Characteristics of skeletal muscle as a secretory organ

Abstract

Growing evidence has shown that skeletal muscle cells can secrete bioactive proteins into the extracellular milieu. Secretion of many of those proteins is accelerated in response to exercise and muscle contraction and can regulate functions of several organs via autocrine, paracrine, and endocrine routes; this is referred to as the myokine theory. Habitual exercise leads to various health benefits such as metabolic improvement, anti-inflammation, and muscle building, which are at least partly caused by myokines including specific interleukins. Comprehensive analysis suggests that skeletal muscle cells can secrete over 100 proteins, many of which remain unknown. However, recent studies have identified additional novel myokines. Some secreted proteins can improve nutrient metabolism and muscle/bone mass; others exert anti-inflammatory and anti-tumorigenesis effects. The concept of myokines could be expanded to not only include high molecule weight proteins but also small peptides and noncoding RNA and applied to various fields including nutrition and medicine.