Solanum nigrum L. (SNL) has been used in folk medicine for its anti-inflammatory activity. We isolated only the SNL glycoprotein from SNL and found that it was cytotoxic at low concentration. With respect to cytotoxicity, we investigated whether purified SNL glycoprotein is able to regulate protein kinase C (PKC) alpha activation and nuclear factor (NF)- kappaB activities in 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced tumor promotion, and whether it has an apoptosis-inducing effect in MCF-7 cells using western blot analysis. In addition, to elucidate the relationship between PKCalpha and NF-kappaB, inhibitory studies were performed with staurosporine (an inhibitor of phospholipid/calcium-dependent protein kinase) and pyrrolidine dithiocarbamate (an inhibitor of NF-kappaB activation). To verify induction of apoptosis by the SNL glycoprotein, we performed DNA fragmentation and nuclear staining assays using ethidium bromide and bisbenzamide H33342. The results in this study indicated that SNL glycoprotein induces apoptosis through modulation of PKCalpha and NF-kappaB activity in MCF-7 cells. In fact, SNL glycoprotein interfered with PKCalpha membrane translocation and inhibited NF-kappaB (p50) protein activity in MCF-7 cells stimulated with TPA (61.68 ng/mL, 100 nM) dose-dependently. Regarding the apoptotic-inducing effect, nucleosomal DNA fragmentation and nuclear staining by SNL glycoprotein in MCF-7 cells were shown. Collectively, the data demonstrate that SNL glycoprotein is a potential natural anticancer agent because of its ability to induce apoptosis in MCF-7 cells.