Effects of EGCG on activation of the IGF/IGF-1R system in human hepatoma cells Free

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The insulin like growth factor (IGF)/IGF-1 receptor (IGF-1R) system, which includes IGF, IGF-1R, and IGF binding proteins (IGFBPs), plays an important role in the development of human hepatoma. We previously reported that in human colon cancer cell lines EGCG, the major biologically active component of green tea, inhibits activation of the receptor tyrosine kinases (RTKs) EGFR, HER2, and HER3, and that this is associated with inhibition of multiple downstream signaling pathways (Shimizu, M., et al. Clin Cancer Res 11, 2005). Since IGF-1R is also a RTK, in this study we examined the effects of EGCG on the activity of IGF/IGF-1R system in human hepatoma cells. We found that the level of phosphorylated (i.e. activated) form of the IGF-1R protein (p-IGF-1R) was increased in a series of human hepatoma cell lines when compared with the Hc normal human hepatocyte cell line. EGCG preferentially inhibited the growth of human hepatoma cell line HepG2 cells when compared with Hc cells. Treatment of HepG2 cells with EGCG caused a decrease in the p-IGF-1R protein and its downstream signaling molecules including the p-ERK, p-Akt, p-Stat-3, and p-GSK-3b proteins, both in the absence or presence of ligand stimulation. EGCG also caused a decrease in the levels of both IGF-1 and IGF-2 proteins, but caused an increase in the levels of the IGFBP-3 protein by ELISA assays. Semiquantitative RT-PCR studies indicated EGCG caused a decrease in the levels of IGF-1 and IGF-2 mRNAs in HepG2 cells. These findings suggest that EGCG can overcome the stimulatory effects of IGFs on the IGF-1R dependent signaling pathway, thus expanding the roles of EGCG as an inhibitor of critical RTKs involved in hepatoma cell proliferation. These results provide further evidence that EGCG may be useful in the chemoprevention or treatment of liver cancer.