Role Of Simvastatin On Attenuating Ionizing Radiation-induced Thymus Damage

Backgrounds and Objectives:Radiotherapy is frequently applied to control tumogenesis in the clinic, since it is able toinhibit the proliferation and induce apoptosis of many kinds of cancer cells. However,ionizing radiation could also cause severe normal tissue damage. Acute, subchoronic andlate choronic side effects (including pro-inflammatory and pro-fibrotic responses as well ascell death) maybe caused by radiotherapy, which would impose a huge burden on cancerpatients. Thus, it is necessary to develop safe and effective radioprotective agents. Statins(HMG-CoA reductase inhibitors) are a class of drugs used to lower cholesterol levels byinhibiting HMG-CoA reductase. Futhermore, statins were reported to have pleiotropiceffects involved in stress reponse, proliferation and apoptosis. Interestingly, though themolecular mechanisms involved are still unclear, preclinical data concordantly show apromising radioprotective capacity of statins. This study investigated the protective effectof simvastatin (SMV) on ionizing radiation (IR)-induced normal thymus apoptosis as wellas small intestine and bone marrow damage, and explored the mechanisms involved in Akt,Sirt-1, p53, bcl-2and PARP.Methods and results:1. Simvastatin significantly attenuates IR-induced damage of small intestine and bonemarrow.Male C57/BL6mice were randomly divided into4groups: Control (C), SMV, Radiation(4Gy, R), Radiation+SMV (RS). Mice in SMV and RS groups were given simvastatin of20mg/kg/d for consecutive14days while mice in C and R groups were given vehicle (0.5%CMC). After that, mice in R and RS groups were exposed to4Gy γ-60Co IR, andHE/TUNEL staining of small intestine/bone marrow was conducted to examine IR-inducedinjury and apoptosis. It was found that IR induced severe morphological damage in smallintestine and bone marrow of mice, including shortening the villi and vacuuming the bone,and exacerbated cell apoptosis in small intestine and bone marrow. Pretreatment with SMVsignificantly mitigated IR-induced injury and inhibited IR-induced apoptosis in smallintestine and bone marrow of mice. 2. Simvastatin significantly attenuates IR-induced oxidative stress and apoptosis inthymusApoptosis and SOD/MDA level of thymus was examined using TUNEL staining,Transmission Electron Microscopy (TEM) and SOD/MDA assay kit. It was found thatsimvastatin pretreatment significantly inhibited IR-induced oxidative stress and apoptosisin thymus of mice.3. Radioprotective effect of simvastatin on apoptosis in the thymus of mice is related toAkt/bcl-2pathwayProtein of thymus was extracted. Changes of Akt, Sirt-1, p53, bcl-2and PARPexpression were determined1d,3d and7d after IR exposure by immunoblotting. It wasfound that simvastatin inhibited IR-induced thymus apoptosis possibly via activatingAkt/bcl-2pathway.4. Simvastatin significantly inhibited IR-induced apoptosis of cultured thymocytesThymocytes isolated from male C57/BL6mice (5-7wks) were randomly divided into4groups: Control (C), SMV, Radiation (8Gy), Radiation+SMV (RS). Thymocytes in SMVand RS groups were cultured with simvastatin of20μM while thymocytes in C and Rgroups were treated by vehicle (DMSO). Thymocytes in R and RS groups were exposed to8Gy γ-60Co IR. Apoptosis of thymocytes was examined by Annexin/PI apoptosis assay. Itwas found that simvastatin significantly reduced IR-induced apoptosis thymocytes.5. Radio-protective effect of simvastatin on apoptosis of isolated thymocytes is related toAkt/bcl-2pathwayThymocyte protein was extracted. Changes of Akt, Sirt-1, p53, bcl-2and PARPexpression were examined1h,6h and24h after IR exposure using immunoblotting. It wasfound that simvastatin inhibited IR-induced thymocyte apoptosis via activating Akt/bcl-2pathway.Conclusion:Simvastatin significantly attenuated IR-induced damage of small intestine, bone marrowand thymus; simvastatin significantly reduced IR-induced apoptosis of thymocytes, whichwas possibly related to Akt/bcl-2pathway.