Lung cancer is the leading cause of cancer-related deaths. b-Escin, a triterpene saponin isolated from horse chestnut seeds, was tested for inhibition of lung adenoma and adenocarcinoma induced by the tobacco carcinogen 4-(methyl-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in female A/J mice; and its possible mode of action was evaluated using the H460 human lung cancer cell line. At 6 weeks of age, 35 mice were fed AIN-76A-modified diet, and one week later, lung tumors were induced with a single intraperitoneal (i.p.) injection of 10 mmol NNK/mouse. Three weeks after the NNK treatment, groups of mice were fed either control or experimental diets containing 500 ppm for 20 weeks (10 control, 5 b-escin) or 36 weeks (15 control, 5 b-escin) and evaluated for lung tumor via histopathologic methods. Administration of 500 ppm b-escin significantly suppressed lung tumor (adenoma þ adenocarcinoma) formation by more than 40% (P < 0.0015) at 20 weeks and by 53.3% (P < 0.0001) at 37 weeks. b-Escin inhibited NNK-induced lung adenocarcinoma formation by 65% (P < 0.001) at 20 weeks and by 53% (P < 0.0001) at 37 weeks. Immunohistochemical analysis revealed that lung tumors from mice exposed to b-escin showed significantly reduced aldehyde dehydrogenase (ALDH)1A1 and phospho-Akt (p-Akt) expression when compared with those in mice fed control diet. Aldefluor assay for ALDH revealed that among H460 lung cancer cells treated with different concentrations of b-escin (0-40 mmol/L), the subpopulation of cells with elevated ALDH activity was inhibited significantly. Our findings suggest that b-escin inhibits tobacco carcinogeninduced lung tumor formation by modulating ALDH1A1-positive cells and RhoA/Rock signaling. Cancer Prev Res; 6(10);