Silibinin improves palmitate-induced insulin resistance in C2C12 myotubes by attenuating IRS-1/PI3K/Akt pathway inhibition

The present study investigated the effect of silibinin, the principal potentialanti-inflammatory flavonoid contained in silymarin, a mixture of flavonolignansextracted from Silybum marianum seeds, on palmitate-induced insulinresistance in C2C12 myotubes and its potential molecular mechanisms. Silibininprevented the decrease of insulin-stimulated 2-NBDG(2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxy-D-glucose) uptake and thedownregulation of glutamate transporter type 4 (GLUT4) translocation in C2C12myotubes induced by palmitate. Meanwhile, silibinin suppressed the palmitate-induceddecrease of insulin-stimulated Akt Ser473 phosphorylation, which was reversed bywortmannin, a specific inhibitor of phosphatidylinositol-3-kinase (PI3K). We alsofound that palmitate downregulated insulin-stimulated Tyr632 phosphorylation ofinsulin receptor substrate 1 (IRS-1) and up-regulated IRS-1 Ser307 phosphorylation.These effects were rebalanced by silibinin. Considering several serine/threoninekinases reported to phosphorylate IRS-1 at Ser307, treatment with silibinindownregulated the phosphorylation of both c-Jun N-terminal kinase (JNK) and nuclearfactor-κB kinase β (IKKβ), which was increased by palmitate in C2C12 myotubesmediating inflammatory status, whereas the phosphorylation of PKC-θ was notsignificantly modulated by silibinin. Collectively, the results indicated thatsilibinin prevented inhibition of the IRS-1/PI3K/Akt pathway, thus amelioratingpalmitate-induced insulin resistance in C2C12 myotubes.