Simvastatin Attenuates Radiation-induced Tissues Damage In Mice

Backgrounds and Objectives: Radiation is frequently used in cancer therapy to achievelocal tumor control. About half of people with cancer are treated with radiation therapy.Despite of inducing anti-proliferative and cell-killing effects in tumor tissue, normal tissuetoxicity remains the single most important obstacle to uncomplicated cancer cure. Thetoxicity of radiation is associated with induction of acute radiation syndromes involvingthe gastrointestinal tract (GI) and hematopoietic system (HP). It is also believed that cellapoptosis, oxidative stress and inflammation induced by radiation contribute to tissuedamage. Recently, radiation-induced endothelial injury has been the subject ofconsiderable interest relative to radiation toxicities in normal tissue. Endothelialdysfunction appears to play an increasingly important role in the development of radiationresponse[1-3]. The3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductaseinhibitors, also known as statins, are widely used in the clinic for lowering serumcholesterol and decreasing cardiac morbidity and mortality. However, beyond theirwell-known cholesterol-lowering activity, statins also possess pleiotropic biological effectsindependent of their beneficial effects on blood cholesterol levels. Such pleiotropic effectsinclude improving endothelial function, decreasing oxidative stress and inflammation, andregulating immune system. In addition, simvastatin has also been found to mobilizeendothelial progenitor cells (EPCs), a heterogeneous subpopulation of bone marrowmononuclear cells that have an enhanced potential to differentiate into endothelial cells,from the bone marrow and accelerate vascular structure formation. In the current study, wetested our hypothesis that simvastatin was able to decrease the mortality in a60Coγ-radiation-induced mice model, and investigated the radioprotective effect of simvastatinand its potential mechanisms.Methods: Mice were treated intragastrically with simvastatin2,10,20mg/kg/d for2weeks and suffered an8-Gy lethal dose of radiation after the final administration. Thesurvival rate was calculated at the end of a30-day observing period. Mice were radiatedwith a dose of4Gy after20mg/kg/d simvastatin treatment for2weeks. Seven days later,the mice were sacrificed. Morphological changes of jejunum and bone marrow wereobserved and apoptotic cells of both tissues were determined. Peripheral blood cells werecounted, and superoxide dismutase (SOD) activity and malondialdehyde (MDA) level intissues of both thymus and spleen were measured.The expression of apoptosis-relatedproteins in thymus were analysed by Western blot. Results:1. Simvastatin increased the30-day survival rate of mice after8Gy radiation. All of themice in control group did not survive longer than11days after8Gy radiation.Pretreatment with simvastatin2,10mg/kg/d for consecutive14days did not reducethe mortality when compared with the control group. However, simvastatin20mg/kg/d pretreatment significantly increased the30-day survival rate to35%.2. Simvastatin attenuated radiation-induced jejunum injury. Histopathologic examinationfound that the integrity of jejunum structure was damaged and the mean villi heightwas significantly decreased in mice7days after4Gy radiation when compared withthe control ones. Simvastatin20mg/kg/d pretreatment for14days preserved theintegrity of jejunum structure, and increased the mean villi height when compared withthe untreated radiation group. TUNEL staining showed that4Gy radiation inducedlarger number of apoptotic cells per villi in the jejunum tissues of mice compared withthe control ones. However, simvastatin pre-administration dramatically inhibitedradiation-induced apoptosis.3. Simvastatin improved radiation-induced bone marrow damage. The long-termrecovery of the hematopoietic system is relative to replenishment of functionalhematopoietic stem cells and progenitor cells. Thus, histological changes andapoptotic cells of the femoral marrow were examined. The marrow of the radiatedgroup was not replete with hematopoietic cells as compared to the femoral marrowfrom the control mice, but recovery of marrow hematopoiesis was evident in thesimvastatin-treated group. The number of apoptotic cells from isolated femoralmarrow was larger in the radiated group than in the control, and simvastatinpretreatment significantly inhibited apoptosis.4. Simvastatin increased the number of RBC and EPCs. The number of WBC, RBC, PLTand HGB in the peripheral blood was measured7days after radiation. It was foundthat radiation reduced the number of all these cell types, resulting in a significantradiation decrease. Pretreatment of simvastatin significantly accelerated the recoveryof RBC and HGB, but did not change the cell number of WBC and PLT. In addition,the number of EPCs from peripheral blood was determined to find out whetherimprovement of myelosuppression benefited peripheral progenitor cells. Interestingly,it was found that simvastatin pretreatment significantly increased the EPCs numberwhen compared either to the control or radiated group.5. Simvastatin increased SOD activity and decreased MDA level. SOD activity,determined7days after radiation, was significantly lower in the thymus of the radiatedgroup than the control. Two-week pretreatment of simvastatin obviously elevated theSOD activity as compared to the radiated group. Contrast to the result of SOD activity in the thymus, MDA level was significantly reduced after simvastatin pretreatment ascompared to the radiated mice. Consistent with the result of thymus, changes of SODactivity and MDA were similar in the spleen.6. Simvastatin down-regulated p53expression，but not p38in thymus. The expression ofapoptosis-related proteins in thymus were analysed by Western blot on day1,3,7.Western blots revealed that p53was significantly increased in thymus of irradiatedmice when compared to the control, and reduced in simvastatin treated mice.simvastatin treatment reversed the change. It was suggested that simvastatin improvedradiation-induced tissue damage through inhibition of p53and p-p53.Conclusion: Simvastatin enhanced the survival rate and ameliorated tissues damage inradiation-induced mice. The radioprotective effect of simvastatin was possibly related toinhibition of p53and p-p53expression.