Thrombospondin I (TSP I) is an extracellular matrix glycoprotein that influences cell adhesion, motility, and growth. On the basis of its effects on endothelial cell proliferation, TSP I has attracted interest as a potential regulator of solid tumor growth through modulation of tumor blood supply. The regulation of TSP I expression is of critical importance for designing new approaches in tumor therapy. Recently, we have shown that TSP I expression is lost in nickel-transformed cells. In this paper, we identified an activating transcription factor (ATF)/cAMP-responsive element-binding protein binding site as a negative regulatory site in the 5'-flanking sequence of mouse TSP I promoter. We identified ATF-1 as a major component of the ATF/cAMP-responsive element-binding protein binding complex. This Mr 35,000 nuclear ATF-1 protein was shown to be present in higher amounts in nickel-transformed 3T3 cells that do not express TSP 1. Acute treatment of 3T3 cells with NiCl2 resulted in the induction of this transcription factor, and this induction was correlated temporally with the suppression of TSP I expression in the same cells. These results show that nickel exposure causes accumulation of the ATF-1 transcription factor, which is responsible for the down-regulation of transcription of TSP I, and possibly other tumor suppressor genes during nickel-induced cellular transformation.