Analysis of molecular mechanism of radiation-induced cancer and risk estimation

为了阐明辐射致癌的分子机制，并将其应用于风险评估，我们进行了两个实验。首先，我们分析了辐射诱导的小鼠肝癌中的基因表达。用3Gy射线照射B6C3F1小鼠诱发小鼠肝癌，并从同一小鼠的肝癌和正常肝脏中提取γ。我们用差异显示技术鉴定了差异表达基因。我们发现了19个在肝癌中差异表达的基因。5个基因在肝癌中表达下调，14个基因表达上调，其中包括3个新基因CRAD3，它是顺式视黄醇/雄激素脱氢酶(CRAD)家族的成员，Sdf211，它是PMT/RT家族的成员，以及A141-36，其同源性不明。CRAD在雄激素代谢中起着重要作用，它将失活的3羟基-二醇转化为活性的双氢睾酮(氧化3α-羟基类固醇脱氢酶活性：氧化3α更多的HSD活性)，从而增加雄激素活性。实际上，在生理3α-二醇水平上，小鼠肝癌组织中的氧化3α-HSD活性高于正常肝脏。众所周知，双氢睾酮可以促进肝癌的发生。因此，CRAD3的过表达可能通过增加局部双氢睾酮水平来改变辐射诱导的小鼠肝癌的发生。其次，我们试图阐明自发突变在辐射致癌中的作用，因为辐射诱发的癌症和自发癌症之间的突变谱相似是有文献记载的。我们鉴定了小鼠和人的Rev1基因，它是UMUC/DinB/XPV基因家族的成员，在自发突变中起着重要的作用。对小鼠Rev1蛋白的生化分析表明，小鼠Rev1蛋白具有与人Rev1蛋白相同的脱氧胞苷转移酶活性。辐射诱导培养的原代胚胎成纤维细胞表达小鼠Rev1基因。这一观察结果可能很重要，因为生化特性表明，在DNA复制过程中，Rev1蛋白的活性是绕过DNA合成所致的氧化损伤所必需的。较少

To clarify the molecular mechanism of radiation-carcinogenesis, and apply it to the risk estimation, we conducted two experiments. Firstly, we analyzed the expression of genes in radiation-induced-mouse hepatomas. Mouse hepatomas were induced in B6C3F1 mice by 3Gy of ^<60>Coγ-ray exposure, and mRNAs were isolated from hepatomas and normal liver in the same mice. We identified differentially expressed genes by differential display technique. We found nineteen differentially expressed genes in hepatomas. Expressions of five genes were decreased and those of other fourteen genes were increase in hepatomas, including novel three genes named CRAD3 that was a member of cis-retinol/androgen dehydrogenase (CRAD) family, Sdf211 that was a member of Pmt/rt family, and A141-36 whose homology was not identified. CRAD plays an important role in androgen metabolism, which converts inactive 3α-adiol, into active dihydrotestosterone, (oxidative 3α-hydroxysteroid dehydrogenase activities : oxidative 3α … More -HSD activities) and consequently increases androgen activity. Actually, oxidative 3α-HSD activity in mouse hepatomas was found to be higher than that in normal liver at physiological 3α-adiol level. Dihydrotestosterone is well known to promote hepatocarcinogenesis. Therefore, the over-expression of CRAD3 must modify the radiation-induced mouse hepatocarcinogenesis by increasing local dihydrotestosterone level. Secondly, we try to clarify the involvement of spontaneous mutations in radiation carcinogenesis, because similarity of mutation spectra between radiation-induced and spontaneous cancers was well documented. We characterized mouse and human Revl gene which was a member of the UmuC/DinB/XPV gene family, played important roles in spontaneous mutations. Biochemical analysis of the mouse Rev1 protein revealed that the mouse Rev1 protein possessed a deoxycytidyl transferase activity as human REV1 protein. The expression of mouse Rev1 gene of primary embryonic fibroblasts in culture was induced by radiation exposure. This observation might be important, because the biochemical property suggested that the activity of the Rev1 protein was required for bypassing oxidative DNA damage by translesioh DNA synthesis during DNA replication. Less