Endocannabinoid signaling and epidermal differentiation.

Endocannabinoids represent a class of endogenous lipid mediators, that are involved in various biological processes, both centrally and peripherally. The prototype member of this group of compounds, anandamide, regulates cell growth, differentiation and death; this holds true also in the skin, that is the largest organ of the body constantly exposed to physical, chemical, bacterial and fungal challenges. The epidermis is a keratinized multistratified epithelium that functions as a barrier to protect the organism from dehydration, mechanical trauma, and microbial insults, and epidermal differentiation represents one of the best characterized mechanisms of cell specialization. In this review, we shall summarize current knowledge about the main members of the so-called "endocannabinoid system (ECS)", in order to put in a better perspective the manifold roles that they play in skin pathophysiology. In particular, we shall discuss some aspects of the molecular regulation by endocannabinoids of proliferation and terminal differentiation ("cornification") of mammalian epidermis, showing that ECS is finely regulated by, and can interfere with, the differentiation program. In addition, we shall review evidence demonstrating that disruption of this fine regulation might cause different skin diseases, such as acne, seborrhoea, allergic dermatitis, itch, psoriasis and hair follicle regression (catagen), making of ECS an attractive target for therapeutic intervention.