Apoptosis executed by the mammalian caspase family plays a fundamental role in cellular homeostasis. Deregulation of this process is associated with several human diseases. The multimerization of ligand-induced death receptors results in the recruitment of the death inducing signaling complex and autocatalytic activation of initiator caspases, including caspase-8 and -10. However, it is still unclear how initiator caspases trigger and control the early apoptotic signaling pathways, partly because the downstream proteolytic cleavage targets of the initiator caspases are not completely known. Although it is known that a number of proteins are cleaved by various members of the caspase family, the identification of specific cleavage substrates of the initiator caspases 8 and 10, has been hindered by a lack of systematic and broadly applicable strategies for substrate identification. In the present study we constructed a mouse cDNA library and used it to perform a systematic, genomewide screen for novel in vitro substrates of caspase-8 and -10. From this, we successfully identified six putative caspase substrates, including five novel proteins (ABCF1, AKAP1, CPE, DOPEY1 and GOPC1) that may be targeted specifically by the initiator caspases 8 and 10 during the early stages of apoptosis. These findings may provide useful information for elucidating the apoptotic signaling pathways downstream of the death receptors.

Abstract. Low-dose radiation has a variety of effects on cellular activities, including the cell division cycle, apoptosis, proliferation and senescence. However, the effects of low doses of radiation remain controversial. In this study, we examined the effects of low-dose radiation on cellular senescence. We treated MCF7 cells with 0.01 μg/ml doxorubicin to induce replicative senescence, 2 h after exposure to low doses of ionizing radiation of 0.05, 0.1, or 0.2 Gy. The status of p53, senescence-associated ß-galactosidase activity, p38 kinase levels, H2AX levels and ERK/MAPK levels were examined. Low doses of ionizing radiation inhibit doxorubicininduced senescence in human breast cancer MCF7 cells. The phosphorylations of both p38 MAP kinase and p53 induced by doxorubicin were suppressed by low doses of ionizing radiation. The senescence was inhibited without genomic damage, because the level of Á-H2AX protein was not changed. Moreover, low doses of ionizing radiation inhibited senescence through the activation of ERK1/2. The results thus suggest that low doses of radiation suppress doxorubicin-induced replicative senescence through the inhibition of p38-dependent phosphorylation of p53 and by activation of ERK1/2, without genomic damage. Overall, our results suggest that low doses of ionizing radiation may have a protective role against replicative senescence induced by doxorubicin.