AQUAGLYCEROPORINS AND ADIPOSE TISSUE

Abstract

Introduction: Glycerol metabolism in normal condition and in metabolic disorders like type-2 diabetes and obesity could depend at least in part on the transmembrane transport mechanisms of glycerol. Aquaglyceroporins (AQPs) are integral membrane proteins belonging to the water channel protein family (aquaporins) that are permeable to glycerol as well as water. Adipose tissue, the main source of plasma glycerol, expresses high levels of AQP7, the first identified glycerol channel considered the sole AQP of this tissue for a decade. Different studies using AQP7-KO mice models showed contrasting results about the possible onset of obesity in adulthood. Moreover, human subjects homozygous G264V mutant, encoding for a not functional protein, did not show obesity or type 2 diabetes. Some questions arise: 1) the localization of AQP7, 2) the presence of other glycerol channels and 3) the functional role of AQPs in glycerol handling in normal condition and the impact of altered AQPs expression in metabolic syndrome. To this purpose we investigated the questions 1) and 2). Materials and Methods: Realtime RT-PCR, immunoblotting and immunofluorescence were utilized to study the expression and localization of different AQPs in human subcutaneous adipose tissue. Human cultured adipocytes were used to confirm the presence of AQP10 and the possible regulatory role of insulin and isoproterenol on AQP7 and AQP10. Finally, the water and glycerol permeability was measured by a stopped-flow light scattering method both in cultured adipocytes and in adipose plasma membrane vesicles; also the functional effect of AQP10 silencing was investigated. Results and Discussion: AQP7 and AQP10 were expressed in the white adipose tissue and localized both to the plasma membrane and to a thin rim of cytoplasm of the adipocytes. AQP7, but not AQP10, was localized also in the adipose endothelial cells. Cultured adipocytes showed that AQP7 and AQP10 labelling is 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. Water and glycerol permeability measurements confirmed the presence of functioning AQPs and that AQP10 knocked down adipocytes resulted in a 50% glycerol and water permeability reduction. Conclusions: These results and other recently described by the literature seem support the existence in adipose tissue of different AQPs that under hormonal control and in pathological conditions can modify their expression levels and cellular localization, suggesting them as potential therapeutic targets in obesity and metabolic syndrome