- Email: [email protected]
O-glycosylation in liver cancer: Clinical associations and potential mechanisms
Accepted Manuscript O-glycosylation in liver cancer: Clinical associations and potential mechanisms Kung-Hao Liang, Chau-Ting Yeh PII:
S2542-5684(17)00053-8
DOI:
10.1016/j.livres.2017.12.005
Reference:
LIVRES 32
To appear in:
Liver Research
Please cite this article as: Liang KH, Yeh CT, O-glycosylation in liver cancer: Clinical associations and potential mechanisms, Liver Research (2018), doi: 10.1016/j.livres.2017.12.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT LR2016-017 (唐) Review Article
O-glycosylation in liver cancer: Clinical associations and potential
Kung-Hao Liang a, b, Chau-Ting Yeh a, c, *
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mechanisms☆
Liver Research Center, Chang Gung Memorial Hospital, Taoyuan, Taipei, China
b
Medical Research Department, Taipei Veterans General Hospital, Taipei, China
c
Molecular Medicine Research Center, Chang Gung University, Taoyuan, Taipei, China
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☆
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a
Edited by Peiling Zhu and Genshu Wang
*Corresponding author. Liver Research Center, Chang Gung Memorial Hospital, Taoyuan, Taipei, China.
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E-mail address: [email protected] (C.-T. Yeh)
ARTICLE INFO
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Article history:
Received 15 December 2016
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Received in revised form 9 December 2017
Accepted 12 December 2017
ACCEPTED MANUSCRIPT ABSTRACT Liver cancer can be an aggressive disease, and is highly prevalent in Asia and Africa. However, its currently approved therapeutic strategies are far from satisfactory. Recent progress in genomic, proteomic and glycomic profiling technologies have enabled the
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identification of biomarkers that significantly correlate with clinical outcomes. Many biomarkers are related to O-glycosylation of glycoproteins, which belong to an important but
discussed potential underlying mechanisms.
O-Glycosylation Post-translational modification Glycobiology Biomarkers
1. Introduction
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Glycosyltransferase.
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Hepatocellular carcinoma (HCC)
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Keywords:
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less-explored field of liver cancer biology. Here, we review these clinical studies and
Worldwide, liver cancer is the sixth most commonly diagnosed cancer (782 cases per
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100,000 persons per year), and the second most common cause of death from cancer (745 cases per 100,000 persons per year).1 Hepatocellular carcinoma (HCC) is the major type of liver cancer (78%), followed by cholangiocarcinoma (15%), angiosarcoma, hepatoblastoma and other rare types.2 Chronic hepatitis B and C are the two major causes of HCC, and account for more than 80% of HCC in regions where hepatitis viruses are endemic.3 The etiologies of other types of liver cancers are less well defined. Primary sclerosing cholangitis and liver flukes have been identified as risk factors for cholangiocarcinoma, but account for less than 30% of the disease.4 Liver cancer has many aggressive hallmarks, including rapid
ACCEPTED MANUSCRIPT cell growth, high risk of distant metastasis, effective angiogenesis, and a prominent antiapoptosis phenotype—all driven by aberrant mechanisms that are still incompletely understood.5 Altered glycosylation of proteins has been observed in several cancer types,6,7 including
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liver cancer.8 In all tumor, stromal or immune cells, proteins are manufactured by sequential conjugations of amino acids according to ribonucleic acid (RNA) templates, and then further decorated with various molecules, such as phosphates, glycans or lipids, which critically
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affect the proteins’ shape, localization and functions. Glycosylation is a major type of posttranslational embellishment which occurs in the Golgi apparatus, endoplasmic reticulum or
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cytosols of the cells. Glycosylation links to either oxygen (O) or nitrogen (N) atoms of amino acids, and are referred to as O-linked or N-linked glycosylation, respectively. They are involved in many complex biological mechanisms, exerting extensive physiological effects in a wide range of diseases. In this short review, we focused on O-linked glycosylation in liver
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cancer.
O-linked glycosylation is based on oxygen atoms in serine, threonine or tyrosine residues.
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Glycans that conjugate to oxygen atoms include N-acetylgalactosamine (GalNAc), fucose, glucose, galactose, N-acetylglucosamine (GlcNAc), mannose and xylose.9 In human, the
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glycan–protein conjugations are mediated by specific enzymes. For example, fucose–protein conjugation is mediated by O-fucosyltransferases (POFUT1 and POFUT2); glucose–protein conjugation is by O-glucosyltransferase (POGLUT1), GlcNAc–protein conjugation is by OGlcNAc transferase (OGT), O-mannose modification is mediated by O-mannosyltransferases (POMT1 and POMT2), and O-xylose by O-xylosyltransferase (XYLT1 and XYLT2). In the GalNac–protein
conjugation,
a
total
of
20
members
of
the
polypeptide
N-
acetylgalactosaminyltransferase (GALNT) family are responsible for transferring GalNAc from UDP–CalNAc to the substrate glycoproteins.10 After the first carbohydrate molecule is
ACCEPTED MANUSCRIPT added to the protein backbone, subsequent conjugations can proceed by other enzymes including glycosyltransferases such as core 1 synthase, glycoprotein-N-acetylgalactosamine3beta-galactosyltransferase1 (C1GalT1), C1GALT1 specific chaperone 1 (Cosmc), and sialyltransferase such as ST3 beta-galactoside-alpha-2,3-Sialyltransferase 1 (ST3Gal1) (Fig.
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1).
Fig. 1. An example of O-linked protein glycosylation. This event occurs on the serine (S), threonine (T) or tyrosine residue of the proteins. The sugar moieties were added on to these substrates by
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glycosyltransferases. Lectins can bind to the glycosylated proteins.
Although protein glycosylation is believed to underlie many cellular activities, its exact
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functions in liver cancer are mostly unknown due to the complexity of mechanisms. However, genomic, proteomic and glycomic profiling technologies have greatly improved in recent
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years. Commercially available microarrays for evaluating single nucleotide polymorphisms in high density are now available, enabling genome-wide association studies of various cancerrelated clinical phenotypes. High performance glycan and lectin-arrays have also been developed.11,12 The associations between the “omics” measurements and clinical events or outcomes of liver diseases in large patient cohorts have revealed significant biomarkers, including many O-glycan biomarkers that can both be useful for cancer diagnosis and treatments (see Table 1 for some examples). These biomarkers in turn offered valuable novel insights into liver cancer biology. Here we reviewed the recent progress along these lines.
ACCEPTED MANUSCRIPT Because of the ongoing development of this field, this review was written based on authors’ knowledge rather than a thorough literature search. Table 1 Potential diagnosis and prognosis O-glycosylation markers. Diseases
Clinical relevance
Colorectal cancer
Prognosis
GalNAc-T3
Gastric cancer
Prognosis
GalNAc-T3
Lung cancer
Prognosis
GalNAc-T14
Breast cancer
Diagnosis
MUC2 O-glycan
Ulcerative colitis
Prognosis
Shibao K et al.40
Onitsuka K et al.41
Gu C et al.42
Wu C et al.43
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GalNAc-T3
Reference
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Markers
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profile
Larsson JM et al.44
Prognosis
Blixt O et al.45
Prognosis
Berois N et al.46
Prognosis
Kitada S et al.47
Hepatocellular carcinoma
Prognosis
Wu YM et al.21
B3GNT3
Neuroblastoma
Prognosis
Ho WL et al.48
GalNAc-T3
Ovarian cancer
Prognosis
Wang ZQ et al.49
GalNAc-T2
Neuroblastoma
Prognosis
Ho WL et al.50
GalNAc-T3
Oral squamous cell
Prognosis
Harada Y et al.51
Breast cancer
GalNAc-T9
Neuroblastoma
GalNAc-T3
Renal cell carcinoma
C1GALT1
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MUC1 glycoforms
carcinoma
Hepatocellular carcinoma
Prognosis
Liu Y et al.52
GalNAc-T6
Endometrial cancer
Prognosis
Kurita T et al.53
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GalNAc-T4
Abbreviations: GalNAc-T, N-acetylgalactosaminyltransferase; MUC, mucin; C1GALT1, core 1
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synthase, glycoprotein-N-acetylgalactosamine3-beta-galactosyltransferase-1; B3GNT3, beta1,3-N-acetylglucosaminyltransferase-3.
2. O-linked glycosyltransferase/glycosidase and liver cancer Glycosyltransferases have pivotal funtions in the initiation and continuation of O-linked glycosylation. The GalNAc-T glycosyltransferases, encoded by the GALNT gene family in the human genome, catalyzes the addition of GalNAc to oxygen atoms of amino acids. Recently, it was found that genomic variants in one member of the family, GALNT14, were
ACCEPTED MANUSCRIPT associated with chemotherapeutic responses of late-stage HCC patients in a genome-wide association study.13 One leading variant (rs9679162) was subsequently found to be associated with overall survival (OS) of late-stage HCC patients14,15 and the time-to-complete response of intermediate-stage HCC patients who were treated by transcatheter arterial
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chemoembolization.16 Tandem-mass spectrometry analyses showed that the genotypes were associated with the glycosylation of death-receptor 5 protein in liver tissue,16 which played a role in extrinsic apoptosis signaling of cancer cells. Interestingly, other members of this
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family also exerted regulatory functions in liver cancer growth. For example, GALNT2 expression suppressed malignant HCC phenotypes through modification of the epidermal
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growth factor (EGF) receptor,17 whereas GALNT1 and GALNT10 expressions enhanced the malignant character of HCC through modulating EGF signaling and microRNA-122 mediated processes, respectively.18,19
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The GalNac-T enzymes facilitate addition of GalNAc to serine and threonine residues, forming an antigen called Tn-antigen.20 Afterwards, another glycosyltransferase, C1GalT1, catalyzes the addition of the sugar molecule, “Gal”, forming a T-antigen.20 The C1GalT1
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mRNA levels in surgical specimens from a cohort of 72 HCC patients were found to correlate inversely with post-surgery OS, and positively with tumor stage and metastasis.21 Whether all
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three of these factors (C1GalT1 mRNA levels, tumor stage and metastasis) independently affected OS was not clear in multivariate analysis. The C1GalT1 enzyme was also shown to promote tumor invasion and metastasis by altering glycosylation of integrin-β1.22
O-GlcNAc modification is mediated by the glycosyltransferase OGT, whereas glycan removall of the glycans is mediated by glycosidase (OGA, a.k.a., MEGA5). A recent study of a cohort of HCC patients showed that higher the levels of OGA levels in their tumor
ACCEPTED MANUSCRIPT specimens were associated with longer recurrence-free survival after liver transplantation.23
Beta-galactoside alpha-2,6-sialyltransferase (ST6Gal I) is an important enzyme for the sialylation of O-glycoproteins.24 By dividing surgically treated HCC patients into subgroups
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according to the relative ST6Gal I activities in their tumor and non-tumor tissues, the subgroup with stronger tumor-tissue ST6Gal I activity had longer OS.25 Sialyation of Oglycoproteins may affect their binding to lectins, a type of endogenous glycan-binding
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3. O-Glycoproteins and liver cancer
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proteins, some of which are associated with liver cancer.24
Mucins are glycoproteins that contain a serine/threonine-rich domain that often exists in a heavily O-glycosylated state. The mucin-16 glycoprotein (MUC16), also known as cancer antigen-125 (CA-125), is widely used as a biomarker for the diagnosis of ovarian cancer.26 A
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recent analysis of HCC tissues showed that 51 out of 438 samples (11%) manifested somatic MUC16 mutations,27 the highest mutation rate observed among all mucins (in comparison, the second highest mutation rate is 6.4% in MUC4).27 The high mutation rate of MUC16
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suggests an oncogenic role in HCC. Additionally, HCC patients were observed to have higher serum CA-125 levels than controls.28,29 CA-125 was also associated with either spontaneous
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bacterial peritonitis or HCC, particularly in patients with liver cirrhosis.30 The combination of CA-125 and alpha-fetoprotein (AFP), another glycoprotein used in diagnosing HCC, has better sensitivity and specificity than either one used alone.31 However, high CA-125 has also been associated with cirrhosis, a pre-condition of HCC, rather than HCC itself.32 Tissue MUC16 level is associated with OS in patients who have undergone surgery for HCC.33 Furthermore, a study of cardiac metastasis in HCC patients linked tissue MUC1 levels to distant metastasis.34
ACCEPTED MANUSCRIPT The cancer antigen CA19-9, also known as sialyl-Lewis-A, is an antigen found in glycolipids and O-glycoproteins.35 It is an impotent biomarker for cancers rising from the digestive tract and pancreas. Elevated CA19-9 levels are found in patients with cholangiocarcinoma without primary sclerosing cholangitis.36 serum CA19-9 levels at the time of HCC diagnosis are
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reportedly associated with mortality, independent of other clinical variables such as liver function (Child–Pugh) score, AFP, Barcelona Clinic Liver Cancer stage, and Model for End-
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Stage Liver Disease.35
4. Proposed mechanisms of O-glycosylation in liver cancer
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The clinical endpoints of these studies included liver cancer occurrence, recurrence, metastasis and mortality. The biomarkers they revealed were glycoproteins (e.g., MUC16, MUC1), and glycosyltransferases (e.g., GALNT14, GALNT1, GALNT2, C1GALT1, OGA and ST6Gal I). Substrates of these glycosyltransferases identified so far include death
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receptor 5,37 EGF receptor,17,18 integrin and mucins,22 which are all cell surface proteins. Besides the well-known function of death receptors in cell apoptosis and of EGF receptor in tumor cell growth, noval mechanisms related to cancer cell growth have been continuously
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proposed.38 However, one interesting theory not related to cell growth is worth our attention: Altered glycosylation of tumor proteins has been suggested to attenuate the attack of natural
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killer cells.39 The presentation of tumor antigens by the major histocompatibility complex class I-related chain A (MICA) can bind to the nature killer-activating receptor, NKG2D. The aberrant glycoprotein, however, attracts galectin-3, which stands between MICA and NKG2D, and can thereby hamper the nature killer-activation.39 In some studies, such as the genome-wide association study, glycosyltransferase variants associated with HCC outcomes caused ubiquitous O-glycosylation changes throughout subjects’ bodies. Immunological alterations were therefore a mechanism under consideration. Other possible mechanisms included biological environmental changes, such as alterations of angiogenesis and pre-
ACCEPTED MANUSCRIPT metastasis tissue alterations. These fields are potentially important in studies of Oglycosylation, but have not been widely explored.
5. Conclusions
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In this short review, we presented current studies on clinical associations between Oglycosylation and liver cancer. Several potential mechanisms related to cancer growth have been proposed. However, other possible relationships that are not directly linked to cancer
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cell growth, yet critically affect tumor biology, are worth further investigation.
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Authors’ contributions
K.-H. Liang and C.-T. Yeh conceived the project, K.-H. Liang and C.-T. Yeh reviewed literature, K.-H. Liang and C.-T. Yeh wrote and approved the final manuscript.
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Conflict of interest
The authors declare that they have no conflict of interest.
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Acknowledgments
This project was supported by Chang Gung Medical Foundation (CMRPG1B0571,
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CIRPG3B0032, CMRPG3F1601).
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2017;37:3905-3910.
S2542-5684(17)00053-8
DOI:
10.1016/j.livres.2017.12.005
Reference:
LIVRES 32
To appear in:
Liver Research
Please cite this article as: Liang KH, Yeh CT, O-glycosylation in liver cancer: Clinical associations and potential mechanisms, Liver Research (2018), doi: 10.1016/j.livres.2017.12.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT LR2016-017 (唐) Review Article
O-glycosylation in liver cancer: Clinical associations and potential
Kung-Hao Liang a, b, Chau-Ting Yeh a, c, *
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mechanisms☆
Liver Research Center, Chang Gung Memorial Hospital, Taoyuan, Taipei, China
b
Medical Research Department, Taipei Veterans General Hospital, Taipei, China
c
Molecular Medicine Research Center, Chang Gung University, Taoyuan, Taipei, China
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☆
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a
Edited by Peiling Zhu and Genshu Wang
*Corresponding author. Liver Research Center, Chang Gung Memorial Hospital, Taoyuan, Taipei, China.
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E-mail address: [email protected] (C.-T. Yeh)
ARTICLE INFO
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Article history:
Received 15 December 2016
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Received in revised form 9 December 2017
Accepted 12 December 2017
ACCEPTED MANUSCRIPT ABSTRACT Liver cancer can be an aggressive disease, and is highly prevalent in Asia and Africa. However, its currently approved therapeutic strategies are far from satisfactory. Recent progress in genomic, proteomic and glycomic profiling technologies have enabled the
RI PT
identification of biomarkers that significantly correlate with clinical outcomes. Many biomarkers are related to O-glycosylation of glycoproteins, which belong to an important but
discussed potential underlying mechanisms.
O-Glycosylation Post-translational modification Glycobiology Biomarkers
1. Introduction
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Glycosyltransferase.
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Hepatocellular carcinoma (HCC)
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Keywords:
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less-explored field of liver cancer biology. Here, we review these clinical studies and
Worldwide, liver cancer is the sixth most commonly diagnosed cancer (782 cases per
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100,000 persons per year), and the second most common cause of death from cancer (745 cases per 100,000 persons per year).1 Hepatocellular carcinoma (HCC) is the major type of liver cancer (78%), followed by cholangiocarcinoma (15%), angiosarcoma, hepatoblastoma and other rare types.2 Chronic hepatitis B and C are the two major causes of HCC, and account for more than 80% of HCC in regions where hepatitis viruses are endemic.3 The etiologies of other types of liver cancers are less well defined. Primary sclerosing cholangitis and liver flukes have been identified as risk factors for cholangiocarcinoma, but account for less than 30% of the disease.4 Liver cancer has many aggressive hallmarks, including rapid
ACCEPTED MANUSCRIPT cell growth, high risk of distant metastasis, effective angiogenesis, and a prominent antiapoptosis phenotype—all driven by aberrant mechanisms that are still incompletely understood.5 Altered glycosylation of proteins has been observed in several cancer types,6,7 including
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liver cancer.8 In all tumor, stromal or immune cells, proteins are manufactured by sequential conjugations of amino acids according to ribonucleic acid (RNA) templates, and then further decorated with various molecules, such as phosphates, glycans or lipids, which critically
SC
affect the proteins’ shape, localization and functions. Glycosylation is a major type of posttranslational embellishment which occurs in the Golgi apparatus, endoplasmic reticulum or
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cytosols of the cells. Glycosylation links to either oxygen (O) or nitrogen (N) atoms of amino acids, and are referred to as O-linked or N-linked glycosylation, respectively. They are involved in many complex biological mechanisms, exerting extensive physiological effects in a wide range of diseases. In this short review, we focused on O-linked glycosylation in liver
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cancer.
O-linked glycosylation is based on oxygen atoms in serine, threonine or tyrosine residues.
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Glycans that conjugate to oxygen atoms include N-acetylgalactosamine (GalNAc), fucose, glucose, galactose, N-acetylglucosamine (GlcNAc), mannose and xylose.9 In human, the
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glycan–protein conjugations are mediated by specific enzymes. For example, fucose–protein conjugation is mediated by O-fucosyltransferases (POFUT1 and POFUT2); glucose–protein conjugation is by O-glucosyltransferase (POGLUT1), GlcNAc–protein conjugation is by OGlcNAc transferase (OGT), O-mannose modification is mediated by O-mannosyltransferases (POMT1 and POMT2), and O-xylose by O-xylosyltransferase (XYLT1 and XYLT2). In the GalNac–protein
conjugation,
a
total
of
20
members
of
the
polypeptide
N-
acetylgalactosaminyltransferase (GALNT) family are responsible for transferring GalNAc from UDP–CalNAc to the substrate glycoproteins.10 After the first carbohydrate molecule is
ACCEPTED MANUSCRIPT added to the protein backbone, subsequent conjugations can proceed by other enzymes including glycosyltransferases such as core 1 synthase, glycoprotein-N-acetylgalactosamine3beta-galactosyltransferase1 (C1GalT1), C1GALT1 specific chaperone 1 (Cosmc), and sialyltransferase such as ST3 beta-galactoside-alpha-2,3-Sialyltransferase 1 (ST3Gal1) (Fig.
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1).
Fig. 1. An example of O-linked protein glycosylation. This event occurs on the serine (S), threonine (T) or tyrosine residue of the proteins. The sugar moieties were added on to these substrates by
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glycosyltransferases. Lectins can bind to the glycosylated proteins.
Although protein glycosylation is believed to underlie many cellular activities, its exact
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functions in liver cancer are mostly unknown due to the complexity of mechanisms. However, genomic, proteomic and glycomic profiling technologies have greatly improved in recent
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years. Commercially available microarrays for evaluating single nucleotide polymorphisms in high density are now available, enabling genome-wide association studies of various cancerrelated clinical phenotypes. High performance glycan and lectin-arrays have also been developed.11,12 The associations between the “omics” measurements and clinical events or outcomes of liver diseases in large patient cohorts have revealed significant biomarkers, including many O-glycan biomarkers that can both be useful for cancer diagnosis and treatments (see Table 1 for some examples). These biomarkers in turn offered valuable novel insights into liver cancer biology. Here we reviewed the recent progress along these lines.
ACCEPTED MANUSCRIPT Because of the ongoing development of this field, this review was written based on authors’ knowledge rather than a thorough literature search. Table 1 Potential diagnosis and prognosis O-glycosylation markers. Diseases
Clinical relevance
Colorectal cancer
Prognosis
GalNAc-T3
Gastric cancer
Prognosis
GalNAc-T3
Lung cancer
Prognosis
GalNAc-T14
Breast cancer
Diagnosis
MUC2 O-glycan
Ulcerative colitis
Prognosis
Shibao K et al.40
Onitsuka K et al.41
Gu C et al.42
Wu C et al.43
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GalNAc-T3
Reference
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Markers
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profile
Larsson JM et al.44
Prognosis
Blixt O et al.45
Prognosis
Berois N et al.46
Prognosis
Kitada S et al.47
Hepatocellular carcinoma
Prognosis
Wu YM et al.21
B3GNT3
Neuroblastoma
Prognosis
Ho WL et al.48
GalNAc-T3
Ovarian cancer
Prognosis
Wang ZQ et al.49
GalNAc-T2
Neuroblastoma
Prognosis
Ho WL et al.50
GalNAc-T3
Oral squamous cell
Prognosis
Harada Y et al.51
Breast cancer
GalNAc-T9
Neuroblastoma
GalNAc-T3
Renal cell carcinoma
C1GALT1
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MUC1 glycoforms
carcinoma
Hepatocellular carcinoma
Prognosis
Liu Y et al.52
GalNAc-T6
Endometrial cancer
Prognosis
Kurita T et al.53
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GalNAc-T4
Abbreviations: GalNAc-T, N-acetylgalactosaminyltransferase; MUC, mucin; C1GALT1, core 1
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synthase, glycoprotein-N-acetylgalactosamine3-beta-galactosyltransferase-1; B3GNT3, beta1,3-N-acetylglucosaminyltransferase-3.
2. O-linked glycosyltransferase/glycosidase and liver cancer Glycosyltransferases have pivotal funtions in the initiation and continuation of O-linked glycosylation. The GalNAc-T glycosyltransferases, encoded by the GALNT gene family in the human genome, catalyzes the addition of GalNAc to oxygen atoms of amino acids. Recently, it was found that genomic variants in one member of the family, GALNT14, were
ACCEPTED MANUSCRIPT associated with chemotherapeutic responses of late-stage HCC patients in a genome-wide association study.13 One leading variant (rs9679162) was subsequently found to be associated with overall survival (OS) of late-stage HCC patients14,15 and the time-to-complete response of intermediate-stage HCC patients who were treated by transcatheter arterial
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chemoembolization.16 Tandem-mass spectrometry analyses showed that the genotypes were associated with the glycosylation of death-receptor 5 protein in liver tissue,16 which played a role in extrinsic apoptosis signaling of cancer cells. Interestingly, other members of this
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family also exerted regulatory functions in liver cancer growth. For example, GALNT2 expression suppressed malignant HCC phenotypes through modification of the epidermal
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growth factor (EGF) receptor,17 whereas GALNT1 and GALNT10 expressions enhanced the malignant character of HCC through modulating EGF signaling and microRNA-122 mediated processes, respectively.18,19
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The GalNac-T enzymes facilitate addition of GalNAc to serine and threonine residues, forming an antigen called Tn-antigen.20 Afterwards, another glycosyltransferase, C1GalT1, catalyzes the addition of the sugar molecule, “Gal”, forming a T-antigen.20 The C1GalT1
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mRNA levels in surgical specimens from a cohort of 72 HCC patients were found to correlate inversely with post-surgery OS, and positively with tumor stage and metastasis.21 Whether all
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three of these factors (C1GalT1 mRNA levels, tumor stage and metastasis) independently affected OS was not clear in multivariate analysis. The C1GalT1 enzyme was also shown to promote tumor invasion and metastasis by altering glycosylation of integrin-β1.22
O-GlcNAc modification is mediated by the glycosyltransferase OGT, whereas glycan removall of the glycans is mediated by glycosidase (OGA, a.k.a., MEGA5). A recent study of a cohort of HCC patients showed that higher the levels of OGA levels in their tumor
ACCEPTED MANUSCRIPT specimens were associated with longer recurrence-free survival after liver transplantation.23
Beta-galactoside alpha-2,6-sialyltransferase (ST6Gal I) is an important enzyme for the sialylation of O-glycoproteins.24 By dividing surgically treated HCC patients into subgroups
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according to the relative ST6Gal I activities in their tumor and non-tumor tissues, the subgroup with stronger tumor-tissue ST6Gal I activity had longer OS.25 Sialyation of Oglycoproteins may affect their binding to lectins, a type of endogenous glycan-binding
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3. O-Glycoproteins and liver cancer
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proteins, some of which are associated with liver cancer.24
Mucins are glycoproteins that contain a serine/threonine-rich domain that often exists in a heavily O-glycosylated state. The mucin-16 glycoprotein (MUC16), also known as cancer antigen-125 (CA-125), is widely used as a biomarker for the diagnosis of ovarian cancer.26 A
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recent analysis of HCC tissues showed that 51 out of 438 samples (11%) manifested somatic MUC16 mutations,27 the highest mutation rate observed among all mucins (in comparison, the second highest mutation rate is 6.4% in MUC4).27 The high mutation rate of MUC16
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suggests an oncogenic role in HCC. Additionally, HCC patients were observed to have higher serum CA-125 levels than controls.28,29 CA-125 was also associated with either spontaneous
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bacterial peritonitis or HCC, particularly in patients with liver cirrhosis.30 The combination of CA-125 and alpha-fetoprotein (AFP), another glycoprotein used in diagnosing HCC, has better sensitivity and specificity than either one used alone.31 However, high CA-125 has also been associated with cirrhosis, a pre-condition of HCC, rather than HCC itself.32 Tissue MUC16 level is associated with OS in patients who have undergone surgery for HCC.33 Furthermore, a study of cardiac metastasis in HCC patients linked tissue MUC1 levels to distant metastasis.34
ACCEPTED MANUSCRIPT The cancer antigen CA19-9, also known as sialyl-Lewis-A, is an antigen found in glycolipids and O-glycoproteins.35 It is an impotent biomarker for cancers rising from the digestive tract and pancreas. Elevated CA19-9 levels are found in patients with cholangiocarcinoma without primary sclerosing cholangitis.36 serum CA19-9 levels at the time of HCC diagnosis are
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reportedly associated with mortality, independent of other clinical variables such as liver function (Child–Pugh) score, AFP, Barcelona Clinic Liver Cancer stage, and Model for End-
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Stage Liver Disease.35
4. Proposed mechanisms of O-glycosylation in liver cancer
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The clinical endpoints of these studies included liver cancer occurrence, recurrence, metastasis and mortality. The biomarkers they revealed were glycoproteins (e.g., MUC16, MUC1), and glycosyltransferases (e.g., GALNT14, GALNT1, GALNT2, C1GALT1, OGA and ST6Gal I). Substrates of these glycosyltransferases identified so far include death
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receptor 5,37 EGF receptor,17,18 integrin and mucins,22 which are all cell surface proteins. Besides the well-known function of death receptors in cell apoptosis and of EGF receptor in tumor cell growth, noval mechanisms related to cancer cell growth have been continuously
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proposed.38 However, one interesting theory not related to cell growth is worth our attention: Altered glycosylation of tumor proteins has been suggested to attenuate the attack of natural
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killer cells.39 The presentation of tumor antigens by the major histocompatibility complex class I-related chain A (MICA) can bind to the nature killer-activating receptor, NKG2D. The aberrant glycoprotein, however, attracts galectin-3, which stands between MICA and NKG2D, and can thereby hamper the nature killer-activation.39 In some studies, such as the genome-wide association study, glycosyltransferase variants associated with HCC outcomes caused ubiquitous O-glycosylation changes throughout subjects’ bodies. Immunological alterations were therefore a mechanism under consideration. Other possible mechanisms included biological environmental changes, such as alterations of angiogenesis and pre-
ACCEPTED MANUSCRIPT metastasis tissue alterations. These fields are potentially important in studies of Oglycosylation, but have not been widely explored.
5. Conclusions
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In this short review, we presented current studies on clinical associations between Oglycosylation and liver cancer. Several potential mechanisms related to cancer growth have been proposed. However, other possible relationships that are not directly linked to cancer
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cell growth, yet critically affect tumor biology, are worth further investigation.
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Authors’ contributions
K.-H. Liang and C.-T. Yeh conceived the project, K.-H. Liang and C.-T. Yeh reviewed literature, K.-H. Liang and C.-T. Yeh wrote and approved the final manuscript.
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Conflict of interest
The authors declare that they have no conflict of interest.
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Acknowledgments
This project was supported by Chang Gung Medical Foundation (CMRPG1B0571,
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CIRPG3B0032, CMRPG3F1601).
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