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肝癌发展过程中促血管生成因子NogoB的转录调节机制
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摘要
肝癌是我国常见恶性肿瘤之一,我国每年约有11万人死于肝癌。本室于1998年克隆的基因NogoB,又称RTN4B,属于定位于内质网膜的reticulon (RTN)蛋白家族。研究发现NogoB在神经和肌肉系统中发挥一定的作用。还有文献报道称NogoB能够诱导血管内皮细胞的迁移和粘附,并参与血管损伤后的重塑。而我们发现NogoB在正常肝组织中不表达但在肝癌组织中异常高表达。因此,我们推测并在以前的研究中证实了肝癌中高水平表达的NogoB能够促进肝癌组织内的血管生成,从而促进肝癌的发生发展。为了探讨NogoB在正常肝脏中不表达而在肝癌组织中高水平表达的原因,我们对它的转录调节机制进行了一系列研究。
     随着肿瘤的快速发展,距离血管超过180μm的肿瘤细胞会发生坏死。这与氧气可扩散的距离相当。为了适应缺氧的环境,肿瘤细胞通过调节信号通路,诱导出新生血管,从而为快速持续增殖的肿瘤细胞提供充足的氧气和代谢物。缺氧诱导因子(HIF-1a)即是一个调控缺氧诱导的血管生成的关键转录因子。HIF-1α广泛存在于人和哺乳动物体内,它调控的众多靶基因都能通过促进内皮细胞的增殖和迁移而参与调节血管生成,从而使肿瘤细胞适应缺氧微环境。基于这些研究结果,我们推测新的促血管生成因子NogoB在肝癌中的高水平表达很可能也是受HIF-1α调节的。通过比较癌和癌旁组织样本中NogoB和HIF-1α的表达量,我们发现NogoB和HIF-1α都是在肝癌组织中上调表达。我们利用大鼠肝动脉结扎模型,结扎后的肝脏处于低氧环境并且诱导了NogoB的表达。用缺氧模拟化合物CoCl2处理大鼠原代肝细胞后,NogoB也能够上调表达。而后,我们用缺氧模拟化合物DFO处理肝癌细胞系,发现其中的NogoB表达量也逐渐增加,而缺氧(1%02)也可以使NogoB的表达量在肝癌细胞系中逐渐增加。为了研究NogoB受缺氧诱导是否由HIF-1a调节,我们利用荧光素酶报告系统发现HIF-1a能增强NogoB启动子的转录活性并存在剂量依赖。用慢病毒干扰内源HIF-1α后,缺氧条件下NogoB启动子的激活显著下降,这说明缺氧可以通过HIF-1a上调NogoB的转录。为了确定HIF-1是否能直接调节NogoB以及NogoB启动子上有效的HRE的位置,我们对NogoB启动子进行了生物信息学分析,发现存在两个可能的H1F-1α应答元件(HRE)。进而我们对NogoB启动子做了分段缺失,发现转录起始位点上游-4041—-991碱基之间存在HIF-1a应答元件。然后我们对转录起始位点上游-3829—-3822碱基的HRE进行点突变,在1%02或DFO处理的条件下NogoB启动子的转录活性均没有提高,说明缺氧条件下,HIF-1通过该HRE激活NogoB的启动子。然后,我们用染色质免疫沉淀法证实HIF-1α在1%02和化学模拟缺氧条件下均能在体内与NogoB启动子DNA序列上的该HRE直接结合。并且凝胶阻滞实验进一步证实HIF-1和HRE的结合是直接的,并且鉴定了具体的结合位点序列。我们的研究发现促血管生成因子NogoB受缺氧条件诱导上调表达,并通过体内体外实验探讨了HIF-1α调节NogoB表达的机制。
     虽然缺氧是NogoB在肿瘤中诱导表达的重要机制,然而,许多肿瘤促血管生成因子如VEGF,在受缺氧诱导的同时也能通过其他转录因子应答肿瘤细胞的内外刺激。AP-1是一种bZIP蛋白形成的二聚体转录因子,包括Jun、Fos、ATF、MAF等蛋白家族。它可由两个Jun蛋白形成同源二聚体或由一个Jun和一个Fos蛋白形成异源二聚体,通过结合启动子上的TPA应答元件(TRE)调节靶基因的转录,从而参与肿瘤的转化、增殖、浸润、转移和血管生成等过程。我们发现与癌旁组织相比,AP-1也和NogoB一样在癌组织中上调表达。瞬时表达外源AP-1和用TPA激活内源AP-1能够提高NogoB的表达水平和启动子的转录活性。并且对NogoB转录激活是通过Jun而不是Fos实现的。A-Fos是AP-1的一个显性负突变体,而它可以抑制AP-1或TPA激活的NogoB转录活性。另外A-Fos只能抑制AP-1诱导的转录活性,而对基础转录活性的抑制作用有限,说明AP-1并不负责NogoB的基础表达。通过对NogoB启动子进行分析,我们发现存在三个可能的AP-1结合位点(TRE)。我们对NogoB启动子做了分段缺失,发现转录起始位点上游-991—-770之间的区域对于AP-1诱导NogoB转录十分重要。对启动子上这三个TRE做点突变,发现转录起始位点上游-782—-776碱基之间是AP-1应答元件。染色质免疫沉淀法证实AP-1能在体内与NogoB启动子序列上的这段DNA结合。
     综上所述,本文研究发现肝癌中HIF-1 a和AP-1能够转录激活促血管生成因子NogoB的表达,即肝肿瘤快速增殖产生的缺氧微环境激活了NogoB的表达,同时肝癌中高水平的AP-1也参与了NogoB的转录激活。从而为阐明NogoB在肝癌中异常高水平表达的分子机制提供了理论依据。
Hepatocarcinoma (HCC) is one of the common malignant tumors in China.110 thousands of people died of HCC every year. NogoB, also known as RTN4B, was cloned by our lab in 1998. It is a member of reticulon (RTN) protein family which is localized in the endoplasmic reticulum. It is found that NogoB plays dual roles in both nerve and muscle system. It is also reported that NogoB takes part in vascular remodeling by promoting migration and adhesion of vascular endothelial cells. However, our group found that NogoB is expressed highly and seretedly in liver cancer compared to its expression in adjacent tumor tissue. So we presumed and confirmed in the former studies that highly expressed NogoB plays positive roles in tumor angiogenesis, and then promotes tumorigenesis in HCC. To explore the reason of the specific characterization of NogoB in HCC, we worked on its mechanism of transcriptional regulation.
     Along with the uncontrolled proliferation, tumor cells that were located more than 180μm away from blood vessels were observed to necrose. This is similar to the distance that oxygen diffuses as it passes from the capillary to cells. In response to the hypoxia condition, cancer cells undergo genetic changes to induce new blood vessel to provide rapidly proliferating tumor cells with an adequate supply of oxygen and metabolites. Hypoxia inducible factor (HIF)-la is one of the key regulator in hypoxia induced angiogenesis. HIF-1a is a transcription factor widely expressed in human and mammalian. Its multiple target genes have been proved to modulate angiogenesis by promoting proliferation and migration in endothelial cells. Based on the evidence, we speculated that the overexpression of novel pro-angiogenesis factor NogoB is also regulated by HIF-la in HCC.
     By comparing the expression patterns of NogoB in HCC and its normal counterpart by Western blot analyses, we found that both NogoB and HIF-la was significantly up-regulated in HCC samples in comparison to the adjacent non-tumorous liver tissues. Then we use rat hepatic artery ligation (HAL) model to make the ligated liver physiological hypoxia, and this induced the expression of NogoB. Deferoxamine (DFO) and CoCl2 could mimic hypoxia conditions by means of stabilizing HIF-1α. In CoCl2 treated rat primary liver cells, increased expression of NogoB was also observed. By using hepatocarcinoma cell lines as a model, we found that in response to DFO or hypoxic condition, the expression of NogoB increased in a time-dependent manner. To determine whether elevated expression of NogoB was mediated by HIF-la, we utilized luciferase assay. The result showed that transfected HIF-la . increased the NogoB promoter activity in a dose-dependent manner and lentivirus delivery of HIF-1a siRNA reduced the NogoB promoter activity in hypoxia condition. Analysis of the NogoB promoter showed two predicted hypoxia response elements (HRE). To identify the HIF-la binding site in NogoB promoter, a series of NogoB promoter deletion constructs was generated. The promoter region responsible to hypoxia span from-4041 to-991 relative to transcription start site. Then a site-directed mutant was constructed at putative HRE located-3829 to-3822 upstream of transcription start site. No significant increase of NogoB promoter activity was detected either in hypoxia or in DFO treated culture. Chromatin immunoprecipitation assay (ChIP) proved that HIF-la protein is bound to the distal HRE. A gel mobility shift assay confirmed that the interaction between HIF-1 and the HRE in the NogoB promoter is direct. Our observation suggeted that hypoxia may play a central role in NogoB upregulation in HCC and further elucidate the molecular mechanisms responsible for hypoxia mediated induction of NogoB expression in HCC cells.
     That hypoxia strongly induced NogoB is an important mechanism of induction in tumor. However, many tumor-associatied angiogenetic factors could also be elevated by other transcriptional factors in response to diverse simuli. The AP-1 (activator protein 1) transcription factor, known as basic leucine-zipper (bZIP) protein, is a dimeric complex which is composed of Jun, Fos, ATF and Maf protein families. The Jun proteins could form homodimers or heterodimers with Fos proteins which dimerize with Jun proteins to enhance their DNA-binding activity. By binding to the TPA response element (TRE) in the promoter, AP-1 complex transcriptional regulates the expression of target genes and thereby plays a role in the proliferation, invasion, metastasis, transformation and angiogenesis of tumors. Our data indicated both NogoB and AP-1 was significantly up-regulated in HCC samples in comparison to the adjacent non-tumorous liver tissues. Transient expression of AP-1 and activation of endogenous AP-1 by treatment with TPA increased the NogoB expression and its promoter activity. And the induction of NogoB promoter activity was mediated by c-Fos but not c-Jun. A-Fos, a dominant negative construct of AP-1, showed to inhibit AP-1-dependent and TPA induced NogoB promoter activity. The A-Fos was able to prevent AP-1 induction of the promoter activity, but only slightly decreased the basal activity of the promoter. This indicated that constitutive NogoB expression is only slightly due to the effect of AP-1. Bioinformatic analysis revealed three putative AP-1 response elements. Sequential deletion of the promoter revealed that the region located upstream of transcription initiation site (-991/-770) was essential for AP-1 induction of NogoB transcription. Site-directed mutation analysis of three potential AP-1 binding sites suggested that the distal element at-782bp to-776bp was responsible for the induction, which was confirmed by chromatin immunoprecipitation.
     In conclusion, our results provided the molecular basis that the overexpression of NogoB was mainly induced by HIF-la in hypoxic condition. Meanwhile AP-1 also played an important role in regulation of NogoB in response to diverse simuli.
引文
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