Apigenin, a plant
flavone, potentially activates wild-type p53 and induces apoptosis in
cancer cells. We conducted detailed studies to understand its mechanism of action. Exposure of human
prostate cancer 22Rv1 cells, harboring wild-type p53, to growth-suppressive concentrations (10-80 microM) of
apigenin resulted in the stabilization of p53 by phosphorylation on critical
serine sites, p14ARF-mediated downregulation of
MDM2 protein, inhibition of
NF-kappaB/p65 transcriptional activity, and induction of p21/WAF-1 in a dose- and time-dependent manner.
Apigenin at these doses resulted in ROS generation, which was accompanied by rapid
glutathione depletion, disruption of mitochondrial membrane potential, cytosolic release of
cytochrome c, and apoptosis. Interestingly, we observed accumulation of a p53 fraction to the mitochondria, which was rapid and occurred between 1 and 3 h after
apigenin treatment. All these effects were significantly blocked by pretreatment of cells with the
antioxidant N-acetylcysteine, p53 inhibitor
pifithrin-alpha, and
enzyme catalase.
Apigenin-mediated p53 activation and apoptosis were further attenuated by p53
antisense oligonucleotide treatment. Exposure of cells to
apigenin led to a decrease in the levels of Bcl-XL and Bcl-2 and increase in Bax, triggering
caspase activation. Treatment with the
caspase inhibitors Z-VAD-FMK and
DEVD-CHO partially rescued these cells from
apigenin-induced apoptosis. In vivo,
apigenin administration demonstrated p53-mediated induction of apoptosis in 22Rv1
tumors. These results indicate that
apigenin-induced apoptosis in 22Rv1 cells is initiated by a ROS-dependent disruption of the mitochondrial membrane potential through transcriptional-dependent and -independent p53 pathways.