Toll-like receptor 4 (TLR4) activation by lipopolysaccharides (LPS) increases proinflammatory cytokine production and upregulation of muscle atrophy signaling pathways. Muscle contractions can suppress LPS/TLR4 axis activation by reducing the protein expression of TLR4 on immune cells. However, the mechanism by which muscle contractions decrease TLR4 remains undefined. Moreover, it is not clear whether muscle contractions affect TLR4 expressed on skeletal muscle cells. The purpose of this study was to uncover the nature and mechanisms by which stimulated myotube contractions using electrical pulse stimulation (EPS) as an in vitro model of skeletal muscle contractions affect TLR4 expression and intracellular signaling to combat LPS-induced muscle atrophy. C2C12 myotubes were stimulated to contract via EPS with and without subsequent LPS exposure. We then examined the isolated effects of conditioned media (CM) collected following EPS and soluble TLR4 (sTLR4) alone on LPS-induced myotube atrophy. Exposure to LPS decreased membrane-bound and sTLR4, increased TLR4 signaling (decreased inhibitor of κBα), and induced myotube atrophy. However, EPS decreased membrane-bound TLR4, increased sTLR4, and prevented LPS-induced signaling and myotube atrophy. CM, which contained elevated levels of sTLR4, prevented LPS-induced upregulation of atrophy-related gene transcripts muscle ring finger 1 (MuRF1) and atrogin-1 and reduced myotube atrophy. Recombinant sTLR4 added to media prevented LPS-induced myotube atrophy. In summary, our study provides the first evidence that sTLR4 has anticatabolic effects by reducing TLR4-mediated signaling and atrophy. In addition, the study reveals a novel finding, by demonstrating that stimulated myotube contractions decrease membrane-bound TLR4 and increase the secretion of sTLR4 by myotubes.NEW & NOTEWORTHY Excessive Toll-like receptor 4 (TLR4) activation causes muscle atrophy. Muscle contractions can limit TLR4 activation on immune cells, but its impact on TLR4 expressed on skeletal muscle cells remains unclear. Here, we demonstrate in C2C12 myotubes for the first time that stimulated myotube contractions reduce membrane-bound TLR4 and increase soluble TLR4, preventing TLR4-mediated signaling and myotube atrophy. Further analyses revealed soluble TLR4 independently prevents myotube atrophy, supporting a potential therapeutic role in combating TLR4-mediated atrophy.
Keywords: C2C12; conditioned media; electrical pulse stimulation; muscle atrophy; soluble receptor.