Background
Prostate cancer (PCa) is the most frequently diagnosed malignancy and the second leading cause of cancer-related deaths in men [
1]. Despite great advances in systemic and individualized treatments of PCa in the last decades, the incidence of PCa remains markedly increasing in China [
2,
3]. The skeleton is the most common site for PCa metastases, which severely affects the quality of life and survival time of PCa patients [
4]. Therefore, better understanding of the underlying mechanisms responsible for bone metastasis of PCa facilitates the identification of novel therapeutic targets for bone metastasis of PCa.
Since identified in several decades ago [
5], the central roles of the NF-κB pathway in physiologic and pathologic processes, including inflammation and tumorigenesis, have been well documented [
6,
7]. Constitutive activation of NF-κB signaling has been reported in numerous human cancers, which promotes the initiation, progression and metastasis of malignancies [
8‐
12]. Notably, ubiquitination- and phosphorylation-mediated signaling transduction has been identified as an important regulatory mechanism for the activation of NF-kB signaling [
13,
14]. After binding to respective ligands, the receptors recruit multiple receptor-associated factors, such as tumor necrosis factor receptor-associated factor (TRAFs), function as a ubiquitin ligase via inducing the K-63 polyubiquitination of receptor-interacting protein 1 (RIP1), resulting in activation of transforming growth factor β–activated kinase-1 (TAK1)/TAB2/3 complex. Activated TAK1/TAB2/3 complex phosphorylates and activates inhibitor of NF-kB kinase (IKK)-α/β/γ complex, leading to nuclear translocation and activation of NF-kB [
14,
15]. Furthermore, several lines of evidence have reported that NF-κB signaling plays a critical role in the bone metastasis of cancers [
9,
16,
17]. Importantly, NF-κB activation has also been reported to be associated with the metastatic phenotype of PCa progression [
18,
19]. A study by Chen et al. reported that NF-κB signaling activity promoted the development of PCa bone metastasis [
19]. However, the underlying mechanism of constitutive activation of NF-κB signaling in the bone metastasis of PCa needs to be further elucidated.
MicroRNAs (miRNAs) are a series of small non-coding RNAs with 18–24 nucleotides that post-transcriptionally regulate target genes via binding to specific sequences in the 3′ untranslated region (3’UTR) of downstream target genes, leading to mRNA degradation and/or translational inhibition [
20]. Abundant evidence has revealed that the aberrant expression of miRNAs was associated with progressive and metastatic phenotypes of cancers [
21‐
24]. Our previous studies in combination with other literatures have shown that several dysregulation of miRNAs were crucial mediators in the bone metastasis of PCa [
25‐
28]. As one of the originally discovered miRNAs, dysregulation of miR141-3p is implicated in the progression and metastasis of various cancers [
29,
30].Importantly, a study by Liu and colleagues has demonstrated that bone metastatic PCa cells PC-3 expressed little endogenous miR-141-3p [
29], suggesting that low expression of miR-141-3p may play an important role in the bone metastasis of PCa. However, the clinical significance and biological roles of miR-141-3p in the bone metastasis of PCa remain unclear.
In this study, we report that miR-141-3p is downregulated in bone metastatic PCa tissues compared with non-bone metastatic PCa tissues. miR-141-3p expression inversely correlates with the clinicopathological characteristics and bone metastasis status in PCa patients. Furthermore, upregulating miR-141-3p represses, while silencing miR-141-3p enhances EMT, invasion and migration of PCa cells in vitro. Importantly, upregulating miR-141-3p significantly inhibits bone metastasis of PC-3 cells in vivo. Our results further demonstrate that ectopic expression of miR-141-3p suppresses activity of NF-κB signaling via targeting TRAF5 and TRAF6, which further inhibits invasion, migration and bone metastasis in PCa. The analysis of clinical correlation shows that miR-141-3p inversely correlates with TRAF5 and TRAF6 expression, as well as with NF-κB signaling activity and downstream target genes of NF-κB signaling in human PCa tissues. Taken together, these findings clarify a novel mechanism responsible for constitutive activation of NF-κB signaling in bone metastasis of PCa, determining that miR-141-3p play a tumor-suppressive role in bone metastasis of PCa.
Methods
Cell culture
The human PCa cell lines 22RV1, PC-3, VCaP, DU145, LNCaP and normal prostate epithelial cells RWPE-1 were obtained from Shanghai Chinese Academy of Sciences cell bank (China). RWPE-1 cells were grown in defined keratinocyte-SFM (1×) (Invitrogen). PC-3, LNCaP and 22Rv1 cells were cultured in RPMI-1640 medium (Life Technologies, Carlsbad, CA, US) supplemented with penicillin G (100 U/ml), streptomycin (100 mg/ml) and 10% fetal bovine serum (FBS, Life Technologies). DU145 and VCaP cells were grown in Dulbecco’s modified Eagle’s medium (Invitrogen) supplemented with 10% FBS. The C4-2B cell line was purchased from the MD Anderson Cancer Center and maintained in T-medium (Invitrogen) supplemented with 10% FBS. All cell lines were grown under a humidified atmosphere of 5% CO2 at 37 °C.
Plasmid, small interfering RNA and transfection
The human miR-141-3p gene was PCR-amplified from genomic DNA and cloned into a pMSCV-puro retroviral vector (Clontech, Japan). The pNFκB-luc and control plasmids (Clontech, Japan) were used to examine the activity of transcription factor quantitatively. The 3′-untranslated region (3’UTR) regions of the human TRAF5 and TRAF6 were PCR-amplified from genomic DNA and cloned into pmirGLO vectors (Promega, USA), and the list of primers used in cloning reactions is presented in Additional file
1: Table S1. Anti-miR-141-3p, small interfering RNA (siRNA) for the TRAF5 and TRAF6 knockdown and corresponding control siRNAs were synthesized and purified by RiboBio. Transfection of miRNA, siRNAs, and plasmids was performed using Lipofectamine 3000 (Life Technologies, USA) according to the manufacturer’s instructions.
Total miRNA from tissues or cells was extracted using the mirVana miRNA Isolation Kit (Ambion). Messenger RNA (mRNA) and miRNA were reverse transcribed from total mRNA using the RevertAid First Strand cDNA Synthesis Kit (Thermo Fisher, USA) according to the manufacturer’s protocol. Complementary DNA (cDNA) was amplified and quantified on the CFX96 system (BIO-RAD, USA) using iQ SYBR Green (BIO-RAD, USA). The primers are provided in Additional file
2: Table S2. Real-time PCR was performed according to a standard method, as described previously [
31]. Primers for U6 and miR-141-3p were synthesized and purified by RiboBio (Guangzhou, China). U6 or glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was used as the endogenous controls. Relative fold expressions were calculated with the comparative threshold cycle (2
-ΔΔCt) method.
Western blotting
Nuclear/cytoplasmic fractionation was separated using the Cell Fractionation Kit (Cell Signaling Technology, USA) according to the manufacturer’s instructions, and the whole cell lysates were extracted with RIPA Buffer (Cell Signaling Technology). Western blotting was performed according to a standard method, as described previously [
32]. Antibodies against E-cadherin (Cat# 3195), Vimentin (Cat# 5741), Fibronectin (Cat# 4706), TRAF1 (Cat# 4710), TRAF5 (Cat# 41658) and TRAF6 (Cat# 8028) were purchased from Cell Signaling Technology, p65 (cat# 10745–1-AP) from Proteintech, and p84 (Cat#:PA5–27816) from Invitrogen. The membranes were stripped and reprobed with an anti–α-tubulin antibody (Sigma-Aldrich, USA) as the loading control.
Luciferase assay
Cells (4 × 10
4) were seeded in triplicate in 24-well plates and cultured for 24 h and performed as previously described [
33]. Cells were transfected with 100 ng of the pNFκB reporter luciferase plasmid, or pmirGLO-TRAF5–3′UTR, or –TRAF6–3′UTR luciferase plasmid, plus 5 ng pRL-TK the Renilla plasmid (Promega) using Lipofectamine 3000 (Invitrogen) according to the manufacturer’s recommendations. Luciferase and Renilla signals were measured 36 h after transfection using a Dual Luciferase Reporter Assay Kit (Promega) according to the manufacturer’s protocol.
miRNA immunoprecipitation
Cells were co-transfected with HA-Ago2, followed by HA-Ago2 immunoprecipitation using anti-HA-antibody, as previously described [
34]. Real-time PCR analysis of the IP material was performed to test the association of the mRNA of SOCS1 and TNIP1 with the RISC complex.
Invasion and migration assays
The invasion and migration assays were performed using Transwell chamber consisting of 8-mm membrane filter inserts (Corning) with or without coated Matrigel (BD Biosciences) respectively as described previously [
35]. Briefly, the cells were trypsinized and suspended in serum-free medium. Then, 1.5 × 10
5 cells were added to the upper chamber, and lower chamber was filled with the culture medium supplemented with 10% FBS. After incubation for 24–48 h, cells passed through the coated membrane to the lower surface, where cells were fixed with 4% paraformaldehyde and stained with haematoxylin. The cell count was performed under a microscope (×100).
Animal study
All mouse experiments were approved by The Institutional Animal Care and Use Committee of Sun Yat-sen University and the approval-No. was L102012016110D. For the bone metastasis study, BALB/c-nu mice ((5–6 weeks old, 18–20 g)) were anaesthetized and inoculated into the left cardiac ventricle with 1 × 10
5 PC-3 cells in 100 μl of PBS. Bone metastases were monitored by bioluminescent imaging (BLI) as previously described [
36]. Osteolytic lesions were identified on radiographs as radiolucent lesions in the bone. The area of the osteolytic lesions was measured using the Metamorph image analysis system and software (Universal Imaging Corporation), and the total extent of bone destruction per animal was expressed in square millimeters. Each bone metastasis was scored based on the following criteria: 0, no metastasis; 1, bone lesion covering <1/4 of the bone width; 2, bone lesion involving 1/4~1/2 of the bone width; 3, bone lesion across 1/2~3/4 of the bone width; and 4, bone lesion >3/4 of the bone width. The bone metastasis score for each mouse was the sum of the scores of all bone lesions from four limbs. For survival studies, mice were monitored daily for signs of discomfort, and were either euthanized all at one time or individually when presenting signs of distress, such as a 10% loss of body weight, paralysis, or head tilting.
Patients and tumor tissues
A total of 141 archived PCa tissues, including 89 non-bone metastatic PCa tissues and 52 bone metastatic PCa tissues were obtained during surgery or needle biopsy at The First People’s Hospital of Guangzhou City (Guangzhou, China) between January 2008 and October 2016. Patients were diagnosed based on clinical and pathological evidence, and the specimens were immediately snap-frozen and stored in liquid nitrogen tanks. For the use of these clinical materials for research purposes, prior patient’ consents and approval from the Institutional Research Ethics Committee were obtained. The clinicopathological features of the patients are summarized in Additional file
3: Table S3. The median of miR-141-3p expression in PCa tissues was used to stratify high and low expression of miR-141-3p.
Statistical analysis
All values are presented as the mean ± standard deviation (SD). Significant differences were determined using the GraphPad 5.0 software (USA). Student’s t-test was used to determine statistical differences between two groups. The chi-square test was used to analyze the relationship between miR-141-3p expression and clinicopathological characteristics. P < 0.05 was considered significant. All experiments were repeated three times.
Discussion
The primary results of the current study provide novel visions into the critical role of miR-141-3p in the repressive activation of NF-κB signaling, which further inhibits bone metastasis of PCa. Here, we reported that miR-141-3p expression was reduced in bone metastatic PCa tissues, and low expression of miR-141-3p correlated with PSA levels, Gleason grade and bone metastasis status in PCa patients. Our results further indicate that miR-141-3p inhibits NF-κB signaling in PCa cells via directly targeting TRAF5 and TRAF6, which further suppresses bone metastasis of PCa. Therefore, our results indicate miR-141-3p plays a tumor suppressive role in bone metastasis of PCa via inhibiting NF-κB signaling.
Extensive studies have shown that NF-κB signaling was constitutively activated in a variety of human cancer types and is associated with tumor initiation, progression and metastasis [
6,
37]. Helbig and colleagues has demonstrated that NF-kappaB promoted the motility of breast cancer cells by transcriptionally up-regulating the expression of CXCR4 [
38]; furthermore, aberrant activation of NF-κB signaling regulates multiple genes expression, including VEGF and IL-8, which are important for lung tumorigenesis via induction of angiogenesis [
39]. Accumulating literatures have demonstrated that activation NF-κB signaling is essential for the bone metastasis of cancers [
16,
19]. Park et al. reported that constitutive activation of NF-κB signaling in breast cancer cells promoted the bone resorption characteristic of osteolytic bone metastasis. The mediating gene involved in osteolytic bone metastasis of breast cancer was a key target of NF-κB signaling: granulocyte macrophage-colony stimulating factor (GM-CSF) promoted osteolytic bone metastasis of breast cancer cells by stimulating osteoclast development [
17]. Importantly, Chen et al. reported that NF-κB signaling was crucial for the development of PCa bone metastasis [
19]. However, the underlying mechanism of constitutive activation of NF-κB signaling in the bone metastasis of PCa remains poorly known. In this study, our results revealed that TRAF5 and TRAF6 were direct targets of miR-141-3p in PCa cells. In turn, downexpression of miR-141-3p constitutively activated NF-κB signaling through upregulating TRAF5 and TRAF6 in PCa cells. Importantly, inhibition of NF-κB signaling activity by specific inhibitors of NF-κB signaling LY2409881 and JSH-23attenuated the stimulatory effects of silencing miR-141-3p on invasion and migration of PCa cells. Taken together, our results uncover a novel regulatory mechanism for NF-kB activation in bone metastasis of PCa.
As an adaptor protein of NF-kB signaling cascades, TRAFs protein family transduct signal via binding to tumor necrosis factor (TNF) receptor cytoplasmic domains and mediating TNF-induced activation of NF-kB signaling [
14,
15]. Accumulating literatures have demonstrated that overexpression of TRAFs promotes the progression and aggressiveness of cancers via activating NF-kB signaling. For example, TRAF2 has been reported to upregulated in 15% of human epithelial cancer due to amplification and rearrangement, which contributes to the constitutive activation of NF-kB signaling [
40]. Compagno et al. reported that somatic mutation of TRAF5 sustained the activity of NF-kB signaling, which was associated with most aggressive subtype, activated B-cell-like Diffuse large B-cell lymphoma [
41]. Moreover, a study by Starczynowski and colleagues showed that TRAF6 exhibited concomitant mRNA overexpression and gene amplification in RAS-driven lung cancers. TRAF6 overexpression in NIH3T3 cells promoted anchorage-independent growth and tumor formation via activating NF-κB signaling [
42]. Thus, further exploring the mechanisms of regulation of TRAFs would increase our knowledge of the biologic basis of the constitutive activation of NF-kB in cancer and provide novel insights for tumor therapy. In this study, our results found that miR-141-3p directly suppressed the expression of TRAF5 and TRAF6, which further constrained the NF-kB signaling activity. Therefore, our results indicate that miR-141-3p inactivates NF-kB signaling via targeting TRAF5 and TRAF6, which further inhibits bone metastasis of PCa.
Abundant studies have shown that miR-141-3p was downexpressed in multiple human cancers, including hepatocellular carcinoma, prostate cancer, breast cancer, renal cell carcinoma, pancreatic cancer and gastric cancer and that low expression of miR-141-3p promoted cancer cell invasion and metastasis via varying mechanisms [
29,
30,
43‐
46]. However, recent literatures reported that miR-141-3p was upregulated in nasopharyngeal carcinoma and acted as an oncogenic miRNA [
47,
48]. These studies indicate that the pro- and anti-cancer roles of miR-141-3p are tumor-type dependent. Furthermore, low expression of miR-141-3p has been reported to be closely associated with metastatic phenotype of cancers [
49,
50]. Importantly, miR-141-3p expression levels has been reported to be downexpressed in bone metastatic PCa cells PC-3 [
29], suggesting that low expression of miR-141-3p plays an important role in the bone metastasis of PCa. However, the clinical significance and biological roles of miR-141-3p in bone metastasis of PCa remain largely unknown. In this study, our results revealed that miR-141-3p expression was reduced in human bone metastatic PCa tissues and cells. Low expression of miR-141-3p was positively associated with serum PSA level, Gleason grade and distant bone metastasis status in PCa patients. Our results further revealed that miR-141-3p repressed the activity of NF-κB signaling via targeting TRAF5 and TRAF6, which further inhibited the EMT, invasion, migration and bone metastasis of PCa cells in vitro and in vivo. Collectively, our findings indicate that miR-141-3p plays an important role in the bone metastasis of PCa.