Esculetin, a phenolic compound, has been shown to inhibit the growth of colon tumors in animal studies. However, the roles of signaling pathways and cell cycle regulation in the esculetin-induced inhibition of cancer cell growth, remain to be elucidated. The present study suggests a novel mechanism for the Ras/ERK1/2 pathway in esculetin-treated human colon cancer HCT116 cells. The treatment of cells with esculetin resulted in significant growth inhibition and G1 phase cell cycle arrest, which led to the down-regulation of cyclin and cyclin-dependent kinase (CDK) expressions. This G1 phase cell cycle arrest was associated with the up-regulation of p27KIP expression. In addition, ERK1/2 was activated by esculetin. The pre-treatment of cells with the MEK1/2-specific inhibitor, PD98059, blocked the p27KIP expression induced by esculetin. Blockage of the ERK1/2 function consistently prevented the inhibition of cell proliferation and decreased G1 phase cell cycle protein levels. Furthermore, Ras activation was increased by the esculetin treatment. Transient transfection of the dominant negative Ras (RasN17) mutant gene abolished both the ERK1/2 activity and p27KIP expression induced by esculetin. Finally, the overexpression of RasN17 suppressed the esculetin-induced reduction in cell proliferation and cell cycle proteins. In conclusion, these results indicate that the Ras/ERK1/2 pathway is mediated by the p27KIP1 induction, leading to a reduction in cyclin/CDK complexes in the esculetin-induced inhibition of colon cancer cell growth. Overall, these findings indicate that the molecular action of esculetin has therapeutic potential for the treatment of colon malignancies.

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