Deletion of C/EBP homologous protein (Chop) in C57Bl/6 mice dissociates obesity from insulin resistance

Endoplasmic reticulum (ER) stress has been implicated in the development of type 2 diabetes, via effects on obesity, insulin resistance and pancreatic beta cell health. C/EBP homologous protein (CHOP) is induced by ER stress and has a central role in apoptotic execution pathways triggered by ER stress. The aim of this study was to characterise the role of CHOP in obesity and insulin resistance. Metabolic studies were performed in Chop (-/-) and wild-type C57Bl/6 mice, and included euglycaemic-hyperinsulinaemic clamps and indirect calorimetry. The inflammatory state of liver and adipose tissue was determined by quantitative RT-PCR, immunohistology and macrophage cultures. Viability and absence of ER stress in islets of Langerhans was determined by electron microscopy, islet culture and quantitative RT-PCR. Systemic deletion of Chop induced abdominal obesity and hepatic steatosis. Despite marked obesity, Chop (-/-) mice had preserved normal glucose tolerance and insulin sensitivity. This discrepancy was accompanied by lower levels of pro-inflammatory cytokines and less infiltration of immune cells into fat and liver. These observations suggest that insulin resistance is not induced by fat accumulation per se, but rather by the inflammation induced by ectopic fat. CHOP may play a key role in the crosstalk between excessive fat deposition and induction of inflammation-mediated insulin resistance.