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Year : 2015  |  Volume : 52  |  Issue : 7  |  Page : 144--147

The role of ppGalNAc-T family in breast cancer development and progression

R Yang, H Zhang, Y Ma, S Gong, J Niu, J Ma, A Zhong 
 Department of Respiratory Medicine, Yidu Central Hospital of Weifang, Qingzhou, China

Correspondence Address:
A Zhong
Department of Respiratory Medicine, Yidu Central Hospital of Weifang, Qingzhou


Glycosylation of proteins is an essential process in all eukaryotes. Mucin-type O-linked glycosylation is an evolutionarily conserved protein modification as a kind of glycosylation of proteins. The role of O-glycosylation was well documented in multiple cancers. While in breast cancer, the enzymes that catalyzed the initiation of O-glycosylation remained elusive. In this review, we briefly introduced the process of the initiation of O-glycosylation and summarized the roles of enzymes that catalyzed the initiation step of O-glycosylation in the breast cancer carcinogenesis, development, and progression. Finally, we summarized some attempts exploring the therapy against aberrant O-glycosylation.

How to cite this article:
Yang R, Zhang H, Ma Y, Gong S, Niu J, Ma J, Zhong A. The role of ppGalNAc-T family in breast cancer development and progression.Indian J Cancer 2015;52:144-147

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Yang R, Zhang H, Ma Y, Gong S, Niu J, Ma J, Zhong A. The role of ppGalNAc-T family in breast cancer development and progression. Indian J Cancer [serial online] 2015 [cited 2021 Jun 19 ];52:144-147
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Glycosylation of proteins is an essential process in all eukaryotes, and a great diversity in types of protein glycosylation exists in animals, plants, and microorganisms. Mucin-type O-linked glycosylation is an evolutionarily conserved protein modification found in diverse species including mammals, fish, insects, worms, and some types of fungi. [1] The key initiation step is catalyzed by a large homologous polypeptide N-acetylgalactosamine (GalNAc)-transferase (GalNAc-T; GalNAc-Ts have also appeared in the literature as polypeptide ppGalNAcT, ppGalNAc-T, ppGalNAc T, and GalNAcT) family that catalyzes the first step in the biosynthesis forming the GalNAcα1-O-serine (Ser)/threonine (Thr) linkage in O-glycoproteins. Thus, the large number of enzymes controlling the initiation step makes mucin-type O-glycosylation unique among other types of protein glycosylation. All other types of protein glycosylation are controlled by one or two isoenzymes or in the case of N-glycosylation a complex of proteins. The O-GalNAc residues are further processed by the addition of different monosaccharides catalyzed by 30 or more distinct glycosyltransferases. GalNAc O-glycosylation is initiated in the Golgi apparatus after most protein folding events have taken place. [2]

Breast cancer is a malignant tumor that seriously affects females' physical and psychological health; it is the leading cause of cancer death among female. [3],[4],[5] According to the global cancer statistics, about 1.2 million women get breast cancer every year in the world on average; breast cancer accounted for 23% of the total cancer cases and 14% of the cancer deaths in 2008. [6] In Western countries and areas, both death rate and its morbidity take the first place among females' cancers. It is estimated that 235,030 new cases would be diagnosed, and 40,430 estimated deaths would occur in the United States in 2014. [7] Moreover, its incidence increased dramatically over the past 30 years and documented a peak incidence represented by the middle-age group (45-59 years), which emerged in the last 20 years. The incidence peak moved from the 40 to 44 years group in the previous two decades to the 50-54 years group in the most recent decade in Shanghai. [8] The high chance of relapse and mortality rates are two characteristics of Stage III breast cancer. [8] Breast cancer is characterized by overexpression of ERBB2, BCL2, and mutations involving in genes TP53, BRCA1, BRCA2, and other tumor suppressor genes and also characterized by the presence of estrogen and progesterone receptors in cancerous cells. [9],[10] At present, various countries are investing a large number of human resources and material resources in research on genesis and development of breast cancer so as to take positive prevention measures. [11],[12]

The extended ppGalNAc-T family and their functional involvement in the cancer metastasis cascade were well reviewed by Beaman and Brooks. [13] The exact role of ppGalNAc-T family members' function in breast cancer progression remained unclear. In this review, we summarized the functions of ppGalNAc-T family members in breast cancer to better understand breast cancer progression and seek new methods for therapy.

 The Initiation of O-glycosylation and its Function in Breast Cancer

O-glycosylation is a multi-step process initiated in the Golgi apparatus after most protein folding events have taken place. It is comprised O-glycosylation initiation, core extension, and O-glycan capping. [14]

The initiation of O-glycosylation is catalyzed by polypeptide N-acetylgalactosamine transferase (ppGalNAc-T or GALNT) family. It is thought that all of the ppGalNAc-T gene family, except for ppGalNAc-T4, arose from an ancestral ppGalNAc-T gene and that throughout evolution various paralogs formed. To date, 20 ppGalNAc-T isozymes have been identified and 17 of these characterized functionally in humans. ppGalNAc-Ts are located throughout the Golgi apparatus. All of the ppGalNAc-Ts except for-T20 share a Type II membrane structure composed of a short N-terminal cytoplasmic tail, a hydrophobic membrane spanning domain, a stem region (90-470 amino acids in length), a luminal catalytic domain (~230 amino acids long), and unique to the ppGalNAc-Ts, a C-terminal ricin-like lectin domain (~120 amino acids in length) which has a binding affinity for a-GalNAc monosaccharides. [13] Babino et al. first revealed that Tn antigen is a precancerous biomarker in breast tissue and serum in n-nitrosomethylurea-induced rat mammary carcinogenesis in 2000. [15]

 The Critical Role of ppGalNAc-T Family in Breast Cancer Progression

The ppGalNAc-Ts play an important role in normal cell function. For instance, the initiation of protein O-glycosylation by ppGalNAcT-1 provides a distinctive repertoire of advantageous functions that support vascular responses, humoral immunity, [16] normal heart valve development, and cardiac function. [17]

The ppGalNAc-T family plays important roles in multiple processes in the cancer metastatic cascade. [13] Abnormal O-glycans such as Tn antigen are found in over 90% of breast cancers. The first well-documented GALNT is 6 in breast cancer. Berois et al. raised that GALNT6 was a new immunohistochemical breast cancer marker. [18] Freire et al. results indicate that GALNT6 mRNA could be a specific marker applicable to the molecular diagnosis of breast cancer cells dissemination. [19] Park et al. implied that overexpression of GALNT6 might contribute to mammary carcinogenesis through aberrant glycosylation and stabilization of mucin-1 (MUC1). [20] At the same time, Park et al. identified fibronectin that was O-glycosylated in vivo and thereby stabilizing by GALNT6. Knockdown of fibronectin abrogated the disruptive proliferation caused by introduction of GALNT6 into epithelial cells, suggesting that GALNT6-fibronectin pathway should be a critical component for breast cancer development and progression. [21]

Niang et al. found that GalNAc-T4 putatively modulates the estrogen regulatory network through FOXA1 (a key transcription factor, functions to promote estrogen-responsive gene expression by acting as a cofactor to estrogen receptor alpha) glycosylation in human breast cancer cells. [22] Pangeni et al. carried out a bioinformatic screen of genome-wide breast tumor methylation data available at the Cancer Genome Atlas and a broad literature review to identify candidate genes that may contribute to breast to brain metastasis (BBM). This analysis identified 82 candidates. They investigated the methylation status of these genes using combined bisulfite and restriction analysis and identified 21 genes frequently methylated in BBM. They identified that GALNT9 was frequently methylated (55%) and silenced in BBM and infrequently methylated in primary breast tumors. GALNT9 may play a role in the progression of primary breast tumors to brain metastases. [23] Wu et al. reported that GALNT14 was a potential biomarker for breast cancer by immunohistochemistry. [24] Huanna et al. have reported that GALNT14 is heterogeneously expressed in most breast cancers, plays a critical role in the invasion and migration of breast cancers by regulating the activity of matrix metalloproteinase-2 and expression of some EMT genes, such as N-cadherin, vimentin, vascular endothelial growth factor, and transforming growth factor-beta. [25] Song et al. generated mice with mammary epithelial-specific deletion of core 1-derived O-glycans and found that deficiency of core 1 O-glycosylation impaired the localization of MUC1, a major O-glycoprotein, on the apical surfaces of mammary epithelium and loss of core 1-derived O-glycans decreases breast cancer development in mice. [26]

Initiation of GalNAc-type O-glycosylation in the endoplasmic reticulum promotes cancer cell invasiveness. [27] This process markedly stimulates cell migration and is constitutively activated in more than 60% of breast carcinomas. Chia et al. identified 12 negative regulators of O-glycosylation that all control GalNAc-T subcellular localization. ERK8, an atypical MAPK with high basal kinase activity, is a strong hit and is partially localized at the Golgi. Its inhibition induces the relocation of GalNAc-Ts, but not of KDEL (Lys-Asp-Glu-Leu-COO-) receptors, revealing the existence of two separate COPI (Coat Protein I)-dependent pathways. [28] A diagram that summarized reported GALNTs' function and pathway in breast cancer is shown in [Figure 1].{Figure 1}

 Exploring the Therapy against Aberrant O-glycosylation

In 1997, Babino et al. raised a group of antibodies that recognize tumor-associated antigens arising from an aberrant O-glycosylation. [29] Blixt et al. found that autoantibodies to specific cancer-associated glycoforms of MUC1 were more frequently and at higher levels in early stage breast cancer patients than in women with benign breast disease or healthy women. Association of strong antibody response with reduced rate and delay in metastases suggests that autoantibodies can affect disease progression. [30]

During the last decade, an increasing number of GalNAc-transferase isoforms have been cloned and their substrate-specificities partly characterized. These differences in substrate specificities have been exploited for in vitro site-directed O-glycosylation. In GlycoPEGylation, polyethylene glycol is transferred to recombinant therapeutics to specific acceptor sites directed by GalNAc-transferases. GalNAc-transferases have also been used to control density of glycosylation in the development of glycopeptide-based cancer vaccines. The membrane-associated MUC1 has long been considered a target for immunotherapeutic and immunodiagnostic measures since it is highly overexpressed and aberrantly O-glycosylated in most adenocarcinomas, including breast, ovarian, and pancreatic cancers. Using vaccines mimicking the glycosylation pattern of cancer cells, it is possible to overcome tolerance in transgenic animals expressing the human MUC1 protein as a self-antigen providing important clues for an improved MUC1 vaccine design. The present review will highlight some of the potential applications of site-directed O-glycosylation. [31] Liu et al. produced the anti-human ppGalNAc-T2 monoclonal antibody (MAb) 5F3 which might be a therapy for breast cancer. [32]

Using synthetic Tn-based vaccines, Mazal et al. have generated a panel of anti-Tn MAbs which could be of important clinical value, notably due to the increasing interest of this antigen in anticancer vaccine design as well as for the development of anti-Tn antibodies for in vivo diagnostic and therapeutic strategies. [33] Stuchlová Horynová et al. found that in breast cancer, elevated levels of antibodies recognizing aberrant MUC1 are associated with better outcome whereas in IgA nephropathy, the antibodies recognizing aberrant IgA1 are part of the pathogenetic process. [34]


GalNAc type O-glycosylation is an abundant modification of proteins. The initiation of this modification is catalyzed by 20 GalNAc-transferases. In recent years, the dialectical role of the regulation of enzymes that initiate O-glycosylation of proteins is raised. GALNT6 is the most well-documented enzyme contributed to breast cancer development and progression. GALNT4 and GALNT14 are reported as regulator for breast cancer development, invasion, and migration. Some investigators focus on the regulation of GALNTs. ERK8 is one of the negative regulators of GALNTs' activity. Methylation might also be a regulation modification of GALNTs and might be relative to breast cancer metastasis. However, the role of other members of GALNTs family remained elusive.

Because of the great diversity and specificity of O-glycosylation, the oncoproteins that could be O-glycosylated were outstanding for breast cancer treatment. The antibody targeted to the aberrant O-glycosylated could recognize the oncoproteins with the aberrant carbohydrate chain which can enhance the specificity. Thus, the aberrant O-glycosylated oncoproteins and enzymes catalyzed O-glycosylaion might serve as novel target in breast cancer treatment.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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