Confocal Raman spectroscopy as a tool to assess advanced glycation end products on solar-exposed human skin

Abstract

Advanced glycation end products (AGEs) consist of a complex and heterogeneous class of compounds frequently associated with the skin aging process. The formation of AGEs and its accumulation in the body have been related as the pathophysiological key to important age-related diseases, such as diabetes mellitus and Alzheimer's disease. In addition, AGEs are also related to skin aging as result of ultraviolet (UV) light exposure in both senescent and photo aged skin. Thus, establishing novel approaches for spatially assessing AGEs appears as a potentially useful strategy toward the investigation of the mechanistic role of these compounds in aging-related processes. Herein, we propose a direct and non-destructive approach to assess the biological changes on the human skin highly exposed to solar irradiation. Confocal Raman spectroscopy (CRS) was used to evaluate the presence and the differences in the AGEs content, by using reference spectra of the major AGEs present in human skin. Two different groups were considered from volunteers who had a daily history of high sun exposure: samples collected from the inner face of the arm (sun-protected or low direct sun exposure, SP) and samples from the forearm dorsum (sun-exposed or high direct sun exposure, SE). In this study, the main AGEs products were found to be carboxymethyl-lysine (CML), glucosepane (GLU) and pentosidine (PEN) for both groups; while CML were predominant at the stratum corneum and epidermis, GLU and PEN were found to be the main AGEs in dermis. Multivariate statistical analysis revealed a high discrimination rate in the stratum corneum region at around 93.8 %, while in the layers of the epidermis and dermis this value was found to be 68.8 %, according to the degree of discrimination of SP and SE groups. In sum, this investigation shows that CRS is able to assess AGEs in vivo and non-invasively on human skin and provided a better understanding on the effects and changes resulting from skin chronological aging caused by glycation under conditions of high exposure to UV irradiation.