Novel mechanism of inhibition of nuclear factor-κB DNA-binding activity by diterpenoids isolated from Isodon rubescens

Abstract

The development of specific inhibitors that can block nuclear factor-κB (NF-κB) activation is an approach for the treatment of cancer, autoimmune, and inflammatory diseases. Several diterpenoids, oridonin, ponicidin, xindongnin A, and xindongnin B were isolated from the herb Isodon rubescens. These compounds were found to be potent inhibitors of NF-κB transcription activity and the expression of its downstream targets, cyclooxygenase-2 and inducible nitric-oxide synthase. The mechanisms of action of the diterpenoids against NF-κB are similar, but significant differences were also identified. All of the diterpenoids directly interfere with the DNA-binding activity of NF-κB to its response DNA sequence. Oridonin and ponicidin have an additional impact on the translocation of NF-κB from the cytoplasm to nuclei without affecting IκB-α phosphorylation and degradation. The effect of these compounds on the interaction of NF-κB with consensus DNA sequences is unique. Different inhibitory effects were observed when NF-κB bound to various DNA sequences. Both p65/p65 and p50/p50 homodimers, as well as p65/p50 heterodimer association with their responsive DNA, were inhibited. Kinetic studies on NF-κB-DNA interaction indicate that the diterpenoids decrease the Bmax app but have no effect on Kd app. This suggests that this class of compounds interacts with both p65 and p50 subunits at a site other than the DNA binding site and subsequently modulates the binding affinity of the transcription factor toward DNA with different NF-κB binding sequences. The diterpenoid structure could therefore serve as a scaffold for the development of more potent and selective NF-κB inhibitors that target regulated gene transcription. Copyright © 2005 The American Society for Pharmacology and Experimental Therapeutics.