Repeated bouts of resistance exercise attenuate mitogen-activated protein-kinase signal responses in rat skeletal muscle Junya Takegaki a , Kohei Sase a, b , Satoshi Fujita b, * a Ritsumeikan Global Innovation Research Organization, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga, 525-8577, Japan b Faculty of Sport and Health Science, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga, 525-8577, Japan article info Article history: Received 2 September 2019 Accepted 12 September 2019 Available online xxx Keywords: Resistance exercise Mechanistic target of rapamycin complex 1 Extracellular signal-regulated kinase 1/2 Mitogen-activated protein kinase Muscle hypertrophy abstract Resistance exercise training induces skeletal muscle hypertrophy, but repeated bouts gradually attenuate this anabolic effect. Attenuation of mechanistic target of rapamycin complex 1 (mTORC1) activation by repetitive resistance exercise is involved in this process, but the mechanism leading to inactivation of mTORC1 remains unclear. In this study, we investigated repetition-dependent changes in mitogen- activated protein kinases (MAPKs) and the 90-kDa ribosomal S6 kinase (p90RSK), upstream regulators of mTORC1, in a rat resistance-exercise model. Resistance exercise was associated with increased phosphorylation of 70-kDa ribosomal protein S6 kinase (Thr389), but its magnitude was decreased with repeated bouts. Additionally, phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 (Thr202/ Tyr204) and p38 MAPK (Thr180/Tyr182), which are MAPKs, decreased with repeated bouts. A similar result was also observed for p90RSK phosphorylation (Thr573). These observations indicate that repeated bouts desensitized ERK1/2 and p38 MAPK, subsequently attenuating p90RSK phosphorylation. This reduction in p90RSK phosphorylation may have been partly responsible for the blunting of mTORC1 activation by resistance exercise. © 2019 Published by Elsevier Inc. 1. Introduction Skeletal muscle is a very plastic tissue and its mass can be modified by multiple stimuli, including contraction-induced me- chanical stress. Resistance exercise is an effective tool for producing high loads on skeletal muscle, as long-term training induces skel- etal muscle hypertrophy. However, in continuous training, the muscle hypertrophic effect per exercise sessions is attenuated with repetition of exercise [1 ,2], but the mechanisms underlying this attenuation remain unclear. Activation of muscle protein synthesis by resistance exercise accumulates protein in skeletal muscle and induces muscle hy- pertrophy over long-term training. The mechanistic target of rapamycin complex 1 (mTORC1) plays a major role in these phenomena [3,4]. Resistance exercise is known to activate mTORC1; phosphorylation of its downstream target 70-kDa ribo- somal protein S6 kinase (p70S6K) is widely used as an indicator of mTORC1 activity [5e8]. On the other hand, previous studies have reported that repeated bouts of resistance exercise blunt the acti- vation of mTORC1 [1 ,2]. This phenomenon is considered to be the major reason for attenuation of muscle hypertrophy in continuous training; therefore, identifying changes upstream of mTORC1 pro- duced by repetitive bouts of exercise should make a substantial contribution to our understanding of the mechanism. Mitogen-activated protein-kinase (MAPK) signaling is activated by a variety of cellular stresses, including mechanical stress. Muscle contraction caused by resistance exercise is also known to activate extracellular signal-regulated kinase 1/2 (ERK1/2), p38 MAPK, and c-Jun N-terminal kinase (JNK), members of the MAPK family [9, 10]. MAPKs are associated with increased transcriptional activity [11], and previous studies reported that MAPKs activation stimulates mTORC1 activity [12, 13]. In addition to these data, previous studies have shown that ERK activates mTORC1 via the 90 kDa ribosomal S6 kinase (p90RSK) [14, 15]. In a previous ex vivo study, Ryder et al. reported that contraction-induced p90RSK activity is mainly regulated by ERK, but p38 MAPK also plays a role [16]. Thus, Abbreviations: mTORC1, mechanistic target of rapamycin complex 1; MAPK, mitogen-activated protein kinase; p90RSK, 90-kDa ribosomal protein S6 kinase; p70S6K, 70-kDa ribosomal protein S6 kinase; ERK 1/2, extracellular signal- regulated kinase 1/2; JNK, c-Jun N-terminal kinase. * Corresponding author. E-mail addresses: takegaki@fc.ritsumei.ac.jp (J. Takegaki), sh0009iv@ed. ritsumei.ac.jp (K. Sase), safujita@fc.ritsumei.ac.jp (S. Fujita). Contents lists available at ScienceDirect Biochemical and Biophysical Research Communications journal homepage: www.elsevier.com/locate/ybbrc https://doi.org/10.1016/j.bbrc.2019.09.050 0006-291X/© 2019 Published by Elsevier Inc. Biochemical and Biophysical Research Communications xxx (xxxx) xxx Please cite this article as: J. Takegaki et al., Repeated bouts of resistance exercise attenuate mitogen-activated protein-kinase signal responses in rat skeletal muscle, Biochemical and Biophysical Research Communications, https://doi.org/10.1016/j.bbrc.2019.09.050