The present study was undertaken to gain insights into the mechanism of cell cycle arrest caused by
honokiol, a constituent of oriental herb Magnolia officinalis. The
honokiol treatment decreased the viability of PC-3 and LNCaP human
prostate cancer cells in a concentration- and time-dependent manner, which correlated with G0-G1 phase cell cycle arrest. The
honokiol-mediated cell cycle arrest was associated with a decrease in
protein levels of
cyclin D1,
cyclin-dependent kinase 4 (Cdk4), Cdk6, and/or
cyclin E and suppression of complex formation between
cyclin D1 and Cdk4 as revealed by immunoprecipitation using anti-
cyclin D1 antibody followed by immunoblotting for
Cdk4 protein. The
honokiol-treated PC-3 and LNCaP cells exhibited a marked decrease in the levels of total and phosphorylated
retinoblastoma protein (Rb), which correlated with the suppression of transcriptional activity of E2F1. Exposure of PC-3 and LNCaP cells to
honokiol resulted in the induction of p21 (PC-3 and LNCaP) and p53
protein expression (LNCaP). However,
small interfering RNA (
siRNA)-mediated knockdown of either p21 (PC-3 and LNCaP) or p53 (LNCaP)
protein failed to confer any protection against
honokiol-induced cell cycle arrest. The
honokiol treatment caused the generation of
reactive oxygen species (ROS), and the cell cycle arrest caused by
honokiol was partially but significantly attenuated in the presence of
antioxidant N-acetylcysteine. In conclusion, the present study reveals that the
honokiol-mediated G0-G1 phase cell cycle arrest in human
prostate cancer cells is associated with the suppression of
protein level/phosphorylation of Rb leading to inhibition of transcriptional activity of E2F1.