Abstract
Irisin is a browning-stimulating molecule secreted from the fibronectin type III domain containing 5 precursor (FNDC5) by muscle tissue upon exercise stimulation. Despite its beneficial role, there is an unmet and clamorous need to discern many essential aspects of this protein and its mechanism of action not only as a myokine but also as an adipokine. Here we contribute to address this topic by revealing the nature and role of FNDC5/irisin in adipose tissue. First, we show that FNDC5/irisin expression and secretion are induced by adipocyte differentiation and confirm its over-secretion by human obese visceral (VAT) and subcutaneous (SAT) adipose tissues. Second, we show how secreted factors from human obese VAT and SAT decrease PGC1α, FNDC5 and UCP1 gene expression on differentiating adipocytes; this effect over UCP1 is blunted by blocking irisin in obese secretomes. Finally, by stable gene silencing FNDC5 we reveal that FNDC5-KO adipocytes show reduced UCP1 expression and enhanced adipogenesis.
Highlights
Evidences showed an association of irisin circulating levels with markers of glucose and lipid homeostasis disturbance in obesity and with metabolic syndrome[13,14,15,16]
fibronectin type III domain containing 5 precursor (FNDC5) immunoprecipitation in C3H10T1/2 cells secretomes followed by two-dimensional (2-DE) western blotting confirmed the previous immunoblots by detecting an exclusive spot in differentiated adipocytes with the expected molecular weight and isoelectric point for 12 kDa-secreted non-glycosylated irisin (Fig. 1F, marked with an arrow). 2-DE western blot showed specific spots at 12 kDa that disappear using blocked antibody in cell lysates (Fig. 1G, marked with an arrow)
We continue with our previous discovery describing FNDC5/irisin as an adipokine over-secreted by obese adipose tissue, which lead us to hypothesise that adipose-derived FNDC5/irisin might participate on the elevated irisin levels detected on obese individuals[5,10,11,12,13,14,16]
Summary
Evidences showed an association of irisin circulating levels with markers of glucose and lipid homeostasis disturbance in obesity and with metabolic syndrome[13,14,15,16]. We found an association between the reduction of plasma irisin levels and the depletion of lipid metabolism biomarkers in patients with metabolic syndrome under an energy-restricted dietary program[14], and a possible role of circulating irisin as a predictor of the insulin resistance onset in association with weight regain in obese individuals[13]. The mentioned data supports the hypothesis that increased circulating irisin, probably secreted by adipose tissue, may be an adaptive response to counterbalance decreased insulin sensitivity and other metabolism disorders associated with obesity[3,9]. In the present paper we go a step forward by revealing the increasing expression and secretion of FNDC5/irisin during adipocyte differentiation, elucidating FNDC5/irisin secreted isoforms in adipocytes, demonstrating the elevated secretion of this adipokine by human obese adipose VAT and SAT explants, revealing the negative effect of obese adipose secreted factors on FNDC5 expression on normal adipocytes, and more importantly, achieving a FNDC5-KO murine adipocyte cell line that shows decreased UCP1 expression and enhanced adipogenesis
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