Skin barrier defects play an important role in atopic dermatitis (AD) pathogenesis. Loricrin, an important barrier protein suppressed in human AD, is down-regulated by IL-4 in keratinocytes. However, the molecular mechanism is unknown. Since loricrin transcription requires p300/CBP, and Stat6 also recruits this common coactivator for its stimulated factors, we hypothesize that IL-4-activated Stat6 competes for the available endogenous p300/CBP, leading to loricrin transcription inhibition. First, we showed that loricrin is suppressed in the skin of IL-4 transgenic mice, an AD mouse model. In human keratinocytes, IL-4 down-regulation of loricrin is abrogated by a pan-Jak inhibitor, suggesting that the Jak-Stat pathway is involved. To further investigate the downstream molecular mechanism, we transfected HaCat cells with a loricrin promoter and then treated them with either IL-4 or vehicle. Not surprisingly, IL-4 greatly suppressed the promoter activity. Interestingly, this suppression was prevented when we knocked down Stat6, indicating that Stat6 participates in IL-4 regulation of loricrin. A Stat6-specific inhibitor confirmed the knockdown study. Finally, IL-4 suppression of loricrin was reversed with transfection of a CBP expression vector in a dose-dependent manner. Taken together, for the first time, we delineate a molecular mechanism for IL-4 down-regulation of loricin expression in human keratinocytes, which may play an important role in AD pathogenesis.

Summary Interleukin‐19 (IL‐19) plays an important role in asthma by stimulating T helper type 2 (Th2) cytokine production. Interestingly, IL‐4, a key Th2 cytokine, in turn up‐regulates IL‐19 expression in bronchial epithelial cells, so forming a positive feedback loop. In atopic dermatitis (AD), another Th2 disease closely related to asthma, IL‐19 is up‐regulated in the skin. We propose to use IL‐4 transgenic (Tg) mice and human keratinocyte culture to delineate the molecular mechanisms involved in the up‐regulation of IL‐19 in AD. IL‐19 is similarly up‐regulated in the skin of IL‐4 Tg mice as in human AD. Next we show that IL‐4 up‐regulates IL‐19 expression in keratinocytes. Interestingly, the up‐regulation was suppressed by a pan‐Janus kinase (Jak) inhibitor, suggesting that the Jak–signal transducer and activator of transcription (Jak‐STAT) pathway may be involved. Dominant negative studies further indicate that STAT6, but not other STATs, mediates the up‐regulation. Serial 5′ deletion of the IL‐19 promoter and mutagenesis studies demonstrate that IL‐4 up‐regulation of IL‐19 in keratinocytes involves two imperfect STAT6 response elements. Finally, chromatin immunoprecipitation assay studies indicate that IL‐4 increases the binding of STAT6 to its response elements in the IL‐19 promoter. Taken together, we delineate the detailed molecular pathway for IL‐4 up‐regulation of IL‐19 in keratinocytes, which may play an important role in AD pathogenesis.

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Although the pathogenesis of atopic dermatitis (AD) is not entirely clear, AD has been associated with filaggrin loss-of-function mutations, Th2 cytokines, and colonization with the gram-positive cocci S. aureus that contribute to the skin barrier defect and disease severity. To mimic skin scratching in AD patients, we performed scalpel cuts on the backs of filaggrindeficient (ft/ft) and wt (Balb/c) mice. Unexpectedly, w21 days after the skin injury, ft/ft mice but not wt mice spontaneously developed an expanding area of inflammation at the site of skin injury with clinical alopecia, erythema, and scale, histologic acanthosis and grampositive cocci colonization in the stratum corneum, an inflammatory infiltrate of monocytes, eosinophils, mast cells and neutrophils, and increased protein levels of (e.g., IL-1b) and the Th2-related cytokines (e.g., IL-33 and IL-31). Interestingly, topical treatment with a potent corticosteroid ointment (clobetasol) resulted in a greater degree of skin inflammation, and histologic sections revealed a marked increase in abundance of gram-positive cocci throughout the stratum corneum and hair follicles. In contrast, topical treatment with an antibiotic ointment (Neosporin) resulted in resolution of clinical and histologic skin inflammation, decreased inflammatory infiltrate, and reduced levels of proinflammatory and Th2 cytokines. Taken together, these findings provide a new in vivo mouse model of spontaneous AD development to investigate the specific roles of filaggrin deficiency, skin injury, and bacterial skin colonization in the pathogenesis of AD.