SIK1 drives lipid-induced insulin resistance in skeletal muscle by linking TGFβ1-Smad2/3 activation to PDE4-cAMP dysregulation

Excessive lipid accumulation in skeletal muscle contributes to Insulin resistance. Salt-inducible kinase 1 (SIK1) is known to be involved in myogenic differentiation, yet its role in lipid-induced skeletal muscle Insulin resistance remains unclear. Here, we identified the functional role of SIK1 in skeletal muscle Insulin resistance under lipid overload and delineated the underlying signaling mechanisms. In C2C12 myotubes, palmitate markedly increased SIK1 expression and phosphorylation at Thr182, and further impaired insulin-stimulated Akt phosphorylation and glucose uptake. These effects were blocked by SIK1 knockdown or pharmacological inhibition of SIK. The palmitate-induced upregulation of SIK1 and the associated Insulin signaling defects were abolished by inhibition of TGFβ receptor 1 or knockdown of SMAD2/3. Moreover, genetic or pharmacological inhibition of SIK1 restored the palmitate-reduced cAMP levels in myotubes, and inhibition of PDE4 similarly rescued cAMP levels and Insulin signaling, mimicking the effects of SIK1 suppression. Consistent with these in vitro findings, SIK1 and TGFβ1-Smad2/3 signaling were upregulated while cAMP levels were decreased in skeletal muscle of diet-induced obese (DIO) mice. Either SIK inhibition or blockade of TGFβ1-Smad2/3 signaling restored the impaired insulin-stimulated Akt phosphorylation in isolated skeletal muscle. Together, we demonstrate that SIK1 is upregulated under lipid overload via TGFβ1-Smad2/3 signaling, thereby triggering PDE4-dependent cAMP degradation and consequent Insulin resistance in skeletal muscle. These findings establish SIK1 as a critical mediator of lipid overload-induced Insulin signaling defects in skeletal muscle.