Characterization of the promoter region of the porcine opn (osteopontin, secreted phosphoprotein 1) gene. Identification of positive and negative regulatory elements and a 'silent' second promoter.

Abstract

Osteopontin (secreted phosphoprotein-1, Opn) is a phosphorylated glycoprotein expressed by transformed cells, macrophages, activated T-lymphocytes, specialized epithelial cells and bone cells that is characteristically enriched in milk and in the mineralized matrix of bone. The synthesis of Opn by bone cells is regulated by glucocorticoids and growth factors, which promote bone formation, and by the osteotropic hormone calcitriol (1,25-dihydroxycholecalciferol) and retinoic acid, which mediate bone resorption, indicating a bifunctional role for this protein in bone remodelling. To study the transcriptional regulation of the opn gene, two genomic clones (10 and 15 kb) encoding the opn gene were isolated from a porcine liver genomic library cloned into lambda phage. From the 15-kb clone a 4-kb EcoRI fragment containing the first two exons and 2.6 kb of the 5' flanking region of the opn gene was sequenced, and the transcriptional start site determined by primer extension analysis and S1 nuclease mapping. To identify the opn promoter, chimeric chloramphenicol acetyltransferase constructs were prepared using fragments from the first intron and the 5' flanking region of the opn gene. Transient transfection of porcine bone cells with these constructs showed strong promoter activity located within 74 bp upstream from the transcription initiation site. Within this region a TATA sequence, TTTAAA, was identified at positions -26 to -31. However, the highest transcription rate was observed in a construct extending 180 bp upstream that included a CCGCCC Sp1 binding sequence (-63 to -68), and an AP1 site (-74 to -80). Further upstream in the 5' flanking region and within the first intron of the opn, a number of consensus sequences could be identified. Chimeric constructs containing a GGGTCAtatGGTTCA direct repeat consensus sequence for a vitamin D3 response element located at nucleotides -2245 to -2259 responded to the addition of 0.1 microM calcitriol by a 2.5-fold stimulation of transcription, although a greater than 2-fold increase was also observed in shorter constructs -180 to -905 lacking such a consensus sequence. Promoter activity was also exhibited by a region containing a TTTAAA sequence in the first intron that corresponded to the putative promoter site reported for mouse opn in macrophages (Miyazaki, Y., Setoguchi, M., Yoshida, S., Higuchi, Y., Akizuki, S. & Yamamoto, S. (1990) J. Biol. Chem. 265, 14432-14438).(ABSTRACT TRUNCATED AT 400 WORDS)