Background
Head and neck squamous cell carcinomas (HNSCC) are the fifth most common non-skin cancer worldwide and the third most common cancer in developing countries [
1,
2]. HNSCC constitutes up to 90% of all head and neck cancers with an annual incident of 600,000 cases and its overall 5 year survival rate is only 40–50% despite aggressive treatment [
3]. Cisplatin is one of the most common chemotherapeutics being used as a first-line agent in the treatment of HNSCC. Cisplatin exerts its anti-tumor effects through the generation of unrepairable DNA lesions that result in cellular apoptosis via the activation of DNA damage response [
4,
5]. Resistance to cisplatin is a major obstacle to effective cancer therapy because clinically relevant levels of resistance emerge quickly after treatment. Many important signaling pathways, which regulate the expression of genes controlling growth, survival, and chemosensitivity, are involved in development of cisplatin resistance, including mutation or loss of function of tumor suppressor genes such as p53 as well as the over expression, and activation of oncogenic proteins such as HER2, Aurora-A, and members of the BCL-2 family [
3‐
11]. It is essential to improve the efficacy of cisplatin therapy using a mechanism-based approach, so it is urgent to identify the critical molecules and signaling pathways that underlie the development of cisplatin resistance.
B-cell lymphoma 2-associated athanogene-1 (BAG-1), is a multifunctional protein that regulates a variety of cellular processes: proliferation, cell survival, transcription, apoptosis, and motility [
12]. BAG-1 has three isoforms which are produced by the alternative translation initiation of a single mRNA transcript that results in different N-terminus regions. BAG-1 isoforms appear to be differentially localized in cells. BAG-1L is a 50 kDa protein that is localized to the nucleus due to the presence of a nuclear localization signal (NLS). In contrast, a shorter isoform of BAG-1, BAG-1s (36 kDa), exists in the cytoplasm and an intermediate sized isoform, BAG-1M (46 kDa), partitions between the cytoplasm and nucleus via interactions with companion proteins [
13]. Interactions of BAG-1 with various proteins(s)/complexes determines its function in the cell. Well-known interacting partners of BAG-1 isoforms are, BCL-2, Raf-1, Hsc70/Hsp70 system, nuclear hormone receptors (NHR), ubiquitin/proteasome machinery and DNA [
14].
The B-cell lymphoma 2 (BCL-2) protein family is a group of structurally related proteins have opposite functions, and can be classified into two functional subgroups [
15,
16]: Anti-apoptotic proteins including BCL-2, BCL-xL, BCL-W, MCL-1, BCL-B, protect cells from cytotoxic insults such as chemotherapeutic medicine [
17]; Pro-apoptotic proteins, such as BID, BIM, BAD, BAC, BAK. Although BCL-2 protein was investigated in various of cancers apoptosis studies [
18], BCL-xL, a protein encoded by gene BCL2L1, is considered as a more effective marker than BCL-2 [
19].
Currently there are no defined targetable genetic aberrations for HNSCC, and no approved therapies are tied to genetic alterations [
20,
21]. All patients with HNSCC are treated with a largely uniform approach based on stage and anatomic location, typically using surgery, radiation therapy, and chemotherapy alone or in combination [
20,
21]. Cetuximab, an anti-EGFR antibody, is the only approved targeted therapy for HNSCC with a single-agent response rate of 10–13%. Despite the modest response rate, there are no validated predictive biomarkers for benefit from cetuximab therapy [
22,
23].
Previous gene expression studies of other cancers have produced lists of differently expressed genes but have failed to establish how these genes form regulatory networks [
22‐
24]. Systematic examination of datasets for genes and pathways associated with cisplatin-resistant has been limited. Moreover, these studies have ignored genes that do not pass randomly or empirically determined criteria for gene selection. Therefore, we adopted a computational tool, Ingenuity Pathway Analysis (IPA; Ingenuity Systems, Mountain View, CA), to visualize regulatory networks of differentially expressed genes and the corresponding canonical pathways that govern the response to cisplatin treatment.
In this study, we combined microarray technology and IPA, to identify and validate genes with altered expression in cisplatin resistant University of Michigan Squamous Cell Carcinoma (UMSCC) laryngeal cells. We have found that BAG-1 is a gain function gene associated with cisplatin resistance. We also discuss possible individualization of cancer chemotherapy with possible new molecular markers of anticancer resistance.
Methods
Study design
First, we screened UMSCC cell lines for resistance to cisplatin by viability, proliferation by MTT assay. Then we compared resistant and non-resistant cells by gene expression analysis. Next, we confirmed expression of differentially expressed genes in UMSCC cell lines at the protein level by western blot and immunohistochemistry. To further prove our hypothesis, we used the UMSCC cells with three specific inhibitors and siRNA.
Cells and cell culture
UMSCC cells were kindly provided by Dr. Thomas Carey (Department of Otolaryngology/Head and Neck Surgery, University of Michigan, MI). UMSCC cells were maintained in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% fetal bovine serum (FBS), l-glutamine, sodium pyruvate, nonessential amino acids (Life Technologies, Inc.). Adherent monolayer cultures were maintained on plastic plates and incubated at 37 °C in 5% CO2 condition. The cultures were mycoplasma-free and maintained for no longer than 12 weeks after they were recovered from frozen stocks.
Preparation of reagents
Cisplatin was purchased from Sigma-Aldrich and was diluted in phosphate buffered saline (PBS) immediately before each experiment. Propidium iodide and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) were purchased from Sigma-Aldrich. Stock solutions were prepared by dissolving either 0.5 mg of propidium iodide or 2 mg of MTT in 1 mL of PBS. Each solution was filtered, protected from light, stored at 4 °C, and used within 1 month. Bag-1 siRNAs (h) were purchased from Santa Cruz Biotechnology (sc-29211). Specific inhibitors ly29004 (#9901) and U0126 (#9903) were purchased from Cell Signaling, NSC 74859 was purchased from R&D (cat#4655).
Cell proliferation assay
The anti-proliferative activity of cisplatin against UMSCC HNSCC cells in vitro was determined by MTT cell viability assay. Briefly, UMSCC 14A, B and UMSCC 17A, B cells were plated in 96-well plates in medium. After a 24-h attachment period, the cells were incubated for indicated hours in various concentrations of cisplatin or with PBS alone as a control. Cells were then incubated in medium containing 10% FBS and 0.25 mg/mL MTT for 3 h. The cells were then lysed in 100 μL dimethylsulfoxide to release formazan. We used an EL-808 96-well plate reader (BioTek Instruments) set at an absorbance of 570 nm to quantify the conversion of MTT to formazan. The experiment was repeated in triplicate.
Clonogenic survival assay
To determine the sensitivity of the HNSCC cells to cisplatin, we performed a clonogenic survival assay. Cells in culture were treated with the indicated concentrations of cisplatin for 2 h or with PBS alone as a control after which they were cultured for 12 days in medium without cisplatin. The cell medium was changed with fresh new medium every 3 days. At the end of experiment, the cells were stained with 0.5% crystal violet in absolute ethanol, and colonies with more than 50 cells were counted under a dissection microscope.
Agilent microarrays
Total RNA was extracted from each cell line using miRNA Easy Kit (Qiagen, Germantown, MD) and RNA was evaluated using NanoDrop 2000 (Thermo Scientific, Wilmington, DE). Microarray expression experiments were performed on 4 × 44 K whole human genome microarray (Agilent technologies), according to the manufacturer’s instructions, the images of arrays were scanned by using Agilent Scan G2505B and then following the data extraction by using the Feature Extraction Software (Agilent Technologies, GE2-1200_Jun14). Partek Genomic Suite 6.6 (Partek Inc., St. Louis, MO, USA) was used for data visualization, identification of differentially expressed transcripts and hierarchical cluster analysis. Fluorescence intensity data was transformed to log 2 ratios of each sample versus the universal human RNA reference (Stratagene, Santa Clara, CA, USA). Then t-tests were used to identify differentially expressed genes. Analysis of functional pathways was performed by ingenuity pathway analysis (IPA) tool (Ingenuity System Inc., Redwood City, CA, USA). The microarray data had been deposited in National Center for Biotechnology Information Gene Expression Omnibus database GSE102787).
Western blotting
The cultured cells were analyzed by western blotting. UMSCC cells (2 × 10
6 per well) were plated in 100 mm dishes (Costar) in 10 mL medium containing 10% FBS, incubated for 24 h, and then treated with indicated concentration of cisplatin, and PBS as untreated control. Total cell lysates were then obtained and subjected to Western blot analysis as previously described [
25]. The membranes were blocked for 1 h at room temperature with 5% bovine serum albumin in 0.1% Tween 20 in Tris-buffered saline and incubated overnight at 4 °C with anti-BAG-1 (sc-939 1:500), anti-BCL-xL (sc-7195 1:1000), anti-BCL-2 (sc-7382, 1:1000), anti-Akt (Cell Signaling; 1:1000), anti-phosphorylated Akt (Ser473; Cell Signaling; 1:1000), anti-mitogen–activated protein kinase (MAPK, Cell signaling, 1:1000) in 5% non-fat milk in 0.1% Tween 20 tris-buffered saline. Next, the membranes were washed with 0.1% Tween 20 in tris-buffered saline and incubated for 1 h at room temperature in horseradish peroxidase-conjugated anti-rabbit immunoglobulin G (Santa Cruz Biotechnology) to detect EGFR, phosphorylated EGFR, or species-appropriate fluorescently conjugated proteins (goat anti-rabbit IRDye 800 and goat anti-mouse IRDye 800, Invitrogen). The membranes were then analyzed using the SuperSignal West chemiluminescent system (Pierce Biotechnology). To verify equal protein loading, we stripped and re-probed the membranes with anti-GAPDH (sc-47724, 1:5000).
HNSCC tissue array immunohistochemistry for BAG-1 and BCL-xL expression
Immunohistochemical studies of BAG-1, BCL-xL, and phosphorylated AKT (Ser473) were performed on both paraffin-embedded tissue sections from HNSCC tissues arrays (IMH-310), which were purchased from IMGENEX (San Diego, CA). HNSCC tissue arrays contained 58 cases of primary HNSCC and 2 cases of metastatic HNSCC mounted on slides. The tissue arrays were deparaffinized by xylene, and then, re-hydrated with sequential washes of 100%, 75%, 50% ethanol, and PBS. For antigen retrieval, slides were placed in 50 mM Tris–HCl buffer pH 9.0, heated in a decock pressure cooker for 20 min, and then stayed in the buffer for 15 min. Endogenous peroxidase activity was inhibited with 3% hydrogen peroxidase in PBS. Non-specific binding was blocked with 3% normal goat serum for 30 min. Tissue sections arrays were then incubated with anti-BAG-1 or anti-BCL-xL antibodies (Santa Cruz Biotech; Santa Cruz, CA), phosphorylated AKT (Cell signaling) for 1 h at room temperature. Immunodetection was performed using DAB staining systems according to manufacturer’s instructions (ScyTek Laboratories; Logan, UT 84321). All sections were counterstained with haematoxylin. After dehydration with washes of 95 and 100% ethanol and xylene, tissue sections and tissue arrays with permanent mounting medium were covered with glass coverslips, and viewed by light microscope.
Discussion
Patients with HNSCC are treated aggressively with surgery followed by radiation and often with cisplatin [
24]. Although these treatments increase loco-regional control, there are frequently disfiguring and induce high-grade toxicities limiting their effectiveness [
41]. Furthermore, resistance to cisplatin and radiation contributes to tumor recurrence, and options for those who do not respond are limited to palliative care. Targeted therapies for HNSCC are currently limited to experimental agents targeting the EGF receptor [
42]. We used the two pairs of cell lines (each pair of cells were established from the same UMSCC patient) for genome-wide expression analysis to identify cisplatin-resistance candidate genes. Moreover, because previous microarray studies have produced lists of differentially expressed genes [
43] and ignored the genes that did not pass randomly or empirically determined criteria for gene selection, we adopted a computational tool, IPA, to visualize regulatory networks and gene ontology of differentially expressed genes. We identified BAG-1 genes as differentially expressed in the two cisplatin-resistant UMSCC cell lines. We found BCL-xL and AKT were also among the up-regulated genes accompany with BAG-1. This indicates that the ability of HNSCC cells to gain cisplatin resistance is multifactorial and that several mechanisms are encountered simultaneously within the same tumor cells. Therefore, we believe that the genes selected using our microarray approach to be new candidate cisplatin-resistant genes in HNSCC. Tumor response to cisplatin resistance cannot be predicted by one factor and may be determined by a critical balance of expression of several genes. Our selected genes might be helpful in the development of individualized HNSCC chemotherapy. This is likely to have an impact on current clinical practice for eligibility for chemotherapy in patients with HNSCC. The biological basis for the association between high BCL-xL and BAG-1 expression and cisplatin resistance in HNSCC has yet to be determined. Given that both of these proteins are key anti-apoptotic mediators that are part of the mitochondrial (indirect) pathway [
44], it suggested that targeting BAG-1 and/or BCL-xL might be an effective adjuvant therapy in a subset of HNSCCs in future.
The two major forms of cisplatin resistance are intrinsic resistance, in which previously untreated tumor cells are inherently insensitive to the chemotherapeutic agent, and acquired resistance, in which treated tumor cells become insensitive after drug exposure. The various mechanisms of cisplatin resistance have been studied hoping to overcome this major chemotherapeutic obstacle. Research has determined that acquired cisplatin resistance is multifactorial, in that it involves host factors, genetic and epigenetic changes, and numerous molecular events [
45]. Resistance itself may be due to decreased drug accumulation, alteration of intracellular drug distribution, reduced cell-cycle deregulation, increased damaged DNA repair and a reduced apoptotic response [
46]. It was reported that over expression of the multi drug resistant gene (MDR1) is associated with drug-resistant cancer cells. However, little is known about the genes differentially expressed in cisplatin-resistant HNSCC cells [
43]. Recently developed techniques for genome-wide expression analysis hopefully will provide additional information, novel candidate genes associated with cancer drug resistance, and perhaps new therapeutic targets.
The identification of pretreatment molecular markers that can predict response to therapy is of great interest in head and neck oncology and is required to develop personalized treatments that maximize survival while minimizing morbidity. Several studies have been performed on drug sensitivity and drug resistance in untreated human cancer cell lines and drug-exposed cells using a gene expression microarray technologies [
47‐
49]. These studies showed correlations between gene expression and drug activity and the genes differentially expressed in drug-sensitive and drug-resistant cancer cells. In addition, several gene expression microarray studies have been performed to identify genes with altered expression in HNSCC [
45,
50]. From these studies, numerous genes have been associated with the development and progression of head and neck cancer, some of which will be used as novel chemotherapeutic targets to treat or prevent HNSCC.
It had been theorized that each cancer cell represents a different pattern of drug-resistant gene expression signature, even within cells clonally derived from the same cancer, and may be expected to exhibit considerable heterogeneity with respect to drug resistance [
45]. Here, we still suggest that targeting BAG-1 and/or BCL-xL in HNSCCs might improve the therapeutic ration of adjuvant therapy in a subset of HNSCCs.
Authors’ contributions
SL and BR carried out cell culture and western blot; HG, SL and YZ for IHC, XS for siRNA knockdown, PJ and DS for gene array analysis; ZX and QZ wrote the manuscript; YL conceived the study, participated in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.