Modulation of Tight Junctions and the Permeability Barrier in Betaine-Treated Keratinocyte Cultures

Abstract BackgroundTight junctions (TJs) between keratinocytes are important for regulating the inside-out and outside-in permeability barriers. Betaine is a natural intracellular osmolyte utilized for instance in cosmetics. We determined by liquid chromatography that betaine content in human skin samples was higher in the epidermis than in the dermis. This led us to further investigate the effect of betaine on TJs by utilizing different in vitro models: topical application of betaine on normal human epidermal keratinocytes (NHEKs) differentiated for 5-days, freshly isolated NHEKs differentiated for 3 days and betaine applied during the differentiation process of organotypic rat epidermal keratinocytes (REKs) differentiated for 12-15 days. ResultsTopical betaine in amount range of 100 to 500 µM increased transepithelial electrical resistance (TEER) in 5-day-differentiated NHEKs, and downregulated occludin mRNA in concentrations 100-250 µM, but not claudin-4 (CLDN4) or zonula occludens-1 (ZO-1). In REKs, betaine applied during differentiation process had no effect on mannitol permeation or the expression of occludin, ZO-1, transforming growth factor beta 2 (TGF-β2), or cluster of differentiation (CD147). However, in freshly isolated differentiating human epidermal keratinocytes, betaine treatment increased TJ protein ZO-1 and the differentiation marker involucrin at protein level. By indirect immunofluorescence, prominent staining and co-localization for TJ proteins ZO-1 and claudin-1 was detected in freshly isolated differentiating keratinocytes treated with betaine. ConclusionsTaken together, the results provide evidence that betaine modulates TJ proteins in differentiating epidermal keratinocytes and promotes formation of TJs. These results suggest a role for betaine in the regulation of the epidermal barrier and as a natural ingredient in topical cosmetics.