Abstract
The aquaglyceroporin AQP7 is a pore-forming transmembrane protein that facilitates the transport of glycerol across cell membranes. Glycerol is utilized both in carbohydrate and lipid metabolism. It is primarily stored in white adipose tissue as part of the triglyceride molecules. During states with increased lipolysis, such as fasting and diabetes, glycerol is released from adipose tissue and metabolized in other tissues. AQP7 is expressed in adipose tissue where it facilitates the efflux of glycerol, and AQP7 deficiency has been linked to increased glycerol kinase activity and triglyceride accumulation in adipose tissue, leading to obesity and secondary development of insulin resistance. However, AQP7 is also expressed in a wide range of other tissues, including kidney, muscle, pancreatic β-cells and liver, where AQP7 also holds the potential to influence whole body energy metabolism. The aim of the review is to summarize the current knowledge on AQP7 in adipose tissue, as well as AQP7 expressed in other tissues where AQP7 might play a significant role in modulating whole body energy metabolism.
Highlights
Far, 13 members of mammalian aquaporins (AQPs) have been identified (AQP0–12), and most of them have been well characterized
AQP7 is expressed in adipose tissue where it facilitates the efflux of glycerol, and AQP7 deficiency has been linked to increased glycerol kinase activity and triglyceride accumulation in adipose tissue, leading to obesity and secondary development of insulin resistance
AQP7 deficiency is linked to increased TG accumulation and development of obesity; when considering targeting of AQP7 in anti-obesity therapy, the wide range of tissues where AQP7 is expressed should be taken into account
Summary
13 members of mammalian aquaporins (AQPs) have been identified (AQP0–12), and most of them have been well characterized. The second group, called aquaglyceroproteins, is permeable to small uncharged molecules, such as urea or glycerol in addition to water. AQP6, AQP8, AQP11 and AQP12 belong to the third subgroup called unorthodox aquaporins whose function is still being elucidated [1]. The deduced amino acid sequence of human AQP7 (hAQP7) is 342 aa long [7,8], while in mice (mAQP7), the protein is 303 aa in length [8]. The hAQP7 sequence is 67% identical to mAQP7, and both share 68% and 79% sequence homology with rat AQP7 (rAQP7), respectively. Experiments in Xenopus oocytes expressing AQP7 showed increased water permeability [3,5,6], along with glycerol and urea uptake [6]. AQP7 is expressed in a wide range of tissues and this review will focus on the tissues where current knowledge suggests a role for AQP7 in whole body energy metabolism
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