Novel Mechanism of Inhibition of Nuclear Factor-κB DNA-Binding Activity by Diterpenoids Isolated from Isodon rubescens

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 B_{max app} but have no effect on K_{d 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.