INDUCTION OF ACTIVATING TRANSCRIPTION FACTOR-1 BY NICKEL AND ITS ROLEAS A NEGATIVE REGULATOR OF THROMBOSPONDIN-I GENE-EXPRESSION

Thrombospondin I (TSP I) is an extracellular matrix glycoprotein that influences cell adhesion, motility, and growth. On the basis of its ef fects on endothelial cell proliferation, TSP I has attracted interest as a potential regulator of solid tumor growth through modulation of t umor blood supply. The regulation of TSP I expression is of critical i mportance 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 reg ulatory site in the 5'-flanking sequence of mouse TSP I promoter. We i dentified ATF-1 as a major component of the ATF/cAMP-responsive elemen t-binding protein binding complex. This M(r)35,000 nuclear ATF-1 prote in was shown to be present in higher amounts in nickel-transformed 3T3 cells that do not express TSP I. Acute treatment of 3T3 cells with Ni Cl2 resulted in the induction of this transcription factor, and this i nduction was correlated temporally with the suppression of TSP I expre ssion in the same cells. These results show that nickel exposure cause s 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.