Cyathula prostrata ethanol extract activates the extrinsic pathway of apoptosis in HeLa and U937 cell lines

Cyathula prostrata is a medicinal plant that is used in tropical Africa, Asia and Australia to treat many ailments including rheumatic fever, dysentery, wounds and eye trouble (Burkill, 1995). Sowemimo et al. (2009) reported cytotoxicity against a cervical cancer cell line, HeLa, at a concentration of 250 μg/mL. Due to the reported cytotoxicity, the mode of cell death caused by an ethanolic C. prostrata extract was investigated. Dose-response assays were carried out using HeLa and U937 cell lines and IC50 values of 100.8 μg/mL and 64.43 μg/mL, respectively, were achieved. Cisplatin was used as a positive control for all experiments. Cytotoxicity of the plant extract was not evident when treating normal peripheral blood mononuclear cells. Progression of cells through the cell cycle, apoptosis and mitochondrial membrane potential was investigated. Arrest of cells in the G0/G1 phase of the cell cycle was evident after 24 h of exposure to the plant extract in both cell lines, but not due to increases in p21 levels. Caspase 8 activation was evident and no depolarisation of the mitochondrial membrane and cytochrome c release was seen in both cell lines. Phosphatidylserine translocation was investigated and confirmed the onset of apoptosis. Thus, it is deduced that exclusive activation of the extrinsic pathway of apoptosis is caused by the treatment of HeLa and U937 cells with ethanolic C. prostrata.