Bad as a converging signaling molecule between survival PI3-K/Akt and death JNK in neurons after transient focal cerebral ischemia in rats

Bad, a proapoptotic Bcl- 2 family protein, plays a critical role in determining cell death/ survival. The phosphatidylinositol 3- kinase ( PI3- K)/ Akt pathway and the c- Jun N- terminal kinase ( JNK) pathway are thought to be involved in regulation of Bad. Therefore, the present study was performed to clarify the role of Bad as a common target of the PI3- K/ Akt and JNK pathways after transient focal cerebral ischemia ( tFCI) in rats. We found that Akt activity increased at 3 h and then decreased, whereas JNK activity increased 7 to 24 h in the peripheral area after tFCI. Administration of LY294002, a PI3- K- specific inhibitor, exacerbated DNA fragmentation, whereas administration of SP600125, a JNK- specific inhibitor, attenuated it. Inhibited by LY294002, phospho- Bad ( Ser136) expression increased in the peripheral area 3 h after tFCI, with suppression of Akt activity. Furthermore, phospho- Bad ( Ser136) and phospho- Akt ( Ser473) were colocalized. Decreases in phospho- Bad ( Ser136) and Bad/ 14- 3- 3 dimerization and increases in Bcl- XL/ Bad or Bcl- 2/ Bad dimerization observed 7 to 24 h after tFCI, were prevented by SP600125 administration, with inhibition of JNK activity. The present study indicates that signal predominance varies from PI3- K/ Akt- mediated survival signaling to JNK- mediated death signaling with the development of neuronal damage in the peripheral area after tFCI. This study also suggests that PI3- K/ Akt has a role in Bad inactivation, whereas the JNK pathway is involved in Bad activation. We conclude that Bad may be an integrated checkpoint of PI3- K/ Akt- mediated survival signaling and JNK- mediated death signaling and that it contributes to cell fate in the peripheral area after cerebral ischemia.