INVOLVEMENT OF JUND IN TRANSCRIPTIONAL ACTIVATION OF THE ORPHAN RECEPTOR GENE NUR77 BY NERVE GROWTH-FACTOR AND MEMBRANE DEPOLARIZATION IN PC12 CELLS

nur77, an immediate early gene that encodes an orphan nuclear receptor , is rapidly and transiently induced by nerve growth factor (NGF) stim ulation or membrane depolarization in the rat pheochromocytoma-derived cell line PC12. The Nur77 protein can act as a potent transcription a ctivator and may function to regulate the expression of downstream gen es in response to extracellular stimuli. We show here that activation of nur77 by NGF treatment and membrane depolarization is signalled thr ough distinct pathways. These distinct signals appear to converge on t he-same transcription factors acting on the same promoter elements. We show that nur77 activation by both processes requires two cis-acting AP1-like elements, NAP1 and NAP2, which contain the core sequence TGCG TCA centered at 67 and 38 nucleotides upstream of the transcription st art site. The NAP elements can confer inducibility by NGF and membrane depolarization on an otherwise unresponsive heterologous promoter. We identified JunD as a key mediator of nur77 activation by reason of th e following observations. (i) JunD, but not CREB or other members of t he Fos/Jun family, is a component of NAP binding activity in PC12 cell nuclear extracts. (ii) JunD, but not other Fos/Jun family members, sp ecifically transactivates the nur77 promoter through the NAP elements (iii) A dominant-negative mutant of JunD effectively abolishes the act ivation of nur77 by either NGF treatment or membrane depolarization. T hese data draw a contrast between the regulation of nur77 with that of c-fos, in which the sequence requirements for activation by NGF treat ment and membrane depolarization appear separable, and CREB appears to play a role in activation by both NGF and membrane depolarization.