Introduction
Colorectal cancer (CRC) is one of the leading causes of cancer-related deaths worldwide. CRC arises through the accumulation of genetic mutations and epigenetic alterations that result in the transformation of normal colon epithelial cells into adenomas, which may be the precursor of CRC, and ultimate into cancers. Alongside the “normal → adenoma → cancer” pathological progression, numerous molecular alterations have been identified [
1],[
2]. Understanding the underlying molecular mechanisms of CRC might help guide clinical therapies. For example, current clinical guidelines recommend checking the KRAS mutation status before treating CRC patients with EGFR inhibitors.
Human replication factor C (RFC) is a multimeric protein consisting of five distinct subunits that are highly conserved through evolution [
3]. The RFC family functions as clamp loaders that load PCNA onto DNA in an ATP-dependent process during DNA synthesis [
4]-[
7]. In addition, RFCs play an important role in DNA repair activities following DNA damage [
8],[
9]. Among the RFCs, the
RFC4 gene, that encodes the fourth largest subunit of the RFC complex, has been reported by cDNA microarray or meta-analysis to be deregulated in diverse malignancies, including prostate cancer, cervical cancer, and head and neck squamous cell carcinomas [
10]-[
13]. However, the role of RFCs in cancer initiation and progression remains unclear. In the current study, we investigated the expression levels of
RFC4 in CRCs, and determined the potential biological function of RFC4 in CRC.
Materials and methods
Data mining
The expression of RFC4 mRNA in CRC tissues was obtained by The Cancer Genome Atlas (TCGA), Gene Expression Omnibus databases (GEO) and BioGPS database (Biogps.org). For TCGA analysis, we queried The Cancer Genome Atlas [
http://tcga-data.nci.nih.gov/] for colon cancer patients. “Level 3 of Exp-Gene” files from COAD Data Matrix Datasets were downloaded and used to extract mRNA expression for
RFC4. For GEO data analysis, we downloaded and analyzed data publicly available from the GEO [
http://www.ncbi.nlm.nih.gov/geo/; 82 samples from GSE9348 and 105 samples from GSE5206 and 64 samples from GSE8671]. “Series Matrix” Files were used to extract mRNA expression for
RFC4. The genes amplification folds were analyzed and visualized by Graph Prism6 (GraphPad software Inc., San Diego, CA).
Patients and tissue specimens
First, to detect the expression dynamics of RFC4 in CRC, 30 paired freshly frozen specimens and 49 paired formalin-fixed, paraffin-embedded (FFPE) specimens from primary CRC tissues and patient-matched normal colonic tissues were obtained from the 6th Affiliated Hospital of Sun Yat-sen University (Guangzhou, China). Each normal colonic tissue was acquired from a distance of at least 10 cm from the tumor margin. Second, to determine the clinical relevance of RFC4 in CRC, 331 FFPE CRC tumor specimens resected between January 2000 and December 2006 were obtained from the pathology archives at the 1st Affiliated Hospital of Sun Yat-sen University (Guangzhou, China) for tissue microarray analyses. Among the 331 patients, 91 patients (27.5%) had been censored as death and 97 patients (29.3%) had developed distant metastasis or local recurrence after a median follow-up of 73.0 months (range 1-122). None of the patients had received neoadjuvant chemotherapy or radiotherapy, and the status of all samples was confirmed by pathologists after resection. Tumor tissues were staged according to the 7th edition of the Union for International Cancer Control Tumor-Node-Metastasis (TNM) staging system. Written informed consents for using tissue samples for research purposes were obtained from all patients. This study was approved by the Institutional Review Board of the 1st and 6th Affiliated Hospital of Sun Yat-Sen University and all clinical and pathological data of the enrolled patients were collected from the Institutional Review Board approved CRC database, which was maintained by specialists.
Cell lines
The human CRC cell lines, SW480 and DLD1, were obtained from the Culture Collection of the Chinese Academy of Science (Shanghai, China) and cultured in RPMI 1640 media. All media were supplemented with 10% fetal bovine serum, 100 U/ml penicillin, and 100 μg/ml streptomycin. The cells were cultured at 37°C in a humidified atmosphere of 95% air and 5% CO2.
RNA extraction and quantitative PCR
All RNA extractions were performed using the Trizol Reagent (Life Technologies, Carlsbad, CA, USA) according to the manufacturer’s protocols. For first-strand complementary DNA synthesis, total RNA was reverse-transcribed with an oligo-dT primer using the RevertAid™ First Strand cDNA Synthesis Kit (Fermentas, Burlington, Canada). Quantitative PCR (qPCR) reactions were performed with an ABI PRISM® 7500 Fast Real-time PCR System (Applied Biosystems, Foster City, CA, USA) and a SYBR Green qPCR Mix Kit (Takara, Japan). β-actin expression was used as the normalization control. The following temperature profiles were used: initial heating at 95°C for 10 min, followed by 40 cycles of denaturation at 95°C for 15’s, annealing at 60°C for 60’s, and extension at 95°C for 15’s. The primers used were:
RFC4 forward: 5′-GCGGAAACCTGAGGAACGAGCC-3;
RFC4 reverse: 5′-TGGCAGCTACTCCTCGATCCTTG-3;
β-actin forward: 5′-TGGATCAGCAAGCAGGAGTA-3;
β-actin reverse: 5′-TCGGCCACATTGTGAACTTT-3.
Data were analyzed using the 2-ΔΔCt method.
Western blotting
Total cellular proteins were extracted from tissues or cells, separated by SDS-PAGE, and transferred onto a polyvinylidene difluoride membrane (Pall, New York, USA). Membranes were blocked with 5% nonfat milk in 1% Tween-PBS (PBST) and then probed overnight with anti-RFC4 rabbit polyclonal antibody (1:1000, Epitomics, Burlingame, CA, USA) or anti-ACTB antibody (1:1000, Proteintech, Chicago, IL, USA). After three washing steps of 10 min in PBST, membranes were incubated with species-appropriate fluorescently-conjugated secondary antibodies (1:10000 in PBST, LI-COR Biosciences, Lincoln, NE, USA) for 1 h at room temperature. The immunoreactive signals were detected using the two-color fluorescent western blotting Odyssey infrared imaging system (LI-COR Biosciences).
Construction of tissue microarrays (TMAs)
To validate the overexpression of RFC4 and to define the clinical roles of RFC4 in CRC, two TMAs were constructed. The first contained tissue cores of 49 paired specimens of primary CRC tissues and matched normal colonic tissues. The second contained additional spots of 331 resected CRC specimens. For each case, two cores (1 mm diameter) were taken from the selected tumor area, and two additional cores (1 mm diameter) were taken from histologically confirmed normal adjacent colorectal mucosa to construct the TMAs using Tissue Array (Alphelys, MINIPORE, Plaisir, France). The TMAs were performed as we described previously [
14].
Immunohistochemistry (IHC) staining
The TMAs were deparaffinized in xylene and rehydrated with graded ethanol. Sections were then heated in antigen retrieval solution (EDTA, pH 9.0) for 20 min and incubated with 3% H2O2 for 10 min. The sections were then incubated with anti-RFC4 rabbit polyclonal antibody (1:800, Epitomics) at 4°C overnight. The sections were then treated with the secondary antibody for 15 min at room temperature and stained with 3, 3′-diaminobenzidine until brown granules appeared in the membrane, cytoplasm, or nucleus (Dako, EnvisionSystem/DAB-chromogen, Glostrup, Denmark). The sections were counterstained with hematoxylin for 2 min at room temperature. A negative control was employed by exchanging the specific primary antibody with non-immune serum immunoglobulins at a 1:200 dilution.
Evaluation of IHC staining and selection of the optimal cutoff score
The immunoreaction was semi-quantitatively scored by assessing the extent and intensity of nuclear staining. Weak cytoplasmic staining was identified in some cases. The intensity of nuclear staining was scored as follows: negative (0), weak (1), moderate (2), and strong (3). The extent of nuclear staining was graded into quartiles according to the percentage of stained cells in each field: 0-25% (1), 26-50% (2), 51-75% (3) and 76-100% (4). The merged overall score (staining intensity plus staining extent) was then calculated. IHC staining was scored independently by two blinded pathologists to the clinical data (Huang and Fan).
To generate a logical cutoff score for further analysis, the nuclear RFC4 expression scores of 49 paired CRC and matched normal colonic tissues were subjected to the receiver operating characteristic curve (ROC) analysis. The score localized closest to the point at both maximum sensitivity and specificity (0.0, 1.0) was selected as the cutoff score leading to the greatest number of specimens which were correctly classified as tumor or normal.
RFCsiRNA gene knockdown
To knockdown endogenous RFC4 expression, cell lines were transfected with 20 nM of duplexed siRNA (RiboBio, Guangzhou, China) using Lipofectamine® RNAiMAX Reagent (Life Technologies). Duplex siRNAs included the following: RFC4 siRNA #1 (sense 5′-CAACUCAGCUCGUCAAUCAdTdT-3, antisense 3′-dTdTGUUGAGUCGAGCAGUUAGU-5); RFC4 siRNA #2 (sense 5′-GACCAAGGAUCGAGGAGUAdTdT-3, antisense 3′-dTdTCUGGUUCCUAGCUCCUCAU-5); RFC4 siRNA #3 (sense 5′-GAAAGUGAUUACAGACAUUdTdT-3, antisense 3′-dTdTCUUUCACUAAUGUCUGUAA-5). Duplexes of siRNA with non-specific sequences were used as scrambled siRNA. SW480 and DLD1 cells were transfected with RFC4 siRNA or scrambled siRNA according to the manufacturer’s instructions.
Cell proliferation assays
Cells were plated at equal densities in 96-well plates for 24 h and then transiently transfected with the indicated siRNAs (6 biological replicates per condition). Cell proliferation was assessed using the Vybrant MTT Cell Proliferation Assay Kit (Invitrogen) according to the manufacturer’s instructions. Absorbance was measured at 490 nm.
The effect of RFC4 on proliferation was also tested by the 5-ethynyl-2′-deoxyuridine (EdU) cell proliferation assay using the Cell-Light™ EdU DNA Cell Proliferation Kit (RiboBio, Guangzhou, China). Briefly, cells (1 × 104) were seeded in each well of 96-well plates for transfection with RFC4 siRNA or scrambled siRNA. After incubation at 37°C under 5% CO2 for 96 h, 50 μM EdU was added and the cells were incubated for another 2 h. Cells were then fixed with 4% paraformaldehyde and stained with Apollo® Dye Solution for proliferating cells. Nucleic acids in all cells were stained with Hoechst 33342. The extent of cell proliferation was calculated according to the manufacturer’s instructions. Images were taken using a fluorescence microscope (Olympus FSX100). All experiments were performed in triplicate.
Cell cycle analysis
Cells were trypsinized, washed with cold PBS, and fixed overnight in 70% ethanol pre-chilled to -20°C. For the measurement of DNA content, cells were stained with a propidium iodide solution (50 mg/mL propidium iodide, 100 mg/mL RNase A, 0.05% Triton X-100 in PBS) and incubated at 37°C in the dark for 30 min. DNA content was assessed using a BD FACSCanto II flow cytometer (BD Biosciences) with ModFit LT software (Verity Software House, Topsham, ME).
Statistical analyses
Overall survival (OS) time was measured from the date of surgery to the date of death due to any cause, or the date of the last clinical follow-up. Disease-free survival (DFS) time was defined as the time from surgery to the date confirmed local failure, distant metastases, or death due to disease or treatment, whichever occurred first. Kaplan-Meier survival curves were evaluated by the log-rank test. Univariate Cox regression analyses were performed to determine the prognostic significance of individual clinicopathologic variables. Multivariate analysis to test statistical independence and the significance of multiple predictors was performed by stepwise Cox analysis with backward selection using the Akaike Information Criterion. Wilcoxon matched-pairs, signed-rank test were used to assess IHC score within groups (cancer and matched normal tissues). Chi-square tests were used to compare dichotomized RFC4 groups and baseline clinicopathologic factors. Mann-Whitney U test were used to compare continuous variables and categorical variables. The level of significance for all tests was defined as P <0.05. All statistics were performed using SPSS v. 17.0 (SPSS, Inc., Chicago, IL).
Discussion
RFC family members play important roles in eukaryotic DNA replication and the repair of DNA damage. Members of the RFC family may also be involved in several other biological processes such as S-phase checkpoint regulation, sister chromatid cohesion and genome maintenance [
8],[
15],[
16]. Because cancer cells are actively replicating, RFCs are presumably deregulated in cancer. Indeed, this was evidenced by previous studies demonstrating that high expression of RFC family members occurs in diverse types of cancer. For example, amplification of
RFC3 is frequently found in esophageal adenocarcinoma [
17], while RFC2 is overexpressed in nasopharyngeal carcinoma [
18].
The current study showed that RFC4 is overexpressed in CRC and that increased RFC4 expression is significantly associated with poorly differentiated and advanced tumor TNM stage. Furthermore, high RFC4 expression predicts an unfavorable outcome in CRC patients. These findings demonstrate that RFC4 may play an oncogenic role in CRC. The vast majority of published studies on the RFC family have focused on their biological functions. However, the role of RFCs in the development and progression of cancer remains unclear. Thus, we further investigated the relationship between RFC4 and the proliferation of cancer cells. The knockdown of RFC4 expression in CRC cell lines by siRNA resulted in a significant decrease in cell proliferation assessed by both the MTT and Edu assays. This result further supported our contention that RFC4 is involved in DNA replication in cancer cells.
Our data showed that knockdown of RFC4 by siRNA significantly increases the number of CRC cells in the S-phase. This finding is consistent with the effect of RFC4 in hepatocellular carcinoma and breast cancer [
10],[
19] and indicates that RFC4 acts as a sensor in cell cycle checkpoint control. Knockdown of RFC4 contributes to the decrease in intra-S phase checkpoints and allowing more cells with DNA damage to enter the S-phase where they are unable to combine with EdU and subsequently fails to prevent cells with DNA damage from entering mitosis. In addition, RFC4, together with PCNA, is required for large loop DNA repair synthesis [
20]. Other studies demonstrated that RFC recruits DNA polymerase delta to sites of nucleotide excision repair but is not required for the recruitment of PCNA [
21]. Regardless, the DNA damage checkpoint is critical for repair in response to DNA damage caused by some chemotherapeutic agents. Interestingly, a previous study revealed that the downregulation of RFC4 can enhance the cytotoxic effects of doxorubicin and camptothecin in hepatocellular carcinoma cells [
19]. Together, these findings indicate that decreasing RFC4 may sensitize colon cancer cells to DNA damaging agents that are commonly used in chemotherapy.
Acknowledgments
This work was supported in part by the Program of Introducing Talents of Discipline to Universities, the International S&T Cooperation Program of China (ISTCP), State key clinical specialist construction projects, National Natural Science Foundation of China (81201920, 81472257, YL), Guangdong Natural Science Foundation (S2013010013607, YL)and the Medical Scientific Foundation of Guangdong Province (A2011214).
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Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
JX, LF, JP and JW contributed to the study design, manuscript drafting and provided fund for this study. YL, ZY and LW participated in the design of the study. ZY, YL, JC and MH collected the FFPE CRC patient tissues and related clinicpathologic information and performed the statistical analysis. YH and XF carried out the immunohistochemistry assays. All authors read and approved the final manuscript.