Biophysical Assessment of Human Aquaporin-7 as a Water and Glycerol Channel in 3T3-L1 Adipocytes

The plasma membrane aquaporin-7 (AQP7) has been shown to be expressed in adipose tissue and its role in glycerol release/uptake in adipocytes has been postulated and correlated with obesity onset. However, some studies have contradicted this view. Based on this situation, we have re-assessed the precise localization of AQP7 in adipose tissue and analyzed its function as a water and/or glycerol channel in adipose cells. Fractionation of mice adipose tissue revealed that AQP7 is located in both adipose and stromal vascular fractions. Moreover, AQP7 was the only aquaglyceroporin expressed in adipose tissue and in 3T3-L1 adipocytes. By overexpressing the human AQP7 in 3T3-L1 adipocytes it was possible to ascertain its role as a water and glycerol channel in a gain-of-function scenario. AQP7 expression had no effect in equilibrium cell volume but AQP7 loss of function correlated with higher triglyceride content. Furthermore it is also reported for the first time a negative correlation between water permeability and the cell non-osmotic volume supporting the observation that AQP7 depleted cells are more prone to lipid accumulation. Additionally, the strong positive correlation between the rates of water and glycerol transport highlights the role of AQP7 as both a water and a glycerol channel and reflects its expression levels in cells. In all, our results clearly document a direct involvement of AQP7 in water and glycerol transport, as well as in triglyceride content in adipocytes.已有研究证实，质膜水通道蛋白7（aquaporin-7，AQP7）在脂肪组织中表达，其在脂肪细胞甘油释放与摄取过程中的功能已被提出，并与肥胖发病相关。但部分研究对此观点提出了质疑。基于此，本研究重新评估了AQP7在脂肪组织中的精准定位，并分析了其在脂肪细胞中作为水通道和/或甘油通道的功能。对小鼠脂肪组织进行分级分离后发现，AQP7同时存在于脂肪细胞组分与基质血管组分（stromal vascular fractions）中。此外，AQP7是脂肪组织及3T3-L1脂肪细胞中唯一表达的水甘油通道蛋白（aquaglyceroporin）。通过在3T3-L1脂肪细胞中过表达人源AQP7，本研究在功能获得性（gain-of-function）实验模型中证实了其作为水通道与甘油通道的功能。AQP7的表达对细胞平衡体积无显著影响，但AQP7功能缺失与细胞内甘油三酯（triglyceride）含量升高相关。此外，本研究首次报道了水通透性与细胞非渗透体积（non-osmotic volume）之间的负相关关系，这一结果支持了"AQP7缺失细胞更易发生脂质积累"的观测结论。同时，水转运速率与甘油转运速率之间存在显著正相关，这既印证了AQP7同时作为水通道与甘油通道的功能，也反映了细胞内AQP7的表达水平。综上，本研究结果明确证实了AQP7直接参与脂肪细胞的水和甘油转运过程，并与脂肪细胞内甘油三酯含量密切相关。