Metabolic flux signatures of the ER unfolded protein response in vivo reveal decreased hepatic de novo lipogenesis and mobilization of lipids from adipose tissue to liver

Abstract

The unfolded protein response in the endoplasmic reticulum (UPRER) is involved in a number of metabolic diseases, including non-alcoholic fatty liver disease. Here, we characterize the UPRER induced metabolic changes in mouse liver through in vivo metabolic labeling and mass spectrometric analysis of proteome and lipid fluxes. We induced ER stress in vivo via tunicamycin treatment and measured rates of proteome-wide protein synthesis, de novo lipogenesis and cholesterol synthesis serially over a three-day period, thereby generating a metabolic "signature" of the UPRER over time. Synthesis of most proteins was suppressed under ER stress conditions, including proteins involved in lipogenesis, consistent with reduced de novo lipogenesis at 48 and 72 hours. Electron microscopy revealed striking morphological changes to ER and H&E staining showed lipid droplet enriched livers under ER stress. Pre-labeling of adipose tissue prior to ER stress induction revealed mobilization of lipids from adipose to the liver. Interestingly, the source of these lipids was uptake of free fatty acids, not whole triglycerides or phospholipids from lipoproteins, as demonstrated by replacement of the triglyceride-glycerol moiety in liver concurrently with increased incorporation of labeled palmitate from adipose. We also induced ER stress by a high-fat diet and observed similar metabolic flux signatures, suggesting that this mechanism may play a role in the progression of fatty liver disease. This flux-based approach provides a powerful tool to identify novel regulators of ER stress and potential targets for pharmacological intervention.