There is an emerging evidence that plumbagin (5-hydroxy-2-methyl-1, 4-naphthoquinone) may have potential as a chemotherapeutic agent. However, the growth inhibitory mechanisms of plumbagin have remained unexplored. The aim of the study was to determine whether plumbagin-induced cell death in human cervical cancer cell line, ME-180, exhibited biochemical characteristics of apoptosis and to check whether N-acetyl-l-cysteine (NAC), which is a free radical scavenger, can reverse the cytotoxic effects of plumbagin. It can be concluded from the results that plumbagin inhibits the growth of ME-180 cells in a concentration and time-dependent manner. The cytotoxic effect of plumbagin induced cell death is through the generation of reactive oxygen species (ROS) and subsequent induction of apoptosis as demonstrated by the present data. Treatment of cells with plumbagin caused loss of mitochondrial membrane potential (DeltaPsi(m)), and morphological changes characteristic of apoptosis, such as the translocation of phosphatidyl serine, nuclear condensation, and DNA fragmentation. Moreover, plumbagin-induced apoptosis involved release of mitochondrial cytochrome c and apoptosis inducing factor (AIF), thus activation of caspase-dependent and -independent pathways, as shown by the plumbagin-mediated activation of caspase-3 and -9. Our results also show that pretreatment of ME-180 cells with NAC blocks plumbagin-induced loss of DeltaPsi(m) and subsequent release of cytochrome c, AIF, and caspase-9 and -3 activation, thus inhibiting the apoptotic ability of plumbagin.

Previous studies have shown that reduction in BRCA1 mRNA and protein can result in increased proliferation of BG-1 ovarian cancer cells in both in vitro and in vivo conditions, suggesting that BRCA1 may normally act as a growth inhibitor in these cells. Also, there are other reports that suggest that wild-type BRCA1 protein may repress estrogen receptor (ER) function either directly or indirectly. However, response to antiestrogen drugs in BRCA1-blocked ER-positive ovarian cancer cells has not been reported, and this served as the rationale for this study. We analyzed the effect of tamoxifen, emodin, and plumbagin in BRCA1-blocked ER-positive BG-1 ovarian cancer cells. For all three drugs, BRCA1-blocked cells were more sensitive than the corresponding control cells as assessed by MTT assay; however, only plumbagin showed a statistically significant difference in mean viability (P < 0.05). All three drugs induced loss of mitochondrial membrane potential (DeltaPsi(m)), nuclear condensation, DNA fragmentation, and morphological changes, as observed after 6 h of drug treatment, suggesting apoptosis induction in both BRCA1-blocked and control cells. However, apoptosis induction was greater in BRCA1-blocked cells, the efficacy being in the order of plumbagin > tamoxifen > emodin. The dose of plumbagin needed to kill 50% was 5 microM in the control cells and 2.68 microM for the BRCA1-blocked cells, indicating that the latter was about twofold more sensitive to plumbagin than the wild-type cells. This throws light on the fact that plumbagin may have chemotherapeutic potential as an anticancer agent in BRCA1-mutated ovarian cancer patients.