Cinnamaldehyde (Cin) has been shown to be effective in inducing apoptotic cell death in a number of human
cancer cells. The aim of this study was to investigate the effect of
pifithrin-alpha (PFTalpha; a specific p53 inhibitor) and
mitogen-activated protein kinases (MAPKs) inhibitors [namely
SP600125 (a specific JNK inhibitor),
SB203580 (a specific p38 inhibitor) and
PD98059 (a specific ERK inhibitor)] on apoptotic signaling transduction mechanism induced by Cin in human
hepatoma PLC/PRF/5 (CD95-negative) cells. Using XTT assay, Cin exhibited a powerful cytotoxic effect and apoptotic induction in PLC/PRF/5 cells. Apoptosis was elicited when cells were treated with 1 microM Cin as characterized by morphological changes and the appearance of
phosphatidylserine on the outer surface of the plasma membrane. Cin down-regulated the expression of Bcl-(XL), up-regulated mutant p53 and Bax
proteins and promoted
caspase-3 to active forms, as well as cleaving
poly (ADP-ribose) polymerase (PARP) in a time-dependent pattern. This could be supported by the activation and phosphorylation of MAPKs, including JNK, ERK and p38
kinases. Pre-incubation with PFTalpha and specific MAPK inhibitors significantly diminished the effect of Cin-induced apoptosis. The activities of anti-apoptotic (Bcl-(XL)) and pro-apoptotic (Bax)
proteins were remarkably affected by PFTalpha and
PD98059 pre-treatment. PFTalpha effectively blocked PARP cleavage in cells treated with Cin, and also markedly prevented the phosphorylation of JNK, p38 and ERK
proteins. These results suggest that p53 induction and MAPK signaling pathways are required for Cin-mediated apoptosis in PLC/PRF/5 cells.