In
genetic hemochromatosis (GH),
iron overload affects mainly parenchymal cells, whereas little
iron is found in reticuloendothelial (RE) cells. We previously found that RE cells from GH patients had an inappropriately high activity of
iron regulatory protein (IRP), the key regulator of intracellular
iron homeostasis. Elevated IRP should reflect a reduction of the
iron pool, possibly because of a failure to retain
iron. A defect in
iron handling by RE cells that results in a lack of feedback regulation of intestinal absorption might be the basic abnormality in GH. To further investigate the capacity of
iron retention in RE cells of GH patients, we used
inflammation as a model system as it is characterized by a block of
iron release from macrophages. We analyzed the
iron status of RE cells by assaying IRP activity and
ferritin content after 4, 8, and 24 hours of incubation with
lipopolysaccharide (LPS) and
interferon-gamma (IFN-gamma).
RNA-bandshift assays showed that in monocytes and macrophages from 16 control subjects, IRP activity was transiently elevated 4 hours
after treatment with LPS and IFN-gamma but remarkably downregulated thereafter. Treatment with NO donors produced the same effects whereas an
inducible Nitric Oxide Synthase (iNOS) inhibitor prevented them, which suggests that the NO pathway was involved. Decreased IRP activity was also found in monocytes from eight patients with
inflammation. Interestingly, no late decrease of IRP activity was detected in
cytokine-treated RE cells from 12 GH patients.
Ferritin content was increased 24 hours
after treatment in monocytes from normal subjects but not in monocytes from GH patients. The lack of downregulation of IRP activity under inflammatory conditions seems to confirm that the control of
iron release from RE cells is defective in GH.