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.