Dipartimento di Bioscienze, Biotecnologie e Scienze Farmacologiche, Università degli Studi di Bari Aldo Moro, Bari, Italy.
Biol Cell. 2012 Jun;104(6):342-51. doi: 10.1111/boc.201100061. Epub 2012 Mar 23.
Lipolytic glycerol, released from adipocytes, flows through the bloodstream to the liver, where its utilisation in supplying hepatocyte gluconeogenesis is rate-limited by the permeation step. An aquaglyceroporin expressed in hepatocytes, aquaporin-9 (AQP9), has been often linked to liver uptake of glycerol. However, the truthfulness of this postulation and the potential existence of additional pathways of glycerol import by hepatocytes have never been assessed directly. Here, we define the identity and extent of liver glycerol transport and evaluate the correlation between hepatic AQP9 expression and glycerol permeability (P(gly) ) in AQP9(+/+) wild-type mice in different nutritional states and circulating insulin levels. The liver P(gly) of AQP9 null mice is also assessed.
By stopped-flow light scattering, facilitated diffusion of glycerol into hepatocytes was indicated by the low Arrhenius activation energy (3.5 kcal/mol) and strong inhibition by phloretin, an AQP9 blocker, that characterised the transport. Although fasting markedly increased hepatic AQP9, a straight parallelism was seen both in quantitative and time-space terms between P(gly) and AQP9 protein in AQP9(+/+) mice kept in fed or fasted/refed states. In line with these findings, the highest P(gly) (P(gly) ≈ 14.0 × 10(-6) cm/s at 20°C) at 18-h fasting coincided with the highest percent of phloretin inhibition (63%). Besides being markedly lower than that in AQP9(+/+) mice, the liver P(gly) of the AQP9 null mice did not increase during fasting. Reverse-transcription PCR analysis showed lack of compensation by AQP3 and AQP7, the other known murine glycerol facilitators, in AQP9 null mice.
Overall, these results experimentally prove major functional significance for AQP9 in maximising liver glycerol import during states requiring increased glucose production. If any, alternative facilitated pathways would be of minor importance in transporting glucogenetic glycerol into hepatocytes during starvation. Refining the understanding of liver AQP9 in metabolic and energy homeostasis may reveal helpful for therapeutic purposes.
脂肪细胞释放的脂解甘油通过血液循环进入肝脏，在肝脏中，其用于供应肝细胞糖异生的利用受到渗透步骤的限制。在肝细胞中表达的水通道蛋白 aquaporin-9 (AQP9) 常与肝脏摄取甘油有关。然而，这种推测的真实性以及肝细胞摄取甘油的其他潜在途径的存在从未被直接评估过。在这里，我们确定了肝脏甘油转运的身份和程度，并评估了不同营养状态和循环胰岛素水平下 AQP9(+/+)野生型小鼠肝 AQP9 表达与甘油通透性 (P(gly)) 之间的相关性。还评估了 AQP9 缺失小鼠的肝 P(gly)。
通过停流光散射，甘油易位进入肝细胞表明，低 Arrhenius 活化能 (3.5 kcal/mol) 和 AQP9 阻断剂 phloretin 的强抑制作用，这是运输的特征。尽管禁食显著增加了肝 AQP9，但在 AQP9(+/+) 小鼠保持禁食/再喂养状态或禁食状态时，无论是在定量还是时空方面，P(gly) 与 AQP9 蛋白之间都存在直接的平行关系。与这些发现一致的是，禁食 18 小时时最高的 P(gly) (P(gly) ≈ 14.0 × 10(-6) cm/s 在 20°C) 与 phloretin 抑制的最高百分比 (63%) 相吻合。除了明显低于 AQP9(+/+) 小鼠外，AQP9 缺失小鼠的肝 P(gly) 在禁食期间也没有增加。逆转录 PCR 分析显示，在 AQP9 缺失小鼠中，AQP3 和 AQP7 缺乏补偿，AQP3 和 AQP7 是已知的其他小鼠甘油促进剂。
总的来说，这些结果从实验上证明了 AQP9 在需要增加葡萄糖生成的状态下最大限度地促进肝脏甘油摄取方面具有重要的功能意义。如果有的话，替代的易位途径在饥饿期间将葡萄糖生成甘油转运到肝细胞中时的重要性较小。深入了解肝脏 AQP9 在代谢和能量稳态中的作用可能有助于治疗目的。
