Research progresses in roles of LncRNA and its relationships with breast cancer

Oncogenic c-Myc, which is located at chromosome 8q24, is one of the proto-oncogenic genes which is most frequently involved in human carcinogenesis [34]. Myc-induced transformation leads to the conversion from glucose to glutamine as the oxidizable substrate which is essential to maintain TCA cycle activity. c-Myc binds to the promoters and induces the expression of several crucial regulatory genes which are involved in glutaminolytic metabolism. It has been demonstrated that supra-physiological levels of Myc expression associated with oncogenic transformation are both necessary and sufficient for the induction of glutaminolysis to the excessive level that results in “glutamine addiction” specific to tumor cells.

The c-Myc mediates elevation of glutaminolysis in cancer cells. c-Myc promotes both glutamine uptake and glutamine catabolism. Because of c-Myc-mediated metabolic reprogramming, cancer cells tend to exhibit “glutamine addiction”. This is a typical example of metabolic reprogramming in cancer cells with oncogene-addiction, suggesting a potential “Achilles’ heel” of tumor cells that are addicted to glutamine metabolism in manner that is mediated by c-Myc [35]. The c-Myc/Max complex inhibits the ectopic differentiation of both types of artificial stem cells. Whereas c-Myc plays a fundamental role as a “double-edged sword” promoting both induced pluripotent stem cells generation and malignant transformation. Collectively, although the further research is warranted to develop the effective anti-tumor therapeutic strategy targeting Myc family, we should always catch up with the current advances in the complex functions of Myc family in highly-malignant and heterogeneous tumor cells to realize the precision medicine [36].

As a transcription factor, MYC’s primary mode of transformation is through the pro-tumorigenic transcriptional dysregulation of a wide variety of processes including proliferation, cell size, apoptosis, and metabolism. The alternative strategies of targeting MYC-driven cancers via selective inhibition of cellular pathways, like metabolism, that may selectively kill MYC-overexpressing cells have attractive therapeutic potential [37].

In a subset of neuroendocrine breast carcinoma tissues, the catalytic activity of EZH2 increases the number of the complex composed of N-Myc, AR, and EZH2-PRC2. Enhanced levels of EZH2 protein expression and EZH2 catalytic activity play a crucial role both in murine models overexpressing N-Myc and in human castration-resistant prostate cancer cells. N-Myc redirects EZH2 activity to N-Myc target gene promoters, resulting in the transcriptional suppression, whereas EZH2 inhibition reverses N-Myc-driven genetic regulation. Importantly, N-Myc sensitizes tumor cells to EZH2 inhibitors both in vitro and in vivo. In addition, N-Myc amplification rarely occurs in other lung cancer patho-histologic findings. N-Myc amplification also occurs in approximately 40% of neuroendocrine/small-cell prostate cancers, is commonly seen concurrently with amplification of the aurora kinase A gene (AURKA), and associated with poor prognosis.

Notably, downregulation of Myc interactor (NMI), a gatekeeper of epithelial phenotype, in breast tumors promotes mesenchymal, invasive and metastatic phenotype of the cancer cells. Aberrant miR-29 expression may account for reduced NMI expression in breast tumors and mesenchymal phenotype of cancer cells that promotes invasive growth. Reduction in NMI levels has a positive feedback machinery on miR-29 levels [36].

N-myc (MYCN), a member of the Myc family of basic-helix–loop–helix–zipper (bHLHZ) transcription factors, is a central regulator of many vital cellular processes. Overexpression of N-myc has been seen in a subset of BCs and correlates with poor prognostic features. Because phosphorylation of N-myc is directly regulated by Gsk3β, and indirectly by upstream signaling through the PI3K/Akt/mTOR pathway, targeting PI3K/Akt/mTOR may be an effective therapeutic approach [38]. NMI is an inducible protein whose expression is compromised in advanced stage breast cancer. Analysis of mRNA of NMI and miR-29 from patient derived breast cancer tumors showed a strong, inverse relationship between the expression of NMI and the miR-29. The related studies also revealed that in the absence of NMI, miR-29 expression is upregulated due to unrestricted Wnt/β-catenin signaling resulting from inactivation of GSK3β. Reduction in NMI levels has a feed-forward impact on miR-29 levels [39].

After neoadjuvant chemotherapy, 82.4% of patients showed pathologic partial response, with only 9.8% showing pathologic complete response. In multivariate analysis, MYC immunoreactivity and high MYC gain defined as MYC/nucleus ≥ 5 were significant predictor factors for pathologic partial response. MYC may have a role in chemosensitivity to AC and/or docetaxel drugs. The analysis of MYC amplification may help in the identification of patients that may have a better response to AC + T treatment [40].

The growth of ER-positive BC is mostly dependent upon estrogen, so anti-estrogen therapies are major means for treating this type of BC in clinical practices. Nevertheless, these therapies can’t completely inhibit the growth of BC cells. BCAR4 (breast cancer antiestrogen resistance 4), as a type of LncRNA connected with resistance to tamoxifen which is an endocrine drug, may be detected in 10–27% BC samples. For patients with metastatic BC who are treated by tamoxifen, relatively high level of BCAR4 mRNA is associated with high degree of tumor malignancy and short survival. Certain report has suggested that in human BC cells ZR-75-1 and MCF7, cells can proliferate without estrogen and with various antiestrogen factors owing to artificial expression of BCAR4. Besides, BCAR4-induced resistance to endocrine drugs doesn’t depend upon estrogen receptors. Therefore, BCAR4 promotes BC by strengthening drug resistance of BC cells. As a type of highly transformed genes, it makes the growth of cancer cells not depend upon estrogen and drug-resistant to antiestrogen therapies. As a result, it contributes to the genesis of tumors in vivo. Meanwhile, some researchers consider that BCAR4-based high tumor-specific expression would be used for treating anti-estrogen BC as target. The research of Godinho et al. has suggested that both ERBB2 and ERBB3 are upregulated in the BC cell ZR that expresses BCAR4. In their research, the activation of ERBB2/ERBB3 signaling pathway is considered to be the reason why BCAR4 promotes drug resistance of BC. BCAR4-positive patients with BC would benefit from BCAR4-targeted therapies. Shi et al. [41] have discovered that Lnc-ATB is upregulated in BC cells and tissues which are resistant to trastuzumab (i.e. a type of monoclonal antibody for immunotherapies and drug for targeted therapies). It can be induced and activated by TGF-β. By competitively binding with miR-200c, it may promote the resistance to trastuzumab and induce invasion-metastasis cascade to upregulate expressions of ZEB1 and ANF-217. As a result, EMT is caused. In addition, researchers have discovered that high-level Lnc-ATB is connected with the resistance of BC to trastuzumab. These research findings imply that perhaps Lnc-ATP would cause EMT and resistance to trastuzumab in patients with BC as a downstream factor of TGF-β. Furthermore, Jiang et al. [42] have reported that LncRNA HIF1A-AS2 and AK124454 may not only stimulate proliferation and invasion of BC cells, but also play certain roles in promoting resistance of triple-negative BC to paclitaxel. Possibly used as signals for recurrence of BC, HIF1A-AS2 and AK124454 can be also utilized for strengthening therapeutic effects of paclitaxel as a target.

It is thus clear that some LncRNAs may increase resistance of BC cells to endocrine drugs and those for targeted therapies, so as to promote the genesis of BC.

Recently, Liu et al. [61] have reported a new LncRNA known as NF-KappaB interacting LncRNA (NKILA). Upregulated by NF-κB, it binds with NF-κB/IκB to produce stable composites, so as to directly cover the phosphorylated structural domains of IκB. As a consequence, IKK (IκB kinase) induced IκB phosphorylation and NF-κB are activated. Importantly, NKILA may prevent excessive activation of the NF-κB in mammary epithelial cells under inflammatory stimuli. In MDA-MB-231 (i.e. a BC cell line), NKILA may enhance apoptosis and reduce invasion. To sum up, NKILA may control invasion and metastasis of BC by inhibiting activation of NF-κB.

Thus, it may be inferred that some LncRNAs inhibit genesis and development of BC. In terms of its mechanism, it mainly plays its roles in inhibiting genesis and development of BC by suppressing proliferation of BC cells, or promoting apoptosis of these cells, or inhibiting invasion and metastasis of the cells. Nevertheless, no many LncRNAs have been discovered to be effective for inhibiting BC, and very few of them have been investigated from the perspective of their inhibition mechanism. It is hoped that researchers can discover more LncRNAs that can inhibit BC and examine them more clearly through their efforts.

Li et al.’s study explored the mechanism underlying long non-coding RNA ROR regulating autophagy on tamoxifen resistance in BC. These results indicate that inhibition of long non-coding RNA ROR reverses resistance to tamoxifen by inducing autophagy in BC [62]. The expression levels of chemoresistance-associated long non-coding RNA (CRALA), a newly discovered long non-coding RNA, were measured by quantitative real time-PCR in 79 pre-treatment biopsied primary BC samples. Small interfering RNAs were used to knockdown CRALA expression. The effect of CRALA on chemosensitivity was evaluated using cell growth assay. The study findings indicate that CRALA expression may be an important biomarker for predicting the clinical response to chemotherapy and prognosis in BC patients. It is possible to target CRALA to reverse chemoresistance in BC patients [44]. Gu et al. [63] investigates the role of LncRNA growth arrest-specific transcript 5 (GAS5) in tamoxifen resistance in BC. Their study demonstrates that GAS5 enhances the efficacy of tamoxifen in the treatment of BC and could be a novel prognostic biomarker.

BC antiestrogen resistance 4 (BCAR4) was identified in a functional screen for genes involved in tamoxifen resistance. BCAR4 is a strong transforming gene causing estrogen-independent growth and antiestrogen resistance, and induces tumor formation in vivo. Due to its restricted expression, BCAR4 may be a good target for treating antiestrogen-resistant BC [64]. BCAR4 may have clinical relevance for tumour aggressiveness and tamoxifen resistance. The research of Godinho et al. [65] suggests that BCAR4-positive breast tumours are driven by ERBB2/ERBB3 signalling. Patients with such tumours may benefit from ERBB-targeted therapy.

Ding et al. [66] have studied the possible applications of LncRNAs in diagnosing BC as a type of LncRNAs that exist among genes. They have found that lincRNA-BC2 and lincRNA-BC5 are generally upregulated by over twice in BC specimens compared with normal mammary tissues, whereas lincRNA-BC4 and lincRNA-BC5 are downregulated. For stage 3 BC, the expression of LincRNA-BC4 is significantly low, whereas the expression of lincRNA-BC5 is significantly high, and the expression of lincRNA-BC2 has significant positive correlations with LNM (lymph node metastasis). In particular, the expression of numerous LncRNAs is found to be highly associated with different molecular classification [67]. Furthermore, some research has suggested that LncRNAs in serum may be used for diagnosing genesis of BC. For instance, the expression of RP11-445H22.4 increases significantly in serum of patients with BC. The sensitivity and specificity for diagnosing BC with this are up to 92% and 74% respectively among Chinese people [68]. Liu et al. [69] aimed to develop a long noncoding RNA (LncRNA) expression signature that can predict response to tamoxifen. A set of LncRNAs (LINC01191, RP4-639F20.1 and CTC-429P9.3) associated with distant metastasis-free survival was established. Estrogen receptor-positive BC patients in the training series could be classified into high- and low-risk groups with significantly different distant metastasis-free survival values based on this signature. The LncRNA signature may have possible clinical implications in the selection of high-risk patients for tamoxifen therapy. It is thus clear that certain LncRNAs may be employed for diagnosing BC as biomarkers.

Zhao et al. [70] have discovered a range of LncRNAs which may be used for differentiating patients with BC from low to high risks. The high expression of LINC00324 and low expressions of PTPRG-AS1 or SNHG17 are related to the relatively longer survival. Another study has suggested that SPRY4-IT1 promotes proliferation of BC cells by upregulating the expression of ZNF703. In addition, increased expression of SPRY4-IT1 is connected with higher tumor volume and more severe pathological staging in patients with BC [71]. Hence, SPRY4-IT1 is a type of new prognostic biomarkers and potential candidates of therapeutic targets.

The overexpression of HOTAIR in BC tissues is discovered to be connected with higher invasiveness and metastasis, which implies that HOTAIR would become biomarkers for predicting overall survival. The expression of HOTAIR may independently forecast if ER-positive BC is metastatic. MALAT1 is discovered to have higher expression in primary BC and its expression further increases in the course of metastasis [72]. On the contrary, the expression of BC040587 [73], NBAT1 [74] and EGOT [75] is discovered to be downregulated in BC tissues, which is associated with poor prognosis. Additionally, the high expression of LINC00472 in BC tissues is connected with poor invasiveness of BC cells and relatively good therapeutic effects [76]. Related clinical and in vivo studies suggest that LINC00472 is a type of tumor inhibitors, so it is possibly valuable for judging prognosis and predicting therapeutic effects of BC in clinical practices. This suggests that some LncRNAs may be used for judging prognosis of BC as biomarkers and have a great prospect for application in clinical practices.

MALAT1 is a highly conserved LncRNA that is highly expressed in several types of cancer, including BC. It has been demonstrated by in vivo and in vitro studies that MALAT1 promotes proliferation, tumor development and metastasis of TNBC. In addition, the expression of MALAT1 has been reported to be negatively correlated to the survival of ER negative, lymph node negative patients of the Her-2 and TNBC molecular subtypes. It is particularly noteworthy that a recent study using genetic interventions with MALAT1 antisense nucleotides has achieved good effects for suppressing cancer development in mouse models with luminal B BC. These studies suggested that MALAT1 is expected to become a new biomarker for prognosis of BCs and a potential target for treating them [77].

LncRNAs have potential value for being used for predicting drug resistance and survival of BC as markers. The overexpression of BCAR4 (a type of LncRNAs) is discovered to be effective for forecasting the resistance to tamoxifen which is a type of estrogenic drugs. On the other hand, LINC00160 and LINC01016 (i.e. two kinds of LinRNAs) are discovered to be highly expressed in ER-positive BC compared with ER-negative BC and normal tissues, which is discovered to have significant negative correlations with the overall survival of luminal A BC [78]. Additionally, a special BC subtype may be identified by detecting whether CCAT2 is overexpressed, having poor responses to adjuvant chemotherapies based on cyclophosphamide, methotrexate and CMF [79]. At last, the research of Chen et al. [80] has shown that the overexpression of ROR (i.e. a kind of LncRNAs) is correlated to chemotherapy resistance, which suggests that ROR may be used for predicting such resistance as an indicator. It may be inferred from the above that some LncRNAs are likely to become markers for predicting drug resistance and survival of BC.

Therefore, researchers are exploring the applications of LncRNAs from multiple perspectives (mainly including diagnosis of BC, judgment of prognosis, prediction of drug resistance and survival) based on their existing knowledge about them. Although it still needs some time to put them into practices, it is believed that the explorations of their applications will be more elaborate with the promotion of pertinent studies in terms of their breadth and depth. Meanwhile, new applications will be constantly developed.