Radiation oncologists maximize tumor kill by using various dosing schemes that encompass timing parameters, size of dose/fraction, type of ionizing radiation, and various geometric approaches. For most radiation therapy, the limitation of treatment is defined by injury to normal tissue. Exposure of tissue to radiation leads to chemical and biochemical events, which because of the presence of different cell populations results in cells dying at various times as a consequence of their differing inherent radiation sensitivities. At the cellular level, the biochemical target that ionizing radiation damages to cause death is DNA. The architectural changes resulting from their death and the type of cell(s) that repopulates could account for the clinically described acute and chronic phases of radiation-induced injury. However at the tissue level the effects of ionizing radiation may not be limited to direct injury and cell death from damaged DNA. Tissue injury may not result from only the initiating damage, but also as a result of the initial damage inducing chemokines or cytokine/cell or cell/cell or cell/matrix cascade(s) that result in cellular death. Numerous cytokines, such as TNF-a, IL-1, PDGF, FGF, and TGF-b, have been reported to be increased within irradiated tissue. Within the context of wound healing and radiation-induced injury there has been growing interest in the importance of transforming growth factor-beta (TGF-b). TGF-b has been found to be elevated in circulation following irradiation. In tissue, TGF-b is reported to be elevated as early as 6 hours after and as long as 20 years following irradiation. We have now shown that animals lacking the TGF-beta activated signaling molecule, Smad3, are relatively resistant to radiation-induced injury. Therefore we were interested in obtaining small molecules that might interfere with TGF-beta signaling through the Smad3 pathway. One of the several compounds tested was the quinazolone alkaloid halofuginone, which had previously been shown to inhibit collagen synthesis and decrease bleomycin-induced lung fibrosis. Following irradiation of the leg, halofuginone (1, 2.5, or 5 g) was injected into the intraperitoneum each day for 3 or 5 months and mice were followed for several months thereafter. Halofuginone ameliorated radiation-induced injury (P <0.01 for all time points, at all doses). We are pursing the mechanism of action of the drug. It does downregulate the TGF-beta type II receptor, inhibit Smad3 and Smad2 phosphorylation and upregulate expression of Smad7. The upregulation is Smad7 is independent of Smad3, JNK, and NFkB pathways. We are attempting to determine the exact molecular mechanism by which halofuginone induces Smad7 and downregulates the TGF-beta type II receptor.
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