Rescue of sirtuin inhibitor-dependent decrease in claudin-4 expression and paracellular barrier property in keratinocytes by epigallocatechin gallate

The barrier function of granular layer in the skin is mainly sustained by claudin-1 (CLDN1) and CLDN4, tight junctional components. We recently found that the activity of sirtuin-2 (SIRT2), an anti-aging molecule, is decreased with aging in keratinocytes, leading to the attenuation of CLDN4 expression and paracellular barrier function. SIRT2 may be a novel target for enhancing skin barrier function in elderly people. In vitro SIRT2 activity assay showed that epigallocatechin gallate (EGCG) and green tea extract (GT) have a potent ability to activate SIRT2. Tenovin-1 (Ten-1), a sirtuin-1/2 inhibitor, decreased the SIRT2 activity in human keratinocyte-derived HaCaT cells, which was rescued by EGCG and GT. Ten-1 decreased the protein level of CLDN4, which was rescued by EGCG, whereas CLDN1 expression was changed by neither Ten-1 nor EGCG. Ten-1 decreased the tight junctional localization of CLDN4, transepithelial electrical resistance, and paracellular permeability to FD4, a fluorescence paracellular flux marker, which were rescued by EGCG. Ten-1 increased the acetylation level of CLDN4, which was inhibited by EGCG without affecting NAD⁺ content, a substrate for SIRT2. The protein levels of wild-type and K191A mutant were decreased by Ten-1, whereas that of K196A was not. Furthermore, Ten-1 increased the acetylation levels of WT and K191A mutant. We suggest that Ten-1 decreases CLDN4 expression mediated by the acetylation of K196 of CLDN4 and EGCG is useful to protect from aging-induced dysfunction of paracellular barrier in the keratinocytes.