Calpain activation is upstream of caspases in radiation-induced apoptosis

The molecular events involved in apoptosis induced by ionizing radiation re main 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 fo r 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 al so 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 demonst rate that calpain is activated within 15 min after radiation exposure, conc omitant 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.