caspase 3 recruiting contrasts with the caspase3 freedom of the pathway we identified, which, together with the established cell autonomy of the brand new pathway, argues against a role for DNA damage induced external signaling downstream of chk1 exhaustion. Certainly, the AO reactivity of p53,chk1MO,casp8MO zebrafish embryos didn’t change from that of p53,chk1MO types. Stopping Tipifarnib ic50 death receptor signaling with a fadd MO also failed to affect AO discoloration. Therefore, extrinsic signaling like mitochondrial signaling doesn’t appear to play a crucial role downstream of chk1 reduction. The only real caspase whose depletion blocked the Chk1 suppressed path was caspase 2, a poorly known however extremely preserved caspase with characteristics of both initiator and executioner caspases. In three separate experiments, p53,chk1MO,casp2MO1 embryos consistently showed a mean 6 fold decrease in AO labeling in contrast to p53,chk1MO embryos. casp2 MO1, which targets the splice donor site of intron 4, resulted in marked reductions in casp2 mRNA levels and to aberrant continuing transcripts lacking exon 4. A-second casp2 MO paid off IR caused demise in embryos, and a mismatch model of casp2 MO1 had no effect. Totally, these epistasis studies identify a book atm/atr casp2 apoptotic system as a key mechanism whereby Chk1 destruction radiosensitizes p53 mutant zebrafish embryos without recruiting the classical Urogenital pelvic malignancy mitochondrial and death receptor pathways. We next investigated if the DNA damage induced apoptotic pathway suppressed by Chk1 in zebrafish is conserved in human cancer cells defective in p53 signaling. To prevent Chk1 in these cells, we used the indolocarbazole small compound Go 6976, that has greater specificity compared to the commonly used Chk1 chemical UCN 01. In HeLa cells, caspase 2 bosom was readily apparent at 2-4 hpIR within the pres-ence of Go 6976. This effect was synergistic since neither IR nor Go 6976 alone caused substantial increases in cleaved caspase 2 levels compared to basal levels observed in get a handle on cells. Furthermore, caspase2 cleavage tightly correlated with a powerful radiosensitizing effect. By comparison, the levels of Doxorubicin ic50 cleaved caspase 3 in Go 6976 treated cells at 2-4 hpIR were negligible and did not differ from those observed in irradiated cells not confronted with the chemical. Moreover, equally caspase 2 cleavage and concomitant cellular radiosensitization were insensitive to overexpression of human BCL2, although caspase 3 cleavage was com-pletely eliminated in this context. Synergistic activation of caspase 2-by Go 6976 and IR did not generate or contain cytochrome c release from the mitochondria at 24 hpIR. Together, these studies show that Chk1 inhibition and IR synergize to activate caspase 2 and trigger BCL2 and mitochondria in-dependent cell death in p53 defective human cells, in keeping with our zebrafish information.