Activation of β-Catenin Signalling Increases StarD7 Gene Expression in JEG-3 Cells

StarD7 gene encodes a protein that belongs to the StAR-related lipid transfer proteins involved in intracellular transport and metabolism of lipids. It has been previously documented that StarD7 has a wide-spread mRNA expression in trophoblastic tissues and several tumour cell lines with highest levels in both choriocarcinoma JEG-3 and JAR cells, hepatocellular carcinoma HepG2, and colorectal adenocarcinoma HT-29 cells. To understand the molecular mechanisms that regulate the expression of the human StarD7 gene, we have cloned and characterized the 5′-flanking region of the gene. Transient transfections of several 5′deleted StarD7-promoter-firefly luciferase constructs into JEG-3 cells indicated that the −312/+157 region contains the gene minimal promoter. In addition, sequence analysis of a 1.6 kb gene fragment revealed the presence of a TATA-less promoter as well as multiple regulatory motifs, including one regulatory element corresponding to the T-cell factor 4 (TCF4) binding site. Inhibition of glycogen synthase kinase-3β (GSK3β), a component of Wnt/β-catenin signalling, increased both StarD7 mRNA and protein expression as well as its promoter activity. Co-transfection experiments in JEG-3 cell line revealed that the StarD7 promoter is activated by TCF4 transcription factor and by its β-catenin coactivator. Moreover, site-directed mutagenesis of the TCF4 site located −614/−608 bp relative to the transcription start site markedly diminished StarD7 promoter activity. Chromatin immunoprecipitation analysis demonstrated that β-catenin and TCF4 are bound in vivo to the StarD7 gene promoter in JEG-3 cells treated with lithium chloride. Collectively, these studies show that β-catenin and TCF4 activate the human StarD7 gene interacting with its promoter region through Wnt/β-catenin signalling.