ROLE OF FERRITIN IN THE CONTROL OF THE LABILE IRON POOL IN MURINE ERYTHROLEUKEMIA-CELLS

In vitro studies have shown that ferritin iron incorporation is mediat ed by a ferroxidase activity associated with ferritin H subunits (H-Ft ) and a nucleation center-associated with ferritin L subunits (L-Ft). To assess the role played by the ferritin subunits in regulating intra cellular iron distribution, we transfected mouse erythroleukemia cells with the H-Ft subunit gene mutated in the iron-responsive element, St able transfectants displayed high H-Ft levels and reduced endogenous L -Ft levels, resulting in a marked change in the H:L subunit ratio from 1:1 in control cells to as high as 20:1 in some transfected clones. T he effects of H-Ft overexpression on the labile iron pool were determi ned in intact cells by a novel method based on the fluorescent metallo sensor calcein, H-Ft overexpression resulted in a significant reductio n in the iron pool, from 1.3 mu m in control cells to 0.56 mu m in H-F t transfectants, and in higher buffering capacity following iron loads . A fraction of the H-Ft-associated iron was labile, available to cell -permeant, but not cell-impermeant, chelators. The results of this stu dy provide the first in vivo direct demonstration of the capacity of H -Ft to sequester cell iron and to regulate the levels of the labile ir on pool.