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 ions. A tight regulation of cell iron uptake and storage by iron regulatory proteins (IRP) of keratinocytes and fibroblasts avoids these damaging reactions. We report here that about 40 J/cm2 of UVA are required to inactivate half of the binding capacity of apo-IRP-1 to iron responsive elements (IRE) of RNA whereas 15 J/cm2 already inhibit half of the holo-IRP-1 aconitase activity. No increase in the holo-IRP-1 activity is observed during the apo-IRP-1 photoinactivation suggesting that UVA does not trigger a shift between these two forms. As opposed to holo-IRP-1, which contains a 4Fe-4S cluster, apo-IRP-1 has no UVA chromophore. Thus it should be inactivated indirectly by reactive oxygen species generated by the UVA-induced endogenous photo-oxidative stress. The apo-IRP-1 photoinactivation is weakly prevented by the lipophilic oxyradical scavenger vitamin E but not by the hydrophilic azide anion, a singlet oxygen quencher or by diethyldithiocarbamate, a superoxide dismutase inhibitor. However, full protection against photoinactivation 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 photoinactivation of the apo and holo forms, the light dose-independent effect of the sulfhydril group reagent, 2-mercaptoethanol and the partial protection brought by the ferric ion complexing agent desferrioxamine suggest that the photochemistry of the 4Fe-4S cluster of the holo form plays little, if any, role in the photoinactivation of the apo-IRP-1/IRE interaction. It is concluded that the apo/holo equilibrium is irreversibly destroyed by UVA irradiation.
Giordani
A
A
Muséum National d'Histoire Naturelle, Laboratoire de Photobiologie, Paris, France.
Martin
M E
ME
Beaumont
C
C
Santus
R
R
Morlière
P
P
D000154:Aconitate Hydratase / Q000378:metabolism*; D002478:Cells, Cultured; D005347:Fibroblasts / Q000201:enzymology / Q000378:metabolism / Q000528:radiation effects; D006801:Humans; D035925:Iron-Regulatory Proteins; D007506:Iron-Sulfur Proteins / Q000378:metabolism*; D015603:Keratinocytes / Q000201:enzymology / Q000378:metabolism / Q000528:radiation effects; D016601:RNA-Binding Proteins / Q000378:metabolism*; D012867:Skin / Q000166:cytology / Q000201:enzymology / Q000378:metabolism / Q000528:radiation effects*; D014466:Ultraviolet Rays
D035925:Iron-Regulatory Proteins; D007506:Iron-Sulfur Proteins; D016601:RNA-Binding Proteins; D000154:Aconitate Hydratase