Caffeic acid phenethyl ester-mediated Nrf2 activation and IκB kinase inhibition are involved in NFκB inhibitory effect: Structural analysis for NFκB inhibition

Caffeic acid phenethyl ester (CAPE) is an active component of propolis from honeybee. We investigated potential molecular mechanisms underlying CAPE-mediated nuclear factor kappa beta (NF kappa B) inhibition and analyzed structure of CAPE for its biological effect. CAPE attenuated expression of NF kappa B dependent luciferase stimulated with TNF-alpha or LPS and suppressed LPS-mediated induction of iNOS, a target gene product of NF kappa B. In HCT116 cells, CAPE interfered with TNF-alpha dependent I kappa B alpha degradation and subsequent nuclear accumulation of p65, which occurred by direct inhibition of inhibitory protein kappaB kinase (IKK). CAPE increased the expression of Nrf2-dependent luciferase and heme oxygenase-1, a target gene of Nrf2, and elevated the nuclear level of Nrf2 protein, indicating that CAPE activated the Nrf2 pathway. In HCT116 cells with stable expression of Nrf2 shRNA, CAPE elicited a reduced inhibitory effect on TNF-alpha-activated NF kappa B compared to scramble RNA expressing control cells. On the other hand, the NF kappa B inhibitory effect of CAPE was diminished by removal or modification of the Michael reaction acceptor, catechol or phenethyl moiety in CAPE. These data suggest that CAPE inhibits TNF-alpha-dependent NF kappa B activation via direct inhibition of IKK as well as activation of Nrf2 pathway, in which the functional groups in CAPE may be involved. (C) 2010 Elsevier B.V. All rights reserved.