Humulone, a bitter
acid derived from hop (Humulus lupulus L.), possesses antioxidative, anti-inflammatory and other biologically active activities. Although
humulone has been reported to inhibit chemically induced mouse skin
tumor promotion, the underlying mechanisms are yet to be elucidated. Since an inappropriate over-expression of
cyclooxygenase-2 (COX-2) is implicated in
carcinogenesis, we investigated effects of
humulone on COX-2 expression in mouse skin stimulated with the
tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Topical application of
humulone (10 mumol) significantly inhibited TPA-induced epidermal COX-2 expression.
Humulone also diminished TPA-induced
DNA binding of
nuclear factor-kappaB (
NF-kappaB) and
activator protein-1 (AP-1). Pre-treatment with
humulone attenuated TPA-induced phosphorylation of p65 and nuclear translocation of
NF-kappaB subunit
proteins.
Humulone blunted TPA-induced activation of inhibitory kappaB (
IkappaB) kinase (IKK) in mouse skin, which accounts for its suppression of phosphorylation and subsequent degradation of
IkappaBalpha. An in vitro
kinase assay revealed that
humulone could directly inhibit the catalytic activity of IKKbeta.
Humulone suppressed the activation of
mitogen-activated protein kinases (MAPKs) in TPA-treated mouse skin. The roles of extracellular signal-regulated
protein kinase-1/2 and
p38 MAPK in TPA-induced activation of
NF-kappaB in mouse skin had been defined in our previous studies. The present study revealed that topical application of
SP600125, a pharmacological inhibitor of
c-Jun-N-terminal kinase (JNK), abrogated the activation of
AP-1 and the expression of COX-2 in TPA-treated mouse skin. Taken together,
humulone suppressed TPA-induced activation of
NF-kappaB and
AP-1 and subsequent expression of COX-2 by blocking upstream
kinases IKK and JNK, respectively, which may account for its antitumor-promoting effects on mouse skin
carcinogenesis.