2 macro phages. Inhibitors of p38 MAPK have been shown to up regulate these iNOS expression in IL 1 stimulated rat mesangial cells, in LPS IFN stimulated RAW264. 7 macro phages, in interferon mannose capped lipoarabinomannan stimulated RAW264. 7 macro phages and in LPS stimulated J774. A1 macrophages. In this study, a novel p38 MAPK inhibitor SB220025 increased LPS induced NO production with an EC50 of 100 nM, which is close to its IC50 value of p38 MAPK inhibition. Furthermore, a structurally related inactive control compound SB202474 had no effect. These results together suggest that the observed increase in NO production and iNOS expression was due to inhibition of p38 MAPK. SB203580 inhibits the p38 and p38 isoforms at equipotent efficiency, but does not inhibit p38 or p38?.

To our knowledge there is no published data about the isoform specificity of SB220025. In the present study both SB203580 and SB220025 had similar effect on LPS induced NO production, thus it is likely that the observed effects are mediated by p38 and or p38. J774 macrophages were found to express p38 mRNA and p38 protein at relatively high levels whereas only low amounts of p38 mRNA were detected. Similar pattern of p38 and p38 expression was reported by Lui et al. in rat renal mesangial cells, in which p38 MAPK inhibition was also found to increase iNOS expres sion. In their further transfection experiments, Lui et al. found that p38 mutant and p38 wild type iso forms inhibited IL 1 induced iNOS expression suggesting that the two isoforms have reciprocal effects on iNOS expression in renal mesangial cells.

Our results show that inhibition of p38 enhances iNOS expression and NO pro duction in macrophages activated by LPS but further stud ies are required to clarify the roles of different p38 MAPK isoforms in that process. The mechanisms how p38 MAPK inhibitors enhance iNOS expression and NO production have been unclear. The present data suggest that inhibition of p38 enhances JNK activity that results in stabilization of iNOS mRNA and enhanced iNOS protein expression. Our results are in line with those of Avdi et al. in which inhibition of p38 MAPK by SB203580 led to increased activity of JNK in human neutrophils. The inhibition of p38 MAPK was found to reduce the activity of protein phosphatase 2A which resulted in reduced dephosphorylation and increased activity of JNK. Various protein phosphatases are able to dephosphorylate MAPKs and are thus impor tant regulators of MAPK activity. It is possible that p38 MAPK regulates the activity of protein phosphatase 2A or some other protein phosphatase and inhibition of p38 MAPK by SB220025 reduces protein phosphatase Dacomitinib activity, which leads to increased JNK activity observed in the present study.