Basal cancer cell survival involves JNK2 suppression of a novel JNK1/c-Jun/Bcl-3 apoptotic network.
BACKGROUND
The regulation of apoptosis under basal (non-stress) conditions is crucial for normal mammalian development and also for normal cellular turnover in different tissues throughout life. Deficient regulation of basal apoptosis, or its perturbation, can result in impaired development and/or disease states including cancer. In contrast to stress-induced apoptosis the regulation of apoptosis under basal conditions is poorly understood. To address this issue we have compared basal- and stress-induced apoptosis in human epithelial cells of normal and cancerous origins. For this purpose we focussed our study on the opposing pro-apoptotic JNK/anti-apoptotic NFkappaB pathways.


RESULTS
Combinatorial RNAi plus gene knockout were employed to access and map basal regulatory pathways of apoptosis. Follow-on, in-depth analyses included exogenous expression of phosphorylation mutants and chromatin immunoprecipitation. We demonstrate that basal apoptosis is constitutively suppressed by JNK2 in a range of human cancer cell lines. This effect was not observed in non-cancer cells. Silencing JNK2 by RNAi resulted in JNK1-dependent apoptosis of cancer cells via up-regulation of the AP-1 factor c-Jun. Unexpectedly we discovered that JNK1 and c-Jun promote basal apoptosis in the absence of "activating phosphorylations" typically induced by stress. Hypo-phosphorylated c-Jun accumulated to high levels following JNK2 silencing, auto-regulated its own expression and suppressed expression of Bcl-3, an unusual IkappaB protein and regulator of NFkappaB. Basal apoptosis was mediated by components of the TNFalpha response pathway but was mechanistically distinct from TNFalpha-induced apoptosis.


CONCLUSIONS
Our results demonstrate that mechanistically distinct pathways operate to regulate apoptosis in mammalian cells under basal (physiological) versus stress-induced conditions. We also describe a novel apoptotic network which governs the basal survival of cancer cells. Such information is crucial for understanding normal cellular turnover during mammalian development and subsequently throughout life. This information also opens new avenues for therapeutic intervention in human proliferative disease states including cancer.
Ahmed
Shafiq Uddin
SU
YCR P53 Research Unit, Department of Biology, University of York, York, United Kingdom.
Milner
Jo
J
D017209:Apoptosis; D000081242:B-Cell Lymphoma 3 Protein; D045744:Cell Line, Tumor; D002470:Cell Survival; D015972:Gene Expression Regulation, Neoplastic; D020868:Gene Silencing; D006801:Humans; D048055:Mitogen-Activated Protein Kinase 8 / Q000378:metabolism*; D048056:Mitogen-Activated Protein Kinase 9 / Q000378:metabolism*; D016328:NF-kappa B / Q000378:metabolism; D009369:Neoplasms / Q000378:metabolism; D010766:Phosphorylation; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D016755:Proto-Oncogene Proteins c-jun / Q000378:metabolism*; D034622:RNA Interference; D014157:Transcription Factors / Q000378:metabolism*; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism
D000081242:B-Cell Lymphoma 3 Protein; C000631344:BCL3 protein, human; D016328:NF-kappa B; D011518:Proto-Oncogene Proteins; D016755:Proto-Oncogene Proteins c-jun; D014157:Transcription Factors; D014409:Tumor Necrosis Factor-alpha; D048056:Mitogen-Activated Protein Kinase 9; D048055:Mitogen-Activated Protein Kinase 8