Daidzein and
genistein are
isoflavones found in soybean.
Genistein is known to exhibit anticarcinogenic activities and inhibit
tyrosine kinase activity. However, the underlying molecular mechanisms of the chemopreventive activities of
daidzein and its metabolite,
equol, are not understood. Here we report that
equol inhibits 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced neoplastic transformation of JB6 P+ mouse epidermal cells by targeting the
MEK/ERK/p90RSK/
activator protein-1 signaling pathway. TPA-induced
neoplastic cell transformation was inhibited by
equol, but not
daidzein, at noncytotoxic concentrations in a dose-dependent manner.
Equol dose-dependently attenuated TPA-induced activation of
activator protein-1 and c-fos, whereas
daidzein did not exert any effect when tested at the same concentrations. The TPA-induced phosphorylation of ERK1/2, p90RSK, and Elk, but not
MEK or
c-Jun N-terminal kinase, was inhibited by
equol but not by
daidzein. In vitro
kinase assays revealed that
equol greatly inhibited MEK1, but not Raf1,
kinase activity, and an ex vivo
kinase assay also demonstrated that
equol suppressed TPA-induced MEK1
kinase activity in JB6 P+ cell lysates.
Equol dose-dependently inhibited neoplastic transformation of JB6 P+ cells induced by
epidermal growth factor or H-Ras. Both in vitro and ex vivo pull-down assays revealed that
equol directly bound with
glutathione S-transferase-MEK1 to inhibit MEK1 activity without competing with
ATP. These results suggested that the antitumor-promoting effect of
equol is due to the inhibition of cell transformation mainly by targeting a
MEK signaling pathway. These findings are the first to reveal a molecular basis for the anticancer action of
equol and may partially account for the reported chemopreventive effects of soybean.