Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

MAP4K3 (GLK)

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101713

Synonyms

Historical Background

The serine/threonine kinase GCK-like kinase (GLK) is first identified by screening a human skeletal muscle cDNA library using degenerate oligonucleotide PCR primers (Diener et al. 1997). The nucleotide sequence of GLK encodes an open reading frame of 894 amino acids with a calculated molecular mass of 100 kDa. GLK shares 57% amino acid identity with MAP4K2 (also named germinal center kinase; GCK) and 49% amino acid identity with MAP4K1 (also named hematopoietic progenitor kinase 1 (HPK1)) (Hu et al. 1996; Diener et al. 1997). Thus, GLK is classified as a member of the MAP kinase kinase kinase kinase (MAP4K) family and also named MAP4K3. The MAP4K3/GLK protein contains a conserved N-terminal kinase domain, three proline-rich regions, and a conserved C-terminal citron homology domain (Fig. 1). Three splice variants of human MAP4K3/GLK are listed at NCBI website (Fig. 1). The mRNA levels of GLK in the heart, brain, placenta, lung, liver, skeletal muscle, kidney, and pancreas are detectable by Northern blotting analysis using a radioactive MAP4K3/GLK probe (Diener et al. 1997).
MAP4K3 (GLK), Fig. 1

The structural domains of MAP4K3/GLK. MAP4K3/GLK is composed of an N-terminal kinase (KD) domain, proline-rich motifs in the middle region, and a citron homology (CNH) domain in the C-terminal region. The identified phosphorylation site is indicated. Three splice variants of human MAP4K3 with their accession numbers are shown

MAP4K3/GLK Induces JNK Activation

The MAP4K family is a subfamily of the mammalian Ste20-like serine/threonine kinase family (Chuang et al. 2016b). All of the six members of the MAP4K family induce the activation of the JNK subgroup of MAP kinases through the MAP3K-MAP2K kinase cascade (Hu et al. 1996; Diener et al. 1997; Yao et al. 1999; Chuang et al. 2016b). MAP4K3/GLK signaling induces JNK activation through MAP3K1 (also named MEKK1) and MKK4 (also named SEK1) in HEK293 cells. In addition to MAP4K3/GLK autophosphorylation, endogenous MAP4K3/GLK in Hela cells is activated by UV radiation and TNF-α stimulation (Diener et al. 1997). Thus, MAP4K3/GLK is a physiological activator of JNK in the responses to stress and inflammation; duration of JNK activation determines cell apoptosis or proliferation (Chen et al. 1996a, b; Chen and Tan 2000) (Fig. 2).
MAP4K3 (GLK), Fig. 2

Summary of the identified signal transduction pathways of MAP4K3/GLK

MAP4K3/GLK Positively Regulates mTOR Signaling and Cell Growth

MAP4K3/GLK activates S6K and 4E-BP1, which are the mTOR downstream molecules (Findlay et al. 2007). Moreover, the kinase activity of ectopically expressed GLK in HEK293T cells is enhanced by a treatment of amino acids; this induced kinase activity is dependent on Ser-170 phosphorylation of MAP4K3/GLK (Yan et al. 2010). The Ser-170 phosphorylation of MAP4K3/GLK is dephosphorylated by PP2AT61ε upon amino acid restriction (Fig. 2). Thus, MAP4K3/GLK positively regulates mTOR signaling and subsequently promotes cell growth in epithelial cells (Fig. 2); however, whether MAP4K3/GLK activates mTOR kinase activity still needs to be clarified.

MAP4K3/GLK Directly Activates PKC-θ During T-Cell Activation

Two members of the MAP4K family kinases, MAP4K1 and MAP4K4, have been shown to modulate T-cell activation (Shui et al. 2007; Chuang et al. 2014, 2016a; 2017). MAP4K3/GLK kinase activity in T lymphocytes is induced by T-cell receptor (TCR) signaling (TCR crosslinking by anti-CD3 antibody) (Chuang et al. 2011). To date, the kinases that phosphorylate and regulate MAP4K3/GLK remain unknown. The kinase activity of MAP4K3/GLK in T cells is regulated by the direct interaction between MAP4K3/GLK and the adaptor protein SLP-76. Although another SLP-76-interacting protein MAP4K1/HPK1 is a negative regulator of TCR signaling (Shui et al. 2007), MAP4K3/GLK plays as a positive regulator during TCR signaling. The activated MAP4K3/GLK directly interacts with PKC-θ and phosphorylates PKC-θ at Thr-538, which is required for PKC-θ catalytic activation (Chuang et al. 2011; Wang et al. 2012). MAP4K3/GLK overexpression in T cells induces activation of the PKC-θ-IKK-NF-κB axis, leading to T-cell activation and proliferation (Fig. 2). Conversely, the activation of PKC-θ and IKK is abolished in the purified primary T cells of MAP4K3/GLK-deficient mice. The T-cell-mediated immune responses and antibody productions are also impaired in MAP4K3/GLK-deficient mice, indicating MAP4K3/GLK plays a critical role in T-cell activation through phosphorylating PKC-θ (Chuang et al. 2011). In addition, HIP-55 (also named mAbp1 and SH3P7) originally identified as a MAP4K3/GLK- and MAP4K1/HPK1-interacting protein (Ensenat et al. 1999), which is also required for T-cell activation (Han et al. 2005). It is likely that the adaptor protein HIP-55 may be an upstream regulator or downstream target of MAP4K3/GLK.

MAP4K3/GLK Is a Biomarker and Therapeutic Target for Autoimmune Diseases

The expression of MAP4K3/GLK and the activation of PKC-θ and IKK are drastically enhanced in peripheral blood T cells from autoimmune disease patients with systemic lupus erythematosus (SLE) or rheumatoid arthritis (RA) (Chuang et al. 2011; Chen et al. 2013). The frequencies of MAP4K3/GLK-positive T cells are correlated with SLE disease activity index (SLEDAI; r = 0.773) of SLE patients and with 28-joint disease activity score (DAS28; r = 0.606) of RA patients, respectively. Similarly, the expression levels of MAP4K3/GLK are also increased in peripheral blood T cells from patients with autoimmune adult-onset Still’s disease (AOSD) (Chen et al. 2012). The frequencies of MAP4K3/GLK-positive T cells are correlated with the disease activity scores (r = 0.599) and serum IL-17 levels (r = 0.787) of AOSD patients; the frequencies of MAP4K3/GLK-positive T cells in AOSD patients are decreased after effective therapy.

IL-17-producing CD4+ T (Th17) cells are the main pathogenic cells for autoimmune diseases. MAP4K3/GLK-deficient mice are resistant to the induction of experimental autoimmune encephalomyelitis (EAE) model, which is mediated by Th17 cells (Chuang et al. 2011). The IL-17 levels in the serum and Th17 population in the brain during EAE induction are drastically reduced in MAP4K3/GLK-deficient mice compared to those in wild-type cells. Moreover, similar attenuated disease severity and reduced IL-17 production are shown in collagen-induced arthritis (CIA) mouse model using MAP4K3/GLK-deficient mice (Chuang et al. 2011; Chen et al. 2013). Taken together, GLK is a biomarker and therapeutic target for autoimmune diseases.

MAP4K3/GLK Is a Biomarker for Cancer Recurrence

MAP4K4 (also named HGK) is a critical regulator of cancer migration and invasion (Collins et al. 2006; Liang et al. 2008); therefore, MAP4K3/GLK signaling may be also involved in tumorigenesis. The MAP4K3 protein levels are overexpressed in tumor tissues compared to adjacent noncancerous tissues of non-small cell lung cancer (NSCLC) or hepatocellular carcinoma (HCC) patients (Ho et al. 2016; Hsu et al. 2016). Another publication reported that overexpression of MAP4K3/GLK in lung cancer cell lines may be due to the reduction of the microRNA let-7c, which targets and downregulates MAP4K3/GLK mRNA (Zhao et al. 2014). Paradoxically, the mRNA levels of MAP4K3/GLK are not increased in NSCLC patients (Hsu et al. 2016); therefore, GLK overexpression in NSCLC may be due to its translational or posttranslational regulation instead of GLK upregulation through let-7c reduction. Lung cancer and hepatocellular carcinoma (HCC) are the first and the third leading causes of cancer death worldwide, respectively. Moreover, cancer recurrence remains the major cause of death after curative resection for NSCLC and HCC. MAP4K3/GLK protein levels in tumor tissues of NSCLC patients detected by immunoblotting are correlated with increased recurrence risks and poor recurrence-free survival rates. The analyses of MAP4K3/GLK proportion score in noncancerous liver tissues of HCC patients detected by immunohistochemistry show similar results. Notably, the correlations of the recurrence risks or the recurrence-free survival rates with MAP4K3/GLK protein levels are equivalent to or higher than those with the pathologic stages of NSCLC or HCC patients (Ho et al. 2016; Hsu et al. 2016). Thus, MAP4K3/GLK is a prognostic biomarker for cancer recurrence of NSCLC or HCC, as well as a therapeutic target for cancer (Table 1).
MAP4K3 (GLK), Table 1

Summary of the information of MAP4K3/GLK

MAP4K3

 

Synonyms

Mitogen-activated protein kinase kinase kinase kinase 3, MAPKKKK3, MAP4K3, germinal center kinase-like kinase, GCK-like kinase, GLK

Amino acid

894 aa (human), 894 aa (mouse)

Phosphorylation site

Ser-170

Direct interacting protein

SLP-76, PKC-θ, HIP-55

Upstream activator

SLP-76

Downstream substrate

PKC-θ (Thr-538 phosphorylation)

Stimuli for kinase activity

UV radiation, TNF-α, amino acids, T-cell receptor stimulation

Summary

MAP4K3/GLK is a serine/threonine kinase of the Ste20-like family. The kinase activity of MAP4K3/GLK is induced by UV radiation, TNF-α stimulation, amino acid treatment, or T-cell receptor signaling. The kinases regulating GLK remain unknown, while the adaptor protein SLP-76 directly interacts with MAP4K3/GLK and mediates MAP4K3/GLK activation during T-cell receptor signaling. In addition, amino acid-induced Ser-170 phosphorylation of MAP4K3/GLK is required for GLK activation during nutrient-sufficient condition. The miRNA let-7c targets MAP4K3/GLK mRNA, leading to inhibition of MAP4K3/GLK expression. MAP4K3/GLK overexpression in cell lines positively regulates the MEKK1-MKK4-JNK kinase cascade, the mTOR pathway, and the PKC-θ-IKK-NF-κB axis. To date, only one direct substrate of MAP4K3/GLK has been identified; MAP4K3/GLK directly interacts with PKC-θ and phosphorylates PKC-θ at Thr-538, leading to activation of the IKK-NF-κB axis in T cells. The data derived from MAP4K3/GLK-deficient mice demonstrate that MAP4K3/GLK plays a critical role in T-cell-mediated immune responses and autoimmunity. The studies using human clinical samples further indicate that MAP4K3/GLK overexpression occurs in T cells of autoimmune disease patients and in tumor tissues of cancer patients. In conclusion, translational research reveals that MAP4K3/GLK is a biomarker and therapeutic target for autoimmune diseases and cancer.

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Immunology Research Center, National Health Research InstitutesZhunanTaiwan