TINCR: An lncRNA with dual functions in the carcinogenesis process

Long non-coding RNAs (lncRNAs) have prominent roles in the pathogenesis of human cancers. Several studies have shown oncogenic or tumor suppressor roles of lncRNAs in different human tissues. Thus, these transcripts have been regarded as putative targets in treatment of cancer. The lncRNA terminal differentiation-induced non-coding RNA (TINCR) has an especial position in this regard, as it exerts different opposite roles in the pathogenesis of different human cancers. While it is up-regulated in gastric, esophageal, bladder and breast cancer; it is down-regulated in glioma, retinoblastoma and prostate cancer. Notably, data regarding expression profile of this lncRNA in a number of human cancers such as colon cancer, squamous cell carcinoma, non-small cell lung cancer (NSCLC) and hepatocellular carcinoma (HCC) are controversial. Expression level of this lncRNA has been associated with clinical outcome in patients with gastric cancer, colorectal cancer, NSCLC and head and neck squamous cell carcinoma. Moreover, Kaplan-Meier analyses have shown correlation between expression levels of TINCR and patients survival in patients with lung cancer and HCC. A number of cellular pathways such as Wnt/β-catenin, ERK1/2‐SP3 and MAPK signaling pathways have been identified as targets of this lncRNA in different cancers. Moreover, the rs8113645, rs2288947 and rs8105637 within this lncRNA have been associated with risk of gastric and colorectal cancer. In conclusion, although the role of TINCR in the carcinogenesis is essential, based on the conflicting data regarding the direction of effect of this lncRNA, therapeutic targeting of this lncRNA is a complicated issue which should be considered in a tissue-specific or even individualized manner.


Introduction
Long non-coding RNAs (lncRNAs) as a group of transcripts with sizes more than 200 nt are considered as important regulators of genes expression and carcinogenesis process. These transcripts participate in multiple cellular processes such as epigenetic regulation of gene expression, modulation of expression at transcriptional and post-transcriptional levels, thus influencing cell proliferation, apoptosis, migration and stability of the genome [1]. Several lncRNAs have been demonstrated to affect the carcinogenesis process [2]. Notably, they usually exert either oncogenic roles or tumor suppressor role in human cancers [3]. Yet, a number of lncRNAs have recently identified that their role in the carcinogenesis process depends on the tissue where they are expressed. Among these lncRNAs is the terminal differentiation-induced non-coding RNA (TINCR) [4][5][6][7]. This lncRNA has a 3733 nt length and its expression has been recognized at a late phase of human epidermal differentiation [8]. It regulates expression of several important differentiation genes such as FLG, LOR, ALOXE3, ALOX12B, ABCA12, CASP14 and ELOVL3 at post transcriptional level. Notably mutations in a number of these genes have been associated with skin diseases [8]. Functional studies have revealed the presence of a TINCRa 25-nucleotide 'TINCR box' motif in target mRNAs that mediates interaction between these mRNAs and TINCR [8]. Additional in silico analyses predicted the role of TINCR in esophageal development as well [9]. Based on the data provided by Human Protein Atlas RNA-seq dataset, TINCR has been shown to have specific expression in skin, placenta and esophagus [9]. Subsequent studies reported its aberrant expression in a wide array of human malignancies. The most important note about this lncRNA is its different roles in the pathogenesis of different cancers. In this review, we summarize the available data regarding the role of this lncRNAs in human cancers.  silencing of or forced over-expression of this lncRNA in cell proliferation, apoptosis of invasive properties of these cells. TINCR silencing in human gastric cancer cell lines has decreased cell proliferation and colony formation [6]. In the following sections, we describe the role of this lncRNA in different cancer cell lines.

Gastric cancer
Expression of TINCR has been significantly higher in gastric cancer cell line compared with a human gastric epithelial cell line [10]. Notably, in gastric cancer cell lines, the nuclear transcription factor SP1 has a prominent role in induction of expression of this lncRNA. The oncogenic role of TINCR in gastric cancer cells is mediated through its interaction with STAU1 protein. This interaction can modulate stability and expression of KLF2 and subsequently regulate expression of cyclindependent kinase genes [6]. Moreover, E2F1 enhances expression of TINCR in gastric cancer cells. Forced over-expression of E2F1 enhances gastric cancer cells proliferation, while its silencing reduces cell proliferation through hindering cell cycle progression in these cells. This transcription factor enhances growth of gastric cancer cells via induction of TINCR expression. TINCR interacts with STAU1 protein to affect stability and expression of the CDKN2B transcript, thus enhancing the proliferation of these cells [11].

Colorectal cancer
In colorectal cancer cells, mechanistic studies revealed conflicting results. Small interfering RNA (siRNA)-mediated silencing of TINCR in HCT116 and HCT8 cells suppressed cell proliferation, inhibited colony forming capacity and decreased cells migration and invasion [12]. On the other hand, another study in SW620 and HTC116 cells demonstrated the role of this lncRNA in suppression of proliferation and migration. Yet, a radioresistant colorectal cancer cell line (SW620R) showed over-expression of TINCR. Notably, TINCR silencing decreased radioresistance of these cells [13].

Non-small cell lung cancer (NSCLC)
NSCLC is another cancer type in which mechanistic studies of TINCR showed conflicting results. This lncRNA has been shown to interact with BRAF to enhance its kinase activity, therefore resulting in activation of MAPK pathway [14]. On the other hand, other have shown that this lncRNA decreases expression of miR-21 in NSCLC and suppresses cancer cell migration and invasion [15].

Hepatocellular carcinoma (HCC)
In HCC, two distinct studies showed association between over-expression of this lncRNA and induction of apoptosis via regulation of P53 and enhancement of cell proliferation via modulation of the miR-214-5p/ROCK1 axis, respectively [16,17]. Consistent with the latter study, depletion of this lncRNA in other HCC cell liens decreased oncogenic behavior through modulation of miR-218-5p/DDX5/AKT signaling [18].

Breast cancer
SP1 has a similar role in induction of TINCR expression in breast cancer cell lines. Up-regulation of this lncRNA in these cells enhanced cell proliferation, anchorage-independent growth and inhibited cell apoptosis in these cells. Notably, TINCR exerts its oncogenic role through competing with miR-7 and modulating KLF4 expression [19].
Another study in breast cancer demonstrated the role of TINCR in stimulation of tumorigenesis through modulating expression of miR-125b and its target gene ERBB2. Through this molecular axis, TINCR inhibits apoptosis in breast cancer cells [20]. Finally, TINCR has been shown to be over-expressed in trastuzumab-resistant breast cancer cells compared with sensitive cells. Its silencing has changed the trastuzumab resistance phenotype and reversed the attained epithelial-mesenchymal transition (EMT) in these cells. Its interaction with miR-125b has been shown to release HER-2 and prompt trastuzumab resistance. The CREBbinding protein-mediated H3K27 acetylation at the promoter region of TINCR has been suggested as the underlying mechanism for over-expression of TINCR in breast cancer [21].

Other cancers
In esophageal squamous cell carcinoma cells (SCC), siRNA-mediated silencing of TINCR repressed cell proliferation, migration and invasion. Moreover, this approach led to induction of apoptosis and inhibition of cell cycle progression [22]. A single study in oral SCC verified the results of this study [23]. However, another study in diverse types of SCC originated from cervix, head and neck and lung implied a tumor suppressor role for this lncRNA as TINCR silencing by siRNA enhanced cell growth and migration of these cells [24].
Other studies in glioma [25], prostate cancer [5] and retinoblastoma [26] indicated a tumor suppressor role for TINCR. Table 1 summarizes the results of cell line studies that assessed expression and function of TINCR in various malignancies.

In vivo studies
A number of studies have investigated the effects of TINCR silencing or over-expression in xenograft animal models. In gastric cancer, the results of in vivo studies were consistent with the proposed oncogenic role of TINCR from cell line studies since two independent studies showed decreased tumor growth following TINCR knock-down in xenograft animal models [4,6]. In colorectal cancer, two in vivo studies revealed inconsistent results. Zhang et al. have compared the tumor growth in BALB/c mice after subcutaneous injection of HCT116-shTINCR or HCT116-NC. Notably, they reported larger sizes and more rapid growth of tumors in sh-TINCR group. Furthermore, these tumors had higher ki-67 proliferation index, higher rate of metastatic foci in the lung and the liver compared with the control group. Thus, they suggested that TINCR silencing intensely stimulates tumor growth and metastasis [7]. On the other hand, Yu et al. have shown that stable knockdown of TINCR in HCT116 cells inhibits cancer cells growth and metastasis in BALB/c nude mice [12]. Two independent in vivo studies in breast cancer reached the similar results confirming the oncogenic role of TINCR in this kind of cancer [19,21]. Notably, the latter also verified the effects of this lncRNA in induction of Trastuzumab resistance [21]. Besides, TINCR knock-down has suppressed growth of NSCLC in xenograft model through modulating expression of miR-29b [28]. However, experiments in a mouse xenograft model which was established through subcutaneous injection of glioma cells have shown lower growth rate and tumor weight in TINCR overexpressing cells. Furthermore, protein levels of Ki-67 and RPL36 were lower in TIN-CR-overexpressing tumors. Thus, TINCR has been shown to suppress glioma growth in vivo [25]. Table 2 shows summary of studies which assessed function of TINCR in animal models.

Human studies
Several studies have compared expression of TINCR between human tumor samples and non-cancerous samples from the same tissues. A number of studies have reported up-regulation of this lncRNA in gastric, esophageal, bladder and breast cancer tissues compared with the corresponding non-cancerous tissues [4,6,28,31]. However, other studies have demonstrated down-regulated of TINCR in glioma, retinoblastoma and prostate cancer [5,25,26]. Notably, data regarding expression profile of this lncRNA in a number of human cancers such as colon cancer, squamous cell carcinoma, non-small cell lung cancer and HCC are controversial [7,12,[14][15][16]22,23,30]. Expression level of this lncRNA has been associated with clinical outcome in patients with gastric cancer [6], colorectal cancer [12], head and neck SCC [24], nonsmall cell lung cancer [14] and some other cancers. Notably, Kaplan-Meier analyses have shown correlation between expression levels of this lncRNA and patients survival in both directions. This issue has been reported in patients with lung cancer [14,15] and HCC [18,30]. Table 3 summarizes the results of studies which reported aberrant expression of TINCR in clinical samples.
Based on the importance of lncRNAs in differentiating the disease status in clinical samples, a number of studies have assessed the diagnostic power of TINCR in different cancer types. This approach has been performed on both tissue samples and plasma samples, Based on the measured area under curve (AUC) values in receiver operating characteristic (ROC) curves, the best diagnostic power has been reported in colorectal cancer where plasma levels of this lncRNA could differentiate patients from healthy controls with 92% accuracy [12]. Moreover, expression levels of this lncRNA could discriminate oral SCC tissues from adjacent non-cancerous tissues with diagnostic power of 87% [23]. Table 4 summarizes the results of studies which assessed diagnostic power of this lncRNA in clinical samples.
TINCR has a number of single nucleotide polymorphisms (SNPs) which might affect expression or function of this lncRNA. Based on the importance of this lncRNA in human malignancies, these SNPs are expected to modify the risk of cancer. Ma et al. have assessed association between four tag SNPs (rs8113645, rs2288947, rs8105637, rs12610531) across the entire TINCR locus and risk of gastric cancer in a Chinese population [32]. Notably, they reported associations between rs8113645 and rs2288947 variant alleles and decreased risks of gastric cancer in the assessed population. Genotypes having A allele of rs8113645 and G allele of rs2288947 reduced risk of this cancer. Particularly, the observed associations were more prominent in younger persons, males, nonsmokers, and persons from rural areas. The functional role of the rs8113645 has been verified through the observed lower tissue expression of TINCR in GA + AA genotype carriers [32]. In another study, Zheng et al. assessed association between three tag SNPs (rs2288947, rs8105637 and rs12610531) and colorectal cancer in a Chinese population. They reported associations between rs2288947 and rs8105637 SNPs and risk of this cancer in their cohort of patients. Notably, the mentioned SNPs were cancer metastasis to lymph nodes as well [33]. Table 5 summarizes the results of studies which assessed association between TINCR SNPs and risk of cancer.

Conclusions
Several lncRNAs have been reported to affect carcinogenesis process [3]. Among the newly assessed lncRNAs in this regard is the TINCR. TINCR is an lncRNA whose function in human malignancies is tissuesspecific. This means that TINCR exert oncogenic effects in some human tissues and tumor suppressive effects in others. This observation can be explained by a tissue-specific influence of this lncRNA on distinct signaling pathways and molecular targets which are prominent in each tissue. However, a more complicated issue has been raised when assessing function of this lncRNAs in some tissues such as colon, lung and hepatic tissues in which different studies revealed controversial results regarding the role of this lncRNA. In cancer cell lines, this observation might be explained by the diversity of cancer cell liens, passage number and other culture conditions. Yet, when this issue is assessed in clinical samples, it best reflects the heterogeneity of cancerous cells and their behavior and necessitates a personalized approach for treatment of cancer patients. Perhaps, the presence of other genetic or environmental factors is important in determination of the role of this lncRNA.
TINCR has a sponging effect on a number miRNAs. For instance, in gastric tissues, it exerts a competing endogenous RNA (ceRNA) role to modulate PDK1 expression by sponging miR-375 [4]. In colorectal and hepatic tissues, TINCR has been shown to exert its effects through sponging miR-7-5p and miR-214-5p, respectively [12,18]. Finally, in lung tissues, it regulates cell proliferation through sponging miR-29b [28]. Thus, a possible explanation for tissue-specific role of this lncRNA might be the relative abundance of its targets in each tissue.
Although several studies have addressed the potential of lncRNAspecific strategies for cancer treatment [34], modalities that alter expression and function of TINCR should be applied with especial caution. A theophylline controllable RNA interference strategy has been successfully switches expression of this lncRNA in bladder cancer tissues [31]. Such controllable molecular approaches are promising methods in cancer biology. However, if these strategies are being tested in clinical settings, more strict criteria should be applied for selection of patients and cancer types.
In addition to the mechanistic studies which showed regulatory effects of TINCR on cancer-related pathways such as Wnt/β-catenin, ERK1/2-SP3 and MAPK signaling pathways and important tumor suppressor genes such as P53, the presence of genomic variants within this lncRNA that influence risk of human cancer support the importance of TINCR in the carcinogenesis. Meanwhile, different roles of this lncRNA in diverse tissues further support the critical role of tissue context or tumor microenvironment in this process. Thus, a holistic point of view which is brought by a system biology method is required for understanding the complex interaction network that determines or modifies the role of TINCR in each tissue or context.

Declaration of competing interest
The authors declare they have no conflict of interest.

Acknowledgment
This study is financially supported by Shahid Beheshti University of