Fyn is a novel target of (−)‐epigallocatechin gallate in the inhibition of JB6 Cl41 cell transformation

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Abstract
The cancer preventive action of (−)‐epigallocatechin gallate (EGCG), found in green tea, is strongly supported by epidemiology and laboratory research data. However, the mechanism by which EGCG inhibits carcinogenesis and cell transformation is not clear. In this study, we report that EGCG suppressed epidermal growth factor (EGF)‐induced cell transformation in JB6 cells. We also found that EGCG inhibited EGF‐induced Fyn kinase activity and phosphorylation in vitro and in vivo. Fyn was implicated in the process because EGF‐induced JB6 cell transformation was inhibited by small interfering RNA (siRNA)‐Fyn‐JB6 cells. With an in vitro protein‐binding assay, we found that EGCG directly bound with the GST‐Fyn‐SH2 domain but not the GST‐Fyn‐SH3 domain. The Kd value for EGCG binding to the Fyn SH2 domain was 0.367 ± 0.122 µM and Bmax was 1.35 ± 0.128 nmol/mg. Compared with control JB6 Cl41 cells, EGF‐induced phosphorylation of p38 MAP kinase (p38 MAPK) (Thr180/Tyr182), ATF‐2 (Thr71) and signal transducer and activator of transcription 1 (STAT1) (Thr727) was decreased in siRNA‐Fyn‐JB6 cells. EGCG could inhibit the phosphorylation of p38 MAPK, ATF‐2, and STAT1. The DNA binding ability of AP‐1, STAT1, and ATF‐2 was also decreased in siRNA‐Fyn‐JB6 cells. Overall, these results demonstrated that EGCG interacted with Fyn and inhibited Fyn kinase activity and thereby regulated EGF‐induced cell transformation. Inhibition of Fyn kinase activity is a novel and important mechanism that may be involved in EGCG‐induced inhibition of cell transformation.