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In order to illustrate the mechanism of cell cycle arrest and apoptosis induced by botanical insecticide, the effects of cell proliferation inhibition and apoptosis induction by azadirachtin A were investigated in Spodoptera litura cell line Sl-1 in vitro. The cell proliferation, cell cycle division, mitotic and apoptotic figure of nucleus, and the expression of related proteins were detected by a variety of methods, including MTT assay, flow cytometry, fluorescence microscope observation, and western blot technique. Sl-1 cells grew in a dose-dependent inhibition manner and the proliferation inhibition rates were 18.95%, 33.45% and 41.00%, after 48 h of treatment with azadirachtin A at the concentrations of 0.1 mu g/mL, 0.5 mu g/mL and 1.0 mu g/mL, respectively. Sl-1 cell cycle was arrested in G2/M phase with lower expression of cyclin B1. The apoptosis rates were found to be 19.71%, 23.40% and 24.91% after 48 h of treatment with 0.1, 0.5, and 1.0 mu g/mL of azadirachtin A. which increased with the activities of caspase-3. The up-regulation of p53 was observed before apoptosis and cell cycle arrest occurred. These findings suggest that p53 protein was involved in cell cycle arrest, apoptosis induction and cell proliferation inhibition when the insect cultured cells were treated with azadirachtin A. (C) 2010 Elsevier Inc. All rights reserved.