Induction of Cytosolic Phospholipase A2a Is Required for Adipose Neutrophil Infiltration and Hepatic Insulin Resistance Early in the Course of High-Fat Feeding

In established obesity, inflammation and macrophage recruitment likely contribute to the development of insulin resistance. In the current study, we set out to explore whether adipose tissue infiltration by neutrophils that occurs early (3 days) after initiating a high-fat diet (HFD) could contribute to the early occurrence of hepatic insulin resistance and to determine the role of cytosolic phospholipase A2a (cPLA2a) in this process. The 3-day HFD caused a significant upregulation of cPLA2a in periepididymal fat and in the liver. A specific antisense oligonucleotide (AS) effectively prevented cPLA2a induction, neutrophil infiltration into adipose tissue (likely involving MIP-2), and protected against 3-day HFD–induced impairment in hepatic insulin signaling and glucose over-production from pyruvate. To sort out the role of adipose neutrophil infiltration independent of cPLA2a induction in the liver, mice were injected intraperitoneally with anti–intracellular adhesion molecule-1 (ICAM-1) antibodies. This effectively prevented neutrophil infiltration without affecting cPLA2a or MIP-2, but like AS, prevented impairment in hepatic insulin signaling, the enhanced pyruvate-to-glucose flux, and the impaired insulin-mediated suppression of hepatic glucose production (assessed by clamp), which were induced by the 3-day HFD. Adipose tissue secretion of tumor necrosis factor-a (TNF-a) was increased by the 3-day HFD, but not if mice were treated with AS or ICAM-1 antibodies. Moreover, systemic TNF-a neutralization prevented 3-day HFD-induced hepatic insulin resistance, suggesting its mediatory role. We propose that an acute, cPLA2a-dependent, neutrophil-dominated inflammatory response of adipose tissue contributes to hepatic insulin resistance and glucose overproduction in the early adaptation to high-fat feeding.