Ceramide promotes apoptosis in lung cancer-derived A549 cells by a mechanism involving c-Jun NH2-terminal kinase

Ceramide regulates diverse signaling pathways involving cell senescence, the cell cycle, and apoptosis. Ceramide is known to potently activate a number of stress-regulated enzymes, including the c-Jun NH2-terminal kinase (JNK). Although ceramide promotes apoptosis in human lung cancer-derived A549 cells, a role for JNK in this process is unknown. Here, we report that ceramide promotes apoptosis in A549 cells by a mechanism involving JNK. The JNK inhibitor SP600125 proved effective at protecting cells from the lethal effects of ceramide. To understand which JNK-mediated pathway may be involved, a number of JNK target proteins were examined, including the transcription factor, c-jun, and the apoptotic regulatory proteins Bcl-X-L and Bim. A549 cells exhibited basal levels of phosphorylated c-jun in nuclear fractions, revealing that active c-Jun is present in these cells. Ceramide was found to inhibit c-jun phosphorylation, suggesting that JNK-mediated phosphorylation of c-jun is not likely involved in ceramide-induced apoptosis. Ceramide did not promote Bcl-X-L phosphorylation. On the other hand, ceramide promoted phosphorylation of Bim and induced translocation of active JNK from the nucleus to the cytoplasm and mitochondrial fraction. Ceramide-mediated changes in localization of JNK were consistent with the observed changes in phosphorylation status of c-Jun and Bim. Furthermore, ceramide promoted Bim translocation to the mitochondria. Mitochondrial localization of Bim has been shown recently to promote apoptosis. These results suggest that JNK may participate in ceramide-induced apoptosis in A549 cells by a mechanism involving Bim.