Abstract: Purpose: Epidermal growth factor receptor (EGFR)/protein kinase B (PKB, also named Akt) signaling pathway is over-expressed and abnormally activated in most human solid tumors. EGFR/Akt signaling activation has been known to promote cell proliferation, which is a key factor for tumor survival. Tannins are a class of plant polyphenols, which have been reported for their antitumor activity. However, it remains unclear how tannic acid regulates EGFR/Akt signaling pathway in tumors. Methods: In the present study, human malignant glioma U87 cells with over-activated EGFR/Akt signaling pathway were used as a model to clarify the mechanism underlying the inhibitory effect of tannic acid on cell proliferation and EGFR/Akt signaling pathway. Results: Tannic acid significantly inhibited the phosphorylation of Akt and its two downstream effector proteins, 4EBP-1 and S6, whose phosphorylation are important for protein translation and tumor cells proliferation. Tannic acid also suppressed the proliferation of human glioma U87 cells. Further investigation revealed that tannic acid could block the recruitment of TRAF6 to plasma membrane and decrease the amount of Akt directly interacted with TRAF6, leading to the inhibition of Akt phosphorylation in human glioma U87 cells. In order to validate the role of tannic acid in TRAF6 recruitment, we expressed a constitutively activated EGFR vIII mutant in human glioma U87 cells and found that the inhibition of tannic acid on EGFR phosphorylation, leading to the decrease of TRAF6 plasma membrane recruitment, required the existence of 2-7 exons of EGFR extracellular domain; and the inhibition of tannic acid in EGFR phosphorylation. Conclusion: EGFR is the receptor of tannic acid in human glioma U87 cells. Tannic acid manipulates TRAF6 recruitment and inhibits Akt activation, thereby executing the signal transduction of tannic acid across the cell plasma membrane into tumor cells. These results provide a possible mechanism for the potential anti-tumor therapy with food-derived tannin.

Key words: tannic acid (TA), tumor necrosis receptor-associated factor 6 (TRAF6), protein kinase B(Akt), recruitment, epidermal growth factor receptor (EGFR)