Human HDAC3: mechanism of activation and proteasomal degradation

描述（由申请人提供）：组蛋白脱乙酰酶（HDAC）是基因调控的关键参与者，也是癌症治疗的公认靶点。HDAC活性的抑制阻止癌细胞生长并促进细胞死亡。由于目前的HDAC抑制剂被设计为靶向保守的催化位点，因此它们区分HDA不同亚型的能力固有地受到限制。这些泛HDAC抑制剂产生严重的副作用，限制了它们的使用。最近的研究表明，HDAC 3在基因调控和肿瘤发生中起着独特的作用，并且其在多种癌症类型中的过表达与不良的生存和预后相关。HDAC 3的消耗阻止肿瘤增殖，诱导坏死，并改善小鼠模型的存活率。HDAC 3耗尽后细胞的表型变化表明HDAC 3是目前临床试验中使用的HDAC抑制剂的大多数有益作用的原因。因此，重要的是要制定具体的方法， 抑制癌细胞中HDAC 3功能。 该应用程序的重点是HDAC 3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HDAC 3激活和降解的独特机制有望在癌症中特异性抑制HDAC 3。这项研究是创新的，因为我们最近发现了HDAC 3通过蛋白酶体途径激活和降解的新调控机制。HDAC 3与其他HDAC的不同之处在于它必须经历活化步骤才能成为功能性酶。这种激活需要HDAC 3与CoR的结合。在激活之前，无活性的游离HDAC 3与Hsc 70和TRiC分子伴侣紧密相关。这些分子伴侣从HDAC 3-CoR复合物的成熟活性形式中释放，表明分子伴侣在抑制HDAC 3活性中的作用。我的实验室最近表明，自由分子伴侣相关的HDAC 3也不稳定，并且容易通过蛋白酶体途径快速周转。重要的是，我们发现HDAC 3的独特C-末端在偶联CoR结合、伴侣蛋白消除和随后的HDAC 3活化中起着先前未被认识的作用。这些结果提供了证据，存在一个无活性和不稳定的中间HDAC 3复合物含有CoRs和Hsc 70伴侣。我们假设从该中间复合物到完全活性和稳定的HDAC 3-CoR复合物的转变需要HDAC 3中的构象变化，这是由CoR和HDAC 3的C-末端之间的相互作用引起的。我们将进一步测试这一假设，通过描绘的机制，CoR和HDAC 3的C-末端依赖性激活HDAC 3和确定因素，介导蛋白酶体降解的HDAC 3。更好地了解HDAC 3激活和降解的具体机制应该为开发专门针对癌症中HDAC 3的新药提供见解。这类药物有望显著改善临床结果，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.