Background
Pancreatic cancer is a highly invasive malignant tumor worldwide characterized by late diagnosis and limited therapeutic options [
1]. Based on tumor resectability, patients with pancreatic cancer have been classified into four categories: resectable, borderline resectable, locally advanced, and metastatic [
2]. In spite of recent advances in the early diagnosis and treatment of pancreatic cancer, complete surgical resection is the only way to offer a chance of cure for patients with pancreatic cancer. Most cases (up to 80%) are diagnosed at advanced stages and lose the opportunity to carry out surgery. Chemotherapy combinations, mainly including gemcitabine plus nab-paclitaxel and FOLFIRINOX (made up of folinic acid, fluorouracil, irinotecan, and oxaliplatin) remain the primary treatment for patients with advanced disease [
3,
4]. However, they have shown limited efficacy and severe adverse effects, and the combined five-year survival rate for pancreatic cancer is very low at just 5% to 10%. PARP inhibitors, such as olaparib, have gained approval for use in patients with germline
BRCA1 or
BRCA2 mutation [
5]. Compared with placebo, olaparib improved median progression-free survival from 3.8 to 7.4 months, but there is no significant difference in overall survival between the groups. Thus, pancreatic cancer remains a deadly malignancy with limited options for effective therapy, and further development of more effective and better-tolerated treatments is warranted.
The ubiquitin–proteasome system provides opportunities for pharmacological intervention owing to their crucial roles in cancer [
6]. A series of critical enzymes including ubiquitin-activating enzymes (E1), ubiquitin-conjugating enzymes (E2), ubiquitin-ligating enzymes (E3), and deubiquitinases are involved in the ubiquitin signaling, controlling post-translational ubiquitination of proteins and regulating protein stability [
6]. To initiate protein modification by ubiquitin, an active site cysteine of E1 reacts with the C-terminus of ubiquitin. Then the ubiquitin is transferred from E1 to E2 followed by E3 directly or indirectly promoting the transfer of ubiquitin from E2 to the target substrate proteins. Removal ubiquitin from modified substrates is accomplished by deubiquitinases [
7].
There are more than 600 known E3 ubiquitin ligases and they have been classified into three general types based on unique topology and mechanism: homologous to E6AP C-terminus (HECT), really interesting new gene (RING), and RING-between-RING (RBR) [
8]. RING-type E3s represent the largest class of E3s in humans, are characterized by a conserved structure stabilized by two Zn
2+ atoms, and catalyze the direct transfer of ubiquitin from E2-ubiquitin to a CO-bound substrate protein depending on the RING finger domain. Many of the RING-type E3s have been demonstrated as effective molecular targets for treating human cancers, including pancreatic cancer [
9‐
11]. Among them, the inhibitor of apoptosis proteins (IAPs), including family members cIAP1/2 and XIAP, are implicated in cancer initiation, metastasis, and progression [
12]. In pancreatic cancer, cIAP1 is over-amplified and high cIAP2 expression is correlated with resistance to chemotherapeutic drugs [
12]. Co-expression of cIAP1/2 has been associated with an unfavorable prognosis of pancreatic cancer. Similar associations have been found between XIAP levels and pancreatic cancer prognosis [
13,
14]. Accumulated evidence supports that IAPs play critical roles in numerous cellular processes such as induction of the epithelial-mesenchymal transition (EMT), DNA repair, and activation of NF-κB signaling, and have been demonstrated as promising targets for cancer therapy [
12]. To date, medicinal chemistry programs directed at IAPs have resulted in clinically evaluated molecules, and only very few compounds targeting IAPs entered clinical trials and are still at the early phase [
15].
Recent studies have demonstrated that targeting the E2-E3 axis is a potential strategy for developing anticancer agents [
6]. The human proteome contains about 50 E2s and numerous studies have reported that E2s also serve an important role in the carcinogenesis and progression of pancreatic cancer [
16]. UbcH10 expression at mRNA and protein levels are both upregulated in pancreatic ductal adenocarcinoma and is significantly correlated with poor overall survival [
17]. UBE2C is upregulated in pancreatic cancer and associated with clinical stage, lymph node metastasis, and survival [
18]. Silencing UBE2C inhibits cell proliferation by inducing cell cycle arrest and apoptosis and decreases the migration in vitro and in vivo. UBE2T has been recently identified as an oncogenic protein that is highly expressed in pancreatic cancer tissues and cell lines, and overexpression of UBE2T significantly promotes pancreatic cancer cell proliferation, migration, and invasion [
19]. Increased UBE2S level promotes EMT via the VHL/HIF-1α/STAT3 signaling by attenuating the activity of the promoter [
20]. UBE2N interacts with TRIM11 to decrease the sensitivity of Panc1 and BxPC3 cells to gemcitabine [
21]. Collectively, targeting E2 enzymes may be an attractive strategy to modulate E3 ubiquitin ligases for pancreatic cancer therapy. Identifying E2s that collaborate with IAPs may also provide new molecular targets for developing effective anticancer agents.
In this study, we examined the expression of E2 UbcH5c in pancreatic cancer tissues and analyzed its association with IAPs expression and the prognosis of pancreatic cancer patients. We further identified a small-molecule UbcH5c inhibitor DHPO and investigated its anticancer activity and mechanisms of action in pancreatic cancer models in vitro and in vivo. Our data suggested that UbcH5c may be a promising target for anti-pancreatic cancer drug development.
Methods
Cell lines and chemicals
Cell lines Panc1, BxPC3, SW1990, HPAC, CFPAC1, AsPC1, Capan2, and HPNE were purchased from American Type Culture Collection (ATCC; Rockville, MD, USA). Panc1, BxPC3, SW1990, and HPNE were cultured in DMEM medium (Gibco/Life Technologies, Darmstadt, Germany) supplemented with 10% fetal bovine serum (FBS) (Thermo Fisher Scientific, Waltham, MA, USA). CFPAC1 and AsPC1 were cultured in RPMI 1640 medium (Gibco/Life Technologies, Darmstadt, Germany) supplemented with 10% FBS. Capan2 was cultured in IMDM medium (Gibco/Life Technologies, Darmstadt, Germany) supplemented with 20% FBS. HPAC was cultured in DMEM/F12 medium supplemented with 10% FBS and 0.002 mg/ml insulin, 0.005 mg/ml transferrin, 40 ng/ml hydrocortisone, and 10 ng/ml epidermal growth factor. All cell culture media were supplemented with 1% penicillin and streptomycin (Thermo Fisher Scientific/Life Technologies, Waltham, MA, USA) and incubated at 37 °C in a humidified atmosphere of 95% air and 5% CO
2. The isolation and purification of DHPO were performed as previously [
22]. TNF-α (Cat. No. Z01001) was purchased from GenScript. The anti-GAPDH (Cat. No. 5174S, 1/1000), anti-phospho-IκBα (Cat. No. 2859S, 1/1000), anti-IκBα (Cat. No. 4814S, 1/1000), anti-phospho-NF-κB p65 (Cat. No. 3033S, 1/1000), anti-NF-κB p65 (Cat. No. 8242S, 1/1000), anti-c-Myc (Cat. No. 18583S, 1/1000), anti-Mcl-1 (Cat. No. 94296S, 1/1000), and anti-UbcH5c (Cat. No. 4330S, 1/1000) antibodies were purchased from Cell Signaling Technology (Beverly, MA, USA).
In vitro assays for determination of DHPO’s anticancer activity
All the assays used to evaluate the effects of DHPO on pancreatic cancer cell viability (CCK8 assays, Nuoyang Biotech, Hangzhou, China), colony formation, cell apoptosis (Annexin V-FITC Apoptosis Staining/Detection Kit, BD, USA), cell migration (wound healing assay), and cell invasion (transwell invasion assay) were performed as described previously [
23,
24].
Western blot
The methodology used here was described elsewhere [
25]. In brief, the total protein was extracted from cells using RIPA buffer containing phosphatase and protease inhibitors (Sigma-Aldrich, St. Louis, MO, USA). The cell lysates were evaluated for their protein concentrations, resolved by SDS-PAGE, and transferred to polyvinylidene difluoride (PVDF) membrane (Millipore, Germany). The membranes were blocked in 5% skim milk and incubated with primary antibody followed by incubation with appropriate secondary antibody. ECL Chemiluminescent Substrate Reagent Kit (Biosharp, Hefei, China) was used to detect protein bands.
Immunofluorescence
Confocal immunofluorescence was performed as described earlier [
26]. In short, cells were seeded on glass coverslips overnight, treated with DHPO for 24 h followed by exposure to TNF-α (20 ng/ml) for 15 min. The treated cells were fixed and incubated with primary antibody followed by incubation with secondary antibody. Coverslips were then mounted with an anti-fade mounting solution supplemented with 4’,6-diamino-2-phenylindole (DAPI). The images were then captured using a confocal microscope (Leica Microsystems, Wetzlar, Germany).
Real-Time qRT-PCR
The mRNA expression levels of genes were determined by Real-Time qRT-PCR as described previously [
26]. Total RNA of cell samples was extracted using RNA-Quick Purification Kit (YiShan Biotechnology Co. LTD, Shanghai, China) according to the manufacturer’s instructions. The concentrations of isolated RNA samples were determined using NanoDrop (Thermo Fisher Scientific, Wilmington, DE, USA). The cDNA was synthesized using Fast-All-in-One RT Kit (YiShan Biotechnology Co. LTD, Shanghai, China) according to the manufacture’s protocols. Real-Time PCR was performed using 2xSuper SYBR Green Qpcr Master Mix (YiShan Biotechnology Co. LTD, Shanghai, China) and by means of CFX96™ PCR instruments (Bio-Rad Laboratories, Hercules, CA, USA). The data were analyzed using Bio-Rad CFX Manager™ Software version 3.1 (Bio-Rad Laboratories, Hercules, CA, USA). GAPDH was used as an internal control to normalize individual gene expression levels. Primer sequences were shown in Additional file 1: Table S
1.
Stable transfection
Lenti-shUbcH5c and their corresponding control vectors were constructed by and purchased from Hanbio (Shanghai, China). Stable transfection was performed according to the manufacturer's instructions. Then the stably transfected cells were screened by puromycin for 2 weeks.
Panc1 cells were treated with 5 μM DHPO, or an equal volume of 1‰ DMSO as controls, and were incubated in a humidified atmosphere of 5% CO
2 at 37℃ for 24 h. Cell samples were collected and added into Trizol reagent (Invitrogen, CA, USA), and then handed over to LC-Bio Technology Co., Ltd (Hangzhou, China) for the subsequent mRNA library construction and sequencing. Hisat2, StringTie, and Ballgown are used to compare the filtered CleanData with the human genome (
ftp://ftp.ensembl.org/pub/release-101/fasta/homo_sapiens/dna/), and to perform initial assembly and FPKM quantification of genes or transcripts. All genes were used to perform PCA. PCA results were visualized in a two-dimensional coordinate space according to two major principal components. The differentially expressed mRNAs were selected with |log2FC|> 0.58 and
FDR < 0.05 by R package edgeR. The genes with log2FC < -0.58 and
FDR < 0.05 were selected to perform GO enrichment and KEGG enrichment analyses using DAVID. GSEA was performed with Broad GSEA software (version 4.0.2) using the hallmark gene sets (h.all.v.7.4.symbols.gmt) in MSigDB for pathway annotation.
Quantitative proteome analysis
Panc1 cells were treated with 5 μM DHPO or DMSO for 24 h, and the cell samples were collected for proteome analysis with TMT technology by LC-Bio Technology Co., Ltd. Quantitative proteome analysis involved the following six steps in turn: protein extraction, SDS-PAGE separation, filter-aided sample preparation (FASP Digestion), TMT labeling, peptide fractionation with reversed-phase (RP) chromatography, and mass spectrometry analysis. Six samples were sequentially labeled with 126, 127, 128, 129, 130, and 131. MS/MS raw files were processed using the MASCOT engine (Matrix Science, London, UK; version 2.6) embedded into Proteome Discoverer 2.2. A peptide and protein false discovery rate of 1% was enforced using a reverse database search strategy. All proteins were used to perform PCA. PCA results were visualized in a two-dimensional coordinate space according to two major principal components. Proteins with |log2FC|> 0.26 and FDR (Student’s t-test followed by multiple test correction through the Benjamini–Hochberg method) < 0.05 were considered to be differentially expressed proteins. The top 150 downregulated proteins with FDR smaller than 0.05 were selected to perform pathway and process enrichment analysis, and protein–protein interaction enrichment was performed using Metascape. All these procedures were performed with the default configuration.
Molecular docking
Molecular docking was carried out using Autodock 4.2 software. The crystal structure of UbcH5c (PDB code: 5EGG) was retrieved from the Research Collaboratory for Structural Bioinformatics Protein Data Bank (RCSB PDB,
http://www.rcsb.org/pdb/). The standard structure of DHPO (PubChem CID: 286761) was retrieved from the PubChem Compound Database (
https://pubchem.ncbi.nlm.nih.gov/). The Lamarckian genetic algorithm (LGA) was used to implement molecular docking.
Surface plasmon resonance analysis
The binding affinity of DHPO to UbcH5c was evaluated using a Biacore 8 k system (GE Healthcare, Sweden) at 25 °C. After diluting with 50 mM acetic acid sodium acetate buffer, 20 μg/ml of UbcH5c protein was immobilized onto a CM5 sensor chip at a flow rate of 30 μl/min in PBS containing 5% (v/v) DMSO and 0.05% (v/v) Tween-20. Gradient concentrations of DHPO were injected into the flow system and analyzed, respectively. The UbcH5c protein and DHPO were allowed to associate for 120 s and then dissociate for 180 s. After dissociation, the sensor chip was washed with running buffer and regenerated with pure water containing 10% DMSO for 30 s. The injection mode of the sample is multi-cycle kinetic injection. The concentration of 0 is deducted in all the data to obtain the final binding dissociation curve.
Cellular thermal shift assays
After treatment with DHPO or DMSO for 1 h, Panc1 and SW1990 cells were digested with trypsin, centrifuged, then resuspended in pre-cooled PBS supplemented with protease inhibitors, and transferred to the PCR tubes. PCR tubes were heated at their designated temperature for 3 min in the PCR instrument, then removed and incubated at room temperature for 3 min, treated in liquid nitrogen for 3 min, and circulated 3 times. Then the protein was detected and quantified by Western blotting analysis.
Orthotopic pancreatic cancer mouse model
The establishment and treatment of orthotopic models were performed as reported previously [
26]. 50 μl Panc1-luc/SW1990-luc cell suspension was injected into the pancreas of male SCID mice. When tumor volume was confirmed, the mice were randomly divided into two groups: the control group only received controls while the DHPO treatment group was administered by intraperitoneal injection. For the Panc1 orthotopic model, DHPO injection was at a dose of 5 mg/kg/day, 7 days/week, for 5 weeks. For the SW1990 orthotopic model, DHPO injection was at a dose of 10 mg/kg/day, 7 days/week, for 4 weeks. The tumor volume was monitored once every 7 days by fluorescence imaging via an IVIS imaging system. The body weight was measured every three days as a surrogate marker for toxicity. At the end of the treatment, the mice were sacrificed and their tumors and major organs were excised for subsequent experiments. Hematoxylin and eosin staining and scoring were performed as described previously [
27].
Statistical analysis
The data were analyzed using the Prism software program (version 9) (Graph Pad Software Inc., San Diego, CA, USA). Data were expressed as mean ± SEM. Statistical analyses between control and treatment groups were performed by student’s t-test. P < 0.05 was considered to be statistically significant.
Discussion
New therapeutic targets are critically needed for the development of promising anti-pancreatic cancer therapeutic drugs [
34‐
36]. We have recently proposed that UbcH5c is a potential therapeutic target in human cancer [
29]. Indeed, there is increasing evidence that UbcH5c is overexpressed in human cancer, such as esophageal squamous carcinoma and breast cancer [
37‐
39]. UbcH5c was also found to be upregulated in plasma samples of colorectal cancer, and the overexpression of UbcH5c promotes metastasis of colorectal cancer. Here we found that UbcH5c is overexpressed in pancreatic tumor tissues compared with non-tumor tissues and predicts a poor prognosis in patients with pancreatic cancer. Our experimental data indicated that UbcH5c may serve as a promising biomarker and therapeutic target for the treatment of pancreatic cancer patients. Further characterization and verification of UbcH5c need to be carried out in more clinical samples from multicenter cohorts and some important clinicopathologic features should also be considered.
It is worth noting that selective inhibition of UbcH5c remains an active area of drug development. To date, several UbcH5c inhibitors have been developed to inactivate UbcH5c by forming a covalent adduct in recent pharmacodynamics studies [
40], and they have been reported to exert anti-inflammatory activity and increase drug sensitivity in various models [
30,
39,
41]. Here, we identified a novel UbcH5c inhibitor, DHPO, a sesquiterpene lactone isolated from
Inula plants, which is widely distributed in China and has been used as a traditional Chinese medicine Jinfeicao for treating various diseases, including cancer [
22,
42]. This compound has been reported to show potent Nrf2-activating and neutral protective activities [
43], but its role in cancer is not clear. The present study indicated that DHPO could directly bind to UbcH5c, potently inhibit pancreatic cancer cell proliferation, induce apoptosis, and prevent migration and invasion in vitro. Moreover, DHPO suppressed pancreatic tumor growth and metastasis in vivo, without causing any obvious host toxicity. Our work represents the first attempt to demonstrate that targeting UbcH5c could be an alternative strategy to control pancreatic cancer progression. However, more specific and effective UbcH5c inhibitors with higher binding affinity are expected to be identified via virtual screening of other compound libraries and validation of cell-based assays. The structural optimization of DHPO is also a possible strategy to develop more specific UbcH5c inhibitors. Moreover, DHPO may also exert anticancer efficacy in other types of cancer with high expression levels of UbcH5c. The efficacy and safety of DHPO and other UbcH5c inhibitors should be evaluated in more clinically relevant cancer models in the future.
Recent studies have reported that UbcH5c acts as an oncoprotein by inducing the ubiquitination and degradation of dozens of proteins and thereby causing NF-κB activation [
29]. It is becoming clear that NF-κB has multiple roles in cell proliferation, survival, migration, tumorigenesis, and metastasis in cancer; inhibition of this pathway could be a therapeutic option [
44]. Over the past few years, a great number of small molecules mainly targeting proteasome [
45] or IKK [
46] have been developed to block NF-κB activation and some of them are being investigated in clinical trials. However, limited or no efficacy of these small molecules has been observed in pancreatic cancers [
47], leading us to identify new targets upon which the development of new agents inhibiting NF-κB will be based. Based on our results, we proposed the molecular mechanism of DHPO against pancreatic cancer (Fig.
7e). This compound may directly bind to UbcH5c and disrupt the E2/E3 interaction of the UbcH5c/cIAPs complex, thus inhibiting the polyubiquitination of receptor-interacting protein 1 (RIP1) and NF-κB essential modulator (NEMO). Consequently, the IKK-β-mediated phosphorylation of IκBα and NF-κB is blocked by DHPO, thereby repressing the transcription of downstream target genes in pancreatic cancer cells. Further, we demonstrated that UbcH5c KD cells were resistant to DHPO treatment, showing less sensitivity in inhibiting cell viability. Nonetheless, these results must be interpreted with caution and a number of limitations should be borne in mind. First, the specificity of DHPO for UbcH5c over other E2s is not clear. Second, DHPO affects ubiquitination of which members of cIAPs should be validated using cutting-edge methods. Third, UbcH5c was also involved in many cancer-related signaling pathways, including the p53 signaling pathway [
39,
48], p62-mediated autophagy [
49], YAP pathway [
50], and DNA repair pathways [
37], so whether DHPO also inhibits pancreatic cancer via these pathways needs to be further explored. In addition, DHPO may have secondary molecular targets, which should also be studied in the future.
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