Lipophilic prodrugs of amino acids and vitamin E as osmolytes for the compensation of hyperosmotic stress in human keratinocytes.

Skin keratinocytes are subjected to changing osmotic conditions and evolved counteracting mechanisms. Particularly, the expression of osmolyte transporters serves for the maintenance of cell volume in a hypertonic environment. In this study, we show that hyperosmotic stress significantly decreases the proliferation in HaCaT keratinocytes. Supplementation of the culture medium with the amino acids glycine, sarcosine, betaine, taurine and proline restored the proliferation indicating osmoprotective properties of these substances. Amino acids are highly polar molecules and therefore unable to penetrate into deeper epidermal layers after topical application. Thus, we utilized a prodrug concept in which the tested amino acids are coupled to a lipophilic moiety. Ethyl glycinate as a first model compound also showed an osmoprotective effect. In addition, improved penetration of the glycine derivative into deeper epidermal layers could be demonstrated. The prodrug concept was further developed by using the lipid soluble antioxidant alpha-tocopherol as a lipophilic moiety. The derivatives d,l-alpha-tocopheryl-(mono-) glycinate (TMG) and d,l-alpha-tocopheryl-(mono-) prolinate caused an increase in proliferation of HaCaT keratinocytes under salt stress and a decrease in apoptosis induced by hypertonic conditions. Furthermore, the osmoprotective effect of d,l-TMG could be corroborated in normal human keratinocytes. Therefore, it seems feasible that amino acids and their lipophilic derivatives may help to improve the osmotic balance and the hydration of skin. Clinical and cosmetic indications such as atopic eczema, UV exposed skin or aged skin may benefit from this new concept.