WHY DO HIGH OXIDATIVE MUSCLE FIBERS REMAIN SMALL – A ROLE FOR A HIGH RATE OF PROTEIN DEGRADATION?

Introduction Muscle fiber size and oxidative capacity are inversely related [1], and training induced hypertrophy and increased mitochon- drial density are mutually exclusive [2]. Paradoxically, high oxidative fibers contain a large potential for protein synthesis, but remain small compared to low oxidative fibers [1]. The aim of this study was to obtain insight in the mechanisms underlying this paradox. Meth- ods Soleus muscle (SOL, largely containing high oxidative fibers) and extensor digitorum longus muscle (EDL, largely containing low oxidative fibers) from 6 male Wistar rats were used to determine mRNA concentrations of growth factors, their binding proteins, E3- ligases and their transcription factors by qPCR. Activation of translational signaling pathways was assessed by Western blot. Results IGF-I mRNA concentration was 2.7–fold higher and myostatin concentration 2.5–fold lower (p<0.02) in SOL than in EDL, indicating higher growth factor induced protein synthesis in SOL. mRNA levels of respective growth factor inhibitors (IGFBP-4, follistatin) were also higher in SOL (p<0.005), which suggests that growth factor signaling in SOL is more inhibited than in EDL. PGC-1alpha concentrations (involved in mitochondrial biogenesis) were higher in SOL (p<0.01). Protein levels of total (t-)AKT and phospho(p-)AKT, p-mTOR, and t-S6 were similar. P-S6 and p-AMPK concentrations were higher in SOL (p<0.02). Surprisingly, despite inhibited growth factor signaling in SOL, we found a higher ratio p-S6/t-S6 (p<0.01) indicating a high capacity for protein translation compared to EDL. mRNA concentrations of factors involved in proteasome-mediated degradation (MaFbx, MuRF, NF-kB, FOXOs) were higher in SOL than in EDL (p<0.02). Discussion Several mecha- nisms may underlie the paradox between size and oxidative metabolism in a steady state situation: 1) inhibitors of IGF-I and myostatin are abundantly expressed in high oxidative fibers, and may attenuate the AKT-mTOR pathway; 2) the AKT-mTOR pathway may be inhibit- ed by high PGC-1alpha and p-AMPK levels [3]; 3) elevated expression levels of FOXOs, NF-kB and the E3-ligases suggest that in high oxidative SOL, a large capacity for protein synthesis is accompanied by high E3-ligase activity and subsequent proteasome-mediated degradation. We conclude that signaling for biosynthesis of mitochondria is likely accompanied by an enhanced rate of degradation of contractile protein and hence limited hypertrophy. References 1. Van Wessel T, De Haan A, Van der Laarse WJ, Jaspers RT (2010) Eur J Appl Physiol, 110, 665-694. 2. Hawley J (2009) Appl Physiol Nutr Metab, 34, 355-361. 3. Chan A, Dyck J (2005) Can J Physiol Pharmacol, 83, 24-28.