PI3K-dependent mRNA expression of IL-6 and IL-8 in OLCs

Objectives: Cytokines, such as interleukin-6 (IL-6) and interleukin-8 (IL-8), are mediators that play an important role during pro-inflammatory responses to pathogenic microorganisms. In odontoblasts, signaling pathways leading to IL-6 and IL-8 mRNA induction in response to Streptococcus mutans have not been elucidated. The aim of the present study was to investigate the involvement of the phosphatidylinositol-3-kinase (PI3K) in IL-6 and IL-8 gene expression in odontoblast-like cells when exposed to heat-killed S. mutans.

Methods: Cultured human odontoblast-like cells (OLCs) (Dulbecco's modified Eagle's medium) were pre-treated with a specific inhibitor for PI3K (LY 294002) and subsequently stimulated with heat-killed S. mutans for 6 and 24 hours. After stimulation, RNA was extracted from the cells and cDNA synthesis was performed. Gene expression of IL-6 and IL-8 was analyzed by real-time polymerase chain reaction and normalized to the gene expression of beta-actin. Cell survival was determined for stimulation experiments.

Results: The gene expression of IL-6 and IL-8 was significantly increased in response to heat-killed S. mutans (p=0.002 and p<0.001, respectively). After 6 hours, the mRNA expression of IL-6 and IL-8 was significantly higher than after 24 hours of stimulation (p=0.019 and p<0.001, respectively). Pre-treatment with the inhibitor LY 294002 blocked the induced gene expression of IL-6 and IL-8 (p=0.002 and p<0.001, respectively). No differences in viable cell counts were found after stimulation with heat-killed S. mutans and/or pre-treatment with LY 294002 compared to the unstimulated control.

Conclusions: The results of the present study showed that the gene expression of IL-6 and IL-8 was induced by heat-killed S. mutans via signaling pathways mediated by PI3K. These findings indicate that odontoblasts respond to cariogenic bacteria with increased gene expression of pro-inflammatory mediators and hence participate in immune processes.