GSK-3 at the heart of cardiometabolic diseases: Isoform-specific targeting is critical to therapeutic benefit

Abstract

Glycogen synthase kinase-3 (GSK-3) is a family of serine/threonine kinases. The GSK-3 family has 2 isoforms, GSK-3α and GSK-3β. The GSK-3 isoforms have been shown to play overlapping as well as isoform-specific-unique roles in both, organ homeostasis and the pathogenesis of multiple diseases. In the present review, we will particularly focus on expanding the isoform-specific role of GSK-3 in the pathophysiology of cardiometabolic disorders. We will highlight recent data from our lab that demonstrated the critical role of cardiac fibroblast (CF) GSK-3α in promoting injury-induced myofibroblast transformation, adverse fibrotic remodeling, and deterioration of cardiac function. We will also discuss studies that found the exact opposite role of CF-GSK-3β in cardiac fibrosis. We will review emerging studies with inducible cardiomyocyte (CM)-specific as well as global isoform-specific GSK-3 KOs that demonstrated inhibition of both GSK-3 isoforms provides benefits against obesity-associated cardiometabolic pathologies. The underlying molecular interactions and crosstalk among GSK-3 and other signaling pathways will be discussed. We will briefly review the specificity and limitations of the available small molecule inhibitors targeting GSK-3 and their potential applications to treat metabolic disorders. Finally, we will summarize these findings and offer our perspective on envisioning GSK-3 as a therapeutic target for the management of cardiometabolic diseases.

Keywords: Cardiometabolic disease; Fibrosis; Glycogen synthase kinase; Heart failure; Hypertrophy; Myocardial infarction; Obesity.

Fig.1:. Domain structure of GSK-3 isoforms:

Comparison of domains and phosphorylation sites of the two GSK-3 isoforms Some kinases that phosphorylate GSK-3 isoforms are also shown. GSK-3 = Glycogen synthase kinase-3; Fyn= Proto-oncogene tyrosine-protein kinase; ILK = Integrin-linked kinase; MEK = Mitogen-activated protein kinase kinase; p38 MAPK = p38 mitogen-activated protein kinase; p70S6K = p70S6 kinase; p90RSK = p90 ribosomal S6 kinase; PKA = Protein kinase A; PKC = Protein kinase C; PyK2 = Protein tyrosine kinase 2; Ser = Serine; Tyr=Tyrosine; Thr=Threonine.

Fig.2:. GSK-3α promotes myofibroblast transformation and fibrosis:

In the classical pro-fibrotic TGF-β1/SMAD pathway, the binding of ligands to the receptor initiates a signaling cascade that leads to phosphorylation of the R-SMADs (SMAD-2 and SMAD-3). R-SMADs associate with Co-SMAD (SMAD-4), resulting in nuclear translocation and induction of target gene transcription. In fibroblasts, IL-11 mediates pro-fibrotic effects through the RAF-MEK-ERK signaling cascade. We identified that GSK-3α mediates pro-fibrotic effects through IL-11 and ERK signaling, which operates independently of the classical pro-fibrotic TGF-β1/SMAD-3 pathway. α-SMA = Alpha-smooth muscle actin; ERK = Extracellular signal-regulated kinase; GSK-3β = Glycogen synthase kinase-3β; gp130 = Glycoprotein 130; IL-11 = Interleukin 11; IL-11RA = Interleukin 11 receptor alpha; MEK = Mitogen-activated protein kinase kinase; MMP = Matrix metalloproteinases; Raf = Rapidly accelerated fibrosarcoma; SMAD = Suppressor of Mothers against Decapentaplegic; TGF-β = Transforming growth factor beta.

Fig. 3:. GSK-3β prevents fibrosis through negative regulation of TGF-β1/SMAD-3 and Wnt/β-catenin signaling pathway:

GSK-3β directly interacts with SMAD-3 and negatively regulates SMAD-3 activity. Hence, deletion/inhibition of GSK-3β leads to hyperactivation of TGF-β1/SMAD-3 signaling and excessive fibrosis. In Wnt/β-catenin signaling, GSK-3β is required for phosphorylation and proteasomal degradation of β-catenin. Activation of Wnt signaling inhibits GSK-3β and leads to stabilization and nuclear translocation of β-catenin, which further activates fibrotic gene programs. APC = Adenomatous Polyposis Coli; β-TrCP = β-transducin repeat-containing protein; CKI = Cyclin-dependent kinase inhibitor; DVL = Disheveled; GSK-3β = Glycogen synthase kinase 3β; SMAD = Suppressor of Mothers against Decapentaplegic; TGF-β1 = Transforming growth factor beta 1.

Fig. 4:. Emerging roles of GSK-3α in the pathophysiology of cardiac diseases:

In resting cells, GSK-3α inhibits cyclin E1 and E2F1 transcription factors. Hence, inhibition of GSK-3α promotes cardiomyocyte proliferation and differentiation. Under pathological conditions, GSK-3α upregulates BAX expression and contributes to apoptosis and mitochondrial dysfunction. In cardiomyocytes, GSK-3α regulates the transcription of genes involved in FA uptake and storage through PPARα. BAX = Bcl-2 Associated X-protein; Cyc E1 = Cyclin E1; E2F1 = E2F Transcription factor 1; GSK-3α = Glycogen synthase kinase 3 alpha; PPAR = Peroxisome proliferator-activated receptor; RTK = receptor tyrosine kinases; S6K = Ribosomal protein S6 kinase beta-1.

Fig. 5:. GSK-3β plays a predominant role in cardiomyocyte biology:

GSK-3β modulates activities of various transcription factors and proteins such as NFAT, GATA4, β-catenin, myocardin, and Eukaryotic translation initiation factor 2Bε (eIF2Bε) and acts as a negative modulator of cardiomyocyte hypertrophy. GSK-3β promotes autophagy by regulating mTOR signaling through TSC2 and ULK1. The active form of GSK- 3β mobilizes to the sarcomere, associates with myofilament, and contributes to length-dependent activation. abLIM-1 = Actin binding LIM protein 1; Ca²⁺ = Calcium ion; eIF2Bε = Eukaryotic translation initiation factor 2Bε; GATA = Gata binding protein 4; GSK-3β = Glycogen synthase kinase 3 beta; LRP = Low-density lipoprotein receptor-related protein; mTOR = Mammalian target of rapamycin; NFAT = Nuclear factor of activated T cells; Rheb = Ras homolog enriched in brain; RTK = Receptor tyrosine kinases; TSC = Tuberous sclerosis protein; ULK-1 = Unc-51-like kinase 1.

Fig. 6:. Role of GSK-3 in glucose metabolism and insulin signaling:

In resting conditions, GSK-3 is in an active state and negatively regulates glycogen synthase (GS). Insulin inhibits GSK-3 activity through RTK-mediated phosphorylation thereby promoting GS activation and glycogen synthesis. GSK-3 phosphorylates Insulin Receptor Substrate-1 (IRS-1) and inhibits insulin signaling either by intervening in insulin receptor -IRS1 interactions or by promoting proteasomal degradation of IRS-1. GS = Glycogen synthase; GSK-3 = Glycogen synthase kinase 3; IRS = Insulin receptor substrate 1; PBK = PDZ-binding kinase; PDK = Phosphoinositide-dependent protein kinase; PKB = Protein kinase B.