ROLE OF THE JUN KINASE PATHWAY IN THE REGULATION OF C-JUN EXPRESSION AND APOPTOSIS IN SYMPATHETIC NEURONS

When deprived of nerve growth factor (NGF), developing sympathetic neu rons die by apoptosis. This death is associated with an increase in th e level of c-Jun protein and is blocked by expression of a c-Jun domin ant negative mutant, Here we have investigated whether NGF withdrawal activates Jun kinases, a family of stress-activated protein kinases th at can stimulate the transcriptional activity of c-Jun by phosphorylat ing serines 63 and 73 in the transactivation domain and which can acti vate c-jun gene expression, We found that sympathetic neurons containe d high basal levels of Jun kinase activity that increased further afte r NGF deprivation. In contrast, p38 kinase, another stress-activated p rotein kinase that can also stimulate c-jun gene expression, was not a ctivated after NGF withdrawal, Consistent with Jun kinase activation, we found using a phospho-c-Jun-specific antibody that c-Jun was phosph orylated on serine 63 after NGF withdrawal. Furthermore, expression of a constitutively active form of MEK kinase 1 (MEKK1), which strongly activates the Jun kinase pathway, increased c-Jun protein levels and c -Jun phosphorylation and induced apoptosis in the presence of NGF, Thi s death could be prevented by cc-expression of SEKAL, a dominant negat ive mutant of SAPK/ERK kinase 1 (SEK1), an activator of Jun kinase tha t is a target of MEKK1, In contrast, expression of SEKAL alone did not prevent c-Jun expression, increases in c-Jun phosphorylation, or cell death after NGF withdrawal, Thus, activation of Jun kinase and increa ses in c-Jun phosphorylation and c-Jun protein levels occur at the sam e time after NGF withdrawal, but c-Jun levels and phosphorylation are regulated by an SEK1-independent pathway.