Differential expression of collagen types I and III in consequential and primary fibrosis in irradiated mouse colon.

These studies were undertaken to understand further the pathogenesis of consequential and primary fibrosis in mouse colon after irradiation. The distal 2.5 cm of colon of C3Hf/Kam mice was irradiated with either a single dose of 27 Gy or a split dose of 2 x 14.75 Gy separated by 10 days to induce a consequential or primary fibrotic lesion, respectively. The amount of total collagen in the two lesions was quantified by hydroxyproline, and tensile strength, an assay of tissue rigidity, was measured as a function of dose and time after irradiation. The relative distribution of collagen types I, III and IV in the colon was visualized by immunohistochemistry. Collagen types I, III and IV were quantified by immunoblot techniques, and in situ hybridization was used to identify and score the cells producing procollagen mRNA types I and III as a function of time after irradiation. The hydroxyproline and tensile strength measurements demonstrated that both lesions contained significantly increased amounts of collagen compared to controls. However, the ulcerated lesion of consequential fibrosis contained three times as much collagen and required a three- to fourfold increase in the peak force to rupture the colon as did the non-ulcerative lesion of primary fibrosis. The fibrosis accompanying the consequential lesion contained elevated levels of both collagen types I and III, but primary fibrosis contained only elevated levels of type I collagen compared to controls. The in situ hybridization studies showed cells producing increased amounts of procollagen mRNA 8 and 25 weeks before the elevated levels of collagen were detected for consequential and primary fibrosis, respectively. The cells producing the excess collagen mRNA were identified as fibroblasts. No distinction between the two lesions could be made based on the cell types producing the collagen. The distribution of labeled cells was localized to tissue areas showing specific immunofluorescence for the collagen types. These data show that the two histologically distinct lesions in irradiated colon, consequential and primary fibrosis, contain different collagen types and quantities of each type, suggesting that the underlying pathogenesis of these two lesions may be different.