Platycodin D induces mitotic arrest in vitro, leading to endoreduplication, inhibition of proliferation and apoptosis in leukemia cells

Platycodin D (PD), a major constituent of triterpene saponins in Platycodon grandiflorum, has also become an interesting candidate for cancer chemotherapy; however, little is known about apoptotic mechanisms on cancer cells. We herein investigated the mechanisms that are related to PD-induced antiproliferation and cell death in human leukemia cells (U937, THP-1 and K562 cells). Cell growth was assessed with proliferation assays, cell counting, flow cytometry, phase contrast microscopy and Western blot assay. Microtubule (MT) formation was measured with immunofluorescent staining and in vitro tubulin polymerization assay. Apoptotic effect was analyzed by assessing increase in annexin V-staining and caspase-3 activity. Treatment of synchronized leukemia cells with varying concentrations of PD resulted in significant mitotic arrest and endoreduplication (END) via downregulation of Cdc2/cyclin 131 and upregulation of wee1 expression, and elevated the Cdk2 protein via downregulation of p21 within 48 hr. We also researched PD's induction of polyploidy through the MT polymerization. Immunofluorescent microscopy and Western blot analysis revealed that PD significantly caused MT polymerization in leukemia cells. We also found that very high concentrations of PD (>200 mu M) were required to directly induce MT polymerization in vitro. Finally, PD exposure induced apoptosis in U937 cells through caspase-3-dependent PARP and lamin A cleavage. We conclude that the primary antileukemia activity of PD is induction of endoreduplication and mitotic arrest, as a consequence of suppressing spindle MT dynamics and in promoting apoptosis in human leukemia cells. (C) 2008 Wiley-Liss, Inc.