Identification of human hepatic stimulator substance gene promoter and demonstration of dual regulation of AP1/AP4 cis-acting element in different cell lines

Human hepatic stimulator substance (hHSS) is a newly identified growth-promoting factor in the liver. HSS is capable of stimulating hepatic regeneration in partial hepatectomized rats, thus, promoting growth of hepatic tumor cells. To understand and elucidate the transcriptional regulation of hHSS gene, the 4890 bp of 5′-flanking region of the gene have been isolated and sequenced. The transcriptional start site, located at 248 nt upstream from the ATG starting codon, was identified by 5′-rapid amplification cDNA end (5′-RACE). The classical promoter sequences, such as TATA box or GAATT were not identified in the promoter region, instead a GC-rich segment was formed (>70%) by expanding to a longer than 400 bp, and immediately upstream from the ATG start codon. The transient transfection assays, using promoter deletion constructs, showed that hHSS promoter was efficiently capable in driving the reporter expression not only in HepG2 cells, but also in Cos7 cells. A region spanning nucleotides in the range of −447 to −358 bp revealed a negative regulation on promoter activity in HepG2 cells, but with positive regulation in Cos7 and Hela cells. The promoter activity was obviously influenced by AP1/AP4 (−375/−369 nt) mutation in these three cell lines. EMSAs showed that the site was recognized by AP1 in HepG2 cell, and only by an AP4 protein in Cos7 cells. The c-Jun bound to the promoter was further verified by supershift in HepG2 cells and human liver tissue. Chromatin immuno-precipitation (ChIP) demonstrated that there was a direct association of c-Jun with hHSS promoter in HepG2 cells. The c-Jun strongly suppressed hHSS promoter activity in transient expression analyses in HepG2 cells. Mutations in the AP1 binding sites rescued suppression caused by c-Jun, suggesting this was a direct regulation of the hHSS promoter. In contrast, there was no significant effect in c-Jun over-expressed Cos7 and Hela cells. The tissue-specific function of c-Jun in hHSS promoter activity may in part help explain the differences in biology function of hHSS between liver and non-liver cells.