4.1. HOTAIR Is a Powerful Prognostic Factor in Breast Cancer (BC)
Dysregulation of
HOTAIR in various cancers is associated with metastasis and tumor progression.
HOTAIR was found to be highly up-regulated in primary, as well as metastatic, breast tumors, and its elevation correlated with both metastasis and poor survival rate [
11,
30].
HOTAIR expression is increased in primary breast tumors and metastases (as much as 2000-fold), and
HOTAIR expression in primary tumors is a powerful predictor of eventual metastasis and death, independent of known clinicopathologic risk factors. Gupta
et al. [
11] measured
HOTAIR levels in an independent panel of 132 primary breast tumors (stage I and II) with extensive clinical follow-up. Nearly one-third overexpressed
HOTAIR by more than 125-fold when compared to normal breast epithelia. A high level of
HOTAIR is a significant predictor of subsequent metastasis (
p = 0.0004) and death (
p = 0.005). Stable overexpression of
HOTAIR by several hundredfold through retroviral transduction promoted colony growth in soft agar. Conversely, depletion of
HOTAIR with siRNA in the MCF7 cell line, which expresses endogenous
HOTAIR, decreased matrix invasiveness, thus demonstrating the function of
HOTAIR in vitro. In addition, Gupta
et al. labeled control and
HOTAIR-expressing MDA-MB-231 cells with firefly luciferase and grafted the cells into mammary fat pads. Two weeks later, the number of luciferase foci in the lung fields was higher in mice with
HOTAIR-expressing primary tumors than in control mice, which suggests that
HOTAIR promotes lung metastasis
in vivo.
4.1.1. HOTAIR and the Methylation Status of Down-Stream CpG Islands
The prognostic value of
HOTAIR in breast cancer was widely accepted until an article published at the end of 2012 challenged the prevailing view. Lu
et al. [
41] analyzed DNA methylation in 348 primary breast cancer tissues with methylation-specific PCR and found a positive correlation between DNA methylation and
HOTAIR expression. Methylation was associated with unfavorable disease characteristics. However, no significant associations were found between
HOTAIR expression and clinical or pathologic features. Patients with high
HOTAIR expression had lower risks of relapse and mortality than those with low
HOTAIR expression. These findings suggest that intergenic DNA methylation may be important in regulating
HOTAIR expression and that
HOTAIR expression may not be an independent prognostic marker in breast cancer. However, the findings require further validation in independent studies.
4.1.2. HOTAIR Function in Breast Cancer Is Associated with Estrogen Receptor Status
Recent studies showed that
HOTAIR is an independent prognostic factor of metastases in estrogen receptor (ER)-positive primary breast cancer. A retrospective study of 164 primary breast cancer patients by Sørensen
et al. [
15] found that a high level of
HOTAIR was significantly associated with worse prognosis (
p = 0.012). The association was even stronger when only ER-positive tumor samples were considered (
p = 0.0086). Bhan
et al. [
42] demonstrated that
HOTAIR requires transcriptional induction by E2. The
HOTAIR promoter contains multiple functional estrogen response elements. In the presence of E2, ERs and various ER coregulators bind to the promoter. Meanwhile, the level of H3K4 trimethylation, histone acetylation, and RNA polymerase II recruitment is enriched at the
HOTAIR promoter. In contrast, knockdown of ERs down-regulates E2-induced
HOTAIR expression. Given these findings, therapeutic methods that target
HOTAIR or regulate ER status may help suppress the progression of breast cancer.
4.2. HOTAIR Is a Potential Prognostic Factor in Gastroenteric Tumors
As in breast cancer,
HOTAIR also induces genome-wide PRC2 retargeting, participates in epigenetic regulation and promotes metastases in colorectal cancer (CRC) [
11,
13,
43]. Analysis of approximately 100 cancer tissues showed that
HOTAIR expression was significantly higher in cancer tissues than in normal tissue samples (
p = 0.002). After dividing the cancer tissues into a high
HOTAIR expression group (
n = 20) and a low expression group (
n = 80) using a
HOTAIR/GAPDH ratio of 0.273, further analysis revealed that the high expression group exhibited less differentiated histology (
p = 0.024), greater tumor depth (
p = 0.039), greater liver metastases (
p = 0.006), and worse prognosis (
p = 0.0046). Transduction of lentiviral vectors encoding
HOTAIR into the HCT116 cell line promoted cell invasion in Matrigel (
p < 0.05). Conversely, siRNA-induced knockdown of
HOTAIR decreased cell invasion (
p < 0.05), further suggesting that
HOTAIR is related to liver metastases and is a potential prognostic factor in colorectal cancer [
13]. On the other hand,
HOTAIR suppression sensitized cancer cells to tumor necrosis factor α (TNF-α), induced apoptosis, and rendered the cells more sensitive to the chemotherapeutic agents cisplatin and doxorubicin [
13], indicating that
HOTAIR could be a target for therapy.
Svoboda M,
et al. [
43] also assessed the prognostic value of
HOTAIR expression. On one hand, they underlined the prognostic potential of
HOTAIR expression level in tumor tissues of CRC patients, both in univariate analysis (
p = 0.046) and multivariate analysis (
p = 0.048). What is more, they demonstrated that
HOTAIR relative expression in tumor and paired blood are positively correlated (
R = 0.43,
p = 0.03). In univariate analysis,
HOTAIR levels in blood were associated with higher mortality of patients (Cox’s proportional hazard, hazard ratio = 5.9, 95% confidence interval: 1.3–26.1,
p = 0.019). This means that the
HOTAIR expression in blood can also be an independent prognostic marker in CRC. As a kind of surrogate sample of tumor tissue, the non-invasively obtained HOTAIR data may have vast clinical potential in prognosis. Moreover, HOTAIR overexpression-either in tumor tissue or in blood-may identify patients that would require more intensive care of personally tailored treatment.
EZH2 and SUZ12, the components of PRC2, are overexpressed in several cancers. In particular, SUZ12 is reportedly overexpressed in colorectal cancer. Gene pathway analysis indicated that HOTAIR-regulated gene sets included CDH1 (E-cadherin) target genes, whose expression is lost in metastatic cancer cells of the mesenchymal phenotype. Thus, HOTAIR might cooperate with PRC2 to maintain mesenchymal and undifferentiated cancer cells. For these reasons, HOTAIR or SUZ12 might be a treatment target in colorectal cancer.
Studies of
HOTAIR in gastric cancer (GC) have just begun, but its relevance has already been demonstrated. Using soft agar assays, Endo
et al. [
17] showed that the anchorage-independent growth of gastric cancer cells depended on
HOTAIR expression. They grafted gastric cancer cells with increased or suppressed
HOTAIR expression into the tail vein or peritoneal cavity of immunodeficient mice. The
HOTAIR-expressing group showed greater venous invasion, more frequent lymph node metastases, and lower overall survival rate than the
HOTAIR-suppressed group.
HOTAIR-expressing cells were also more likely to induce liver metastases. These findings suggest that
HOTAIR has prognostic value in gastric cancer
in vitro and
in vivo. However, the molecular mechanism of
HOTAIR in gastric cancer is poorly understood. Recently published data suggested it may function as a miRNA sponge and induces down-regulation of
miR-331-3p, thereby modulating the derepression of
HER2 expression and promoting migration and invasion of gastric cancer cells [
44,
45]. Meanwhile, Xu
et al. [
37] found that inhibition of
HOTAIR could reduce invasiveness and reverse EMT process in these cells. These results also indicate that
HOTAIR may be a therapeutic target in the treatment of gastric cancer.
4.3. HOTAIR Is a Potential Prognostic Factor of Hepatoma Metastases and Relapse after Hepatic Resection or Liver Transplantation
In primary hepatocellular carcinoma (HCC),
HOTAIR overexpression is not observed in every cancer patient. However, a study of HCC patients showed that patients with
HOTAIR overexpression exhibit larger tumor size, worse prognosis, and an increased risk of metastasis when compared to patients with normal
HOTAIR levels [
12]. In a study of 63 patients after hepatectomy, Geng
et al. [
46] found that
HOTAIR was associated with hepatocellular growth and concluded that
HOTAIR might be a biomarker for predicting lymph node metastasis. In some cancer patients, the
HOTAIR level of the cancer biopsy was higher than adjacent normal tissue; and this cohort showed higher risk of relapse. Knockdown of
HOTAIR down-regulated proteins related to cell motility and metastasis, such as matrix metalloproteinase (MMP)-9 and vascular endothelial growth factor (VEGF), and decreased proliferation of Bel7402, a hepatoma cell line. On the other hand,
HOTAIR levels were increased in tumor samples from patients with lymph node metastasis.
In some patients who underwent liver transplantation, Yang
et al. [
31] also observed higher
HOTAIR expression in cancer tissues than in adjacent normal tissues and found that
HOTAIR was an independent prognostic factor for relapse (
p = 0.001). The survival period after relapse was shorter in these patients. Similarly, in a hepatoma cell line, interfering
HOTAIR function with specific siRNA affected viability and invasiveness and increased the sensitivity to TNF-α-induced apoptosis and cisplatinum/doxorubicin treatment. Thus, after liver transplantation,
HOTAIR is a promising prognostic factor and a potential target for treatment.
Recently, the research team of Yang reported their study on the role and molecular mechanism of
HOTAIR in HCC progression. They found that
HOTAIR suppression significantly increased the expression of RNA binding motif protein 38 (RBM38), and the expression levels of RBM38 were significantly lower than adjacent normal tissues. Moreover, RBM38 suppression by siRNA strategy also reversed the cell migration and invasion compared with
HOTAIR-knockdown cells [
47]. These results suggest that RBM38 is one of the direct target of
HOTAIR in HCC progression, and underlines the potential of
HOTAIR as a therapy target.
VEGF and MMP-9 play important roles in hepatocellular carcinoma progression. Because these proteins are regulated by HOTAIR, treatments that target HOTAIR might suppress growth. On the other hand, drugs that target VEGF or MMP-9 directly might also be helpful for hepatoma treatment.
In summary, the lncRNA
HOTAIR is important in breast, gastroenteric, and liver cancers. The signaling molecules most likely to interact with
HOTAIR in these cancers are shown in
Table 2, and some of the related pathways that are most likely to be applied to therapeutic strategy are enumerated in
Table 3,
Table 4,
Table 5 and
Table 6.
Table 2.
HOTAIR-related signaling molecules.
Table 2.
HOTAIR-related signaling molecules.
Cancer Type | lncRNA | Related Molecules | Reference |
---|
BC | HOTAIR | PRC2/LSD1/E2/BRCA1 | [11,30] |
CRC | HOTAIR | EZH2/SUZ12/CDH1 | [11,13] |
HCC | HOTAIR | VEGF/MMP-9 | [31] |
Table 3.
HOTAIR-related signaling pathways/processes in BC.
Table 3.
HOTAIR-related signaling pathways/processes in BC.
Pathways | Signaling Molecules | Biological Processes |
---|
ErbB signaling pathway | HER2 | cell migration/evasion |
NF-kappa B signaling pathway | HER2 | DNA degradation/cell survival |
PI3K-Akt signaling pathway | BRCA1/EZH2 | DNA repair/cell proliferation; angiogenesis |
TGF-β signaling pathway | twist/snail/miR-10b | EMT process; Cell growth/survival; Cell migration/invasion |
estrogen signaling pathway | E2 | Cell cycle/cell adhension; apoptosis |
ubiquitin mediated proteolysis | BRCA1/E2 | Degradation of target protein |
Table 4.
HOTAIR-related signaling pathways/processes in CRC.
Table 4.
HOTAIR-related signaling pathways/processes in CRC.
Pathways | Signaling Molecules | Biological Processes |
---|
TGF-β signaling pathway | snail | EMT process |
P53 signaling pathway | EZH2/CDK1 | Cell cycle arrest |
Cell cycle | CDH1/EZH2/CDK1 | Cell cycle arrest |
Ubiquitin mediated proteolysis | CDH1 | Degradation of target protein |
Table 5.
HOTAIR-related signaling pathways/processes in GC.
Table 5.
HOTAIR-related signaling pathways/processes in GC.
Pathways | Signaling Molecules | Biological Processes |
---|
TGF-β signaling pathway | snail | EMT process |
RNA interfere | miR-331-3p | miRNA sponge/HER2-mRNA protection; cell signaling networks |
Table 6.
HOTAIR-related signaling pathways/processes in HCC.
Table 6.
HOTAIR-related signaling pathways/processes in HCC.
Pathways | Signaling Molecules | Biological Processes |
---|
VEGF signaling pathway | VEGF | cell proliferation/migration; sustained angiogenesis |
TNF signaling pathway | MMP-3/MMP-9 | remodeling of extracellular matrix |
PPAR signaling pathway | MMP-1 | adipocyte differentiation |
Wnt signaling pathway | MMP-7 | cell cycle |
mRNA surveillance pathway | RBM38/p53 | gene overexpression |
TGF-βsignaling pathway | miR-10b | EMT process; cell growth and survival; cell migration and invasion |