title: Reduced hepatic aquaporin-9 and glycerol permeability are related to insulin resistance in non-alcoholic fatty liver disease.
abstract: OBJECTIVE
  Glycerol represents an important metabolite for the control of lipid accumulation and hepatic gluconeogenesis. We investigated whether hepatic expression and functionality of aquaporin-9 (AQP9), a channel mediating glycerol influx into hepatocytes, is impaired in non-alcoholic fatty liver disease (NAFLD) and steatohepatitis (NASH) in the context of insulin resistance.
  
  
  METHODS
  Liver biopsies were obtained from 66 morbid obese patients undergoing bariatric surgery (66% women, mean body mass index (BMI) 46.1±1.0 kg m(-2)) with available liver echography and pathology analysis of the biopsies in this cross-sectional study. Subjects were classified according to normoglycemia (NG), impaired glucose tolerance (IGT) or type 2 diabetes (T2D). Hepatic expression of AQP9 was analyzed by real-time PCR, western blotting and immunohistochemistry, while glycerol permeability (P(gly)) was measured by stopped-flow light scattering.
  
  
  RESULTS
  AQP9 was the most abundantly (P<0.0001) expressed aquaglyceroporin in human liver (AQP9>>>AQP3>AQP7>AQP10). Obese patients with T2D showed increased plasma glycerol as well as lower P(gly) and hepatic AQP9 expression. The prevalence of NAFLD and NASH in T2D patients was 100 and 65%, respectively. Interestingly, AQP9 expression was decreased in patients with NAFLD and NASH as compared with those without hepatosteatosis, in direct relation to the degree of steatosis and lobular inflammation, being further reduced in insulin-resistant individuals. The association of AQP9 with insulin sensitivity was independent of BMI and age. Consistent with these data, fasting insulin and C-reactive protein contributed independently to 33.1% of the hepatic AQP9 mRNA expression variance after controlling for the effects of age and BMI.
  
  
  CONCLUSIONS
  AQP9 downregulation together with the subsequent reduction in hepatic glycerol permeability in insulin-resistant states emerges as a compensatory mechanism whereby the liver counteracts further triacylglycerol accumulation within its parenchyma as well as reduces hepatic gluconeogenesis in patients with NAFLD.
authors.affiliation: 1] Metabolic Research Laboratory, Clínica Universidad de Navarra, Pamplona, Spain [2] CIBER Fisiopatología de la Obesidad y Nutrición (CIBERobn), Instituto de Salud Carlos III, Pamplona, Spain.
authors.forename: A
authors.initials: A
authors.lastname: Rodríguez
authors.affiliation: Department of Biosciences, Biotechnologies and Biopharmaceutics, University of Bari Aldo Moro, Bari, Italy.
authors.forename: P
authors.initials: P
authors.lastname: Gena
authors.affiliation: Metabolic Research Laboratory, Clínica Universidad de Navarra, Pamplona, Spain.
authors.forename: L
authors.initials: L
authors.lastname: Méndez-Giménez
authors.affiliation: Department of Biosciences, Biotechnologies and Biopharmaceutics, University of Bari Aldo Moro, Bari, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Rosito
authors.affiliation: 1] CIBER Fisiopatología de la Obesidad y Nutrición (CIBERobn), Instituto de Salud Carlos III, Pamplona, Spain [2] Department of Surgery, Clínica Universidad de Navarra, Pamplona, Spain.
authors.forename: V
authors.initials: V
authors.lastname: Valentí
authors.affiliation: 1] CIBER Fisiopatología de la Obesidad y Nutrición (CIBERobn), Instituto de Salud Carlos III, Pamplona, Spain [2] Department of Surgery, Clínica Universidad de Navarra, Pamplona, Spain.
authors.forename: F
authors.initials: F
authors.lastname: Rotellar
authors.affiliation: Department of Pathology, Clínica Universidad de Navarra, Pamplona, Spain.
authors.forename: I
authors.initials: I
authors.lastname: Sola
authors.affiliation: Department of Anesthesia, Clínica Universidad de Navarra, Pamplona, Spain.
authors.forename: R
authors.initials: R
authors.lastname: Moncada
authors.affiliation: Department of Endocrinology and Nutrition, Clínica Universidad de Navarra, Pamplona, Spain.
authors.forename: C
authors.initials: C
authors.lastname: Silva
authors.affiliation: 1] Department of Biosciences, Biotechnologies and Biopharmaceutics, University of Bari Aldo Moro, Bari, Italy [2] Centro di Eccellenza di Genomica in campo Biomedico ed Agrario (CEGBA), Bari, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Svelto
authors.affiliation: Department of Endocrinology and Nutrition, Clínica Universidad de Navarra, Pamplona, Spain.
authors.forename: J
authors.initials: J
authors.lastname: Salvador
authors.affiliation: 1] Department of Biosciences, Biotechnologies and Biopharmaceutics, University of Bari Aldo Moro, Bari, Italy [2] Centro di Eccellenza di Genomica in campo Biomedico ed Agrario (CEGBA), Bari, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Calamita
authors.affiliation: 1] Metabolic Research Laboratory, Clínica Universidad de Navarra, Pamplona, Spain [2] CIBER Fisiopatología de la Obesidad y Nutrición (CIBERobn), Instituto de Salud Carlos III, Pamplona, Spain [3] Department of Endocrinology and Nutrition, Clínica Universidad de Navarra, Pamplona, Spain.
authors.forename: G
authors.initials: G
authors.lastname: Frühbeck
mesh_terms: D000328:Adult; D020346:Aquaporins / Q000378:metabolism*; D015153:Blotting, Western; D002097:C-Reactive Protein / Q000378:metabolism; D003430:Cross-Sectional Studies; D015536:Down-Regulation; D005234:Fatty Liver / Q000378:metabolism* / Q000503:physiopathology; D005260:Female; D018149:Glucose Intolerance / Q000378:metabolism; D005990:Glycerol / Q000378:metabolism; D006801:Humans; D007333:Insulin Resistance; D008099:Liver / Q000378:metabolism* / Q000473:pathology; D008297:Male; D065626:Non-alcoholic Fatty Liver Disease / Q000378:metabolism* / Q000503:physiopathology; D010539:Permeability; D060888:Real-Time Polymerase Chain Reaction; D014280:Triglycerides / Q000378:metabolism
chemical_list: C109669:AQP9 protein, human; D020346:Aquaporins; D014280:Triglycerides; D002097:C-Reactive Protein; D005990:Glycerol