BINDING OF PHOSPHORYLATED SP1 PROTEIN TO TANDEM SP1 BINDING-SITES REGULATES ALPHA(2) INTEGRIN GENE CORE PROMOTER ACTIVITY

The alpha(2) beta(1) integrin, a collagen/laminin receptor, is express ed by a variety of cell types, including epithelial cells, mesenchymal cells, and hematopoietic cells. To understand the molecular mechanism s that regulate expression of the alpha(2) beta(1) integrin in cells w ith megakaryocytic differentiation, we characterized the 5' flanking r egion of the alpha(2) integrin gene and identified three distinct regu latory regions, including a core promoter, a silencer, and megakaryocy te enhancers in the distal 5' flank (Zutter et al, Blood 96:3006, 1995 and Zutter et al, J Biol Chem 269:463, 1994). We now focus on the cor e promoter of the alpha(2) integrin gene located between bp -30 and -9 2 that is required for transcriptional activity of the alpha(2) integr in gene. Sequence analysis identified two Sp1 consensus sites and a po tential AP2 site, Gel retardation assays showed that nuclear proteins from uninduced K562 cells and K562 cells induced to become megakaryocy tic bound specifically to the core promoter region (bp -30 to bp -92) producing two DNA-protein complexes. In addition, nuclear extracts fro m cells induced along the megakaryocyte lineage produced a selective i ncrease in the slower migrating complex, Site-directed mutagenesis of the 5', the 3', or both Sp1 binding sites suggested that both Sp1 bind ing sites are required for full promoter activity and for DNA-protein complex formation. DNA footprinting also showed specific protection of the 5' Sp1 site by nuclear extracts from uninduced K562 cells and pro tection of both the 5' and the 3' Sp1 sites by nuclear extracts from i nduced K562 cells. Sp1 protein-DNA complex formation was dependent on Sp1 phosphorylation. The faster migrating DNA-protein complex was enha nced by dephosphorylation; the slower migrating DNA-protein complex wa s diminished or lost. (C) 1997 by The American Society of Hematology.