Abstract #2985: Fisetin, a dietary flavonoid activates tumor suppressor LKB1 and inhibits Akt/ mTOR signaling in human non-small cell lung cancer cells

Lung cancer is the leading cause of cancer mortality worldwide. Despite recent advances in radiotherapy and chemotherapy modalities, the five-year survival rates have not improved substantially from the disease. One way to reduce the incidence of lung cancer is through chemoprevention. The use of naturally occurring or synthetic agents to prevent, inhibit or reverse lung carcinogenesis would therefore, greatly benefit public health. Fisetin (3,3',4',7-tetrahydroxyflavone), is found in fruits andvegetables, such as strawberry, apple, persimmon, grape, onionand cucumber. In the present study, we determined the effect of fisetin on the inhibition of cell-growth and suppression of Akt/mammalian target of rapamycin (mTOR) signaling in human non-small cell lung cancer (NSCLC) cells. Treatment of fisetin (5-20 \#956;M; 48 h) was found to result in a decrease in the viability of A549 and H1792 cells but had only minimal effect on normal human bronchial epithelial cells. Since A549 cells are LKB1-mutant, we selected H1792 cells for further studies. Fisetin treatment was found to reduce the formation of H1792 cell colonies in a dose-dependent manner. The Akt/mTOR pathway is considered a central regulator of protein synthesis, cell proliferation, differentiation and survival in lung cancer. Treatment of cells with fisetin caused decrease in the protein expression of PI3K (p85 and p110) and inhibition of the phosphorylation of Akt and mTOR. The mTOR integrates mitogenic signals and intracellular nutrient levels to activate eukaryotic initiation factor (eIF) 4E-binding protein-1 (4E-BP1) and the 40S ribosomal protein S6 kinase (p70S6K), which controls protein translation and cell cycle progression. Fisetin caused inhibition of the phosphorylation of p70S6K1, eIF-4E and 4E-BP1, downstream targets of mTOR in H1792 cells. We also found that fisetin treatment to cells dose-dependently inhibited the constituents of mTOR signaling complex such as Rictor, Raptor, G\#946;L and PRAS40. The LKB1 tumor suppressor is a serine/threonine kinase that is mutationally inactivated in the autosomal dominant Peutz-Jeghers syndrome as well as in sporadic lung adenocarcinomas. Fisetin treatment to cells caused activation of LKB1 as well as the phosphorylation of AMP-activated protein kinase (AMPK\#945;), both known to down-regulate mTOR. The activity of mTOR is negativelyregulated by tuberous sclerosis complex 1 and 2 (TSC1 and TSC2,also known as hamartin and tuberin). Akt phosphorylates TSC2 and disrupts the TSC1/TSC2complex, leading to activation of mTOR. Fisetin treatment to cells also caused dose-dependent activation of TSC2 in H1792 cells. Our results show that fisetin suppressed Akt/mTOR signaling and activated LKB1 in NSCLC cells.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 2985.