Reduction of PPARγ Expression by γ-irradiation as a Mechanism Contributing to Inflammatory Response in Rat Colon: Modulation by the 5-ASA Agonist

Radiation-induced intestinal injuries, including inflammation and immune response, remain a limiting factor in the effectiveness of pelvic radiotherapy and in the patient's quality of life during and after treatment. Peroxisome proliferation-activated receptor (PPAR) agonists are now emerging as therapeutic drugs for various inflammatory diseases that are characterized by impaired PPAR expression. The purpose of this study was to investigate the profile of PPAR expression in rat colonic mucosa 3 and 7 days after abdominal γ-irradiation (10 Gy). We tested whether irradiation-induced acute inflammatory response could be modulated pharmacologically with the antiinflammatory properties of 5-aminosalicylic acid (5-ASA) (250 mg/kg/day), which is a PPAR activator. Irradiation drastically reduced mRNA and protein levels of PPARα and -γ and of the heterodimer retinoid X receptor (RXR)α at 3 days postirradiation. 5-ASA treatment normalized both PPARγ and RXRα expression at 3 days postirradiation and PPARα at 7 days. By promoting PPAR expression and its nuclear translocation, 5-ASA interfered with the nuclear factor (NF)-κB pathway, both by reducing irradiation-induced NF-κB p65 translocation/activation and increasing the expression of nuclear factor-κB inhibitor (IκB) mRNA and protein. Therefore, 5-ASA prevents irradiation-induced inflammatory processes as well as expression of tumor necrosis factor α, monocyte chemotactic protein-1, inducible nitric-oxide synthase, and macrophage infiltration. In addition, 5-ASA restores the interferon γ/signal transducer and activator of transcription (STAT)-1 and STAT-3 concentrations that were impaired at 3 and 7 days postirradiation and are correlated with suppressor of cytokine signaling-3 repression. Collectively, these results indicate that PPAR agonists may be effective in the prevention of inflammatory processes and immune responses during and after pelvic radiotherapy.