The IL-6-dependent activation of the JAK/STAT pathway plays a central role in the induction of the acute phase response in the liver. In a search for new inhibitors of the IL-6-mediated signal transduction in HepG2 cells using secreted alkaline phosphatase (SEAP) as reporter gene, four novel cyclopentenones, 2-(1-chloropropenyl)-4,5-dihydroxycyclopent-2-enone (CPDHC, 1), 4,5-dihydroxy-2-propenylcyclopent-2-enone (DHPC, 2), 5-hydroxy-2,3-dimethylcyclopent-2-enone (HDC, 3), and 4-methyl-5-methylenecyclopent-3-en-1,2-diol (MMCD, 4) were isolated from fermentations of the ascomycete strain A23-98. CPDHC (1) inhibits the IL-6-induced SEAP expression with IC50 values of 4.0-5.3 mu M (0.75-1 mu g/ml). The compounds DHPC (2), HDC (3), and MMCD (4) which are structurally closely related to CPDHC (1) showed no inhibitory effects on the IL-6-induced SEAP expression in HepG2 cells. Studies on the mode of action revealed that CPDHC (1) affects the IL-6-dependent pathway by inhibiting the tyrosine phosphorylation of the STAT3 and STAT1 as well as the serine phosphorylation of the Stat3 transcription factor. In addition, CPDHC (1) and DHPC (2) inhibit the AP-1 and NF-kappa B mediated SEAP expression in transiently transfected HeLa S3 cells with IC50 values of 10-15 mu M (23 mu g/ml) and 50-100 mu M (8-16 mu g/ml) respectively. Our results indicate that CPDHC inhibits the NF-kappa B pathway by preventing the phosphorylation and proteolytic degradation of the I kappa B alpha protein. The novel cyclopentenones may represent lead compounds for the development of new antiinflammatory drugs.