Skin aging is a multifactorial process in which formation of advanced glycation end products (AGE) are major factors influencing extrinsic and intrinsic aging. AGE are formed by a non-enzymatic reaction between reducing sugar and amino acid residues in proteins such as collagen. This non-enzymatic Maillard reaction leads to the formation of autofluorescent protein crosslinks in the extracellular matrix of the dermis. The early stage of AGE formation is associated with cellular oxidative stress, inflammatory reactions and denaturation of essential metabolic enzymes. AGE accumulation in the skin could be in the long-term responsible for a reduction of its elasticity, an increase of deep wrinkles and the formation of dark age spots below the epidermis.
The goal of this project was to identify potent glycation inhibitors using a combination of fluorescence based in vitro assays and non-invasive in vivo methods in human skin. The characteristic auto-fluorescence signal of AGE was measured in a biochemical assay to analyse the anti-glycation properties in vitro. In this assay an extract of Citrus paradisi inhibited the formation of AGE in a dose dependent manner in comparison to a glycation inducer. The efficacy of the Citrus paradisi extract is further evaluated by two different in vivo methods. Dermal AGE accumulation increases the auto-fluorescence signal of skin. Therefore fluorescence measurements of collagen and elastin-crosslinks with a fiber optic probe at excitation/emission wavelengths of 370/440nm will be used to detect directly changes of AGE content in the skin. Additionally, skin thickness and the density of the dermis containing echogenic collagen will be analysed by high frequency ultrasound scanner (DermaScan C).
In conclusion, Citrus paradisi extract showed promising anti-glycation activity in vitro, whereas demonstration of its in vivo performance is still under investigation. Rational screening and a scientific approach of testing potent AGE inhibitors requires a variety of well-established in vitro and in vivo methods, thereby not only focusing on the inhibition of the Maillard reaction but also on restoring the protective capacity of the skin.