**3. Chemical factors that influence barrier function**

#### **3.1 Ions**

Lipid metabolism is regulated by a series of enzymes in the epidermis (Feingold & Elias 2000) and each of them has optimal conditions of pH (Mauro 1998), concentrations of other ions (Denda 1999), etc. for activity. For example, the pH value of the healthy stratum corneum is kept acidic because the lipid-processing enzymes have an acidic optimal pH. Mauro et al. demonstrated that topical application of a basic buffer after barrier disruption delayed the repair process because the basic condition perturbed lipid processing (Mauro 1998).

It has also been shown that topical application of calcium or potassium reduced barrier repair [Lee 1992], while magnesium and a mixture of calcium and magnesium salts accelerated the repair process [Denda 1999]. Topical application of an aqueous solution containing 10 mM magnesium chloride, magnesium sulfate, and magnesium lactate accelerated barrier repair. Application of magnesium bis(dihydrogen phosphate) or magnesium chloride in PBS solution did not affect the barrier recovery rate. Application of 10 mM calcium chloride aqueous solution delayed barrier repair, but a mixture of calcium chloride and magnesium chloride accelerated it when the calcium-to-magnesium molar ratio was lower than 1. Application of the mixture also improved the condition of dry, scaly skin induced by SDS treatment. These results suggest that ions are important in barrier homeostasis.

#### **3.2 Hexose**

Hexose is known to influence the stability of phospholipid bilayers. Therefore, the effects of topical application of all 12 stereoisomers of dextro-hexose on the epidermal barrier recovery rate after barrier disruption were evaluated (Denda 2011). Immediately after tape stripping, a 0.1 M aqueous solution of each hexose was applied on hairless mouse skin. Among the 8 dextro-aldohexoses, topical application of altose, idose, mannose and talose

Physical and Chemical Factors that Improve Epidermal Permeability Barrier Homeostasis 205

with aging was improved by topical application of cholesterol (Ghadially 1996) or mevalonic acid (Haratake 2000), presumably because the delay of the aged skin was caused

Sex hormones are strongly associated with epidermal permeability barrier homeostasis (Hanley 1996). Moreover, when the balance of these hormones alters at menopause or during the menstrual cycle, skin sensitivity or barrier function is changed. These results suggest that the relative composition of hormones influences barrier function and skin sensitivity. We recently studied the effects of topical application of sex hormones on the permeability barrier recovery rate of hairless mice after tape stripping (Tsutusmi and Denda 2007). Application of androgens, testosterone and androsterone, delayed barrier recovery. The delay was blocked by application of beta-estradiol. Application of progesterone also delayed barrier recovery. However, the delay was enhanced by the application of beta-estradiol. These results suggest that the alteration of the sex hormone balance at menopause or during the estradiol cycle might be the cause of skin problems at

Epidermal keratinocytes express a series of receptors, which were originally found in the central nervous system as neurotransmitter receptors. These receptors can be categorized

Among the former group, receptors that act as calcium ion or chloride ion permeable channels plays crucial roles in epidermal permeability barrier homeostasis. Topical application of calcium channel agonists delays barrier recovery, while antagonists accelerate barrier repair (Denda et al. 2002a) (Denda 2003) (Fuziwara 2003). Topical application of chloride ion channel agonists accelerates barrier recovery (Denda 2003)

The G-protein coupled receptors influence intracellular cAMP level, which plays a crucial role in epidermal barrier homeostasis (Denda 2003b). Increase of intracellular cAMP in epidermal keratinocytes by topical application of forskolin delays barrier recovery, while cAMP antagonist treatment accelerates barrier recovery. Activation of dopamine 2-like receptors (Fuziwara 2005), melatonin receptors, and serotonin receptor (type 5-HT1) decreases intracellular cAMP and consequently accelerates barrier recovery, while activation of adrenergic 2 receptors increases the intracellular cAMP and delays barrier repair (Denda 2003b). Barrier disruption induces an increase of intracellular cAMP. Thus, topical application of agonists of receptors that reduce the intracellular cAMP level accelerates

Histamine receptors are related to skin barrier function [Ashida and Denda 2001]. Three different types of histamine receptors, H1, H2, H3, and H4 have been reported. First, topical application of histamine H1 and H2 receptor antagonists accelerated barrier repair. Histamine itself, H2 receptor agonist, and histamine releaser delayed barrier repair. Histamine H3 receptor antagonist and agonist did not affect the barrier recovery rate. Topical application of H1 and H2 receptor antagonists prevented the epidermal hyperplasia induced by barrier disruption under low humidity. The mechanism of the

by a decrease of cholesterol synthesis.

the corresponding period of time.

barrier repair. (Denda 2004, Denda 2005)

**4.2 Neurotransmitters** 

(Denda 2002b).

**4.1 Endocrine factors** 

**4. Biochemical factors that influence barrier function** 

two groups: ionotropic receptors and G-protein-coupled receptors.

accelerated barrier recovery, while allose, galactose, glucose and gulose had no effect. Among the 4 dextro-ketohexoses, psicose, fructose, sorbose and tagatose all accelerated barrier recovery. Because the effects of hexoses on the barrier recovery rate appeared within one hour, the mechanism is unlikely to be genomic. Instead, these hexoses may influence phase transition of the lipid bilayers of lamellar bodies and cell membrane, a crucial step in epidermal permeability barrier homeostasis.
