三篇Nature探讨BET抑制剂的癌症耐药性机制
2016/01/26
延续着去年两篇发表在Nature上的文章,在本期1月21日Nature上的一篇文章探讨了BET抑制剂的表观基因组靶向药物的潜力,并解释了它们的耐药性机制,为改善其临床反应提供了潜在途径。


许多现代的癌症药物以突变激活的蛋白为靶向,但这种治疗方法有一定的局限性。只有相对少量的突变出现在人类肿瘤中,耐药性也快速发展。针对表观基因组——化学修饰后的DNA,相关的组蛋白和促进DNA进行染色质包装的其他蛋白质;所有这些都影响基因的表达,而且是一个很好的替代方法。延续着去年两篇发表在Nature上的文章,在本期1月21日Nature上的一篇文章探讨了BET抑制剂的表观基因组靶向药物的潜力,解释了它们的耐药性机制,为改善其临床反应提供了潜在途径。

1
BET抑制剂的发现

长期以来,大家都承认肿瘤细胞具有鲜明的表观基因组学特征,从而导致肿瘤促进转录因子,如Myc过量表达。转录因子是具有挑战性的治疗靶点,因为它们缺乏可以很容易地用药物来靶向的结构。但是,随着我们对表观基因组调节因子如何影响基因表达的更深的认识,其中许多似乎是“药物”,为避开这个障碍提供了一个潜在的方法。

在这些因子中bromodomain蛋白家族,包括BET亚家族(BRD2、3、4和T)。BET蛋白含有两个bromodomains,每个与已标记了乙酰基团的组蛋白结合,使BET蛋白在基因组的特定位点招募细胞的转录机制开展对基因表达的调节。BET亚家族成员如BRD4,可调节Myc基因的转录,参与了多种肿瘤(尤其是血液中的癌症)过程。因此可作为治疗的候选目标。

几年前,一些小分子BET抑制剂首次被发现,并显示能有效破坏癌细胞的增殖。这种效应似乎反映了BET介导的Myc基因的表达受到抑制。在白血病和淋巴瘤的早期临床试验中,已经令人鼓舞。研究人员正在寻找在哪些其他疾病的情况下,这些抑制剂可能起作用,并预测所将不可避免地出现的耐药性机制。

2
BET抑制剂在急性髓细胞性白血病中可能的耐药性机制

2015年两个研究关注了BET抑制因子在急性髓细胞性白血病(AML)中的可能耐药性性机制。Rathert等筛选了小鼠白血病细胞生存所需的染色质修饰因子。他们证实AML细胞需要BRD4,也确定其他几个因子的抑制活性赋予AML细胞耐JQ1。在对JQ1耐药的白血病细胞中,作者观察到调节基因表达的DNA的增强子区域特定表观基因特征的变化。这些变化意味着,myc基因表达可以无需通过BRD4激活。

Rathert等人还发现,已知驱动肿瘤发生的Wnt信号通路相关的基因,在耐药细胞中的表达上调。Wnt激活足以促进对JQ1的耐药性,在耐药细胞可能是通过增强子驱动MYC转录。最后,作者发现在其他一些类型的癌症和白血病患者中对JQ1的耐药性机制相同。他们的数据表明,BET抑制剂会和Wnt通路抑制剂结合扩大作用。

Fong等人以一种不同的方法,通过不断使小鼠AML细胞暴露在BET抑制剂中,让这些细胞产生耐BET抑制,最终产生抗药性克隆群。本实验也表明Wnt激活在耐药性中起作用。此外,耐药细胞具有干细胞特征,这表明AML肿瘤干细胞群,或其子集,不回应BET抑制剂。


BET抑制: BET蛋白BRD4可以结合乙酰基(K)由组蛋白将DNA包装成染色质。BRD4上调表达促癌的Myc基因改变细胞的转录机制。用BET抑制剂治疗可以防止BRD4–染色质结合,阻止MYC转录,但癌细胞迅速产生耐药性。Rathert和Fong等人报告说,在急性髓细胞性白血病(AML),耐药性是通过Wnt信号通路的活化授予,导致DNA和MYC激活的β-连环蛋白结合。相比之下,Shu等人发现抗性在三阴性乳腺癌(TNBC)中是源于酪蛋白激酶2(CK2)激活酶。CK2磷酸化(P)BRD4,允许BRD4结合到转录调节蛋白MED1激活MYC。

3
BET在三重阴性乳腺癌中的可能耐药性机制

在1月21日最新的研究中,Shu等人探讨了在人乳腺癌中BET的抑制作用。通过分析一组乳腺癌细胞株,他们观察到,一个癌症亚型——“三重阴性”乳腺癌是对BET抑制敏感的。像Fong等人,作者通过在JQ1中培养敏感的三阴性细胞来构建对BET的获得性耐药细胞,然后分析耐药性细胞的特征。耐药性细胞仍然依赖于BRD4,但这种依赖不涉及蛋白质的bromodomains。

一个称为MED1的广泛存在的转录活化调节蛋白,在耐药细胞中比在敏感细胞中和BRD4结合得更紧密。作者分析这种更紧密的结合是由于酪蛋白激酶2(CK2)介导的BRD4磷酸化的增加。结合产生了不依赖于bromodomain的,BRD4介导的MYC转录活性激活。这些数据表明,使用CK2和BET抑制剂组合治疗三阴性乳腺癌可能防止癌症耐药性。

虽然以前的研究已经表明,在动物和细胞模型中药物对三阴性乳腺癌有效性,值得注意的是,这些药物至今未能在人体中对抗肿瘤。因此,乐观情绪应该暂缓一下。

总结

总的来说,这三个报告显示,BET抑制剂可能有一个比以前已被实现的更广泛的潜力。他们还强调了与其他药物相结合,可能克服先天性和获得性的耐药性。虽然报导的耐药机制似乎反映了适应药物压力,但阻力的根源仍然未知。是否一个特定的突变导致Wnt和CK2的激活作用?或是这些适应性变化,通过可逆的表观遗传机制驱动抗性?理解完整的耐药性机制仍需定义。

需要注意的是,临床抑制Wnt和CK2尚未开发。因此,从这些研究中出现的假设不能在临床上进行测试。然而,这三份报告提供了一个良好的基础上,来建立一个对耐药机制更好的了解,能够在临床上预期应用。

所有文章仅代表作者观点,不代表本站立场。如若转载请联系原作者。
查看更多
  • BET inhibitor resistance emerges from leukaemia stem cells

    Bromodomain and extra terminal protein (BET) inhibitors are first-in-class targeted therapies that deliver a new therapeutic opportunity by directly targeting bromodomain proteins that bind acetylated chromatin marks1, 2. Early clinical trials have shown promise, especially in acute myeloid leukaemia3, and therefore the evaluation of resistance mechanisms is crucial to optimize the clinical efficacy of these drugs. Here we use primary mouse haematopoietic stem and progenitor cells immortalized with the fusion protein MLL–AF9 to generate several single-cell clones that demonstrate resistance, in vitro and in vivo, to the prototypical BET inhibitor, I-BET. Resistance to I-BET confers cross-resistance to chemically distinct BET inhibitors such as JQ1, as well as resistance to genetic knockdown of BET proteins. Resistance is not mediated through increased drug efflux or metabolism, but is shown to emerge from leukaemia stem cells both ex vivo and in vivo. Chromatin-bound BRD4 is globally reduced in resistant cells, whereas the expression of key target genes such as Myc remains unaltered, highlighting the existence of alternative mechanisms to regulate transcription. We demonstrate that resistance to BET inhibitors, in human and mouse leukaemia cells, is in part a consequence of increased Wnt/β-catenin signalling, and negative regulation of this pathway results in restoration of sensitivity to I-BET in vitro and in vivo. Together, these findings provide new insights into the biology of acute myeloid leukaemia, highlight potential therapeutic limitations of BET inhibitors, and identify strategies that may enhance the clinical utility of these unique targeted therapies.

    展开 收起
  • Transcriptional plasticity promotes primary and acquired resistance to BET inhibition

    Following the discovery of BRD4 as a non-oncogene addiction target in acute myeloid leukaemia (AML), bromodomain and extra terminal protein (BET) inhibitors are being explored as a promising therapeutic avenue in numerous cancers. While clinical trials have reported single-agent activity in advanced haematological malignancies6, mechanisms determining the response to BET inhibition remain poorly understood. To identify factors involved in primary and acquired BET resistance in leukaemia, here we perform a chromatin-focused RNAi screen in a sensitive MLL–AF9;NrasG12D-driven AML mouse model, and investigate dynamic transcriptional profiles in sensitive and resistant mouse and human leukaemias. Our screen shows that suppression of the PRC2 complex, contrary to effects in other contexts, promotes BET inhibitor resistance in AML. PRC2 suppression does not directly affect the regulation of Brd4-dependent transcripts, but facilitates the remodelling of regulatory pathways that restore the transcription of key targets such as Myc. Similarly, while BET inhibition triggers acute MYC repression in human leukaemias regardless of their sensitivity, resistant leukaemias are uniformly characterized by their ability to rapidly restore MYC transcription. This process involves the activation and recruitment of WNT signalling components, which compensate for the loss of BRD4 and drive resistance in various cancer models. Dynamic chromatin immunoprecipitation sequencing and self-transcribing active regulatory region sequencing of enhancer profiles reveal that BET-resistant states are characterized by remodelled regulatory landscapes, involving the activation of a focal MYC enhancer that recruits WNT machinery in response to BET inhibition. Together, our results identify and validate WNT signalling as a driver and candidate biomarker of primary and acquired BET resistance in leukaemia, and implicate the rewiring of transcriptional programs as an important mechanism promoting resistance to BET inhibitors and, potentially, other chromatin-targeted therapies.

    展开 收起
  • Response and resistance to BET bromodomain inhibitors in triple-negative breast cancer

    Triple-negative breast cancer (TNBC) is a heterogeneous and clinically aggressive disease for which there is no targeted therapy. BET bromodomain inhibitors, which have shown efficacy in several models of cancer, have not been evaluated in TNBC. These inhibitors displace BET bromodomain proteins such as BRD4 from chromatin by competing with their acetyl-lysine recognition modules, leading to inhibition of oncogenic transcriptional programs. Here we report the preferential sensitivity of TNBCs to BET bromodomain inhibition in vitro and in vivo, establishing a rationale for clinical investigation and further motivation to understand mechanisms of resistance. In paired cell lines selected for acquired resistance to BET inhibition from previously sensitive TNBCs, we failed to identify gatekeeper mutations, new driver events or drug pump activation. BET-resistant TNBC cells remain dependent on wild-type BRD4, which supports transcription and cell proliferation in a bromodomain-independent manner. Proteomic studies of resistant TNBC identify strong association with MED1 and hyper-phosphorylation of BRD4 attributable to decreased activity of PP2A, identified here as a principal BRD4 serine phosphatase. Together, these studies provide a rationale for BET inhibition in TNBC and present mechanism-based combination strategies to anticipate clinical drug resistance.

    展开 收起
发表评论 我在frontend\modules\comment\widgets\views\文件夹下面 test