Overexpression of inducible nitric oxide synthase (iNOS) and the resultant overproduction of NO has been implicated in neuronal inflammatory diseases. Leptomycin B (LMB), a metabolite of Streptomyces, has been identified as a specific inhibitor of CRM1 nuclear export receptor. In this study, we evaluated the effect of LMB on lipopolysaccharide (LPS)-induced iNOS expression in BV2 cells, a murine microglial cells and the associated mechanisms. LMB strongly inhibited LPS-induced iNOS protein and mRNA expressions in BV2 cells in which 10 ng/ml of LMB (18 nM) was sufficient to greatly down-regulate iNOS by LPS, suggesting the potency of LMB to inhibit iNOS. The data of iNOS promoter-driven luciferase assay further suggested that the LMB inhibitory effect was in part due to inhibition of iNOS transcription. However, LPS-induced activation of various intracellular signaling proteins, such as nuclear factor-κB (NF-κB), extracellular signal-regulated kinases (ERKs), p38s, and c-Jun N-terminal kinases (JNKs), whose activations are known to be important for iNOS expression by LPS in BV2 cells, were not affected in the presence of LMB. Together, these results suggest that LMB inhibits iNOS expression in response to LPS in BV2 microglia, and the inhibition seems to be associated with blockage of CRM1-mediated iNOS mRNA nuclear export and also in part transcriptional down-regulation of iNOS, but not through modulation of NF-κB and the mitogen-activated protein kinase signaling pathways.

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