Honokiol induces paraptosis and apoptosis and exhibits schedule-dependent synergy in combination with imatinib in human leukemia cells

Honokiol, an active component isolated and purified from Chinese traditional herb magnolia, has been shown to inhibit growth and induce apoptosis in different cancer cell lines. This study shows that honokiol can induce a cell death distinct from apoptosis at lower concentrations. The death was characterized by cytoplasmic vacuolization with the endoplasmic reticulum swelling and accompanied by apoptosis at higher concentrations in NB4 and K562 cells. The two death processes may be in sequence at lower concentrations and in parallel with the increase of honokiol concentration. Membrane-associated cytotoxicity was involved in honokiol-induced paraptosis and apoptosis. Furthermore, honokiol inhibited concentration-dependent cell adhesion to extracellular matrix for NB4 cells. In addition, the cytotoxicity of honokiol combined treatment with imatinib was schedule- and concentration-dependent and the sequential administration of honokiol before imatinib appeared to be more beneficial in K562 cells. Taken together, the data suggest that honokiol induced a novel cell death pathway and there was cross-talk between apoptotic and non-apoptotic programmed cell death caused by honokiol in leukemia cells. Moreover, honikiol exhibited schedule- dependent synergy in combination with imatinib and sequential administration of imatinib followed by honokiol could be the optimal sequence to combine these two drugs in K562 cells.