Calpain activation is upstream of caspases in radiation-induced apoptosis

The molecular events involved in apoptosis induced by ionizing radiation remain unresolved, In this paper we show that the cleavage of fodrin to a 150 kDa fragment is an early proteolytic event in radiation-induced apoptosis in the Burkitts' Lymphoma cell line BL30A and requires 100 mu m zVAD-fmk for inhibition. Caspases-1, -3, -6 and -7 were shown to cleave fodrin to the 150 kDa fragment in vitro and all were inhibited by 10 mu M zVAD-fmk. We also show that the in vitro cleavage of fodrin by calpain is inhibited by 100 mu M zVAD-fmk as was the calpain-mediated hydrolysis of casein, We demonstrate that calpain is activated within 15 min after radiation exposure, concomitant with the cleavage of fodrin to the 150 kDa fragment whereas caspase-3 is activated at 2 h correlating with the cleavage of fod rin to the 120 kDa fragment. These results Support a role for calpain in the early phases of the radiation-induced apoptosis pathway, upstream of the caspases.