Post-translational modifications of Hsp90 that impact drug efficacy

热休克蛋白90(Hsp90)是一种分子伴侣，在癌细胞中经常被激活、突变或过度表达，影响癌细胞的增殖和存活，是许多信号蛋白稳定和发挥功能所必需的。HSP90是一种构象灵活的蛋白质，它与一组不同的辅助伴侣结合，这取决于氨基末端结合口袋的ATP或ADP占有率。Hsp90自身的核苷酸交换和ATP水解，在辅助伴侣的帮助下，驱动Hsp90伴侣机器结合、伴侣和释放客户蛋白。Hsp90伴侣机器的循环对其功能至关重要。虽然ATP结合和水解已被令人信服地涉及调控Hsp90循环，但越来越多的证据表明，Hsp90的各种翻译后修饰，包括磷酸化、乙酰化和其他修饰，提供了额外的重叠或平行水平的调控。更全面地了解这些不同的蛋白质修饰是如何在细胞水平上调节并相互作用以调节Hsp90伴侣活性的，对于设计新的方法来抑制这一医学上重要的分子靶点至关重要。调节Hsp90不同翻译后修饰的信号通路的协调是很有可能的。理解不同修饰之间的相互作用无疑是一项困难的工作，但这些知识将极大地增加我们对Hsp90功能如何在复杂的细胞环境中进行调控的理解。这些信息可能提供一种独特的方法来特异性阻断癌细胞中的Hsp90功能，因此将是设计Hsp90抑制剂与其他分子靶向药物联合临床试验的重要考虑因素。更透彻地了解翻译后修饰在调节Hsp90功能中所起的作用，肯定会提高这种联合疗法的有效性。HSP90受到一系列影响其功能的翻译后修饰，包括乙酰化。组蛋白脱乙酰酶(HDAC)抑制剂和HDAC6的敲除可诱导Hsp90乙酰化并抑制其活性。然而，直接确定Hsp90乙酰化的功能后果还有待于绘制特定位点的图谱。我们已经证明了Hsp90K294是乙酰化的。突变分析表明，K294的乙酰化状态是客户蛋白和辅伴侣结合的重要决定因素。在酵母中，在K294位不能乙酰化的Hsp90突变体与WT或模拟构成乙酰化的突变体相比，活性和伴侣功能降低。这些数据表明，K294的乙酰化/去乙酰化在调节Hsp90伴侣循环中起着重要作用。进一步的研究将集中在Hsp90中特定氨基酸的磷酸化以及这些翻译后变化对Hsp90与抑制药物亲和力的影响。

Heat shock protein 90 (Hsp90) is a molecular chaperone required for the stability and function of many signaling proteins that are often activated, mutated or over-expressed in cancer cells and that underly cancer cell proliferation and survival. Hsp90 is a conformationally flexible protein that associates with a distinct set of co-chaperones depending on ATP or ADP occupancy of an amino-terminal binding pocket. Nucleotide exchange and ATP hydrolysis by Hsp90 itself, with the assistance of co-chaperones, drive the Hsp90 chaperone machine to bind, chaperone, and release client proteins. Cycling of the Hsp90 chaperone machine is critical to its function. Although ATP binding and hydrolysis have been convincingly implicated in regulating the Hsp90 cycle, growing evidence suggests that various post-translational modifications of Hsp90, including phosphorylation, acetylation, and other modifications, provide an additional overlapping or parallel level of regulation. A more complete understanding of how these various protein modifications are regulated and interact with each other at the cellular level to modulate Hsp90 chaperone activity is critical to the design of novel approaches to inhibit this medically important molecular target. Coordination of signaling pathways that mediate distinct post-translational modifications of Hsp90 is highly likely. Understanding the cross-talk between various modifications will no doubt be a difficult undertaking, but such knowledge will add greatly to our appreciation of how Hsp90 function is regulated in the complex milieu of the cell. Such information may provide a unique approach to specific interdiction of Hsp90 function in cancer cells and will thus be an important consideration in designing clinical trials of Hsp90 inhibitors in combination with other molecularly targeted drugs. A more thorough understanding of the role that post-translational modifications play in modulating Hsp90 function will certainly improve the effectiveness of such combination therapies. Hsp90 is subject to an array of posttranslational modifications that affect its function, including acetylation. Histone deacetylase (HDAC) inhibitors and knockdown of HDAC6 induce Hsp90 acetylation and inhibit its activity. However, direct determination of the functional consequences of Hsp90 acetylation has awaited mapping of specific sites. We have demonstrated that Hsp90 K294 is acetylated. Mutational analysis of K294 shows that its acetylation status is a strong determinant of client protein and cochaperone binding. In yeast, Hsp90 mutants that cannot be acetylated at K294 have reduced viability and chaperone function compared to WT or to mutants that mimic constitutive acetylation. These data suggest that acetylation/deacetylation of K294 plays an important role in regulating the Hsp90 chaperone cycle. Further studies will focus on phosphorylation of specific amino acids in Hsp90 and of the affect of these post-translational changes on Hsp90 affinity for inhibitory drugs.