Role of the PDK1–PKB–GSK3 pathway in regulating glycogen synthase and glucose uptake in the heart

In order to investigate the importance of the PDK1–PKB–GSK3 signalling network in regulating glycogen synthase (GS) in the heart, we have employed tissue specific conditional knockout mice lacking PDK1 in muscle (mPDK1−/−), as well as knockin mice in which the protein kinase B (PKB) phosphorylation site on glycogen synthase kinase-3α (GSK3α) (Ser21) and GSK3β (Ser9) is changed to Ala. We demonstrate that in hearts from mPDK1−/− or double GSK3α/GSK3β knockin mice, insulin failed to stimulate the activity of GS or induce its dephosphorylation at residues that are phosphorylated by GSK3. We also establish that in the heart, both GSK3 isoforms participate in the regulation of GS, with GSK3β playing a more prominent role. This contrasts with skeletal muscle where GSK3β is the major regulator of insulin-induced GS activity. Despite the inability of insulin to stimulate glycogen synthesis in hearts from the mPDK1−/− or double GSK3α/GSK3β knockin mice, these animals possessed normal levels of cardiac glycogen, demonstrating that total glycogen levels are regulated independently of insulin’s ability to stimulate GS in the heart and that mechanisms such as allosteric activation of GS by glucose-6-phosphate and/or activation of GS by muscle contraction, could operate to maintain normal glycogen levels in these mice. We also demonstrate that in cardiomyocytes derived from the mPDK1−/− hearts, although the levels of glucose transporter type 4 (GLUT4) are increased 2-fold, insulin failed to stimulate glucose uptake, providing genetic evidence that PDK1 plays a crucial role in enabling insulin to promote glucose uptake in cardiac muscle.