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Is Smad3 a Major Player in Signal Transduction Pathways Leading to Fibrogenesis?*

Anita B. Roberts, PhD; Ester Piek, PhD; Erwin P. Böttinger, MD; Gillian Ashcroft, MD; James B. Mitchell, PhD; Kathleen C. Flanders, PhD
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*From the Laboratory of Cell Regulation and Carcinogenesis (Drs. Roberts, Piek, Ashcroft, and Flanders) and Radiation Oncology Branch (Dr. Mitchell), National Cancer Institute, Bethesda, MD; and Department of Medicine (Dr. Böttinger), Albert Einstein College of Medicine, Bronx, NY.

Correspondence to: Anita B. Roberts, PhD, Chief, Laboratory of Cell Regulation and Carcinogenesis, National Cancer Institute, Bldg 41, Room C629, 41 Library Dr, MSC 5055, Bethesda, MD 20892-5055; e-mail: Robertsa@dce41.nci.nih.gov



Chest. 2001;120(1_suppl):S43-S47. doi:10.1378/chest.120.1_suppl.S43
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Transforming growth factor (TGF)-β plays a central role in fibrosis, contributing both to the influx and activation of inflammatory cells, as well as to activation of fibroblasts to elaborate extracellular matrix. In the past few years, new insight has been gained into signal transduction pathways downstream of the TGF-β receptor serine-threonine kinases with the identification of a family of evolutionarily conserved Smad proteins. Two receptor-activated Smad proteins, Smad2 and Smad3, are phosphorylated by the activated TGF-β type I receptor kinase, after which they partner with the common mediator, Smad4, and are translocated to the nucleus to where they participate in transcriptional complexes to control expression of target genes. We have shown in wound healing studies of mice null for Smad3, that loss of this key signaling intermediate interferes with the chemotaxis of inflammatory cells to TGF-β as well as with their ability to autoinduce TGF-β. Moreover, studies with mouse embryo fibroblasts null for Smad3 show that TGF-β–dependent induction of c-Jun and c-Fos, important in induction of collagen as well as in autoinduction of TGF-β, is mediated by Smad3. Based on these observations, we hypothesize that loss of Smad3 will confer resistance to fibrosis and result in reduced inflammatory cell infiltrates, reduced autoinduction of TGF-β, important to sustain the process, and reduced elaboration of collagen. Preliminary observations in a model of radiation-induced fibrosis confirm this hypothesis and suggest that inhibitors of Smad3 might have clinical application both to improve wound healing and to reduce fibrosis.

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