Apoptotic dell death through lipid kinase/PKB/AFX and stress signaling pathways

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PKB is a downstream target of PI 3-kinase in the growth factor-signaling pathway. It is proposed that AFX, a forkhead transcription factor, is phosphorylated by PKB, and that AFX localizes in cytosol while phosphorylated and is translocated into cell nucleus, when dephosphorylated, to induce apoptosis. We revealed that two residues of three phosphorylation consensus sites by PKB in AFX, such as Thr32, Ser197, and Ser262, are phosphorylated, and that Ser197 is dephosphorylated immediately after the removal of growth factors in cells by the analysis of point mutants using the phosphorylation site-specific antibodies. On the other hand, we had shown that PKB is activated by heat shock, and thus compared the activation mechanisms of PKB by growth factor and heat shock. It was shown that heat shock generates the active form of PKB in a manner independent of PI 3-kinase without phosphorylation of PKB, which is essential for growth factor-induced activation of the enzyme. It has been established that PKB is recruited to the plasma membrane from cytosol through its PH domain upon growth factor stimulation. In contrast, we revealed that PKB is accumulated in the perinuclear region by the association with Hsp27, a low molecular chaperon protein, in heat-shocked cells. Therefore, PKB is supposed to be activated by different mechanisms to prevent apoptosis by inhibiting the nuclear translocation of AFX. Furthermore, it was shown that PKC, having the catalytic domain highly homologous to that of PKB, is activated by stress signal and enhances apoptosis presumably through the regulation of some lipid metabolizing enzymes. It is necessary to analyze the targets of PKB and PKC in the regulation of stress-induced apoptosis.