3.1. Glycan-related genes expression of HBV-HCC and adjacent tissues
A better understanding of glyco-gene expression changes may facilitate the exploration of association between glycosylation and HBV-HCC. The differences in glyco-gene expression between HBV-HCC and adjacent samples were investigated using an integrated strategy with a combination of transcriptomics, glycomics and glycoproteomics (Fig. 1A). First, GSE135631 and GSE94660 gene expression profiles, including 15 and 21 pairs of HBV-HCC and adjacent tissues respectively, were retrieved from the GEO database. A total of 7502 and 7839 genes were differentially expressed in GSE135631 and GSE94660, as shown in Fig. 1B, in which top 100 differentially expressed glyco-genes (DEGGs) were highlighted (Fig. 1B, Tab. S4). Partial Least Squares Discrimination Analysis (PLS-DA) of these DEGGs exhibited a clear separation between HBV-HCC and adjacent tissues (Fig. 1C). The expression patterns of all DEGGs across HBV-HCC and adjacent tissues from the combined two GSE study were shown as a heatmap (Fig. 1D). Next, functional enrichment analysis revealed that the main molecular function categories were carbohydrate binding, transferring glycosyl and hexosyl group activity (Fig. 1E).
Protein-protein interaction (PPI) network of these DEGGs was constructed in an attempt to explicit the molecular mechanisms in the progression of HBV-HCC. Clustering analysis revealed ten major modules, in which protein glycosylation, GPI anchor biosynthetic process and glycosylation processes were mostly enriched (Fig. 1F; S1). Hub genes with a high degree of connectivity in the PPI network are significantly enriched in the process of fucosylation (FUT4, FUT6, FUT2 and FUT1), GPI anchor biosynthesis (PIGM, PIGV, PIGT and GPAA1), galactosylation (B4GALT3) and galNAcylation (B3GNT3).
Furthermore, FUTs family expression at mRNA levels were determined by RT-qPCR in HBV-HCC and adjacent tissues (Fig. 1G), revealed an elevated level of FUT1, FUT2, FUT4, FUT5, FUT8 and FUT10, as well as a reduced level of FUT3, FUT6, FUT7, FUT9, FUT11 and FUT12 in HBV-HCC tissues. Collectively, these data indicated that changes in glycosylation especially fucosylation were closely correlated to HBV-HCC.
3.2. N-glycan profiles of normal, hepatitis, cirrhosis and HCC serum samples
In our studies, N-glycans in HC, CHB, LC and HBV-HCC serum samples (Tab. S1) were profiled by MALDI-TOF/TOF-MS to reveal abnormal N-glycosylation. Representative MS spectra of N-glycans with signal-to-noise ratios > 5 were displayed and annotated (Fig. 2; Tab. S5). A total of 36 distinct m/z N-glycan structures were identified, with 31, 32, 25 and 25 N-glycans present in HC, CHB, LC and HBV-HCC samples, respectively. There were 21 N-glycans found in all groups but with different intensities.
The expression pattern of identified N-glycans in HC, CHB, LC and HBV-HCC were exhibited in Fig. 3A. Hierarchical clustering revealed that LC/HBV-HCC were clearly separated from HC and CHB, however, LC/HBV-HCC could not be separated from each other. Furthermore, quantitative comparison analysis revealed that 25, 24, and 22 N-glycan structures were differentially expressed in CHB, LC and HBV-HCC versus HC. Of these N-glycans, 11 N-glycans were concertedly down-regulated, and 7 N-glycans were concertedly up-regulated in the progression of HBV-HCC. Notably, 6 of 7 up-regulated N-glycans were fucosylated (Fig. 3A).
N-glycans were classified into three types, including high mannose, complex and hybrid. Relative abundance of hybrid and complex N-glycans were increased in CHB, LC and HBV-HCC versus HC, while which of high mannose N-glycans were decreased (Fig. 3B). Consistent with elevated FUTs expression at mRNA levels, relative abundance of total fucosylation were up-regulated, of which mono-fucosylation levels were increased, while bi-fucosylation levels were decreased (Fig. 3C). Other terminal modification sialylation levels were increased, followed by a decrease in the progression of HBV-HCC (Fig. 3D). Significantly higher levels of bi-antennary structures were found in CHB, LC and HBV-HCC versus HC, while mono-antennary structures was progressively reduced (Fig. 3E). In combination of transcriptomic and glycomic analysis revealed that fucosylation levels were up-regulated in the progression of HBV-HCC.
3.3 Site-specific glycoproteomic profiling in HBV-HCC serum samples
To decode the biological function of specific N-glycosylation especially fucosylation in the progression of HBV-HCC, intact glycoproteomic analysis of HC, CHB, LC and HBV-HCC serum samples (Tab. S2) were performed. A total of 1114 glycopeptides were identified (Fig. 4A; Tab. S6), of which, 1019, 1011, 999 and 982 glycopeptides, including 864 in common were found in HC, CHB, LC and HBV-HCC serum samples, respectively. The identified glycopeptides represented 129 glycosites were modified with 102 glycan structures. These N-glycans contained 8 high mannose, 13 hybrid, 77 complex and 4 paucimannose subtypes (Fig. 4B).
Quantitative comparison analysis revealed that a total of 288 unique glycopeptides from 73 glycoproteins were differentially expressed using the cutoff of fold change > 2 or < 0.5 and p value < 0.05 in CHB, LC and HBV-HCC versus HC (Fig. 4C). Differentially expressed glycopeptides (DEGPeps) were mainly decorated with complex type N-glycans, of which sialylated N-glycans accounted for the largest proportion, followed by fucosylated N-glycans and bi-antennary structures (Fig. 4D).
To understand the association between glycoprotein trajectories and the progression of HBV-HCC, hierarchical clustering for all DEGPeps was performed, with which three cluster (I-IV) were generated (Fig. 5A&B; Tab. S7). Glycopeptides in “cluster I” were continually increased in the progression of HBV-HCC, encompassing 27 proteins. These DEGPeps were significantly enriched in endopeptidase inhibitor activity, acute-phase response, blood microparticle and complement and coagulation cascades by functional enrichment analysis (Fig. 5C). Glycopeptides in “cluster II” were significantly decreased, followed by an increase (39 proteins), and mainly involved in the peptidase regulator activity, complement activation, blood microparticle and complement and coagulation cascades. Glycopeptides in “cluster III” were slightly decreased, followed by an increase (25 proteins), and enriched in regulation of humoral immune response, blood microparticle and complement and coagulation cascades. Glycopeptides in “cluster IV” (31 proteins) were decreased and primarily connected with peptidase regulator activity, acute-phase response, blood microparticle, complement and coagulation cascades. (Fig. S2).
Taking into account the escalating malignancy of HBV-HCC progression, 49 glycopeptides with fucosylation attached in “cluster I” were screened, which were consisted of 23 fucosylated N-glycans and 25 peptides from 21 protiens (Fig. 5D). These fucosylated N-glycans contained three to seven HexNAc, three to eight hexose, up to four fucose, and two sialic acid. Among 21 identified glycoproteins, 16 contained 1 glycosite, 3 contained 2 glycosites, and 1 contained 3 glycosites. Notably, among these glycopeptides, IGHA1-340-N5H5F1S2 was identified with highest PSM score.
3.4 Site-specific fucosylation on IGHA1 and IGHG2
We further mapped N-glycan structures on each glycosite of IGHA1 and IGHG2 based on our glycoproteomics data. A total of 30 and 5 unique intact glycopeptides were identified from IGHA1 and IGHG2, which were comprised 3 glycosites (144N#LT and 340N#VS for IGHA1, 176N#ST for IGHG2) and 31 glycans (Fig. 6A). Among these glycopeptides, fucosylation was accounted for 46% of all glycans on glycopeptides, and the majority of fucosylated intact glycopeptides were up-regulated in the progression of HBV-HCC.
Next, we screened out 8 and 3 site-specific target glycopeptides from IGHA1 and IGHG2 respectively, which were all fucosylated and gradually elevated in the progression of HBV-HCC (Fig. 7A). Consistent with the glycoproteomic results, LCA-based ELISA analysis confirmed the elevated levels of fucosylated IGHA1 and IGHG2 in HBV-HCC serum (Fig. 7B). In summary, combined results of glycoproteomic and ELISA analysis revealed that IGHA1 and IGHG2 are highly fucosylated and fucosylation on IGHA1 and IGHG2 were up-regulated in the progression of HBV-HCC, implied that fucosylated of IGHA1 and IGHG2 might be involved in the initiation and development of HBV-HCC.