Keratinocytes take up serum-derived retinol (vitamin A) and metabolize it to all-trans-retinoic acid (atRA), which binds to the nuclear retinoic acid receptor (RAR). We previously reported that serum-affected keratinocyte differentiation and function; namely, it inhibited keratinization, decreased loricrin (LOR) and claudin (CLDN) 1 expression, increased keratin (K) 4 and CLDN4 levels, and reduced paracellular permeability in three-dimensional (3D) cultures of mouse keratinocytes (COCA). Contrarily, RAR inhibition reversed these changes. Here, we aimed to examine whether atRA exerted the same effects as serum, and whether it was involved in the differential oral mucosa keratinization among animal species. Porcine oral mucosal keratinocytes, which form non-keratinized epithelium in vivo, established keratinized epithelium in 3D cultures. Both mouse and porcine sera induced non-keratinized epithelium at 0.1% in COCA 3D cultures. Although atRA caused the same changes as serum, its effective concentration differed. atRA inhibited keratinization at 0.1 nM and 1 nM in porcine or human keratinocytes and COCA, respectively. Furthermore, atRA upregulated CLDN7 in the cytoplasm but not in cell-cell contacts. These atRA-induced changes were reverted by RAR inhibition. The results indicate that serum-induced changes are probably due to the effect of serum-derived atRA, and that mouse keratinocytes require higher atRA concentrations to suppress keratinization than porcine and human keratinocytes. We propose that the lower susceptibility of mouse keratinocytes to atRA, rather than a lower retinol concentration, is a possible reason for the keratinization of mouse oral mucosal epithelium.

中文翻译：

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小鼠角质形成细胞对视黄酸敏感性的降低可能与口腔和食管粘膜上皮的角质化有关。

角质形成细胞吸收血清来源的视黄醇（维生素A）并将其代谢为全反式视黄酸（atRA），后者与核视黄酸受体（RAR）结合。我们以前曾报道过血清影响的角质形成细胞的分化和功能。也就是说，它在小鼠角质形成细胞（COCA）的三维（3D）培养中抑制了角化作用，降低了loricrin（LOR）和claudin（CLDN）1的表达，增加了角蛋白（K）4和CLDN4的水平，并降低了细胞旁通透性。相反，RAR抑制逆转了这些变化。在这里，我们旨在检查atRA是否发挥与血清相同的作用，以及它是否参与了动物物种之间口腔黏膜角化的差异。在体内形成未角化上皮的猪口腔粘膜角质形成细胞在3D培养物中建立了角化上皮。小鼠和猪血清在COCA 3D培养物中均诱导0.1％的非角质化上皮细胞。尽管atRA引起的变化与血清相同，但其有效浓度却有所不同。atRA在猪或人的角质形成细胞和COCA中分别以0.1 nM和1 nM抑制角化作用。此外，atRA在细胞质中上调CLDN7，但在细胞间接触中不上调。这些atRA诱导的变化通过RAR抑制得以恢复。结果表明，血清诱导的变化可能归因于血清来源的atRA的作用，并且小鼠角质形成细胞比猪和人角质形成细胞需要更高的atRA浓度来抑制角质化。我们提出，小鼠角质形成细胞对atRA的敏感性较低，而不是视黄醇浓度较低，这是小鼠口腔粘膜上皮角质化的可能原因。