Pleiotropic effects of O-glycosylation in colon cancer

Molecular mechanisms and pathophysiological implications of mucin-type O-glycosylation dysregulation in colorectal cancer progression

Colorectal cancer (CRC) is among the most prevalent malignancies globally, with 1.9 million new cases annually. While CRC pathogenesis has been widely attributed to the adenoma-carcinoma and serrated sequences, our study highlights the critical and multifaceted role of O-glycosylation impairment in this malignancy. Mucin-type O-glycosylation, a key post-translational modification, exerts significant effects on tumor cells, impacting their proliferation, migration, and invasiveness. Additionally, its influence on the immune response to CRC presents novel perspectives for potential therapeutic interventions. The authors conducted a systematic literature review following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, using databases such as Google Scholar, PubMed, and Scopus. In this article, we provide a comprehensive analysis of the mechanisms underlying mucin-type O-glycosylation disruption in CRC and examine how these mechanisms could serve as biomarkers for early diagnosis and personalized treatment strategies. Our findings contribute to a more detailed understanding of CRC pathogenesis and offer promising directions for innovative diagnostic and therapeutic approaches, which in the future may lead to improved patient prognosis.

HEXB Drives Raised Paucimannosylation in Colorectal Cancer and Stratifies Patient Risk

Noninvasive prognostic markers are needed to improve the survival of colorectal cancer (CRC) patients. Toward this goal, we applied untargeted systems glycobiology approaches to snap-frozen and formalin-fixed paraffin-embedded tumor tissues and peripheral blood mononuclear cells from CRC patients spanning different disease stages and matching controls to faithfully uncover molecular changes associated with CRC. Quantitative glycomics and immunohistochemistry revealed that noncanonical paucimannosidic N-glycans are elevated in CRC tumors relative to normal adjacent tissues. Cell origin-focused glycoproteomics enabled using the well-curated Human Protein Atlas combined with immunohistochemistry of CRC tumor tissues recapitulated these findings and indicated that the paucimannosidic proteins were in part from tumor-infiltrating monocytes (e.g., MPO, AZU1) and of CRC cell origin (e.g., LGALS3BP, PSAP). Biosynthetically explaining these observations, N-acetyl-β-D-hexosaminidase (Hex) subunit β (HEXB) was found to be overexpressed in CRC tissues relative to normal adjacent colorectal tissues and colocalization and enzyme inhibition studies confirmed that HEXB facilitates paucimannosidic protein biosynthesis in CRC cells. Employing a sensitive, quick, and robust enzyme activity assay, we then showed that Hex activity was elevated in plasma and peripheral blood mononuclear cells from patients with advanced CRC relative to controls and those with early-stage disease. Surveying a large donor cohort, the plasma Hex activity was found to be raised in CRC patients relative to normal controls and correlated with the 5-year survival of CRC patients indicating that elevated plasma Hex activity is a potential disease risk marker for patient outcome. Our glycoproteomics-driven findings open avenues for better prognostication and disease risk stratification in CRC.

Unraveling the role of C1GALT1 in abnormal glycosylation and colorectal cancer progression

C1GALT1 plays a pivotal role in colorectal cancer (CRC) development and progression through its involvement in various molecular mechanisms. This enzyme is central to the O-glycosylation process, producing tumor-associated carbohydrate antigens (TACA) like Tn and sTn, which are linked to cancer metastasis and poor prognosis. The interaction between C1GALT1 and core 3 synthase is crucial for the synthesis of core 3 O-glycans, essential for gastrointestinal health and mucosal barrier integrity. Aberrations in this pathway can lead to CRC development. Furthermore, C1GALT1's function is significantly influenced by its molecular chaperone, Cosmc, which is necessary for the proper folding of T-synthase. Dysregulation in this complex interaction contributes to abnormal O-glycan regulation, facilitating cancer progression. Moreover, C1GALT1 affects downstream signaling pathways and cellular behaviors, such as the epithelial-mesenchymal transition (EMT), by modifying O-glycans on key receptors like FGFR2, enhancing cancer cell invasiveness and metastatic potential. Additionally, the enzyme's relationship with MUC1, a mucin protein with abnormal glycosylation in CRC, highlights its role in cancer cell immune evasion and metastasis. Given these insights, targeting C1GALT1 presents a promising therapeutic strategy for CRC, necessitating further research to develop targeted inhibitors or activators. Future efforts should also explore C1GALT1's potential as a biomarker for early diagnosis, prognosis, and treatment response monitoring in CRC, alongside investigating combination therapies to improve patient outcomes.

Mass Spectrometry-Based Glycoproteomic Workflows for Cancer Biomarker Discovery

Glycosylation has a clear role in cancer initiation and progression, with numerous studies identifying distinct glycan features or specific glycoproteoforms associated with cancer. Common findings include that aggressive cancers tend to have higher expression levels of enzymes that regulate glycosylation as well as glycoproteins with greater levels of complexity, increased branching, and enhanced chain length1. Research in cancer glycoproteomics over the last 50-plus years has mainly focused on technology development used to observe global changes in glycosylation. Efforts have also been made to connect glycans to their protein carriers as well as to delineate the role of these modifications in intracellular signaling and subsequent cell function. This review discusses currently available techniques utilizing mass spectrometry-based technologies used to study glycosylation and highlights areas for future advancement.

Altered O-glycomes of Renal Brush-Border Membrane in Model Rats with Chronic Kidney Diseases

Chronic kidney disease (CKD) is defined as a decrease in renal function or glomerular filtration rate (GFR), and proteinuria is often present. Proteinuria increases with age and can be caused by glomerular and/or proximal tubule (PT) alterations. PT cells have an apical brush border membrane (BBM), which is a highly dynamic, organized, and specialized membrane region containing multiple glycoproteins required for its functions including regulating uptake, secretion, and signaling dependent upon the physiologic state. PT disorders contribute to the dysfunction observed in CKD. Many glycoprotein functions have been attributed to their N- and O-glycans, which are highly regulated and complex. In this study, the O-glycans present in rat BBMs from animals with different levels of kidney disease and proteinuria were characterized and analyzed using liquid chromatography tandem mass spectrometry (LC-MS/MS). A principal component analysis (PCA) documented that each group has distinct O-glycan distributions. Higher fucosylation levels were observed in the CKD and diabetic groups, which may contribute to PT dysfunction by altering physiologic glycoprotein interactions. Fucosylated O-glycans such as 1-1-1-0 exhibited higher abundance in the severe proteinuric groups. These glycomic results revealed that differential O-glycan expressions in CKD progressions has the potential to define the mechanism of proteinuria in kidney disease and to identify potential therapeutic interventions.

Towards structure-focused glycoproteomics

Facilitated by advances in the separation sciences, mass spectrometry and informatics, glycoproteomics, the analysis of intact glycopeptides at scale, has recently matured enabling new insights into the complex glycoproteome. While diverse quantitative glycoproteomics strategies capable of mapping monosaccharide compositions of N- and O-linked glycans to discrete sites of proteins within complex biological mixtures with considerable sensitivity, quantitative accuracy and coverage have become available, developments supporting the advancement of structure-focused glycoproteomics, a recognised frontier in the field, have emerged. Technologies capable of providing site-specific information of the glycan fine structures in a glycoproteome-wide context are indeed necessary to address many pending questions in glycobiology. In this review, we firstly survey the latest glycoproteomics studies published in 2018–2020, their approaches and their findings, and then summarise important technological innovations in structure-focused glycoproteomics. Our review illustrates that while the O-glycoproteome remains comparably under-explored despite the emergence of new O-glycan-selective mucinases and other innovative tools aiding O-glycoproteome profiling, quantitative glycoproteomics is increasingly used to profile the N-glycoproteome to tackle diverse biological questions. Excitingly, new strategies compatible with structure-focused glycoproteomics including novel chemoenzymatic labelling, enrichment, separation, and mass spectrometry-based detection methods are rapidly emerging revealing glycan fine structural details including bisecting GlcNAcylation, core and antenna fucosylation, and sialyl-linkage information with protein site resolution. Glycoproteomics has clearly become a mainstay within the glycosciences that continues to reach a broader community. It transpires that structure-focused glycoproteomics holds a considerable potential to aid our understanding of systems glycobiology and unlock secrets of the glycoproteome in the immediate future.

O-glycan recognition and function in mice and human cancers

Protein glycosylation represents a nearly ubiquitous post-translational modification, and altered glycosylation can result in clinically significant pathological consequences. Here we focus on O-glycosylation in tumor cells of mice and humans. O-glycans are those linked to serine and threonine (Ser/Thr) residues via N-acetylgalactosamine (GalNAc), which are oligosaccharides that occur widely in glycoproteins, such as those expressed on the surfaces and in secretions of all cell types. The structure and expression of O-glycans are dependent on the cell type and disease state of the cells. There is a great interest in O-glycosylation of tumor cells, as they typically express many altered types of O-glycans compared with untransformed cells. Such altered expression of glycans, quantitatively and/or qualitatively on different glycoproteins, is used as circulating tumor biomarkers, such as CA19-9 and CA-125. Other tumor-associated carbohydrate antigens (TACAs), such as the Tn antigen and sialyl-Tn antigen (STn), are truncated O-glycans commonly expressed by carcinomas on multiple glycoproteins; they contribute to tumor development and serve as potential biomarkers for tumor presence and stage, both in immunohistochemistry and in serum diagnostics. Here we discuss O-glycosylation in murine and human cells with a focus on colorectal, breast, and pancreatic cancers, centering on the structure, function and recognition of O-glycans. There are enormous opportunities to exploit our knowledge of O-glycosylation in tumor cells to develop new diagnostics and therapeutics.