Inhibition of the Nuclear Factor κB (NF-κB) Pathway by Tetracyclic Kaurene Diterpenes in Macrophages

Abstract

The anti-inflammatory action of most terpenes has been explained in terms of the inhibition of nuclear factor κB (NF-κB) activity. Ent-kaurene diterpenes are intermediates of the synthesis of gibberellins and inhibit the expression of NO synthase-2 and the release of tumor necrosis factor-α in J774 macrophages challenged with lipopolysaccharide. These diterpenes inhibit NF-κB and IκB kinase (IKK) activation in vivo but failed to affect in vitro the function of NF-κB, the phosphorylation and targeting of IκBα, and the activity of IKK-2. Transient expression of NF-κB-inducing kinase (NIK) activated the IKK complex and NF-κB, a process that was inhibited by kaurenes, indicating that the inhibition of NIK was one of the targets of these diterpenes. These results show that kaurenes impair the inflammatory signaling by inhibiting NIK, a member of the MAPK kinase superfamily that interacts with tumor necrosis factor receptor-associated factors, and mediate the activation of NF-κB by these receptors. Moreover, kaurenes delayed the phosphorylation of p38, ERK1, and ERK2 MAPKs, but not that of JNK, in response to lipopolysaccharide treatment of J774 cells. The absence of a coordinate activation of MAPK and IKK might contribute to a deficient activation of NF-κB that is involved in the anti-inflammatory activity of these molecules.