Aquaporin-10 Represents an Alternative Pathway for Glycerol Efflux from Human Adipocytes

Abstract

BackgroundGlycerol outflow from adipocytes has been considered for a decade to be mediated by aquaporin-7, an aquaglyceroporin highly expressed in the adipose tissue. Its involvement in glycerol metabolism has been widely studied also in humans. Recent studies in different aquaporin-7 KO mice models pose two different questions 1) the exact localization of aquaporin-7 in human white adipose tissue; 2) the existence of other aquaglyceroporins that work with aquaporin-7 to guarantee glycerol efflux and thus a normal adiposity in humans. To this purpose we investigated the expression, the localization and the functioning of aquaglyceroporin-10 in subcutaneous white adipose tissue, in isolated and cultured differentiated adipocytes.Methodology/Principal FindingsAquaporin-7 and -10 were expressed in the white adipose tissue both at mRNA and at protein level. Immunofluorescence revealed aquaporin-7 and -10 labelling in the human adipose tissue both to the plasma membrane and to a thin rim of cytoplasm of adipocytes. Aquaporin-7, but not aquaporin-10, colocalized with the endothelial marker CD34. Human cultured differentiated adipocytes showed an aquaporin-7 and -10 labelling mainly in the cytoplasm and in the lipid droplets with insulin reinforcing the lipid droplets staining and isoproterenol inducing its translocation to the plasma membrane compartment. Water and glycerol permeability measurements using adipocytes and adipose membrane vesicles confirmed the presence of functioning aquaglyceroporins. Aquaporin-10 silencing in human differentiated adipocytes resulted in a 50% decrease of glycerol and osmotic water permeability.Conclusions/SignificanceThe results indicate that aquaporin-7, differently from mice, is present in both adipocyte and capillary plasma membranes of human adipose tissue. Aquaporin-10, on the contrary, is expressed exclusively in the adipocytes. The expression of two aquaglyceroporins in human adipose tissue is particularly important for the maintenance of normal or low glycerol contents inside the adipocyte, thus protecting humans from obesity.

Metabolic impact of the glycerol channels AQP7 and AQP9 in adipose tissue and liver

“…Furthermore, in transgenic mice with eGFP expression driven by the AQP7 promoter, the expression of eGFP was also only found in the endothelial cells of the small capillaries within the adipose tissue (Lebeck et al 2012a). The localization of AQP7 to the endothelial cells within human adipose tissue has been confirmed by others (Rodriguez et al 2011, Laforenza et al 2013. However, in their studies, they also detected labeling for AQP7 in the adipocytes.…”

“…Immunostaining for AQP7 in adipocyte cell culture models demonstrates a predominant localization in intracellular regions (Kishida et al 2000, Miranda et al 2010, Rodriguez et al 2011, which does not apply to a role in transporting glycerol across the plasma membrane. Stimulation of the cells with epinephrine/isoproterenol have been reported to result in translocation of some of the AQP7 labeling to the plasma membrane domain (Kishida et al 2000, Rodriguez et al 2011, Laforenza et al 2013, in turn suggesting that AQP7 traffics to the plasma membrane in response to lipolytic stimuli.…”

Aquaporin expression in the Japanese medaka (Oryzias latipes, Temminck & Schlegel) in FW and SW: challenging the paradigm for intestinal water transport?

“…In humans, zebrafish and eel the ortholog is permeable to water, glycerol and urea (Ishibashi et al, 2002;MacIver et al, 2009;Tingaud-Sequeira et al, 2010). Recently, it was also found in human adipocytes, where it is co-expressed with two other glyceropores, AQP3 and AQP7 (Rodríguez et al, 2011;Laforenza et al, 2013), and may play a major role in glycerol metabolism. In the Japanese medaka, two paralogs are present: aqp10a and aqp10b.…”

Revisiting the adipocyte: a model for integration of cytokine signaling in the regulation of energy metabolism

“…On the other hand, glycerol 3-phosphate, the second metabolite required for TAG biosynthesis, derives from three different metabolic sources: 1) glucose, since glycerol 3-phosphate is an intermediate metabolite of glycolysis; 2) lipolysis-derived glycerol, which is phosphorylated by the glycerol kinase (GK) enzyme; and 3) aquaglyceroporin (AQP)-mediated glycerol uptake (263). AQP7 is considered the main glycerol channel facilitating glycerol release/uptake in adipocytes (78,117,129,199), but other glycerol channels such as AQP3, AQP5, AQP9, AQP10, and AQP11 also contribute to glycerol transport in fat cells (175,192,193,264). Insulin constitutes an important regulator of adipocyte hypertrophy, but not of hyperplasia, since low levels of insulin receptor are found in preadipocytes (112).…”