Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, C044

Oral Communications

Altered mTOR localization is associated with decreased protein synthesis and loss of body mass in experimental cancer cachexia

T. A. Nissinen1, J. Hentilä1, F. Penna2, A. Lampinen1, V. Fachada1, J. Lautaoja1, T. Holopainen3, O. Ritvos4, R. Kivelä3, J. Hulmi1,4

1. Neuromuscular Research Center, Faculty of Sport and Health Sciences, University of Jyväskylä, Jyväskylä, Finland. 2. Department of Clinical and Biological Sciences, University of Turin, Turin, Italy. 3. Translational Cancer Biology Program, Research Programs Unit, Faculty of Medicine, Wihuri Research Institute, University of Helsinki, Helsinki, Finland. 4. Department of Physiology, Faculty of Medicine, University of Helsinki, Helsinki, Finland.

Cancer cachexia is a debilitating condition associated with marked loss of muscle mass, decreased tolerance to cancer therapies and increased mortality.1 However, the molecular mechanisms of cachexia-associated muscle wasting are still not fully understood. The mechanistic Target of Rapamycin (mTOR) is one of the key proteins involved in the regulation of muscle protein synthesis. Its localization at lysosomes/late-endosomes has been associated with its activation and possibly also with growth regulation in skeletal muscle.2 The aim of this study was to investigate the mechanisms of muscle wasting and its prevention by activin receptor ligand blocking in experimental cancer cachexia. BALB/c mice were anaesthetised with a mixture of ketamine (60-70 mg/kg, i.p.) and xylazine (9 mg/kg, i.p.) and subcutaneously inoculated with C26 cancer cells (n = 15) or vehicle control (n = 9).3 Half of the tumour-bearing mice were treated with soluble activin receptor type IIB (sACVR2B-Fc, 5 mg/kg, i.p., twice a week, n = 8) to block myostatin and activins and thus to prevent cancer-induced muscle loss.3 Tibialis anterior (TA) muscles were collected 11 days after cancer cell inoculation. Protein synthesis was analysed using SUnSET method.4,5 The colocalization of mTOR and LAMP2 was analysed by immunofluorescence from TA cryosections. C26 and sACVR2B-Fc -effects were analysed by Students t-test and correlations by Pearson correlation coefficient. Statistical significance was set at P < 0.05. C26 cancer resulted in reduced colocalization of mTOR with LAMP2, a marker of lysosomes/late-endosomes, indicating decreased mTOR activity based on previous studies2 and supported by our Western blot results of markers of mTOR signaling (S6K1 and rpS6 phosphorylation). Further supporting the role of mTOR localization in cachexia, mTOR-LAMP2 colocalization also correlated with protein synthesis (r = 0.751; P < 0.01) and with the body mass change of the last day of the experiment (r = 0.630; P < 0.01) in untreated mice. Interestingly, muscle mass and mTOR localization were restored by sACVR2B-Fc treatment despite unaltered protein synthesis at this time-point. This discordance may be related to the time-point studied and to observed decreases in food intake and physical activity in the tumour-bearing mice. These results show that cancer-induced muscle wasting is associated with reduced mTOR localization to lysosomes/late-endosomes resulting potentially in attenuated mTOR activation. This provides new insights into the molecular mechanisms underlying decreased protein synthesis and muscle wasting in cancer cachexia. These results also open up new research questions and possible treatment options for cachexia.

Where applicable, experiments conform with Society ethical requirements