Oncogenic Ras induces autophagy and caspase independent cell death through MKK7/ JNK mediated upregulation of Atg5

초록

To prevent development of malignancies, mammalian cells activate cellular disposal programs such as programmed cell death in response to deregulated oncogene expression. However, the molecular basis for the regulation of cellular disposal machinery in response to activated oncogene remains unclear. In this study, we showed that oncogenic H-Ras (H-RasV12) induces autophagic vacuole formation and caspase-independent cell death in Rat2 fibroblasts, and that MKK7/JNK-mediated upregulation of autophagy-related protein, Atg5 is associated with oncogenic Ras-induced autophagy and cell death. Pretreatment of Bafilomycin A1, an autophagy inhibitor, completely attenuates oncogenic Ras-induced caspase-independent cell death as well as autophagy. Selective induction of Atg5 expression among Atg family members was observed in cells overexpressing oncogenic Ras. siRNA targeting of Atg5 clearly attenuated caspase-independent cell death as well as autophagic vacuole formation in response to oncogenic Ras. Oncogenic Ras also markedly induced activation of MAP kinases. siRNA targeting of JNK or c-Jun attenuated oncogenic H-Ras-induced upregulation of Atg5, autophagic vacuole formation and cell death, but inhibition of ERK or p38 MAPK did not. JNK activation kinase MKK7, but not MKK4, was selectively activated in response to oncogenic H-Ras, and inhibition of MKK7 effectively attenuated JNK activation, autophagic vacuole formation and cell death induced by oncogenic H-Ras. These data support a notion that activation of MKK7/JNK signals upregulation of Atg5 in response to oncogenic Ras expression, resulting in induction of autophagy and subsequent caspase-independent cell death. Our findings suggest that in cells acquiring an apoptotic defects oncogenic stress results instead in induction of autophagy, which protects malignant progression.