Betaine, a natural osmolyte, regulates tight junction protein amount and localization in normal human epidermal keratinocytes

Tight junctions (TJs) connecting adjacent keratinocytes to each other are part of skin’s permeability barrier. Betaine is a natural intracellular osmolyte, that is obtained in diet but it is also used as a cosmetic ingredient. We utilized liquid chromatography to determine the betaine amount in the human skin samples. In addition, freshly isolated human epidermal keratinocytes (NHEKs) were used to investigate how betaine affects TJ protein and differentiation markers expression, as well as their localization with indirect immunofluorescence. When betaine content was measured from non-treated human skin samples separated into dermal and epidermal fractions, the betaine content was higher in epidermis than in dermis, indicating that betaine is being transported to the epidermis from dermis. When freshly isolated NHEKs were treated with betaine, TJ protein zonula occludens -1 (ZO-1) and differentiation marker involucrin were increased at protein level. Indirect immunofluorescence staining showed a prominent localization of TJ proteins ZO-1 and claudin-1 at cell-cell contacts in betaine treated keratinocytes. In conclusion, the results indicate that betaine could modulate TJ protein level in the differentiating NHEKs, and promote the formation of TJs. Together with the higher content in epidermis in human skin samples, the results suggest a role for betaine in formation of the skin’s permeability barrier.