Role of the JNK Signal Transduction Pathway in Cell Survival: a Dissertation

The c-Jun NH2-terminal kinases (JNK) are evolutionarily conserved serine/threonine protein kinases that are activated by proinflammatory cytokines, environmental stress, and genotoxic agents. These kinases play key regulatory roles within a cell by coordinating signals from the cell surface to nuclear transcription factors. JNK phosphorylates the amino terminal domain of all three Jun transcription factors (JunB, c-Jun and JunD) all members of the AP-1 family. The activated transcription factors modulate gene expression to generate appropriate biological responses, including cell migration, proliferation, differentiation and cell death. The role of the JNK signaling pathway in cell death/apoptosis is controversial, both pro-apoptotic and pro-survival roles have been attributed to JNK. The mechanism that enables the JNK signaling pathway to mediate both apoptosis and survival is unclear. The aim of this study is to examine the role of TNF-stimulated JNK activation on cell survival. The proinflammatory cytokine TNF, is known to activate JNK and induce apoptosis. To test whether the JNK signaling pathway contributes to TNF-induced apoptosis, the response of wild type and Jnk1-/- Jnk2-/- (JNK deficient fibroblasts) fibroblasts to TNF was examined. JNK deficient fibroblasts are more sensitive to TNF-induced apoptosis than wild-type fibroblasts. The TNF-sensitivity cannot be attributed to altered expression of TNF receptors or defects in the NF-кB or AKT pathways, known anti-apoptotic signal transduction pathways. (In fact, TNF stimulated NF-кB activation provides a major mechanism to account for survival in both wild-type and JNK deficient cells.) However this increased TNF-sensitivity can be attributed to JNK deficiency. Apoptosis is suppressed in JNK deficient cells when transduced with JNK1 retrovirus. These data implicate the JNK signaling pathway in cell survival. The AP-1 family of transcription factors is a target of the JNK signal transduction pathway. In addition JNK is required for the normal expression of the AP-1 family member, JunD. Previous studies have indicated that JunD can mediate survival. Interestingly, JNK deficient and JunD null cells display similar phenotypes: premature senescence and increased sensitivity to TNF induced apoptosis. In fact, the TNF-sensitivity is also suppressed in JNK deficient fibroblasts transduced with JunD retrovirus. Although JunD can replace the survival signaling role of JNK, phosphorylation of JunD is essential to inhibit TNF induced apoptosis. JNK deficient cells transduced with phosphomutant JunD retrovirus maintain TNF-sensitivity. Activated transcription factors modulate gene expression. It is most likely that JunD functions by regulating the expression of key molecules that act to inhibit TNF-stimulated apoptosis. Microarray analysis comparing wild-type with JNK deficient fibroblasts revealed that the expression of the survival gene, cIAP-2, was induced by TNF in only wild-type fibroblasts. Furthermore, protein expression of cIAP-2 was induced by TNF in only wild-type fibroblasts. Analysis of the cIAP-2 promoter revealed two critical NF-кB binding sites and one AP-1 binding site. Luciferase reporter assays indicated key roles for both NF-кB and the AP-1 component, JunD in TNF-induced cIAP-2 gene expression. These experiments establish that the JNK/JunD pathway collaborates with NF-кB pathway to increase the expression of the anti-apoptotic protein cIAP-2 in TNF treated cells. Without this collaboration, the JNK pathway mediates apoptosis. The integration of JNK signaling with other signaling pathways represents a mechanism to account for the dual ability of the JNK pathway to mediate either survival or apoptosis. The dynamic coordination of signals within and between pathways is critical. The future challenge will be to fit the details of individual signaling pathways into the context of signaling networks