Ionizing radiation utilizes c-Jun N-terminal kinase for amplification of mitochondrial apoptotic cell death in human cervical cancer cells

Abstract

Exposure of cells to ionizing radiation induces activation of multiple signaling pathways that play a critical role in controlling cell death. However, the basis for linkage between signaling pathways and the cell-death machinery in response to ionizing radiation remains unclear. Here we demonstrate that activation of c-Jun N-terminal kinase (JNK) is critical for amplification of mitochondrial cell death in human cervical cancer cells. Exposure of HeLa cells to radiation induced loss of mitochondrial membrane potential, release of cytochrome c and apoptosis inducing factor (AIF) from mitochondria, and apoptotic cell death. Radiation also induced transcriptional upregulation of Fas, caspase-8 activation, Bax and Bak activation, and phosphorylation and downregulation of Bcl-2. Inhibition of caspase-8 attenuated Bax and Bak activation, but did not affect phosphorylation and downregulation of Bcl-2. Expression of a mutant form of Bcl-2 (S70A-Bcl-2) completely attenuated radiation-induced Bcl-2 downregulation. Interestingly, inhibition of JNK clearly attenuated radiation-induced Bax and Bak activation, and Bcl-2 phosphorylation as well as Fas expression. In addition, dominant-negative form of c-Jun inhibited radiation-induced Fas expression and Bax and Bak activation. These results indicate that the JNK-c-Jun pathway is required for the transcriptional upregulation of Fas and subsequent activation of Bax and Bak, and that JNK, but not c-Jun, is directly associated with phosphorylation and downregulation of Bcl-2 in response to ionizing radiation. These results suggest that ionizing radiation can utilize JNK for amplification of mitochondrial apoptotic cell death in human cervical cancer cells.