Fructose induces synthesis and reduces oxidation of liver fatty acids through ChREBP activation

A Rebollo1, M Baena1, N Roglans1,2, M Alegret1,2, JC Laguna1,2. 1University of Barcelona, Pharmacology Unit, School of Pharmacy, Spain, 2NetCenter for Biomedical Research in the Physiopathology of Obesity and Nutrition (CIBERobn), Pharmacology Unit, School of Pharmacy (University of Barcelona), Spain

Background and aims: We have previously shown that fatty liver and hypertriglyceridemia induced by liquid fructose (F) require the simultaneous induction of synthesis and reduction of oxidation of liver fatty acids (Roglans et al., Hepatology 2007;45(3):778-88). We sought to determine the molecular mechanism involved.

Methods: Female Sprague-Dawley rats (fSDR) had free access to water (control C, n=8) or to a 10 % (w/v) F solution (n=12). After 7 and 14 days, animals were sacrificed by decapitation under isoflurane anesthesia and plasma and liver samples were obtained for determining plasma analytes, liver triglycerides, liver enzyme activities and expression of enzymes and transcription factors related to fatty acid metabolism. To confirm possible molecular mechanisms, FaO rat hepatoma cells, and human hepatocytes were incubated for 24 hours with 25 mM F (n=4 C and n=4 F, for each treatment).

Results: F-fed fSDR ingested more calories than C rats after 7 (1.24 fold, f, vs C values, p<0.05) and 14 days (1.27 f, p<0.05) of F supplementation, mainly due to an increased consumption of liquid F (2.3 and 2.8 f, for 7 and 14 days, respectively, p<0.05,) not completely compensated by a decrease in the ingestion of solid food (0.78 f for 7 and 14 days, p<0.05). F-fed fSDR had a reduced activity of liver fatty acid β-oxidation at 7 (0.55 f, p<0.01) and 14 days (0.57 f, p<0.01) of F supplementation and an increased liver lipogenesis, as exemplified by an increased liver expression of stearoyl-CoA desaturase 1 (12.0 f, p<0.01, for 7 days; 4.4 f, p<0.05, for 14 days). Nevertheless, only fSDR supplemented with liquid F for 14 days presented hypertriglyceridemia (1.5 f, p<0.05), hepatic steatosis (1.9 f, p=0.06) and reduced expression of liver PPARα mRNA (0.36 f, p<0.01) and protein (0.75 f, p<0.05). Cultured liver cells in the presence of F also had reduced PPARα expression (0.5 f, p<0.01 for FaO cells; 0.69 f, p<0.05 for human hepatocytes vs C values). Both in vivo and in vitro, the reduction of PPARα expression was accompanied by an increase in ChREBP (2 f, p<0.05 for liver tissue; 1.4 f, p<0.05 for FaO cells vs C values) and its main target gene, L-PK (2.1 f, p<0.01 for liver tissue; 1.9 f, p<0.001 for FaO cells; 1.86 f, p<0.05, for human hepatocytes vs C values). FaO cells cultured in the presence of 25 mM glucose did not show modification in the expression of PPARα and L-PK genes.

Conclusions: ChREBP is probably responsible for both the increased synthesis and decreased oxidation of liver fatty acids produced by F administration. We are now transfecting FaO cells with a siRNA against ChREBP in order to confirm the involvement of ChREBP activation by fructose in the reduction of PPARα expression.

Financial aid by grant SAF2010-15664 (MICINN), SGR09-00413 (Generalitat de Catalunya) and FPCNL.