Inactivation of iron responsive element-binding capacity and aconitase function of iron regulatory protein-1 of skin cells by ultraviolet A

The ultraviolet-A (UVA) component of sunlight produces in cutaneous cells a highly toxic oxidative stress mediated by redox cycling reactions of Fe io ns, A tight regulation of cell iron uptake and storage by iron regulatory p roteins (IRP) of keratinocytes and fibroblasts avoids these damaging reacti ons, We report here that about 40 J/cm(2) of WA are required to inactivate half of the binding capacity of apo-IRP-1 to iron responsive elements (IRE) of RNA whereas 15 J/cm(2) already inhibit half of the holo-IRP-1 aconitase activity, No increase in the holo-IRP-1 activity is observed during the ap o-ERP-l photoinactivation suggesting that UVA does not trigger a shift betw een these two forms, As opposed to holo-ERP-l, which contains a 4Fe-4S clus ter, apo-IRP-1 has no UVA chromophore, Thus it should be inactivated indire ctly by reactive oxygen species generated by the UVA-induced endogenous pho to-oxidative stress. The apo-IRP-1 photo-inactivation is weakly prevented b y the lipophilic oxyradical scavenger vitamin E but not by the hydrophilic azide anion, a singlet oxygen quencher or by diethyldithiocarbamate, a supe roxide dismutase inhibitor. However, full protection against photoinactivat ion of the apo form is observed after incubation with N-acetylcysteine but the latter only partially protects the aconitase function of the holo-IRP-1 from photoinactivation, The marked difference in the kinetics of photoinac tivation of the apo and hole forms, the light dose-independent effect of th e sulfhydril group reagent, 2-mercaptoethanol and the partial protection br ought by the ferric ion complexing agent desferrioxamine suggest that the p hotochemistry of the 4Fe-4S cluster of the hole form plays little, if any, role in the photoinactivation of the apo-IRP-1/IRE interaction, It is concl uded that the apo/holo equilibrium is irreversibly destroyed by UVA irradia tion.