1999 ANNUAL SCIENTIFIC MEETING 67 crystalline structure. Under conditions of dry relative humidity, a phase change in the lipid to solid crystals may occur. This creates "holes" or defects in the bilayer that can permit moisture to penetrate through the stratum corneum. Application of formulations containing lipid fluidizing agents can slow down the phase change of the lipid to solid crystals under conditions of dry relative humidity. This will maintain the epidermal lipid in the more fluid, liquid crystalline form. Such lipid fluidizers have been identified in the literature and by experimentation with model stratum corneum lipids in vitro (Froebe et al, J. Soc. Cosmet. Chem. 41, 51-65, 1990 Mattai et al J. Soc. Cosmet. Chem. 44, 89- 100, 1993). Using differential scanning calorimetry to track the heat of fusion of the phase changes the above authors determined what materials will reduce the heat of fusion. Lipid fluidizers identified by these in vitro techniques are glycerine and maleated soybean oil (glyceride acid). Glycerine has been reported to be a well known moisturizer / skin protectant for centuries. Perhaps more than its humectant properties, its mechanism of action is via fluidization of stratum corneum lipid. Metabolic Strategies to Repair Aging Skin Barrier Also a more contemporary strategy being explored to repair and enhance barrier is to control the activity of the barrier lipid synthesizing enzymes with mediators of their activity (see review by Harris et al J. Invest. Dermat. 109, 783-7, 1997). The extracellular lipids of the stratum corneum, which are comprised mainly of cholesterol, fatty acids, and ceramides, are essential for epidermal permeability barrier function. Moreover, disruption of the permeability barrier results in increased cholesterol, fatty acid, and ceramide synthesis in the underlying epidermis. This increase in lipid synthesis has been shown to be due to increased activities of HMG-CoA reductase, acetyl-CoA carboxylase, along with fatty acid synthase, and serine palmitoyl transferase, key enzymes of cholesterol, fatty acid, and ceramide synthesis, respectively. It has been determined that the mRNA levels for the key enzymes required for synthesis of these three classes of lipids increase coordinately during barrier recovery. By northern blotting techniques, the steady-state mRNA levels for HMG-CoA reductase, HMG-CoA synthase, farnesyl pyrophosphate synthase, and squalene synthase, key enzymes for cholesterol synthesis, all increased significantly after barrier disruption by either acetone or tape stripping. Additionally, the steady-state mRNA levels of acetyl-CoA carboxylase and fatty acid synthase, required for fatty acid synthesis, as well as serine palmitoyl transferase, the rate-limiting enzyme of de novo ceramide synthesis, also increased. Inhibitors of the activity of these enzymes can also lead to delayed barrier repair. For example application of lovastatin, beta chloroalanine, or TOFA to in vivo to hairless mouse skin with acetone damaged or tape stripped barrier, lead to delayed recovery of TEWL (water loss). These substances inhibit enzymes mentioned above that are involved in cholesterol, sphingolipid, or fatty acid synthesis respectively. It is, therefore, very reasonable that activators of these enzymes should potentiate barrier repair.

68 JOURNAL OF COSMETIC SCIENCE Strategies using Regulation of Gene Expression to Repair Aging Skin Barrier From the above findings it is postulated that regulation of the expression of genes controlling all of the underlying barrier repair entities such as the enzymes discussed above is a more plausible approach to repairing damaged barrier in the future. Nuclear hormone receptors are transcription factors that regulate many cellular functions including cell differentiaton and proliferation. Nuclear hormone receptors which heterodimerize with RXR, such as retinoic acid receptor (RAR) and the vitamin D receptor have been shown to regulate keratinocyte proliferation and differentiation. In transgenic mice that overexpress either an RAR or RXR dominant negative mutant, epidermal differentiation is abnormal. Retinoic acid is the first FDA aproved drug that treats some of the problems of aging skin, especially wrinkles and fine lines, mottlied hyperpigmentation, and tactile roughness of facial skin (PDR 1999). It is also indicated for years for the treatment of acne. It is a nuclear hormone receptor agonist and upregulates transcription of specific regions of the DNA and ultimately translation of RNA to express specific proteins that induce proliferation of keratinocytes at least in in vitro studies. This shift in balance appears to lead to resolution of certain abnormalities of skin and to the development and perscription marketing of Renova© for photodamage. Other nuclear hormone activators like Vitamin D3 (calcipotriene) activate the Vitamin D receptor and have a slightly different effect on skin. This activator also heterodimerizes with RXR but tends to shift the cellular phenotype balance to enhanced differentiation. This is substantiated by the expression of differentiation markers, e.g. filaggrin, involucrin, and others and a decrease in keratin K67, a marker for proliferation (Gottlieb, A. J. Cutan. Path. 23,419-430, 1996). This drug appears in the perscription drug- Dovonex and is an effective treatment for psoriasis, a hyperproliferative skin disease. It seems plausible that such a drug may have applications to treatment of aging skin problems involving abnormal barrier like dry, rough, wrinkled, hyperpigmented skin. The underlying question is whether shifting the balance in these lesions to a differentiated cellular phenotype will ameliorate the condition. The newest class of nuclear hormone receptors are peroxisome proliferator activated receptors (PPARs) and the alpha form also heterodimerizes with RXR. PPARc• is activated by many chemicals including the lipid lowering drug clofibrate and by a variety of fatty acids (Feingold, K., J. Cosmet. Sci. 186-189, 1999). It has been shown that PPARc• activators stimulate epidermal barrier development in fetal skin explants and in utero and increased differentiation in keratinocytes. In induced hyperproliferative conditions in animal models, such as by tape stripping or essential fatty acid deficiency, activators of PPAR successfully ameliorated the condition and switched the skin to a differentiated phenotype thereby correcting the cutaneous pathology. It remains to be seen as to the future value of these nuclear hormone receptor agonists in treating the diseases of the aging skin.