Skeletal Muscle as an Auto-, Para- and Endocrine Organ: The Role of Myokines in Muscle Metabolism and Other Metabolic Organs

Skeletal musculature represents the largest organ in the human body, playing a vital role in systemic metabolism, physiological functions, and glucose homeostasis. Skeletal muscles are also a significant source of multiple humoral factors, including myokines, which are, as part of the muscular secretome, involved in cellular signaling within and outside of the muscle. Myokines are a group of cytokines that exert a major influence on muscle metabolism through autocrine mechanisms and are involved in para- or endocrine regulation in organs outside of muscle tissue, such as the pancreas, adipose tissue, liver, heart, bone, gastrointestinal tract, and brain. In the future, these findings could be crucial for the identification of important biomarkers used for the monitoring of physical activity in the treatment of pathologies such as intensive care-associated muscle wasting, sarcopenia, diabetes, neurodegenerative diseases, etc. As skeletal muscle tissue is intrinsically linked to multiple types of tissues and organs metabolically, functionally, and most importantly, regionally, there can be a significant overlap between the auto- and paracrine effects of myokines, depending on the presence of that myokine. The following section will discuss the auto-, para-, and endocrine effects of some of the myo-inducible cytokines on skeletal muscles and adjacent tissue types.