Inhibition of nuclear factor kappaB by phenolic antioxidants: interplay between antioxidant signaling and inflammatory cytokine expression.

Phenolic antioxidants inhibit the induction of inflammatory cytokines by inflammatory stimuli. Here, we analyzed the mechanism by which the antioxidants inhibit LPS-induced expression of tumor necrosis factor alpha (TNFalpha) in macrophages. Hydroquinone and tert-butyl hydroquinone, prototypes of phenolic antioxidants, block lipopolysaccharide (LPS)-induced transcription of TNFalpha and a nuclear factor (NF)-kappaB-mediated reporter gene expression, suggesting NF-kappaB as a target in the inhibition. Analyses of the NF-kappaB activation pathway revealed that the antioxidants do not inhibit LPS-induced activation of the IkappaB kinase activity, degradation of IkappaBalpha, or translocation of activated NF-kappaB into the nucleus, but they do block the formation of NF-kappaB/DNA binding complexes. In vitro experiments showed that the antioxidants do not directly interfere with DNA binding of NF-kappaB. Structure-activity analyses suggest that inhibition of NF-kappaB function involves the redox cycling property of the antioxidants. These findings implicate a redox-sensitive factor important for the binding of NF-kappaB to its DNA recognition sequence as a target molecule in the inhibition of NF-kappaB function and inflammatory cytokine expression by phenolic antioxidants.