Introduction
Bladder urothelial carcinoma (BUC) is the ninth most common cancer with high mortality worldwide [
1]. Timely surgical treatment or radiotherapy has become the standard therapeutic approaches for patients with localized and non-muscle-invasive BUC. However, these treatments are often insufficient in recurrent or distant metastatic diseases, especially for muscle-invasive BUC, which usually forms micro-metastases [
2]. Hence, systemic chemotherapy has been concurrently employed to control BUC and alleviate symptoms.
Cisplatin-based chemotherapy is the first-line treatment for most cancers, including bladder cancer, breast cancer, and colorectal cancer. Although this treatment achieved an ideal reduction in the risk of bladder cancer-induced death, the overall response rate is less than 50% in clinical settings [
3]. It has been suggested that the complex histological subtypes, genomic effects, and acquired cisplatin resistance might attenuate its efficiency [
4‐
6], and thus, there is substantial room to improve the therapeutic response. Baltazar et al. found that overexpression of CD147 and monocarboxylate transporter 1 might weaken cisplatin sensitivity in bladder cancer [
7]. Other groups indicated that silencing of the pyruvate kinase M2 in vitro could overcome cisplatin resistance [
8,
9]. Unfortunately, the molecules that associated with the efficacy of cisplatin remain unclear [
10]. Therefore, there is an essential need to further identify the underlying mechanisms of cisplatin resistance.
Genome-wide analyses using the clustered regularly interspaced short palindrome repeat-associated nuclease Cas9 (CRISPR-Cas9) system is an emerging new tool with high efficiency and flexibility, which is an ideal choice for investigating gene functions [
11,
12]. The CRISPR library has been reported to target thousands of genes and enable negative or positive alteration screening both in vivo and in vitro. Thus, it could be used to evaluate the specific relationship between genomic variants and drug response [
13]. For example, Xu et al. demonstrated that ELP5 might be a candidate gene responsible for gemcitabine sensitivity in gallbladder cancer cells in the CRISPR screen [
14]. James et al. found that loss of MSH2 promoted cisplatin resistance in BUC cells using CRISPR screening [
15].
In order to understand the underlying principle of cisplatin sensitivity in BUC, we investigated genes associated with cisplatin response in T24 cells using CRISPR screen. The results showed that HNRNPU inhibition weakened the tolerance of cisplatin treatment by promoting cisplatin-mediated cell cycle arrest and apoptosis, in addition to reducing cell migration. Furthermore, we identified that the loss of HNRNPU regulated the interphase chromosome structure of multiple genes, which might be associated with the drug response in BUC.
Materias and methods
Cell culture
Human bladder cancer cells (T24, RT4, HT1197, SW780, RT112, HT1376) and human embryonic kidney HEK293T cells were obtained from ATCC. T24 and HEK293T were maintained in RPMI 1640 medium. The SW780, HT1197, HT1376, and RT112 cells were maintained in EMEM. RT4 cells were maintained in DMEM. All culture media were supplemented with 10% fetal bovine serum (Gibco), 100 mg/mL penicillin, and streptomycin solution (Thermo).
Chemotherapeutic drug cytotoxicity assay
The bladder cancer cell lines were seeded in 96-well plates at 5,000 cells per well and incubated overnight to allow cell attachment. Different concentrations (0–100 μM) of cisplatin, paclitaxel, or doxorubicin were added to the designated wells and the plates were incubated for 3 days. After incubation, 10 µL CCK-8 solution was added to each well and incubated at 37 °C for 2 h, the light absorbance of each sample was then measured at 450 nm using a microplate reader (Biotek). The IC50 values were calculated using GraphPad Prism 8.1.
CRISPR screening
The sgRNA library was obtained from Addgene. HEK 293 T cells were plated on 10-cm dishes to clone the lentiCRISPR vector and the control sgRNA (sgCtrl), following the protocol of GeCKO v2. Next, T24 cells were infected with slow virus at 0.4 functional multiplicity of infection (MOI) and selected by 2 µg/mL puromycin treatment for 48 h after transfection. Subsequently, 2 × 106 transduced T24 cells were treated with 0.25 µM cisplatin or an equal volume of dimethylformamide (DMF) for 7 or 14 days. Finally, the cells were collected for subsequent genomic DNA analyses.
Genomic DNA was isolated using the TIANamp Genomic DNA Kit (Tiangen, China), and amplified by PCR, according to the manufacturer’s protocol. DNA fragments were selected with 2% agarose gels and subjected to HiSeq2500 (Illumina) for sequencing. The data were then analyzed using the MAGeCK software. The candidate genes in the DMF group were defined by a fold change < 2 or > -2 and Bayes facto r < 0. For the cisplatin-treated group, the gene was set at a fold change < -2 and Bayes factor > 2. The KEGG and BioCarta databases were used to determine the gene ontology (GO) and pathway analyses, respectively.
High-content screening (HCS) analysis
To identify the effect of mixed sgRNAs, transfected T24 cells were seeded into 96-well plates at a density of 2000 cells per well, and then treated with 0.6 µM cisplatin or an equal volume of DMF for 5 days. To examine the function of single sgRNA of one gene, 1 µM cisplatin or an equal volume of DMF was used to treat cells for 5 days. The cells were then incubated with 1:1000 Annexin V Alexa Fluor 488 to measure the percentage of apoptotic cells, 10 nM TMRE to examine the mitochondrial membrane potential, and 5 µM Draq5 to determine the nuclear morphology according to a previous study [
16]. Finally, the cells were evaluated using the opera QEHS high-content screening system (PerkinElmer, USA). The images were collected and analyzed using the Acapella software (version 2.0; PerkinElmer, USA).
Real-time quantitative PCR (RT-qPCR)
Total RNA was isolated with TRIZOL Reagent (Sigma-Aldrich) and reverse transcribed using a PrimeScript RT Master Mix kit (Takara, Japan) according to the manufacturer’s protocol. mRNA expression was determined by RT-PCR. The 2
−ΔΔCT method was used to quantify the mRNA expression. The primer sequences are listed in Supplementary Table S
1.
Western blot
The total protein lysates were prepared, subjected to SDS-PAGE, and transferred to PVDF membranes. To quantify the expression level of HNRNPU, membranes were probed with anti-HNRNPU antibody (catalog# 34,095, Cell Signaling Technology Inc., Danvers, MA). The loading control GAPDH was probed by anti-GAPDH antibody (Thermo Fisher Scientific Inc., Waltham, MA). Rabbit IgG HRP conjugates (Cell Signaling Technology Inc., Danvers, MA) were used as secondary antibody. To visualize the bands, ECL detection reagents were used (Thermo Fisher Scientific Inc., Waltham, MA). The protein band densitometry was measured using ImageJ software (NIH, MD).
Cell proliferation assay
T24 cells were seeded in 96-well plates at 5000 cells per well. After treatment with 1 µM cisplatin or DMF for 1, 2, 3, 4, and 5 days, cell proliferation was examined using the Cell Counting kit-8 (CCK-8, Dojindo, Japan) following the manufacturer’s protocol. In brief, cells were collected at different time points and a 10 µL CCK-8 solution was added to each well. After incubation at 37 °C for 2 h, the light absorbance of each sample was measured at 450 nm using a microplate reader (Biotek).
Flow cytometry
Cells were plated onto 6-cm dishes at 105 cells per dish. After treatment with 1 µM cisplatin or DMF for 5 days, the cells were collected. For the apoptosis assay, cells were incubated with 10 μg/mL RNase A for 20 min, followed by treatment with 50 μg/mL propidium iodide (PI) solution for 20 min. Subsequently, the cells were stained with 10 μL Annexin V-FITC reaction reagent for 20 min. All the processes were performed at room temperature in the dark. For the cell cycle assay, the cells were fixed in 70% ethanol at -20 °C overnight after cisplatin treatment, followed by incubation with 50 μg/mL propidium iodide (PI) and 10 μg/mL RNase A at 37 °C for 30 min. Finally, the cells were analyzed using a flow cytometer (BD Biosciences), according to the manufacturer’s instructions.
Cell apoptosis assay
After transfection, the cell culture supernatant of each experimental group was collected in a 5-mL centrifuge tube, washed once with D-Hanks. The cells were then trypsinized, the culture supernatant was terminated, and the cells were collected in the same 5-mL centrifuge tube. Cells were centrifuged at 1500 rpm for 5 min, the supernatant was discarded, and the cells were washed three times in PBS. The final density of the cell suspension was 1 × 106—1 × 107 cell/mL. A 1 × staining buffer was used to resuspend the cell pellet; 5 µL Annexin V-APC was used for staining, and cell pellets were transferred to a flow cytometer for analysis.
Trans well assay
A total of 1.5 × 105 cells were plated on the upper chamber of the transwell system, with 200 μL serum-free medium. Culture medium (800 μL) supplemented with 10% FBS was added to the lower chamber. After 12 h, the non-migrating cells were carefully removed using a cotton bud, and the migrated cells were fixed with 4% paraformaldehyde and incubated with 0.1% crystal violet. Cells were counted under a light microscope.
ATAC-sequencing
After different treatments, the T24 cells were collected and incubated with 25
\(\mu\) L transposase reaction solution (2 μL Tn5 transposase and 12.5 μL of TD buffer) at 37 °C for 1 h. Subsequently, the reaction was stopped by adding 25 μL EDTA solution. Next, the DNA fragments were enriched by PCR. The library was amplified using the following protocol: 72 °C for 5 min, 98 °C for 1 min, 15 cycles of 98 °C for 10 s, 63 °C for 30 s, and 72 °C for 1 min. Finally, the fragments were sequenced using a HiSeq2500 (Illumina, USA). Peak calling was analyzed using MACS2 [
17,
18].
ATAC-seq data analysis
Raw ATAC-seq fastq data were first cleaned using Cutadapt (3.4) to trim low-quality and adapter sequences. The cleaned sequence data was then aligned to the human hg19 genome reference using bowtie2 (2.4.4) in the ‘very-sensitive’ mode. Samtools (0.1.19) was then used to extract uniquely mapped alignments of high quality. To call ATAC-seq peaks, MACS2 (2.0.9) was used with the ‘nomodel’ option. The ‘shift’ and ‘extsize’ parameter of MACS2 was set as 100 and 200, respectively. The identified peaks were then annotated to the nearest genes by the homer (4.8). Pathway enrichment and motif analysis were performed using the online tools Enrichr and MEME-Suite, respectively.
TCGA cohort analysis
The TCGA bladder cancer cohort was used for analysis. UALCAN, CbioPortal, and GEPIA were used for pan-cancer analysis and expression analysis of the HNRNPU gene [
19,
20]. Promoter region methylation analysis was conducted using the UALCAN software. Survival and co-expression analyses were performed using the GEPIA web interface. GEPIA2021, along with the bioinformatics tools CIBERSORT, EPIC, and quanTIseq, was used to generate immune cell expression profiles of HNRNPU using the TCGA bladder cancer cohort.
Microarray assay
The control cells (sgCtrl) and sgHNRNPU-transfected cells were treated with 1 µM cisplatin or DMF for 5 days. Total RNA from each group was isolated using the RNeasy Mini Kit (Qiagen), following the manufacturer’s protocol. Next, the samples were labeled and hybridized using the Affymetrix GeneChip Human Genome U133 Plus 2.0 Array, according to the manufacturer’s protocol. The data were analyzed using GeneSpring 12.6 software as previously described [
21].
Tumor xenograft model
T24 cells (1 × 107 cells) with or without HNRNPU knockdown were suspended in 200 μL PBS and subcutaneously injected into each flank of 4–6-week-old BALB/c nu/nu female mice. The tumor volume was measured every 4 days. The mice were sacrificed after 28 days. Cisplatin was administered by tail injection at a dose of 2 mg/kg every other day. After 4 weeks, the mice were euthanized, the tumor was isolated, and the weight of the tumor was measured. Animal experiments were approved by the Xuzhou Medical University Animal Care and Use Committee.
Statistical analysis
All experiments were independently repeated at least three times. The data were analyzed using GraphPad Prism 8.0 software, and shown as the mean ± standard deviation (SD). Two-tailed Student’s t-test was performed to compare the differences between two groups. One-way analysis of variance was performed to analyze the differences among multiple groups. Statistical significance was set at P < 0.05.
Discussion
Cisplatin-based chemotherapy is a mainstay of BUC management. Previous studies have shown that DNA is a crucial target of cisplatin. Once activated, cisplatin can bind to the N7 reactive center, leading to DNA damage in cancer cells, inhibiting cell division, and inducing cell death [
28,
29]. Unfortunately, cisplatin resistance is a common trait in cancer treatment. Moreover, some patients were not benefited from cisplatin chemotherapy because of the substantial side effects, such as hepatotoxicity and cardiotoxicity [
28‐
30]. Hence, identifying the essential genes that associated with cisplatin response is important. Previous studies have shown that genome-wide CRISPR screening combined with NGS can be a useful tool to investigate potential genes or proteins to guide cisplatin treatment [
15,
31]. In this study, we found that HNRNPU is involved in the response of bladder cancer to cisplatin, but not doxorubicin or paclitaxel in vitro. A low level of HNRNPU significantly improved cisplatin sensitivity compared to the other genes.
HNRNPU, which plays an essential role in cell survival against DNA damage, prompted DNA double-strand break repair, DNA-end resection, and ATR-dependent signaling via homologous recombination [
32]. Down-regulation of HNRNPU expression led to inhibition of cell proliferation and was associated with good prognosis in cancer patients. In addition, Zhao et al. reported that this gene might promote cisplatin-induced apoptosis and increase drug sensitivity in lung squamous cell carcinoma [
33]. HNRNPU is a possible marker for the diagnoses of multiple cancers, including pancreatic ductal adenocarcinoma, nasopharyngeal carcinoma, gastric cancer, and clear cell renal cell carcinoma [
34‐
37]. Here, we identified that HNRNPU was significantly associated with cisplatin sensitivity using CRISPR screening. In a panel of bladder cancer cell lines, a significant correlation was found between HNRNPU protein level and the cytotoxicity of cisplatin. Downregulation of HNRNPU is profoundly related to cisplatin chemosensitivity. Furthermore, we demonstrated that knockout of HNRNPU in combination with cisplatin treatment induced apoptosis, S-phase arrest, and inhibited cell migration in a cisplatin-resistant bladder cancer cell line. By analysis of the TCGA bladder cancer cohort, we also found that high HNRNPU is negatively correlated with patient survival, indicating its negative effect on cancer treatment. In a validation analysis of another cohort of bladder cancer (PMID: 23,897,969), the association between high HNRNPU and poor survival outcome was also observed, though the statistic result is not significant, which may be due to the limited case number (Supplementary Fig.
7).
In order to demonstrate the mechanism by which HNRNPU is linked to cisplatin sensitivity, we performed ATAC-sequencing and microarray assays on the HNRNPU KO and control cells. The resulting data showed that the interphase chromosome structure of the majority of genes was downregulated when HNRNPU expression was inhibited in T24 cells. Among them, most genes showed gene expression, transcription, and focal adhesion. Interestingly, from the microarray data of HNRNPU-depleted T24 cells after cisplatin treatment, we observed that cisplatin significantly modified the response to DNA damage, and hence influenced cell cycle arrest and apoptosis. Our results demonstrated that HNRNPU is involved in sensitization to cisplatin by regulating the ATM and DNA double-strand break repair pathways.
We further investigated the downstream genes regulated by HNRNPU, which mediates the chemosensitivity of HNRNPU. CCK-8 assay with the expression profile assays revealed that only knockdown of NF1 or overexpression of ERBB3 could reverse the effect of HNRNPU knockout on cisplatin sensitivity. NF1 is a tumor suppressor that regulates RAS via modulating GTPase activity [
38]. Mymryk et al. reported that cisplatin inhibits chromatin remodeling and transcription factor binding of NF1 in mice [
39]. In melanoma and lung cancer, loss of NF1 conferred chemoresistance to cancer cells through inhibition of kinases and the RAS signaling pathways [
26,
40]. In the present study, we found that HNRNPU knockout induced upregulation of NF1, and knockdown of NF1 reversed the effect of HNRNPU knockout on enhancing chemosensitivity, indicating that the effect of HNRNPU was mediated by regulating NF1 expression. However, how HNRNPU determines NF1 activity is not fully understood. The regulatory axis of HNRNPU and NF1 warrants further study. Moreover, the role of ERBB3 was not verified because of the failed overexpression of ERBB3 protein in cells, and the relationship between ERBB3 and HNRNPU should be explored in the future.
Acknowledgements
The authors acknowledge the supports from Shao-yuan Wu, School of Life Sciences, Jiangsu Normal University, Shanghai KR Pharmtech, Inc. , Ltd. and Shanghai Genechem Co., Ltd. We also appreciate the assistance received in data processing and consultation from Intanx Life (Shanghai) Co. Ltd. We would like to thank UNIWINSCI. INC (New York) for editing the manuscript.
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