Induction of apoptosis and cellular senescence in mice lacking transcription elongation factor, Elongin A.

Elongin A is a transcription elongation factor that increases the overall rate of mRNA chain elongation by RNA polymerase II. To gain more insight into the physiological functions of Elongin A, we generated Elongin A-deficient mice. Elongin A homozygous mutant (Elongin A(-/-)) embryos demonstrated a severely retarded development and died at between days 10.5 and 12.5 of gestation, most likely due to extensive apoptosis. Moreover, mouse embryonic fibroblasts (MEFs) derived from Elongin A(-/-) embryos exhibited not only increased apoptosis but also senescence-like growth defects accompanied by the activation of p38 MAPK and p53. Knockdown of Elongin A in MEFs by RNA interference also dramatically induced the senescent phenotype. A study using inhibitors of p38 MAPK and p53 and the generation of Elongin A-deficient mice with p53-null background suggests that both the p38 MAPK and p53 pathways are responsible for the induction of senescence-like phenotypes, whereas additional signaling pathways appear to be involved in the mediation of apoptosis in Elongin A(-/-) cells. Taken together, our results suggest that Elongin A is required for the transcription of genes essential for early embryonic development and downregulation of its activity is tightly associated with cellular senescence.

Cyclin A/cdk2 activation is involved in hypoxia-induced apoptosis in cardiomyocytes.

Cardiomyocytes are terminally differentiated cells characterized as withdrawal cell-cycle machinery, but nonetheless they are known to express cell-cycle regulators. Because many proteins related to the cell cycle induce apoptosis in proliferating cells, we examined the involvement of these proteins in the apoptosis pathway in cardiomyocytes. Primary rat cardiomyocytes were exposed to a severe hypoxic condition to induce apoptosis. The apoptosis rate of cardiomyocytes increased to approximately 40% under 24 hours of hypoxia as evaluated by the TUNEL method. The cyclin A protein level assessed by immunoblot analysis accumulated in a time-dependent manner in cardiomyocytes, but there was no increase in nonmyocytes. Hypoxia increased the activity of cyclin A-associated kinase but not the activity of cyclin E-associated kinase, and the apoptosis was inhibited by infection of dominant-negative cdk2 adenovirus, suggesting that cyclin A and its associated kinase play significant roles in the apoptosis of cardiomyocytes. To investigate the cyclin A-mediated apoptosis, we infected cultured cells with cyclin A adenovirus. Apoptosis was induced in 63+/-12% of the infected cardiomyocytes in contrast to only 12+/-3% of the LacZ-infected control cells. In addition, the cells in the hypoxic condition showed an increase in caspase-3 activity and a subsequent decrease in p21(cip1/waf1) protein, which is partly cleaved by caspase-3. These findings confirm that cyclin A-associated kinase mediates hypoxia-induced apoptosis in cardiomyocytes, and they also suggest that additional elements of the cell-cycle-dependent machinery participate in this mechanism.