Honokiol inhibits TNF-α-stimulated NF-κB activation and NF-κB-regulated gene expression through suppression of IKK activation

Honokiol, a small molecular weight lignan originally isolated from Magnolia officinalis, shows anti-angiogenic, anti-invasive and antic-proliferative activities in a variety of cancers. In this study, we investigated whether honokiol affects the transcription factor nuclear factor-kappa B (NF-kappa B) which controls a large number of genes involved in angiogenesis, metastasis and cell survival. We observed that the tumor necrosis factor-alpha (TNF-alpha)-induced NF-kappa B activation was blocked by honokiol in four different cancer cell lines as evidenced by EMSA. Honokiol did not directly affect the NF-kappa B-DNA binding. Immunoblot experiments demonstrated that honokiol inhibited the TNF-alpha-stimulated phosphorylation and degradation of the cytosolic NF-kappa B inhibitor I kappa B alpha. Further-more, honokiol suppressed the intrinsic and TNF-alpha-stimulated upstream I kappa B kinases (IKKs) activities measured by a non-radioactive kinase assay using immuno-precipitated IKKs, suggesting a critical role of honokiol in abrogating the phosphorylation and degradation of I kappa B alpha. In a HeLa cell NF-kappa B-dependent luciferase reporter system, honokiol suppressed luciferase expression stimulated by TNF-alpha and by the transient transfection and expression of NIK (NF-kappa B-inducing kinase), wild type IKK beta, constitutively active IKK alpha and IKK beta, or the p65 subunit. Honokiol was also found to inhibit the nuclear translocation and phosphorylation of p65 subunit of NF-kappa B. RT-PCR results showed that honokiol suppressed NF-kappa B-regulated inflammatory and carcinogenic gene products including MMP-9, TNF-alpha, IL-8, ICAM-1 and MCP-1. In line with the observation that NF-kappa B activation may up-regulate anti-apoptotic genes, it was shown that honokiol enhanced TNF-alpha-induced apoptotic cell death. In summary, our results demonstrate that honokiol suppresses NF-kappa B activation and NF-kappa B-regulated gene expression through the inhibition of IKKs, which provides a possible mechanism for its anti-tumor actions. (c) 2005 Elsevier Inc. All rights reserved.