Iroquois homeobox 1 (IRX1) belongs to the Iroquois homeobox family known to play an important role during embryonic development. Interestingly, however, recent studies have suggested that IRX1 also acts as a tumor suppressor. Here, we use homozygous knockout mutants of zebrafish to demonstrate that the IRX1 gene is a true tumor suppressor gene and mechanism of the tumor suppression is mediated by repressing cell cycle progression. In this study, we found that knockout of zebrafish Irx1 gene induced hyperplasia and tumorigenesis in the multiple organs where the gene was expressed. On the other hands, overexpression of the IRX1 gene in human tumor cell lines showed delayed cell proliferation of the tumor cells. These results suggest that the IRX1 gene is truly involved in tumor suppression. In an attempt to identify the genes regulated by the transcription factor IRX1, we performed microarray assay using the cRNA obtained from the knockout mutants. Our result indicated that the highest fold change of the differential genes fell into the gene category of cell cycle regulation, suggesting that the significant canonical pathway of IRX1 in antitumorigenesis is done by regulating cell cycle. Experiment with cell cycle blockers treated to IRX1 overexpressing tumor cells showed that the IRX1 overexpression actually delayed the cell cycle. Furthermore, Western blot analysis with cyclin antibodies showed that IRX1 overexpression induced decrease of cyclin production in the cancer cells. In conclusion, our in vivo and in vitro studies revealed that IRX1 gene functionally acts as a true tumor suppressor, inhibiting tumor cell growth by regulating cell cycle.

中文翻译：

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Iroquois Homeobox 1通过调节细胞周期进程，在多个器官中充当真正的肿瘤抑制剂。

Iroquois同源盒1（IRX1）属于Iroquois同源盒家族，已知在胚胎发育过程中起重要作用。但是，有趣的是，最近的研究表明IRX1也可以作为肿瘤抑制因子。在这里，我们使用斑马鱼的纯合敲除突变体来证明IRX1基因是真正的抑癌基因，并且其抑制肿瘤的机制是通过抑制细胞周期进程来介导的。在这项研究中，我们发现敲除斑马鱼Irx1基因会在表达该基因的多个器官中引起增生和肿瘤发生。另一方面，IRX1基因在人肿瘤细胞系中的过表达表明肿瘤细胞的细胞增殖延迟。这些结果表明IRX1基因确实参与了肿瘤抑制。为了鉴定受转录因子IRX1调控的基因，我们使用从敲除突变体获得的cRNA进行了微阵列分析。我们的结果表明，差异基因的最高倍数变化属于细胞周期调节的基因类别，这表明IRX1在抗肿瘤发生中的重要经典途径是通过调节细胞周期来完成的。用细胞周期阻滞剂处理过表达IRX1的肿瘤细胞的实验表明，IRX1过表达实际上延迟了细胞周期。此外，用细胞周期蛋白抗体进行的蛋白质印迹分析表明，IRX1过表达诱导癌细胞中细胞周期蛋白的产生减少。总之，我们的体内和体外研究表明，IRX1基因在功能上可充当真正的肿瘤抑制因子，