Phorbol ester activation of the neuronal nicotinic acetylcholine receptor alpha 7 sub-unit gene: Involvement of transcription factor Egr-1

alpha-Bungarotoxin-sensitive neuronal nicotinic acetylcholine receptors fro m bovine adrenomedullary chromaffin cells are up-regulated by long-term exp osure to phorbol esters. The rise in receptor density is paralleled by an i ncrease in transcripts corresponding to the alpha 7 subunit, which is a com ponent of this receptor subtype. Transcriptional activation of the alpha 7 subunit gene is evidenced in reporter gene transfection experiments, in whi ch phorbol esters increase alpha 7 promoter activity by up to 14-fold. Abou t 80% of this activation is abolished when at least two of the three sites for the immediate-early transcription factor Egr-1, present in the proximal promoter region of the alpha 7 subunit gene, are mutated simultaneously. I n addition, phorbol esters elevate both Egr-1 mRNA and Egr-1 protein levels in chromaffin cells, whereas electrophoretic mobility shift assays show th at the Egr-1 component of the complexes that originate at the alpha 7 promo ter increases in cells treated with phorbol esters. These results suggest t hat the transcription factor Egr-1 is involved in triggering expression of alpha-bungarotoxin-sensitive nicotinic receptors in response to external st imuli, such as the ones resulting from phorbol ester treatment, and support our previous hypothesis that the alpha 7 subunit gene is one of the specif ic targets for Egr-1.