Although radiation-induced fibrosis has long been characterized by excess fibroblast proliferation and extracellular matrix deposition, the origin of cell activation in these complications of radiotherapy or radiation accidents is still controversial. The present work was designed to test the hypothesis that the abnormal production of TGFβ1 in irradiated tissues results in continuous signals for tissue repair and long-term cell activation. We examined gene expression of this growth factor in a well-characterized pig model of radiation-induced fibrosis, using Northern-blot and slot-blot hybridizations and indirect immunofluorescence. We found that the TGF-β1 mRNA level was increased 19-fold in the irradiated skin during the early erythematous phase, which started 3 weeks after irradiation. During the later phases of fibrosis, from 6 to 12 months after irradiation, the TGF-β1 gene was highly expressed in the repaired skin and the underlying muscular fibrotic tissue, with 10- and 8-fold maximal increases, respectively. In addition, we found that the β-actin mRNA level was increased in the fibrotic tissue. Immunostaining for TGF-β1 revealed the presence of the protein in endothelial cells of capillaries, in myofibroblasts, and in the collagenous matrix of the fibrotic tissue. These results suggest that TGF-β1 may be one of the key cytokines involved in the cascade of events that leads to radiation-induced fibrosis, at both early and late stages.

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