TRANSCRIPTIONAL ACTIVATION OF THE INDUCIBLE NUCLEAR RECEPTOR GENE NUR77 BY NERVE GROWTH-FACTOR AND MEMBRANE DEPOLARIZATION IN PC12 CELLS

nur77 is an immediate-early gene inducible by nerve growth factor or membrane depolarization in the rat pheochromocytoma cell line PC12 and by serum growth factors in fibroblasts. The nur77-encoded protein is a member of the steroid/thyroid hormone receptor superfamily and can act as a potent transcription activator. The induction of nur77 in PC12 cells is rapid and transient, with kinetics similar to those of the c-fos protooncogene. Induction does not require de novo protein synthesis. Whereas transcriptional activation of c-fos by nerve growth factor in PC12 cells requires a 20-base pair serum response element in its promoter, there is no such sequence in the nur77 promoter. To understand the mechanism for the activation of nur77, we have analyzed the inducibility of a series of transfected nur77 minigenes using an S1 nuclease protection assay. We identified the sequence 22-86 nucleotides upstream of the transcription start site as necessary and sufficient for nur77 induction by nerve growth factor and membrane depolarization in PC12 cells. Sequences farther upstream enhance the induction. Analysis of base substitution mutations allowed us to identify three sequence elements within this region that are essential for induction. These sequence elements include two copies of an AP1-like element and a GC-rich sequence. Unlike transcriptional activation of c-fos, the sequence requirements for the activation of nur77 by nerve growth factor and membrane depolarization cannot be readily separated. Taken together, our data suggest that activation of nur77 and c-fos by nerve growth factor occurs through different mechanisms in PC12 cells.