A powerful mitochondria-targeted iron chelator affords high photoprotection against solar ultraviolet A radiation

Mitochondria are the principal destination for labile iron, making these organelles particularly susceptible to oxidative damage on exposure to ultraviolet A (UVA, 320–400 nm), the oxidizing component of sunlight. The labile iron-mediated oxidative damage caused by UVA to mitochondria leads to necrotic cell death via adenosine triphosphate depletion. Therefore, targeted removal of mitochondrial labile iron via highly specific tools from these organelles may be an effective approach to protect the skin cells against the harmful effects of UVA. In this work, we designed a mitochondria-targeted hexadentate (tricatechol-based) iron chelator linked to mitochondria-homing SS-like peptides. The photoprotective potential of this compound against UVA-induced oxidative damage and cell death was evaluated in cultured primary skin fibroblasts. Our results show that this compound provides unprecedented protection against UVA-induced mitochondrial damage, adenosine triphosphate depletion, and the ensuing necrotic cell death in skin fibroblasts, and this effect is fully related to its potent iron-chelating property in the organelle. This mitochondria-targeted iron chelator has therefore promising potential for skin photoprotection against the deleterious effects of the UVA component of sunlight

Dual selective iron chelating probes for the monitoring of mitochondrial labile iron pools

Mitochondria-targeted peptides incorporating dual fluorescent and selective iron chelators have been designed as novel biosensors for the mitochondrial labile iron pool.</p

Thiothymidine plus low-dose UVA kills hyperproliferativehuman skin cells independently of their human papilloma virus status

The thymidine analogue 4-thiothymidine (S4TdR) is a photosensitizer for UVA radiation. The UV absorbance spectrum of S4TdR and its incorporation into DNA suggests that it might act synergistically with nonlethal doses of UVA to selectively kill hyperproliferative or cancerous skin cells. We show here that nontoxic concentrations of S4TdR combine with nonlethal doses of UVA to kill proliferating cultured skin cells. Established cell lines with a high fraction of proliferating cells were more sensitive than primary keratinocytes or fibroblasts to apoptosis induction by S4TdR/UVA. Although S4TdR plus UVA treatment induces stabilization of p53, cell death, as measured by apoptosis or clonal survival, occurs to a similar extent in both p53 wild-type and p53-null backgrounds. Furthermore, different types of human papilloma virus E6 proteins, which protect against UVB-induced apoptosis, have little effect on killing by S4TdR/UVA. S4TdR/UVA offers a possible therapeutic intervention strategy that seems to be applicable to human papilloma virus–associated skin lesions

Design of novel fluorescent mitochondria-targeted peptides with iron-selective sensing activity

Mitochondrial labile iron (LI) plays a crucial role in oxidative injuries and pathologies. At present, there is no organelle-specific sensitive iron sensor which can reside exclusively in the mitochondria and reliably monitor levels of LI in this organelle. In the present study, we describe the development of novel fluorescent and highly specific mitochondria iron sensors, using the family of mitochondria-homing ‘SS-peptides’ (short cell-permeant signal peptides mimicking mitochondrial import sequence) as carriers of highly specific iron chelators for sensitive evaluation of the mitochondrial LI. Microscopic analysis of subcellular localization of a small library of fluorescently labelled SS-like peptides identified dansyl (DNS) as the lead fluorophore for the subsequent synthesis of chimaeric iron chelator-peptides of either catechol (compounds 10 and 11) or hydroxypyridinone (compounds 13 and 14) type. The iron-sensing ability of these chimaeric compounds was confirmed by fluorescent quenching and dequenching studies both in solution and in cells, with compound 13 exhibiting the highest sensitivity towards iron modulation. The intramolecular fluorophore–chelator distance and the iron affinity both influence probe sensitivity towards iron. These probes represent the first example of highly sensitive mitochondria-directed fluorescent iron chelators with potential to monitor mitochondrial LI levels.</jats:p

Identification of potentially cytotoxic lesions induced by UVA photoactivation of DNA 4-thiothymidine in human cells

Photochemotherapy—in which a photosensitizing drug is combined with ultraviolet or visible radiation—has proven therapeutic effectiveness. Existing approaches have drawbacks, however, and there is a clinical need to develop alternatives offering improved target cell selectivity. DNA substitution by 4-thiothymidine (S4TdR) sensitizes cells to killing by ultraviolet A (UVA) radiation. Here, we demonstrate that UVA photoactivation of DNA S4TdR does not generate reactive oxygen or cause direct DNA breakage and is only minimally mutagenic. In an organotypic human skin model, UVA penetration is sufficiently robust to kill S4TdR-photosensitized epidermal cells. We have investigated the DNA lesions responsible for toxicity. Although thymidine is the predominant UVA photoproduct of S4TdR in dilute solution, more complex lesions are formed when S4TdR-containing oligonucleotides are irradiated. One of these, a thietane/S5-(6-4)T:T, is structurally related to the (6-4) pyrimidine:pyrimidone [(6-4) Py:Py] photoproducts induced by UVB/C radiation. These lesions are detectable in DNA from S4TdR/UVA-treated cells and are excised from DNA more efficiently by keratinocytes than by leukaemia cells. UVA irradiation also induces DNA interstrand crosslinking of S4TdR-containing duplex oligonucleotides. Cells defective in repairing (6-4) Py:Py DNA adducts or processing DNA crosslinks are extremely sensitive to S4TdR/UVA indicating that these lesions contribute significantly to S4TdR/UVA cytotoxicity