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.

Contrasting effects of excess ferritin expression on the iron-mediated oxidative stress induced by tert-butyl hydroperoxide or ultraviolet-A in human fibroblasts and keratinocytes

Iron and/or ferritin accumulation are known to occur under pathological conditions in many inflammatory skin diseases or in human skin chronically exposed to UV light. Under such conditions, ferritin is believed to play an effective protective role in accommodating and 'deactivating' excess 'free' iron produced by the inflammatory process or the UV illumination. The present study compares the relationship between ferritin over-expression and effects of an oxidative stress induced chemically by tert-butyl hydroperoxide or photochemically by UV-A radiation. As shown by immunoassay, cultured MRC 5 and HS 68 fibroblasts treated for at least one day with transferrin or overnight with non-toxic concentrations of the ferric nitrilotriacetate complex express up to 10 times more ferritin than untreated cells, whereas a five-fold increase is obtained with NCTC 2544 keratinocytes. In all cases a parallel increase in soluble cellular iron is measured by inductive plasma emission spectroscopy. The superoxide dismutase and catalase activities and total glutathione levels are not modified by the iron treatment, whereas a transient increase in the Se-dependent glutathione peroxidase activity of keratinocytes is observed after a short incubation with the iron complex. In keratinocytes and fibroblasts, ferritin over-expression after iron treatment markedly inhibits lipid peroxidation but, paradoxically, not the mortality induced by tert-butyl hydroperoxide. In contrast, this excess ferritin does not protect cells from both the peroxidation and mortality induced by moderate doses (30 J/cm2) of UV-A radiation. As a consequence, protection against oxidative damage by excess ferritin synthesis clearly depends on the nature of the oxidative stress on cell targets and it seems to be of lesser importance in the case of photochemically induced oxidation.

Ultraviolet A radiation induces immediate release of iron in human primary skin fibroblasts: the role of ferritin

In mammalian cells, the level of the iron-storage protein ferritin (Ft) is tightly controlled by the iron-regulatory protein-1 (IRP-1) at the posttranscriptional level. This regulation prevents iron acting as a catalyst in reactions between reactive oxygen species and biomolecules. The ultraviolet A (UVA) radiation component of sunlight (320-400 nm) has been shown to be a source of oxidative stress to skin via generation of reactive oxygen species. We report here that the exposure of human primary skin fibroblasts, FEK4, to UVA radiation causes an immediate release of "free" iron in the cells via proteolysis of Ft. Within minutes of exposure to a range of doses of UVA at natural exposure levels, the binding activity of IRP-1, as well as Ft levels, decreases in a dose-dependent manner. This decrease coincides with a significant leakage of the lysosomal components into the cytosol. Stabilization of Ft molecules occurs only when cells are pretreated with lysosomal protease inhibitors after UVA treatment. We propose that the oxidative damage to lysosomes that leads to Ft degradation and the consequent rapid release of potentially harmful "free" iron to the cytosol might be a major factor in UVA-induced damage to the skin.