Ribosome biogenesis may augment resistance training-induced myofiber hypertrophy and is required for myotube growth in vitro

Abstract

Stec MJ, Kelly NA, Many GM, Windham ST, Tuggle SC, Bamman MM. Ribosome biogenesis may augment resistance training-induced myofiber hypertrophy and is required for myotube growth in vitro. Am J Physiol Endocrinol Metab 310: E652-E661, 2016. First published February 9, 2016 doi:10.1152/ajpendo.00486.2015.-Resistance exercise training (RT) is the most effective method for increasing skeletal muscle mass in older adults; however, the amount of RT-induced muscle growth is highly variable between individuals. Recent evidence from our laboratory and others suggests ribosome biogenesis may be an important factor regulating RT-induced hypertrophy, and we hypothesized that the extent of hypertrophy is at least partly regulated by the amount of RT-induced ribosome biogenesis. To examine this, 42 older adults underwent 4 wk of RT aimed at inducing hypertrophy of the knee extensors (e.g., 2 sets of squat, leg press, and knee extension, 10 -12 repetition maximums, 3 days/wk), and vastus lateralis muscle biopsies were performed pre-and post-RT. Post hoc K-means cluster analysis revealed distinct differences in type II myofiber hypertrophy among subjects. The percent change in type II myofiber size in nonresponders (Non; n ϭ 17) was Ϫ7%, moderate responders (Mod; n ϭ 19) ϩ22%, and extreme responders (Xtr; n ϭ 6) ϩ83%. Total muscle RNA increased only in Mod (ϩ9%, P Ͻ 0.08) and Xtr (ϩ26%, P Ͻ 0.01), and only Xtr increased rRNA content (ϩ40%, P Ͻ 0.05) and myonuclei/type II fiber (ϩ32%, P Ͻ 0.01). Additionally, Mod and Xtr had a greater increase in c-Myc protein levels compared with Non (e.g., approximately ϩ350 and ϩ250% vs. ϩ50%, respectively, P Ͻ 0.05). In vitro studies showed that growth factor-induced human myotube hypertrophy is abolished when rRNA synthesis is knocked down using the Pol I-specific inhibitor CX-5461. Overall, these data implicate ribosome biogenesis as a key process regulating the extent of RT-induced myofiber hypertrophy in older adults. skeletal muscle; ribosome; rRNA; c-Myc; Pol I LOSS OF SKELETAL MUSCLE MASS with aging (i.e., sarcopenia) and disease (e.g., cachexia) is associated with poor clinical outcomes (23, 24) and higher all-cause mortality (44); thus, maximizing mechanisms of muscle regrowth is a high priority. Restoration of muscle mass and contractile function is dependent on an accumulation of various protein pools (myofibrillar, mitochondrial, etc.) within myofibers that leads to overall myofiber hypertrophy. To achieve this, the primary focus to date has been on mechanisms regulating translation initiation. Many have reported acute induction of the relevant signaling pathways in response to various hypertrophy stimuli (protein nutrition, anabolic drugs, exercise) (3, 10, 14, 17, 27, 41), and we have shown that some of these induced signals regulate myofiber hypertrophy (e.g., eIF2Bε) (29). On the other hand, little attention has been given to other potentially rate-limiting processes, including overall ribosomal capacity, which is dependent on efficiency and the number of available ribosomes...

Molecular Regulation of Exercise-Induced Muscle Fiber Hypertrophy

“…Correlative evidence supporting satellite cell–mediated myonuclear addition during human RT-induced myofiber hypertrophy gained traction in the mid-2000s (e.g., Olsen et al 2006; Petrella et al 2006, 2008) and studies reporting modest hypertrophy with no appreciable myonuclear addition (Petrella et al 2006, 2008; Verdijk et al 2009) fueled the concept of reaching a domain ceiling that led to a “demand” for nuclear addition. This concept was reaffirmed recently as moderate myofiber hypertrophy was accomplished in older adults in the absence of myonuclear addition, whereas nuclear addition accompanied greater hypertrophy (Stec et al 2016). New research leveraging the Pax7-DTA satellite cell depletion mouse and myonuclear transcription rates lends additional support for myonuclear reserve capacity (i.e., expansion of the domains before demanding additional nuclei) (Kirby et al 2016).…”

“…This strategy has yielded valuable insight into potential cellular and molecular regulators of human RT-induced myofiber hypertrophy (Bamman et al 2007; Kim et al 2007; Petrella et al 2008; Mayhew et al 2011; Thalacker-Mercer et al 2013). We have since validated the utility of the model (Stec et al 2016) and today similar strategies used by a number of research teams continue to bear fruit (Davidsen et al 2011; Prestes et al 2015; Ahtiainen et al 2016; Ogasawara et al 2016). Of course all participants in RT trials benefit to varying degrees in other ways (Bamman et al 2007; Bickel et al 2011; Churchward-Venne et al 2015) (e.g., strength gains, functional mobility improvements, etc.)…”

“…Using total muscle RNA as a surrogate of rRNA (80%–85% of total RNA), the magnitude of change in total muscle RNA over the course of RTappears linked to the magnitude of myofiber (Stec et al 2016) and muscle (Figueiredo et al 2015) hypertrophy. Via K-means cluster analysis to phenotype human subjects based on the degree of type II myofiber hypertrophy, Stec et al found that only extreme responders successfully up-regulated muscle rRNA expression (indicative of ribosomal biogenesis), and the biopsied muscles of both moderate and extreme responders increased c-Myc protein content (a major upstream regulator of ribosome biogenesis).…”

“…Via K-means cluster analysis to phenotype human subjects based on the degree of type II myofiber hypertrophy, Stec et al found that only extreme responders successfully up-regulated muscle rRNA expression (indicative of ribosomal biogenesis), and the biopsied muscles of both moderate and extreme responders increased c-Myc protein content (a major upstream regulator of ribosome biogenesis). Additionally, follow-up studies in vitro showed that growth factor–induced human myotube hypertrophy was prevented when de novo ribosome biogenesis was blocked by a specific inhibitor of rRNA transcription (Stec et al 2016), a finding recently advanced and linked to mTOR regulation during hypertrophy in C2C12 myotubes (von Walden et al 2016). …”

Progressive resistance‐loaded voluntary wheel running increases hypertrophy and differentially affects muscle protein synthesis, ribosome biogenesis, and proteolytic markers in rat muscle

“…Recent research has reported that an acute bout of resistance exercise in rodents West et al, 2016) and humans (Stec et al, 2015) increases ribosome content or 45S pre-rRNA (which is a precursor to 28S, 18S and 5.8S rRNA (Chaillou et al, 2014 (Kirby et al, 2015) similarly reported that 14 days of synergist ablation in 5-mo-old mice, which elicited a 71% increase in plantaris mass, induced a twofold increase in ribosome content and increased the mRNA expression of numerous ribosomal proteins. In humans, Stec et al (Stec et al, 2016) reported that 4 weeks of lower body resistance training increased vastus lateralis mixed-fibre total RNA levels in "extreme responders"…”