The effect of N-acetylcysteine on the UVB-induced inhibition of epidermal DNA synthesis in rat skin

Photoprotection by antioxidants against UVB-radiation-induced damage in pig skin organ culture

Topically applied antioxidants constitute an important group of protective agents against skin damage induced by ultraviolet radiation. The current study was performed to investigate whether a recently developed ex vivo pig skin model was suitable for short-term studies of the mechanism(s) of UVB-radiation-induced skin damage; the protective effect of topical application of alpha-tocopherol, l-ascorbic acid, alpha-lipoic acid, glutathione ethylester and N-acetylcysteine was tested. Increasing doses of the antioxidants were applied topically on ex vivo pig skin explants and allowed to penetrate for 60 min. Epidermal antioxidant bioavailability was measured before and 60 min after exposure to an ultraviolet B (UVB) radiation of 7.5 kJ/m2. Cell viability (trypan blue dye exclusion) and apoptosis were measured 48 h later in isolated keratinocytes. UVB-radiation-induced epidermal lipid peroxidation was determined immediately after exposure of the skin to a UVB dose of 28 kJ/m2. All antioxidants tested became bioavailable in pig skin epidermis, and none of them were depleted after UVB-radiation exposure. Increasing doses of the antioxidants tested decreased UVB-radiation-induced cell death and apoptosis. The highest doses of antioxidants prevented UVB-radiation-induced lipid peroxidation; alpha-lipoic acid only tended to decrease lipid peroxidation. In conclusion, a single topical dose of the above antioxidants on ex vivo pig skin can reduce UVB-radiation-induced oxidative stress and lipid peroxidation and thereby reduce apoptotic stimuli and cell death. Furthermore, the ex vivo pig skin model was a useful tool for testing compounds for their antioxidant activity.

The mechanism of N-acetylcysteine photoprotection is not related to dipyrimidine photoproducts

Topical application of N-acetylcysteine prior to UVB irradiation of BALB/c mice has previously been shown to inhibit systemic suppression of the contact hypersensitivity response. Formation of cis-urocanic acid, however, is not affected. Besides urocanic acid, UV-induced DNA damage has been held responsible for the initiation of suppression of the contact hypersensitivity response. Therefore, the possible inhibitory effect of N-acetylcysteine on UVB-induced cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts has been investigated. No effect on the photoproducts studied is observed, suggesting that N-acetylcysteine exerts its photoprotective effect during the post-initiation phase of photoimmunosuppression.

Suppression of UVB-induced c-fos and c-jun expression in human keratinocytes by N-acetylcysteine

Irradiation of human keratinocytes with UVB results in the early induction of proto-oncogenes c-fos and c-jun, members of the AP-1 protein family of transcription factors. To explore a possible involvement of oxidant stress in triggering this UVB-induced early gene response, we investigated in human keratinocytes the effect of N-acetylcysteine (NAC), a thiocompound with antioxidant activities, on UVB-induced c-fos, and c-jun expression. Normal human keratinocytes were irradiated with either 16 or 32 mJ cm-2 UVB, which induces a temporary inhibition of DNA synthesis, without compromising cell survival. Preincubation with 1 and 3 mM NAC suppressed c-jun and c-fos induction by UVB in a dose-dependent fashion. These applied concentrations of NAC were not toxic to the keratinocytes, as determined by Trypan Blue exclusion assay and completely suppressed c-jun and c-fos induction by the chemical cadmium chloride (oxidative stress). These results indicate that oxidative stress, at least in part, mediates the transcriptional activation of c-fos and c-jun in human keratinocytes after UVB irradiation.

The protective effect of N-acetylcysteine on UVB-induced immunosuppression by inhibition of the action of cis-urocanic acid

A recent study has shown that N-acetylcysteine (NAC) not only has sun-protective properties but also inhibits the UVB-induced suppression of contact hypersensitivity (CHS) in mice. Because NAC does not absorb any UVA (320-400 nm radiation) or UVB (290-320 nm radiation) we have studied the underlying mechanism of protection. Irradiation of solutions of plasmid DNA with UVC (200-290 nm radiation) (10 J m-2) resulted in the formation of cyclobutane pyrimidine dimers, but the extent to which this occurred was not affected by the presence of NAC as was determined by an in vitro T4 endonuclease assay. N-acetylcysteine proved not to have any effect on the photoisomerization of trans-urocanic acid (UCA) to its cis-form in vitro; at equilibrium, approximately 55% cis-UCA was formed. The same percentage was also found in vivo on exposure of mice to UVB (15 kJ m-2). Topical application of NAC 30 min prior to irradiation did not have any influence as well on the photoisomerization of trans- to cis-UCA. These in vivo experiments were performed under the same conditions used previously to show the protective effect of NAC against UVB-induced suppression of CHS. We conclude that this protection of NAC is at least partly based on interference in the role of cis-UCA in UVB-induced suppression of CHS. This conclusion is supported by the observation that NAC completely inhibits the suppression of CHS by cis-UCA administered to mice that were always kept in the dark. In the same range of doses as used in the present study, it was shown in our previous study that NAC alone does not affect the CHS response.

UV radiation protecting efficacy of cysteine derivatives, studies with UVA-induced binding of 8-MOP and CPZ to rat epidermal biomacromolecules in vivo

With the aim of optimizing the UV radiation protecting efficacy of N-acetylcysteine (NAC), the following topically applied cysteine derivatives were investigated: N-acetylcysteine ethylester (NACET), S-acetylcysteine ethylester (SACET), cysteine ethylester (CYSET), N,S-diacetylcysteinamide (SNACA), N,S-diacetylcysteine (SNAC) and N,S-diacetylcysteine ethylester (SNACET). As a measure for protection the inhibition of in vivo irreversible photobinding of the labelled phototoxic drugs chlorpromazine (CPZ) and 8-methoxypsoralen (8-MOP) to rat epidermal biomacromolecules was used. The duration of protection of the cysteine derivatives was shortened by S-acetylation, N-acetylation and carboxyl derivatization. Compounds with a free thiol group showed a long-lasting presence in the stratum corneum, probably by the formation of mixed disulphides with proteins. The intrinsic protecting efficacy with respect to the total epidermis increased in the order CYSET < SNACET,SNACA,SACET < NACET, SNAC,NAC. The results of this study are discussed in view of susceptibility to oxidation, epidermal bioavailability and metabolic activation. With respect to the viable epidermis we postulate that NACET and SNAC have the most promising properties as UV protective agents.