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

The Vitamin D Receptor Regulates Glycerolipid and Phospholipid Metabolism in Human Hepatocytes

“…We were unable to amplify Aqp3 from mouse liver, which is in agreement with the low expression reported in mouse liver [19] and with the substantial expression of AQP3 reported in human liver [20,21].…”

“…The highest expression levels of human AQP3 were observed in colon, small intestine, kidney and liver [21]. However, liver AQP3 expression is null in rats [21] and very low in mice [19]. Functional studies of aquaglyceroporins are scarce in humans, and mutations in the genes encoding AQP3, AQP7, and AQP9 show divergent phenotypes to those observed in deficient mice [35].…”

Gut microbiota-associated bile acid deconjugation accelerates hepatic steatosis in ob/ob mice

“…In this study, C57BL/6J ob/ob mice and C57BL/6J wild‐type (lean) controls were used because ob/ob mouse is a well‐established model of hepatic steatosis with a high translational value into the human metabolic fatty liver (Gena et al . ). We found net liver weight as well as liver weight/body weight were increased in ob/ob mice compared to those in their lean controls, possibly due to hepatic fat deposit in these animals.…”

Electrostatic attraction of weak monoacid anions increases probability for protonation and passage through aquaporins

“…A 7 Å resolution projection structure from 2D crystals (22), however, indicates that the AQP9 pore is slightly wider than that of the prototypical GlpF from Escherichia coli (9). AQP9 is expressed in hepatic cells (23) where it channels glycerol in for gluconeogenesis (24,25) and urea out to release metabolic nitrogen waste (26). The distribution of AQP9 in the brain, in particular at the blood-brain barrier, in glia cells, and in catecholaminergic neurons, suggests physiological functions in energy provision in the form of ketone bodies, and in the protection of neurons in pathological ischemia by channeling L-lactate (27,28).…”