Satellite cell regulation in response to acute damaging lengthening contraction: Interleukin-6 mediated STAT3 induced proliferation

Abstract

<p><strong>Background:</strong> Although the satellite cell (SC) is a key regulator of muscle adaptation following exercise, the regulation of human muscle SC function in response to damaging exercise remains largely unexplored. STAT3 signalling, mediated via interleukin-6 (IL-6), has recently come to the forefront as a potential regulator of SC proliferation. The early response of the SC population in human muscle to muscle-lengthening contractions (MLC) as mediated by STAT3 has not been studied.</p> <p><strong>Methodology/Principal Findings:</strong> Twelve male subjects (21 ± 2 y; 83 ± 12 kg) performed 300 maximal MLC of the quadriceps femoris at a velocity of 180˚·s^-1 over a<br />55° range of motion with muscle samples (<em>vastus lateralis</em>) and blood samples (<em>antecubital vein</em>) taken prior to exercise (PRE), 1 hour (T1), 3 hours (T3) and 24 hours (T24) post-exercise. Cytoplasmic and nuclear fractions of muscle biopsies were purified and analyzed for total and phosphorylated STAT3 (p-STAT3) by western blot. p-STAT3 was detected in cytoplasmic fractions across the time course, peaking at T24 (p<0.01 vs. PRE). Nuclear total and p-STAT3 were not detected at appreciable levels. However, immunohistochemical analysis revealed a progressive increase in the proportion of SCs expressing p-STAT3 with ~60% of all SCs positive for p-STAT3 at T24 (p<0.001 vs. PRE). Additionally, <em>cMYC</em>, a STAT3 downstream gene, was significantly up-regulated in SCs at T24 versus PRE (p<0.05). Whole muscle mRNA analysis revealed induction of the STAT3 target genes 1L-6, SOCS3, cMYC (peaking at T3, p<0.05), IL-6Rα and GP130 (peaking at T24, p<0.05). In addition, <em>MYF5</em> mRNA was up-regulated at T24 (p<0.05) with no appreciable change in <em>MRF4</em> mRNA.</p> <p><strong>Conclusions/Significant Findings:</strong> We have demonstrated that IL-6 mediated induction of STAT3 signaling occurred exclusively in the nuclei of SCs in response to MLC. An increase in the number of cMYC⁺ SCs indicated that human SCs were induced to proliferate under the control of STAT3 signaling.</p>