Transcription factor NF-kappaB is constitutively active in many human chronic inflammatory diseases and
cancers.
Epoxyquinone A monomer (EqM), a synthetic derivative of the
natural product epoxyquinol A, has previously been shown to be a potent inhibitor of
tumor necrosis factor-alpha (
TNF-alpha)-induced activation of
NF-kappaB, but the mechanism by which EqM inhibits
NF-kappaB activation was not known. In this report, we show that EqM blocks activation of
NF-kappaB by inhibiting two molecular targets:
IkappaB kinase IKKbeta and
NF-kappaB subunit p65. EqM inhibits
TNF-alpha-induced
IkappaBalpha phosphorylation and degradation by targeting IKKbeta, and an
alanine substitution for Cys179 in the activation loop of IKKbeta makes it resistant to EqM-mediated inhibition. EqM also directly inhibits
DNA binding by p65, but not p50; moreover, replacement of Cys38 in p65 with Ser abolishes EqM-mediated inhibition of
DNA binding. Pretreatment of cells with
reducing agent dithiothreitol dose-dependently reduces EqM-mediated inhibition of
NF-kappaB, further suggesting that EqM directly modifies the
thiol group of Cys residues in
protein targets. Modifications of the exocyclic
alkene of EqM substantially reduce EqM's ability to inhibit
NF-kappaB activation. In the human SUDHL-4
lymphoma cell line, EqM inhibits both proliferation and
NF-kappaB DNA binding, and activates
caspase-3 activity. EqM also effectively inhibits the growth of human
leukemia, kidney, and
colon cancer cell lines in the NCI's
tumor cell panel. Among six
colon cancer cell lines, those with low amounts of constitutive
NF-kappaB DNA-binding activity are generally more sensitive to growth inhibition by EqM. Taken together, these results suggest that EqM inhibits growth and induces cell death in
tumor cells through a mechanism that involves inhibition of
NF-kappaB activity at multiple steps in the signaling pathway.