Helenalin, an anti-inflammatory sesquiterpene lactone from Arnica, selectively inhibits transcription factor NF-kappa B

Alcoholic extracts prepared form Arnicae flos, the collective name for flowerheads from Arnica montana and A. chamissonis ssp. foliosa, are used therapeutically as anti-inflammatory remedies. The active ingredients mediating the pharmacological effect are mainly sesquiterpene lactones, such as helenalin, 11 alpha,13-dihydrohelenalin, chamissonolid and their ester derivatives. While these compounds affect various cellular processes, current data do not fully explain how sesquiterpene lactones exert their anti-inflammatory effect. We show here that helenalin, and, to a much lesser degree, 11 alpha,13-dihydrohelenalin and chamissonolid, inhibit activation of transcription factor NF-kappa B. This difference in efficacy, which correlates with the compounds' anti-inflammatory potency in vivo, may be explained by differences in structure and conformation. NF-kappa B, which resides in an inactive, cytoplasmic complex in unstimulated cells, is activated by phosphorylation and degradation of its inhibitory subunit, I kappa B. Helenalin inhibits NF-kappa B activation in response to four different stimuli in T-cells, B-cells and epithelial cells and abrogates kappa B-driven gene expression. This inhibition is selective, as the activity of four other transcription factors, Oct-1, TBP, Spl and STAT 5 was not affected. We show that inhibition is not due to a direct modification of the active NF-kappa B heterodimer. Rather, helenalin modifies the NF-kappa B/I kappa B complex, preventing the release of I kappa B. These data suggest a molecular mechanism for the anti-inflammatory effect of sesquiterpene lactones, which differs from that of other nonsteroidal anti-inflammatory drugs (NSAIDs), indomethacin and acetyl salicylic acid.