Importance of long non-coding RNAs in the pathogenesis, diagnosis, and treatment of prostate cancer

Long non-coding RNAs (lncRNAs) are regulatory transcripts with essential roles in the pathogenesis of almost all types of cancers, including prostate cancer. They can act as either oncogenic lncRNAs or tumor suppressor ones in prostate cancer. Small nucleolar RNA host genes are among the mostly assessed oncogenic lncRNAs in this cancer. PCA3 is an example of oncogenic lncRNAs that has been approved as a diagnostic marker in prostate cancer. A number of well-known oncogenic lncRNAs in other cancers such as DANCR, MALAT1, CCAT1, PVT1, TUG1 and NEAT1 have also been shown to act as oncogenes in prostate cancer. On the other hand, LINC00893, LINC01679, MIR22HG, RP1-59D14.5, MAGI2-AS3, NXTAR, FGF14-AS2 and ADAMTS9-AS1 are among lncRNAs that act as tumor suppressors in prostate cancer. LncRNAs can contribute to the pathogenesis of prostate cancer via modulation of androgen receptor (AR) signaling, ubiquitin–proteasome degradation process of AR or other important signaling pathways. The current review summarizes the role of lncRNAs in the evolution of prostate cancer with an especial focus on their importance in design of novel biomarker panels and therapeutic targets.


Introduction
Prostate cancer is the most commonly diagnosed cancer among males being responsible for 27% of all diagnosed cases (1). It also accounts for the greatest number of deaths from cancer among men after lung cancer (1). A number of risk factors have been identified for prostate cancer among them are age, ethnicity, genetics, family history, obesity, and smoking (2,3). Prostate cancer is developed via a multistep process, starting from prostatic intraepithelial neoplasia and being evolved to localized, advanced prostate cancer with local invasion and metastatic prostate cancer, respectively (4). The aggressiveness of prostate cancer is best described by the Gleason grading system (5). The hormone responsiveness is an important feature in this cancer resulting in tumor regression following castration (6). Therefore, androgen deprivation therapy has been suggested as the regular therapeutic regimen for prostate cancer. However, resistance to this therapeutic modality can develop (4).
Identification of the underlying cause of initiation and progression of prostate cancer is an imperative step in development of novel therapies for this kind of malignancy. Moreover, it can facilitate design of novel biomarkers for early detection of cancers. Long non-coding RNAs (lncRNAs) are promising transcripts for both purposes (7)(8)(9). These transcripts have sizes more than 200 nucleotides and are responsible for a variety of regulatory mechanisms at different levels of gene expression regulation (10). Aberrations in the expression of lncRNAs might be representative of certain phases of cancer progression, and can be used to predict early progression of cancer or induction of cancer-related signaling pathways (11,12).
Therefore, these transcripts have attained much attention during recent years for their contribution in the pathogenesis of almost all kinds of cancers, including prostate cancer. The current review summarized the role of lncRNAs in the evolution of prostate cancer with an especial focus on their importance in design of novel biomarker panels and therapeutic targets. We used PubMed and Google Scholar databases with the key words "lncRNA" or "long non-coding RNA" and "prostate cancer". Then, we screened the obtained articles and included the relevant ones in the manuscript. Finally, we tabulated the data obtained from these articles for the purpose of better classification of the data.

Up-regulated lncRNAs in prostate cancer
Using quantitative real time PCR method, several lncRNAs have been shown to be over-expressed in prostate cancer tissues compared with adjacent non-cancerous tissues or benign prostate hyperplasia (BPH) samples, representing an oncogenic role for these transcripts in the progression of prostate cancer (Table 1). Small nucleolar RNA host genes (SNHGs) are among the mostly (Continued)  DANCR expression has been correlated with PSA level, Gleason score and T stage in these patients. DANCR expression not only can be used for prostate cancer diagnosis, but also can predict poor prognosis of this type of cancer with high diagnostic value. Mechanistically, up-regulation of DANCR or down-regulation of miR-214-5p could enhance proliferation and migration, preclude apoptosis, and induce activity of TGF-b signaling (45). DANCR can also target miR-185-5p to increase expression of LIM and SH3 protein 1 promoting prostate cancer through the FAK/PI3K/AKT/ GSK3b/snail axis (46). In addition, MALAT1 has been found to regulate glucose metabolism through modulation of MYBL2/mTOR axis (47). Moreover, in vitro and in vivo studies have shown the importance of MALAT1/miR-140/BIRC6 axis in the progression of prostate cancer (48). In fact, MALAT1 acts as a molecular sponge for miR-140 to enhance expression of the anti-apoptotic protein BIRC6 (48). In turn, expression and activity of MALAT1 have been shown to be regulated by miR-423-5p, a miRNA that impedes activity of MALAT1 in enhancement of proliferation, migration, and invasiveness of prostate cancer cells (49). Most importantly, upregulation of miR-423-5p could enhance survival and decrease metastasis formation in a xenograft model of prostate cancer (49). In addition, MALAT1 has a possible diagnostic value in prostate cancer. Expression levels of PCA3 and MALAT1 in urinary exosomes have been shown to be superior to the currently used clinical parameters in detection of prostate cancer, particularly high-grade ones (51).
NEAT1 has also been shown to regulate aerobic glycolysis to affect tumor immunosurveillance by T cells in this type of cancer (13). It can also promote progression of prostate cancer through modulation of miR-766-5p/E2F3 axis (54).
CTBP1-AS is reported as the antisense-RNA transcript positively regulated by androgen and promotes castrationresistant prostate cancer tumor growth (123). This lncRNA is localized in the nucleus and its levels are mostly increased in prostate cancer. It enhances both hormone-dependent and castration-resistant tumor growth. From a mechanistical point of view, CTBP1-AS suppresses the expression of CTBP1 through recruitment of PSF and histone deacetylases. It also exerts androgen-dependent function through inhibition of tumorsuppressor genes and enhancement of cell cycle progression (123).
Epigenetic repression of AR corepressor is an important mechanism for AR activation. ARLNC1 is also regulated by androgen and upregulates AR mRNA stability by binding to the 3'-UTR. In line with this, ARLNC1 silencing leads to inhibition of AR expression and suppression of AR signaling as well as of growth of prostate cancer. In fact, ARLNC1 has a role in the preservation of a positive feedback loop that induces AR signaling in the course of prostate cancer progression (124). In addition to these lncRNAs, several CRPC-specific AR-regulated lncRNAs are important for overexpression of AR and its variant. These AR-regulated lncRNAs are over-expressed in CRPC tissues. An experiment in these cells has shown that knock-down of PRKAG2-AS1 and HOXC-AS1 leads to suppression of CRPC tumor growth in addition to inhibition of expression of AR and AR variant. Mechanistically, PRKAG2-AS1 modulates the subcellular localization of the splicing factor, U2AF2. This splicing factor is involved in the AR splicing system (125).
SChLAP1 is another up-regulated lncRNA in prostate cancer whose up-regulation is associated with poor patient outcomes, such as metastases and prostate cancer specific mortality. It has a critical role in invasiveness and metastasis. Functionally, SChLAP1 influences the localization and regulatory function of the SWI/ SNF complex (126).
PCAT-1 is another up-regulated lncRNA in prostate cancer which enhances cell proliferation through cMyc. Mechanistically, PCAT-1-associated proliferation depends on stabilization of cMyc protein. Moreover, cMyc has an essential role in a number of PCAT-1-induced expression alterations (127).
HOTAIR as regarded as an AR-repressed lncRNA is upregulated after androgen deprivation therapy and in CRPC. Mechanistically, HOTAIR binds to the AR protein to inhibit its interactions with the E3 ubiquitin ligase MDM2, thus suppressing AR ubiquitination and its degradation. Therefore, HOTAIR induces androgen-independent AR activation and drives the AR-mediated transcriptional program in the absence of androgen (128). Another study has shown that NEAT1 induces oncogenic growth in prostate tissue through changing the epigenetic marks in the target genes promoters to induce their transcription (129). Moreover, PCGEM1 and PRNCR1 bind to AR and enhance selective looping of ARbound enhancers to target gene promoters (130). Similarly, SOCS2-AS1 interacts with AR for co-factor interaction (131).
The importance of other up-regulated lncRNAs in prostate cancer is summarized in Figure 1 and Table 1.

Down-regulated lncRNAs in prostate cancer
A number of other lncRNAs have been found to act as tumor suppressors in prostate cancer (Table 2). For instance, LINC00893 can inhibit progression of this type of cancer via modulation of miR-3173-5p/SOCS3/JAK2/STAT3 axis (132). Similarly, the sponging effect of LINC01679 on miR-3150a-3p has a role in inhibition of progression of prostate cancer through affecting expression of SLC17A9 (133). MIR22HG is another tumor suppressor lncRNA that acts as a molecular sponge for miR-9-3p (134). The tumor suppressor role of RP1-59D14.5 in prostate cancer is mediated through activation of the Hippo signaling and enhancement of autophagy (135). Moreover, MAGI2-AS3 has been shown to inactivate STAT3 signaling and suppress proliferation of prostate cancer cells through acting as a miR-424-5p sponge (136). NXTAR is another tumor suppressor lncRNA that modulates expression of androgen receptor (AR) and resistance to enzalutamide (137). Totally, the number of identified tumor suppressor lncRNAs in prostate cancer is far below that of oncogenic lncRNAs (Figure 2). Table 2 summarizes the information about tumor suppressor lncRNAs in prostate cancer.

Contribution of lncRNAs variants in prostate cancer
Contribution of single nucleotide polymorphisms (SNPs) within GAS5, POLR2E, MEG3, MALAT1 and HOTAIR in the risk of prostate cancer has been assessed in different ethnic groups (Table 3). Three SNPs within GAS5 have been the subject of these investigations. First, rs145204276 (delCAAGG) is located within the promoter region of GAS5. Compared with subjects carrying ins/ins genotype, cases with ins/del or del/del genotype of this polymorphism have shown decreased risk of pathological lymph node metastasis (152). The rs17359906 in GAS5 is another SNP whose A allele has been shown to be a risk allele for prostate cancer. Similarly, A allele of rs1951625 SNP within GAS5 has been associated with higher risk of this cancer. Both rs17359906 G > A and rs1951625 G > A have been associated with high plasma level of PSA. Most importantly, the recurrence-free survival of patients with prostate cancer has been lowest in patients having AA genotype of rs17359906 and highest in those having GG genotype. Similar findings have been reported for the rs1951625 (153).
A systematic review and meta-analysis of 5 studies on the role of rs3787016 within POLR2E has revealed increased susceptibility to prostate cancer for carriers of T allele in all genotype models (154). The results of other studies on contribution of lncRNAs SNPs in prostate cancer are shown in Table 3.

Importance of lncRNAs as prognostic factors in prostate cancer
Several studies have indicated the importance of dysregulation of lncRNAs in the prediction of survival times of patients with prostate cancer (Table 4). Overall, up-regulation of oncogenic lncRNAs is predictive of lower survival time of patients in terms of overall survival or progression-free survival. For tumor suppressor lncRNAs, an opposite effect has been seen.

Discussion
Several lncRNAs have been shown to contribute to the pathogenesis of prostate cancer via modulation of AR signaling, ubiquitin-proteasome degradation process of AR or other important signaling pathways. Some of them such as PCA3 are highly specific for this kind of cancer, representing an appropriate biomarker for prostate cancer (151). Others might be over-/underexpressed in a variatey of cancers, being therapeutic targets for a wide range of human malignnacies. The observed differences in expression of some lncRNAs between castration-resistant prostate cancer and androgen deprivation therapy-responsive cases imply the importance of these transcripts in defining response of patients to this therapeutic modality and represent these transcripts as targets for management of resistance to this therapy.  -Gleason score and expression of UBE2R2-AS1 are independent prognostic factors for OS of PC patients. (13)

SNHG17
52 PTNTs Its high expression is associated with poor BCR-free survival.
Over expression of SNHG17 is associated with poor OS in PC patients.
Its expression is an independent prognostic factor for OS in patients with PC. (14) LINC00893 66 PTNTs Its low expression is correlated with poorer OS. -- LINC01679 55 PTNTs Its low expression is correlated with reduction in DFS. --

SNHG3
30 PTNTs Its high expression is associated with shorter OS time.

NEAT1
50 PTNTs Its high expression is associated with lower survival rate. --

SNHG3
50 PTNTs Its upregulation is associated with shorter OS and BMFS.
Its high expression is an independent risk factor for death and progression in patients with PCa. (32)

DLEU2
Prostate tumor tissues from TCGA database Its high expression is correlated with lower survival rate.
Its upregulation is associated with a poor progression-free interval.
Its upregulation is independently associated with a poor progression-free interval.
HOXD-AS1 36 PCa and 9 BPH cases Its high expression is associated with shorter PSA.
Serum exosomal HOXD-AS1 in conjunction with tumor stage is a prognostic factor for PRFS.
Serum exosomal HOXD-AS1 is an independent prognostic factor for PFS DANCR is an independent prognostic indicator for PCa.
SNHG17 53 PTNTs Its high expression is associated with poor OS time. --

PVT1
RNA-Seq data from TCGA-PRAD database Its high expression is associated with poor vital survival rates.
Its expression is associated with OS and relapse-free survival.
Its high expression is an independent prognostic factor (74)

(Continued)
Although numerous prostate cancer-specific or prostate cancerassociated lncRNAs have been recognized, few lncRNAs have been verified in independent patient cohorts or approved for using in clinical settings. The most important milestone in the field of lncRNA research is probably approval of urinary PCA3 as a biomarker for detection of prostate cancer by the United States Food and Drug Administration (158). This lncRNA is a promising factor for urine test for prostate cancer and has a superior performance compared with PSA in urinary detection of this disorder. Further reseraches are needed to find other appropriate lncRNA biomarkers for this kind of cancer.
LncRNA profiles can also been used to identify prostae cancer patients that benefit from radiotherapy. For instance, UCA1 has beens shwon to mediate radiosensitivity in prostate cancer cell lines and therefore might be a marker to predict response to radiotherapy in these patients. This lncRNA affects radiosensitivity through influencing cell cycle progression (159).
The importance of lncRNAs in the mediation of cell proliferation, invasiveness and metastasis has potentiated them as therapeutic targets for prostate cancer. The results of animal studies have been promising particularly for some AR-regulated lncRNAs. However, clinical studies are missing in this field.
Notably, LncRNAs are also involved in drug resistance in prostate cancer cells, thus they are proper candidates for therapeutic targeting (160). For instance, HORAS5 up-regulation can trigger taxane resistance in CRPC cells through upregulation of BCL2A1. HORAS5 silencing can reduce resistance of prostate cancer cells to cabazitaxel and enhance the efficacy of chemotherapy (161).
Although expression profile of lncRNAs have been comprhensively assessed in tumoral tissues of patients with prostate cancer, less effort has been made for analysis of their expression in urine or serum samples. Based on the availability of these sources for non-invasive diagnostic procedures, future studies should focus on these biofluids to facilitate early detection of prostate cancer via non-invasive methods.
Taken together, lncRNAs have been found to contribute to the pathogenesis of prostate cancer through various mechanisms. These transcripts can be used as targets for therapeutic interventions in this kind of cancer.

Author contributions
MT and AB designed and supervised the study. SG-F wrote the draft and revised it. EB, BH, and AK collected the data and designed the figures and tables. All authors contributed to the article and approved the submitted version.

Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

PTNTs
Its high expression is positively associated with OS of patients with PCa. --

RAMS11
42 PTNTs Its upregulation is correlated with poorer OS and DFS. -- SNHG9 495 PCa tissues and 52 adjacent prostate tissues Its high expression is associated with poor prognosis.
Its expression level is associated with poorer PFS.
Its expression is independently associated with PFS in PCa patients.

LINC00641
23 PTNTs Its low expression is associated with lower survival rate. -- Publisher's note All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.