Upregulation and spatial shift in the localization of the mannose 6-phosphate/insulin-like growth factor II receptor during radiation enteropathy development in the rat

Abstract

Background and purpose: Transforming growth factor β1 (TGF- β1) appears to play an important role in the pathogenesis of chronic radiation-induced fibrosis in the intestine and several other organs. TGF- β1 is secreted as a non-biologically active complex and its function depends on activation. In vitro data suggest that the mannose 6-phosphate/insulin-like growth factor- β (M6P/IGF-II) receptor is involved in the mechanism of TGF- β1 activation. Thus, we used a rat model of radiation enteropathy to examine the potential role of the M6P/IGF-II receptor in the in vivo regulation of TGF- β1 activity and localization. Materials and methods: A scrotal hernia containing a loop of small intestine was created in male rats. The intestine in the scrotum was exposed to 0, 12, or 21 Gy single dose X-radiation. Groups of rats were euthanized 1 day and 2, 6 and 26 weeks after irradiation. Histopathologic injury was assessed with a radiation injury score (RIS). Computerized image analysis was used to identify M6P/IGF-II receptor-positive cells and to quantify extracellular matrix-associated TGF- β1 immunoreactivity. Changes in urokinase plasminogen activator (uPA), tissue-like plasminogen activator (tPA) and plasminogen activator inhibitor-1 (PAI-1) immunoreactivity were also assessed. Results: In normal (sham-irradiated) intestine, M6P/IGF-II immunoreactivity was confined to relatively weak, but specific epithelial staining. Irradiated intestine exhibited a highly significant time- and dose-dependent increase in the number of M6P/IGF-II receptor-positive cells ( P<0.001). There was a striking spatial shift of M6P/IGF-II receptor immunoreactivity from epithelium during the early post-radiation phase to stromal cells, most notably fibroblasts during the later stages of injury. Irradiated intestine exhibited distinct co-localization of M6P/IGF-II receptor-positive cells and extracellular matrix-associated TGF- β1 in areas of histopathologic injury. There were highly significant associations between the number of M6P/IGF-II receptor-positive stromal cells and TGF- β1 immunoreactivity ( P<0.001), radiation-induced fibrosis ( P<0.001) and RIS ( P<0.001). Endothelial tPA immunoreactivity decreased significantly after irradiation ( P<0.001), whereas uPA and PAI-1 immunoreactivity levels appeared to be unchanged. Conclusions: M6P/IGF-II receptor upregulation may be a key factor in the in vivo control of TGF- β1 activity and responsible for the tissue specificity of TGF- β1 action after irradiation.