Time-dependent expressed markers and their characterization for premature senescence induced by ionizing radiation in MCF7

Recently, senescence has been suggested as a defense mechanism to block sporadic induction of cancer cells. Radiation treatment induces proliferating cancer cells to turn into non-proliferating senescent cells in vitro. To characterize transcriptional reprogramming after radiation treatment, we measured the gene expression profiles of MCF7 at different time points after treatment. In these experiments, we found that IR induced premature senescence in MCF7 cells, and IR treatment resulted in significant changes in the expression of 305 marker genes (less than 1% FDR), which were strongly correlated (|r|>0.9) with IR treatment in a time-dependent manner. Functional analysis of these markers indicated that the dynamics of cytoskeletal structure and lysosomal activity gradually increased. The expression of marker genes for modulator proteins, that were responsible for the dynamics of actin stress fibers and focal adhesion, displayed a particularly strong positive correlation with senescence-associated (SA) morphological changes through time. We also observed a strong induction of genes related to lysosomal metabolic activity, which was accompanied by an increase in the number of SA-beta-Gal positive cells. However, the expression of genes for the cell cycle progression, the post-transcription and the translation activities gradually decreased after radiation treatment. Especially, we observed clear cell cycle arrest specifically at the S and G2/M phases with consistent down-regulation of genes for microtubule assembly/disassembly or spindle biogenesis. Gene expression profiles were obtained from human MCF7 cells after ionizing radiation (6 Gy) at day 0 (no ionizing radiation), day 1 (after ionizing radiation), day 2, day 3, and day 4.

近年来，细胞衰老（senescence）被认为是一种防御机制，可阻断偶发的癌细胞诱导事件。体外实验中，放疗可使增殖性癌细胞转化为非增殖性的衰老细胞。为刻画放疗后的转录重编程过程，本研究检测了MCF7细胞在经放疗处理后不同时间点的基因表达谱。实验结果显示，电离辐射（IR）可诱导MCF7细胞发生过早衰老；且IR处理导致305个标记基因的表达发生显著变化（错误发现率FDR<1%），这些基因的表达与IR处理呈强相关性（|相关系数r|>0.9），且具有时间依赖性。对上述标记基因的功能分析表明，细胞骨架结构动力学与溶酶体活性均呈逐渐增强的趋势；其中，负责调控肌动蛋白应力纤维（actin stress fibers）与黏着斑（focal adhesion）动力学的调控蛋白标记基因，其表达随时间推移与衰老相关（SA）形态变化呈现出极强的正相关性。本研究同时观察到，与溶酶体代谢活性相关的基因被显著诱导表达，且这一现象伴随衰老相关β-半乳糖苷酶（SA-beta-Gal）阳性细胞数量的增加。然而，放疗后，参与细胞周期进程、转录后调控及翻译活动的基因表达量逐渐下调。尤为关键的是，本研究观察到细胞周期出现明确的阻滞，且特异性地发生在S期与G2/M期；同时参与微管组装/去组装或纺锤体生物发生的基因呈现持续下调。本研究的基因表达谱来源于经6 Gy电离辐射处理的人MCF7细胞，采样时间点分别为第0天（未接受电离辐射）、第1天、第2天、第3天及第4天。