In addition, cyclohex imide, a known JNK activator, also increased LPS induced iNOS mRNA expression in a similar manner as SB220025. The stimulatory effect of cycloheximide on iNOS mRNA expression was reversed by SP600125, suggesting that the quality control effect of cycloheximide is at least partially mediated through increased JNK activity. Up regulatory role for JNK in iNOS expression has been previously shown in IL 1 IFN stimulated human fetal astrocytes, in LPS IFN stimulated RAW264. 7 macrophages, IL 1 stimulated rat primary mesangial cells and LPS stimulated J774. A1 macrophages. Regulation of iNOS mRNA stability seems to be an impor tant mean to regulate iNOS expression. However, the mechanisms regulating iNOS mRNA stability are poorly known.

HuR is a mRNA stabilizing factor, which has been shown to bind an AU rich sequence element in the 3 untranslated region of human iNOS mRNA and to stabi lise iNOS mRNA. Tristetraprolin seems to have a role as a mRNA stabilizing factor for human iNOS while the KH type splicing regulatory protein has been identified as a destabilizing factor. Heterogeneous nuclear ribonucleoproteins I and L have been reported to interact with murine iNOS mRNA. In addition, dex amethasone, protein kinase C? and adrenergic stimulation have been shown to regulate iNOS mRNA stability. Recently, we have shown that JNK inhib itor SP600125 reduces iNOS mRNA stability. In the present study, treatment with SB220025 had no effect on iNOS mRNA levels when measured 4 h after LPS stimula tion, whereas a two fold increase in mRNA levels was observed 10 h after LPS.

Furthermore, mRNA levels decreased slower in SB220025 treated cells than in cells treated with LPS alone. These results together suggest that SB220025 increases iNOS mRNA expression by stabilis ing mRNA. Also actinomycin D seems to have a stabilising effect on iNOS mRNA. Actinomycin D has previously been reported to stabilise mRNAs of transferrin receptor and cyclooxygenase 2 but the mechanisms are not known in detail. Conclusion The present results show that inhibition of p38 MAPK enhances JNK activity, which leads to stabilisation of iNOS mRNA, and to increased iNOS expression and NO production. p38 MAPK regulates activity of JNK pathway and Batimastat it is therefore important to consider whether results obtained by inhibiting p38 MAPK might result from increased JNK activity rather than from reduced p38 MAPK activity directly. Finally, based on our results, it seems that JNK is an important post transcriptional regu lator of LPS induced iNOS expression and NO produc tion.