Withaferin A strongly elicits IκB kinase β hyperphosphorylation concomitant with potent inhibition of its kinase activity

The transcription factor NF kappa B plays a critical role in normal and pathophysiological immune responses. Therefore, NF kappa B and the signaling pathways that regulate its activation have become a major focus of drug development programs. Withania somnifera (WS) is a medicinal plant that is widely used in Palestine for the treatment of various inflammatory disorders. In this study we show that the leave extract of WS, as well as its major constituent withaferin A (WA), potently inhibits NF kappa B activation by preventing the tumor necrosis factor-induced activation of I kappa B kinase beta via a thicialkylation-sensitive redox mechanism, whereas other WS-derived steroidal lactones, such as withanolide A and 12-deoxywithastramonolide, are far less effective. To our knowledge, this is the first communication of I kappa B kinase beta inhibition by a plant-derived inhibitor, coinciding with MEK1/ERK-dependent Ser-181 hyperphosphorylation. This prevents I kappa B phosphorylation and degradation, which subsequently blocks NF kappa B translocation, NF kappa B/DNA binding, and gene transcription. Taken together, our results indicate that pure WA or WA-enriched WS extracts can be considered as a novel class of NF kappa B inhibitors, which hold promise as novel anti-inflammatory agents for treatment of various inflammatory disorders and/or cancer.