Role Of Toll-like Receptor 4 In Cast Immobilization-induced Muscle Atrophy

Abstract

The mechanism of immobilization-induced skeletal muscle atrophy is still unclear. Interestingly, inflammatory cytokines cause muscle protein degradation, through the activation of the ubiquitin proteasome in skeletal muscle cells. Therefore, local inflammation of skeletal muscles may play an important role in immobilization-induced muscle atrophy. Recent studies have shown that bed rest causes an up-regulation in Toll-like receptor (TLR) 4 and inflammatory cytokine expression in human skeletal muscles. TLR4 induces the production of inflammatory cytokines in response to activation by endogenous ligands. Therefore, it is possible that increased TLR4 expression affects the severity of immobilization-induced muscle atrophy. PURPOSE: We have examined the role of TLR4 in cast immobilization-induced skeletal muscle atrophy using TLR4-defective C3H/HeJ mice. METHODS: C3H/HeN mice and C3H/HeJ mice were divided into control and cast-immobilization groups. Cast immobilization was imposed for 14 days. Muscle atrophy in the gastrocnemius muscle was evaluated by analyzing the muscle mass and cross-sectional area of the muscle fiber. Gene expressions in the gastrocnemius were evaluated by real time-Reverse Transcriptase -Polymerase Chain Reaction. RESULTS: Cast immobilization resulted in an increase in TLR4 mRNA expression (3.2-fold) in the gastrocnemius, and in a decrease in muscle mass of C3H/HeN mice. However, no difference in a decrease in muscle mass (-16.1 ± 1.7% in C3H/HeN, -15.9 ± 1.7% in C3H/HeJ), or in the gastrocnemius fiber cross-sectional area at day 14, in the cast immobilized group. Cast immobilization-induced increase in ubiquitin ligase mRNA was not affected by defective TLR4 (Atrogin-1; 5.7-fold increase in C3H/HeN, 5.9-fold in C3H/HeJ and MuRF-1, 7.3-fold in C3H/HeN, and 6.7-fold in C3H/HeJ). In control groups, TNF-α mRNA expression in C3H/HeJ mice was lower than that in C3H/HeN mice. However, this variable did not differ between the cast-immobilized groups of the two mouse models at day 14. CONCLUSIONS: Our findings in C3H/HeJ mice suggest that TLR4 may not play an essential role in immobilization-induced muscle atrophy.