The role of sphingosine 1-phosphate in the TNF-α induction of IL-8 gene expression in lung epithelial cells

Tumor necrosis factor-α (TNF-α) is an important cytokine involved in the pathogenesis of inflammatory diseases of the lung. Interleukin-8 (IL-8), a C-X-C chemokine, is induced by TNF-α and initiates injury by acting as a chemoattractant for neutrophils and other immune cells. Although sphingolipids such as ceramide and sphingosine 1-phosphate (S1-P) have been shown to serve as signaling molecules in the TNF-α inflammatory response, their role in the TNF-α induction of IL-8 gene expression in lung epithelial cells is not known. We investigated the role of sphingolipids in the TNF-α induction of IL-8 gene expression in H441 lung epithelial cells. We found that TNF-α induced IL-8 mRNA levels by increasing gene transcription, and the stability of IL-8 mRNA was not affected. Exogenous S1-P but not ceramide or sphingosine increased IL-8 mRNA levels and IL-8 secretion. Dimethylsphingosine, an inhibitor of sphingosine kinase, partially inhibited TNF-α induction of IL-8 mRNA levels indicating the importance of intracellular increases in S1-P in the IL-8 induction. S1-P induction of IL-8 mRNA was due to an increase in gene transcription, and the stability of IL-8 mRNA was not affected. S1-P induction of IL-8 mRNA was associated with an increase in the binding activity of AP-1 but the activities of NF-κB and NF IL-6 were unchanged. S1-P induced the phosphorylation of ERK, p38 and JNK MAPKs. Pharmacological inhibitors of ERK and p38 but not JNK partly inhibited S1-P induction of IL-8 mRNA levels. These data show that increases in the intracellular S1-P partly mediate TNF-α induction of IL-8 gene expression in H441 lung epithelial cells via ERK and p38 MAPK signaling pathways and increased AP-1 DNA binding.