Cellular Sources of Transforming Growth Factor-β Isoforms in Early and Chronic Radiation Enteropathy

The three mammalian transforming growth factor (TGF)-β isoforms (TGF-β1, TGF-β2, and TGF-β3) differ in their putative roles in radiation-induced fibrosis in intestine and other organs. Furthermore, tissue specificity of TGF-β action may result from temporal or spatial changes in production and/or activation. The present study examined shifts in the cell types expressing TGF-β mRNA relative to TGF-β 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-β1, TGF-β2, or TGF-β3 transcripts were identified by in situ hybridization with digoxigenin-labeled riboprobes. Intestinal wall TGF-β 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-β isoforms. Two weeks after irradiation, regenerating crypts, inflammatory cells, smooth muscle cells, and mesothelium exhibited increased TGF-β1 expression and, to a lesser degree, TGF-β2 and TGF-β3 expression. Twenty-six weeks after irradiation, TGF-β2 and TGF-β3 expression had returned to normal. In contrast, TGF-β1 expression remained elevated in smooth muscle, mesothelium, endothelium, and fibroblasts in regions of chronic fibrosis. Extracellular matrix-associated TGF-β1 immunoreactivity was significantly increased at both observation times, whereas, TGF-β2 and TGF-β3 immunoreactivity exhibited minimal postradiation changes. Intestinal radiation injury is associated with overexpression of all three TGF-β isoforms in regenerating epithelium. Radiation enteropathy was also associated with sustained shifts in the cellular sources of TGF-β1 from epithelial cells to cells involved in the pathogenesis of chronic fibrosis. TGF-β2 and TGF-β3 did not exhibit consistent long-term changes. TGF-β1 appears to be the predominant isoform in radiation enteropathy and may be more important in the mechanisms of chronicity than TGF-β2 and TGF-β3.