2.1 Cell culture
Human epithelial type 2 (HEp-2, (ATCC Cat# CCL-23, RRID:CVCL_1906), passage number 9 - 18) cells were grown up to 80 % confluence in DMEM/Ham’s F12 supplemented with 10 % FBS (Biowest, Nuaillé, France), 2 mM L-glutamine (Merck Millipore, Massachusetts, USA) and penicillin/streptomycin (Merck Millipore, Massachusetts, USA) in a humidified incubator at 37 °C and 5 % CO2.
2.2 Participants characteristics and tissue samples
All tissue samples from colorectal cancer patients were taken at Magdeburg University Hospital and are listed in Table 1. A total of 15 patients were included in the study, with randomization designed to ensure representation across a broad spectrum of colorectal cancer classifications.
2.3 Ethics approval
This study was performed in line with the principles of the Declaration of Helsinki. Approval was granted by the Ethics Committee of Medical faculty of University Hospital Magdeburg (33/01, amendment 43/14).
Informed consent was obtained from all individual participants included in the study.
2.4 Tissue sample preparation
Patient samples (~ 50 mg of weight) from tumor and adjacent normal tissue were homogenized on ice in lysis buffer containing 25 mM HEPES (pH 7.5), 75 mM NaCl, 0.5 % Triton X-100, 0.5 % Nonidet P40, 0.5% Na-deoxycholate, 0.1 % SDS, 10 % Glycerol and 1 mM EDTA in the presence of different protease inhibitors (10 mM NaF, 20 mM Na4P2O7, 1 mM PMFS, and 1 μg / ml aprotinin) using a Fastprep-24 5G homogenizer (matrix E-tubes, 3 intervals a‘ 30 sec, acceleration of 8 m / sec) and cleared from nuclei and tissue debris by centrifugation at 10.000 × g for 10 min.
2.5 Antibodies
The following antibodies were used in this study: anti-𝛾H2AX (Cell Signaling Technology, Cat# 2577, RRID:AB_2118010, Massachusetts, USA), anti-N-PSIP1 (Santa Cruz Biotechnology, Cat# sc-101087, RRID:AB_2171222, Texas, USA), anti-UBC13 (Santa Cruz Biotechnology, Cat# sc-376470, RRID:AB_11150503, Texas, USA), anti-mouse-IgGκ BP-HRP (Santa Cruz Biotechnology, Cat# sc-516102, RRID:AB_2687626, Texas, USA).
2.6 Protein determination
Tissue samples from tumor and adjacent non-tumor were homogenized in lysis buffer and protein content was determined by Bradford assay (#ADV01, Cytoskeleton Inc., Denver, USA).
2.7 Sodium Dodecyl Sulfate (SDS) – PolyAcrylamid Gel Electrophoresis
SDS-page was performed using standard protocols 12. Briefly, protein lysate, equivalent to 40 µg protein, was loaded onto an SDS-page with 12 % separation gel. Electrophoresis was performed in an Owl™ system (Thermo Fisher Scientific, Massachusetts, US) at 45V for 90 min, followed by 120 V for 100 min.
2.8 Immunoblotting
Immunoblotting was performed, as described elsewhere 12. Briefly, protein transfer was performed in a Biometra FastblotTM blotting chamber (Analytik Jena GmbH+Co. KG, Jena, Germany) for 2 h at 50 mA / gel. Membranes were afterwards stained with ponceau S-solution (0.5% (w/v)) to verify protein transfer. Membrane was blocked with 2 % milk in TBS-T for 1 h at RT, followed by incubation with primary antibodies for 1 h, RT. Primary antibodies were diluted as follows: anti-N-PSIP1 (1:1000), anti-Ubc13 (1:1000), anti-RPA32 (1:1000) and anti-yH2AX (1:1000)-antibodies were diluted in 2 % BSA / TBS / 0.1 % Tween-20 (Carl Roth, Karlsruhe, Germany), anti-GAPDH (1:10000) antibody was diluted in 2 % milk powder / TBS / 0.1 % Tween-20 (Carl Roth, Karlsruhe, Germany). Secondary antibody (anti-mouse-IgGκ BP-HRP) were diluted 1:5000 in 5 % milk powder / TBS / 0.1 % Tween-20) and incubated for 1 h at RT. The protein bands were visualized using ECL-solution (100 µl of 250 mM Luminol, 100 µl of 90 mM p-coumaric acid, 10 µl H2O2 (30 %) in 20 ml 1 M Tris/HCl, pH 8.5) and intensity was quantified by ImageJ software (1.53c 26). Original images can be found in Supplement 1.
2.9 RNA sequencing
MRNA Sequencing data and related clinical information on colon cancer was obtained from GDC cohort from “The Cancer Genome Atlas Program” (TCGA) 15 research network and can be downloaded directly from xenabrowser (TCGACOAD.htseq_fpkm.tsv). Datasets are also available on request via Microsoft OneDrive. The patients with a lack of follow-up records were excluded. Finally, a total of 521 patients were enrolled in this study, including 41 adjacent samples and 480 cancer samples.
2.10 Statistical analysis
All data were statistically analyzed with the statistical computing language R ≥ 4.2.1 16. For the analysis of mRNA data the packages DSeq and limma were used and applied according to their default settings. The corresponding code is available in Suppl. 2. Adjusted p-values less than 0.05 were considered as significant. Experiments were conducted with at least three biological replicates. For the analysis of band intensity, representing the expression of protein of interest (LEDGF/p75, UBC13, 𝛾H2AX and GAPDH) compared to whole protein amount) one-way analysis of variance (ANOVA) and comparison between two groups (unequal sample size of tumor and non-tumor tissue) was performed by paired student t-test. P-value of lower than 0.05 was considered statistically significant.