Interplay between Pur? and Egr-1 in the Transcriptional Regula-tion of Amyloid Precursor Protein Gene Expression

Background: One of the pathological hallmarks of Alzheimer’s disease is the presence of fibrillary amyloid-β deposits, which result from cleavage of the amyloid precursor protein. Understanding the regulatory mechanism of the amyloid precursor protein gene expression is crucial for comprehending the genesis and development of Alzheimer’s disease. The nucleic acid binding protein, Purα, is best characterized as a transcriptional factor (TF) with affinity to singlestrand G/C-rich regions. In a previous study, we demonstrated that the Purα protein can downregulate amyloid precursor protein (APP) promoter activity, but the mechanism underlying this downregulation requires further investigation. To better understand this mechanism, we analyzed the characteristic of the APP promoter and found that another transcriptional factor, namely Egr-1, can bind the APP promoter and may exert transcriptional regulatory effects on APP gene expression. Therefore, the interaction between these two transcriptional factors may explain the mechanism in regulating APP gene expression. Methodology/Principal Findings: The binding sites of Purα and Egr-1 on the APP promoter 5’-UTR were identified, and reporter plasmids in which the binding sites for Purα and Egr-1 were deleted have been constructed. A luciferase assay was performed, and the results demonstrated that both Purα and Egr-1 lost their regulatory effects when these binding sites were deleted. The luciferase results also demonstrated that Purα can suppress the effects of Egr-1 on APP promoter activities. The electrophoresis mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) assay results demonstrated that both Purα and Egr- 1 can competently bind to the APP promoter. The endogenous Egr-1 expression was disturbed with the HDAC inhibitor and suramin, and the Egr-1 expression level affected the APP promoter activities and APP gene expression. Purα can also suppress the endogenous expression of Egr-1. Conclusion/Significance: The mechanism through which Purα regulates AβPP gene expression may involve its interaction with Egr-1, which is a positive regulator of the APP promoter. Because both transcriptional factors possess the binding sites in the APP promoter 5’-UTR and the position of these sites are overlapped, there may exist a displacement mechanism for these two transcriptional factors. In addition, Purα also suppresses the endogenous Egr-1 expression. All of these findings explain the mechanism through which Purα regulates APP gene expression.