The known unknowns of apolipoprotein glycosylation in health and disease

The intracellular chylomicron highway: novel insights into chylomicron biosynthesis, trafficking, and secretion

PURPOSE OF REVIEW

Chylomicron biosynthesis plays a vital role in supplying essential lipids and lipid soluble vitamins to peripheral tissues for various functions. Despite this, the intracellular synthesis, trafficking, and secretion of chylomicrons remains only partly understood. The purpose of this review is to summarize the role of established proteins in this process and bring attention to recently identified proteins to provide an up-to-date model of chylomicron biosynthesis.

RECENT FINDINGS

Recently, several proteins have been shown to play a role in the initial formation and lipidation of chylomicrons at the endoplasmic reticulum (ER), which include: TM6SF2, PLA2G12B, PRAP1, and SURF4. In addition, mitochondria have been implicated in chylomicron metabolism, but mechanistic insight is missing. The trafficking of chylomicrons from the ER to the Golgi, and the subsequent trafficking from the Golgi to the basolateral side of enterocytes, however, remains a mystery.

SUMMARY

Progress in the chylomicron biosynthesis field is largely associated with findings in VLDL biosynthesis. In addition, increased insight in events after prechylomicrons leave the ER is needed. Given the important role of chylomicron biosynthesis in whole-body lipid metabolism, further research into the molecular mechanisms is warranted.

Plasma protein corona of liposomes loaded with a phospholipid-allocolchicinoid conjugate enhances their anti-inflammatory potential

Today colchicine is considered as a possible new treatment for cardiovascular diseases. Its physiological effects have been shown to be primarily due to the intra-leukocyte concentrations. Nanoparticulate formulations could help accumulation of colchicine in phagocytic cells. Previously we formulated liposomes loaded with a colchicine analog in the form of an enzyme-responsive conjugate with phosphatidylcholine (aC-PC) and showed acceptable stability of the formulation in human plasma. Here, we investigated how protein coronas formed on a series of aC-PC-bearing liposomes in human plasma affected their interactions with leukocytes and endothelial cells. Liposome-protein complexes were analyzed by shotgun proteomics. Liposomes 25C with the highest load of aC-PC (25 %) were distinguished by a three times more massive protein corona and specific profile of proteins, including enrichment with ApoD and galectin-3-binding protein, which may affect the inflammation-associated signaling. Differences in the protein coronas did not noticeably affect liposome uptake by cultured monocytes and endotheliocytes, although the level of uptake decreased in the presence of plasma proteins. Nor did the composition of liposomes affect the course of phagocytosis by leukocytes in the blood ex vivo. The effects of protein coronas were manifested in the suppression of the production of inflammatory chemokine MCP-1 (and to a much lesser extent IL-8) by stimulated peripheral blood monocytes about 1.5 times compared with naked liposomes. In the case of liposomes 25C the inhibition was complete. These liposomes are considered the most promising for further preclinical studies.

The emerging role of glycans and the importance of sialylation in cardiovascular disease

Glycosylation is the process by which glycans (i.e. 'sugars') are enzymatically attached to proteins or lipids to form glycoconjugates. Growing evidence points to glycosylation playing a central role in atherosclerosis. Glycosylation occurs in all human cells and post-translationally modifies many signalling molecules that regulate cardiovascular disease, affecting their binding and function. Glycoconjugates are present in abundance on the vascular endothelium and on circulating lipoproteins, both of which have well-established roles in atherosclerotic plaque development. Sialic acid is a major regulator of glycan function and therefore the process of sialylation, in which sialic acid is added to glycans, is likely to be entwined in any regulation of atherosclerosis. Glycans and sialylation regulators have the potential to present as new biomarkers that predict atherosclerotic disease or as targets for pharmacological intervention, as well as providing insights into novel cardiovascular mechanisms. Moreover, the asialoglycoprotein receptor 1 (ASGR1), a glycan receptor, is emerging as an exciting new regulator of lipid metabolism and coronary artery disease. This review summarises the latest advances in the growing body of evidence that supports an important role for glycosylation and sialylation in the regulation of atherosclerosis.

Strategies to verify equimolar peptide release in mass spectrometry-based protein quantification exemplified for apolipoprotein(a)

OBJECTIVES

Quantitative protein mass spectrometry (MS) is ideally suited for precision diagnostics and for reference standardization of protein analytes. At the Leiden Apolipoprotein Reference Laboratory we apply MS strategies to obtain detailed insight into the protein-to-peptide conversion in order to verify that quantifier peptides are not partly concealed in miscleaved protein backbone.

METHODS

Apolipoprotein(a) (apo(a)) was digested in a non-optimal manner to enhance the number of miscleaved peptides that were identified by high resolution liquid chromatography tandem-MS measurements. The protein-to-peptide conversion was carefully mapped with specific attention for miscleaved peptides that contain an apo(a) quantifier peptide. Four different isotopologues of each apo(a)-quantifier peptide were applied to evaluate linearity of internal peptide standards during measurement of specific real-life samples.

RESULTS

Two apo(a) quantifier peptides that were concealed in two different miscleaved peptides were included into a multiple reaction monitoring list in our targeted MS-based apo(a) quantifications to alert for potential protein digestion discrepancies. The presence of miscleaved peptides could be ruled out when applying our candidate reference measurement procedure (RMP) for apo(a) quantification.

CONCLUSIONS

These data further corroborate the validity of our apo(a) candidate RMP as higher order method for certification of commercial Lp(a) tests that is endorsed by the International Federation of Clinical Chemistry and Laboratory Medicine. MS-based molecular detection and quantification of heterogeneous apo(a) proteoforms will allow manufacturers' transitioning from confounded lipoprotein(a) [Lp(a)] mass levels into accurate molar apo(a) levels.

Advancing the Metabolic Dysfunction-Associated Steatotic Liver Disease Proteome: A Post-Translational Outlook

Metabolic dysfunction-associated steatotic liver disease (MASLD) is a prevalent liver disorder with limited treatment options. This review explores the role of post-translational modifications (PTMs) in MASLD pathogenesis, highlighting their potential as therapeutic targets. We discuss the impact of PTMs, including their phosphorylation, ubiquitylation, acetylation, and glycosylation, on key proteins involved in MASLD, drawing on studies that use both human subjects and animal models. These modifications influence various cellular processes, such as lipid metabolism, inflammation, and fibrosis, contributing to disease progression. Understanding the intricate PTM network in MASLD offers the potential for developing novel therapeutic strategies that target specific PTMs to modulate protein function and alleviate disease pathology. Further research is needed to fully elucidate the complexity of PTMs in MASLD and translate these findings into effective clinical applications.

Establishing the Role of Liver Fatty Acid-Binding Protein in Post-Golgi Very-Low-Density Lipoprotein Trafficking Using a Novel Fluorescence-Based Assay

The liver plays a crucial role in maintaining lipid homeostasis by converting toxic free fatty acids into VLDL, which the body uses for energy. Even minor changes in VLDL formation and secretion can result in serious health conditions such as atherosclerosis and non-alcoholic fatty liver disease. Despite the importance of VLDL, the proteins and signaling pathways involved in its regulation remain largely unknown. This study aims to develop a novel methodology to study intracellular VLDL transport events and explore the role of liver fatty acid-binding protein (LFABP) in VLDL transport and secretion. Current methods to study VLDL are often tedious, time-consuming, and expensive, underscoring the need for an alternative approach. We designed a new immunofluorescence-based assay to track the formation and secretion of VLDL in cells over time using fluorescently tagged TopFluor oleic acid. Confocal microscopy confirmed that TopFluor oleic acid enters hepatocytes and colocalizes with the ER, Golgi, and plasma membrane. Additionally, the collection of cell culture media revealed that TopFluor was incorporated into VLDL particles, as confirmed by fluorescence readings and ApoB100 immunoblots. This novel assay provides a valuable tool for further research into the mechanisms of VLDL regulation and the development of potential therapeutic targets for related diseases. Utilizing this assay, we identified LFABP as a key regulatory protein in post-Golgi VLDL trafficking. Our data suggest that LFABP plays a crucial role in this process, and its functional impairment leads to reduced VLDL secretion.

Effect of N-glycosylation on secretion, degradation and lipoprotein distribution of human serum amyloid A4

Serum amyloid A (SAA) is a family of apolipoproteins predominantly synthesized and secreted by the liver. Human SAA4 is constitutively expressed and contains an N-glycosylation site that is not present in other SAA subtypes. SAA4 proteins are not fully glycosylated, resulting in the presence of both glycosylated and non-glycosylated forms in human plasma. The efficiency of N-glycosylation in SAA4 is known to be influenced by some reasons such as genetic polymorphism and metabolic disorders. However, the specific role of N-glycosylation in SAA4 remains largely unexplored. This study aimed to investigate how N-glycosylation affects the secretion, degradation, and lipoprotein distribution of SAA4. Initially, we designed and constructed an SAA4 plasmid vector to compare with the expression pattern of endogenous SAA4. The exogenous SAA4 was partially N-glycosylated, analogous to endogenous SAA4 in human hepatocellular carcinoma cells. Subsequently, we created a non-glycosylated mutant by replacing asparagine 76 with glutamine. Immunoblotting assays showed that the disruption of N-glycans did not affect the secretion and degradation of SAA4. Furthermore, we analyzed the lipoprotein profiles of SAA4 in the conditioned medium derived from transfected cells. The results revealed that non-glycosylated mutant SAA4 exhibited a distinct lipoprotein distribution compared to wild-type SAA4. Our findings suggest that N-glycosylation may be a key regulator of the distribution of SAA4 in lipoproteins, shedding light on the previously unknown physiological activities of human SAA4.

Effect of yogurt probiotic liquid and powder supplementation on hematology and biochemistry blood levels of layer-phase laying hens

Objective

The aim of this study was to determine the best form and concentration of yogurt probiotics that can reduce leucocyte, neutrophil, lymphocyte, alkaline phosphatase, and cholesterol levels to normal limits.

Materials and Methods

There were 35 Isa Brown laying hens aged 40 weeks with health conditions chosen as the object of research. We used a completely randomized design method with seven treatments. Treatments included P0: basal feed; T1: basal feed and probiotic liquid 2%; T2: basal feed and probiotic liquid 3%; T3: basal feed and probiotic liquid 4%; T4: basal feed and probiotic powder 2%; T5: basal feed and probiotic powder 3%; and T6: basal feed and probiotic powder 4%. On day 35 of the research, we collected blood samples. We analyzed the data using analysis of variance, followed by Duncan's multiple range test.

Results

This showed that probiotic liquid and powdered yogurt had a significant effect (p < 0.05) on all parameters. Supplementation with 4% probiotic powder reduced neutrophil (53.96%), lymphocyte (27.84%), and N/L ratios (36.25%); alkaline phosphatase levels (53.6%); and cholesterol levels (ApB 26.65% and LDL 42.4%) compared to the control.

Conclusion

This study shows that all probiotic supplementation shows improvement in the studied parameters, but the addition of 4% probiotic powder had the best result for reducing neutrophil, lymphocyte, N/L ratio, alkaline phosphatase, and cholesterol levels compared to the control and probiotic liquid.

ERICH4 is not involved in the assembly and secretion of intestinal lipoproteins

BACKGROUND AND AIMS

The small intestine plays a central role in lipid metabolism, most notably the uptake of dietary fats that are packaged into chylomicrons and secreted into the circulation for utilisation by peripheral tissues. While microsomal triglyceride transfer protein (MTP) is known to play a key role in this pathway, the intracellular assembly, trafficking, and secretion of chylomicrons is incompletely understood.

METHODS AND RESULTS

Using human transcriptome datasets to find genes co-regulated with MTTP, we identified ERICH4 as a top hit. The gene encodes for glutamate-rich protein 4, a protein of unknown function. REACTOME gene-function prediction tools indicated that ERICH4 is involved in intestinal lipid metabolism. In addition, GWAS data point to a strong relationship between ERICH4 and plasma lipids. To validate ERICH4 as a lipid gene, we generated whole-body Erich4 knockout (Erich4-/-) mice. ERICH4 deficiency, however, did not result in changes in body weight and composition, food intake, circulating plasma lipids, energy absorption and excretion, and tissue weights compared to controls. Additionally, there were no morphological abnormalities seen in the small intestine. Challenging mice with a high-fat diet did not give rise to a phenotype either.

CONCLUSIONS

Despite prediction tools indicating ERICH4 as a strong candidate gene in intestinal lipid metabolism, we here show that ERICH4 does not play a role in intestinal lipid metabolism in mice. It remains to be established whether ERICH4 plays a role in human lipid metabolism.

Gliflozins, sucrose and flavonoids are allosteric activators of lecithin-cholesterol acyltransferase

Lecithin-cholesterol acyltransferase (LCAT) serves as a pivotal enzyme in preserving cholesterol homeostasis via reverse cholesterol transport, a process closely associated with the onset of atherosclerosis. Impaired LCAT function can lead to severe LCAT deficiency disorders for which no pharmacological treatment exists. LCAT-based therapies, such as small molecule positive allosteric modulators (PAMs), against LCAT deficiencies and atherosclerosis hold promise, although their efficacy against atherosclerosis remains challenging. Herein we utilized a quantitative in silico metric to predict the activity of novel PAMs and tested their potencies with in vitro enzymatic assays. As predicted, sodium-glucose cotransporter 2 (SGLT2) inhibitors (gliflozins), sucrose and flavonoids activate LCAT. This has intriguing implications for the mechanism of action of gliflozins, which are commonly used in the treatment of type 2 diabetes, and for the endogenous activation of LCAT. Our results underscore the potential of molecular dynamics simulations in rational drug design.

Analysis of N- and O-linked site-specific glycosylation by ion mobility mass spectrometry: State of the art and future directions

Glycosylation, the major post-translational modification of proteins, significantly increases the diversity of proteoforms. Glycans are involved in a variety of pivotal structural and functional roles of proteins, and changes in glycosylation are profoundly connected to the progression of numerous diseases. Mass spectrometry (MS) has emerged as the gold standard for glycan and glycopeptide analysis because of its high sensitivity and the wealth of fragmentation information that can be obtained. Various separation techniques have been employed to resolve glycan and glycopeptide isomers at the front end of the MS. However, differentiating structures of isobaric and isomeric glycopeptides constitutes a challenge in MS-based characterization. Many reports described the use of various ion mobility-mass spectrometry (IM-MS) techniques for glycomic analyses. Nevertheless, very few studies have focused on N- and O-linked site-specific glycopeptidomic analysis. Unlike glycomics, glycoproteomics presents a multitude of inherent challenges in microheterogeneity, which are further exacerbated by the lack of dedicated bioinformatics tools. In this review, we cover recent advances made towards the growing field of site-specific glycosylation analysis using IM-MS with a specific emphasis on the MS techniques and capabilities in resolving isomeric peptidoglycan structures. Furthermore, we discuss commonly used software that supports IM-MS data analysis of glycopeptides.

An LC-MS-based designated comparison method with similar performance to the Lp(a) reference measurement procedure to guide molar Lp(a) standardization

BACKGROUND

The 2022 consensus statement of the European Atherosclerosis Society (EAS) on lipoprotein(a) (Lp(a)) recognizes the role of Lp(a) as a relevant genetically determined risk factor and recommends its measurement at least once in an individual's lifetime. It also strongly urges that Lp(a) test results are expressed as apolipoprotein (a) (apo(a)) amount of substance in molar units and no longer in confounded Lp(a) mass units (mg/dL or mg/L). Therefore, IVD manufacturers should transition to molar units. A prerequisite for this transition is the availability of an Lp(a) Reference Measurement Procedure (RMP) that allows unequivocal molecular detection and quantification of apo(a) in Lp(a). To that end an ISO 17511:2020 compliant LC-MS based and IFCC-endorsed RMP has been established that targets proteotypic peptides of apolipoprotein(a) (apo(a)) in Lp(a). The RMP is laborious and requires highly skilled operators. To guide IVD-manufacturers of immunoassay-based Lp(a) test kits in the transition from mass to molar units, a Designated Comparison Method (DCM) has been developed and evaluated.

METHODS

To assess whether the DCM provides equivalent results compared to the RMP, the procedural designs were compared and the analytical performance of DCM and RMP were first evaluated in a head-to-head comparison. Subsequently, apo(a) was quantified in 153 human clinical serum samples. Both DCM and RMP were calibrated using external native calibrators that produce results traceable to SRM2B. Measurement uncertainty (MU) was checked against predefined allowable MU.

RESULTS

The major difference in the design of the DCM for apo(a) is the use of only one enzymatic digestion step. The analytical performance of the DCM and RMP for apo(a) is highly similar. In a direct method comparison, equivalent results were obtained with a median regression slope 0.997 of and a median bias of - 0.2 nmol/L (- 0.2%); the intermediate imprecision of the test results was within total allowable error (TEa) (CVa of 10.2% at 90 nmol/L).

CONCLUSIONS

The semi-automated, higher throughput, LC-MS-based method for Lp(a) meets the predefined analytical performance specifications and allowable MU and is hence applicable as a higher order Designated Comparison Method, which is ideally suited to guide IVD manufacturers in the transition from Lp(a) mass to molar units.

VLDL Biogenesis and Secretion: It Takes a Village

The production and secretion of VLDLs (very-low-density lipoproteins) by hepatocytes has a direct impact on liver fat content, as well as the concentrations of cholesterol and triglycerides in the circulation and thus affects both liver and cardiovascular health, respectively. Importantly, insulin resistance, excess caloric intake, and lack of physical activity are associated with overproduction of VLDL, hepatic steatosis, and increased plasma levels of atherogenic lipoproteins. Cholesterol and triglycerides in remnant particles generated by VLDL lipolysis are risk factors for atherosclerotic cardiovascular disease and have garnered increasing attention over the last few decades. Presently, however, increased risk of atherosclerosis is not the only concern when considering today's cardiometabolic patients, as they often also experience hepatic steatosis, a prevalent disorder that can progress to steatohepatitis and cirrhosis. This duality of metabolic risk highlights the importance of understanding the molecular regulation of the biogenesis of VLDL, the lipoprotein that transports triglycerides and cholesterol out of the liver. Fortunately, there has been a resurgence of interest in the intracellular assembly, trafficking, degradation, and secretion of VLDL by hepatocytes, which has led to many exciting new molecular insights that are the topic of this review. Increasing our understanding of the biology of this pathway will aid to the identification of novel therapeutic targets to improve both the cardiovascular and the hepatic health of cardiometabolic patients. This review focuses, for the first time, on this duality.

Human lipocalins bind and export fatty acids through the secretory pathway of yeast cells

The activation of fatty acids to their acyl-CoA derivatives is a crucial step for their integration into more complex lipids or their degradation via beta-oxidation. Yeast cells employ five distinct acyl-CoA synthases to facilitate this ATP-dependent activation of acyl chains. Notably, mutant cells that are deficient in two of these fatty acid-activating (FAA) enzymes, namely, Faa1 and Faa4, do not take up free fatty acids but rather export them out of the cell. This unique fatty acid export pathway depends on small, secreted pathogenesis-related yeast proteins (Pry). In this study, we investigate whether the expression of human fatty acid-binding proteins, including Albumin, fatty acid-binding protein 4 (Fabp4), and three distinct lipocalins (ApoD, Lcn1, and Obp2a), could promote fatty acid secretion in yeast. To optimize the expression and secretion of these proteins, we systematically examined various signal sequences in both low-copy and high-copy number plasmids. Our findings reveal that directing these fatty-acid binding proteins into the secretory pathway effectively promotes fatty acid secretion from a sensitized quadruple mutant model strain (faa1∆ faa4∆ pry1∆ pry3∆). Furthermore, the level of fatty acid secretion exhibited a positive correlation with the efficiency of protein secretion. Importantly, the expression of all human lipid-binding proteins rescued Pry-dependent fatty acid secretion, resulting in the secretion of both long-chain saturated and unsaturated fatty acids. These results not only affirm the in vitro binding capabilities of lipocalins to fatty acids but also present a novel avenue for enhancing the secretion of valuable lipidic compounds. Given the growing interest in utilizing yeast as a cellular factory for producing poorly soluble compounds and the potential of lipocalins as platforms for engineering substrate-binding specificity, our model is considered as a powerful tool for promoting the secretion of high-value lipid-based molecules.

Integration of Web-Based Tools to Visualize, Integrate, and Interpret Glycogene Expression and Glycomics Data

Glycosylation is the most abundant and diverse post-translational modification occurring on proteins. Glycans play important roles in modulating cell adhesion, growth, development, and differentiation. Changes in glycosylation affect protein structure and function and contribute to disease processes. Therefore, understanding glycosylation patterns is key for the identification of targets for the diagnosis of diseases, cellular states, and therapy. Glycosylation is a non template-driven process governed by the action of numerous enzymes and substrate availability that varies among cell types and species. Therefore, qualitative and quantitative assessment of global glycosylation and individual glycans remains challenging because it requires integration of multiple complex data types. Glycan structure and quantity data are often integrated with assessments of gene expression to aid contextualization of observed glycosylation changes within biological processes. However, correlating glycogene expression to the glycan structure is challenging because transcriptional changes may not always concur with the final gene product; there is often a lack of information on nucleotide sugar pools, and the final glycan structure is the result of many different glycogenes acting in concert. To overcome these challenges, interactive online tools are emerging as key resources for facilitating the analysis and integration of glycomics and glycogene expression data. Importantly, these tools work in concurrence with glycan biosynthetic schemes and therefore provide a clear indication of the molecular pathways where the glycan and glycogene are involved. In this chapter, we describe the applications of four freely available online tools that can be used for integrated visualization, interpretation, and presentation of RNAseq and glycomics results.

Developing an SI-traceable Lp(a) reference measurement system: a pilgrimage to selective and accurate apo(a) quantification

In the past decade a remarkable rebirth of serum/plasma lipoprotein(a) (Lp(a)) as an independent risk factor of cardiovascular disease (CVD) occurred. Updated evidence for a causal continuous association in different ethnic groups between Lp(a) concentrations and cardiovascular outcomes has been published in the latest European Atherosclerosis Society (EAS) Lp(a) consensus statement. Interest in measuring Lp(a) at least once in a person's lifetime moreover originates from the development of promising new Lp(a) lowering drugs. Accurate and clinically effective Lp(a) tests are of key importance for the timely detection of high-risk individuals and for future evaluation of the therapeutic effects of Lp(a) lowering medication. To this end, it is necessary to improve the performance and standardization of existing Lp(a) tests, as is also noted in the Lp(a) consensus statement. Consequently, a state-of-the-art internationally endorsed reference measurement system (RMS) must be in place that allows for performance evaluation of Lp(a) field tests in order to certify their validity and accuracy. An ELISA-based RMS from Northwest Lipid Research Laboratory (University of Washington, Seattle, USA) has been available since the 1990s. A next-generation apo(a)/Lp(a) RMS is now being developed by a working group from the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC). The envisioned apo(a) RMS is based on the direct measurement of selected proteotypic fragments generated after proteolytic digestion using quantitative protein mass spectrometry (MS). The choice for an MS-based RMS enables selective measurement of the proteotypic peptides and is by design apo(a) isoform insensitive. Clearly, the equimolar conversion of apo(a) into the surrogate peptide measurands is required to obtain accurate Lp(a) results. The completeness of proteolysis under reaction conditions from the candidate reference measurement procedure (RMP) has been demonstrated for the quantifying apo(a) peptides. Currently, the candidate apo(a) RMP is endorsed by the IFCC and recommendations for suitable secondary reference materials have been made in a recent commutability study paper. Ongoing efforts toward a complete apo(a) RMS that is listed by the Joint Committee on Traceability in Laboratory Medicine (JCTLM) are focused on the peptide-based calibration and the establishment of a network of calibration laboratories running the apo(a) RMS in a harmonized way. Once completed, it will be the holy grail for evaluation and certification of Lp(a) field methods.

A Novel ELISA for the Quantification of Serum Levels of 2 Glycosylated Variants of Apolipoprotein J: Biomarkers for Myocardial Ischemia

BACKGROUND

Previous studies have pointed out a potential role of ApoJ-Glyc as a biomarker of cardiac ischemia. The aim of this study was to validate the analytical performance of 2 novel ELISAs against 2 different glycosylated ApoJ variants, ApoJ-GlycA2 and ApoJ-GlycA6.

METHODS

The analytical measuring range, limit of blank (LoB), lower limit of quantification (LoQ), precision, accuracy, recovery, cross-reactivity, and stability were evaluated in serum samples.

RESULTS

The analytical measuring range was 500-16 000 ng/mL for ApoJ-GlycA2 and 125-4000 ng/mL for ApoJ-GlycA6, with a LoB of 455 ng/mL and 121 ng/mL for ApoJ-GlycA2 and ApoJ-GlycA6, respectively. The LoQ was 500 ng/mL for ApoJ-GlycA2 and 125 ng/mL for ApoJ-GlycA6. The assay performance fulfills the acceptance criteria established in the European Medicines Agency Guideline on bioanalytical method validation. Specifically, the calibration range variability is <15% for ApoJ-GlycA2 and ApoJ-GlycA6; the accuracy is <15% for ApoJ-GlycA2 and ApoJ-GlycA6; the between- and within-run precision is <15% for ApoJ-GlycA6 and ≤20% for ApoJ-GlycA2; and the total allowable error is <30% for ApoJ-GlycA2 and ApoJ-GlycA6. Cross-reactivity studies revealed the absence of cross-reactivity with endogenous components of the matrix (using ApoJ-depleted serum), with nonglycosylated ApoJ and with transferrin (as a high abundant N-glycosylated serum protein). Both ApoJ-GlycA2 and ApoJ-GlycA6 measurements were stable after storage of serum samples at -80°C for 24 months.

CONCLUSIONS

The newly developed ELISAs to quantify ApoJ-GlycA2 and ApoJ-GlycA6 serum levels showed an acceptable analytical performance according to European Medicines Agency guidelines on bioanalytical method validation in terms of precision, accuracy, recovery, cross-reactivity, and stability.

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.