Characterization of the human insulin-induced gene 2 (INSIG2) promoter: the role of Ets-binding motifs.

Insulin-induced gene 2 (INSIG2) and its homolog INSIG1 encode closely related endoplasmic reticulum proteins that regulate the proteolytic activation of sterol regulatory element-binding proteins, transcription factors that activate the synthesis of cholesterol and fatty acids in animal cells. Several studies have been carried out to identify INSIG2 genetic variants associated with metabolic diseases. However, few data have been published regarding the regulation of INSIG2 gene expression. Two Insig2 transcripts have been described in rodents through the use of different promoters that produce different noncoding first exons that splice into a common second exon. Herein we report the cloning and characterization of the human INSIG2 promoter and the detection of an INSIG2-specific transcript homologous to the Insig2b mouse variant in human liver. Deletion analyses on 3 kb of 5'-flanking DNA of the human INSIG2 gene revealed the functional importance of a 350-bp region upstream of the transcription start site. Mutated analyses, chromatin immunoprecipitation assays, and RNA interference analyses unveiled the significance of an Ets-consensus motif in the proximal region and the interaction of the Ets family member SAP1a (serum response factor (SRF) accessory protein-1a) with this region of the human INSIG2 promoter. Moreover, our findings suggest that insulin activated the human INSIG2 promoter in a process mediated by phosphorylated SAP1a. Overall, these results map the functional elements in the human INSIG2 promoter sequence and suggest an unexpected regulation of INSIG2 gene expression in human liver.