Focal adhesion proteins FAK and paxillin increase in hypertrophied skeletal muscle.

Components of signaling pathways for mechanotransduction during load-induced enlargement of skeletal muscle have not been completely defined. We hypothesized that loading of skeletal muscle would result in an adaptive increase in the expression of two focal adhesion complex (FAC)-related proteins, focal adhesion kinase (FAK) and paxillin, as well as increased FAK activity. FAK protein was immunolocalized to the sarcolemmal region of rooster anterior latissimus dorsi (ALD) myofibers in the middle of the ALD muscle. FAK (77 and 81%) and paxillin (206 and 202%) protein concentrations per unit of total protein in Western blots increased significantly after 1.5 and 7 days, but not after 13 days, of stretch-induced hypertrophy-hyperplasia of the ALD muscle. FAK autokinase activity in immunoprecipitates was increased after 1.5, 7, and 13 days in stretched ALD muscles. To determine whether increased FAK and paxillin protein concentrations are associated with hypertrophy and/or new fiber formation, two additional experiments were performed. First, during formation of primary chicken myotubes (a model of new fiber formation), FAK protein concentration (63%), FAK activity (157%), and paxillin protein concentration (97%) increased compared with myoblasts. Second, FAK (112% and 611%) and paxillin (87% and 431%) protein concentrations per unit of total protein in the soleus muscle increased at 1 and 8 days after surgical ablation of the synergistic gastrocnemius muscle (a model of hypertrophy without hyperplasia). Thus increases in components of the FAC occur in hypertrophying muscle of animals and in newly formed muscle fibers in culture. Furthermore, increased FAK activity suggests a possible convergence of signaling at the FAC in load-induced growth of skeletal muscle.