The HGF/c-Met pathway is an important regulator of signaling pathways responsible for invasion and
metastasis of most human
cancers, including
prostate cancer. Exposure of DU145 prostate
tumor cells to HGF stimulates the
PI3-kinase and MAPK pathways, leading to increased scattering, motility, and invasion, which was prevented by the addition of EGCG. EGCG acted at the level of preventing phosphorylation of tyrosines 1234/1235 in the
kinase domain of the c-Met receptor without effecting dimerization. HGF-induced changes were independent of the formation of
reactive oxygen species, suggesting that EGCG functioned independent of its
antioxidant ability. ECG, another
tea polyphenol, was as effective as EGCG, while EGC and EC were less effective. EGCG added up to 4 h after the addition of HGF still blocked cell scattering and reduced the HGF-induced phosphorylation of c-Met, Akt, and Erk, suggesting that EGCG could act both by preventing activation of c-Met by HGF and by attenuating the activity of pathways already induced by HGF. HGF did not activate the MAPK and PI3-K pathways in cells treated with
methyl-beta-cyclodextrin (mCD) to remove
cholesterol. Furthermore, subcellular fractionation approaches demonstrated that only phosphorylated c-Met accumulated in
Triton X-100 membrane insoluble fractions, supporting a role for
lipid rafts in regulating c-Met signaling. Finally, EGCG treatment inhibited
DiIC16 incorporation into
membrane lipid ordered domains, and
cholesterol partially inhibited the EGCG effects on signaling. Together, these results suggest that
green tea polyphenols with the R1 galloyl group prevent activation of the c-Met receptor by altering the structure or function of
lipid rafts.