Fructose supplementation impairs rat liver autophagy through mTORC activation without inducing endoplasmic reticulum stress

Supplementation with 10% liquid fructose to female rats for 2 weeks caused hepatic steatosis through increased lipogenesis and reduced peroxisome proliferator activated receptor (PPAR) alpha activity and fatty acid catabolism, together with increased expression of the spliced form of X-binding protein-1 (Rebollo et al., 2014). In the present study, we show that some of these effects are preserved after sub-chronic (8 weeks) fructose supplementation, specifically increased hepatic expression of lipid synthesis-related genes (stearoyl-CoA desaturase, x 6.7-fold; acetyl-CoA carboxylase, x 1.6-fold; glycerol-3-phosphate acyltransferase, x 1.65-fold), and reduced fatty add beta-oxidation (x 0.77-fold), resulting in increased liver triglyceride content (x 1.69-fold) and hepatic steatosis. However, hepatic expression of PPAR alpha and its target genes was not modified and, further, livers of 8-week fructose-supplemented rats showed no sign of unfolded protein response activation, except for an increase in p-IRE1 levels. Hepatic mTOR phosphorylation was enhanced (x 1.74-fold), causing an increase in the phosphorylation of UNC-51-like kinase 1 (ULK-1) (x 2.8-fold), leading to a decrease in the ratio of LC3B-II/LC3B-I protein expression (x 0.39-fold) and an increase in the amount of the autophagic substrate p62, indicative of decreased autophagy activity. A harmful cycle may be established in the liver of 8-week fructose-supplemented rats where lipid accumulation may cause defective autophagy, and reduced autophagy may result in decreased free fatty add formation from triglyceride depots, thus reducing the substrates for beta-oxidation and further increasing hepatic steatosis. In summary, the length of supplementation is a key factor in the metabolic disturbances induced by fructose: in short-term studies, PPAR alpha inhibition and ER stress induction are critical events, whereas after subchronic supplementation, mTOR activation and autophagy inhibition are crucial. (C) 2014 Elsevier B.V. All rights reserved.