The cellular labile iron pool and intracellular ferritin in K562 cells

The labile iron pool (LIP) harbors the metabolically active and regulatory forms of cellular iron. We assessed the role of intracellular ferritin in the maintenance of intracellular LIP levels. Treating K562 cells with the permeant chelator isonicotinoyl salicylaldehyde hydrazone reduced the LIP from 0.8 to 0.2 micromol/L, as monitored by the metalo-sensing probe calcein. When cells were reincubated in serum-free and chelator-free medium, the LIP partially recovered in a complex pattern. The first component of the LIP to reappear was relatively small and occurred within 1 hour, whereas the second was larger and relatively slow to occur, paralleling the decline in intracellular ferritin level (t1/2= 8 hours). Protease inhibitors such as leupeptin suppressed both the changes in ferritin levels and cellular LIP recovery after chelation. The changes in the LIP were also inversely reflected in the activity of iron regulatory protein (IRP). The 2 ferritin subunits, H and L, behaved qualitatively similarly in response to long-term treatments with the iron chelator deferoxamine, although L-ferritin declined more rapidly, resulting in a 4-fold higher H/L-ferritin ratio. The decline in L-ferritin, but not H-ferritin, was partially attenuated by the lysosomotrophic agent, chloroquine; on the other hand, antiproteases inhibited the degradation of both subunits to the same extent. These findings indicate that, after acute LIP depletion with fast-acting chelators, iron can be mobilized into the LIP from intracellular sources. The underlying mechanisms can be kinetically analyzed into components associated with fast release from accessible cellular sources and slow release from cytosolic ferritin via proteolysis. Because these iron forms are known to be redox-active, our studies are important for understanding the biological effects of cellular iron chelation.