Adipocytes are described as newly introduced

Adipocytes are described as newly introduced members of the immune system, as they release cytokines such as interleukin 6 and TNF-α in addition to adipokines such as irisin, adiponectin, and leptin. Discovered by Boström et al, irisin is made up of 112 amino acids, and is synthesized in the muscular tissue. White adipose tissue also secretes FNDC5/irisin; therefore, irisin also behaves as an adipokine and induces the browning of white adipose tissue and protects against diet induced obesity and diabetes. Interestingly, the concentration of irisin rises remarkably following exercise in both mice and humans. Some animal model studies have shown that irisin prolongs life span, reduces body weight by increasing total rgd peptide expenditure, and decreases insulin resistance caused by diet-induced obesity. By contrast, one study involving patients with polycystic ovary syndrome, which is a disease associated with increased insulin resistance, reported significantly increased irisin levels in these patients. This result was interpreted as a protective mechanism against development of DM in patients with polycystic ovary syndrome. In addition, Park et al found increased levels of irisin in patients with metabolic syndrome. In Behçet\'s disease, which is characterized by endothelial dysfunction and atherosclerosis that are triggered by hyperglycemia and chronic inflammation, irisin levels were found to be low. It was stated that lower irisin levels could be related to atherosclerosis. A negative correlation was observed between irisin levels and HOMA-IR scores. In our study, although we also found a negative correlation between irisin levels and HOMA-IR scores, this was not statistically significant. There is a strong association between psoriasis and insulin resistance, suggesting a possible role for irisin in etiopathogenesis of psoriasis, as irisin is known to reduce weight by increasing total energy expenditure, and decrease insulin resistance caused by diet. The serum irisin levels were found to be decreased in patients with type 2 DM, and this was associated with insulin resistance. One study involving 581 patients with psoriasis found a significant association between psoriasis and type 2 DM. Another study found significantly increased insulin levels and HOMA-IR scores in patients with psoriasis vulgaris. Naldi et al observed a significant association between BMI and psoriasis. In our study, there was no significant association between psoriasis and BMI; however, there was a significant association between psoriasis and levels of glucose and HbA1c, as well as HOMA-IR scores. Some studies have found a positive correlation between irisin and low-density lipoprotein cholesterol. Wen et al detected a positive correlation between irisin and HDL levels in patients with chronic renal failure. Similar to our results, Icli et al did not find any correlation between irisin and lipid profile. Cytokines such as TNF-α have important roles in the pathogenesis of psoriasis. As is already known, TNF-α is an important cytokine for inflammatory, infectious, and malignant states. TNF-α has been found to be elevated in the serum of patients with psoriasis, and in psoriatic skin. Furthermore, TNF-α level has been shown to decrease in serum and skin lesion following treatment. Moreover, TNF-α antagonists are effectively used in the treatment of psoriasis. In one study with mice model, following short-term administration of the cytokines TNF-α and interleukin-1β to mice, an inflammatory state was induced, which reduced the release of irisin from skeletal muscle. Signaling pathways leading to psoriasis development are, however, not fully clear yet; one study has emphasized that peroxisome proliferator-activated receptor-γ might cause development of metabolic syndrome in patients with psoriasis. Moreover, peroxisome proliferator-activated receptor-γ coactivator α plays a primary role in lipid and carbohydrate metabolism and inflammation, and irisin synthesis is regulated through its activation.