[The epidermis as metabolically active tissue: regulation of lipid synthesis by the barrier function]

Numerous investigations have shown that the lipids of the horny layer play an important role in the epidermal barrier function. These lipids consist of sphingolipids, cholesterol, and free fatty acids in nearly equimolar proportions. If the barrier function is disturbed--i.e. in case of lipid extraction or a diet deficient of essential fatty acids--we find an increased synthesis of free fatty acids, cholesterol, and non-saponifiable lipides in the epidermis. Covering the skin with a Latex wrap prevents an increased lipid synthesis. The synthesis of cholesterol depending on the barrier function is regulated by the enzyme HMG CoA reductase. The regulation process involves both the quantity and the activity (phosphorylation) of the enzyme. Acute disruption of the permeability barrier results in an increased synthesis of cholesterol in the lower epidermis, whereas in case of chronic barrier disorders, the specific increase takes place in the upper dermis. A reduction of the cholesterol synthesis by the HMG CoA reductase inhibitor Lovastatin leads to a disturbed permeability barrier and epidermal hyperplasia.