Interleukin-1 receptor-associated kinase-M in gingival epithelial cells attenuates the inflammatory response elicited byPorphyromonas gingivalis

Abstract

Recent studies have revealed that negative regulatory molecules, including interleukin-1 receptor-associated kinase-M (IRAK-M), control the overactivation of Toll-like receptor (TLR) signaling. The role of IRAK-M in human gingival epithelial cells (HGECs), which express TLRs, remains unclear. The present study examined the role of IRAK-M on interleukin-8 and macrophage chemoattractant protein-1 (MCP-1) expression in HGECs stimulated with Porphyromonas gingivalis and TLR ligands. Primary HGECs and an SV40 T-antigen-immortalized HGEC line (epi 4) were stimulated with live or heat-killed P. gingivalis, P. gingivalis lipopolysaccharide or the synthetic lipopeptide PAM(3)CSK(4), and subsequent expression of IRAK-M, interleukin-8 and MCP-1 was evaluated at the mRNA and protein levels. The effects of IRAK-M on interleukin-8 and MCP-1 expressions were evaluated by IRAK-M-specific RNA interference (RNAi)-based loss-of-function assay. All tested stimulants up-regulated the expression of IRAK-M in HGECs. The P. gingivalis lipopolysaccharide or PAM(3)CSK(4) increased MCP-1 expression, whereas live P. gingivalis down-regulated the MCP-1 expression in HGECs. However, IRAK-M RNAi increased the expression of MCP-1 irrespective of up- or down-regulation mediated by the respective stimulants. Interleukin-8 gene expression, up-regulated by all tested stimulants, was further enhanced by IRAK-M RNAi. In contrast, IRAK-M RNAi had no effect on the interleukin-8 protein levels, irrespective of the stimulant, indicating that post-translational modification, not IRAK-M, controls interleukin-8 protein expression. Interleukin-1 receptor-associated kinase-M appeared to have distinct regulatory roles on the interleukin-8 and MCP-1 produced by HGECs, further suggesting an important role for interleukin-8 in the immune response to periodontopathic bacteria.