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We have examined whether rapamycin, an immunosuppressive drug, may exert part of its antifibrotic activity by directly targeting fibroblast extracellular matrix deposition. Incubation of human lung fibroblast (WI-26) cultures with rapamycin led to dose-and time-dependent reduction in the expression of types I and III collagens, both at the protein and mRNA levels. Rapamycin had no effect on collagen promoter activity but accelerated mRNA decay, indicating post-transcriptional control of collagen gene expression. In contrast, rapamycin significantly enhanced the expression of interstitial collagenase (MMP-1) at the protein and mRNA levels and transcriptionally. We determined that rapamycin efficiently activates AP-1- Fibrosis is a reactive process involving different pathophysiological events such as attraction of blood-born cells (e.g. leukocytes, platelets, activated lymphocytes), alteration of the microvasculature, and activation of resident mesenchymal cells (fibroblasts, endothelial cells, pericytes) leading to excessive extracellular matrix (ECM) 2 deposition (1-4). Possible explanations for the excessive deposition of collagen observed during the fibrotic process include both an increased biosynthesis or reduced degradation of ECM components, particularly that of fibrillar collagens, by fibroblasts. An accumulation of collagen may originate from accelerated production of collagen resulting from enhanced collagen gene transcription and/or mRNA stabilization in response to soluble factors present in the microenvironment. Alternatively, reduced matrix metalloproteinase (MMP) expression and subsequent inhibition of collagen degradation may also contribute to the fibrotic process (1). Thus, identifying molecules that may either affect collagen production negatively or MMP expression positively is of utmost importance to define novel therapeutic means against fibrosis. In this context, rapamycin (sirolimus), a Streptomyces fungus macrolide antibiotic with potent immunosuppressive properties, is currently used for the prevention of graft rejection in kidney transplant recipients (5-7). Several experimental studies have shown that rapamycin is also effective in preventing liver or pulmonary fibrogenesis in animal models (8 -10).At the cell membrane, rapamycin binds to the immunophilin FK506-binding protein (FKBP12). This rapamycin⅐FKBP12 complex interacts with the rapamycin binding domain of mTOR, a serine-threonine kinase, and thus inactivates mTOR known to control proteins that regulate mRNA translation initiation and G 1 progression in T cells (11). mTOR is a transducer that may be initiated by insulin, growth factors, and amino acids to activate downstream targets and regulate cell growth and proliferation as well as metabolic homeostasis (12,13). It has been shown that ribosomal protein S6 kinases 1 and 2 (S6K-1 and S6K-2) and the eukaryotic initiation factor 4E-binding protein (4E-BP1) are downstream targets of mTOR. Rapamycin induces translational arrest by preventing phosphorylation of S6K-1 and 4E-BP1 by ...