Abstract: Tannic acid (TA) possesses anti-tumor activity toward a broad range of tumor cells. However, it is hard for tannic acid to enter cells across the plasma membrane based on its chemical structure. How the signal of tannic acid is transduced into cells across the plasma membrane remains unclear. In the present study, we used human malignant glioma U87 cells, in which the epidermal growth factor receptor (EGFR) and its downstream signaling are over-activated, as a model to clarify the role of tannic acid in cell proliferation, EGFR phosphorylation, extracellular signal-regulated kinase (ERK) and signal transducer and activator transcription (STAT) 1/3 signaling activation. The results showed that tannic acid treatment obviously inhibited the proliferation of human glioma U87 cells in a time- and dose-dependent manner. Tannic acid dramatically inhibited the expression of B-cell lymphoma-2 (Bcl-2) protein but promoted the expression of Bcl-2 associated X (Bax) protein, thus resulting in a significant enhancement of Bax/Bcl-2 ratio in human glioma U87 cells. Furthermore, we found that tannic acid enhanced the level of Bax in the mitochondria and promoted the release of cytochrome c (Cyt c) to the cytoplasm, implicating the initiation of cell apoptosis. These results confirm the inhibitory role of tannic acid in cell proliferation. Further investigations revealed that tannic acid significantly inhibited the whole level of EGF-stimulated EGFR phosphorylation in human glioma U87 cells. By detecting the phosporylation of individual tyrosine sites of EGFR, we uncovered that tannic acid specifically weakened the phosphorylation of EGFR at Y1045 and Y1068, but had no effect at Y845 and Y992, indicating a inhibitory role of tannic acid in site-specific EGFR phosphorylation. Finally, tannic acid also blocked the phosphorylation and activation of ERK and STAT1/3, both of which were regulated by the phosphorylation of EGFR at Y1045 and Y1068. In summary, tannic acid inhibited site-specific phosphorylation of EGFR in human glioma U87 cells. As a consequence, the phosphorylation and activation of ERK and STAT1/3 were also inhibited by tannic acid, thereby accounting for the inhibition of the proliferation of human glioma U87 cells by tannic acid and laying a theoretical foundation for the anti-tumor activity tannic acid.

Key words: tannic acid, epidermal growth factor receptor, phosphorylation, U87 cells, anti-tumor activity