The protein p73 is a structural and functional homologue of the p53 tumour-suppressor protein but, unlike p53, it is not induced in response to DNA damage. The tyrosine kinase c-Abl is activated by certain DNA-damaging agents and contributes to the induction of programmed cell death (apoptosis) by p53-dependent and p53-independent mechanisms. Here we show that c-Abl binds to p73 in cells, interacting through its SH3 domain with the carboxy-terminal homo-oligomerization domain of p73. c-Abl phosphorylates p73 on a tyrosine residue at position 99 both in vitro and in cells that have been exposed to ionizing radiation. Our results show that c-Abl stimulates p73-mediated transactivation and apoptosis. This regulation of p73 by c-Abl in response to DNA damage is also demonstrated by a failure of ionizing-radiation-induced apoptosis after disruption of the c-Abl-p73 interaction. These findings show that p73 is regulated by a c-Abl-dependent mechanism and that p73 participates in the apoptotic response to DNA damage.

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Duct-to-duct reconstruction with continuous suture combined with an external stent represents a useful technique for LDLT utilizing the right lobe, but biliary complications remain significant.

Activation of the c-Abl protein tyrosine kinase by certain DNA-damaging agents contributes to downregulation of Cdk2 and G 1 arrest by a p53-dependent mechanism. The present work investigates the potential role of c-Abl in apoptosis induced by DNA damage. Transient transfection studies with wild-type, but not kinase-inactive, c-Abl demonstrate induction of apoptosis. Cells that stably express inactive c-Abl exhibit resistance to ionizing radiation-induced loss of clonogenic survival and apoptosis. Cells null for c-abl are also impaired in the apoptotic response to ionizing radiation. We further show that cells deficient in p53 undergo apoptosis in response to expression of c-Abl and exhibit decreases in radiation-induced apoptosis when expressing inactive c-Abl. These findings suggest that c-Abl kinase regulates DNA damage-induced apoptosis.

The c-Abl protein tyrosine kinase is activated by ionizing radiation (IR) and certain other DNA-damaging agents. The present studies demonstrate that c-Abl associates constitutively with protein kinase C d (PKCd). The results show that the SH3 domain of c-Abl interacts directly with PKCd. c-Abl phosphorylates and activates PKCd in vitro. We also show that IR treatment of cells is associated with c-Abl-dependent phosphorylation of PKCd and translocation of PKCd to the nucleus. These ®ndings support a functional interaction between c-Abl and PKCd in the cellular response to genotoxic stress.

The c-Abl nonreceptor tyrosine kinase is activated in cells exposed to ionizing radiation (IR) 1 and certain other DNAdamaging agents (1-4). IR induces DNA double-strand breaks (5) and thereby activates the DNA-dependent protein kinase (DNA-PK) (6 -8). Recent work has shown that DNA-PK phosphorylates and activates c-Abl (9). Other studies have demonstrated that c-Abl interacts with the ataxia telangiectasia mutated (ATM) gene product and that ATM may activate c-Abl in the response to genotoxic stress (10, 11). Whereas cells deficient in DNA-PK or ATM are hypersensitive to killing by IR (12, 13), c-Abl-deficient cells are resistant to IR-induced apoptosis (14). Activation of c-Abl by genotoxic stress is associated with interaction of c-Abl with the p53 tumor suppressor in the G 1 arrest response (15,16). Other signals dependent on c-Abl activation include induction of the stress-activated protein kinase and p38 mitogen-activated protein kinase by genotoxic agents (1,2,17). The findings that c-Abl contributes to the regulation of p53 and certain stress-induced kinases associated with apoptosis have provided support for the activation of c-Abl as a pro-apoptotic signal (14). In this context, expression of c-Abl is associated with G 1 phase growth arrest and induction of apoptosis (14,18,19).Recombination plays a fundamental role in the repair of DNA damage. In Escherichia coli, the RecA protein mediates repair of double-strand breaks by initiating pairing and strand exchange between homologous DNAs (20). Identification of structural homologs of RecA in yeast, Xenopus laevis, mouse, and human cells has supported conservation of similar repair functions throughout evolution (21-25). ScRad51, the RecA homolog in Saccharomyces cerevisiae, is required for DNA damage-induced mitotic recombination (21). ScRad51 converts DNA double-strand breaks to recombinational intermediates, and rad51 mutants accumulate these breaks during meiosis (21). The finding that human Rad51 (HsRad51) promotes homologous pairing and strand exchange reactions in vitro has suggested that Rad51 may also play a role in recombinational repair in man (26). Whereas yeast deficient in Rad51 are viable (21), targeted disruption of the rad51 gene in mice results in an embryonic lethal phenotype (27,28). These findings in rad51 Ϫ/Ϫ mice have suggested that mammalian Rad51 has an essential role in cell proliferation and/or maintenance of genomic stability.The present studies demonstrate that c-Abl associates with Rad51. We show that c-Abl phosphorylates Rad51 on Tyr-54 in vitro and in irradiated cells. Importantly, phosphorylation of Rad51 by c-Abl inhibits Rad51 function in DNA strand exchange assays. MATERIALS AND METHODSCell Culture-U-937 cells, HeLa cells, 293 embryonal kidney cells, and mouse embryo fibroblasts (Abl Ϫ/Ϫ , Abl ϩ ) (29) were grown as described (1). Irradiation was performed using a Gammacell 1000 (Atomic Energy of Canada) with a 137 Cs source emitting at a fixed dose of 0.21 gray min Ϫ1 as determined by dosimetry. Immunoprecipitations ...

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