Author: Bin GuoPublisher:
ISBN:
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Languages : en
Pages :
Book Description
Skeletal muscle comprises 30-40% of total body weight and contributes to movement, breathing, metabolism, and immune responses. The size/mass of skeletal muscle significantly affects its function; thus, it is important for human health and development. Protein turnover, the balance between protein synthesis and degradation, is critical for skeletal muscle size control. As ribosomes translate genetic information into functional proteins, an adequate quantity of ribosomes is required to fulfill the need for protein synthesis. Human and mouse ribosomes are composed of ~80 ribosomal proteins (r-proteins) and four ribosomal RNAs (rRNAs). The process to generate ribosomes requires all three RNA polymerases (Pol I, Pol II, and Pol III), while the initial and rate-limiting step is the transcription of rRNA genes (rDNA) by Pol I in the nucleolus. The overarching aim of this dissertation was to investigate how external and internal challenges modulate ribosome biogenesis, specifically rDNA transcription, to affect skeletal muscle size control. Previous studies suggest that chemotherapeutic agents (CAs), first-line antineoplastic treatments in a wide variety of cancers, can exacerbate the loss of skeletal muscle in cancer patients. Thus, we first investigated the detrimental consequences of CAs on myotubes. In vitro experiments using three commonly used CAs (paclitaxel, doxorubicin, and marizomib) revealed that myotube protein synthesis was diminished by CA treatments, and ribosomal capacity was compromised via the suppression of rDNA transcription. To further understand the potential mechanisms that control rRNA synthesis, the next study was designed to evaluate the effect of one specific type of chemotherapeutic agent, proteasome inhibitors. Proteostatic balance is essential for cellular function, so protein synthesis and degradation need to be carefully orchestrated to support skeletal muscle homeostasis and adaptations. Using mature myotubes, we observed that inhibition of the ubiquitin-proteasome system activity by MG-132 resulted in suppressed muscle anabolism, as determined by diminished ribosomal capacity, reduced protein synthesis rates, and impeded myotube hypertrophy. In parallel, the nucleolar structure of the myotubes was dispersed and p53 protein accumulated in response to acute exposure to MG-132, indicating that p53-related nucleolar stress is associated with suppressed rDNA transcription. In addition to external stresses, the third study was designed to investigate the effect of Pol I-specific internal challenge by loss of transcription initiation factor 1A (TIF-1A) in skeletal muscle and cultured myotubes using tamoxifen-dependent conditional knockout and shRNA-mediated knockdown, respectively. In adult mice, we found that ablation of TIF-1A did not impede the maintenance of muscle mass. In C2C12 myotubes, while depletion of TIF-1A suppressed rDNA transcription and reduced rRNA content at the basal stage, it did not affect myotube hypertrophy in response to high serum stimulation. These data strongly suggest that TIF-1A is dispensable for the size control of adult skeletal muscle. Together, results from the current dissertation present an important initial exploration and provide a further understanding on the potential mechanisms by which external and internal challenges affect ribosome biogenesis and skeletal muscle size control. Our findings power future studies to investigate potential clinical therapies to prevent muscle loss in aging, chronic diseases, and treatments.
