S100A9 Exacerbates the Inflammation in Rosacea through Toll-Like Receptor 4/MyD88/NF-κB Signaling Pathway.

Rosacea is a chronic inflammatory skin disorder characterized by immune response-dependent erythema and pustules. S100A9, a proinflammatory alarmin, has been associated with various inflammation-related diseases. However, the specific role of S100A9 in rosacea remains unexplored. Therefore, our objective was to unravel the role of S100A9 in the pathogenesis of rosacea and its underlying molecular mechanisms. In this study, we show that expression levels of S100A9 were elevated in both the lesions and serum of patients with papulopustular rosacea as well as in lesions of the LL37-induced rosacea-like mouse model. Moreover, the upregulation of S100A9 was correlated with clinical severity and levels of inflammatory cytokines. In addition, we demonstrated that S100A9 promoted the production of proinflammatory factors in HaCaT cells by activating toll-like receptor 4/MyD88/NF-κB signaling pathways. Notably, inhibition of S100A9 suppressed the progression of rosacea-like dermatitis and inflammatory responses in the LL37-induced rosacea-like mouse model through toll-like receptor 4/MyD88/NF-κB signaling pathways. In conclusion, this study illustrated that S100A9 participates in the pathogenesis of rosacea by upregulating toll-like receptor 4/MyD88/NF-κB signaling pathways, thereby promoting rosacea-associated skin inflammation. These results not only expand our understanding of the potential role of S100A9 in the development of rosacea but also offer greater insight toward targeted therapies.