BACKGROUND Cachexia is the direct cause of at least 20% of cancer-associated deaths. Muscle wasting in skeletal muscle results in weakness, immobility, and death secondary to impaired respiratory muscle function. Muscle proteins are massively degraded in cachexia; nevertheless, the molecular mechanisms related to this process are poorly understood. Previous studies have reported conflicting results regarding the amino acid abundances in cachectic skeletal muscle tissues. There is a clear need to identify the molecular processes of muscle metabolism in the context of cachexia, especially how different types of molecules are involved in the muscle wasting process. METHODS New in situ -omics techniques were used to produce a more comprehensive picture of amino acid metabolism in cachectic muscles by determining the quantities of amino acids, proteins, and cellular metabolites. Using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging, we determined the in situ concentrations of amino acids and proteins, as well as energy and other cellular metabolites, in skeletal muscle tissues from genetic mouse cancer models (n = 21) and from patients with cancer (n = 6). Combined results from three individual MALDI mass spectrometry imaging methods were obtained and interpreted. Immunohistochemistry staining for mitochondrial proteins and myosin heavy chain expression, digital image analysis, and transmission electron microscopy complemented the MALDI mass spectrometry imaging results. RESULTS Metabolic derangements in cachectic mouse muscle tissues were detected, with significantly increased quantities of lysine, arginine, proline, and tyrosine (P = 0.0037, P = 0.0048, P = 0.0430, and P = 0.0357, respectively) and significantly reduced quantities of glutamate and aspartate (P = 0.0008 and P = 0.0124). Human skeletal muscle tissues revealed similar tendencies. A majority of altered amino acids were released by the breakdown of proteins involved in oxidative phosphorylation. Decreased energy charge was observed in cachectic muscle tissues (P = 0.0101), which was related to the breakdown of specific proteins. Additionally, expression of the cationic amino acid transporter CAT1 was significantly decreased in the mitochondria of cachectic mouse muscles (P = 0.0133); this decrease may play an important role in the alterations of cationic amino acid metabolism and decreased quantity of glutamate observed in cachexia. CONCLUSIONS Our results suggest that mitochondrial dysfunction has a substantial influence on amino acid metabolism in cachectic skeletal muscles, which appears to be triggered by diminished CAT1 expression, as well as the degradation of mitochondrial proteins. These findings provide new insights into the pathobiochemistry of muscle wasting.

中文翻译：

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恶病质骨骼肌中氨基酸的排列紊乱是由线粒体功能障碍引起的。

背景技术恶病质是导致至少20％的癌症相关死亡的直接原因。骨骼肌肌肉萎缩会导致呼吸肌功能受损继发虚弱，不动和死亡。肌肉蛋白质在恶病质中大量降解。然而，与此过程有关的分子机制了解甚少。先前的研究报道了关于恶病质骨骼肌组织中氨基酸丰度的矛盾结果。明确需要在恶病质的背景下识别肌肉代谢的分子过程，尤其是在肌肉消瘦过程中如何涉及不同类型的分子。方法：采用新的原位组学技术，通过确定氨基酸的数量，对恶病质肌肉中的氨基酸代谢产生更全面的了解，蛋白质和细胞代谢产物。使用基质辅助激光解吸/电离（MALDI）质谱成像，我们确定了遗传小鼠癌症模型（n = 21）的骨骼肌组织中氨基酸和蛋白质以及能量和其他细胞代谢产物的原位浓度并来自癌症患者（n = 6）。获得并解释了三种单独的MALDI质谱成像方法的组合结果。线粒体蛋白和肌球蛋白重链表达的免疫组织化学染色，数字图像分析和透射电子显微镜对MALDI质谱成像结果进行了补充。结果检测到恶病质小鼠肌肉组织中的代谢紊乱，赖氨酸，精氨酸，脯氨酸和酪氨酸的含量显着增加（P = 0.0037，P = 0.0048，分别为P = 0.0430和P = 0.0357），并显着降低了谷氨酸和天冬氨酸的含量（P = 0.0008和P = 0.0124）。人的骨骼肌组织显示出相似的趋势。大多数改变的氨基酸是通过分解参与氧化磷酸化的蛋白质而释放的。在恶病质的肌肉组织中观察到能量电荷减少（P = 0.0101），这与特定蛋白质的分解有关。此外，阳离子氨基酸转运蛋白CAT1的表达在恶病质小鼠肌肉的线粒体中显着降低（P = 0.0133）。这种减少可能在恶病质中观察到的阳离子氨基酸代谢变化和谷氨酸含量减少中起重要作用。结论我们的结果表明，线粒体功能障碍对恶病质骨骼肌的氨基酸代谢有重要影响，这似乎是由CAT1表达降低以及线粒体蛋白降解引起的。这些发现为肌肉消瘦的病理生物化学提供了新的见解。