Insulin potentiates lipopolysaccharide-induced IL-6 expression through epigenetic remodeling in adipocytes: in vitro and in vivo mechanistic study.

Interleukin-6 (IL-6) is a central mediator of chronic low-grade inflammation associated with metabolic disease. Because obesity is characterized by elevated circulating insulin and metabolic endotoxemia, we investigated whether insulin modulates lipopolysaccharide (LPS) induced IL-6 expression in adipocytes and examined the underlying epigenetic mechanisms. Insulin priming markedly enhanced LPS-induced Il6 mRNA expression (25.33 ± 0.833-fold) and protein levels (181.8 ± 2.754 pg/ml) in 3T3-L1 mouse adipocytes. Similar synergistic effects were observed in primary mouse (Il6 mRNA; 1.364 ± 0.287-fold and protein; 298.6 ± 13.79-pg/ml) and human adipocytes (Il6 mRNA; 12.99 ± 0.912-fold and protein; 1441 ± 68.69-pg/ml). In vivo, mice treated with insulin followed by LPS exposure exhibited significantly higher Il6 expression in peripheral blood mononuclear cells and adipose tissue compared to either treatment alone. Pharmacological inhibition of PI3K signaling suppressed this effect and AKT phosphorylation. Mechanistically, epigenetic profiling revealed that insulin increased histone H3 lysine 9 acetylation (H3K9ac), an active chromatin marker, in a PI3K-dependent manner. Chromatin immunoprecipitation-quantitative polymerase chain reaction (ChIP-qPCR) analysis demonstrated an enhanced H3K9 acetylation at the NF-κB and CREB loci at the distal region and CREB/NF-IL6 locus at the proximal region of the Il6 promoter following combined insulin and LPS stimulation; this effect was significantly attenuated upon blockade of insulin signaling. This synergistic induction was dependent on H3K9 acetylation, indicating that metabolic and inflammatory signals converge at the Il6 promoter to promote chromatin remodeling and transcriptional co-activator recruitment. Collectively, these findings demonstrate that insulin synergizes with LPS to amplify IL-6 mediated inflammation in adipocytes through epigenetic remodeling of the Il6 locus, linking hyperinsulinemia to chronic inflammation in obesity and insulin resistance.