Mohannad E Qazzaz , Vijay J. Raja, Daniel Roth, Natacha Olieric, Andrea E. Prota, Kuan-Hon Lim, Toh-Seok Kam, Anne Straube, Michel O. Steinmetz and Tracey D. Bradshaw
Natural products play a pivotal role in medicine especially in the cancer arena. Many drugs that are currently used in cancer chemotherapy originated from or were inspired by nature. Jerantinine B (JB) is one of seven novel Aspidosperma indole alkaloids isolated from the leaf extract of Tabernaemontana corymbosa. Preliminary antiproliferative assays revealed that JB significantly inhibited growth and colony formation in human cancer cell lines. Flow cytometric assays were then employed to determine whether JB affected cell cycle progression. Stark cell cycle perturbation revealed i) profound G2/M cell cycle arrest (inferring inhibition of microtubule dynamics); ii) emergence of pre-G1 populations (indicative of apoptosis induction). Indeed, 1 μM JB comprehensively inhibited tubulin polymerisation; in addition microtubule dynamicity was dose-dependently inhibited (IC50 0.385 μM). Confocal microscopy exposed multipolar spindles within cells undergoing failed mitoses and apoptotic morphology following treatment with JB. Dual annexin-V / PI staining, dose-dependent accumulation of cleaved-PARP and caspase 3/7 activation, in addition reduced Bcl-2 and Mcl-1 expression confirmed dose- and time-dependent apoptosis. Furthermore, JB provoked significant increases in reactive oxygen species (ROS). Polo-like kinase 1 (PLK1; an early trigger for the G2/M transition) was also dose-dependently inhibited by JB (IC50 1.5μM). Crystal structure studies confirmed that JB acetate binds to the colchicine binding site of tubulin. Such promising data advocate further preclinical evaluation, in vivo testing and development of JB as a potential chemotherapeutic agent.
Keywords: Natural products, Cell cycle, Tubulin, PLK1, Reactive oxygen species, Apoptosis, Crystal structure,