Chemicals
Etoposide (MCE, China, Cat#HY-13629), MNNG (MCE, China, Cat#HY-128612), CDDP (MCE, China, Cat#HY-17394), 5-FU (MCE, China, Cat#HY-90006), CBL0137 (MCE, China, Cat#HY-18935A).
Antibodies
WB: The following antibodies were from commercial sources: anti-Cleaved Caspase -3 (CST, USA, Cat#9664, 1:1000); anti-Caspase-3 (ABclonal, China, Cat#A2156, 1:1000); anti-phospho-MLKL (Abcam, USA, Cat#ab196436, 1:1000); anti-MLKL (ABclonal, China, Cat#A5579, 1:1000); anti-FLAG-Tag (ABclonal, China, Cat#AE004, 1:1000); anti-ZBP1 (Adipogen, Switzerland, Cat#AG-20B-0010-C100, 1:1000); anti-GSDME-N-terminal (Abcam, USA, Cat#ab215191, 1:1000); anti-phospho-RIPK1 (CST, USA, Cat#31122, 1:1000); anti-RIPK1 (BD, USA, Cat# 610458, 1:1000); anti-phospho-RIPK3 (CST, USA, Cat#91702, 1:1000); anti-RIPK3 (CST, USA, Cat#95702, 1:1000); anti-Caspase-6 (CST, USA, Cat#9762, 1:1000); anti-Cleaved Caspase-6 (CST, USA, Cat#9761T, 1:1000); anti-γ-H2AX (CST, USA, Cat#9718, 1:1000); anti-H2AX (ABclonal, China, Cat#A11463, 1:1000); anti-phospho-p53(CST, USA, Cat#9284, 1:1000) and GAPDH (HUABIO, China, Cat# ET1601-4, 1:100000).
IF/IHC: The following antibodies were from commercial sources: anti-dsRNA J2 (SCICONS, Hungary, Cat#10010200, 1:200); anti-Z-DNA/Z-RNA [Z22] (Absolute antibody, UK, Cat#T2028A11, 1:200); anti-DDDDK-Tag (ABclonal, China, Cat#AE004, 1:200); anti-ZBP1(Proteintech, China, Cat#No. 13285-1-AP, 1:100)
Plasmids and Lentiviral particles
The mammalian cell expression plasmids of Flag-tagged mouse ZBP1 and Flag-tagged mouse ZBP1-mutZα1α2 (N46A/Y50A/N122A/Y126A) were purchased from Synbio Technologies (Suzhou, China). For generating the knockout cell lines by the CRISPR-Cas9 approach, gRNA sequences were firstly cloned into pLenti-CRISPRv2 vector. The gRNA targeting sequences are as follows:
sgRIPK3 #1 sense: 5′ -CTCTGGGTCCAAGTACGCTA-3′
sgRIPK3 #1 antisense: 5′ -TAGCGTACTTGGACCCAGAG-3′
sgRIPK3 #2 sense: 5′ -GCTCTGGGTCCAAGTACGCT-3′
sgRIPK3 #2 antisense: AGCGTACTTGGACCCAGAGC-3′
sgCasp-6 #1 sense: 5′ -CAGGTTGTCTCTGTCTGCGT-3′
sgCasp-6 #1 antisense: 5′ - ACGCAGACAGAGACAACCTG -3′
sgCasp-6 #2 sense: 5′ -CGTTGGTGCCCCGCCTCTCT-3′
sgCasp-6 #2 antisense: 5′ - AGAGAGGCGGGGCACCAACG-3′
For generating lentivirus, HEK293T cells were co-transfected with pCMV-VSV-G and
pCMV-dr8.2-dvpr and either empty or sgRNA targeted plasmids. After 24 hours, supernatant was collected and this lentiviral preparation was used to infect cells. After 24 hours of infection, cells were selected with puromycin for a further 48 hours.
Cell culture and transfection
All the cell lines used in this study were obtained from the American Type Culture Collection (ATCC, Manassasa, VA). Wild-type (WT), ZBP1-deficient and Zα1α2-mutated mouse dermal fibroblast (MDF) cells were isolated from postnatal day 2 of C57BL/6 mouse skin. All cells were maintained in DMEM culture media supplemented with 10% FBS (v/v), 2 mM of L‐glutamine and 100 U/ml of penicillin/streptomycin. Cells were grown at 37℃ in a humidified atmosphere with 5% CO2 and were harvested in all experiments from exponentially growing cultures.
The plasmids were transfected with Lipo293F™ transfection reagent (Beyotime, China) according to the manufacturer's protocol. After 24 hours, the cell lysates were analyzed by immunoblotting.
Cell treatment and cell death assays
Cells were treated with ETO (50 μM), MNNG (0.5 mM), CDDP (50 μM) or 5-FU (50 μg/mL) for indicated time to induce cell death. If there are any differences, a detailed description will be given in the figure legend. Cell death was examined by propidium iodide (PI) staining. Briefly, cells were trypsinized, collected by centrifugation, washed once with PBS buffer and then resuspended in PBS containing 5 μg/mL of PI. The stained cells were subjected to flow cytometry using a BD FACS Aria II (BD Biosciences, USA). The proportions of PI‐positive cells were quantified with FlowJo™ Software (BD Biosciences, USA).
Immunoblotting
Cell lysates were prepared for immunoblotting analysis using RIPA buffer supplemented with protease/phosphatase inhibitors and PMSF (MCE, Shanghai, China). The RIPA buffer consisted of 10 mM Tris-HCl [pH 8.0], 1 mM EDTA, 0.5 mM EGTA, 140 mM NaCl, 0.1% SDS, 1% Triton X-100, 50 mM NaF, 40 mM glycerophosphate, and 0.1 mM sodium vanadate. Cell lysates were separated by SDS‐PAGE and analyzed by immunoblotting. The proteins were visualized by enhanced chemiluminescence according to the manufacturer's instructions (Tanon, China).
Immunoprecipitation
Cells were harvested and lysed in immunoprecipitation lysis buffer (Beyotime, China, Cat#P0013) with protease/phosphatase inhibitors and PMSF. After centrifugation, the cell lysate supernatants were immunoprecipitated with anti-FLAG magnetic beads (15 μl) (MCE, China, Cat#HY-K0207) at 4℃ overnight with rotation. Following extensive washing with lysis buffer, the proteins bound to magnetic beads were eluted by boiling in 1×SDS sample buffer (30 μl). Finally, the precipitated proteins were subjected to immunoblotting using the specified antibodies. Image J (NIH, USA) software was used for greyscale analysis of western blotting. For DNase I or RNase A treatment, cell lysates were incubated with 25 U/mL DNase I (NEB, USA, Cat#M0303S) or 1 mg/mL RNase A (MCE, China, Cat# HY-129046A) for 1 hour at 4 °C. Subsequent immunoprecipitation and western blotting were then performed.
Immunofluorescence microscopy
For tissue staining, paraffin sections of tissues were baked at 65 °C for 1 hour, and then deparaffinized. Antigen retrieval was performed by boiling the sections in Citrate Antigen Retrieval Solution (Beyotime, China, Cat#P0081) for 20 minutes. For cell staining, cells were plated on 24-well glass slides (JingAnBiological, China, Cat#J24001), and cultured for 24 hrs before use in experiments. Following ETO (50 μM) or CBL0137 (5 μM) treatmente, cells were fixed in 4% paraformaldehyde for 15 min and permeabilized with 0.1% Triton-X100 for 5 min. To detect dsRNA and Z-DNA in cells, cells were subjected to proteinase K treatment (0.008 U/mL) for 20-40 min at 37°C post-fixation. If required, the cells will then be treated with RNase A (1mg/mL), RNase III (1 mg/mL) or DNase I (25 U/ml) for 1 hour at 37°C. Subsequently, cells or tissue sections were blocked with 5% BSA-Ⅴ (Solarbio, China, Cat#A8020) in PBS, and then incubated overnight with primary antibodies at 4°C. After washing with PBS for three time, slides were incubated with fluorophore-conjugated secondary antibodies for 1 hour at room temperature and stained with DAPI for 5 min. Following an additional three washes in PBS, slides were mounted in ProLong Gold antifade reagent (Thermo Fisher Scientific). The staining images were acquired using the confocal laser scanning microscope (Nikon, Japan).
Proximity Ligation Assay
The Proximity Ligation Assay (PLA) was performed using the Duolink® In Situ Red Starter Kit (Sigma, USA, Cat#DUO92101) according to the manufacturer's instructions. Briefly, cells were cultured on confocal culture dishes and then fixed in 4% paraformaldehyde (PFA) for 15 minutes, washed three times, permeabilized with 0.1% Triton X-100 for 5 minutes, and then blocked with Duolink blocking solution for 1 hour at 37°C. Cells then incubate overnight at 4°C with primary antibodies diluted in Duolink antibody diluent. Subsequently, secondary antibodies were applied, followed by ligation and signal amplification steps according to the manufacturer's instructions. The PLA dots were visualized using a Nikon confocal microscope.
Real-time (RT) PCR
To measure relative gene expression by RT-PCR, total cellular RNA was isolated using TRIzol reagent (ThermoFisher, USA). The extracted RNA was reversely transcribed into cDNA using the Strand cDNA Synthesis kit (Transgen, China, Cat#AH321-01) according to the manufacturer’s instructions. Levels of mRNA encoding for RLTR45-int, RLTR1B-int or IL-6 were measured by real-time PCR using PerfectStart® Green qPCR SuperMix (Universal Passive Reference Dye) (Transgen, China, Cat#AQ602-01) in the QuantStudio 7 Flex Real-Time PCR system (ThermoFisher, USA). The ratio for the mRNA was normalized to GAPDH. All reagents, buffers and containers used for RNA work were RNase-free grade, to eliminate RNase contaminants in experiments described in this section and other relevant sections. The primer sequence of RLTR45-int and RLTR1B-int have been reported previously19. The primer sequence are as follows:
RLTR45-int sense: 5′ -CCCCGAGAAGAGGGTCAAAA-3′
RLTR45-int antisense: 5′ -AGCGGGAACTTGGTGGTATC-3′
RLTR1B-int sense: 5′ - AATCCACTGTCTCCTGCGTG-3′
RLTR1B-int antisense: 5′ - CCCCACTCAACTCCCGATTC-3′
mIL-6 sense: 5′ -CAATGGCAATTCTGATTGTATG-3′
mIL-6 antisense: 5′ -AGGACTCTGGCTTTGTCTTTC-3′
mGAPDH sense: 5′ -GTTGTCTCCTGCGACTTCA-3′
mGAPDH antisense: 5′ - GGTGGTCCAGGGTTTCTTA-3′
RNA immunoprecipitation (RIP)
RIP assays were performed following the instructions of the BeyoRIP™ RIP Assay Kit (Beyotime, China, Cat# P1801S). The cell lysates were mixed with magnetic beads bound to the corresponding antibodies, and incubated at 4 °C overnight. RNA was purified with protease K solution and extracted by using RNA isolated Total RNA Extraction Reagent. Subsequent RT-PCR was performed using the primers for RLTR45-int and RLTR1B-int.
Clinical samples
Tumor and matched noncancerous colonic tissues were obtained from the first surgical resection at Xinqiao Hospital. All study participants provided written informed consent and volunteered to participate in the experiment. All samples were collected with the approval of the local ethics Committee and the institutional review board of the hospital. Detailed clinic information of the CRC patients is shown in Supplementary Table II.
Animal experiment
All animal care and experimental procedures complied with the National Institutes of Health guidelines and were approved by the animal care and use committee of Tongji University. Animal studies are reported in compliance with the ARRIVE guidelines39. C57BL/6 mice were purchase from SLAC ANIMAL (Shanghai, China) unless mentioned otherwise. ZBP1-/- and ZBP1mutZα1α2 (N46A/Y50A/N122A/Y126A) mice with C57BL/6 background were generated by Bangyao Biologicals (Shanghai, China) with CRISPR-Cas9 gene editing technology. Mice were bred and housed under SPF conditions in individually ventilated cages with a 12 h light/dark cycle and stable temperature (25 °C). All mice used in the experiments were between the age of 8–10 weeks. We used Quick Genotyping Assay Kit for Mouse Tail (Beyotime, China, Cat#D7283S) to identify the genotype of mice. The genotyping primer sequence are as follows:
ZBP1 sense: 5′ - CCTAAGTGTCCCAGGCCATA-3′
ZBP1 antisense: 5′ -ATTTGCCTGGTCTCCAGATG-3′
To establish mouse models of chemotherapy-induced toxicity, ETO was injected intravenously at a dose of 20 mg/kg; CDDP was injected intraperitoneally at a dose of 25 mg/kg. Saline injection was used in control group. Five days after injection, the mice were sacrificed using CO2 inhalation. The small intestines and lungs were collected for haematoxylin and eosin (H&E) and immunohistochemistry staining.
H&E staining and pathological analysis
The small intestine and lung tissues were fixed in formalin overnight and subsequently embedded in paraffin blocks for sectioning. The tissue sections were then subjected to H&E staining, which facilitated the visualization of the nucleus and cytoplasm, respectively. The pathological images of livers were captured using an upright microscope.
The average number of surviving crypts, villi and neutrophils within the alveolar space were counted in 20 randomly selected fields from each mouse (n=6 per group). Acute lung injury scores were calculated according to previously described method40. Briefly, a 3-point scale was used to score the severity of the lung injury (0=none, 1=medium, and 2=severe); the number of neutrophils in the alveolar and interstitial space (0=none, 1=1-5, and 2=>5), the formation of hyaline membranes (0=none, 1=1, and 2=>1), the proteinaceous debris filling the airspaces(0=none, 1=1, and 2=>1) and the extent of alveolar septal thickening (0=<2x, 1=2-4x, and 2=>4x) were graded. Each grade was then multiplied by a factor and then summed to obtain the final score for each mouse.
For spleen injury assessment, the spleens were firstly weighed and then homogenized. The remaining lymphocytes were counted by hemocytometer after lysing the red blood cells using an ACK (ammonium-chloride-potassium) buffer. The mice's weights were recorded daily for a period of five days.
Immunohistochemistry
For immunohistochemistry analysis, intestinal tissues from mouse or patient were fixed in formalin for a minimum of 24 hours, followed by paraffin embedding and sectioning. The sections were then deparaffinized in xylene, rehydrated, and washed with PBS. Antigen retrieval was performed by boiling the sections in Citrate Antigen Retrieval Solution (Beyotime, China, Cat#P0081) for 20 minutes. Subsequently, the sections were blocked with 10% normal goat serum in PBS at room temperature for 1 hour and stained with dsRNA J2 antibodies (SCICONS, Hungary, Cat#10010200, 1:200) overnight. Signals were then developed using VECTASTIN ABC Elite kit (Vector Laboratories, USA) and DAB Substrate Kit (Vector Laboratories, USA).
Four human tissues, the staining sections were then reviewed and scored as follows: Cell with <25% staining was rated as negative staining (-, 1); cell with 10-49% staining was scored as (+, 2); cell with 50-74% staining was scored as (++, 3); cell with 75-100% staining was scored as (+++, 4). According to the score of staining intensity, 0 point for no positive staining (negative), 1 point for light yellow particles (weak positive), 2 points for brown particles (strong positive), and 3 points for brown particles (strong positive). The final score was defined as staining number score multiplied by staining color score41.
RNA-seq and TE RNA-seq analysis
Total RNA was isolated using RNeasy kit (Qiagen). Purified total RNA was quantified by Qubit (Invitrogen, USA) and analyzed by an Agilent Bioanalyzer to assess RNA integrity. rRNA-depleted RNA was generated using a NEBNext® rRNA Depletion Kit (New England Biolabs, Cat#E6310S). The directional RNA library was constructed with a NEBNext® Ultra II Directional RNA Library Prep Kit for Illumina® (New England Biolabs, Cat#E7760L) according to the manufacturer’s instructions. Library sequencing was performed by Novogene (Tianjin, China) using an Illumina HiSeq 4000 platform.
RNA-seq data processing was performed as described previously42. For TE analysis, the reads were mapped to the mouse genome (mm10) using the STAR aligner (v2.5.4b)43. For bi-directional ERV transcripts analysis, the strand specific RNA-seq fastq data were aligned to mm10 genome using STAR, and then the forward and revers strand were separated by samtools(version 1.17)44 with the -f flag parameter. The counts for each gene or TE family were counted using scTE942. DESeq2 (v1.20.0) was used for data normalization and differential expression analysis45. Differentially expressed TEs were defined by a Benjamini-Hochberg-corrected P value < 0.05 and an absolute fold change > 2 (for genes).
Statistical analysis
The student’s t-test and one-way analysis of variance (ANOVA) were used for comparison among all different groups represented with the mean values ± standard errors. Log-rank (Mantel-Cox) test was performed for survival curve analysis using GraphPad Prism 8 (GraphPad Software, USA). All experiments were repeated at least three times with similar results. p < 0.05 was considered statistically significant.