The saturated fatty acid palmitate induces insulin resistance through Smad3-mediated down-regulation of FNDC5 in myotubes.

Elevated plasma free fatty acid (FFA) levels are associated with insulin resistance and can cause lipotoxicity in skeletal muscles. In response to FFAs, skeletal muscle can secrete a variety of cytokines. Irisin, one such muscle-secreted cytokine, can improve glucose tolerance, glucose uptake, and lipid metabolism. It is produced by the transmembrane protein fibronectin type Ⅲ domain containing 5 (FNDC5) by specific proteases. The purpose of this study was to investigate the regulatory mechanisms of the FNDC5 response to palmitate and their relationships with insulin resistance in C2C12 myotubes. RNA sequencing analysis results from C2C12 myotubes treated with palmitate showed that palmitate could activate the TGF-β signaling pathway. Palmitate directly affected the expression of Smad3, but not its phosphorylation level, in C2C12 myotubes. Furthermore, knockdown and knockout of Smad3 alleviated the inhibitory effect of palmitate on the expression of FNDC5. In contrast, overexpression of Smad3 aggravated the inhibition of FNDC5 expression. There is a Smad3 binding motif in the -660 bp to -649 bp region of the Fndc5 promoter. CRISPR/Cas9 knockout of this region also alleviated the inhibition of FNDC5 expression in response to palmitate. More importantly, inhibition of FNDC5 expression mediated by Smad3 led to a decrease in insulin sensitivity in C2C12 myotubes. Collectively, these findings suggest that palmitate could induce insulin resistance through Smad3-mediated down-regulation of the Fndc5 gene.