Nitric oxide-mediated induction of ferritin synthesis in J774 macrophages by inflammatory cytokines: role of selective iron regulatory protein-2 downregulation.

Cytokine-treated macrophages represent a useful model to unravel the molecular basis of reticuloendothelial (RE) iron retention in inflammatory conditions. In the present study, we showed that stimulation of murine macrophage J774 cells with interferon (IFN)-gamma/lipopolysaccharide (LPS) resulted in a nitric oxide-dependent modulation of the activity of iron regulatory proteins (IRP)-1 and 2, cytoplasmic proteins which, binding to RNA motifs called iron responsive elements (IRE), control ferritin translation. Stimulation with cytokines caused a small increase of IRP-1 activity and a strong reduction of IRP-2 activity accompanied by increased ferritin synthesis and accumulation. Cytokines induced only a minor increase of H chain ferritin mRNA, thus indicating that IRP-2-mediated posttranscriptional regulation plays a major role in the control of ferritin expression. This was confirmed by direct demonstration that the translational repression function of IRP was impaired in stimulated cells. In fact, translation in cell-free extracts of a reporter transcript under the control of an IRE sequence was repressed less efficiently by IRP-containing lysates from cytokine-treated cells than by lysates from control cells. Our findings throw light on the role of IRP-2 showing that: (1) this protein responds to a stimulus in opposite fashion to IRP-1; (2) when abundantly expressed, as in J774 cells, IRP-2 is sufficient to regulate intracellular iron metabolism in living cells; and (3) by allowing increased ferritin synthesis, IRP-2 may play a role in the regulation of iron homeostasis in RE cells during inflammation.