Transferrin-dependent uptake and distribution of iron in osteoclast-like cells

Iron is an essential micro component and is involved in numerous critical cellular processes and in energy production. While its roles in oxygen transport and in oxidative phosphorylation are well documented, it remains to be elucidated, whether iron modulates specific cellular processes in different organs. Iron deficiency has been found to lead to a decrease in the capacity of osteoclasts to dissolve amorphous calcium phosphate. Furthermore, levels of iron in the cellular environment led to significant changes in the levels of transcripts encoding iron transport proteins. Within the present study, the uptake of iron by osteoclasts and the kinetics of intracellular transport were analyzed. For this purpose, M-CSF (Macrophage-Colony Stimulating Factor) dependent non-adherent osteoclast progenitor cells were differentiated to osteoclasts in media containing M-CSF and RANKL (Receptor Activator of NF-κB Ligand). For the upregulation of iron transport capacity, media were supplemented with Deferoxamine, an iron complexor, rendering the cultures virtually iron-free. To analyze iron uptake by osteoclast like cells, holo-transferrin, loaded with ⁵⁵Fe was added to the cells and iron uptake was quantitated in whole cell lysates and in fractionated cells. The data demonstrates that Deferoxamine-treated osteoclasts absorb higher quantities of iron as compared to untreated control cells. By density gradient centrifugation, cell associated iron can be separated into two major pools. Pool I represents non-transferrin associated iron in cytoplasmic fractions, while pool II contains transferrin/ transferrin receptor associated iron. Within 4 h of incubation in iron-deficient medium, pool I disappears, as does transferrin. Pool II iron and transferrin receptor, however, remain detectable. Furthermore, iron peaks did not associate with ferritin nor with mitochondria, demonstrating that these two mechanisms of iron storage did not become activated during the course of the study. The data thus demonstrates that iron uptake by osteoclasts can be modulated by exogenous iron and that cell associated iron forms either a labile iron pool of free iron that is lost within a short period of time or a vesicular pool of non-transferrin bound iron that remains stable over the experimental period. No further trafficking of iron into ferritin particles or mitochondria was detected.