Characterization of the role of Sp1 and NF-Y in differential regulation of PTTG/securin expression in tumor cells

Abstract

Pituitary tumor transforming gene (PTTG), also known as securin, is a regulator of cell division that is overexpressed in many tumors. Its expression is cell cycle regulated, although its transcriptional regulation is yet to be determined. The 5' RACE analysis of the human testis mRNA revealed the existence of a previously unreported transcription start site at 317 bp upstream of the translation start site (ATG). This gene is known to be composed of five exons and four introns, which is now changed to six exons and five introns. To map the promoter region, and to understand its regulation, we designed several fusion constructs of the 5' flanking region of PTTG including the sequence from nucleotide -1373 to -3 (relative to the translation start site) to a luciferase reporter gene. Transient transfection of these constructs in prostate cancer cell line (PC-3) and fibroblast cell line (HS27) confirmed the existence of promoter for PTTG between nucleotides -161 and -3 (in relation to translation start site). The 5' and 3' deletion analysis of the PTTG flanking region and electrophoretic mobility shift assays revealed binding of Sp1 and NF-Y transcription factors within nucleotides -540 to -500. Chromatin immunoprecipitation (ChIP) assays of the HS27 and PC-3 cells revealed the binding of Sp1 protein to PTTG promoter sequence in vivo. Site-directed mutagenesis of the Sp1 consensus sequence resulted in approximately 70% reduction of the overall transcriptional activation of the PTTG promoter, whereas mutation of the NF-Y sequence resulted in approximately 25% reduction. Deletion of both Sp1 and NF-Y consensus sequences resulted in 90% loss of PTTG promoter activity. It was further observed, by Western blot analysis, that the levels of Sp1 protein are higher in PC-3 cells when compared to levels in HS27 cells, possibly contributing to a tissue-specific effect. Our studies indicate an important role of Sp1 in transcription regulation of PTTG expression in tumors.