Ectoine disperses keratin and alters hydration kinetics in stratum corneum

Abstract

Moisturizing compounds are commonly applied topically to human stratum corneum (SC). Many types of molecular species are employed, most commonly including humectants and occlusives. We find new evidence of keratin dispersion caused by the moisturizing compound ectoine (1,4,5,6-tetrahydro-2-methyl-4-pyrimidinecarboxylic acid), and provide the first characterization of its impacts on the hydration kinetics and biomechanics of SC. A second compound, 2-(2-hydroxyethoxy)ethylguanidine succinate (HEG) was investigated for comparison. A suite of biomechanical and biochemical assays including FTIR, drying stress, and cellular cohesion were used. Studies were conducted on normal, lipid-extracted, and lipid plus natural moisturizing factor extracted SC. Ectoine was found to improve the dispersity and hydration of keratin bundles in corneocytes. It also decreased rates of stress development in lipid extracted SC when exposed to a dry environment by ∼30% while improving stress reduction during rehydration by ∼20%. Peak stresses were increased in harsh drying environments of <5% RH, but SC swelling measurements suggest that water retention was improved in ambient conditions. Further, changes up to ∼4 J/m² were seen in cohesion after ectoine treatments, suggesting corneodesmosome interactions. HEG was tested and found to disperse keratin without impacting corneodesmosomes. These results indicate that keratin dispersants produce beneficial effects on SC hydration kinetics, ultimately resulting in higher SC hydration under ambient conditions.

Keywords: AE, acetone/ether; AE/W, acetone/ether followed by water; Chemical analysis; DCB, double cantilever beam; Drying stress; HDX, hydrogen-deuterium exchange; HEG, 2-(2-hydroxyethoxy)ethylguanidine succinate; Hydration; Skin barrier; Skin physiology/structure.

Fig. 1

Keratin dispersing agents ectoine and HEG, including (a) their molecular structure and (b) a schematic representation of the state of a corneocyte before and after treatment with these dispersants and lipid/NMF extractions.

Fig. 2

(a) Illustration of the method for measuring keratin hydration and dispersity via hydrogen-deuterium exchange (HDX). (b) Bar graph of the degree of HDX for untreated, lipid plus NMF extracted, and 0.7% w/w ectoine treated human SC (tape-stripped). (c) Average change in cadaver SC thickness ± SD after soaking in water, either with or without 7% w/w ectoine. (d) Drying stress profiles of the cadaver SC obtained in order to study the effect of 7% w/w ectoine on stress relaxation during humidification processes. Each curve was normalized to the peak stress attained by untreated SC in order to facilitate comparison.

Fig. 3

(a) Change in peak drying stress after various exposures, normalized to control values. (b) Change in stress development or reduction rate compared to untreated control rates. (c) Ratio of stress rate during hydration to stress rate during drying.

Fig. 4

Cadaver SC graded delamination behavior following treatment with 7% (w/w) aqueous solution of ectoine after (a) NMF extraction via distilled water exposure, (b) lipid + NMF extraction via AE/W exposure, or (c) lipid extraction via AE exposure. Note in each of these cases the substantial change in cohesion energy gradient after treatment with ectoine. (d) The predicted G_c for each condition at a depth of 6 μm into the tissue, based on linear fits for the data in a-c.