Phorbol ester activation of the neuronal nicotinic acetylcholine receptor alpha7 subunit gene: involvement of transcription factor Egr-1

Abstract

alpha-Bungarotoxin-sensitive neuronal nicotinic acetylcholine receptors from bovine adrenomedullary chromaffin cells are up-regulated by long-term exposure to phorbol esters. The rise in receptor density is paralleled by an increase in transcripts corresponding to the alpha7 subunit, which is a component of this receptor subtype. Transcriptional activation of the alpha7 subunit gene is evidenced in reporter gene transfection experiments, in which phorbol esters increase alpha7 promoter activity by up to 14-fold. About 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 alpha7 subunit gene, are mutated simultaneously. In addition, phorbol esters elevate both Egr-1 mRNA and Egr-1 protein levels in chromaffin cells, whereas electrophoretic mobility shift assays show that the Egr-1 component of the complexes that originate at the alpha7 promoter increases in cells treated with phorbol esters. These results suggest that the transcription factor Egr-1 is involved in triggering expression of alpha-bungarotoxin-sensitive nicotinic receptors in response to external stimuli, such as the ones resulting from phorbol ester treatment, and support our previous hypothesis that the alpha7 subunit gene is one of the specific targets for Egr-1.