Mitogen-activated protein kinase-activated protein kinase 2 (MK2) in skeletal muscle atrophy and hypertrophy.

Skeletal muscle is a highly plastic tissue. Overall muscle growth (hypertrophy) or muscle wasting (atrophy) results from alterations in intracellular signaling pathways with important regulatory steps occurring in the nucleus as well as in the cytoplasm. Previous studies have identified components of the Akt/mTor pathway as well as the p38 MAPK pathway as important for skeletal muscle hypertrophy and/or atrophy. The present study tests the hypothesis that MK2, a substrate of p38 which following phosphorylation, can be exported from the nucleus in a complex with p38, may be important for skeletal muscle growth. The expression of MK2 was examined in denervated mouse hind-limb (atrophic) and hemidiaphragm (transiently hypertrophic) muscles. MK2 mRNA expression decreased after denervation in both atrophic (48% of innervated controls, P < 0.001) and hypertrophic muscle (34% of innervated controls, P < 0.01) but MK2 protein expression decreased only in atrophic muscle (32% of innervated controls, P < 0.01). The level of T205 phosphorylated MK2 increased after denervation in both atrophic (fourfold increase, P < 0.01) and hypertrophic muscles (almost sevenfold increase, P < 0.001) whereas the level of T317 phosphorylated MK2 (necessary for nuclear export) increased after denervation in hypertrophic muscle (nearly threefold increase, P < 0.001) but not in atrophic muscle. Logarithmically transformed relative changes in MK2 phosphorylated at T317 correlated well (r(2) = 0.7737) with relative changes in muscle weight. The results suggest a role for MK2 in the regulation of muscle mass, a role which, at least in part, may be related to determining the subcellular localization of p38 in muscle fibers.