Background
Ovarian cancer has the highest mortality rate among all gynecologic malignancies [
1], with a 5-year survival rate of < 40% [
2,
3]. Epithelial ovarian cancer (EOC) is the most common subtype, accounting for 90% of all ovarian cancers. Patients with EOC are usually given chemotherapy, and the most popular drugs include platinum drugs, cisplatin, and carboplatin [
4,
5]. Yet, while most EOC tumors initially respond well to platinum-based therapy, about 75% of patients experience disease relapse due to high therapeutic resistance [
6‐
8]. In order to improve the prognosis of EOC patients, it is of urgent importance to elucidate the underlying mechanisms that promote platinum resistance in EOC.
Estrogen can induce the growth of cancer cells through multiple pathways, including estrogen receptor (ER)-mediated pathways [
9]. Sulfation is the main pathway for estrogen metabolism [
10]. Sulfation of active estradiol (E
2) forms inactive estradiol sulfate, which can be reactivated following desulfation by estrogen sulfatase. 3'-phosphoadenosine 5'-phosphosulfate (PAPS) synthase (PAPSS) catalyzes the biosynthesis of PAPS, which serves as the universal sulfonate donor compound for all sulfotransferase reactions [
11]. In humans, PAPSS exists in two isoforms: PAPSS1 (3’-phospoadenosine 5’-phosphosulfate synthase 1) and PAPSS2(3’-phospoadenosine 5’-phosphosulfate synthase 2) [
12,
13]. PAPSS1 is localized to the nucleus, while PAPSS2 is found in the cytoplasm [
14,
15]. PAPSS1 sequentially synthesizes the biologically active sulfate form, the substrate for cell sulfonation reactions.
Sulfonation has been largely overlooked in the context of oncology. Recent evidence has suggested that the sulfonation pathway may contribute to carcinogenesis and patient survival. For example, Xu et al
. demonstrated that the overexpression of SULT1E1 and PAPSS1 can block estrogen-stimulated cell proliferation in MCF-7 breast cancer cells [
16]. Also,
PAPSS1 has been suggested as a candidate HCC-susceptibility gene and correlated with poor survival in patients with familial or early onset hepatocellular carcinoma (HCC) [
17,
18]. Alterations in the
PAPSS2 have been associated with bone development diseases, hepatocellular carcinoma, and estrogenic hormone disorder [
19]. A lower expression of PAPSS2 has been correlated with worse survival in patients with colon cancer [
20]. Moreover, recent studies suggested that silencing of PAPSS1 can enhance cisplatin activity in non-small cell lung cancer; also, PAPSS1 expression was negatively correlated with survival rate in patients receiving platinum-based chemotherapy [
21,
22]. However, studies on PAPSS association with cancer are still in their infancy, especially studies assessing PAPSS enzymes with platinum-based chemotherapy. Also, the exact mechanisms of action remain unclear.
EOC is characterized by DNA repair defects [
23], especially the homologous recombination repair (HRR) deficiency. HRR-deficient tumors frequently originate from hormone-enriched tissues, such as breast and ovarian tissue [
24,
25]. It has also been found that estrogen increases genome instability affecting HRR in estrogen receptor-positive (ERα +) EOC cells [
26]. Platinum functions through exacerbating DNA damage; these drugs are considered DNA damage-inducing drugs, which might disrupt the DNA repair pathway, increase reactive oxygen species, and ultimately lead to DNA damage-dependent apoptosis/cell death [
27,
28]. The participation of estrogen and ERs in the development of chemoresistance to cisplatin is observed in multiple cancer types, including breast cancer, non-small cell lung cancer and ovarian cancer [
29‐
31]. Contrary, some studies showed that estrogen decreases resistance to cisplatin in vitro [
32,
33]. Therefore, a better understanding of the contribution of estrogen and ERs to the emergence of resistance to cisplatin in EOC will enable us to identify targets in this pathway in order to restore sensitivity to cisplatin chemotherapy.
In the current study, we determined the therapeutic value of targeting PAPSS1 as a cisplatin modulator in vitro and in vivo by testing the effects of PAPSS1 gene knockdown on cisplatin activity in EOC cells. To understand the interaction of PAPSS1 and ERα on a molecular level, we investigated the expression and their correlation in vitro.
Discussion
To the best of our knolwdge, this is the first study that revealed a relationship between PAPSS1-mediated sulfation and ERα signaling in cisplatin resistance and that PAPSS1 may have an important role in regulating cisplatin resistance in EOC. There have been significant research interests in the clinical impacts of hormone receptors on ovarian cancer concerning both patients’ survival and drug responsiveness. However, the effect of estrogen signaling on EOC platinum-resistance remains controversial. Some studies found that ER activation by estrogen and cisplatin can induce platinum-resistance by increasing the expression of an anti-apoptotic protein [
35,
36]. On the other hand, some other studies demonstrated that estrogen signaling enhances the sensitivity of ovarian cancer cells to chemotherapy agents [
37,
38]. Sulfonation, the main pathway for estrogen metabolism, is commonly associated with the metabolism of xenobiotics that inactivate drugs by increasing their water solubility and biological activity [
39]. This modification is also partially responsible for drug resistance to chemotherapy in cancer treatments [
40]. Given the biological role of PAPSS1, which synthesizes the biologically active form of sulfate, one can speculate on the role of sulfur metabolism and homeostasis in cancer cells when they are first exposed to cytotoxic agents [
41]. Thus, studies on PAPSS1 and drug resistance are still in the early stage and thus, more research is required.
In this study, we found that PAPSS1 was highly expressed in ovarian cancer compared to normal tissue and was also upregulated in ovarian cancer cisplatin-sensitivity or resistance cells (A2780 and SKOV3) than in normal ovarian cells (HOSEpic). CCK-8 assay, colony formation assay, apoptosis and cell cycle analysis further revealed that the knockdown of PAPSS1 inhibits cell proliferation and survival, promotes apoptosis, and increases the number of cells in replicating S phase. Next, we found that the knockdown of PAPSS1 may enhance DNA damage in the presence of low doses of cisplatin and down-regulate HRR DNA repair protein BRCA1. Previous studies have shown that drug transporters, such as multidrug resistance-associated protein 1 (MRP1), influence the sensitivity of cancer cells to chemotherapy [
42,
43]. In this study, we found a decreased expression of MRP1 in PAPSS1-silenced EOC cells. Furthermore, we demonstrated that suppression of PAPSS1 expression inhibits tumor progression by enhancing the in vivo sensitivity of A2780 and SKOV3 cells to cisplatin. Similarly, Leung et al
. found that PAPSS1 knockdown sensitizes non-small cell lung cancer ( (NSCLC) cells to cisplatin in vivo [
21]. Altogether, these data suggest that the effects achieved when cisplatin is combined with PAPSS1 silencing are highly synergistic in EOC cells.
The present study further investigated the clinical value and significance of PAPSS1. The results of the Spearman chi-square test (Table
1) indicated that the high nucleus PAPSS1 was positively associated with the FIGO stage, histological subtype, platinum resistance, metastasis and recurrence in patients with ovarian cancer. Based on the present findings, we propose that PAPSS1 is a relevant oncology target in ovarian cancers and provides a novel strategy for ovarian cancer treatment.
Studies show that the hormone receptor, especially ER, is significantly associated with improved OS in patients with EOC [
44]. In postmenopausal patients with advanced-stage HGSOC, a poorer survival outcome was associated with low functional ER pathway activity [
45]. Some studies revealed that ER-positive breast cancers receiving anti-hormone and/or chemotherapy might lose their ER expression, which in turn leads to the disease's evolution to higher aggressiveness and drug resistance [
46]. At the same time, a recent study demonstrated that depleting ERα in EOC cells up-regulates HRR activity and HRR gene expression [
47]. This study found that both PAPSS1 and ERα are prognostic factors in EOC and are associated with platinum sensitivity. An inverse correlation of the gene expression between
ESR1 and
PAPSS1 was revealed in ROC plotter datasets of ovarian cancer. We also observed an inverse correlation between PAPSS1 and ERα in EOC and that the combination of low PAPSS1 and high ERα expression was associated with a survival benefit in EOC. ER pathway activity is consistent with the previous finding that the regulation of DNA repair activity is strongly associated with outcomes and response to chemotherapy in EOC [
48]. However, the results here provide an alternative explanation by establishing a molecular connection between PAPSS1-mediated sulfation, ERα signaling and DNA repair. Our novel finding that the reduction in PAPSS1-mediated sulfation is indirectly responsible for the impairment of DNA repair mechanisms up-regulates ERα activity and estrogen-responsive gene expression, leaving PAPSS1-silencing EOC cells more sensitive to stimulation by cisplatin.
Hormonal ERa-targeted therapy, such as tamoxifen, fulvestrant, and aromatase inhibitors, prevents disease recurrence and reduces mortality from ERa-positive breast cancer. However, the positive response to ERa-targeted therapy in ovarian cancer is limited [
49‐
51]. Traditionally, due to the estrogen etiology of ovarian cancer, estrogen replacement is not comprehensively recommended for most patients [
52]. On the contrary, hormone replacement therapy (HRT) benefits the survival of EOC patients who have undergone surgical treatment [
53,
54]. So far, the role of estrogen in EOC is still debated. Our results confirmed that EOC cells had higher ERα and estrogen-responsive gene expression by reducing the expression of PAPSS1 can sensitize tumors to cisplatin. Based on the present findings and previous reports, we hypothesized that ERα might be a PAPSS1-binding partner in EOC cells. Therefore, there is documented interplay between PAPSS1 and ER-signaling in tumorigenesis that may account for cisplatin resistance but also may be exploited for therapeutic development.
Despite our important findings, the present study has several limitations. First, this analysis was limited to two selected EOC cells and one basic anticancer drug. The exact mechanism through which PAPSS1 enhances the activity of the other cytotoxic agents, molecularly targeted drugs, and cancer immunotherapy drugs need to be further explored. Further studies on the precise molecular mechanisms involved would be required to explore this possibility.
Methods
Patients and tissue specimens
In total, 75 specimens of EOC and 31 normal ovarian epithelium tissues from benign tumor patients were analyzed in this study. EOC patients were 36–72 years old (mean age, 49.72 years); 18 cases were stage I-II, and 57 were stage III-IV. Histopathology and tumor grade were determined via pathology. None of the patients had been subjected to chemotherapy or radiotherapy before surgery, and all were treated with systemic platinum-based chemotherapy following surgery with a median follow-up period of 45 months. For experiments on platinum-based chemotherapy resistance, patients with EOC were divided into two groups (platinum-resistant and platinum-sensitive) according to the criteria described below. Patient response to chemotherapy was mainly evaluated according to National Comprehensive Cancer Network guidelines (version 1.2017, ovarian cancer) [
55]: ‘sensitive’ vs. ‘resistant’ disease at 6 months.
Ovarian cancer tissue microarray (HOvaC160Su01) was obtained from Outdo Biotech Co Ltd (Shanghai, People’s Republic of China). Clinicopathological factors, such as age, FIGO stage, histologic grade, tumor size, lymph node metastasis, recurrence and platinum resistance, were collected from the database (
http://www.superchip.com.cn/biology/tissue.html).
This study was approved by the Medical Ethics Committee of the First Affiliated Hospital of Anhui Medical University, and informed consent was obtained from 2017 to 2019.
Sampling
All patient tissues were snap-frozen in liquid nitrogen within 30 min after resection and stored at -80°C. Frozen sections were then analyzed by RT-PCR analysis and immunohistochemistry (IHC).
Reagents
Cisplatin was purchased from Hansoh Pharmaceutical Co. Ltd (Lianyungang, China). E2 was obtained from Sigma-Aldrich (St. Louis, MO, USA).
Immunohistochemistry (IHC)
Tissues with a tumor cell ratio > 60% were directly included in the study. Formalin-fixed, paraffin-embedded OC samples and animal tissue was freshly cut. The sections (4 μm) were then incubated with polyclonal PAPSS1 antibody (1:1500, Abcam) and monoclonal ERα antibody (1:1000, Santa Cruz).
For sections of data analysis, an intensity score represented the average intensity of the positive cells: 0 (none); 1 (weak); 2 (intermediate); and 3 (strong). The proportion and intensity scores were then multiplied to obtain a total score ranging from 0 to 12.
Cell lines and cell culture
Human chemoresistant ovarian cancer cells SKOV3 and chemosensitive ovarian cancer cells A2780 were purchased from Shanghai Huiying Biological Co. Ltd. Human ovarian surface epithelial cells (HOSEpic) were obtained from iCell Bioscience Inc (Shanghai, China). HOSEpic and A2780 were cultured in complete Roswell Park Memorial Institute (RPMI) 1640 medium (Hyclone, USA) supplemented with 10% newborn calf serum (NBCS, Gibco), while SKOV3 were cultured in McCoy’s 5A medium (BI, Israel) supplemented with 10% fetal bovine serum (FBS, Gibco) at 37°C in a humidified atmosphere containing 5% CO2.
SiRNA transfections
Small interfering RNA (siRNA) against the PAPSS1 gene was synthesized by GenePharma (Shanghai, China). The non-targeting si-NC was used as a negative control. Each siRNA was transfected into cells using Lipofectamine RNAiMAX (Thermo Fisher Scientific, USA) according to the manufacturer’s instructions and then incubated for 48 h. Cell transfection efficiency was verified by qRT-PCR and Western blot.
Cell counting kit-8 (CCK-8) assay
Cells were seeded into 96-well plates at a concentration of 1 × 104 cells with 100 μl of medium per well; 5 replicate wells were set at the same time. After cellular adhesion, cells were exposed to a gradually increased concentration (0, 5, 10, 15 and 20 µM, were used for A2780 cell adherence; 0, 50, 100, 150 and 200 µM were used for SKOV3 cell adherence) of cisplatin for 24 h. Then, 10μL of a sterile CCK-8 (Beyotime Biotechnology, China) was added to each well and incubated for another 4 h at 37 °C. The absorbance at 570 nm was determined using a microplate reader (Thermo, USA).
Cells were seeded into 6-well plates at a concentration of 1 × 103 cells per well. After incubation for 14 days, the colonies were fixed with 4% formaldehyde for 15 min, stained with 1% crystal violet for 30 min, washed with distilled water, dried overnight, and counted the next day. Clonal formation rate (CFR) was calculated using the following formula: number of colonies containing > 50 cells(CFR = [(no. of colonies formed/no. of cells seeded) × 100%]).
Flow cytometry
A2780 and SKOV3 cells transfected with si-NC and siPAPSS1 were harvested for 48 h. Apoptosis was induced by cisplatin (2 or 50 μM) for 24 h. The cells were harvested in trypsin and washed twice with cold phosphate-buffered saline (PBS). After centrifugation, the cells were stained using the annexin V-FITC/propidium iodide Apoptosis Detection Kit (KeyGen BioTECH, Nanjing, China), following the manufacturer's instruction. Apoptotic cells were uncovered using flow cytometry (BD Bioscience, USA). For the cell cycle analysis, cells were single-stained with PI with the BD Cycle test plus DNA reagent Kit (BD Biosciences,USA). Data were analyzed using Cell Quest software (BD Biosciences, USA).
Hochest 33342 stainnig
Cells were seeded into a 6-well plate sat a concentration of 1 × 105 cells per well. Then, 24 h after cisplatin treatment, cells were fixed, washed twice with PBS and stained with Hoechst 33342 according to the manufacturer’s instructions (Beyotime Biotechnology, China). Cells were observed under a fluorescence microscope (Olympus, Japan).
Quantitative real-time PCR
Total RNA was extracted from the tissues and cells using TRIzol reagent (Invitrogen, USA) according to the manufacturer’s protocol. The RNA was then subjected to reverse transcription to synthesize complementary DNA (cDNA) using the PrimeScript RT reagent Kit (TaKaRa, Japan). Quantitative real-time PCR was performed using the SYBR Green PCR master mix (TaKaRa, Japan) on the Light Cycler 96 Real-time System (Roche, Switzerland). The following primers were used: PAPSS1, BRCA1, BRCA2, MRP1, MRP2, CCND1, ESR1, and β-actin.The messenger RNA (mRNA) levels were calculated using 2−ΔΔCT and normalized to β-actin mRNA levels.
Immunofluorescence
The immunofluorescence was performed on the fixed cells grown on the round glass coverslips (Thermo Fisher Scientific, USA) in 35 mm cell culture dishes. The cells were incubated with primary antibody against H2AX (Abcam, ab195188,1:50) overnight at 4 °C, followed by rhodamine-conjugated anti-mouse secondary antibodies incubation for 1 h, and DAPI (Beyotime Biotechnology, China) as a nuclear stain. The cells were then examined under confocal fluorescence imaging microscope (TCSSP5; Leica, Mannheim, Germany).
Western blotting
Total cell lines and tissues were harvested with ice-cold PBS and lysed in a lysis buffer containing a protease inhibitor cocktail. The proteins were quantified using BCATM Protein Assay Kit (Pierce, Appleton, USA). The Western blotting was performed according to the standard protocol. The primary antibodies used were: PAPSS1, ERα, CCND1, p-AKT, Bax, Bcl-2, BRCA1, BRCA2, MRP1, MRP2 and GAPDH. The images were detected with the enhanced chemiluminescence system (Tanon, China) and analyzed with a digital imaging system (Tanon).
Lentiviral transfection
LV3 lentiviral siRNA particles targeting human PAPSS1 (target sequences: GTCTGGACATGCTTCCTAA,ACAAGTTTCATATCACCTT, and GATCGATTCTGAATATGAA) were obtained from GenePharma (Shanghai, China). A2780 and SKOV3 cells were transduced using the LV3 lentiviral siRNA starter kit (GenePharma) following the manufacturer’s instructions, and then selected with 1.5 µg/mL puromycin for 14 days. The clone that was isolated, propagated, and eventually used forthe murine xenograft study was derived from transduction with the PAPSS1-target sequence CCCAGUGCACAAUGGACAUTTAUGUCCAUUGUGCACUGGGTT. A non-silencing control cell line (shSCR) was generated in parallel with the PAPSS1-silenced cells. The shRNA-modified cells were used for the murine xenograft studies described below.
In vivo chemosensitivity assay
Female 5-week-old athymic BALB/c mice were purchased from Bioray Laboratories Inc., Shanghai, China. All the animals were housed in a specific pathogen-free environment with a temperature of 22 ± 1 ºC, relative humidity of 50 ± 1%, and a light/dark cycle of 12/12 h. All animal studies (including the mice euthanasia procedure) were done in compliance with the regulations and guidelines of Huazhong Agricultural University institutional animal care and conducted according to the AAALAC and the IACUC guidelines.
A2780 and SKOV3 cells, stably transfected with shSCR and sh-PAPSS1. A2780 and SKOV3 cells were suspended in PBS (5 × 106 cells/mL) and subcutaneously injected into the upper flank of nude mice (150 μl/mouse). Once the tumor reached a mean volume 25 mm, mice were randomly divided into 6 groups (5 mice per group): PBS, DDP, shSCR + PBS, shSCR + DDP, sh-PAPSS1 + PBS and sh-PAPSS1 + DDP. PBS (0.1 ml) or DDP (0.3 or 3 mg/kg) were peritoneally injected into the mice at 4-day intervals, respectively.
Tumor volumes were examined once a week. Seven weeks after modeling, mice were euthanized, and the primary tumors were excised, paraffin-embedded, formalin-fixed, and subjected to H&E and immunohistochemical (IHC) staining analysis for PAPSS1 (1:1500, Abcam) and Ki67 (1:500, Thermo Fisher) protein expression.
ELISA for measurements of E2
Twenty-four hours post-transfection, the A2780 and SKOV3 cells were treated with cisplatin (2 or 50 μM) for 24 h, respectively. The culture supernatants were harvested, and the concentrations of E2 were measured using ELISA kits (RayBiotech,USA) according to the manufacturer's instructions. The dates were measured at 450 nm by an enzyme-linked immunosorbent assay plate reader (Model 680, Bio-Rad, Hercules, CA, USA). Experiments were performed three times independently.
Luciferase assay
Cells were plated at a density of 10 × 104 per well in 6-well plates 24 h before transfection. Each well was transfected with 0.4 μg of ERE-luciferase plasmid using Lipofectamine-2000 transfection reagent (Invitrogen) according to the manufacturer’s instructions. At 24 h posttransfection, the cells were treated with vehicle, E2 (1 nM), cisplatin (2 or 50 μM), and E2 (1 nM) + cisplatin (2 or 50 μM) for 24 h. Cell lysates were harvested 24 h later using MPER extraction reagent (Pierce), and luciferase assays were performed using the dual luciferase assay kit (Promega) according tothe manufacturer’s instructions. The luciferase activities were normalized to Renilla luciferase activity.
Online database analysis
The online website GEPIA (
http://gepia.cancer-pku.cn/) was used to analyze the PAPSS1 and ESR1 expression in ovarian cancer tissues and normal ovarian tissues. The TNMplot (
https://tnmplot.com/analysis/) was used to employ the expression of PAPSS1 in tumor, normal and metastatic tissues of the ovary. The prognostic value of PAPSS1 and ESR1 expression, PFS and OS for treated with platinum-based chemotherapy were performed using the Kaplan–Meier Plotter platform (
http://kmplot.com/). ROC plotter (
http://www.rocplot.org), was used to detect PAPSS1 and ESR1 expression under different platinum responsiveness.
Statistical analysis
For all quantitative analyses, data were analyzed with the SPSS version 21.0 (SPSS, Chicago, IL, United States) and expressed as the means ± SEM. The statistical comparison was carried out with independent samples T set. Spearman chi-square test was used to analyze the relationship between PAPSS1 expression and clinicopathological characteristics. In addition, Kaplan–Meier analysis was performed to assess the differences in survival rates. Each test was two-sided, and P < 0.05 was considered statistically significant.
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