Though radiation induced fibrosis in normal tissues is a pathologic condition that limits the radiation dose which can be safely delivered in the treatment of tumors, the cellular and molecular events controlling this fibrogenic reaction have not been delineated. Of the multiple growth factors implicated in tissue fibrogenesis during normal wound healing, the cytokine transforming growth factor-beta (TGF-beta) is of particular interest. In addition to its bifunctional role in modifying cellular proliferation (i.e. it stimulates the growth of fibroblasts but inhibits the proliferation of epithelial cells, including hepatocytes), TGF-beta has been shown to be chemotactic for lymphocytes and fibroblasts and stimulatory for the cellular production of collagen and fibronectin. These and other data suggest that TGF-beta is involved in regulating tissue fibrogenesis under both normal and pathological conditions. Recently, we have shown that TGF-beta is significantly increased in pericentral hepatocytes, central veins and sinusoidal vessels of rat liver following irradiation. Additionally, our results demonstrate that elevated concentrations of TGF-beta plays a pivotal role in the hepatic fibrosis formation which occurs subsequent to irradiation. Specifically, we are proposing to determine: 1) the kinetics of TGF-beta expression in irradiated liver as a function of radiation dose; 2) whether the increased TGF-beta observed in irradiated liver results from platelet release during aggregation and/or from increased cellular synthesis; 3) whether TGF-beta observed subsequent to irradiation is present in an active or inactive form; 4) whether the preirradiation levels of TGF-beta in the liver is species dependent; 5) whether the extent of chronic liver fibrosis following radiation treatment can be predicted during treatment by measuring TGF-beta levels in tissue sections by peroxidase immunohistochemistry or in the urine by ELISA techniques; 6) whether the chemical milieu present in an irradiated fibrotic liver (i.e. increased TGF-beta levels in the central lobular region, Addendum B) reduces the capacity of hepatocytes to proliferate in response to mitogenic stimuli and if so, whether this is mediated through changes in receptor function (i.e. EGF, TGF-beta); and 7) the importance of reproductively viable parenchymal hepatocytes in the etiology of chronic radiation hepatitis. In conclusion, during the next grant period we will utilize the rat liver as a model system to investigate the cellular and molecular events controlling tissue fibrogenesis and parenchymal cell homeostasis. By more clearly defining the role which TGF-beta plays in regulating fibrogenesis subsequent to radiation exposure, it may be possible to ultimately reduce the fibrosis formation which often limits the radiation dose which can be safely delivered in the treatment of tumors, and to determine those patients genetically predisposed to its debilitating effects.
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