Aiston S, Hampson L J, Arden C, Iynedjian P B, Agius L
School of Clinical Medical Sciences-Diabetes, University of Newcastle upon Tyne, NE2 4HH, Newcastle upon Tyne, UK.
Diabetologia. 2006 Jan;49(1):174-82. doi: 10.1007/s00125-005-0068-4. Epub 2005 Dec 10.
AIMS/HYPOTHESIS: An insulin signalling pathway leading from activation of protein kinase B (PKB, also known as Akt) to phosphorylation (inactivation) of glycogen synthase kinase-3 (GSK-3) and activation of glycogen synthase is well characterised. However, in hepatocytes, inactivation of GSK-3 is not the main mechanism by which insulin stimulates glycogen synthesis. We therefore tested whether activation of PKB causes inactivation of glycogen phosphorylase.
We used a conditionally active form of PKB, produced using recombinant adenovirus, to test the role of acute PKB activation in the control of glycogen phosphorylase and glycogen synthesis in hepatocytes.
Conditional activation of PKB mimicked the inactivation of phosphorylase, the activation of glycogen synthase, and the stimulation of glycogen synthesis caused by insulin. In contrast, inhibition of GSK-3 caused activation of glycogen synthase but did not mimic the stimulation of glycogen synthesis by insulin. PKB activation and GSK-3 inhibition had additive effects on the activation of glycogen synthase, indicating convergent mechanisms downstream of PKB involving inactivation of either phosphorylase or GSK-3. Glycogen synthesis correlated inversely with the activity of phosphorylase-a, irrespective of whether this was modulated by insulin, by PKB activation or by a selective phosphorylase ligand, supporting an essential role for phosphorylase inactivation in the glycogenic action of insulin in hepatocytes.
CONCLUSIONS/INTERPRETATION: In hepatocytes, the acute activation of PKB, but not the inhibition of GSK-3, mimics the stimulation of glycogen synthesis by insulin. This is explained by a pathway downstream of PKB leading to inactivation of phosphorylase, activation of glycogen synthase, and stimulation of glycogen synthesis, independent of the GSK-3 pathway.
目的/假设：一条从蛋白激酶B（PKB，也称为Akt）激活到糖原合酶激酶-3（GSK-3）磷酸化（失活）以及糖原合酶激活的胰岛素信号通路已得到充分表征。然而，在肝细胞中，GSK-3失活并非胰岛素刺激糖原合成的主要机制。因此，我们测试了PKB激活是否会导致糖原磷酸化酶失活。
我们使用重组腺病毒产生的条件性活性形式的PKB，来测试急性PKB激活在肝细胞中糖原磷酸化酶控制和糖原合成中的作用。
PKB的条件性激活模拟了胰岛素引起的磷酸化酶失活、糖原合酶激活以及糖原合成的刺激。相比之下，抑制GSK-3导致糖原合酶激活，但并未模拟胰岛素对糖原合成的刺激。PKB激活和GSK-3抑制对糖原合酶激活具有累加效应，表明PKB下游存在涉及磷酸化酶或GSK-3失活的汇聚机制。无论糖原合成是由胰岛素、PKB激活还是选择性磷酸化酶配体调节，糖原合成均与磷酸化酶a的活性呈负相关，这支持了磷酸化酶失活在肝细胞胰岛素糖原生成作用中的重要作用。
结论/解读：在肝细胞中，PKB的急性激活而非GSK-3的抑制模拟了胰岛素对糖原合成的刺激。这可以通过PKB下游一条导致磷酸化酶失活、糖原合酶激活以及糖原合成刺激的通路来解释，该通路独立于GSK-3通路。
