) is a potent cytokine that stimulates interstitial collagenase-1 (matrix metalloproteinase-1; MMP-1). In this study, we compared the mechanism(s) by which IL-1b induces collagenase gene expression in two very different cells, normal human foreskin fibroblasts (HFFs) and an aggressive breast cancer cell line, BC-8701 cells. Northern analysis showed that the time course of collagenase induction was distinct in the two cells: although both cells expressed low levels of MMP-1 constitutively, addition of IL-1b increased MMP-1 mRNA in HFFs by 1 h and levels remained high over a 24-h period. In contrast, MMP-1 levels in IL-1b-treated BC-8701 cells did not increase until 4 h, peaked by 12 h and then declined. To analyze the transcriptional response, we cloned and sequenced more than 4,300 bp of the human MMP-1 promoter, and from this promoter clone, we prepared a series of 58-deletion constructs linked to the luciferase reporter and transiently transfected these constructs into both cell types to measure both basal and IL-1b induced transcription. When both cell types were uninduced, promoter fragments containing less than 2,900 bp gave only a minimal transcriptional response, while larger fragments showed increased transcriptional activity. With IL-1b treatment, significant responsiveness (P , 0.001) in HFFs was seen only with the larger fragments, while in the BC-8701 cells, all fragments were significantly induced with IL-1b. Finally, we found that IL-1b stabilized MMP-1 mRNA in normal fibroblasts, but not in BC-8701 breast cancer cells. We conclude that both the transcriptional and posttranscriptional regulation of MMP-1 gene expression by IL-1b is controlled by cell-type specific mechanisms, and we suggest that IL-1 induced MMP-1 expression in tumor cells and in neighboring stromal cells may amplify the invasive ability of tumor cells.