Physical exercise triggers the release of several cytokines/chemokines from working skeletal muscles, but the underlying mechanism(s) by which skeletal muscles decipher and respond to highly complex contractile stimuli remains largely unknown. In an effort to investigate the regulatory mechanisms of the expressions of two contraction-inducible CXC chemokines, CXCL1/KC and CXCL5/LIX, in contracting skeletal muscle cells, we took advantage of our in vitro exercise model using highly developed contractile C2C12 myotubes, which acquire properties similar to those of in vivo skeletal muscle via manipulation of Ca2+ transients with electric pulse stimulation (EPS). Production of these CXC chemokines was immediately augmented by EPS-evoked contractile activity in a manner dependent on the activities of JNK and NF-κB, but not p38, ERK1/2, or calcineurin. Intriguingly, exposure of myotubes to cyclic mechanical stretch also induced expression of these CXC chemokines; however, a much longer period of stimulation (∼12 h) was required, despite rapid JNK phosphorylation. We also demonstrate herein that CXCL1/KC and CXCL5/LIX have the ability to raise intracellular Ca2+ concentrations via CXCR2-mediated activation of pertussis toxin-sensitive Gαi proteins in C2C12 myoblasts, an action at least partially responsible for their migration and differentiation. Although we revealed a possible negative feedback regulation of their own production in response to the contractile activity in differentiated myotubes, exogenous administration of these CXC chemokines did not acutely influence either insulin-induced Akt phosphorylation or GLUT4 translocation in C2C12 myotubes. Taken together, these data shed light on the fundamental characteristics of contraction-inducible CXC chemokine production and their potential roles in skeletal muscle cells.
THE FINE STRUCTURE OF EMBRYONIC CHICK SKELETAL MUSCLE CELLS DIFFERENTIATED IN VITRO
