA. Collectively, these data are consistent with the hypothesis that iron increases macrophage ferroportin mRNA levels by inducing transcription of the ferroportin gene.