Human skin is constantly exposed to environmental stresses such as UV light and pollution. These agents cause
oxidative stress associated with reactive oxygen species (ROS) generation, that will interfere with the normal cellular
redox equilibrium. As ROS are mainly produced within mitochondria, the cellular metabolic activity could be impacted
by UV light.
We dynamically assessed UVA light (representing the majority of solar UV rays reaching Earth surface) effects on
cellular metabolic activity of reconstructed human skin using multiphoton fluorescence lifetime imaging microscopy
Multiphoton FLIM offers non-invasive, label-free quantitative functional information on cellular metabolic activity
based on the endogenous two-photon excited fluorescence (2PEF) of NADH (reduced form of nicotinamide adenine
dinucleotide) and FAD (flavine adenine dinucleotide) metabolic coenzymes.
The experiments were performed in both stratum granulosum and spinosum layers (T-Skin™ model, Episkin™), before
and after (30 min and 2 h) UVA exposure (20 J/cm²; 20 min exposure; 320 – 400 nm).
We observed quasi similar effects in both epidermal layers after UVA exposure:
• Decrease of RedOx ratio NADH / (NADH + FAD) at 30 min and 2 h;
• Increase in the proportion of protein-bound NADH at 2 h, and in the proportion of free FAD as early as 30 min
after UVA exposure;
This study shows that the effects of UVA light on epidermis, can be non-invasively evidenced and followed overtime
using NADH/FAD multiphoton FLIM imaging method. Altogether, these data suggest that epidermal cells respond to
UVA light by promoting oxidative phosphorylation, the most efficient metabolic pathway for ATP production.