Most of the
iron required for erythropoiesis is provided by
heme iron recycling following degradation of senescent erythrocytes by tissue macrophages. Accumulation of biochemical modifications at the red blood cell membrane during ageing (externalisation of
phosphatidyl-serine, peroxydation of membrane-bound
lipoproteins, loss of
sialic acid residues and formation of senescence neoantigens) constitute a series of signals that will allow the macrophage to identify the red blood cells to be eliminated, through interaction with specific receptors. After this initial recognition step, the red blood cell is internalised by phagocytosis, and phagosome maturation, which can comprise recruitment of the endoplasmic reticulum, will favour degradation of red blood cell constituents.
Heme is catabolised by
heme oxygenase 1, anchored in the endoplasmic reticulum membrane. A fraction of the released
iron will be recycled back to the plasma through
ferroportin, a membrane-bound Fe (II) export molecule, and a fraction will retained within the
ferritin molecules, to be released at later stages. Multiple evidence coming from human diseases (
type 4 hemochromatosis) and animal models indicate that
ferroportin is essential for
heme iron recycling by macrophages. Furthermore,
ferroportin seems to be the molecular target of
hepcidin, this circulating
peptide synthesized by the liver and acting as a negative regulator of intestinal
iron absorption and
iron recycling by macrophages. Perturbations in erythrophagocytosis play a physiopathological role in several diseases, including
hemochromatosis,
anemia of chronic disorders and
thalassemia.