# Neurotransmitter 5-HT Further Promotes LL-37-Induced Rosacea-like Inflammation Through HTR3A

## TL;DR

This study shows that the neurotransmitter 5-HT worsens rosacea-like inflammation by acting through the HTR3A receptor, suggesting it as a potential treatment target.

## Contribution

The study identifies HTR3A as a novel mediator of 5-HT's role in LL-37-induced rosacea-like inflammation.

## Key findings

- 5-HT enhances LL-37-induced macrophage inflammation and M1 polarization.
- HTR3A is selectively upregulated by 5-HT and LL-37, with TPS and HTR3A siRNA reducing inflammation.
- 5-HT worsens rosacea-like symptoms in mice, which are reversed by TPS.

## Abstract

Rosacea is a chronic inflammatory skin disease and is usually accompanied by extensive macrophage infiltration. There is growing evidence suggesting that neurotransmitter 5-hydroxytryptamine (5-HT) plays a crucial role in inflammatory reactions. However, the interaction between 5-HT and rosacea is still unclear. Here, we hypothesized that the inflammation of rosacea is partly caused by 5-HT, and we investigated the underlying mechanism. In this study, we employed a rosacea model induced by LL-37, which is usually applicated as a rosacea stimulator, to investigate the effects of 5-HT on rosacea in vitro and in vivo. In LL-37-(4 μM)-induced THP-1-derived macrophages, 5-HT (400 μM) further promoted the secretion of inflammatory cytokines and polarized macrophages towards M1 phenotype, which could promote an inflammatory response. Further research revealed that exposure to LL-37 and 5-HT (L5) selectively upregulated HTR3A mRNA expression but not HTR2A or HTR7 and induced colocalization of 5-HT with HTR3A. Notably, application of antagonist tropisetron (TPS) and siRNA of HTR3A suppressed L5-induced inflammation. Meanwhile, 5-HT (25 μg each injection a total of three times) deteriorated skin erythema, stimulated dermal inflammatory cell infiltration, and promoted the secretion of inflammatory cytokines in LL-37 (40 μL and 320 μM each injection a total of four times) induced rosacea-like mice, while these undesirable effects were reversed by using TPS. Our findings revealed that neurotransmitter 5-HT further promoted LL-37-induced rosacea-like inflammation through HTR3A. Our study highlights HTR3A as a promising therapeutic target, which warrants further in-depth investigation into its clinical applicability.