Cellular sources of transforming growth factor-β isoforms in early and chronic radiation enteropathy

The three mammalian transforming growth factor (TGF)-beta isoforms (TGF-beta 1, TGF-beta 2, and TGF-beta 3) differ in their putative roles in radiation-induced fibrosis in intestine and other organs. Furthermore, tissue specificity of TGF-beta action may result from temporal or spatial changes in production and/or activation. The present study examined shifts in the cell types expressing TGF-beta mRNA relative to TGF-beta 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-beta 1,TGF-beta 2, or TGF-beta 3 transcripts were identified by in situ hybridization with digoxigenin-labeled riboprobes. Intestinal wall TGF-beta 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-beta isoforms, Two weeks after irradiation, regenerating crypts, inflammatory cells, smooth muscle cells, and mesothelium exhibited increased TGF-beta 1 expression and, to a lesser degree, TGF-beta 2 and TGF-beta 3 expression. Twenty-six weeks after irradiation, TGF-beta 2 and TGF-beta 3 expression had returned to normal. In contrast, TGF-beta 1 expression remained elevated in smooth muscle, mesothelium, endothelium, and fibroblasts in regions of chronic fibrosis, Extracellular matrix-associated TGF-beta 1 immunoreactivity was significantly increased at both observation times, whereas, TGF-beta 2 and TGF-beta 3 immunoreactivity exhibited minimal postradiation changes. Intestinal radiation injury is associated with overexpression of all three TGF-beta isoforms in regenerating epithelium, Radiation enteropathy was also associated with sustained shifts in the cellular sources of TGF-beta 1 from epithelial cells to cells involved in the pathogenesis of chronic fibrosis. TGF-beta 2 and TGF-beta 3 did not exhibit consistent long-term changes. TGF-beta 1 appears to be the predominant isoform in radiation enteropathy and may be more important in the mechanisms of chronicity than TGF-beta 2 and TGF-beta 3.