Lipopolysaccharide and IL-1β coordinate a synergy on cytokine production by upregulating MyD88 expression in human gingival fibroblasts

Both lipopolysaccharide (LPS) and interleukin (IL)-1β activate the MyD88-dependent signaling pathways to stimulate proinflammatory cytokine expression. However, it remains unknown how LPS and IL-1β interact with each other to coordinate the stimulation. In this study, we sought to investigate the interaction between LPS and IL-1β on MyD88-dependent signaling pathways in human gingival fibroblasts (HGFs). Results showed that LPS derived from Porphyromonas gingivalis (Pg LPS) and IL-1β cooperatively stimulated mitogen-activated protein kinase (MAPK) and nuclear factor kappa B (NFκB) signaling pathways, and subsequent expression of proinflammatory cytokine expression. Furthermore, our results showed that Pg LPS and IL-1β exerted a synergy on MyD88 expression and knockdown of MyD88 expression by small interfering RNA diminished the synergistic effect of Pg LPS and IL-1β on IL-6 expression, suggesting that upregulation of MyD88 is involved in the coordinated stimulation by Pg LPS and IL-1β of proinflammatory cytokine expression. Finally, our results showed that pharmacological inhibitors for MAPK and NFκB significantly reduced IL-6 secretion stimulated by Pg LPS and IL-1β, indicating that the MyD88-dependent MAPK and NFκB signaling pathways are essential for the upregulation of proinflammatory cytokine expression by Pg LPS and IL-1β. Taken together, this study showed that LPS and IL-1β coordinate a synergy on cytokine production by upregulating MyD88 expression in HGFs.