Radiation-induced fibrosis is an untoward effect of high dose therapeutic and
inadvertent exposure to ionizing radiation. Transforming growth factor-beta
(TGF-beta) has been proposed to be critical in tissue repair mechanisms resulting
from radiation injury. Previously, we showed that interruption of TGF-beta
signaling by deletion of Smad3 results in resistance to radiation-induced injury.
In the current study, a small molecular weight molecule, halofuginone (100 nm),
is demonstrated by reporter assays to inhibit the TGF-beta signaling pathway, by
Northern blotting to elevate inhibitory Smad7 expression within 15 min, and by
Western blotting to inhibit formation of phospho-Smad2 and phospho-Smad3 and to
decrease cytosolic and membrane TGF-beta type II receptor (TbetaRII). Attenuation
of TbetaRII levels was noted as early as 1 h and down-regulation persisted for 24
h. Halofuginone blocked TGF-beta-induced delocalization of tight junction ZO-1, a
marker of epidermal mesenchymal transition, in NMuMg mammary epithelial cells and
suggest halofuginone may have in vivo anti-fibrogenesis characteristics. After
documenting the in vitro cellular effects, halofuginone (intraperitoneum
injection of 1, 2.5, or 5 microg/mouse/day) efficacy was assessed using ionizing
radiation-induced (single dose, 35 or 45 Gy) hind leg contraction in C3H/Hen
mice. Halofuginone treatment alone exerted no toxicity but significantly lessened
radiation-induced fibrosis. The effectiveness of radiation treatment (2 gray/day
for 5 days) of squamous cell carcinoma (SCC) tumors grown in C3H/Hen was not
affected by halofuginone. The results detail the molecular effects of
halofuginone on the TGF-beta signal pathway and show that halofuginone may lessen
radiation-induced fibrosis in humans.