MicroRNA-146a alleviates chronic skin inflammation in atopic dermatitis through suppression of innate immune responses in keratinocytes

Background: Chronic skin inflammation in atopic dermatitis (AD) is associated with elevated expression of proinflammatory genes and activation of innate immune responses in keratinocytes. microRNAs (miRNAs) are short, single-stranded RNA molecules that silence genes via the degradation of target mRNAs or inhibition of translation. Objective: The aim of this study was to investigate the role of miR-146a in skin inflammation in AD. Methods: RNA and protein expression was analyzed using miRNA and mRNA arrays, RT-quantitative PCR, Western blotting, and immunonohistochemistry. Transfection of miR-146a precursors and inhibitors into human primary keratinocytes, luciferase assays, and MC903-dependent mouse model of AD were used to study miR-146a function. Results: We show that miR-146a expression is increased in keratinocytes and chronic lesional skin of patients with AD. miR-146a inhibited the expression of numerous proinflammatory factors, including IFN-gamma-inducible and AD-associated genes CCL5, CCL8, and ubiquitin D (UBD) in human primary keratinocytes stimulated with IFN-gamma, TNF-alpha, or IL-1 beta. In a mouse model of AD, miR-146a-deficient mice developed stronger inflammation characterized by increased accumulation of infiltrating cells in the dermis, elevated expression of IFN-gamma, CCL5, CCL8, and UBD in the skin, and IFN-gamma, IL-1 beta, and UBD in draining lymph nodes. Both tissue culture and in vivo experiments in mice demonstrated that miR-146a-mediated suppression in allergic skin inflammation partially occurs through direct targeting of upstream nuclear factor kappa B signal transducers caspase recruitment domain-containing protein 10 and IL-1 receptor-associated kinase 1. In addition, human CCL5 was determined as a novel, direct target of miR-146a. Conclusion: Our data demonstrate that miR-146a controls nuclear factor kappa B-dependent inflammatory responses in keratinocytes and chronic skin inflammation in AD.