Human HDAC3: mechanism of activation and proteasomal degradation

描述(由申请人提供)：组蛋白脱乙酰酶(HDAC)是基因调控中的关键角色，也是癌症治疗的公认靶点。抑制HDAC活性可阻止癌细胞生长并促进细胞死亡。由于目前的HDAC抑制剂是针对保守的催化部位设计的，因此它们区分不同亚型的HDA的能力固有地有限。这些泛-HDAC抑制剂会产生严重的副作用，限制了它们的使用。最近的研究表明，HDAC3在基因调控和肿瘤发生中起着独特的作用，其在多种癌症类型中的过度表达与不良的生存和预后相关。在小鼠模型中，HDAC3的缺失可以阻止肿瘤的增殖，诱导坏死，并提高存活率。HDAC3耗尽后细胞的表型变化表明，HDAC3是目前临床试验中使用的HDAC抑制剂的大部分有益效果的原因。因此，重要的是要开发出能够具体 抑制癌细胞中的HDAC3功能。这一应用主要集中在HDAC3是如何被其特定的相互作用蛋白-核受体辅阻遏物(CORS)激活和稳定的。这 这项研究意义重大，因为了解HDAC3激活和降解的独特机制有望在癌症中特异性地灭活HDAC3。这项研究具有创新性，因为我们最近发现了通过蛋白酶体途径激活和降解HDAC3的新的调控机制。HDAC3与其他HDAC的不同之处在于，它必须经历一个激活步骤才能成为一种功能酶。这种激活需要HDAC3与CORS结合。在激活之前，非活性的游离HDAC3与Hsc70和TIRE伴侣紧密相关。这些伴侣从HDAC3-COR复合体的成熟活性形式中释放出来，表明伴侣在抑制HDAC3活性方面发挥了作用。我的实验室最近发现，与游离伴侣相关的HDAC3也是不稳定的，容易通过蛋白酶体途径快速转换。重要的是，我们发现HDAC3独特的C-末端在偶联COR结合、伴侣清除和随后的HDAC3激活中发挥着以前未被认识到的作用。这些结果为含有CORs和Hsc70伴侣的非活性和不稳定的中间HDAC3复合体的存在提供了证据。我们推测，从这个中间络合物到完全活性和稳定的HDAC3-COR络合物的转变需要HDAC3的构象变化，这是由于COR和HDAC3的C末端相互作用而导致的。我们将通过描述依赖于COR和HDAC3 C末端的HDAC3激活机制和识别介导HDAC3蛋白酶体降解的因素来进一步检验这一假说。更好地了解HDAC3激活和降解的具体机制将为开发针对癌症中HDAC3的新药提供洞察力。这些药物预计将显著改善临床结果，并可能在治疗各种类型的癌症和白血病方面有更广泛的用途。

DESCRIPTION (provided by applicant): Histone deacetylases (HDACs) are key players in gene regulation and proven targets for cancer therapy. Inhibition of HDAC activity arrests cancer cell growth and promotes cell death. Because the current HDAC inhibitors are designed to target the conserved catalytic site, their capacity for distinguishing different isoforms of HDA is inherently limited. These pan-HDAC inhibitors produce profound side effects that have limited their use. Recent studies have shown that HDAC3 plays a unique role in gene regulation and tumorigenesis, and that its overexpression in multiple cancer types is correlated with poor survival and prognosis. Depletion of HDAC3 arrests tumor proliferation, induces necrosis, and improves survival in mouse models. The phenotypic changes of cells upon depletion of HDAC3 suggest that HDAC3 is responsible for most of the beneficial effects of the HDAC inhibitors currently used in clinical trials. It is thus important to develop approaches that can specifically inhibit HDAC3 function in cancer cells. This application is focused on how HDAC3 is activated and stabilized by its specific interacting proteins, the nuclear receptor corepressors (CoRs). This study is significant because understanding the unique mechanism of HDAC3 activation and degradation carries the promise to specifically inactivate HDAC3 in cancer. This study is innovative because we have recently discovered novel regulatory mechanisms underlying HDAC3 activation and degradation via proteasomal pathways. HDAC3 differs from other HDACs in that it must undergo an activation step to become a functional enzyme. This activation entails the binding of HDAC3 to CoRs. Prior to activation, the inactive free HDAC3 is tightly associated with Hsc70 and TRiC chaperones. These chaperones are released from the mature active form of the HDAC3-CoR complex, suggesting a role for chaperones in the inhibition of HDAC3 activity. My lab has recently shown that the free chaperone-associated HDAC3 is also unstable and subject to rapid turnover by proteasomal pathways. Importantly, we found that the unique C- terminus of HDAC3 plays a previously unrecognized role in coupling CoR binding, chaperone dismissal, and subsequent activation of HDAC3. These results provide evidence for the existence of an inactive and unstable intermediate HDAC3 complex containing both CoRs and Hsc70 chaperones. We hypothesize that the transition from this intermediate complex to a fully active and stable HDAC3-CoR complex requires a conformational change in HDAC3 resulting from the interplay between CoR and the C-terminus of HDAC3. We will further test this hypothesis by delineating the mechanism of CoR- and HDAC3 C-terminus-dependent activation of HDAC3 and identifying factors that mediate proteasomal degradation of HDAC3. A better understanding of the specific mechanisms underlying the activation and degradation of HDAC3 should provide insight for the development of new drugs to specifically target HDAC3 in cancer. Such drugs are expected to significantly improve clinical outcomes and may have a broader use in the treatment of various types of cancers and leukemias.