This study is the first to investigate the anticancer effect of plumbagin in human breast cancer cells. Plumbagin exhibited cell proliferation inhibition by inducing cells to undergo G 2 -M arrest and autophagic cell death. Blockade of the cell cycle was associated with increased p21/WAF1 expression and Chk2 activation, and reduced amounts of cyclin B1, cyclin A, Cdc2, and Cdc25C. Plumbagin also reduced Cdc2 function by increasing the association of p21/WAF1/Cdc2 complex and the levels of inactivated phospho-Cdc2 and phospho-Cdc25C by Chk2 activation. Plumbagin triggered autophagic cell death but not predominantly apoptosis. Pretreatment of cells with autophagy inhibitor bafilomycin suppressed plumbagin-mediated cell death. We also found that plumbagin inhibited survival signaling through the phosphatidylinositol 3-kinase/AKT signaling pathway by blocking the activation of AKT and downstream targets, including the mammalian target of rapamycin, forkhead transcription factors, and glycogen synthase kinase 3B. Phosphorylation of both of mammalian target of rapamycin downstream targets, p70 ribosomal protein S6 kinase and 4E-BP1, was also diminished. Overexpression of AKT by AKT cDNA transfection decreased plumbagin-mediated autophagic cell death, whereas reduction of AKT expression by small interfering RNA potentiated the effect of plumbagin, supporting the inhibition of AKT being beneficial to autophagy. Furthermore, suppression of AKT by plumbagin enhanced the activation of Chk2, resulting in increased inactive phosphorylation of Cdc25C and Cdc2. Further investigation revealed that plumbagin inhibition of cell growth was also evident in a nude mouse model. Taken together, these results imply a critical role for AKT inhibition in plumbagininduced G 2 -M arrest and autophagy of human breast cancer cells.

This study first investigates the anticancer effect of plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone) in human nonsmall cell lung cancer cells, A549. Plumbagin has exhibited effective cell growth inhibition by inducing cancer cells to undergo G 2 /M phase arrest and apoptosis. Blockade of cell cycle was associated with increased levels of p21 and reduced amounts of cyclinB1, Cdc2, and Cdc25C. Plumbagin treatment also enhanced the levels of inactivated phosphorylated Cdc2 and Cdc25C. Blockade of p53 activity by dominant-negative p53 transfection partially decreased plumbagin-induced apoptosis and G 2 /M arrest, suggesting it might be operated by p53-dependent and independent pathway. Plumbagin treatment triggered the mitochondrial apoptotic pathway indicated by a change in Bax/Bcl-2 ratios, resulting in mitochondrial membrane potential loss, cytochrome c release, and caspase-9 activation. We also found that c-Jun NH 2 -terminal kinase (JNK) is a critical mediator in plumbagin-induced cell growth inhibition. Activation of JNK by plumbagin phosphorylated p53 at serine 15, resulting in increased stability of p53 by decreasing p53 and MDM2 interaction. SP600125 (anthra [1,9-cd]pyrazol-6(2H)-one-1,9-pyrazoloanthrone), a specific inhibitor of JNK, significantly decreased apoptosis by inhibiting the phosphorylation of p53 (serine 15) and subsequently increased the interaction of p53 and MDM2. SP6000125 also inhibited the phosphorylation of Bcl-2 (Ser70) induced by plumbagin. Further investigation revealed that plumbagin's inhibition of cell growth effect was also evident in a nude mice model. Taken together, these results suggest a critical role for JNK and p53 in plumbagin-induced G 2 /M arrest and apoptosis of human nonsmall cell lung cancer cells.Lung cancer is one of the leading causes of death in the world, and nonsmall cell lung carcinoma accounts for approximately 75 to 85% of all lung cancers (Raez and Lilenbaum, 2004). Nonsmall cell lung cancers commonly develop resistance to radiation and chemotherapy and often present at stages too late for surgical intervention. Since current treatment modalities are inadequate, novel therapies are needed to reduce the effects of the increasing incidence in pulmonary neoplasm (Raez and Lilenbaum, 2004;Kelly, 2005).The tumor suppressor protein p53 is targeted by a wide variety of intracellular and extracellular stimuli, such as withdrawal of growth factors, hypoxia, irradiation, chemicals, and defects in nucleotide synthesis (Harris and Levine, 2005). The activation of p53 leads, primarily through its transcriptional function, to either apoptosis, eliminating those cells harboring severely damaged DNA, or growth arrest, allowing damaged DNA to be repaired and thereby suppressing tumor formation (Harris Robles et al., 2002;Levine, 2005). Stability and activity of p53 are believed to be regulated in part by posttranslational modifications, such as phosphorylation and acetylation.

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