A single c-Jun N-terminal kinase isoform (JNK3-p54) is an effector in both neuronal differentiation and cell death.

The c-Jun N-terminal kinases (JNKs) mediate degeneration and apoptosis in the brain. Particularly, JNK3 is considered to be a degenerative enzyme with c-Jun as a relevant substrate. The contribution of individual JNK isoforms, however, to pathological as well as to physiological processes remains to be defined. To analyze the effects of a single JNK isoform on neuronal cell death and differentiation, we transfected PC12 cells, which normally express only JNK1 and JNK2, with JNK3-p54. Transfected JNK3 significantly enhanced cell death after UV irradiation (0.5-6 J/cm(2)) and paclitaxel/taxol treatment (1-10 microm). In contrast, in the context of nerve growth factor-induced (10 or 50 ng/ml) differentiation of PC12 cells, JNK3 expression significantly increased the number and length of neurites. This functional dichotomy of JNK3 was mirrored by differential activation and induction of nuclear JNK substrates; although activating transcription factor-2 phosphorylation was enhanced by death signaling in response to UV and taxol, c-Jun protein expression and N-terminal phosphorylation were increased during nerve growth factor-induced differentiation. The absence of significant JNK activation or target phosphorylation in response to H(2)O(2) (60 microm) further supports the hypothesis that JNK isoforms are not merely injury- or stress-specific kinases but also have context-specific physiological functions.