α-Tocopherol-mediated caspase-3 up-regulation enhances susceptibility to apoptotic stimuli

Although α-tocopherol is known as an essential micronutrient involved in various oxidative stress-related processes, its non-antioxidant activities have only been characterized in recent years. In this study, we reveal that (+)-α-tocopherol [RRR-α-tocopherol] enhances cellular susceptibility to both oxidative and non-oxidative apoptosis-inducing stimuli through up-regulation of caspase-3/CPP32 expression in several human cell lines. Exposure of (+)-α-tocopherol pretreated cells to known apoptosis-inducing stimuli, such as Fas, H₂O₂, or etoposide, resulted in an increase in cellular apoptotsis. In addition, (+)-α-tocopherol also elevated the pro-caspase-3 protein level and mRNA expression in a time- and dose-dependent manner, while other tocopherol analogues showed no effect. Experiments using a GC-specific DNA binding agent, mithramycin A, and an electrophoretic mobility shift assay demonstrated that Sp1 might mediate the enhanced expression of caspase-3. Our results also confirmed that (+)-α-tocopherol promotes the expression, but not the activation, of caspase-3 in various human cell lines. These findings provide biological evidence showing that (+)-α-tocopherol can amplify the apoptotic response by up-regulating the expression of pro-caspase-3.