Dual effects of collagenase-3 on melanoma: metastasis promotion and disruption of vasculogenic mimicry

Targeting Matrix Metalloproteinases and Their Inhibitors in Melanoma

Malignant melanoma is one of the most important dermatological neoplasms. The high mortality rate associated with this skin disease is primarily due to the occurrence of metastases, while the diagnosis and treatment of melanoma in its early stages has a favorable prognosis. Early detection is crucial because the success of treatment is directly related to the depth of cancerous growth. The family of matrix metalloproteinases (MMPs) plays a critical role in the initiation and progression of melanoma. Prominent MMPs, including MMP-1, MMP-2, MMP-3, MMP-9, MMP-13, and MMP-14, have been shown to significantly contribute to the development of melanoma. The tumor microenvironment, particularly the extracellular matrix (ECM), has emerged as a critical factor in modulating cancer progression. This review focuses on the role of matrix metalloproteinases and their inhibitors in ECM degradation and the subsequent progression of melanoma, as well as their potential as therapeutic targets.

A Whole-Transcriptomic Analysis of Canine Oral Melanoma: A Chance to Disclose the Radiotherapy Effect and Outcome-Associated Gene Signature

Oral melanoma (OM) is the most common malignant oral tumour among dogs and shares similarities with human mucosal melanoma (HMM), validating the role of canine species as an immunocompetent model for cancer research. In both humans and dogs, the prognosis is poor and radiotherapy (RT) represents a cornerstone in the management of this tumour, either as an adjuvant or a palliative treatment. In this study, by means of RNA-seq, the effect of RT weekly fractionated in 9 Gray (Gy), up to a total dose of 36 Gy (4 weeks), was evaluated in eight dogs affected by OM. Furthermore, possible transcriptomic differences in blood and biopsies that might be associated with a longer overall survival (OS) were investigated. The immune response, glycosylation, cell adhesion, and cell cycle were the most affected pathways by RT, while tumour microenvironment (TME) composition and canonical and non-canonical WNT pathways appeared to be modulated in association with OS. Taking these results as a whole, this study improved our understanding of the local and systemic effect of RT, reinforcing the pivotal role of anti-tumour immunity in the control of canine oral melanoma (COM).

MMP1, MMP9, MMP11 and MMP13 in melanoma and its metastasis - key points in understanding the mechanisms and celerity of tumor dissemination

BACKGROUND

Matrix metalloproteinase (MMP)1, MMP9, MMP11, and MMP13 are overexpressed in malignant melanoma (MM), being associated with tumor invasive phase, metastases, and more aggressive neoplastic phenotypes.

AIM

The main objective of the current study was to correlate the expression of the MMPs with the evolution of MM toward distant metastasis.

PATIENTS, MATERIALS AND METHODS

We designed a retrospective cohort study, including 13 patients with metastatic MM. Data concerning age, sex, localization of the primary lesion and metastasis, and histological and immunohistochemical features (intensity of expression and percent of positive cells for MMPs) were statistically processed.

RESULTS

The time between the diagnosis of primitive melanoma and the diagnosis of metastasis ranged between 0 and 73 months, with a mean value of 18.3 months. The metastases rich in MMP1- and MMP9-positive cells occurred earlier than the metastases with low levels of positive cells. The mean period until metastasis was shorter for the MMP1-expressing tumors than the ones without MMP1 expression. MMP13 expression in the tumor and its metastasis was significantly linked with the time until the metastasis occurrence.

CONCLUSIONS

This study emphasizes the roles of MMP1, MMP9, and MMP13 in the process of metastasis in melanoma and the opportunity to use them as therapeutic targets and surveillance molecules.

Advance in vasculogenic mimicry in ovarian cancer (Review)

Ovarian cancer (OC) is a common and highly prevalent malignant tumor in women, associated with a high mortality rate, easy recurrence and easy metastasis, which is predominantly at an advanced stage when detected in patients. This renders the cancer more difficult to treat, and consequently it is also associated with a low survival rate, being the malignancy with the highest mortality rate among the various gynecological tumors. As an important factor affecting the development and metastasis of OC, understanding the underlying mechanism(s) through which it is formed and developed is crucial in terms of its treatment. At present, the therapeutic methods of angiogenic mimicry for OC remain in the preliminary stages of exploration and have not been applied in actual clinical practice. In the present review, various signaling pathways and factors affecting angiogenic mimicry in OC were described, and the chemical synthetic drugs, natural compound extracts, small-molecule protein antibodies and their associated targets, and so on, that target angiogenic mimicry in the treatment of OC, were discussed. The purpose of this review was to provide new research ideas and potential theoretical support for the discovery of novel therapeutic targets for OC that may be applied in the clinic, with the aim of effectively reducing its metastasis and recurrence rates.

Targeting endothelial permeability in the EPR effect

The characteristics of the primary tumor blood vessels and the tumor microenvironment drive the enhanced permeability and retention (EPR) effect, which confers an advantage towards enhanced delivery of anti-cancer nanomedicine and has shown beneficial effects in preclinical models. Increased vascular permeability is a landmark feature of the tumor vessels and an important driver of the EPR. The main focus of this review is the endothelial regulation of vascular permeability. We discuss current challenges of targeting vascular permeability towards clinical translation and summarize the structural components and mechanisms of endothelial permeability, the principal mediators and signaling players, the targeted approaches that have been used and their outcomes to date. We also critically discuss the effects of the tumor-infiltrating immune cells, their interplay with the tumor vessels and the impact of immune responses on nanomedicine delivery, the impact of anti-angiogenic and tumor-stroma targeting approaches, and desirable nanoparticle design approaches for greater translational benefit.

Immunotherapy checkpoints in ovarian cancer vasculogenic mimicry: Tumor immune microenvironments, and drugs

Vasculogenic mimicry (VM), a vessel-like structure independent of endothelial cells, commonly exists in solid tumors which requires blood vessels to grow. As a special source of blood supply for tumor progression to a more aggressive state, VM has been observed in a variety of human malignant tumors and is tightly associated with tumor proliferation, invasion, metastasis, and poor patient prognosis. So far, various factors, including immune cells and cytokines, were reported to regulate ovarian cancer progression by influencing VM formation. Herein, we review the mechanisms that regulate VM formation in ovarian cancer and the effect of cells, cytokines, and signaling molecules in the tumor microenvironment on VM formation, Furthermore, we summarize the current clinical application of drugs targeting VM formation.

Regulation and Function of Matrix Metalloproteinase-13 in Cancer Progression and Metastasis

Matrix metalloproteinase-13 (MMP-13) is a member of the Matrix metalloproteinases (MMPs) family of endopeptidases. MMP-13 is produced in low amounts and is well-regulated during normal physiological conditions. Its expression and secretion are, however, increased in various cancers, where it plays multiple roles in tumour progression and metastasis. As an interstitial collagenase, MMP-13 can proteolytically cleave not only collagens I, II and III, but also a range of extracellular matrix proteins (ECMs). Its action causes ECM remodelling and often leads to the release of various sequestered growth and angiogenetic factors that promote tumour cell growth, invasion and angiogenesis. This review summarizes our current understanding of the regulation of MMP-13 expression and secretion and discusses the actions of MMP-13 in cancer progression and metastasis.

Strategies to improve the EPR effect: A mechanistic perspective and clinical translation

Many efforts have been made to achieve targeted delivery of anticancer drugs to enhance their efficacy and to reduce their adverse effects. These efforts include the development of nanomedicines as they can selectively penetrate through tumor blood vessels through the enhanced permeability and retention (EPR) effect. The EPR effect was first proposed by Maeda and co-workers in 1986, and since then various types of nanoparticles have been developed to take advantage of the phenomenon with regards to drug delivery. However, the EPR effect has been found to be highly variable and thus unreliable due to the complex tumor microenvironment. Various physical and pharmacological strategies have been explored to overcome this challenge. Here, we review key advances and emerging concepts of such EPR-enhancing strategies. Furthermore, we analyze 723 clinical trials of nanoparticles with EPR enhancers and discuss their clinical translation.

Melatonin impedes prostate cancer metastasis by suppressing MMP-13 expression

Prostate cancer has high metastatic potential. Men with higher urinary levels of the sleep hormone melatonin are much less likely to develop advanced prostate cancer compared with men with lower levels of melatonin. Melatonin has shown anticancer activity in experimental investigations. Nevertheless, the therapeutic effect of melatonin in metastatic prostate cancer has largely remained a mystery. Analyses of Gene Expression Omnibus data and human tissue samples indicated that levels of matrix metallopeptidase 13 (MMP-13) expression are higher in prostate cancer patients than in healthy cancer-free individuals. Mechanistic investigations revealed that melatonin inhibits MMP-13 expression and the migratory and invasive capacities of prostate cancer cells via the MT1 receptor and the phospholipase C, p38, and c-Jun signaling cascades. Importantly, tumor growth rate and metastasis to distant organs were suppressed by melatonin in an orthotopic prostate cancer model. This is the first demonstration showing that melatonin impedes metastasis of prostate cancer by suppressing MMP-13 expression in both in vitro and in vivo models. Thus, melatonin is promising in the management of prostate cancer metastasis and deserves to undergo clinical investigations.

The MSC-MCF-7 Duet Playing Tumor Vasculogenesis and Angiogenesis onto the Chick Embryo Chorioallantoic Membrane

BACKGROUND/AIM

Human mesenchymal stem cells (hMSC) represent a versatile cell population, able to modulate the tumor microenvironment. Our aim was to recreate an open scene for the in vivo interaction between hMSC and the MCF-7 breast cancer cells (MCF-7), in order to enlighten the intimate involvement of hMSC in tumor vasculogenesis and angiogenesis.

MATERIALS AND METHODS

hMSC and MCF-7 were seeded onto the chick embryo chorioallantoic membrane (CAM) and incubated for 7 days. Consecutively, the morphology and the immunohistochemical profile of CAM were assessed.

RESULTS

Following this complex interaction, MCF-7 acquired a more aggressive phenotype, hMSC switched to a vascular precursor phenotype, while CAM underwent a major reset to an earlier stage, with hotspots of angiogenesis, vasculogenesis and hematopoiesis.

CONCLUSION

The hallmark of this study was the establishment of a veritable in vivo experimental model of MSC involvement in tumor vasculogenesis and angiogenesis, allowing further analysis in the field.

Twist1 accelerates tumour vasculogenic mimicry by inhibiting Claudin15 expression in triple-negative breast cancer

The up-regulation of EMT regulator Twist1 has been implicated in vasculogenic mimicry (VM) formation in human triple-negative breast cancer (TNBC). Twist1 targets the Claudin15 promoter in hepatocellular carcinoma cells. Claudin family members are related with TNBC. However, the relationship between Claudin15 and VM formation is not clear. In this study, we first found that Claudin15 expression was frequently down-regulated in human TNBC, and Claudin15 down-regulation was significantly associated with VM and Twist1 nuclear expression. Claudin15 down-regulation correlated with shorter survival compared with high levels. Claudin15 silence significantly enhanced cell motility, invasiveness and VM formation in the non-TNBC MCF-7 cells. Conversely, an up-regulation of Claudin15 remarkably reduced TNBC MDA-MB-231 cell migration, invasion and VM formation. We also showed that down-regulation of Claudin15 was Twist1-dependent, and Twist1 repressed Claudin15 promoter activity. Furthermore, GeneChip analyses of mammary glands of Claudin15-deficient mice indicated that Claudin18 and Jun might be downstream factors of Twist1-Claudin15. Our results suggest that Twist1 induced VM through Claudin15 suppression in TNBC, and Twist1 inhibition of Claudin15 might involve Claudin18 and Jun expression.

The role of vascular mimicry as a biomarker in malignant melanoma: a systematic review and meta-analysis

BACKGROUND

Vasculogenic mimicry (VM) a microvascular system consisting of non-endothelial cells that is newly formed by aggressive tumors, has been proposed as an important therapeutic target in malignant melanoma (MM). We performed a systematic literature review to evaluate the diagnostic and prognostic accuracy of VM status for overall survival of MM patients.

METHODS

The quality of the included studies was evaluated using the QUADAS-2 tool. Diagnostic capacity of VM variables, including sensitivity, specificity, positive likelihood ratio (PLR), negative likelihood ratio (NLR), diagnostic odds ratio (DOR), and the area under summary receiver operating characteristic (SROC), were pooled using Meta-DiSc software.

RESULTS

A retrospective observational study was conducted based on twelve clinical studies including 978 clinically confirmed melanoma patients with proportion (P). VM+ melanoma cells were associated with poor prognosis in 38% of MM group (P = 0.35, 95% confidence intervals (CI): 0.27-0.42, p < 0.001). The pooled sensitivity and specificity were 0.82 (95% CI: 0.79-0.84) and 0.69 (95% CI: 0.66-0.71), respectively. Furthermore, the pooled PLR, NLR, and DOR were 2.56 (95% CI: 1.94-3.93), 0.17 (95% CI: 0.07-0.42), and 17.75 (95% CI: 5.30-59.44), respectively. Furthermore, the AUC of SROC was 0.63, indicating high reliability of VM status as a biomarker. Importantly, subgroup results suggested that VM+ status is a significantly accurate prognostic biomarker when diagnosed by the CD31-/PAS+ staining methods in Asian MM samples (p < 0.001).

CONCLUSIONS

Our findings support the potential of VM status of tumors as a promising prognostic biomarker and emphasize an effective adjuvant therapeutic strategy in the prognosis of Asian MM patients.

Anti-metastatic Properties of Naproxen-HBTA in a Murine Model of Cutaneous Melanoma

The beneficial effects of H₂S-release and of COXs-inhibition have been exploited in the design of novel anti-inflammatory drugs, the H₂S-releasing non-steroidal anti-inflammatory drugs (H₂S-NSAIDs), showing promising potential for chemoprevention in cancers. Here, we evaluated the efficacy of a new H₂S-releasing derivative of naproxen, named naproxen-4-hydroxybenzodithioate (naproxen-HBTA), in reducing metastatic melanoma features, both in vitro and in vivo. The novel H₂S donor has been prepared following a synthetic scheme that provided high yields and purity. In particular, we investigated the effect of naproxen-HBTA in vitro on several metastatic features of human melanoma cells such as proliferation, migration, invasion, and colonies formation and in vivo in a model of cutaneous melanoma. Cell culture studies demonstrated that naproxen-HBTA induced caspase 3-mediated apoptosis and inhibited motility, invasiveness, and focus formation. Finally, daily oral treatment with naproxen-HBTA significantly suppressed melanoma growth and progression in mice. In conclusion, by using this dual approach we propose that the COX-2 and H₂S pathways could be regarded as novel therapeutic targets/tools to generate new treatment options based on "combination therapy" for melanoma.

Epithelial-to-endothelial transition and cancer stem cells: two cornerstones of vasculogenic mimicry in malignant tumors

Vasculogenic mimicry (VM) is a functional microcirculation pattern in malignant tumors accompanied by endothelium-dependent vessels and mosaic vessels. VM has been identified in more than 15 solid tumor types and is associated with poor differentiation, late clinical stage and poor prognosis. Classic anti-angiogenic agents do not target endothelium-dependent vessels and are not efficacious against tumors exhibiting VM. Further insight into the molecular signaling that triggers and promotes VM formation could improve anti-angiogenic therapeutics. Recent studies have shown that cancer stem cells (CSCs) and epithelium-to-endothelium transition (EET), a subtype of epithelial-to-mesenchymal transition (EMT), accelerate VM formation by stimulating tumor cell plasticity, remodeling the extracellular matrix (ECM) and connecting VM channels with host blood vessels. VM channel-lining cells originate from CSCs due to expression of EMT inducers such as Twist1, which promote EET and ECM remodeling. Hypoxia and high interstitial fluid pressure in the tumor microenvironment induce a specific type of cell death, linearly patterned programmed cell necrosis (LPPCN), which spatially guides VM and endothelium-dependent vessel networks. This review focuses on the roles of CSCs and EET in VM, and on possible novel anti-angiogenic strategies against alternative tumor vascularization.

AJUBA promotes the migration and invasion of esophageal squamous cell carcinoma cells through upregulation of MMP10 and MMP13 expression

The LIM-domain protein AJUBA has been reported to be involved in cell-cell adhesion, proliferation, migration and cell fate decision by acting as a scaffold or adaptor protein. We previously identified AJUBA as a putative cancer gene in esophageal squamous cell carcinoma (ESCC). However, the function and underlying mechanisms of AJUBA in ESCC remain largely unknown. In the present study, we detected AJUBA levels in ESCC tumor tissues and in corresponding adjacent non-tumor tissues by immunohistochemistry (IHC) and investigated the function and mechanism of AJUBA in ESCC cells. The IHC results showed that AJUBA levels were significantly higher in ESCC tissues compared with corresponding adjacent non-tumor tissues (P < 0.001). Both in vitro and in vivo experiments showed that AJUBA promoted cell growth and colony formation, inhibited cisplatin-induced apoptosis of ESCC cells, and promoted ESCC cell migration and invasion. RNA sequencing was used to reveal the oncogenic pathways of AJUBA that were involved, and MMP10 and MMP13 were identified as two of the downstream targets of AJUBA. Thus, AJUBA upregulates the levels of MMP10 and MMP13 by activating ERK1/2. Taken together, these findings revealed that AJUBA serves as oncogenic gene in ESCC and may serve as a new target for ESCC therapy.

Macrophage migration inhibitory factor promotes vasculogenic mimicry formation induced by hypoxia via CXCR4/AKT/EMT pathway in human glioblastoma cells

Macrophage migration inhibitory factor (MIF) is over-expressed and secreted in various cancer cells in particular in response to hypoxia. Recent studies have shown that, under hypoxic conditions, glioblastoma (GBM) cells display the ability to drive blood-perfused vasculogenic mimicry (VM). The aim of this study was to investigate the underlying mechanism of MIF in the regulation of hypoxia-induced VM in GBM cells. By analyzing clinical specimens, we observed the co-localization of MIF, C-X-C motif chemokine receptor 4 (CXCR4) and VM in hypoxic regions of gliomas. In vitro, the exposure of GBM cells (U87 and U251) to hypoxia increased the expression of MIF and CXCR4 and induced VMs. Other data demonstrated that ectogenic rhMIF promoted VMs in GBM cells and knock-down endogenous MIF attenuated hypoxia-induced VMs. In addition, we demonstrated that MIF augmented VM formation ability by enhancing the epithelial mesenchymal transition (EMT) through the CXCR4-AKT pathway. Moreover, in vivo, the subcutaneous injection of rhMIF resulted in the progression of EMT and VMs formation. On the contrary, CXCR4-AKT pathway inhibitors blocked the effects of rhMIF on EMT and VMs formation. Collectively, our results support a critical role for the MIF-CXCR4 signaling axis in regulating hypoxia-induced VMs formation, indicating the potential usefulness of MIF as a notable target for the anti-vascularization treatment of GBM.

Protein kinase C inhibitor Gö6976 but not Gö6983 induces the reversion of E- to N-cadherin switch and metastatic phenotype in melanoma: identification of the role of protein kinase D1

Background

Melanoma is a highly metastatic type of cancer that is resistant to all standard anticancer therapies and thus has a poor prognosis. Therefore, metastatic melanoma represents a significant clinical problem and requires novel and effective targeted therapies. The protein kinase C (PKC) family comprises multiple isoforms of serine/threonine kinases that possess distinct roles in cancer development and progression. In this study, we determined whether inhibition of PKC could revert a major process required for melanoma progression and metastasis; i.e. the E- to N-cadherin switch.

Methods

The cadherin switch was analyzed in different patient-derived primary tumors and their respective metastatic melanoma cells to determine the appropriate cellular model (aggressive E-cadherin-negative/N-cadherin-positive metastasis-derived melanoma cells). Next, PKC inhibition in two selected metastatic melanoma cell lines, was performed by using either pharmacological inhibitors (Gö6976 and Gö6983) or stable lentiviral shRNA transduction. The expression of E-cadherin and N-cadherin was determined by western blot. The consequences of cadherin switch reversion were analyzed: cell morphology, intercellular interactions, and β-catenin subcellular localization were analyzed by immunofluorescence labeling and confocal microscopy; cyclin D1 expression was analyzed by western blot; cell metastatic potential was determined by anchorage-independent growth assay using methylcellulose as semi-solid medium and cell migration potential by wound healing and transwell assays.

Results

Gö6976 but not Gö6983 reversed the E- to N-cadherin switch and as a consequence induced intercellular interactions, profound morphological changes from elongated mesenchymal-like to cuboidal epithelial-like shape, β-catenin translocation from the nucleus to the plasma membrane inhibiting its oncogenic function, and reverting the metastatic potential of the aggressive melanoma cells. Comparison of the target spectrum of these inhibitors indicated that these observations were not the consequence of the inhibition of conventional PKCs (cPKCs), but allowed the identification of a novel serine/threonine kinase, i.e. protein kinase Cμ, also known as protein kinase D1 (PKD1), whose specific inhibition allows the reversion of the metastatic phenotype in aggressive melanoma.

Conclusion

In conclusion, our study suggests, for the first time, that while cPKCs don’t embody a pertinent therapeutic target, inhibition of PKD1 represents a novel attractive approach for the treatment of metastatic melanoma.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-3007-5) contains supplementary material, which is available to authorized users.

Human hemokinin-1 promotes migration of melanoma cells and increases MMP-2 and MT1-MMP expression by activating tumor cell NK1 receptors

Receptors and their regulatory peptides are aberrantly expressed in tumors, suggesting a potential tumor therapy target. Human hemokinin-1 (hHK-1) is a tachykinin peptide ligand of the neurokinin-1 (NK1) receptor which is overexpressed in melanoma and other tumor tissues. Here, we investigated the role of hHK-1 and the NK1 receptor in melanoma cell migration. NK1 receptor expression was associated with melanoma metastatic potential. Treatment with hHK-1 significantly enhanced A375 and B16F10 melanoma cell migration and an NK1 receptor antagonist L732138 blocked this effect. MMP-2 and MT1-MMP expression were up-regulated in hHK-1-treated melanoma cells and cell signaling data suggested that hHK-1 induced phosphorylation of ERK1/2, JNK and p38 by way of PKC or PKA. Kinase activation led to increased MMP-2 and MT1-MMP expression and melanoma cell migration induced by hHK-1. Thus, hHK-1 and the NK1 receptor are critical to melanoma cell migration and each may be a promising chemotherapeutic target.

Notch4+ cancer stem-like cells promote the metastatic and invasive ability of melanoma

Sphere formation in conditioned serum‐free culture medium supplemented with epidermal growth factor and basic fibroblast growth factor (tumorospheres) is considered useful for the enrichment of cancer stem‐like cells, also known as tumor‐initiating cells. We used a gene expression microarray to investigate the gene expression profile of melanoma cancer stem‐like cells (MCSLCs). The results showed that MCSLCs highly expressed the following Notch signaling pathway molecules: Notch3 (NM_008716), Notch4 (NM_010929), Dtx4 (NM_172442), and JAG2 (NM_010588). Immunofluorescence staining showed tumorosphere cells highly expressed Notch4. Notch4^high B16F10 cells were isolated by FACS, and Western blotting showed that high Notch4 expression is related to the expression of epithelial–mesenchymal transition (EMT)‐associated proteins. Reduced invasive and migratory properties concomitant with the downregulation of the EMT markers Twist1, vimentin, and VE‐cadherin and the overexpression of E‐cadherin was observed in human melanoma A375 and MUM‐2B cells. In these cells, Notch4 was also downregulated, both by Notch4 gene knockdown and by application of the γ‐secretase inhibitor, DAPT. Mechanistically, the re‐overexpression of Twist1 by the transfection of cells with a Twist1 expression plasmid led to an increase in VE‐cadherin expression and a decrease in E‐cadherin expression. Immunohistochemical analysis of 120 human melanoma tissues revealed a significant correlation between the high expression of Notch4 and the metastasis of melanoma. Taken together, our findings indicate that Notch4+ MCSLCs trigger EMT and promote the metastasis of melanoma cells.

Lentiviral-mediated overexpression of long non-coding RNA GAS5 reduces invasion by mediating MMP2 expression and activity in human melanoma cells

The present study evaluated the effects of long non-coding RNA GAS5 on the migration and invasion of melanoma cells. Using the SK-Mel‑110 melanoma cell line, we stably expressed GAS5, visualized the distribution of GAS5 by RNA fluorescence in situ hybridization (FISH) and examined changes in cell migration and invasion with Transwell assays. In GAS5 overexpressed SK-Mel‑110 cells, migrated and invaded cells decreased by 65.3 and 55.6%, respectively. Moreover, the MMP2 protein level, and its activity was downregulated by 67.9 and 15.8%, respectively. Overexpressing lncRNA GAS5 inhibited the migration and invasion ability of melanoma SK-Mel‑110 cells, partially by decreasing the MMP2 expression and its activity. This study is the first to reveal a potential relationship between lncRNA GAS5 and the migration and invasion of melanoma.