Cytokines in aging and muscle homeostasis.

Cytokines are best known as mediators of host defense responses to infection and other environmental stresses. However, some of these proteins (interleukin-1 and tumor necrosis factor) may modulate clearance and repair processes in skeletal muscle following injury, and others (fibroblast growth factor and platelet-derived growth factor) may be involved with the sustained viability of muscle cells. Muscle repair and vitality also require neuronal contact (influenced by nerve growth factor and ciliary neurotrophic factor) as well as angiogenesis and connective tissue matrix formation (influenced by transforming growth factor beta). Successful muscle aging will depend in part on how well a muscle repairs itself after damage. This includes not only overt injury, but also the daily "wear and tear" that may not be perceived via pain or alterations in function. Age-related loss of muscle mass or function may be the cumulative result of repeated episodes of incomplete repair. Abnormal production or sensitivity to cytokines by aged cells may contribute to these changes in muscle mass and function.