Intramuscular Mechanisms of Cancer Cachexia

 描述(申请人提供)：癌症诱导性恶病质(癌症恶病质)是一种消瘦综合征，主要由于过度的蛋白质分解而导致肌肉质量进行性丧失(肌肉萎缩)。癌症恶病质困扰着约50%的癌症患者，是所有癌症相关死亡的直接原因。然而，由于对癌症恶病质的病因缺乏了解，目前还没有FDA批准的治疗方法。癌症恶病质的一个特征是肌原纤维蛋白的急剧丧失。泛素-蛋白酶体途径通过特定的E3泛素连接酶靶向肌原纤维蛋白进行降解，在各种形式的肌肉萎缩中肌纤维蛋白的丢失起主要作用。目前，人们普遍认为，癌症引起的肌肉萎缩与其他类型的肌肉萎缩具有共同的细胞内机制。例如，E3连接酶MuRF1的升高被认为是包括癌症恶病质在内的所有形式的肌肉萎缩的中心。此外，Akt-FoxO1/3信号通路被认为对所有形式的肌肉萎缩都至关重要，因为它调节MuRF1的表达。然而，最近的数据显示，癌症引起的肌肉萎缩似乎涉及独特的细胞内机制，不同于由生理应激引起的肌肉萎缩，如禁食、废弃或去神经。新的证据表明，MuRF1上调和Akt-FoxO1/3信号通路在动物模型和人类患者的癌症诱导的肌肉萎缩中不是必需的。相反，我们发现，在小鼠癌症恶病质模型中，涉及p38 MAPK激活转录因子C/EBP？的炎症激活信号通路是导致肌肉蛋白质大量降解的原因。相比之下，MuRF1和FoxO1/3不是该模型中肌肉萎缩的原因。此外，我们发现C/EBP？上调以前被忽视的E3，UBR2，以响应肿瘤负担。UBr2是E3连接酶家族的成员之一，它是N末端规则通路的底物识别成分，在各种分解代谢刺激诱导的肌肉蛋白质总泛素化和依赖于ATP的降解中占很大比例。在这个E3家族中，UBR2在携带肿瘤的啮齿动物的肌肉中唯一上调。因此，我们建议验证这一假设，即UBR2是导致癌症恶病质中肌原纤维蛋白过度丢失的关键E3，并通过阐明在癌症恶病质中介导UBR2上调的详细信号机制，我们可以通过以下三个目标来靶向使用现有药物的信号机制来缓解肌肉萎缩。目的1.确定UBr2是否是E3泛素连接酶，与癌症引起的肌肉萎缩有关，并确定其底物。目的2.确定C/EBP？位点特异性乙酰化是否介导了癌症诱导的UBR2上调。目的3.探讨癌诱导C/EBP？乙酰化的信号转导机制。

 DESCRIPTION (provided by applicant): Cancer-induced cachexia (cancer cachexia) is a wasting syndrome featuring progressive loss of muscle mass (muscle wasting) due largely to excessive proteolysis. Afflicting ~50% of all cancer patients, cancer cachexia is the immediate cause of ~1/3 of all cancer-related deaths. However, there is no FDA-approved treatment for cancer cachexia due to the poor understanding of its etiology. A hallmark of cancer cachexia is the dramatic loss of myofibrillar proteins. The ubiquitin-proteasome pathway plays a major role in the loss of myofibrillar proteins in various forms of muscle atrophy by targeting myofibrillar proteins for degradation via specific E3 ubiquitin ligases. Currently, cancer-induced muscle wasting is widely thought to share common intracellular mechanisms with other types of muscle atrophy. For example, elevated E3 ligase MuRF1 is considered central for ALL forms of muscle atrophy including cancer cachexia. In addition, the Akt-FoxO1/3 signaling pathway is thought critical for ALL forms of muscle atrophy because it regulates MuRF1 expression. However, recent data revealed that cancer-induced muscle wasting appears to involve unique intracellular mechanisms distinct from muscle atrophy induced by physiological stress such as fasting, disuse or denervation. Emerging evidence suggests that MuRF1 upregulation and the Akt-FoxO1/3 signaling pathway are non-essential for cancer-induced muscle wasting in animal models as well as human patients. Instead, we found that an inflammation-activated signaling pathway involving p38 MAPK activation of transcription factor C/EBPß is responsible for the bulk of muscle protein degradation in a mouse cancer cachexia model. In contrast, MuRF1 and FoxO1/3 are not responsible for the muscle wasting in this model. Further, we found that C/EBPß upregulates a previously overlooked E3, UBR2, in response to a tumor burden. UBR2 is a member of an E3 ligase family that serves as the substrate recognition components of the N-end rule pathway that accounts for a large portion of total protein ubiquitylation and ATP-dependent degradation of muscle proteins induced by various catabolic stimuli. Within this E3 family, UBR2 is uniquely upregulated in the muscle of tumor-bearing rodents. Thus, we propose to test the hypothesis that UBR2 is a key E3 responsible for the excessive loss of myofibrillar proteins in cancer cachexia, and by elucidating detailed signaling mechanisms that mediate UBR2 upregulation in cancer cachexia we can ameliorate muscle wasting by targeting the signaling mechanisms using existing pharmacological inhibitors through pursuing three aims. Aim 1. To determine whether UBR2 is a key E3 ubiquitin ligase responsible for cancer-induced muscle wasting and identify its substrates. Aim 2. To determine whether site-specific acetylation of C/EBPß mediates cancer-induced UBR2 upregulation. Aim 3. To determine the signaling mechanism that mediates cancer-induced acetylation of C/EBPß.