Irisin (Ir), a recently identified adipo-myokine, cleaved and secreted from the protein FNDC5 in response to physical activity, has been postulated to induce the differentiation of a subset of white adipocytes into brown fat and to mediate the beneficial effects on metabolic homeostasis. Metabolic syndrome (MS), a cluster of factors leading to impaired energy homeostasis, affects a significant proportion of subjects suffering from polycystic ovary syndrome (PCOS). The aim of our study was to investigate the relationship between Ir plasma concentrations and metabolic disturbances. The study group consisted of 179 PCOS patients and a population of 122 healthy controls (both groups aged 25-35 years). A subset of 90 subjects with MS was isolated. A positive association between Ir plasma level and MS in the whole group and in controls was found. In subjects with high adipose body content (>40%), Ir was higher than in lean persons (<30%). Our results showed a significant positive association between Ir concentration and android type of adipose tissue in the whole study group and in the control group. Understanding the role of Ir in increased energy expenditure may lead to the development of new therapeutics for obesity and obesity-related diseases.

Skeletal muscles as an active hormonal compartment in the response of physical activity secrete substances named myokines capable of modulating metabolic processes. Myokines take part in communication between muscles and other tissues. Irisin (Ir) - a newly discovered adipomyokine - is cleaved and secreted to the circulation from a fibronectin type III domain containing protein 5 (FNDC5). The mechanism of Ir action has not been described precisely, and receptors for the molecule are not defined yet, but it has been proposed to promote browning of white adipose tissue into beige fat cells. To date we have distinguished two types of adipose tissue in mammals - white, which not only functions as a store of energy but also can play a pro-inflammatory role (secreting adipokines), and brown adipose tissue. Brown adipose tissue has a high mitochondrial content and can dissipate chemical energy in the form of heat (nonshivering termogenesis). It plays a natural antiobesity role and protects against obesity-related diseases. The development of beige adipose tissue, which in its structure and function is similar to brown adipose tissue, and the possibility to modify its amount through some external factors, are nowadays among the most important targets of research on fat cell biology.

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