Activation of mixed lineage kinase 3 by fine particulate matter induces skin inflammation in human keratinocytes

Fine particulate matter (PM_2.5) induces a range of diseases, including skin disorders, through inflammatory responses. In this study, we investigated the novel mechanisms by which PM_2.5 causes skin inflammation in human keratinocytes HaCaT. We observed increased protein expression of cyclooxygenase-2 (COX-2) and the production of prostaglandin E2 (PGE2) in PM_2.5-treated HaCaT cells. To identify the pathways promoting the expression of these inflammatory proteins, we conducted a phospho-kinase antibody array and confirmed that the phosphorylation levels of JNK and p38 were increased by PM_2.5-treated HaCaT cells. Further investigation of the phosphorylation levels of mitogen-activated protein kinases (MAPKs) and upstream signals revealed that PM_2.5 activated the MKK4/7-JNK-c-Jun and MKK3/6-p38-p70^S6K signaling pathways, while the phosphorylation level of ERK1/2 remained unchanged. HaCaT cells treated with PM_2.5 phosphorylated Mixed-lineage kinase 3 (MLK3), an upstream regulator of p38 and JNK. Furthermore, inhibition of ROS production by N-Acetylcysteine (NAC) treatment inhibited MLK3 phosphorylation. Taken together, ROS production induced by PM_2.5 activated the MLK3 signaling pathway and induced skin inflammation.