Purpose: This study was undertaken to determine the efficacy of honokiol, a constituent of oriental medicinal herb Magnolia officinalis, against human prostate cancer cells in culture and in vivo.

Experimental design: Honokiol-mediated apoptosis was assessed by analysis of cytoplasmic histone-associated DNA fragmentation. Knockdown of Bax and Bak proteins was achieved by transient transfection using siRNA. Honokiol was administered by oral gavage to male nude mice s.c. implanted with PC-3 cells. Tumor sections from control and honokiol-treated mice were examined for apoptotic bodies (terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling assay), proliferation index (proliferating cell nuclear antigen staining), and neovascularization (CD31 staining). Levels of Bcl-2 family proteins in cell lysates and tumor supernatants were determined by immunoblotting.

Results: Exposure of human prostate cancer cells (PC-3, LNCaP, and C4-2) to honokiol resulted in apoptotic DNA fragmentation in a concentration- and time-dependent manner irrespective of their androgen responsiveness or p53 status. Honokiol-induced apoptosis correlated with induction of Bax, Bak, and Bad and a decrease in Bcl-xL and Mcl-1 protein levels. Transient transfection of PC-3 cells with Bak- and Bax-targeted siRNAs and Bcl-xL plasmid conferred partial yet significant protection against honokiol-induced apoptosis. Oral gavage of 2 mg honokiol/mouse (thrice a week) significantly retarded growth of PC-3 xenografts without causing weight loss. Tumors from honokiol-treated mice exhibited markedly higher count of apoptotic bodies and reduced proliferation index and neovascularization compared with control tumors.

Conclusion: Our data suggest that honokiol, which is used in traditional oriental medicine for the treatment of various ailments, may be an attractive agent for treatment and/or prevention of human prostate cancers.