ROS-triggered caspase 2 activation and feedback amplification loop in β-carotene-induced apoptosis

Reactive oxygen species (ROS) and caspases 8, 9, and 3 are reported to be crucial players in apoptosis induced by various stimuli. Recently, caspase 2 has been implicated in stress-induced apoptosis but the exact mechanism remains unclear. In this study, we report that ROS generation led to activation of caspase 2 during β-carotene-induced apoptosis in the human leukemic T cell line Molt 4. The apoptosis progressed by simultaneous activation of caspases 8 and 9, and a cross talk between these initiator caspases was mediated by the proapoptotic protein Bid. Inhibition of caspases 2, 8, 9, and 3 independently suppressed the caspase cascade. The kinetics and function of caspase 2 were similar to those of caspase 3, suggesting its role as an effector caspase. Interestingly, β-carotene-induced apoptosis was caspase 2 dependent but caspase 3 independent. The study also revealed cleavage of the antiapoptotic protein BclXL as an important event during apoptosis, which was regulated by ROS. The mechanistic studies identify a functional link between ROS and the caspase cascade involving caspase 2 and cleavage of BclXL. The interdependence of caspases 8, 9, 2, and 3 in the cascade provides evidence for the presence of an extensive feedback amplification loop in β-carotene-induced apoptosis in Molt 4 cells.