OVERLAPPING EGR-1 AND SP1 SITES FUNCTION IN THE REGULATION OF TRANSCRIPTION OF THE MOUSE THROMBOSPONDIN-1 GENE

We have evaluated the basis for the constitutive and serum-regulated expression of the mouse thrombospondin (TSP) gene in both transiently and stably transfected NIH-3T3 cells. Experiments with deleted and mutated mouse promoter/CAT constructs and gel mobility assays demonstrated that an Egr-1 binding site in the proximal promoter, flanked by overlapping GC boxes and an adjacent CC-rich region, functioned to positively regulate the constitutive activity of the gene. These motifs, and their cognate transcription factors, appear to act in concert, with partial redundancy, so that discrete mutations were only partially effective in reducing transcriptional activity. The Egr-1 site corresponds in position to an NF-Y binding site which functions synergistically with a distal serum-response element to mediate the serum response of the human TSP1 gene. However, neither the Egr-1 motif nor the surrounding proximal promoter region upstream from the TATA box participates in the serum response of mouse TSP1. These experiments add support to the growing realization that similar physiologic responses of homologous genes in mouse and man need not utilize similarly placed cis-acting elements.