Biglycan gene expression in the human leiomyosarcoma cell line SK-UT-1 - Basal and protein kinase A-induced transcription involves binding of Sp1-like/Sp3 proteins in the proximal promoter region

In this study we demonstrate that the gene encoding the small leucine-rich proteoglycan biglycan is expressed in human myometrial tissue and in the human leiomyosarcoma cell line SK-UT-1. Treatment of SK-UT-1 cells with forskolin or 8-bromo-cAMP strongly increased biglycan mRNA and this effect was transcriptional as shown by transient transfection experiments with biglycan promoter-luciferase reporter fusion genes. The cAMP-mediated induction of the transfected biglycan promoter in SK-UT-1 cells was abolished by coexpression of a specific protein kinase A inhibitor, and was mimicked by overexpression of the catalytic subunit (CP) of protein kinase A. By 5' deletion analysis, part of the cAMP response was localized to the segment from residues -78 to -46 of the biglycan promoter. This region conferred strong cAMP responsiveness to a heterologous promoter. Electrophoretic mobility shift and antibody supershift assays identified two specific complexes that contained nuclear proteins antigenically related to the ubiquitous transcription factors Spl and Sp3, respectively. The binding site of these proteins was mapped to a CT-rich sequence extending from -59 to -49 in the biglycan promoter. Mutating this sequence eliminated complex formation and markedly reduced basal and cAMP-dependent promoter activity of transfected reporter genes. In vitro binding studies using recombinant Spl revealed that the nuclear factor binding to the CT element was not Spl but a Spl-like protein(s). Western blot analysis of SK-UT-1 nuclear proteins confirmed expression of Sp3, Spl and nuclear proteins that crossreacted with Spl antibody but according to their molecular weight were not Spl. These results indicate that all cAMP-dependent as well as some basal biglycan transcription in SK-UT-1 cells is mediated through activated protein kinase A and that both functions are conferred at the promoter level through the interaction of Sp1-like/Sp3 factors with the CT element at -59 in the biglycan promoter.