In the present study, we examined the relevance of the PKA activity to LPS-induced IL-6, IL-8, and PGE2 production. We showed that the PKA inhibitor H-89 suppressed LPS-induced IL-8 and PGE2 production, and that the PKA activator dbcAMP enhanced LPS-induced IL-6, IL-8, and PGE2 production. Moreover, we showed that aminophylline and adrenaline enhanced this production. However, H-89, dbcAMP, aminophylline, and adrenaline had no relevance to the production of these molecules in the absence of LPS. These results suggest that the PKA pathway enhances the LPS signaling pathway in HGFs, but that the PKA pathway alone does not enhance LPS-induced IL-6, IL-8, and PGE2 production.
Several reports have demonstrated the relevance of LPS to the PKA activity. LPS enhances cAMP accumulation in a mouse pituitary cell line  and in human bladder epithelial cells . Moreover, LPS enhances cAMP response element binding protein (CREB) phosphorylation and its DNA-binding ability, and these activities are inhibited by H-89 . Although there is no comparable report for HGFs, a similar mechanism may be present.
Additionally, various reports have shown the relevance of the PKA activity to IL-6, IL-8, and PGE2 production. However, whether the PKA pathway enhances or suppresses their production is different in depending on the cell type and genes in question. (1) The PKA pathway enhances LPS-induced IL-6 production. For instance, calcitonin gene-related peptide, which increases cAMP accumulation, enhances LPS-induced IL-6 production in mouse peritoneal macrophages , and H-89 suppresses IL-6 expression in human blood monocytes . In the present study, dbcAMP, aminophylline, and adrenaline enhanced LPS-induced IL-6 production by HGFs to the same extent as by monocytes and macrophages in previous reports [14, 15]. In contrast, H-89 had no relevance to LPS-induced IL-6 production. The reasons for this discrepancy remain to be elucidated. However, we assume that the enhancement of PKA activity by LPS treatment is weak in HGFs. (2) The relevance of the PKA activity to IL-8 production are different among several previous reports. The PKA pathway suppresses LPS-induced IL-8 production, i.e. β-adrenergic agonists and dbcAMP suppress and H-89 enhances LPS-induced IL-8 production by human monocytic THP-1 cells . In contrast, cAMP analogs have been shown to have no relevance to LPS-induced IL-8 production by human macrophages  and by human peripheral blood mononuclear cells . Our results showing that the PKA pathway enhanced LPS-induced IL-8 production by HGFs is different from these previous reports, and therefore may be an important characteristic of HGFs. (3) The PKA pathway enhances LPS-induced PGE2 production. dbcAMP enhanced LPS-induced cyclooxygenase-2 (COX-2) expression and PGE2 production by human blood moncytes  and mouse RAW264.7 macrophage cell line . In contrast, H-89 suppresses LPS-induced COX-2 expression in RAW264.7 cells , These results are consistent to our results with HGFs.
Next, we discuss whether aminophylline at a concentration used in clinic enhances the inflammatory response in periodontal disease. The serum concentrations of theophylline in the therapeutic range are 8 to 20 μg/ml [6, 22, 23]. Concentrations of theophylline above 20 μg/ml lead to adverse effects such as arrhythmia, convulsion, and central nervous symptoms, and concentrations above 60 μg/ml lead to death . The therapeutic range of aminophylline is considered to be similar to that of theophylline, because aminophylline is a dimer of theophylline. In the present study, up to 10 μg/ml of aminophylline did not affect LPS-induced IL-6, IL-8, or PGE2 production. Moreover, although 100 μg/ml of aminophylline enhanced LPS-induced production of these molecules, this concentration of aminophylline is fatal. Therefore, it is unlikely that aminophylline enhances the inflammatory response and exacerbates periodontal disease. Moreover, we conclude that only aminophylline among the drugs we tested does not induce inflammatory responses.