Magnolol suppresses NF-κB activation and NF-κB regulated gene expression through inhibition of IkappaB kinase activation

The mis-regulation of nuclear factor-kappa B (NF-kappa B) signal pathway is involved in a variety of inflammatory diseases that leds to the production of inflammatory mediators. Our studies using human U937 promonocytes cells suggested that magnolol, a low molecular weight lignan isolated from the medicinal plant Magnolia officinalis, differentially down-regulated the pharmacologically induced expression of NF-kappa B-regulated inflammatory gene products MMP-9, IL-8, MCP-1, MIP-1 alpha, TNF-alpha. Pre-treatment of magnolol blocked TNF-alpha-induced NF-kappa B activation in different cell types as evidenced by EMSA. Magnolol did not directly affect the binding of p65/p50 heterodimer to DNA. Immunoblot analysis demonstrated that magnolol inhibited the TNF-alpha-stimulated phosphorylation and degradation of the cytosolic NF-kappa B inhibitor I kappa B alpha and the effects were dose-dependent. Mechanistically, a non-radioactive I kappa B kinases (IKK) assay using immunoprecipitated IKKs protein demonstrated that magnolol inhibited both intrinsic and TNF-alpha-stimulated IKK activity, thus suggesting a critical role of magnolol in abrogating the phosphorylation and degradation of I kappa B alpha. The involvement of IKK was further verified in a HeLa cell NF-kappa B-dependent luciferase reporter system. In this system magnolol 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. Magnolol was also found to inhibit the nuclear translocation and phosphorylation of p65 subunit of NF-kappa B. In line with the observation that NF-kappa B activation may up-regulate anti-apoptotic genes, it was shown in U937 cells that magnolol enhanced TNF-alpha-induced apoptotic cell death. Our results suggest that magnolol or its derivatives may have potential anti-inflammatory actions through IKK inactivation. (c) 2006 Elsevier Ltd. All rights reserved.