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Int J Obes (Lond). 2014 Sep;38(9):1213-20. doi: 10.1038/ijo.2013.234. Epub 2013 Dec 13.

Reduced hepatic aquaporin-9 and glycerol permeability are related to insulin resistance in non-alcoholic fatty liver disease.

Author information

1
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.
2
Department of Biosciences, Biotechnologies and Biopharmaceutics, University of Bari Aldo Moro, Bari, Italy.
3
Metabolic Research Laboratory, Clínica Universidad de Navarra, Pamplona, Spain.
4
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.
5
Department of Pathology, Clínica Universidad de Navarra, Pamplona, Spain.
6
Department of Anesthesia, Clínica Universidad de Navarra, Pamplona, Spain.
7
Department of Endocrinology and Nutrition, Clínica Universidad de Navarra, Pamplona, Spain.
8
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.
9
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.

Abstract

BACKGROUND/OBJECTIVES:

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.

SUBJECTS/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.

PMID:
24418844
DOI:
10.1038/ijo.2013.234
[Indexed for MEDLINE]
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