Sphingosine 1-phosphate 1 and TLR4 mediate IFN-beta expression in human gingival epithelial cells

IFN-beta production is a critical step in human innate immune responses and is primarily controlled at the transcription level by highly ordered mechanisms. IFN-beta can be induced by pattern-recognition receptors such as the TLR4. S1P1 is a G protein-coupled receptor, which has a high affinity for sphingosine 1-phosphate (S1P). Although many of the receptors and signaling pathways leading to the expression of IFN-beta have been identified and characterized, it is still unclear how IFN-beta is regulated in primary human gingival epithelial cells (HGECs). In this study, we demonstrate that S1P1 and TLR4, acting in unison, play an important role in IFN-beta expression at the protein and mRNA level in HGECs. We demonstrate that the expression of both IFN-beta and IFN-inducible protein-10 (CXCL-10) is significantly up-regulated by LPS and S1P or LPS and a specific S1P1 agonist. This enhanced innate immune response is attenuated in HGECs by small interfering RNA knockdown of either TLR4 or S1P1. Moreover, we show that triggering of TLR4 results in the increased expression of S1P1 receptors. Furthermore, we found that IFN-regulatory factor 3 activation was maximized by LPS and S1P through PI3K. Our data show that triggering TLR4 increases S1P1, such that both TLR4 and S1P1 acting through PI3K enhancement of IFN-regulatory factor 3 activation increase IFN-beta expression in epithelial cells. The functional association between TLR4 and the S1P1 receptor demonstrates a novel mechanism in the regulation of IFN-beta and CXCL-10 in human primary gingival epithelial cells.