Xenobiotic receptors

过氧化物酶体增殖物激活受体（PPARA）是一种配体激活的转录因子，是脂质代谢的关键介质。PPARA激动剂被认为是“经典的”非遗传毒性致癌物，慢性PPARA激活会导致啮齿动物的肝细胞增殖、肝肿大，并最终导致肝细胞癌（HCC）。研究表明，Myc癌基因有助于激动剂诱导的肝细胞增殖。MYC上调通过PPARA介导的miR-let7c1下调间接发生，miR-let7c1靶向MYC mRNA降解。使用肝特异性MYC敲除（MYC - dhep）小鼠，用强效PPARA激动剂吡替尼酸（Wy14643）治疗，在体内证实了MYC对肝细胞增殖的贡献。有趣的是，激动剂治疗的MycdHep小鼠对肝细胞增殖和肝肿大有抗性，但不能完全保护。此外，Myc-dHep小鼠对N亚硝基二乙胺（DEN）诱导的HCC仅具有部分保护作用。DEN治疗的Myc-dHep小鼠肿瘤前灶和腺瘤的发生率不到一半。这些数据表明，在这些模型中，MYC并不启动细胞增殖，而是增强了由其他机制启动的增殖作用。最近提出了MYC作用的模型，其中MYC作为转录的一般放大器，进一步提高转录活性基因的表达。在该模型中，MYC结合通过促进RNA聚合酶从“暂停”状态释放从而加速转录速率，从而促进转录延伸。该模型源于这样一个事实，即MYC靶点在不同细胞类型之间差异很大，缺乏靶基因重叠使得很难归因于与MYC激活相关的独特基因特征。支持MYC基因表达扩增的研究主要依赖于微阵列和下一代测序数据，这些数据来自人工调节MYC水平或活性的细胞系。其他研究表明，MYC并不普遍地扩增转录活性基因，而是以肿瘤特异性、组织特异性和细胞类型特异性的方式激活离散的基因集。在体内测试MYC放大器活性的一个主要障碍是需要协调控制一组确定的靶基因和MYC表达水平。转录因子PPARA联合MycdHep小鼠的药理激活为在体内测试MYC放大器模型提供了一个独特的机会。Myc消融仅对PPARA长期激活的增殖作用提供部分保护。这一观察结果可以通过MYC介导的PPARA靶基因扩增的潜在机制来解释。因此，MYC对特定基因的转录扩增可能在激动剂诱导的HCC模型中发挥主要作用。PPARA的慢性激活促进小鼠MYC相关肝细胞癌（HCC）的发生。最近的研究表明，MYC可以作为转录放大器，其中MYC不作为基因表达的“开关”，而是通过刺激转录延伸来加速活性启动子的转录速率。考虑到MYC可能会扩增PPARA靶基因的表达，从而促进细胞增殖和肝癌，我们分析了野生型和肝脏特异性MYC敲除（MycdHEP）小鼠肝脏中PPARA激动剂吡替尼酸的基因表达。在MYC的存在下，PPARA靶基因的一个子集被扩增，包括角蛋白23 （Krt23）。Krt23的诱导作用在MycdHEP小鼠中显著减弱，在Ppara缺失小鼠中完全消失。报告基因检测和染色质免疫沉淀证实了PPARA和MYC与Krt23启动子内的位点直接结合。KRT23在原代肝细胞中的强制表达诱导了细胞周期相关基因。这些数据表明，PPARA激活可提高MYC的表达，从而增强参与细胞增殖的PPARA靶基因的表达。最后，KRT23蛋白在人类hcc中高度升高，表明在小鼠中的研究可翻译为人类。PPARA还被发现影响结直肠癌（CRC），根据2018年美国癌症协会的统计数据，结直肠癌是美国第三大常见癌症，也是美国癌症死亡的第三大原因。结直肠癌与多种病因相关，包括遗传和环境因素。最常见的基因突变包括APC（大肠腺瘤性息肉病）、KRAS和TP53。环境因素，包括炎症、肥胖、糖尿病和饮食也在结肠癌中发挥重要作用。目前，结直肠癌的主要治疗方案是结肠切除术联合化疗，使用5氟尿嘧啶（5FU）等药物。不幸的是，转移性结肠癌的5年生存率只有13.3%。因此，有必要进一步了解驱动结肠癌发生的肿瘤生物学，以确定预防和治疗结肠癌的新药物靶点。尽管在了解结肠癌的发病机制方面已经做出了巨大的努力，但其分子机制尚未完全了解。只有考虑到pparα表达的组织特异性背景，才能全面分析PPARA在炎症性疾病和肿瘤发生中的作用。本研究通过制备绒毛蛋白-cre/ PPARA -floxed （PPARA - die）小鼠，研究肠道PPARA在结肠癌发生中的作用。小鼠结肠肿瘤中PPARA mRNA水平降低。Ppara-dIE小鼠在给予偶氮氧甲烷后，无论是否给予DSS，都比对照小鼠出现更多更大的结肠肿瘤。代谢组学分析显示，与对照组小鼠相比，在给药偶氮甲烷（无论是否使用DSS）后，Ppara-dIE小鼠尿液和结肠中甲基化相关代谢物增加。与对照组小鼠相比，Ppara-dIE小鼠结肠肿瘤中DNA甲基转移酶1 （DNMT1）和蛋白精氨酸甲基转移酶6 （PRMT6）水平升高。PPARA的缺失降低了视网膜母细胞瘤蛋白（RB1）的表达，导致DNMT1和PRMT6的表达增加。DNMT1和PRMT6分别通过DNA甲基化和组蛋白H3R2二甲基化介导的转录抑制，降低肿瘤抑制基因Cdkn1a （P21）和Cdkn1b （p27）的表达。非诺贝特保护人类PPARA转基因小鼠免受氮氧甲烷和dss诱导的结肠癌。人结肠腺癌标本中PPARA和RB1水平低于正常结肠组织，DNMT1和PRMT6水平高于正常结肠组织。小鼠肠道中PPARA的缺失通过增加DNMT1介导的P21甲基化和PRMT6介导的p27甲基化来促进结肠癌的发生。人类结直肠肿瘤组织的PPARA mRNA和蛋白水平低于非肿瘤组织。激活PPARA的药物可能被开发用于化学预防或治疗结肠癌。

Peroxisome proliferator activated receptor alpha (PPARA) is a ligand activated transcription factor and key mediator of lipid metabolism. PPARA agonists are considered "classic" nongenotoxic carcinogens, and chronic PPARA activation causes hepatocyte proliferation, hepatomegaly, and, eventually, hepatocellular carcinoma (HCC) in rodents. Studies have shown that the Myc oncogene contributes to agonist induced hepatocyte proliferation. MYC upregulation occurs indirectly through the PPARA mediated down regulation of miR-let7c1, which targets Myc mRNA for degradation. The contribution of MYC to hepatocyte proliferation was confirmed in vivo using liver specific Myc knockout (Myc-dHep) mice treated with the potent PPARA agonist pirinixic acid (Wy14643). Interestingly, agonist treated MycdHep mice were resistant to, but not completely protected from, hepatocyte proliferation and hepatomegaly. Additionally, Myc-dHep mice were only partially protected from N nitrosodiethylamine (DEN) induced HCC. DEN treated Myc-dHep mice had less than half the incidence of preneoplastic foci and adenomas. These data suggest that, in these models, MYC does not initiate cell proliferation but rather potentiates the proliferative effects initiated by alternate mechanisms. A model of MYC action was recently proposed in which MYC functions as a general amplifier of transcription, further elevating expression of transcriptionally active genes. In this model, MYC binding accelerates transcription rates by facilitating the release of RNA polymerase from its "paused" state, thereby promoting transcript elongation. This model originated from the fact that MYC targets vary greatly between cell types, and the lack of target gene overlap makes it difficult to ascribe a distinct gene signature associated with MYC activation. Studies supporting MYC amplification of gene expression have primarily relied on microarrays and next generation sequencing data derived from cell lines in which MYC levels or activity are artificially modulated. Other studies have suggested that MYC does not universally amplify transcriptionally active genes and instead activates discrete sets of genes in a tumor specific, tissue specific, and cell type specific fashion. A major obstacle for testing MYC amplifier activity in vivo is the need for coordinated control of both a defined set of target genes and MYC expression levels. Pharmacological activation of the transcription factor PPARA in combination with MycdHep mice affords a unique opportunity to test the MYC amplifier model in vivo. Myc ablation only provided partial protection against the proliferative effects of prolonged PPARA activation. This observation could be explained by an underlying mechanism involving the MYC mediated amplification of PPARA target genes. Transcriptional amplification of select genes by MYC may therefore play a major role in agonist induced HCC models. Chronic activation of PPARA promotes MYC linked hepatocellular carcinoma (HCC) in mice. Recent studies have shown that MYC can function as an amplifier of transcription where MYC does not act as an "on off" switch for gene expression but rather accelerates transcription rates at active promoters by stimulating transcript elongation. Considering the possibility that MYC may amplify the expression of PPARA target genes to potentiate cell proliferation and liver cancer, gene expression was analyzed from livers of wild type and liver specific Myc knockout (MycdHEP) mice treated with the PPARA agonist pirinixic acid. A subset of PPARA target genes was amplified in the presence of MYC, including keratin 23 (Krt23). The induction of Krt23 was significantly attenuated in MycdHEP mice and completely abolished in Ppara null mice. Reporter gene assays and chromatin immunoprecipitation confirmed direct binding of both PPARA and MYC to sites within the Krt23 promoter. Forced expression of KRT23 in primary hepatocytes induced cell cycle related genes. These data indicate that PPARA activation elevates MYC expression, which in turn potentiates the expression of select PPARA target genes involved in cell proliferation. Finally, KRT23 protein is highly elevated in human HCCs indicating that the studies in mice are translatable to humans. PPARA was also found to influence colorectal cancer (CRC) which is the third most common cancer and is the third leading cause of cancer death in the United States according to 2018 American Cancer Society statistics. CRC is associated with diverse etiological components including both genetic and environmental factors. The most commonly found genetic mutations include APC (adenomatous polyposis coli), KRAS, and TP53. Environmental factors, including inflammation, obesity, diabetes, and diet also play important roles in colon cancer. Currently the major treatment option for CRC is a combination of colectomy with chemotherapy employing agents such as 5 fluorouracil (5FU). Unfortunately, the 5 year survival rate for metastatic colon cancer is only about 13.3%. Therefore, it is necessary to further understand the cancer biology driving colon carcinogenesis in order to identify novel drug targets for both the prevention and treatment of this disease. Although significant efforts have been undertaken to understand the pathogenesis of colon cancer, the molecular mechanisms are not fully understood. A comprehensive analysis of PPARA's role in inflammatory diseases and tumorigenesis can only be achieved by considering the tissue-specific context of PPARalpha expression. In the current study, villin-cre/Ppara-floxed (Ppara-dIE) mice were generated and the role of intestinal PPARA in colon carcinogenesis studied. Levels of PPARA mRNA were reduced in colon tumors from mice. Ppara-dIE mice developed more and larger colon tumors than control mice following administration of azoxymethane, with or without DSS. Metabolomic analyses revealed increases in methylation related metabolites in urine and colons from Ppara-dIE mice, compared with control mice, following administration of azoxymethane, with or without DSS. Levels of DNA methyltransferase 1 (DNMT1) and protein arginine methyltransferase 6 (PRMT6) were increased in colon tumors from Ppara-dIE mice, compared with colon tumors from control mice. Depletion of PPARA reduced the expression of retinoblastoma protein (RB1), resulting in increased expression of DNMT1 and PRMT6. DNMT1 and PRMT6 decreased expression of the tumor suppressor genes Cdkn1a (P21) and Cdkn1b (p27) via DNA methylation and histone H3R2 dimethylation-mediated repression of transcription, respectively. Fenofibrate protected human PPARA transgenic mice from azoxymethane and DSS-induced colon cancer. Human colon adenocarcinoma specimens had lower levels of PPARA and RB1 and higher levels of DNMT1 and PRMT6 than normal colon tissues. Loss of PPARA from the intestine promotes colon carcinogenesis by increasing DNMT1 mediated methylation of P21 and PRMT6 mediated methylation of p27 in mice. Human colorectal tumors have lower levels of PPARA mRNA and protein than non-tumor tissues. Agents that activate PPARA might be developed for chemoprevention or treatment of colon cancer.