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From the Departments of Surgery and Pathology, University of Arkansas for Medical Sciences, Little Rock, Arkansas
The three mammalian transforming growth factor (TGF)-ß isoforms (TGF-ß1, TGF-ß2, and TGF-ß3) differ in their putative roles in radiation-induced fibrosis in intestine and other organs. Furthermore, tissue specificity of TGF-ß action may result from temporal or spatial changes in production and/or activation. The present study examined shifts in the cell types expressing TGF-ß mRNA relative to TGF-ß immunoreactivity and histopathological injury during radiation enteropathy development. A 4-cm loop of rat small intestine was locally exposed to 0, 12, or 21-Gy single doses of x-irradiation. Sham-irradiated and irradiated intestine were procured 2 and 26 weeks after irradiation. Cells expressing the TGF-ß1, TGF-ß2, or TGF-ß3 transcripts were identified by in situ hybridization with digoxigenin-labeled riboprobes. Intestinal wall TGF-ß immunoreactivity was measured using computerized image analysis, and structural radiation injury was assessed by quantitative histopathology. Normal intestinal epithelium expressed transcripts for all three TGF-ß isoforms. Two weeks after irradiation, regenerating crypts, inflammatory cells, smooth muscle cells, and mesothelium exhibited increased TGF-ß1 expression and, to a lesser degree, TGF-ß2 and TGF-ß3 expression. Twenty-six weeks after irradiation, TGF-ß2 and TGF-ß3 expression had returned to normal. In contrast, TGF-ß1 expression remained elevated in smooth muscle, mesothelium, endothelium, and fibroblasts in regions of chronic fibrosis. Extracellular matrix-associated TGF-ß1 immunoreactivity was significantly increased at both observation times, whereas, TGF-ß2 and TGF-ß3 immunoreactivity exhibited minimal postradiation changes. Intestinal radiation injury is associated with overexpression of all three TGF-ß isoforms in regenerating epithelium. Radiation enteropathy was also associated with sustained shifts in the cellular sources of TGF-ß1 from epithelial cells to cells involved in the pathogenesis of chronic fibrosis. TGF-ß2 and TGF-ß3 did not exhibit consistent long-term changes. TGF-ß1 appears to be the predominant isoform in radiation enteropathy and may be more important in the mechanisms of chronicity than TGF-ß2 and TGF-ß3.
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