Age-dependent response of energy metabolism of human skin to UVA exposure: an in vivo study by 31P nuclear magnetic resonance spectroscopy

Purpose The aim of our study was to evaluate the in vivo energy metabolism of human skin as a function of age, in conditions of rest and after a mild stress caused by a suberythemal UVA irradiation. Methods The kinetics of UVA-induced modifications in high-energy phosphorylated metabolites of young and old skins were non-invasively monitored over a period of 24 h using ³¹P nuclear magnetic resonance spectroscopy. In vivo³¹P spectra were obtained on the ventral aspect of the wrist, using a NMR Imaging Spectrometer equipped with a double-tuned surface coil. Concentrations of phosphocreatine, inorganic phospate, adenosine tri-phosphate, phosphomono and phosphodiesters were calculated from the spectra and results were expressed as relative concentrations. A total of 20 subjects were enrolled in this study (n = 10 for the age group below 25 years and n = 10 for the age group above 55 years). A second experiment was then performed on 10 old subjects (mean age 60) who were treated on one wrist, twice a day for one month prior to UVA irradiation, with a product that contained active ingredients to restore barrier function and modulate the inflammatory response, the other wrist being an untreated control. Results Baseline levels of phosphorylated metabolites were similar in young and old skins. A suberythemal dose of UVA (6 J.cm⁻²) led to a significant decrease in the PCr/Pi ratio (index of energy status) and a significant increase in the PME/PDE ratio (index of cellular turnover rate of lipid-related metabolites) within 1 h. The observed variations were transient and the recovery was complete at T + 24 h post-UVA, although recovery was significantly slower in the older group. The disturbances were significantly reduced after treatment of the older skin with a formula that restored barrier function of the stratum corneum and modulated the inflammatory response. Conclusion (i) baseline levels of energy metabolites in skin do not seem to vary with age; (ii) low dose UVA irradiation induces a rapid response in the energy metabolism of the skin; (iii) the kinetics of the response and recovery after an aggression by UVA suggest that older skin has significantly less energy rebound after a stress situation than younger skin; (iv) the energy reserve in older skin can be protected efficiently against UVA-induced stress by restoring barrier function and modulating the inflammatory response.