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 e xpression of the mouse thrombospondin (TSP) gene in both transiently a nd stably transfected NIH-3T3 cells. Experiments with deleted and muta ted mouse promoter/CAT constructs and gel mobility assays demonstrated that an Egr-1 binding site in the proximal promoter, flanked by overl apping GC boxes and an adjacent CC-rich region, functioned to positive ly regulate the constitutive activity of the gene. These motifs, and t heir cognate transcription factors, appear to act in concert, with par tial redundancy, so that discrete mutations were only partially effect ive in reducing transcriptional activity. The Egr-1 site corresponds i n position to an NF-Y binding site which functions synergistically wit h 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 t he serum response of mouse TSP1. These experiments add support to the growing realization that similar physiologic responses of homologous g enes in mouse and man need not utilize similarly placed cis-acting ele ments.