Identification and quantification of proteins from primary TNBC tissue samples
Twenty-four TNBC patients, ranging in age from 35 years to 65 years, fulfilled the clinical criteria for inclusion in this study. The clinical characteristics of these patients are summarized in Supplementary Table S1. Histologically normal tissues adjacent to the tumor tissues were used as a control here. Representative images of H&E staining presented tumor tissues and adjacent non-tumor tissues from different stage of TNBC patients (Supplementary figure 1a). Microscopic tumor foci from TNBC patients with stage III were indicated by yellow arrows, lymphocytic infiltration was detected surrounding tumor foci, and tumor angiogenesis was found to support the rapid growth of tumor foci (Supplementary figure 1b). A representative case of TNBC tissue from stage II showed the absence of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) expression via IHC staining (Supplementary figure 2).To obtain the proteomic profile, the paired tumor and corresponding para-tumor tissues from 24 TNBC patients were divided into four groups and trypsin into peptides respectively (Figure 1a), 12 clinical specimens from grade I-II tumor tissues were pooled and labeled with iTRAQ115, while specimens from the corresponding para-tumor tissues were pooled and labeled with iTRAQ114. As for grade III patients, 12 specimens from tumor tissues were pooled and labeled with iTRAQ117, while specimens from the corresponding para-tumor tissues were pooled and labeled with iTRAQ116. Four group iTRAQ-labeled peptides were combined and separated by high pH reversed-phase chromatography, then analyzed via LC-MS/MS approach. Scatter plots exhibited good reproducibility of the repeat experiments with the same samples (Supplementary figure 3a). A total of 5,401 protein groups were identified and quantified through off-line high-pH fractionation followed by triplicate low pH LC-MS/MS runs (1% FDR rate on both protein and peptide level). Venn diagram depicted the number of identified proteins for the three replicates (Supplementary figure 3b). In detail, 865 proteins changed in patients with Grade I or II TNBCs, among which 309 were up-regulated (fold change tumor/para-tumor ≥ 1.5, p < 0.05) and 556 were down-regulated (fold change tumor/para-tumor ≤ 0.67, p < 0.05). For patients with Grade III TNBCs, 359 proteins were increased and 672 proteins were decreased. Volcano plots in R were utilized to visualize differential expressions with proteomic results (Figure 1b). Venn diagram comparing the number of differential expressed proteins (DEPs) quantified in different stage of TNBCs was presented in Supplementary figure 3c. Gene ontology (GO) analysis for cellular component of all the changed proteins interpreted that both up-regulated and down-regulated proteins significantly enriched in cytoplasm as well as extracellular space and region (Figure 1c). Differentially expressed proteins were listed in Supplementary Table S2&S3.
Differentially expressed proteins of tumor and para-tumor tissues from TNBCs
GO enrichment analyses of the DEPs were performed using clusterProfiler. The top 15 most significant GO terms (p < 0.05) in down-regulated and up-regulated proteins were shown in Figure 2a. The color represented the p-adjusted values for these terms, and brighter red was more significant. The size of the plot displayed the significant genes by gene ratio from Supplementary Table S4 (# genes related to GO term / total number of sig genes). The protein activation cascade was the most significant enriched GO term, followed by acute inflammatory response, complement activation, and so forth in down-regulated proteins from both Grade I-II and Grade III TNBCs. As for up-regulated proteins, ER to Golgi vesicle-mediated transport and antimicrobial humoral response were significant enriched in Grade I-II TNBCs, while SRP-dependent cotranslational protein targeting to membrane and establishment of protein localization to endoplasmic reticulum were significant enriched in Grade III TNBCs (Figure 2a). IPA functional analysis revealed the top 20 canonical pathways in which DEPs participated. Canonical pathway analysis in tumor vs. para-tumor tissues from grade I-II TNBCs showed that the significant pathways (|z-score| >2) were metabolic process (LXR/RXR activation and FXR/RXR activation) or immunity-related (acute phase response signaling, complement system and GP6 signaling pathway) (Figure 2b). Figure 2c displayed the top 20 canonical pathways enriched from grade III TNBCs, among which coagulation system, EIF2 signaling and mTOR signaling were activated (z-score >2).
Functional analyses of differentially expressed proteins from TNBCs
DEPs were further analyzed by KEGG enrichment analyses. DEPs from grade I-II TNBCs were significantly associated with PPAR signaling pathway (p=3.47e-05), PI3K-Akt signaling pathway (p=4.51e-04) and tyrosine metabolism (p=2.32e-03) (Figure 3a). Whereas genes related to ECM-receptor interaction (p=1.39e-12), focal adhesion (p=3.19e-06), PPAR signaling pathway (p=6.04e-04) and glutathione metabolism (p=8.14e-04) were significantly enriched from DEPs in grade III TNBCs (Figure 4a).
Upstream analyses of DEPs with different stage were run by IPA respectively. The most statistically significant transcription factors were quickly prioritized and then visualized in networks. Transcription regulators including SMARCA4, NEUROG1, CPXM1 and SOCS1 were predicted inhibition while MAP3K7, IL27, HOXD10 and TFEB were predicted activation in tumor vs. para-tumor tissues from grade I-II TNBCs (Figure 3b). Regulators including TP53, BTK, BCL6, SMAD3 and MAPK1 were predicted inhibition while TLR9, NUPR1, JUN and IFNL1 were predicted activation in tumor vs. para-tumor tissues from grade III TNBCs (Figure 4b).
Next, we took a glance at the changed proteins when compared tumors from grade III with tumors from grade I-II TNBCs. KOG function classification of the significant changed proteins was predicted and categorized into 20 subcategories, indicating cell cycle control, amino acid transport, lipid metabolism, protein turnover and so on were involved when TNBC progression (Supplementary figure 4a). Upstream analysis of the changed proteins from tumor tissues with different stage was performed by IPA. Transcription regulators including SOCS1, SMARCA4, SAFB, NEUROG1, CPXM1 and CD3 were predicted inhibition while TFEB, SPDEF, RARB, RABL6, MAP3K7, IL27, IL15, HOXD10 and CXCL12 were predicted activation in grade III vs. grade I-II tumor tissues (Supplementary figure 4b). The upstream regulators shared partial overlap when we checked the list of regulators which were altered using DEPs in tumor vs. para-tumor tissues from grade I-II or grade III TNBCs.
Comparison analyses of differentially expressed proteins from TNBCs
Further cross-dataset comparison analyses were used to delineated trends and similarities in the different datasets, as well as differences in their effects on signaling pathways. Cross-dataset comparison analyses demonstrated that death receptor signaling was activated in grade I-II TNBCs while inhibited in grade III TNBCs. For grade I-II TNBCs, the up-regulated levels of death ligands BID, TBID and CYCS mediated the activation of caspase-9, which then accelerated apoptosis by activating other caspases (Figure 5a). Conversely, MKK4/7, CASP8/10 and XIAP were down-regulated in tumor tissues compared with para-tumor tissues for grade III TNBCs, led to NF-κB signaling pathway inactivation and apoptosis signal transduction inhibition (Figure 5b). This finding highlighted the important molecular events involved in TNBC progression.
Human immune system fights against tumor and sends various immune cells to the tumor tissue to induce antitumor immune response. Macrophages are the most common type of tumor-infiltrating immune cells. Here we conducted IHC experiments to examine the expression of CD68 from TNBC tissues with different stage. TNBC tissues from stage III showed more CD68-positive macrophages infiltrations surrounding tumor foci compared with TNBC tissues from stage I-II (Supplementary figure 5).
Additionally, networks developed by IPA integrated DEPs when compared tumor with para-tumor tissues from TNBC patients. For grade I-II TNBCs, IPA top score networks were predicted, one showed numerous immune related genes around two central hubs (NF-κB and Creb), the other with three central hubs (APOA1, CAV1 and P38 MAPK) associated with lipid metabolism and molecular transport (Supplementary figure 6a). For grade III TNBCs, one with two central hubs (CASP8 and CYCS) associated with inflammatory disease, and the other was focused on 30 DEPs also associated with lipid metabolism (Supplementary figure 6b). IPA top score networks indicated inflammatory disorder and abnormality of lipid metabolism were involved in different stage of TNBCs.