Muscle-derived interleukin-6--a possible link between skeletal muscle, adipose tissue, liver, and brain.

Accumulating evidence exists that regular exercise offers protection against chronic disorders such as cardiovascular diseases, type 2 diabetes, dementia, and depression. Although acute and chronic exercise has numerous consequences, it is still discussed how contracting skeletal muscles mediate metabolic and physiological effects of benefits on health. For years the search for the stimulus that initiates and maintains the change of excitability or sensibility of the regulating centers in exercise has been progressing. For lack of more precise knowledge, it has been called the 'work stimulus,' 'the work factor' or 'the exercise factor.' In other terms, the big challenge for muscle and exercise physiologists has been to determine how muscles signal to central and peripheral organs. Recently, we identified that muscle fibers produce and release the cytokine IL-6 into the circulation during exercise. We further proposed that IL-6 and other cytokines, which are produced and released by skeletal muscles, exerting their effects in other organs of the body, should be named 'myokines.' In line with that adipokines have been suggested as a term, which is restricted to cover cytokines and other peptides which are produced and secreted by adipocytes, we suggest that the term "myokines" should be used exclusively to describe cytokines or other peptides, which are produced and released by muscle fibers per se. Myokines may represent the link from working muscle to other organs such as the adipose tissue, the liver, and the vascular compartments. Here, we review the literature on muscle- and brain-derived IL-6. We further suggest that myokines may also provide an explanation as to how regular muscle activity influences mood, performance, and cognitive function.