Expression of interstitial collagenase (MMP-1) has been detected in stromal ®broblasts of various malignant tumors. Here, we have studied the e ect of three structurally di erent ETS transcription factors (ETS-1, ERGB/Fli-1, and PU.1) on MMP-1 promoter activity in NIH3T3 ®broblasts. ETS-1 increased the activity of 3.8 kb MMP-1 promoter construct up to tenfold, while ERGB/Fli-1 or PU.1 alone had no marked e ect on basal promoter activity. ETS-1 also markedly potentiated enhancement of MMP-1 promoter by both c-Jun and JunB, whereas ERGB/Fli-1 augmented only the e ect of c-Jun. Interestingly, PU.1 abolished induction of MMP-1 promoter by both c-Jun and JunB. Stimulation of MMP-1 promoter by 12-O-tetradecanoyl phorbol-13-acetate and okadaic acid was di erentially augmented by ETS-1 and ERGB/Fli-1, and abrogated by PU.1. Cotransfection studies with MMP-1 promoter 5'-deletion constructs revealed that AP-1 site was necessary for PU.1-elicited suppression. As compared to control cell lines, PU.1-positive stable cells exhibited clearly weaker binding of c-Jun and JunD containing AP-1 complexes to MMP-1 promoter AP-1 element, as well as marked reduction in basal level and induction of c-jun mRNA by 12-O-tetradecanoyl phorbol-13-acetate and okadaic acid, suggesting a novel mechanism for PU.1-mediated inhibition of AP-1 dependent gene expression. These results show that three structurally distinct ETS transcription factors di erently modulate AP-1 dependent upregulation of MMP-1 gene expression.