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

Abstract

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 = 17) was -7%, moderate responders (Mod; n = 19) +22%, and extreme responders (Xtr; n = 6) +83%. Total muscle RNA increased only in Mod (+9%, P < 0.08) and Xtr (+26%, P < 0.01), and only Xtr increased rRNA content (+40%, P < 0.05) and myonuclei/type II fiber (+32%, P < 0.01). Additionally, Mod and Xtr had a greater increase in c-Myc protein levels compared with Non (e.g., approximately +350 and +250% vs. +50%, respectively, P < 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.

Keywords: Pol I; c-Myc; rRNA; ribosome; skeletal muscle.

Fig. 1.

K-means cluster analysis defined three distinct groups [nonresponders (Non), moderate responders (Mod), extreme responders (Xtr)] based on the percent change in type II myofiber cross-sectional area (CSA) from pre- to post-resistance exercise training (RT) (A). Each histogram bar represents an individual subject's type II CSA %change. B: representative ×10 immunohistochemical images. Type I myofibers are stained red, type IIa are stained green, and type IIx are black (negative).

Fig. 2.

Cluster differences in whole tissue total RNA content (A) and rRNA content (B) normalized to tissue weight. C–E: representative electropherogram plots with labeled rRNA peaks. *Significantly different from previous time point (P < 0.05).

Fig. 3.

Cluster differences in the abundance of signaling proteins that regulate ribosome biogenesis. Representative immunoblots are shown in C. *Significantly different from previous time point (P < 0.05).

Fig. 4.

Cluster differences in type II myofiber myonuclear addition from pre- to post-RT. A: representative ×20 immunohistochemical images [nuclei labeled with 4′,6-diamidino-2-phenylindole (DAPI)]. B: average no. of myonuclei per type II myofiber within each responder cluser before resistance training (wk0) and posttraining (wk4). *Significantly different from previous time point (P < 0.05). Non, nonresponder; Mod, modest responder; Xtr, extreme responder.

Fig. 5.

Effects of 24 h of FBS treatment or FBS + CX-5461 on myotube total RNA content (A), protein synthesis measured via the surface sensing of translation (SUnSET) method (B), and myotube volume (G). C–F: representative puromycin immunoblots (C) and ×10 immunocytochemical images (myosin heavy chain stained green; D–F). *Significantly different from control (P < 0.05).