The c-Jun N-terminal kinases (JNKs) are a subfamily of the mitogen-activated protein kinases (MAPKs). The JNKs are encoded by three separate genes (jnk1, jnk-2, and jnk3), which are spliced alternatively to create 10 JNK isoforms that are either p46 or p54 in size. In this study, we found that the p52 form of JNK emerged in human leukemia MOLT-4 or U937 cells following X-irradiation or heat treatment. The accumulation of p52 coincided with the reduction of p54 JNK. On the other hand, the amounts of p46 JNK did not change by X-irradiation. Induction of the p52 form of JNK also paralleled the appearance of the active form of caspase-3 and was suppressed by a caspase-specific inhibitor, Ac-DEVD-CHO, but not by Ac-YVAD-CHO. In vitro cleavage assays indicated that recombinant human JNK1beta2 and JNK2beta2 were cleaved by caspase-3, and that the mutation of aspartic acid at position 413 of JNK1beta2 or 410 of JNK2beta2 to alanine abolished the cleavage. Altogether, our results demonstrated that p54 JNKs, at least JNK1beta2 and JNK2beta2, were new selective targets of caspases in JNK splicing variants, and suggested that the p52 form could serve as a marker of apoptosis. (C) 2003 Elsevier Science (USA). All rights reserved.