Protein profiling of OSCC tissue vs NAT lysates
To identify altered biology and candidate biomarkers for OSCC, we performed mass spectrometry-based proteomics of cancer and adjacent normal tissues of 14 patients. For optimal label-free protein comparison, we ensured equal protein amounts loaded on the gels (Additional file 1: Fig. S1). In total, 5,123 proteins were identified (Additional file 2: Table S1). About 3104 proteins were identified per sample. First, to get an overview of altered biology, we used GSEA. This analysis revealed that secretion was the most significant increased pathway and myogenesis the most decreased pathway (Fig. 1A). The latter may indicate a mucosa sample contamination artifact underscoring a focus on cancer-associated proteins.
Up Regulated protein networks associated with OSCC
To study which proteins and pathways were associated with OSCC tissues, we performed differential analysis. A total of 205 up regulated proteins (Additional file 3: Table S2) were significantly differentially expressed (p<0.01, 2-fold up).
To gain further insight into the tumor biology, we generated a protein-protein interaction network for the 205 upregulated proteins in OSSC and found high connectivity with several tightly connected clusters (Fig. 2a, b, Additional file 4: Table S3). The 6 major clusters were enriched for proteins involved in secretory pathway (Cluster 1), spliceosomal complex assembly (Cluster 2), protein localization to endosome (Clusters 3), immunity (cluster 5) and protein biosynthesis (cluster 6).
Importantly, cancer cells are known to exhibit altered protein secretion to create a favorable tumor microenvironment (28). The up regulation of the secretory pathway may indicate that OSCC tissues secrete more proteins compared to normal tissues. Furthermore, secreted proteins may also be used as non-invasive biomarkers, since they can be secreted into biofluids, including blood or saliva. In addition, the enrichment of biosynthesis pathways of aminoacyl tRNA may also be involved in cancer or/and progression. Aminoacyl tRNA synthetases (ARSs) are vital enzymes that activate amino acids to their related tRNA. Beyond ARSs central role in translation, current studies have discovered their non-translational functions and have additionally associated them to the prognosis of cancer (29, 30). For example, in cancer cells inducde by TNF, human lysyl-tRNA synthetase (KARS) is secreted and triggers proinflammatory signaling in immune cells (31). Furthermore, tRNAs can also be secreted, either soluble or via exosomes (32).
Tumor-promoting Transcription factors in oral cancer
To predict the coordinated regulation of the proteins involved in the differential protein secretion and other biological processes associated with the highly connected protein clusters 1 to 6 (Fig. 2a), we analyzed whether specific transcription factors can regulate these proteins (Table 3). Transcription factor binding motifs were predicted in Cytoscape using iRegulon plugin (version 1.3) (26). We used proteins from cluster 1 to 6 as input proteins for motif and track search in the cis-regulatory control elements of genes of those proteins.
Table 3 List of Transcription factors responsible for different biological processes, and the proteins that are identified in the Secretomes of HNSCC cells
Serial. No.
|
Most connected biological network Clusters
|
Target Genes/Proteins
|
Transcription factors by iRegulon
|
Proteins Secreted in cell line HNSCC secretomes
|
1
|
Secretory Pathway
|
KLC1, LMAN2, SEC24C, SEC23A, SEC23B, SEC16A, SEC13, TFG, SEC24D, EML4, COPB1, COPB2, GOLGB1, COPA, LMAN1, SEC23IP, SURF4, COPG1
|
Creb3L1
|
LMAN2, SEC24C, SEC23A, SEC23B, SEC13, TFG, SEC24D, EML4, COPB1, COPB2, GOLGB1, COPA, LMAN1, COPG1
|
2
|
Spliceosomal complex assembly
|
[SF3B1, SNRNP200, SRSF1, SRSF9]
|
ESSRA
|
[SF3B1, SNRNP200, SRSF1, SRSF9]
|
3
|
Protein localization to Endosome
|
[PACSIN2, RAB35, VPS35]
|
YY
|
[PACSIN2, VPS35]
|
4
|
tRNA aminoacylation for protein translation
|
[AARS1, CARS1, EPRS1, LARS1]
|
ELF2
|
[AARS1, CARS1, LARS1]
|
5
|
Type I interferon pathway/viral process
|
[ADAR, EIF2AK2, IFI16, MX1, OAS2, PLSCR1, STAT1]
|
STAT1
|
[ADAR, EIF2AK2, STAT1]
|
6
|
IRE1-mediate unfolded protein response
|
[DNAJB11, DNAJC3, PDIA5]
|
XBP1
|
[DNAJB11, DNAJC3]
|
We found that Creb3/Creb3L1, ESRRA, YY, ELF2, STAT and XBP1 were the central transcription factors that can regulate the highly connected upregulated cluster-proteins. These TFs were previously reported to be upregulated in oral cancer (33-36) (37-40). Targeting these TFs may alter the expression of many proteins.
SecretomeP/SignalP analysis and OSCC tissue and HNSCC cell line secretome protein annotation of subcellular localization
To further explore the process of secretion, we determined which of the deregulated tissue proteins are predicted to have a signal peptide using the SignalP database (27). To gain more insight in the presumed subcellular localization of the proteins, their annotation of subcellular localization was retrieved from the IPA database (Ingenuity® Systems www.ingenuity.com). Interestingly, when looking at the 205 differentially expressed proteins (p < .01) of OSCC tissue vs. NATs, 37% of the proteins were predicted to be secreted via classical or non-classical secretion mechanisms (Fig. 4A). Overall, the proportion of proteins from the different subcellular origins was comparable between the OSCC tissue vs. NATs samples and the proportion of cytoplasmic proteins (̴ 50%) was much higher in OSCC tissues (Fig. 4B,C).
Protein profiling of HNSCC cell line Secretomes and comparison to OSCC tissue
Biological annotation revealed that the secretome pathway was highly enriched in OSCC tissues.
To further explore the potential of the secretome and annotate OSCC-associated proteins as candidate biofluid markers, we performed LC-MS/MS proteomics on the secretomes of HNSCC cell lines. We selected a diverse panel of 09 Head and neck cancer cell lines to capture the complex tumor biology as much as possible. Cancer-associated proteins that are released may be more likely to be detectable in body fluids like saliva (41). Using an in-depth workflow based on gel fractionation coupled to nanoLC-MS/MS (42), we profiled the secretomes of HNSCC cell lines (Table 2). Prior to mass-spectrometry analysis, input quantities were checked by Coomassie-stained SDS-gel (Additional file 1: Fig. S2).
A total of 4472 cancer cell secretome proteins were identified (Additional file 5: Table S4), of which on average ~3500 proteins per sample. Of these, 1724 secretome proteins were identified (˃5 average counts) in all HNSCC secretomes (Additional file 6: Table S5). Overlap analysis of these robustly identified secretome proteins with the differential OSCC tissue proteins revealed 132 promising OSCC proteins as candidate non-invasive markers. Underscoring their value as potential OSCC marker, 131 proteins were also cancer-associated in the large-scale proteomics analysis of OSCC tissue recently reported by Huang et al. 2021 (Additional file 7: Table S6). (6) In our list of most overexpressed protein in OSCC tissue or HNSCC secretomes, transferrin receptor (TFRC) was the most differentially expressed protein in tumors as compared to normal. Human Protein Atlas data expression of TFRC is high in HNSCC and it is also found abundant across different cancers, indicating that this is a common protein involved in multiple cancers. TFRC plays essential role in the cellular uptake of iron. TFRC is related to lysosomes/endosomes, which can be secreted into saliva and blood (Additional file 1: Fig. S3a-c). In previous studies, TFRC expression rate in OSCC was found to be substantially higher than in dysplasia, implying that the progression of OSCC disease may be linked to the expression of TFRC.(43) Interestingly, functional analysis in vitro and in vivo showed that an anti-TFRC antibody blocking the interaction between transferrin and TFRC and consequently inhibiting iron uptake, lead to the suppression of cell growth and induced apoptosis via iron deprivation.(43) Altogether the results of us and others suggest that TFRC may serve as a promising, cancer biology-linked marker for non-invasive diagnostics in OSCC.
Strikingly, only 23% of the 132 overlapping OSCC tissue and HNSCC secretome candidates had a predicted signal peptide in their sequence (Fig. 3a-d, Additional file 8: Table S7). 77% of the proteins did not contain a classical signal peptide but were predicted to be secreted via non-classical routes, as predicted by the SecretomeP algorithm, which is based on other sequence-derived features and their subcellular localization (27). Remarkably, among the proteins significantly more abundant in the HNSCC cell lines secretomes, the proportion of nuclear proteins was 30%. These secretory proteins might serve as non-invasive biomarkers.
Promising candidate biomarker for distinct clinical applications
Non-invasive detection of OSCC would improve its diagnosis. Therefore, we further annotated the 132 promising proteins from our OSCC tissue proteome (p < 0.01; >2 upregulated) that are likely to be secreted (identified in all 9 cancer cell line secretomes with an average abundance of > 5 counts), for their potential use as non-invasive biomarker. To this end, we explored detectability in public proteomics datasets; salivary proteome healthy vs. OSCC dataset by Chu et al., 2019 (7), human salivary proteome by P. Sivadasan et al., 2015 (8) and Normal Saliva Proteome database (https://salivaryproteome.nidcr.nih.gov/) (Additional file 9: Table S8). Importantly, 106 out of our 132 candidates (Fig. 4, Additional file 10: Table S9), potential OSCC biofluid markers were also identified in the saliva proteome, with 25 top candidates detected in all 3 studies (Table 3). Therefore, these proteins may have the potential to be further exploited for developing a non-invasive biomarker test for the early detection or prognosis of OSCC. THBS2, LGALS3BP and DNAJB11 were potentially useful salivary markers for the detection of OSCC as reported previously (41, 44-46).