Enhanced expression of amyloid precursor protein in response to dibutyryl cyclic AMP is not mediated by the transcription factor AP-2

Abstract

The gene for amyloid precursor protein (APP) is expressed almost ubiquitously, with high levels of mRNA being detected in brain. The basal expression level of the APP gene can be modulated by physiological stimuli, and in this report we demonstrate that the second messenger cyclic AMP can regulate APP mRNA through transcriptional mechanisms. Northern blot analysis showed a 1.8-fold increase in steady-state levels of APP mRNA when the neuroblastoma x glioma hybrid cell line NG108-15 was treated with dibutyryl cyclic AMP. Although the upstream sequences of the APP gene do not contain a canonical cyclic AMP response element, transient transfection assays in NG108-15 cells using different portions of the APP promoter showed an increase in reporter gene activity mediated by sequences located between -303 to -204 and -488 to -2991. Cotransfection assays carried out in HepG2 cells with AP-2, a cyclic AMP-regulated transcription factor, failed to activate the APP promoter through the AP-2 consensus sequence (GCCNNNCGG) located at position -205. Electrophoretic mobility shift analysis revealed that the AP-2 binding activity present in HeLa nuclear extracts fails to recognize the APP AP-2 consensus sequence. We conclude that increases in cyclic AMP levels can lead to an up-regulation of APP gene transcription through at least two different regions of the APP promoter. This increase does not involve the AP-2 consensus sequence present in the APP promoter located at position -205, and, moreover, this putative site is not recognized by the transcription factor AP-2.