OXIDATIVE STRESS INDUCES ACTIVATION OF A CYTOSOLIC PROTEIN RESPONSIBLE FOR CONTROL OF IRON UPTAKE

A cytosolic protein, named iron-responsive element-binding protein (IR E-BP), is sensitive to cellular iron concentration. At low cytosolic i ron level, IRE-BP is activated and binds to stem-loop untranslated reg ions (IRE regions) of transferrin and ferritin mRNAs, activating and i nhibiting their translations, respectively, This concerted mechanism p ermits a fine control of iron homeostasis in the cell, The activity of IRE-BP can be measured by its binding to IRE regions, using a protein band-shift electrophoretic assay, Damage to cells by oxidative stress is known to be mediated by iron, We observed that IRE-BP is rapidly a ctivated by exposure of V79 Chinese hamster ovary cells to H2O2. Howev er, if cell extracts are exposed to H2O2 IRE-BP activation is not obse rved, Therefore, the activation is not a direct consequence of the H2O 2 attack to IRE-BP, The in vivo IRE-BP-activation by H2O2 is not preve nted by hydroxyl radical scavengers or by the iron chelator 1,10-phena nthroline, indicating that Fenton reaction is not involved in the proc ess, In fact, simultaneous exposure of cells to H2O2 and 1,10-phenanth roline produces an even stronger activation than exposure to H2O2 alon e, The interpretation of the mechanism of IRE-BP activation by oxidati ve stress is hampered by the fact that the mechanism of IRE-BP modulat ion by cytosolic iron has not been established, It has been recently s hown that the iron-sulfur cluster in IRE-BP must be completely disasse mbled in order for activation to occur and that this is triggered by l ow iron in the cell, It is likely that IRE-BP senses Fe(II) and that i ts oxidation to Fe(III) by H2O2 or chelation by 1,10-phenanthroline se t up a program for increasing iron uptake, The physiological consequen ces of this activation still has to be assessed, (C) 1995 Academic Pre ss, Inc.