1. Cinnamaldehyde has been shown to be effective in inducing cell apoptosis in a number of human cancer cells. The aim of the present study was to investigate the effect of vitamin E on the apoptotic signalling mechanism induced by cinnamaldehyde in human hepatoma PLC/PRF/5 cells.

2. Using the XTT assay, cinnamaldehyde exhibited a powerful antiproliferative effect on PLC/PRF/5 cells. Apoptosis was elicited when cells were treated with 1 µmol/L cinnamaldehyde, as characterized by the appearance of phosphatidylserine on the outer surface of the plasma membrane.

3. The apoptotic effect induced by cinnamaldehyde could be further supported by the release of cytochrome c, Smac/Diablo and Omi/HtrA2 from mitochondria to the cytosol and activation of caspase 3. Cinnamaldehyde also upregulated the expression of pro‐apoptotic protein (Bax) and down‐regulated the levels of anti‐apoptotic proteins, such as Bcl‐2 and the inhibitor of apoptosis protein family (X‐linked inhibitor of apoptosis protein (XIAP), cellular inhibitor of apoptosis protein (cIAP)‐1 and cIAP‐2).

4. Cinnamaldehyde induces the generation of reactive oxygen species (ROS) in cells. Following the pre‐incubation of PLC/PRF/5 cells with anti‐oxidants, it was found that 100 µmol/L vitamin E significantly diminished the effect of cinnamaldehyde‐induced apoptosis, whereas a lesser effect was seen with on 100 µmol/L N‐acetyl‐l‐cysteine. Vitamin E effectively blocked the release of cytochrome c, Smac/Diablo and Omi/HtrA2 from mitochondria to the cytosol in cells treated with cinnamaldehyde. Vitamin E also markedly suppressed caspase 3 activation. The expression of apoptotic inhibitors (XIAP, cIAP‐1, cIAP‐2) and anti‐apoptotic (Bcl‐2) and pro‐apoptotic (Bax) proteins was affected by vitamin E pretreatment.

5. Taken together, the results suggest that cinnamaldehyde triggers apoptosis possibly through the mitochondrial pathway. Pretreatment with vitamin E markedly prevented cinnamaldehyde‐mediated apoptosis, which was associated with the modulation of XIAP, cIAP‐1, cIAP‐2, Bcl‐2 and Bax protein activity.