Transient Activation Of JNK Research Articles

Hsp70 prevents activation of stress kinases. A novel pathway of cellular thermotolerance.

Harmful conditions including heat shock, oxidative stress, UV, and so forth cause programmed cell death, whose triggering requires activation of the Jun N-terminal kinase, JNK. High levels of Hsp72, a heat-inducible member of Hsp70 family, protect cells against a variety of stresses by a mechanism that is unclear at present. Here we report that elevated levels of Hsp72 inhibit a signal transduction pathway leading to programmed cell death by preventing stress-induced activation of JNK. Stress-induced activation of another stress-kinase, p38 (HOG1), is also blocked when the level of Hsp72 is increased. Similarly, addition of a purified recombinant Hsp72 to a crude cell lysate reduced p38 kinase activation, while depletion of the whole family of Hsp70 proteins with a monoclonal antibody enhanced such activation. In addition, we have found that accumulation of abnormal proteins in cells upon incubation with amino acid analogs causes activation of JNK and p38 kinases, which can be prevented by overproduction of Hsp72. Taken together, these data suggest that, in regulation of JNK and p38 kinases, Hsp70 serves as a "sensor" of the build-up of abnormal proteins after heat shock and other stresses. The inhibitory effect of an increased level of Hsp70 on JNK appears to be a major contributor to acquired thermotolerance in mammalian cells.

Tumor necrosis factor alpha stimulates AP-1 activity through prolonged activation of the c-Jun kinase

Tumor necrosis factor alpha (TNF alpha) has multiple biological functions including the prolonged activation of the collagenase and c-jun genes, which are regulated via their AP-1 binding sites. We show that incubating human fibroblasts with TNF alpha induces prolonged activation of JNK, the c-Jun kinase, which phosphorylates the transactivation domain of c-Jun. Furthermore, an immune complex kinase assay specifically demonstrates that TNF alpha stimulates the activity of JNK1, the recently described predominant form of JNK. TNF alpha also produces a small and transient increase in extracellular signal-regulated kinase (ERK) activity and no measured increase in Raf-1 kinase activity. On the other hand, epidermal growth factor causes a prolonged activation of Raf-1 kinase and ERK activity and a smaller, more transient activation of JNK, whereas the phorbol ester phorbol 12-myristate 13-acetate causes a small stimulation of Raf-1 kinase and a pronounced stimulation of ERK activity. The activation of JNK by TNF alpha does not correlate with Raf-1 or ERK activity. The kinetics of Raf-1, ERK, and JNK induction by epidermal growth factor, phorbol 12-myristate 13-acetate, or TNF alpha indicate distinct mechanisms of activation in human fibroblasts.