Ribosomal Protein S6 Phosphorylation is Elevated in NR4A3 Knockout Muscle

Authors

Benjamin Y. Sung, Brigham Young UniversityFollow
Abigail E. Summers, Brigham Young UniversityFollow
Andrew B. Gold, Brigham Young UniversityFollow
Liana Leininger, Brigham Young University
Jeffery S. Tessem, Brigham Young UniversityFollow
David M. Thomson, Brigham Young UniversityFollow

Abstract

Exercise training produces many muscle and body adaptations that improve health and function. Many of these adaptations are driven by altered gene expression. Nuclear Receptor Subfamily 4 Group A Member 3 (NR4A3; AKA NOR-1) is an orphan nuclear receptor and transcription factor whose expression is among the most upregulated genes after exercise, suggesting that it plays a critical role in the adaptation to training. Recently, a knockdown study in cultured skeletal muscle cells showed that the silencing of NR4A3 led to impaired signaling through the mechanistic target of rapamycin (mTOR), a key molecular driver of muscle mass maintenance. PURPOSE: Determine whether NR4A3 knockout in mice suppresses anabolic signaling through mTOR in muscle tissue. METHODS: Resting gastrocnemius muscles were collected from 10 global NR4A3 knockout (KO) and 9 wild-type (WT) mice and analyzed via western blotting for phosphorylated and total mTOR, p70 ribosomal protein S6 kinase (S6k), eukaryotic initiation factor 4E-binding protein 1 (4EBP1), and ribosomal protein S6 (rpS6). RESULTS: Gastrocnemius size was not different between WT vs. KO mice. No significant differences were noted in signaling protein phosphorylation between WT and KO except for rpS6 phosphorylation, which was elevated in KO muscles by 120%, contrary to our hypothesis. RpS6 is phosphorylated by both S6k (whose phosphorylation was not affected basally) and another kinase, p90 ribosomal protein S6 kinase (p90RSK). p90RSK is regulated by mitogen-activated protein kinase (MAPK) signaling. Therefore, we also measured Erk, p38, and Jnk phosphorylation, which were not significantly affected in KO muscles. CONCLUSION: Our findings indicate that NR4A3 knockout may enhance muscle anabolic potential (via elevated rpS6 action) but not through upstream mTOR or MAPK signaling. Furthermore, this suggests that the inhibition of NR4A3 could enhance muscle growth but not in unstimulated, resting muscle.

Recommended Citation

Sung, Benjamin Y.; Summers, Abigail E.; Gold, Andrew B.; Leininger, Liana; Tessem, Jeffery S.; and Thomson, David M. (2024) "Ribosomal Protein S6 Phosphorylation is Elevated in NR4A3 Knockout Muscle," International Journal of Exercise Science: Conference Proceedings: Vol. 14: Iss. 4, Article 53.
Available at: https://digitalcommons.wku.edu/ijesab/vol14/iss4/53