Background
Prostate cancer (PCa) is the most common malignant tumour and the leading cause of cancer death in men, with an estimated 1,276,000 new cases and 359,000 deaths worldwide in 2018 [
1]. Owing to improved early diagnosis and advanced therapeutic strategies, the mortalities of PCa have been appreciably decreased. Unfortunately, some PCa patients eventually develop more aggressive malignant forms resistant to traditional radiotherapy and chemotherapy, leading to poor prognosis [
2]. Recently, the mutation rate identified in homologous recombination DNA repair genes showed as high as 20–25% in metastatic castration-resistant prostate cancer (mCRPC) and defects in the repair genes can sensitize PCa cells to Poly(ADP-ribose) polymerase inhibitors [
3]. Similar results were also found in bladder cancer [
4,
5]. Notably, emerging evidence has demonstrated that enhancing the immune response is an effective therapeutic option for improving the survival of cancer patients [
6]. Indeed, tumour-induced immunosuppression plays a pivotal role in tumour evasion from host immune surveillance, which provides a complementary approach to the treatment of tumours using immune checkpoint inhibitors [
7]. Accordingly, multiple studies have focused on blocking immune checkpoints, such as targeting CTLA-4 and the PD-1/PD-L1 axis, which has been shown to restore or strengthen T cell antitumour immunity [
8,
9]. In particular, PD-1/PD-L1 is widely recognized as the best immune checkpoint target based on its therapeutic validation in prospective clinical outcomes, including subsets of solid tumours [
10].
PD-1, an important T cell co-inhibitory receptor, is expressed on the surface of antigen-stimulated T cells and plays a fundamental role in suppressing adaptive immune responses and promoting self-tolerance against T cell inflammatory activity. The activation of PD-1 prevents autoimmune diseases and impedes tumour suppression due to immunocompromise [
11]. PD-1 has two known ligands, PD-L1 and PD-L2, of which PD-L1 is the dominant inhibitory ligand associated with immunotherapeutic responses within the tumour microenvironment [
12]. As a critical checkpoint molecule in the regulation of the immune response, PD-1 is expressed in dendritic cells, T/B cells, tumour-associated macrophages, and myeloid-derived suppressor cells [
13]. On the other hand, PD-L1 is highly expressed in a variety of cancer cells, which leads to tumour immune evasion as they interact with PD-1 [
14]. Since the PD-L1/PD-1 interaction is essential for inducing T cell apoptosis and the immune checkpoint response, targeting the PD-1/PD-L1 axis is thought to be a feasible approach for treating aggressive tumours [
15].
Inflammatory signalling regulates the transcription of PD-L1. In particular, IFN-γ, a proinflammatory cytokine, is recognized as the most prominent inducer of PD-L1 [
16]. Additionally, several other cytokines can also upregulate PD-L1, including TNF-α, IL-1β, IL-4, IL-10, and IL-17 [
17]. Multiple transcription factors involved in the JAK/STAT, RAS/MAPK, and PTEN-PI3K/AKT pathways participate in the regulation of PD-L1, such as STAT1, STAT3, IRF1, IRF3, HIF-1α, MYC, JUN, BRD4, and NF-κB [
18]. Since NF-κB plays a substantial role in inflammatory and immune responses [
19], NF-κB is considered to influence PD-L1 expression. RelA (p65) has been reported to upregulate PD-L1 expression in lung cancer cells in response to TNF-α stimulation [
20].
NF-κB plays an essential role in cancer progression and therapeutic resistance [
21]. In general, NF-κB activation is mediated by two major pathways, i. e., the canonical pathway and the noncanonical pathway. The canonical pathway quickly responds to many exogenous stimulants and leads to the nuclear translocation of the p50:RelA dimer followed by IκB phosphorylation and degradation [
22]. The noncanonical pathway is gradually but persistently activated by processing p100:RelB into the p52:RelB dimer [
23]. In addition to the canonical pathway that is widely recognized to be critical for regulating the inflammatory response, the noncanonical pathway is thought to be a key regulator of the immune response [
24]. Nevertheless, in contrast to the well-studied canonical pathway in cancer development, the role of the noncanonical pathway in cancer remains to be fully elucidated.
RelB was initially identified in B-cells and the RelB-activated noncanonical NF-κB pathway has been shown to respond to antigen presentation by DCs, which is associated with inflammation and excessive immune cell infiltration [
25‐
27]. Furthermore, RelB has been implicated in cancer progression, particularly in sex hormone-related cancers, including PCa, breast cancer and endometrial cancer [
28‐
30]. We have recently shown that a high constitutive level of RelB is correlated to PCa radioresistance [
31]. The present study further demonstrates a
cis/trans transcriptional regulatory mechanism by which RelB upregulates PD-L1 in PCa cells. Accordingly, the repression of tumour-derived RelB can promote the T cell immune response by downregulating PD-L1.
Methods
PCa patients
The Nanjing Medical University Affiliated Cancer Hospital (Nanjing, China) collected fresh tumour tissues from newly diagnosed PCa patients before chemotherapy or radiotherapy. The Ethics Committee of Nanjing Medical University approved the study protocol with written informed consent forms obtained from patients enrolled in this study. From a Gleason score of 5 to 9, each group containing 8–10 cases was selected. Total 46 cases of PCa tumour tissue v.s 10 cases normal prostate tissue were analysed to examine the correlation between RelB and PD-L1 expression using immunohistochemistry (IHC) with monoclonal antibodies against human RelB and PD-L1 (Cell Signalling Tech., USA). Briefly, the tissues were fixed in paraffin-embedded slides and then dewaxed with xylene. After washing with ethanol, the tissue slides were further rehydrated by rinsing with dH2O. For IHC, the tissue slides were soaked in 5% BSA buffer for 1 h and then incubated with 400x diluted primary antibodies within 5% BSA buffer at 4 °C overnight. After washing with PBS, the tissue slides were incubated with a biotinylated secondary antibody at room temperature for 30 min. After washing with BPS, a DAB Substrate Kit (Cell Signalling Tech., USA) was used to observe immunostaining images under a microscope. The intensity of IHC staining was scored as negative (score 0), weak (1), medium (2), and strong (3). Total cell positivity was scored as the percentage of positive cells, including no positive cells (0), < 25% (1), 25–50% (2), 50–75% (3), and > 75% (4). The “H-score” was calculated using ∑pi (i + 1) for all slides, in which pi represented the percentage of positive and i represented the staining intensity.
Mice
Animal experiments were performed according to the Institutional Animal Care and Use approved by the Research Committee of Nanjing Medical University (No. IACUC-1901031). For the mouse xenograft PCa tumour growth experiment, five-week-old male C57BL/6 mice (Beijing Vital River Laboratory Animal Technology Co., Ltd., China) were randomly divided into several groups. The mice were subcutaneously injected with 2.5 × 105 RM-1 cells with different levels of RelB and PD-L1 into the left flank. The formed tumours were measured using digital callipers every other day and tumour volume was calculated using a standard formula (V = 0.52 × AB2, where A and B represent the diagonal tumour lengths) at 7, 14 and 21 days after the cell injection. CD4+ and CD8+ T cells were isolated from mouse blood samples and analysed by flow cytometry. The mice were finally sacrificed and tumour tissues were excised for immunoblotting and IHC. For the xenograft PCa tumour metastasis experiment, 5 × 104 RM-1 cells were injected into mice through the tail vein. The mice were euthanized four weeks later to remove lung tumour tissues and analysed by IHC and immunoblots. Additionally, CD4+ and CD8+ T cells isolated from mouse spleen tissues were analysed by flow cytometry.
T cell preparation
Whole blood samples were obtained from healthy donors according to the institutional guidelines with informed consent. Peripheral blood mononuclear cells (PBMCs) were isolated by density centrifugation using Ficoll gradient separation. The cells were washed three times with PBS and suspended in RPMI 1640 media (Gibco, USA). In addition, mouse T cells were isolated from peripheral blood samples and spleen tissues derived from male C57BL/6 mice. The pellets were lysed within RBC lysis buffer for 10 min to remove red blood cells and the tissues were ground and then filtered to remove tissue debris. The cells were cultured in a T cell preconditional RPMI 1640 media supplemented with 1000x mercaptoethanol (Gibco), 100x penicillin-streptomycin (Gibco), 100X glutamine-MAX (Gibco) and 10% heat-inactivated FBS (Gibco). Single cells were collected by centrifugation and T cells were stimulated in media containing 2 μg/ml anti-CD3, 3 μg/ml anti-CD28 and 200 U/ml IL-2 (Biolegend, Inc., USA) for 24 h prior to coculture with PCa cells.
The cultured T cells were analysed by flow cytometry before and after coculture with PCa cells. APC-conjugated anti-human CD4/mouse antibody (Biolegend) and PE-conjugated anti-human CD8/mouse antibody (BioLegend) were used to stain CD4 or CD8 cells. The labelled cells were analysed by a BD FACSCalibur flow cytometer (BD Sciences, USA). In addition, to analyse T cell proliferation, the T cells were labelled with 5 μM CFSE (Biolegend) for 20 min at 37 °C in darkness. Cell staining was blocked by adding cell culture media containing 10% FBS. Flow cytometry data were further analysed with FlowJo software and Modfit software.
RelB manipulation in PCa cells
Human PCa cell lines (PC-3 and DU-145) and mouse PCa cell lines (RM-1) were purchased from the American Type Culture Collection (ATCC, USA). The cell lines were cultured in the recommended media containing 10% FBS and 1% penicillin/streptomycin. The cells were treated with IFN-γ (Novus, USA) to induce PD-L1 expression. RelB was silenced in PC-3 and DU-145 cells by transfection of a plasmid-carrying shRNA duplex targeting RelB (RiboBio Co., Ltd., China) and stable cell clones were selected using G418 (Invitrogen, USA). Additionally, to restore RelB activity in RelB-defective PC-3 cells, a RelB cDNA driven by the CMV promoter in pCMV-Script vector (Stratagene, USA) was transiently transfected into RelB-silenced cells. Furthermore, RelB was knocked out (RelB-KO) in RM-1 cells using a CRISPR/Cas9-based gene-editing system. Briefly, RM-1 cells were transfected with a Cas9-single guide RNA (sgRNA) expression plasmid targeting RelB (sgRNA, 5′-GACGAATACATTAAGGAGAA-3′), followed by puromycin selection. In addition, PD-L1 cDNA was cloned into the pcDNA plasmid and then transfected into RelB-KO RM-1 cells, followed by hygromycin B (Invitrogen) selection to generate a stable cell line.
Immune cytotoxicity
The effect of activated T cells on the survival of PCa cells was determined using cell counting and clonogenic assays, respectively. After coculture with activated T cells at a 1:10 ratio for 3 days, PCa cells were seeded into 96-well plates at a density of 103 cells/well and then cultured for 2 days. The cells were treated with CCK-8 reagent (Dojindo Mol. Tech., Japan), and cell viability was measured as the optical density at 450 nm. For the clonogenic assay, PCa cells were treated with an anti-PD-L1 mAb (Abcam, USA) prior to coculture with activated T cells. Thereafter, 100–200 PCa cells were plated in 6-well plates and continuously cultured to allow colony formation. After washing with 1x PBS twice, the colonies were stained with 1% crystal violet dye for 30 min to form visible cell clones. The cell survival fractions were calculated based on the number of colonies divided by the number of cells efficiently plated.
RNA-seq
The RNA sequencing libraries were constructed by Vazyme Biotech Co., Ltd. (Nanjing, China) using a VAHTS mRNA-seq v2 Library Prep Kit for Illumina. The cDNA libraries were sequenced on an Illumina HiSeq X Ten platform with a 150 bp paired-end module. The raw reads were filtered by removing reads containing adapter, poly-N and low-quality read for subsequent analysis. Cuffdiff (v1.3.0) was used to calculate FPKMs for coding genes in each sample. Genes with corrected
p-value less than 0.05 and the absolute value of log2 (fold change) greater than or equal to 1 were considered as significantly differentially expressed. Custom scripts in R software were used for clustering and heatmap analysis (
https://www.r-project.org). In addition, the altered mRNA expression profile in shRelB cells vs. shCtrl cells was analysed using KEGG pathway enrichment (
https://david.ncifcrf.gov).
Immunofluorescence
PCa cells were seeded in confocal dishes at a density of 2 × 103, washed with cold 1x PBS and fixed with 4% paraformaldehyde in PBS for 15 min. Subsequently, the cells were permeabilized with 1x PBS containing 0.5% Triton X-100 for 20 min and then blocked for 30 min. The cells were incubated with primary antibodies against RelB and PD-L1 at 4 °C overnight and then probed with Alexa Fluor® 647 conjugated (Cell Signalling Tech., USA) or Alexa Fluor® 488 conjugated (Abcam, UK) secondary antibodies for 1 h at room temperature. After counterstaining with DAPI (Invitrogen) for 5 min, fluorescence was visualized and captured using a TCS SP5 MP confocal microscope (Leica Microsystems, Inc., USA).
NF-κB binding activity
Nuclear proteins were extracted from PCa cells using a nuclear and cytoplasmic protein extraction kit (Beyotime Biotech.) according to the manufacture’s instruction. The nuclear extracts (50 μg) were subjected to an NF-κB binding activity kit containing the consensus sequence of the NF-κB element as a standard probe (Abcam, USA) to measure the NF-κB binding activity based on the manufacturer’s protocol.
Luciferase reporter assay
A 2000-bp 5′-flanking region of the human CD274 gene containing a core promoter containing putative multiple NF-κB elements was cloned into the pGL3 vector (Promega, USA) to drive the luciferase reporter gene. Subsequently, a functional NF-κB binding site was mutated using a site-directed mutagenesis system (Thermo Fisher, USA). The luciferase reporter constructs were cotransfected with β-gal into PC-3 cells using Lipofectamine (Invitrogen). After 48 h, the luciferase activity was quantified by a luciferase assay system (Promega, USA) using a luminometer (Berthold Tech., Germany). The β-gal activity was quantified using microplate readers (BioTek, USA). NF-κB-mediated transcriptional enhancement was estimated by the β-gal-normalized luciferase response.
ChIP
RelB binding to the NF-κB elements in the human
CD274 gene was examined by chromatin immunoprecipitation (ChIP) using a SimpleChIP Enzymatic Chromatin IP Kit (Cell Signalling Tech.). A RelB antibody (Santa Cruz Biotech., USA) was applied to pull down chromatin from nuclear extracts isolated from PCa cells. Chromatin without antibody pulldown served as the input control, and IgG (Santa Cruz Biotech.) served as a negative antibody control. The pulled-down fragments were quantified by PCR using relative primers (
Table S1). The amounts of the pulled-down fragments were assessed by normalization to the input control.
EMSA
Electrophoretic mobility shift assay (EMSA) was further performed to confirm the specific RelB binding to the human
CD274 promoter. A double-stranded DNA fragment containing the native or mutated NF-κB binding site were synthesized and annealed. The 3′- terminus of the upper strand was labelled using a probe biotin-labelling kit (Beyotime Biotech., China). EMSA was conducted by incubating the probes with PC-3 cell-derived nuclear proteins within a chemiluminescent EMSA kit (Beyotime) according to the manufacturer’s instructions. The unlabelled wild-type and mutated probes were used as competitors to quantify the specific binding activity. A RelB antibody was used to eliminate the binding activity. The EMSA image was visualized using a BioRad
GelDoc XR+ system. The sequences of the wild-type and mutant probes are listed in
Table S1.
RT-qPCR
The mRNA levels of RelB and PD-L1 were quantified by
reverse
transcription-quantitative PCR (RT-qPCR). Total RNA was extracted from PCa cells using the TRIzol reagent and cDNA was reversely transcribed from mRNA using a PrimeScript™ RT reagent kit (Takara Inc., Japan) according to the manufacturer’s instructions. cDNA was quantified by qPCR with SYBR Premix Ex Taq II (Takara Inc.) using a LightCycle System (Roche, USA). The mRNA level of PD-L1 was estimated by normalizing to GAPDH mRNA. Sequences of the specific PCR primers for PD-L1 and GAPDH are listed in
Table S1.
Immunoblots
PCa tumour tissues and cells were lysed in RIPA lysis buffer containing 1 mM PMSF protein inhibitor (Santa Cruz Biotech.). Cellular and nuclear extracts (50–100 μg) were separated on 10% SDS-polyacrylamide gels and then transferred to PVDF membranes. The membranes were subsequently incubated overnight at 4 °C with the primary antibodies against RelA, RelB, PCNA, and GAPDH (Cell Signalling Tech.), and anti-human PD-L1, anti-mouse PD-L1 (Abcam). The membranes were washed three times with TBST buffer and incubated at room temperature for 2 h with HRP-conjugated secondary antibody (Santa Cruz Biotech. USA). The immunoblots were visualized using an enhanced chemiluminescence detection system (Bio-Rad, USA). The intensities of the blots were quantified using Quantity One software and protein expression was normalized to loading controls such as β-actin and GAPDH.
Statistics
The results are presented as the mean ± standard deviation (SD) from at least three replicates. Significant differences between the experimental groups were analysed by unpaired Student’s t-test. One-way analysis of variance (ANOVA) followed by Dunnett’s or Bonferroni’s multiple comparison test was performed using Prism (GraphPad, San Diego, USA). Statistical significance was accepted at P < 0.05.
Discussion
Although the 5-year survival rates of PCa have steadily increased in the United States, the mortality of PCa has been consistently increasing globally, particularly in East Asia [
2]. Moreover, AR-negative metastatic PCa displays resistance to the most common treatments and leads to poor prognosis; in particular, devastating bone metastasis appears to be a salient and daunting challenge in the control of PCa [
36]. Thus, a comprehensive therapeutic strategy in combination with innovative technologies urgently needs to conquer intractable malignant PCa. In this regard, the advantages of novel cancer immunotherapy have emerged as a prospective method for treating worsened tumours, including advanced PCa [
10,
37].
Mounting evidence has demonstrated that immune checkpoint inhibitors have significantly improved overall survival for subsets of patients with malignant tumours that mostly resist traditional anticancer therapies [
38,
39]. For instance, the therapeutic effects of the blockade of PD-1/PDL-1 and CTLA-4 checkpoints have been adopted to treat various cancers [
40,
41]. PD-L1 is uniquely expressed at high levels in cancers; therefore, therapies targeting PD-1/PD-L1 have been shown to promote remarkable antitumour immunity and acquire promising therapeutic outcomes for several malignant tumours [
14,
42]. Likewise, PD-L1 is profoundly expressed in tumour tissues from mCRPC patients, which suggests that PD-L1 is associated with PCa progression [
43]. Although anti-PD-1 treatment has shown therapeutic results for treating primary PCa, the combination of anti-PD-1 mAb with myeloid-derived suppressor cell (MDSC)-targeted therapy has received a robust synergistic therapeutic response in the treatment of mCRPC [
44]. Nevertheless, less response to anti-PD-L1 agents in metastatic urothelial cancer and mCRPC suggested stromal TGF-β dampening the PD-1/PD-L1 blockade therapy, which indicates the existence of alternative immunosuppressive mechanisms provided from the tumour microenvironment [
45,
46].
It is recognized that pro-inflammatory cytokines produced in the tumour microenvironment induce PD-L1 expression in tumour cells [
47,
48]. In particular, IFN-γ has been defined as a favourable adaptive immune resistance inducer. Other oncogenic cytokines are also involved in inducing PD-L1 expression, including TGF-β, IL-6, IL-10, and IL-17 [
18]. Mechanistically, the expression of PD-L1 in tumour cells can be regulated through transcriptional and posttranscriptional regulation [
17]. Several cytokine/chemokine-inducible transcription factors have been shown to participate in the regulation of PD-L1, such as Myc, NF-κB, Stat3, and Jun/Ap-1 [
18,
49]. Additionally, tumour-suppressive miR-34a appeared to directly downregulate PD-L1 [
50], while oncogenic miR-21 seemed to indirectly upregulate PD-L1 via activation of the PI3K-Akt signalling axis by inhibiting PTEN [
51]. Furthermore, a recent study demonstrated that AKT upregulates PD-L1 by phosphorylating β-catenin and promotes glioblastoma immune evasion [
52].
Transcriptional regulation plays a pivotal role in PCa progression. AR-mediated transcriptional regulation plays a fundamental role in the promotion of AR-dependent PCa [
53]. Nevertheless, PCa patients frequently receive androgen deprivation and AR-inhibitory therapies, and the AR response is ultimately eradicated in the developed AR-negative malignancy [
54,
55]. Notably, NF-κB-mediated transcriptional regulation, in turn, to be activated in mCRPC, functionally sustains PCa progression under androgen-free conditions [
56]. Although the activation of the RelA-based canonical NF-κB pathway has been involved in PCa progression and therapeutic resistance [
57,
58], the role of the RelB-based noncanonical NF-κB pathway is underestimated.
RelA has been reported to upregulate PD-L1 in tumour cells in response to TNF-α and LPS stimulation or by cooperation with RB phosphorylation [
20,
59,
60]. Nevertheless, there is a lack of current evidence that p50/RelA directly binds to the NF-κB element located in the
CD274 promoter. Recently, Antonangeli et al. predicted the canonical NF-κB consensus sequence in the
CD274 promoter region as 5′-GGGRNWYYCC-3′ (where R: A/G, W: A/T, Y: C/T, N: any base) [
61]; however, the proposed site has yet not fully validated. The present study used the standard NF-κB consensus sequence (5′-GGGRNYYYCC-3′) to search the potential NF-κB binding site in the
CD274 promoter. Three putative sites were validated using a systemic
cis/trans transcriptional regulatory approach. The results delineated that a proximal NF-κB enhancer element located in the core promoter region is responsive to RelB-mediated PCa immune evasion by upregulating PD-L1. Consistently, the silencing of tumourous RelB led to enhanced T cell immunity in the suppression of PCa.
Notably, the RelB-based noncanonical NF-κB pathway has been implicated in diverse biological processes, including immunogenicity and tumorigenicity [
62]. RelB function is critical for normal B cell maturation and lymphoid organogenesis [
63,
64]. BAFF-NIK-p52/RelB axis is essential for B cell survival by upregulating Bcl-2 and Bcl-xl [
65]. Additionally, TRAF3- or NIK-deficiency appeared to preclude T cell function by inhibiting the noncanonical NF-κB pathway [
66,
67]. Intriguingly, the results from this study elucidated, for the first time, that tumour-derived RelB hampers T cell function by upregulating PD-L1. Thus, the implication of RelB in both immune cells and tumour cells has emerged as a major concern for tumour immunotherapy. Taken together, insight into RelB-mediated PD-L1 overexpression is anticipated to provide a promising approach for enhancing immune checkpoint blockade therapy through the administration of RelB.
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