Dysregulation of sarcomere stabilizing proteins cause muscle atrophy and weakness during cancer cachexia

 描述(申请人提供)：恶病质的特征是进行性的骨骼肌和体重减轻，影响高达80%的癌症患者。这种肌肉质量的丧失会导致严重的肌肉无力和身体功能下降，并与化疗耐受性降低和手术/放射治疗的并发症增加有关。因此，恶病质降低了癌症患者的生活质量和生存时间，而恶病质本身导致了高达30%的癌症相关死亡。有趣的是，临床前癌症恶病质模型以及恶病质人类癌症患者的肌肉显示出肌节和肌纤维膜完整性的破坏，尽管缺乏损伤刺激，而且有人推测这些破坏可能启动分解代谢过程，导致肌肉萎缩和虚弱。我们实验室未发表的和初步的数据已经确定，对于肌节Z-Disk结构完整性至关重要的脊柱侧凸多肽酶(Ky)和对肌膜营养不良相关蛋白复合体(DAPC)重要的肌球蛋白(Myoc)，在肿瘤进展过程中肌肉萎缩和虚弱之前的时间点，在mRNA和蛋白质水平上都高度下调。此外，初步数据显示，Ky在荷瘤小鼠肌肉中的过度表达可以抑制肌肉纤维萎缩。这些观察结果支持我们的第一个假设，即Ky和Myoc基因的下调是导致肌肉结构丧失的原因，导致癌症恶病质进展过程中肌肉萎缩和虚弱。未发表的对C26荷瘤小鼠骨骼肌中显著下调的基因的-1kb到+1kb近端启动子的生物信息学分析显示，在最常见的共享基序中，有一个与肌细胞增强因子-2(MEF2)保守的共识结合基序。此外，Ky和Myoc基因启动子都含有保守的MEF2结合基序。这一观察结果，再加上荷瘤小鼠MEF2蛋白c(MEF2C)在mRNA和蛋白水平上的降低，支持了我们的第二个假设，即荷瘤宿主骨骼肌中MEF2C转录活性的丧失是导致Ky和Myoc下调的原因，并开始破坏肌肉纤维的完整性和肌肉萎缩。因此，我们的两个特定目标是：特定目标1：验证脊柱后凸多肽酶(Ky)和肌球蛋白(Myoc)下调在癌症引起的肌肉纤维完整性丧失和肌肉萎缩启动中起致病作用的假设。具体目标2：验证MEF2C转录活性丧失是癌症诱导的Ky和Myoc下调并启动肌肉萎缩的假设。这些研究的结果将为涉及蛋白质下调的转录机制提供新的见解，这些机制引发癌症引起的肌肉萎缩和虚弱，为治疗干预开辟新的途径。

 DESCRIPTION (provided by applicant): Cachexia is characterized by progressive skeletal muscle and body weight loss and affects up to 80% of cancer patients. This loss of muscle mass contributes to significant muscle weakness and diminished physical function and is associated with reduced tolerance to chemotherapy and increased complications from surgical/radiotherapeutic treatments. Consequently, cachexia decreases both quality of life and survival time in cancer patients and cachexia itself is responsible for up to 30% of all cancer-related deaths. Interestingly muscles from preclinical models of cancer cachexia as well as cachectic human cancer patients show disruptions in sarcomere and myofiber membrane integrity despite the lack of an injury stimulus, and there is speculation that these disruptions may initiate catabolic processes which lead to the muscle atrophy and weakness. Unpublished and preliminary data from our lab has identified that Kyphoscoliosis peptidase (Ky), which is essential to the structural integrity of the sarcomeric Z-disk, and Myocilin (Myoc), which is important to the sarcolemmal dystrophin associated protein complex (DAPC), are highly downregulated at the mRNA and protein level at time points which precede and parallel muscle atrophy and weakness during tumor progression. Moreover, preliminary data show that overexpression of Ky in the muscles of tumor bearing mice inhibits muscle fiber atrophy. These observations support our first hypothesis that the downregulation of Ky and Myoc are causative to the loss of muscle structure leading to muscle wasting and weakness during the progression of cancer cachexia. Unpublished bioinformatics analyses of the -1kb to +1kb proximal promoters of genes significantly downregulated in skeletal muscle of C26 tumor-bearing mice revealed a conserved consensus binding motif for myocyte enhancing factor-2 (MEF2) among the top most commonly shared motifs. Moreover, both the Ky and Myoc gene promoters contain conserved MEF2 binding motifs. This observation, coupled with the findings that MEF2 protein c (MEF2c) is decreased at the mRNA and protein level in tumor bearing mice, supports our second hypothesis that loss of MEF2c transcriptional activity in skeletal muscle of tumor-bearing hosts is causative in the downregulation of Ky and Myoc, and initiates disruptions in muscle fiber integrity and muscle wasting. Thus, our two specific aims are: Specific Aim 1: To test the hypothesis that the downregulation of Kyphoscoliosis peptidase (Ky) and Myocilin (Myoc) play causative roles in the cancer-induced loss of muscle fiber integrity and the initiation of muscle wasting. Specific Aim 2: To test the hypothesis that loss of MEF2c transcriptional activity is causative in the cancer- induced downregulation of Ky and Myoc and initiates muscle wasting. The results of these studies will provide new insight into transcriptional mechanisms involving protein downregulation which initiate cancer-induced muscle wasting and weakness, opening up new avenues for therapeutic interventions.