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-kappa B (NF-kappa B) 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-kappa B 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-kappa B are similar, but significant differences were also identified. All of the diterpenoids directly interfere with the DNA-binding activity of NF-kappa B to its response DNA sequence. Oridonin and ponicidin have an additional impact on the translocation of NF-kappa B from the cytoplasm to nuclei without affecting I kappa B-alpha phosphorylation and degradation. The effect of these compounds on the interaction of NF-kappa B with consensus DNA sequences is unique. Different inhibitory effects were observed when NF-kappa B 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-kappa B-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-kappa B binding sequences. The diterpenoid structure could therefore serve as a scaffold for the development of more potent and selective NF-kappa B inhibitors that target regulated gene transcription.