Liver glycerol permeability and Aquaporin-9 are dysregulated in a murine model of Non-Alcoholic Fatty Liver Disease

Abstract

One form of liver steatosis, namely Non-Alcoholic Fatty Liver Disease (NAFLD), is a worrisome health problem worldwide<br />characterized by intrahepatic triacylglycerol (TG) overaccumulation. NAFLD is a common feature of metabolic syndrome<br />being often associated with obesity, dyslipidemia and diabetes and mostly closely linked to insulin resistance. The<br />mechanism of NAFLD pathogenesis is object of intense investigation especially regarding complex systems ultimately<br />resulting in excessive TG deposition in hepatocytes. However, scarce is the attention about the relevance of hepatic import<br />of glycerol, the other primary source (as glycerol-3-phosphate) of increased TG in hepatocytes. Obese leptin-deficient (ob/<br />ob) mice, an animal model of NAFLD, were used to evaluate the functional involvement of Aquaporin-9 (AQP9), the major<br />pathway of liver glycerol entry, in hepatosteatosis. By RT-PCR and qPCR, the level of Aqp9 mRNA in the liver of starved obese<br />mice was comparable with the corresponding control lean littermates. By immunoblotting, the AQP9 protein at the<br />hepatocyte sinusoidal plasma membrane of obese mice was markedly lower (33%) than lean mice, a finding fully confirmed<br />by immunohistochemistry. By stopped-flow light scattering, the liver glycerol permeability of ob/ob mice was significantly<br />lower (53%) than lean mice, a finding consistent with both the observed down-regulation of AQP9 protein and increased<br />level of plasma glycerol characterizing obese mice. In summary, our results suggest implication of AQP9 in liver steatosis.<br />The reduction of hepatocyte AQP9 and, consequently, glycerol permeability might be a defensive mechanism to counteract<br />further fat infiltration in liver parenchyma.