JNK pathway is required for TNCB-induced IL-18 expression in murine keratinocytes

The epidermal tissue of the skin is the first line of defense against exposure to microbial, chemical, and physical agents that cause cutaneous immune responses. Epidermal epithelial cells (keratinocytes) produce pro-inflammatory cytokine interleukin-18 (IL-18) potentially relevant for the skin immune responses. Expression of IL-18 was investigated after exposure of murine keratinocytes PAM212 to pro-inflammatory stimuli, allergen TNCB (Trinitrichlorobenzen), LPS (lipopolysaccharide) or PMA (phorbol myristate acetate). IL-18 mRNA transcription and IL-18 secretion were detected by real time-PCR and ELISA, respectively. The results showed that TNCB-induced IL-18 expression in cultured murine keratinocytes in a dose-dependent manner, and it significantly stimulated IL-18 mRNA and protein expression at 10 μM. In addition, both LPS and PMA could increase the expression of IL-18 in murine keratinocytes. To determine the molecular mechanism involved, keratinocytes were pretreated with JNK, p38 or ERK MAPK inhibitors SP600125, SB203580, PD98059. We found that JNK inhibitors could significantly suppress IL-18 expression enhanced by TNCB. Western blot results showed that TNCB could induce the phosphorylation of JNK, but not p38 or ERK1/2. These results suggest that TNCB has an up-regulation effect on IL-18 production in murine keratinocyte cell line PAM212, and that the activation of the JNK signal pathway is the mechanism responsible for TNCB-induced IL-18 gene expression in murine keratinocytes.