J UOEH 40（ 2 ）: 147－156（2018） 147 ［Review］ Roles of Glycogen Synthase Kinase-3 (GSK-3) in Cardiac Development and Heart Disease Fumi Takahashi-Yanaga* Department of Pharmacology, School of Medicine, University of Occupational and Environmental Health, Japan. Yahatanishi-ku, Kitakyushu 807-8555, Japan Abstract : Glycogen synthase kinase-3 (GSK-3) is a cytoplasmic serine/threonine protein kinase which is known to regulate a variety of cellular processes through a number of signaling pathways important for cell proliferation, stem cell renewal, apoptosis and development. Although GSK-3 exists in a variety of tissues, this kinase plays very im- portant roles in the heart to control its development through the formation of heart and cardiomyocyte proliferation. GSK-3 is also recognized as one of the main molecules that control cardiac hypertrophy and fibrosis. Therefore, GSK-3 could be an attractive target for the development of new drugs to cure cardiac diseases. The present review summarizes the roles of GSK-3 in the signaling pathways and the heart, and discusses the possibility of new drug development targeting this kinase. Keywords : GSK-3, cardiac development, cardiac hypertrophy, cardiac fibrosis. （Received February 7, 2018, accepted April 6, 2018） Introduction 1. Glycogen synthase kinase-3 (GSK-3) GSK-3 was originally identified as a kinase that can Glycogen synthase kinase-3 (GSK-3) was identified phosphorylate glycogen synthase to inhibit glycogen in 1980 as a protein kinase that inactivates glycogen synthesis. In general, kinases activate their target pro- synthase, the rate-limiting enzyme in glycogen synthe- teins by phosphorylation. However, in 1980, GSK-3 sis. In the 1990s, GSK-3 was revealed to regulate a was found to be an inactivator of glycogen synthase, number of cellular functions such as cell proliferation, an enzyme involved in converting glucose to glycogen stem cell renewal, apoptosis and development through for storage [1, 2]. different signaling pathways [1, 2]. The GSK-3 family is highly conserved throughout The first studies implicating that GSK-3 plays an evolution and is encoded by two genes, GSK-3α and important role in the heart were published in 2000 and GSK-3β, which exist in a variety of tissues [1, 2]. Al- identified GSK-3 as a negative regulator of hypertro- though the homology of GSK-3α (51 kDa) and GSK-3β phic response in cardiomyocytes [3, 4]. Since then, (47 kDa) is approximately 98% in the catalytic domain, a number of studies have been carried out using vari- the overall homology is approximately 85%, because of ous models to clarify the roles of GSK-3 in the heart. the differences in their C- and N- termini [5]. Due to GSK-3 is now known as a molecule that not only sup- this similarity in their kinase domains, there is no spe- presses cardiac hypertrophy but also regulates cardiac cific inhibitor for either isoform at present. development and inhibits cardiac fibrosis. GSK-3 is a unique kinase, because its activity is gen- *Corresponding Author: Fumi Takahashi-Yanaga, Department of Pharmacology, School of Medicine, University of Occupational and Environmental Health, Japan. Yahatanishi-ku, Kitakyushu 807-8555, Japan, Tel: +81-93-691-7424, Fax: +81-93-601-6264, E-mail: [email protected] 148 F Takahashi-Yanaga erally high in resting cells and phosphorylates substrate 2.2. GSK-3 in the Hedgehog signaling proteins to inhibit their functions. This kinase is inhibited The Hedgehog (Hh) signaling pathway has been in response to cellular signaling mediated by growth fac- identified to play an important role not only during de- tors, cytokines and hormones via the phosphorylation of velopment but also in the maintenance of many adult Ser21 in GSK-3α and Ser9 in GSK-3β [1, 2]. Upon inhibi- structures, especially in proliferating cell populations tion of GSK-3 by various extracellular signaling, down- [13-15]. Three Hh ligands (Sonic Hedgehog, Indian stream effectors are activated to mediate their signaling. Hedgehog and Deser Hedgehog) and two receptors Although the GSK-3 isoforms have similar structures [Patched (Ptch) and Smoothened (Smo)] are identified and overlapping functions, the phenotype of global de- in humans. The activity of the Hh signaling pathway letion of these isoforms is different. The homozygous to express target genes is controlled by the Gli fam- knock out (KO) of GSK-3β (GSK-3β-/-) mice yields an ily (Gli1, Gli2 and Gli3). In the absence of the Hh embryonic-lethal phenotype because of hepatic apopto- ligands, the Gli family proteins are phosphorylated by sis or a cardiac patterning defect [2, 6, 8], while the ho- protein kinase A (PKA), GSK-3, and casein kinase 1 mozygous knock out of GSK-3α (GSK-3α-/-) mice are (CK1), resulting in degradation. The above kinases viable, fertile, and display a body mass similar to their are inactivated upon the binding of Hh to its receptors wild-type littermates [2, 9]. Further, only the β-isoform Ptch and Smo, then Gli translocates into the nucleus has an alternatively spliced variant that encodes GSK- to activate the expression of the Hh signaling target 3β2, which has a 13-residue insert in the kinase domain genes. Thus, GSK-3 plays an important role in con- and is the neuron-specific variant [10, 11]. trolling Hh signaling activity via the proteolysis of the Gli family (Fig. 1, center). 2. GSK-3 and its role in signaling pathways GSK-3 plays central roles in a number of signaling 2.3. GSK-3 in the Wnt/β-catenin signaling pathway pathways important for development and survival, in- (canonical pathway) cluding the growth factor signaling pathway, the Wnt/β- Cell signaling cascades activated by Wnt proteins (col- catenin signaling pathway, and the Hedgehog signaling lectively the Wnt signaling pathways) are well conserved pathway, as described below. throughout evolution. As well as regulating cellular pro- cesses, including proliferation, differentiation, motility 2.1. GSK-3 in insulin and other growth factor signal- and survival/apoptosis, the Wnt signaling pathways play ing pathways key roles in embryonic development and maintenance The control of glycogen synthase is a key step in regu- of homeostasis in mature tissues. Among the described lating glycogen metabolism and glucose storage [1, 2]. Wnt signaling pathways [the canonical pathway (Wnt/β- GSK-3 directly phosphorylates glycogen synthase, catenin pathway) and the non-canonical pathways (the thereby catalytically inactivating glycogen synthase. planar cell polarity (PCP) pathway, the Wnt/Ca2+ path- It had been suggested since 1980 that insulin signal- way, and the protein kinase A pathway)], the canonical ing could inhibit GSK-3 activity, and this supposition Wnt signaling pathway is by far the best characterized was proved in 1995 by Cross et al [12]. They found [16, 17]. that insulin activates phosphatidylinositol-3-kinase The activity of the canonical Wnt signaling pathway (PI3K), which phosphorylates Akt, and activated Akt is dependent on the amount of β-catenin in the cyto- phosphorylates GSK-3 (Ser21 in GSK-3α and Ser9 in plasm. Without Wnt ligands, cytoplasmic β-catenin GSK-3β), resulting in GSK-3 inactivation [12]. Upon is phosphorylated by the “destruction” complex com- GSK-3 inactivation, downstream effector molecules posed of four different proteins [Axin, adenomatous are activated. This PI3K-Akt-GSK-3-effector cascade polyposis coli (APC), CK1α, and GSK-3] and de- is also stimulated by growth factors [e.g., fibroblast graded. Upon binding of Wnt to a receptor complex growth factor (FGF), epidermal growth factor (EGF) comprised of Frizzleds/low-density lipoprotein recep- and platelet derived growth factor (PDGF)] and hor- tor-related protein (Fz/LRP), cytoplasmic Dishevelled mones [1, 2] (Fig. 1, left). (Dvl) inhibits GSK-3. Then β-catenin that escapes GSK-3 and the Heart 149 Growth factor Ptch Wnt Growth factor Hh Frizzled receptor Smo LRP-5/6 active P Axin PI3K active CK1 Dvl P P APC active Akt PKA P GSK‐3 P GSK‐3 inactive β‐catenin inactive CK1 P GSK‐3 β‐catenin inactive Gli translocation translocation active effector CBP β‐catenin P300 Gli TCF/LEF Downstream signaling activation activation of transcription Fig. 1. Roles of GSK-3 in signaling pathways. Without stimulation, GSK-3 activity is high and the activity of the signaling pathway is kept low. Upon binding of a ligand to the receptor, GSK-3 activity is suppressed, thereby activating the signaling pathway. Growth factor signaling (left), Hedgehog signaling pathways (center), and Wnt/β-catenin signaling (right) are shown. PI3K: phosphoinositide 3-kinase, GSK-3: glycogen synthase kinase-3, Smo: Smoothened, Hh: Hedgehog, Ptch: Patched, CK1: casein kinase 1, PKA: protein kinase A, Gli: glioma-associated oncogene homolog, Dvl: Dishevelled, LRP-5/6: low-density lipoprotein receptor-related protein- 5/6, APC: adenomatous polyposis coli, CBP: cAMP response element binding protein (CREB)-binding protein, TCF: T-cell factor, LEF: lymphoid enhancer binding factor, P300: E1A binding protein p300. from degradation translocates into the nucleus and two sources of mesoderm, namely the first heart field forms a complex with members of the T-cell factor/ (FHF) and the second heart field (SHF), which are lymphoid enhancer binding factor (TCF/LEF) fam- components of the cardiac crescent and are established ily of transcription factors to activate the expression during late gastrulation. The cardiac progenitors in the of Wnt target genes. In this process, GSK-3 plays a FHF contribute to the left ventricle with small contri- key role in controlling the amount of β-catenin through butions to the atria, whereas the progenitors in SHF phosphorylation dependent proteolysis. Thus, the ac- contribute to the right ventricle, outflow tract, atria and tivity of the Wnt/β-catenin signaling pathway is main- inflow tract. SHF progenitors (islet 1 expressed cells: ly regulated through GSK-3 activity (Fig.