Carnosic Acid Attenuates TNF-α-Induced Insulin Resistance by Regulating Mitochondrial Function in 3T3-L1 Adipocytes

The disruption of mitochondrial homeostasis is a trigger for insulin resistance. The loss of N-acetyltransferase 1 (Nat1) function, an insulin-sensitivity gene, contributes to mitochondrial dysfunction and insulin resistance. Carnosic acid (CA), a diterpene derived from rosemary, has demonstrated an anti-insulin-resistance effect. This study hypothesized that CA protects against TNF-α-induced insulin resistance in 3T3-L1 adipocytes by regulating mitochondrial dynamics, biogenesis, and function via Nat1. 3T3-L1 adipocytes were pretreated with CA for 12 h, followed by co-treatment with TNF-α for an additional indicated duration. Results showed that treatment of 3T3-L1 adipocytes with TNF-α decreases mitochondrial membrane potential (MMP) and PGC-1α protein levels and alters mitochondrial fission/fusion dynamics. Pretreatment with CA improved these effects. In parallel, CA prevented the TNF-α-induced reduction in Nat1 protein and improved insulin signaling by suppressing the phosphorylation of insulin receptor substrate-1 (IRS-1) at serine³⁰⁷, while restoring the phosphorylation of IRS-1 at tyrosine⁶²⁸ and Akt. Moreover, transfection with Nat1 siRNA inhibited the protective effect of CA against TNF-α-induced reductions in MMP, PGC-1α, and insulin signaling. In conclusion, CA ameliorated TNF-α-induced insulin resistance by reducing mitochondrial dysregulation by Nat1.