Tissue samples
FFPE HCC xenograft samples were collected after sacrificed by isoflurane euthanasia from our previous research[11]. In our last study, BALB/c nude female mice (6–8 weeks, 10–25 g) were purchased from Beijing Vital River Laboratory Animal Technology Co., Ltd. All the animals were housed in an environment with a temperature of 22 ± 1 ºC, the relative humidity of 50 ± 1% and a light/dark cycle of 12/12 hr. Mice in group A were regarded as the negative control, which was injected with PBS. Mice in group E, which were injected with NET-1 siRNA-conjugated targeted nanobubbles and irradiated with low-frequency ultrasound, showed the best antitumor curative effect and the most extended median survival. The group A was renamed group 1, and group E was renamed group 2 to distinguish between two studies. Three pieces of FFPE samples in each group were selected for proteomic analysis.
Total Protein Extraction
The FFPE HCC xenograft samples were dewaxed with octane and then hydrated with graded ethanol. After hydration, the sample was washed twice with PBS. After removing the PBS solution, an appropriate amount of protein lysate (4% SDS, 100 mM Tris, pH = 8.5) was added and incubated at 95℃ for 10 minutes at room temperature, mixed by shaking, and sonicated in an ice-water bath for 5 minutes. The samples were de-crosslinked with a refractive index at 95℃ for 60 min, then reduced by adding an appropriate amount of TCEP and carboxyamidomethylated in CAA at 95℃ for 5 min. The samples were sequentially centrifuged at 12000 g at 4℃ for 15 min. Collecting the supernatant and adding four times volume of pre-cooling acetone at -20℃, and precipitated it at -20℃ for at least 4 h. Centrifuging at 12000 g for 15 min at 4℃. Collect the precipitate and air drying. An appropriate amount of protein solution (6 M urea, 100 mM TEAB, pH = 8.5) was added to dissolve the protein pellet.
Trypsin Treatment
The protein solution was added to flat membrane ultrafiltration (cut off molecular is 10 kDa) tube and was centrifuged at 14,000 g at room temperature for 20 minutes, and the flow-through was discarded. 100 µL of 50 mM TEAB was added, and the sample was centrifuged at 14000 g at room temperature for 20 minutes. The washing procedure was repeated four times. 100 µL of 50 mM TEAB and an amount of 1:50 mass ratio of trypsin were added to the protein and incubated at 37℃ overnight. After being centrifuged at 14000 g for 20 min, an equal volume of 2% formic acid was added. After mixing, the solution was centrifuged at 14,000 g for 20 min at room temperature. The supernatant of flow-through was slowly passed through a C18 desalting column, and then 1 mL washing solution (0.1% formic acid, 4% acetonitrile) was added to wash three times in succession, then 0.4 mL of eluent (0.1% formic acid, 75% acetonitrile) was added to elute twice in sequence, the eluent samples were combined and freeze-dried.
Lc-ms/ms Analysis
Mobile phase A (100% water, 0.1% formic acid) and B solution (80% acetonitrile, 0.1% formic acid) were prepared. The lyophilized powder was dissolved in 10 µL of solution A, centrifuged at 15,000 rpm for 20 min at four ℃, and 1 µg of the supernatant was injected into a home-made C18 Nano-Trap column (2 cm × 75 µm, three µm). Peptides were separated in a home-made analyticalcolumn (15 cm × 150 µm, 1.9 µm) using linear gradient elution, as listed in Additional file 1: Supplemental Table 1. The isolated peptides were analyzed by the Q Exactive series mass spectrometer (Thermo Fisher), with ion source of Nanospray Flex™(ESI), spray voltage of 2.3 kV, and ion transport capillary temperature of 320 °C. Full scan range from m/z 350 to 1500 with resolution of 60000 (at m/z 200), an automatic gain control (AGC) target value was 3 × 10 6, and a maximum ion injection time was 20 ms. The top 20༈40༉ precursors of the highest abundant in the full scan were selected and fragmented by higher-energy collisional dissociation (HCD) and analyzed in MS/MS, where resolution was 15000 (at m/z 200), the automatic gain control (AGC) target value was 5 × 104, the maximum ion injection time was 45 ms, a normalized collision energy was set as 27%, and intensity threshold was 2.2 × 104. The dynamic exclusion parameter was 20 s. The raw data of M.S. detection was named as ".raw".
Data analysis
The identification and quantitation of protein
The resulting spectra from each fraction were searched separately against the homo sapiens uniprot database by the search engines: Proteome Discoverer 2.2 (PD 2.2, Thermo). The search parameters are set as follows: mass tolerance for precursor ion was ten ppm, and mass tolerance for production was 0.02 Da. Carbamidomethyl was specified in PD 2.2 as fixed modifications. Oxidation of methionine (M) and acetylation of the N-terminus was specified in PD 2.2 as variable modifications. A maximum of 2 missed cleavage sites was allowed.
The identified protein contains at least one unique peptide with FDR no more than 1.0%. Proteins containing similar peptides that could not be distinguished by MS/MS analysis were identified as the same protein group. Precursor ion was quantified by a label-free quantification method based on intensity. Mann-Whitney Test statistically analyzed the protein quantitation results for proteins whose quantitation significantly different between experimental and control groups were defined as differentially expressed proteins (DEP).
The Functional Analysis Of Protein And Dep
Gene Ontology (GO) and InterPro (IPR) analysis were conducted using the interproscan-5 program against the non-redundant protein database (including Pfam, PRINTS, ProDom, SMART, ProSiteProfiles, PANTHER)[27], the databases of Clusters of Orthologous Groups (COG) and Kyoto Encyclopedia of Genes and Genomes (KEGG) were used to analyze the protein family and pathway. The probable protein-protein interactions (PPI) were predicted using the STRING-db server[28] (http://string.embl.de/). The enrichment pipeline was used for enrichment analysis of GO, IPR, and KEGG[29].
Statistical analysis
All data are presented as mean ± standard error of the mean (SEM). Statistical analysis was performed by unpaired, two-tailed Students t-test using the GraphPad Prism 8.0 software (GraphPad Software) if not denoted otherwise. Differences were considered statistically significant at p-value < 0.05. Mann-Whitney Test statistically analyzed the protein quantitation results. Proteins were supposed to be significantly differentially expressed when the p-value < 0.05, Fold Change (FC) ≤ 0.05 or p-value < 0.05, FC ≥ 2.0.