RESUMO
The mechanism(s) of c-Myc transcription factor-induced apoptosis is still obscure. The activation of c-Myc has been found to lead into the processing/activation of caspases (caspase-3), but the significance of this for the cell demise is debatable. Here we report that several targets of caspases (PKCdelta, MDM2, PARP, replication factor C, 70 kDa U1snRNP, fodrin and lamins) are cleaved during c-Myc-induced apoptosis in Rat-1 MycER cells, indicating an important role for caspases in the apoptotic process. We further found that the ATM (ataxia telangiectasia mutated)--protein is a novel key substrate of caspases. In in vitro assays, purified recombinant ATM protein was found to be cleaved by the effector caspases 3 and 7. The functional significance of the ATM cleavage is supported by the finding that ectopic expression of ATM protected in part against apoptosis. We also show that c-Myc-induced apoptosis involves loss of mitochondrial transmembrane potential, release of cytochrome c from mitochondria into the cytosol and subsequent processing of caspase-9. The cleavage of caspase-9 is, however, minimal and a much later event than the processing/activation of caspase-3, suggesting that it is not the apical caspase. Evidence is provided that there is, nevertheless, an upstream caspase(s) regulating the functions of caspase-3 and mitochondria. Additionally, it was found that p53 becomes upregulated, together with its transcriptional targets MDM2 and p21, upon c-Myc induction, but this occurs also at a later time than the activation of caspase-3.
Assuntos
Apoptose/genética , Caspases/metabolismo , Genes myc , Mitocôndrias/metabolismo , Proteínas Serina-Treonina Quinases/metabolismo , Clorometilcetonas de Aminoácidos/farmacologia , Animais , Proteínas Mutadas de Ataxia Telangiectasia , Proteínas de Transporte/metabolismo , Caspase 3 , Caspase 7 , Inibidores de Caspase , Proteínas de Ciclo Celular , Células Cultivadas , Inibidores de Cisteína Proteinase/farmacologia , Grupo dos Citocromos c/metabolismo , Proteínas de Ligação a DNA , Ativação Enzimática , Humanos , Isoenzimas/metabolismo , Laminas , Potenciais da Membrana , Proteínas dos Microfilamentos/metabolismo , Proteínas Nucleares/metabolismo , Oligopeptídeos/farmacologia , Poli(ADP-Ribose) Polimerases/metabolismo , Proteína Quinase C/metabolismo , Proteína Quinase C-delta , Proteínas Serina-Treonina Quinases/genética , Ratos , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , Proteína Supressora de Tumor p53/metabolismo , Proteínas Supressoras de TumorRESUMO
H-2 antigens on spleen cell membranes absorb antibody to H-2 antigens and induce both humoral and cellular responses. Liver cell membrane H-2 antigens by contrast also absorb antibody but do not influence cellular response and are tolerogenic for the humoral response. This paper demonstrates that syngeneic liver cells contain a substance which can transform the properties of allogeneic spleen cell membranes into those of allogeneic liver cell membranes, i.e., transform a humoral immunogen into a humoral tolerogen. The process appears to be accompanied by cleavage of an antigen component from the spleen membrane and hence to result in a structural change in the H-2 antigen.
