Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC331
TNF-α inhibits amino acid uptake and activation of the MAPK pathway in isolated intact mammalian skeletal muscle fibres through a COX-dependent pathway
M. Rashid2, G. Mutungi1
1. School of Medicine, University of East Anglia, Norwich, United Kingdom. 2. Department of Orthopaedics, Norfolk and Norwich University Hospital, Norwich, Norfolk, United Kingdom.
Ageing and some chronic illnesses such as AIDS, chronic obstructive pulmonary disease and cancer are characterised by a slow but progressive loss in skeletal muscle mass commonly referred to as sarcopenia and cachexia, respectively. Recent studies suggest that this muscle wasting may arise from the high titres of circulating pro-inflammatory cytokines, especially tumour necrosis factor α (TNF-α; also known as cachexin), associated with both sarcopenia and cachexia (Degens, 2010). However, little is known about the cellular-signal transduction events underlying the acute effects of TNFα in mammalian skeletal muscle fibres. Therefore, the primary aim of this study was to investigate the acute effects of TNF-α and the cellular signal transduction events mediating these effects in isolated intact mouse skeletal muscle fibre bundles. All the experiments were performed at room temperature (~20°C) using small skeletal muscle fibre bundles isolated from a fast-twitch (edl) and slow-twitch (soleus) muscle of adult male CD1 mice. The mice were killed as recommended by the Animals (Scientific Procedures) Act 1986, UK and all the experiments conformed to the local animal welfare committee guidelines. The fibre bundles were then treated with either Ringer’s solution only (controls) or the Ringer’s solution plus 200ngml-1 TNF-α (treated). In some experiments, pharmacological interventions were used to investigate the cellular signal transduction events mediating the acute effects of TNF-α in the muscle fibre bundles. At the end of these experiments, the muscle fibre bundles were processed for either liquid scintillation counting or Western blotting. Our results show that both the fast- and slow-twitch fibres bundles express equal levels of TNF-α receptor 1 protein and that treating them with TNF-α had no effect on the expression of the receptor in both fibre types. However, it led to a decrease in the phosporylation of ERK1/2, JNK and p38 as well as that of their downstream effectors, especially c-JUN. It also led to a significant (<0.05) decrease in the uptake of isoleucine and to an increase in the production of prostaglandin E2α in both fibre types. Furthermore, its effects on the MAPK pathways were partially reversed by pre-treating the muscle fibre bundles with aspirin (a general cyclooxygenase (COX) inhibitor), SC-236 (a selective COX 2 inhibitor) and CAY10526 (a prostaglandin E2α specific inhibitor). From these results we suggest that the catabolic actions of TNF-α in adult mammalian skeletal muscle are mediated through a COX2 dependent inhibition of the MAPK pathway; and that they can be partially reversed aspirin.
Where applicable, experiments conform with Society ethical requirements