Squalene and skin barrier function: From molecular target to biomarker of environmental exposure

Abstract

The human skin naturally faces an aerial oxidative environment. The environment presents however a variable oxidative potential since enhanced by solar rays (UV, Visible) possibly combined to aerial-borne pollutants that most often act as catalysts in the different oxidative pathways. The poly-unsaturated human sebum highly present on the upper parts of the body (face, torso) is therefore a natural “receptor” of these oxidative actions. Comprised at 10-20 % within sebum, Squalene (C30H50) is not only specific to human sebum but its 6 double bonds make it a highly sensitive molecule towards various forms of Reactive Oxygen Species, singlet oxygen included, leading to different per-oxidized by-products. The latter thus appear as natural bio-markers of most oxidative actions upon the cutaneous tissue. Some mechanisms can easily be modelled in vitro, thereby demonstrating the influences of UVA rays, cigarette smoke, particulate matters or some porphyrins that are synthesized by the resident skin flora. These models allow the structures of various forms of squalene peroxides to being determined and to quantify the quenching properties of some known anti-oxidants (Carotenoids, Vitamin E). These chains of events were logically traced in vivo, by comparing the contents of Squalene and Vitamin E in the sebum of subjects living in differently polluted but close geographical locations. The oxidized state of Squalene then represents a reliable biomarker of most oxidative events induced by various environmental factors. Their possible biological impacts upon the skin physiology, which greatly remain to being documented, are discussed.