Perturbation of lamellar granule secretion by sodium caprate implicates epidermal tight junctions in lamellar granule function
Background: Polarized secretion of lamellar granules (LGs) delivers various lipids, proteases, and protease inhibitors into the stratum corneum (SC) of the epithelium. Disruption of LGs is associated with severe cutaneous diseases, but the mechanism of their polarized secretion is not known. On the other hand, recent study shows epidermal tight junctions (TJs) localize in stratum granulosum (SG), and TJs are involved in polarized molecule secretion. Thus, we hypothesized epidermal TJs relate to polarized LGs secretion. Objective: To assess the possibility that epidermal TJs are involved in polarized LGs secretion. Methods: In order to examine LGs secretion, we used fluorescent ceramide (BODIPY^(R) FL C"5-ceramide) and a natural LG cargo, lympho-epithelial Kazal-type-related inhibitor (LEKTI), in cultured normal human epidermal keratinocytes and a reconstructed human epidermis. We investigated their alteration using the medium-chain fatty acid sodium caprate (C10), TJs inhibitor. In addition, LG distribution was observed by electron microscopy. Results: C10 significantly inhibited secretion of both fluorescent ceramide and LEKTI in cultured normal human epidermal keratinocytes and a reconstructed human epidermis. C10 also disturbed the polarized localization of fluorescent ceramide and LEKTI in the reconstructed epidermis. Electron microscopy revealed that a large number of LGs remained in corneocytes in the C10-treated epidermis, rather than being secreted. Conclusion: Our data indicate that C10 perturbs the polarized secretion of LGs. Our study therefore suggests that epidermal TJs are possibly involved in polarized LG secretion and provides new insights into potential of treatments for skin diseases caused by abnormal LG secretion.