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Inhibition of the Tumor Necrosis Factor- Pathway Is Radioprotective for the Lung

Authors' Affiliation: Department of Radiation Oncology, University of Michigan School of Medicine, Ann Arbor, Michigan

Requests for reprints: Ming Zhang, Department of Radiation Oncology, University of Michigan Medical School, 1331 East Ann Street, Room 3030, Ann Arbor, MI 48109-5582. Phone: 734-763-7286; Fax: 734-763-1581; E-mail: mingz{at}med.umich.edu.

Purpose: Radiation-induced lung toxicity limits the delivery of high-dose radiation to thoracic tumors. Here, we investigated the potential of inhibiting the tumor necrosis factor- (TNF-) pathway as a novel radioprotection strategy.

Experimental Design: Mouse lungs were irradiated with various doses and assessed at varying times for TNF- production. Lung toxicity was measured by apoptosis and pulmonary function testing. TNF receptor 1 (TNFR1) inhibition, achieved by genetic knockout or antisense oligonucleotide (ASO) silencing, was tested for selective lung protection in a mouse lung metastasis model of colon cancer.

Results: Lung radiation induced local production of TNF- by macrophages in BALB/c mice 3 to 24 hours after radiation (15 Gy). A similar maximal induction was found 1 week after the start of radiation when 15 Gy was divided into five daily fractions. Cell apoptosis in the lung, measured by terminal deoxyribonucleotide transferase–mediated nick-end labeling staining (mostly epithelial cells) and Western blot for caspase-3, was induced by radiation in a dose- and time-dependent manner. Specific ASO inhibited lung TNFR1 expression and reduced radiation-induced apoptosis. Radiation decreased lung function in BALB/c and C57BL mice 4 to 8 weeks after completion of fractionated radiation (40 Gy). Inhibition of TNFR1 by genetic deficiency (C57BL mice) or therapeutic silencing with ASO (BALB/c mice) tended to preserve lung function without compromising lung tumor sensitivity to radiation.

Conclusion: Radiation-induced lung TNF- production correlates with early cell apoptosis and latent lung function damage. Inhibition of lung TNFR1 is selectively radioprotective for the lung without compromising tumor response. These findings support the development of a novel radioprotection strategy using inhibition of the TNF- pathway.