New Presentation: The Role of glutathione in allergic contact dermatitis

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PhD viva presentation at the University of Liverpool.

Allergic Contact Dermatitis is a skin condition that affects up to twenty percent of the North American and Western European population. The molecular initiating event (MIE) of this type IV delayed hypersensitivity is the formation of antigenic species by covalent modification of endogenous proteins. Cysteine residues, though not the most prevalent residues in skin proteins, are prone to react with small electrophilic molecules and thus play a crucial role in protein haptenation. Glutathione (GSH) is the most prominent antioxidant in cells and contains a cysteine residue, providing a free reactive thiol for GSH conjugation of electrophiles and their subsequent removal. It is also the co-factor of an important set of enzymes involved in the metabolic clearance system, glutathione S-transferases (GST). However, it remains unclear whether the reactivity of small exogenous electrophiles with intracellular GSH could be a key factor in determining the epidermal bioavailability of sensitising chemicals. The GSH detoxification of non-toxic doses of three dinitrohalobenzenes DNCB, DNFB and DNBB as well as two compounds prone to Michael addition (DPCP and DEM) was investigated in the HaCaT cell line. Antioxidant properties of GSH were also studied by treating HaCaT cells with the model aldehydes benzaldehyde and phenylacetaldehyde, which oxidised cysteine residues in chemico but did not suffer a significant depletion of GSH levels at sub-toxic concentrations. The volatility of aldehydes was likely to limit the bioavailability of these compounds intracellularly, possibly increasing the probability of haptenation with cell membrane proteins.

These experiments demonstrated that the defence mechanisms in the HaCaT cell line were providing a rapid response to chemical stress. To investigate the effects of chemical exposure on GSH lifecycle in reconstructed human epidermis (RHE), we attempted repeated cycles of 2 hour exposure to DNCB. For three consecutive treatments, each exposure to DNCB led to GSH depletion. Replenishment to basal level was observed after a 22 hour recovery period. Accumulation of Nrf2 in the cytosol also occurred. The amount of GSH conjugate formed (dinitrophenyl glutathione) increased after each exposure, suggesting that the metabolic capacity of skin may be enhanced in response to exposure to exogenous compounds.

In conclusion, the GSH cycle is fully active in skin and participates in the clearance of some exogenous compounds with electrophilic properties. GSH skin metabolism can also potentially be enhanced after repeated exposures to small quantities of electrophiles. Future risk assessments for skin sensitisation potential of topically applied chemicals could integrate these findings to correlate more realistically to in vivo scenarios.

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