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 (NFκB) inhibition and analyzed structure of CAPE for its biological effect. CAPE attenuated expression of NFκB dependent luciferase stimulated with TNF-α or LPS and suppressed LPS-mediated induction of iNOS, a target gene product of NFκB. In HCT116 cells, CAPE interfered with TNF-α dependent IκBα 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-α-activated NFкB compared to scramble RNA expressing control cells. On the other hand, the NFκB 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-α-dependent NFκB activation via direct inhibition of IKK as well as activation of Nrf2 pathway, in which the functional groups in CAPE may be involved.