Radiobiological factors determining the radiosensitivity of radioresistant cancer cells

为了阐明黑素瘤细胞的放射耐药机制，我们在人类和小鼠黑素瘤细胞的实验系统中证明了以下事实：(1)黑素瘤细胞的辐射剂量-生存曲线在低剂量区域具有较大的肩部部分，对常规分割放疗中常用的剂量表现出明显的放射耐药；(2)照射后黑色素瘤细胞对潜在致死损伤（PLD）有明显的修复作用；(3)黑色素瘤细胞中s期细胞的比例较高，在细胞周期中s期细胞比其他期细胞更耐辐射；(4)左旋多巴预处理黑色素瘤细胞显著降低了黑色素瘤细胞的亚致死辐射损伤能力。在此基础上，我们从细胞动力学、黑色素生成、谷胱甘肽代谢、DNA修复等方面进行研究，进一步阐明黑素瘤细胞辐射耐药的机制。所有产生黑色素的黑色素瘤细胞系在低剂量区域显示出较大的肩部部分，在各种条件下检查PLD的修复较大。同步培养法显示了细胞周期中放射敏感性的变化以及细胞周期各阶段的存活曲线。检测GSH水平和GSH相关酶活性，提示GSH代谢与放射敏感性之间的关系。此外，我们还检测了辐照后的非计划性DNA合成（unscheduled DNA synthesis， UDS），以阐明DNA修复与辐射敏感性的关系。

In an attempt to elucidate the mechanism of radioresistance in melanoma cells, we demonstrated the following facts in the experimental system using human and murine melanoma cells : (1) the radiation dose-survival curve in melanoma cells had a larger shoulder portion in the low dose region, showing a marked radioresistance to doses commonly used in the conventional fractionated radiotherapy ; (2) melanoma cells showed a prominent repair of potentially lethal damage (PLD) after irradiation ; (3) melanoma cells had a relatively high fraction of S-phase cells which were more radioresistant than other phase cells in the cell cycle ; and, (4) pretreatment of melanoma cells with L-dopa significantly reduced the capacity for sublethal radiation damage in melanoma cells.On the basis of these results, we have studied the cell kinetics, melanin production, GSH metabolism, and DNA repair, so as to further elucidate the mechanism of radioresistance of melanoma cells. All of melanin-producing melanoma cell lines showede a larger shoulder portion in the low dose region and a larger repair of PLD in various conditioned examined. The synchronized culture method demonstrated the change of radiosensitivity during the cell cycle and the survival curves of cells in each phase of the cell cycle. GSH levels and GSH-related enzymes activity were examined, suggesting the relationship between the GSH metabolism and radiosensitivity. Further, unscheduled DNA synthesis (UDS) after irradiation were also examined, so as to elucidate the relationship between the DNA repair and radiosensitivity.