Inflammatory events after irradiation: Pivotal role of PPARS.
C. Linard, O. Gremy, M. Benderitter. 33rd Annual meeting of the european society for radiation biology, 25-28/08/2004, Budapest, Hongrie.
Objective: Because irradiation can cause acute enteritis that leads to reduced motility and in a later phase to fibrosis, the small bowel is an important dose-limiting organ following radiotherapy. Pathologic changes may be caused by the early stage of an inflammatory process; we therefore studied the irradiation effect on the imbalance between pro and anti-inflammatory andand apoptotic events. We focalized our attention on peroxisome proliferator-activated receptors (PPARs), which have been recently implicated as regulators of inflammatory responses. Methods: PPARs, cytokines (TNF-a, IL-6, IFN-g, IL-10), neutrophil chemoattractant interleukin 8 (CINC), monocyte chemotactic protein-1 (MCP-1), anti apoptotic Bcl-2 and apoptotic Bax mRNA levels were analyzed by RT-PCR assay at 6 h and 3 days after 10-Gy g whole body irradiation in the ileum mucosa of Wistar rats. Neutrophils infiltration was characterized by myeloperoxidase immunohistochemical assay and apoptotic cells were detected by labeling of DNA strand breaks (TUNNEL). Results: The mRNA for cytokines TNF-a, IL-6 was significantly increased at 6h and persisted at 3 days post irradiation. The expression of IFN-g was significantly elevated (2.4 fold; p<0.01) at 3 days post irradiation. On the other hand, the anti-inflammatory cytokine IL-10 mRNA was markedly lower as early as 6h post irradiation. Analysis of PPARs implicated in the anti-inflammatory and anti-apoptotic events show a drastic decrease of the mRNA levels for PPARa (80%) and PPARg (63%) at 3 days post-irradiation. PPARs repression contributes to overexpressions of MCP-1 and CINC with a 6 fold (p<0.005) and 20 fold (p<0.01) increase respectively. CINC expression correlates with an increase of MPO-positive cells at 3 days post irradiation. TUNNEL assay showed an increasing number of apoptotic cells at the bottom of the villi at 6h. No or very few apoptotic cells were observed at day 3. Interestingly, anti-apoptotic Bcl-2 mRNA level was significantly increased (2.5 fold; p<0.005) whereas apoptotic Bax expression remained unaffected, at 3 days after irradiation. Conclusion: Irradiation induced an an early imbalance between pro-inflammatory (TNF-a, IL-6, IFN-g, CINC, MCP-1) and anti-inflammatory (IL-10, PPARs) mediators in the ileal mucosa. Infiltration of inflammatory cells is enhanced following irradiation. These molecular and cellular changes are correlating bound to an upregulationup regulation of an anti-apoptotic gene at day 3. Our work suggests that PPARs may be serving as key regulators in the irradiation-induced inflammatory response. Further investigation are undertake to confirm our hypothesis.