PL-013 Musclin: a myokine induced by aerobic exercise useful to contrast muscle wasting during cancer
Authors: Piccirillo Rosanna , Cecconi Andrea David Re , Chiappa Michela , Beltrame Luca , Marchini Sergio 
Published: October 2018

Objective Physical activity extends life span of patients affected by certain types of cancer, also by contrasting the associated muscle wasting (i.e. cachexia). The most effective type of physical activity against muscle wasting during cancer seems to be aerobic exercise. So, we asked whether it promotes secretion of proteins by muscles (i.e. myokines) that may contrast cancer cachexia. 

Methods To mimic aerobic exercise, we infected C2C12 myotubes with PGC1α-expressing adenoviruses, because PGC1αis the main transcriptional coactivator involved in muscle adaptation during aerobic exercise. By microarray analysis, we identified a number of putatively secreted proteins inducible by PGC1αthat were further confirmed by Q-PCR. We measured by Q-PCR and WB their expression in Tibialis Anterior (TA) muscle of C26 bearing-mice (i.e. cancer cachexia model) and their plasma levels by ELISA. To induce aerobic exercise adaptations, mice were run on treadmill for 5 consecutive days at a speed of 12 m/min and an uphill inclination of 15° for 45 min/day. Anaerobic exercise-like effects were obtained in plantaris muscle after 7 and 14 days from surgical removal of its synergist muscles (i.e. compensatory hypertrophy). We performed muscle in vivoelectroporation of plasmids for musclin or its receptor (i.e Npr3) and in vitrowe evaluated protein synthesis/degradation of atrophying myotubes treated with supernatants from GFP or PGC1α-overexpressing cells and Luciferase-based experiments.

Results Our microarray and Q-PCR analyses showed musclin as a PGC1α-induced myokine. Conversely, its expression was unchanged in myotubes hypertrophying because of activated AKT (to mimic anaerobic exercise). Dexamethazone-treated myotubes or constitutively active (ca)FoxO3-expressing myotubes undergo atrophy as measured by increased rates of proteolysis and MuRF1 induction. Unlike GFP, musclin was able to contrast the dexamethazone-induced MuRF1 expression in Luciferase assays. Consistently, musclin-containing supernatants of PGC1αexpressing-myotubes restrained the caFoxO3-induced rates of long-lived protein degradation. Among other PGC1α-induced myokines, we found only musclin strongly downregulated in cachectic muscles and plasma of C26-bearing mice even at times when their body weights were not lost yet. Of note, also its receptor Npr3 was downregulated in cachectic muscles. Thus, we electroporated Tibialis Anterior (TA) of C26-bearing mice with musclin-encoding plasmids and found musclin to preserve fiber area. Interestingly, five days of treadmill exercise was able to protect C26-bearing mice from muscle loss with no effect on tumor growth and to rescue the C26-induced downregulation of musclin (but not its receptor) in cachectic muscles and plasma. By contrast, musclin expression did not change in overloaded plantaris of mice, subjected to compensatory hypertrophy. 

Conclusions Musclin is a myokine induced specifically by PGC1α, typically increased upon aerobic exercise. Musclin overexpression is beneficial against muscle wasting during C26 growth or in atrophying myotubes. Overall, musclin could be a good drug option for cancer patients that cannot exercise and are at risk of developing cachexia.