Cyathula prostrata inhibits in vitro cancer cell growth via multiple targets

The in vitro anticancer activity of an 80% ethanol extract of Cyathula prostrata (L.) Blume, an annual branching shrub used by traditional healers in Nigeria to treat cancer was investigated. IC₅₀ values were 100.8µg/ml and 64.4µg/ml for HeLa (cervical cancer) and U937 (myelo-monocytic) cell lines, respectively. Further experiments were performed using 125µg/ml C. prostrata extract and 50µM cisplatin as positive control. More than 80% of the cells were arrested in the G1 phase after 48 hours of C. prostrata treatment. The annexin V-FITC/PI assay revealed an increase in percentage apoptotic cells from 4.9% to 53.1% at 24h. Cell cycle arrest was not accompanied by increased levels of the cyclin-CDK inhibitor p21. Increase in caspase-8 activation was observed in response to treatment with the extract with no cyt-c release from the mitochondria. The lack of cyt-c release was due to no change in mitochondrial membrane potential, which was investigated with the aid of fluorescent mitochondrial dyes and flow cytometric techniques. The results therefore show that C. prostrata extract induces apoptosis via the extrinsic pathway and this activation is independent of the mitochondria. Levels of hTERT, the catalytic subunit of telomerase, were also shown to decrease upon C. prostrata treatment. The findings from this study suggest that the extract acts through multiple targets, by inducing: cell cycle arrest in the G1 phase through an unknown mechanism; apoptosis through an extrinsic death receptor pathway and replicative senescence through inhibition of telomerase.

Keywords: Cyathula prostrata apoptosis, caspase 8, telomerase, cell cycle arrest

Acknowledgement: This work was funded by the African Laser Centre and the National Research Foundation, South Africa.