Vernonia amygdalina Extracts Inhibit Cancer Cell Growth by Disrupting Microtubule Assembly
Daniel Oyugi1,2 and Winston Anderson2
1Department of Natural Sciences, Mississippi Valley State University, Itta Bena, MS
2Department of Biology, Howard University, Washington DC
Vernonia amaygdalina (VA), one of the medicinally-important plants of Africa is considered the most used plant in the genus Vernonia. Previously we reported the in-vitro growth inhibition and anti-proliferative activities of VA extracts on cancer cells. In the present study, we examine whether VA elicits the aforementioned effects by targeting and disrupting cellular microtubule. Using immunocytochemical and fluorescence analyses, we probed the effects of VA fractions on microtubule assembly, disassembly and apoptosis in prostate (DU-145) and breast (MCF-7) cancer cell lines. Cell viability was tested using Calcein-AM Red Orange. Apoptosis was measured using Double Stain Apoptosis Detection Kit (Hoechst 33342 and Propidium Iodide (PI)). Our results indicate that organic and aqueous fractions of VA extracts abrogated the steady state-microtubule pattern into a disassembled form in DU-145. In MCF-7 cells, the fractions caused retraction, condensation and clustering of tubulin protofilaments into aggregates within the cytoplasm. Examination of cell structure and morphology revealed marked cell shrinkage, nuclear fragmentation, chromatin condensation, DNA fragmentation and formation of membrane blebs and apoptotic bodies. Further analysis of cell death by fluorescence staining indicated manifestation of condensed chromatin and nuclear fragmentation, confirming an apoptotic death, with greater quantities of apoptotic phenotypes observed in MCF-7 than in DU-145.Viability assay showed a dose-dependent reduction in viable cells, with petroleum ether and aqueous fractions exhibiting a higher reduction effect (IC50 61.02 µg/mL; 65.82 µg/mL) than methanol fraction (IC50 80.77 µg/mL) in MCF-7 cells. In DU-145 cells, methanol fraction exerted highest viability reduction (IC50 44.21µg/mL) than aqueous (IC50 131.7 µg/mL) and petroleum ether fractions (IC50 130.5 µg/mL). VA fractions induce microtubule disassembly in a fashion similar to Nocodazole, but different to Taxol. Taken together, these observations demonstrate that VA contains biologically active components capable of inhibiting growth and proliferation of cancer cells, exerting their properties via mechanisms that target and trigger disruption of microtubule organization, effectively causing apoptotic death. Key words: Vernonia amygdalina; anti-microtubule; apoptosis; cancer cells.