Differential functional significance of AP-1 binding sites in the promoter of the gene encoding mouse tissue inhibitor of metalloproteinases-3

Abstract

Tissue inhibitor of metalloproteinases-3 (TIMP-3) is an extracellular-matrix-associated protein that suppresses tumorigenicity or invasion in several model systems. We have identified, by in vitro footprinting, six AP-1 (activator protein-1) or AP-1-like binding sites in the mouse TIMP-3 promoter that bind purified c-Jun homodimers. Electrophoretic mobility shift assays revealed that the non-consensus fifth AP-1 binding site (AP-720; nt -720 to -714) had the strongest binding activity for recombinant c-Jun protein, and that the fourth binding site (AP-763; nt -763 to -754) and AP-720 showed strong binding activity for cellular nuclear proteins. Antibody supershift and blocking experiments suggest that AP-720, but not AP-763, binds authentic AP-1 components. Transient transfection reporter assays of deletion constructs showed that the region spanning AP-720 has the highest transcriptional activity, and that sequences 5' to this region (nt -2846 to -747) may contain negative regulatory elements. The deletion construct containing about 500 nt 5' to the transcriptional start, but no AP-1 sites, showed lower but significant activity, suggesting both AP-1-dependent and -independent regulation of the mouse TIMP-3 promoter. Mutational inactivation of AP-720 abolished the activity increment that distinguished the reporter construct containing both AP-720 and sixth AP-1 binding site (AP-617; nt -617 to -611) from that containing only AP-617. In summary, we report here that both AP-1 and non-AP-1 elements contribute to activity, with the non-consensus AP-1 site at -720 showing the greatest functional significance among the AP-1 sites.