MicroRNA heterogeneity in melanoma progression

Abstract

The altered expression of miRNAs has been linked with neocarcinogenesis or the development of human malignancies including melanoma. Of significance, multiple clinical studies have documented that distinct sets of microRNAs (miRNAs) could be utilized as prognostic biomarkers for cancer development or predict the outcomes of treatment responses. To that end, an in-depth validation of such differentially expressed miRNAs is necessary in diverse settings of cancer patients in order to devise novel approaches to control tumor growth and/or enhance the efficacy of clinically-relevant therapeutic options. Moreover, considering the heterogeneity and sophisticated regulation of miRNAs, the precise delineation of their cellular targets could also be explored to design personalized medicine. Given the significance of miRNAs in regulating several key cellular processes of tumor cells including cell cycle progression and apoptosis, we review the findings of such miRNAs implicated in melanoma tumorigenesis. Understanding the novel mechanistic insights of such miRNAs will be useful for developing diagnostic or prognostic biomarkers or devising future therapeutic intervention for malignant melanoma.

Introduction

While much has been discovered regarding the biogenesis and roles of miRNAs in cancer growth and metastasis [1–5], the mechanisms governing these effects have yet to be fully elucidated. Accumulating evidence indicates that miRNAs play crucial roles in essentially all the biological and pathophysiological processes such as cell cycle differentiation, lipid metabolism, inflammation, neurological, cardiovascular and metabolic disorders and cancers including melanoma [5–12]. The general description of miRNA discovery including several enzymatic and non-enzymatic pathways involved in the biogenesis as well as regulation of miRNAs have been identified [reviewed in ref. 5]. In particular, miRNAs have been shown to modulate the growth of melanoma tumors via regulating pathways involved in melanocyte transformation (melanomagenesis), cell cycle progression and apoptosis, epigenetics as well as host versus tumor immune responses [5,13–18].

Of several protein-coding genes, miRNAs are believed to target approximately one-third of human mRNAs, and a single miRNA could target approximately 200 transcripts simultaneously due to its differential target binding patterns [19]. Importantly, the identification of miRNAs regulatory patterns for various species including humans, and the analysis of miRNA sequences annotated in the human genome have led to the understanding of miRNAs locations as well as their gene clusters [20–23]. Of note, due to the frequent expression of miRNAs as polycistronic transcripts, it has been documented that the deregulation of one miRNA member in a gene cluster is accompanied by similar deregulations of other miRNA members within the same gene cluster [24–25]. Thus, it is important to delineate if one miRNA in a cluster can be regulated independently of others. Moreover, considering the fact of sophisticated regulation of miRNAs via cooperativity (i.e. more than one miRNA species can target the same mRNA) as well as multiplicity of their targets (i.e. one miRNA can target hundreds of mRNA species) [26–29], an in-depth analysis of miRNAs is required to precisely validate their roles and mechanisms in human diseases including cancer.

The aberrant expression of miRNAs has been identified in human malignancies including melanoma [30–36]. Notably, differential miRNA expressions have been documented with several clinicopathological variables including tumor growth and metastasis, treatment resistance, cancer reoccurrence as well as patients’ responses to therapeutic agents [37–43]. To that end, it is imperative to utilize distinct sets of miRNAs in order to design effective strategies for an early detection/diagnosis, and therapeutic interventions for cancers [5, 44–45]. In particular, cutaneous melanoma is the most aggressive form of skin cancer, which due to its high metastatic potential, contributes to the majority of skin cancer-related deaths worldwide [46–47]. Of several predisposing risk factors, exposure to ultraviolet (UV) light, melanocyte integrity and homeostatic mechanisms with altered/dysregulated signaling cascades play crucial roles in melanocytes transformation into malignant melanomas [47–50]. Importantly, distinct sets of miRNAs have been identified and validated for their roles in impacting cellular processes and pathways including those governing transformation of normal melanocytes into melanoma [13,51–55].

Of significance, miRNA profiling from tumor tissues (intracellular miRNAs) or serum/plasma samples (extracellular miRNAs) is being extensively explored to identify new biomarkers for cancer diagnosis including melanoma [56–59]. Several studies have utilized the whole miRNome and custom quantitative PCR arrays, to analyze miRNA expression profiles between healthy subjects and melanoma patients as well as in primary melanocytes/keratinocytes cell lines [60–63]. For example, in one such study, when melanoma tissue miRNomes were compared with matching serum samples, the authors observed that distinct sets of miRNAs including miR-30b-5p and miR-122–5p are exclusively present in tumors, and others such as miR-3201 and miR-122–5p are highly expressed in serum samples [63]. In a similar context, miR-21, miR-155, miR-200c, and miR-205 have been shown to be differentially expressed between benign nevi, and primary or metastatic melanoma, and act as tumor suppressors [64–65]. Given these facts, and considering the tumor heterogeneity among patients of any ethnic population, one could hypothesize that according to the personalized medicine model, miRNAs-associated molecular taxonomy could be helpful in predicting the likelihood of patients developing resistance against a particular treatment regimen. The roles and mechanisms of such miRNAs are discussed below in an order of their importance. In addition, the list of such miRNAs with their targets, functions, and cellular responses as per their differential expression levels are summarized in Tables 1 (upregulated/overexpressed) and Table 2 (downregulated).

Table 1:

Summary of upregulated miRNAs

miRNA	Target(s)	Function(s)	Cellular response(s)	References
miR-21	PDCD4, PTEN, SPRY1, STAT3, BTG2, Bax/Bcl-2 ratio, Akt, ERK, NF-kB, FBXO11, BCL6	Regulation of cell proliferation, migration, invasion, and apoptosis	Tumor growth, metastasis, and chemotherapy or radiotherapy sensitivity	66–67, 69, 76–80
miR-155	SKI, MDSC, HIF-1α, CXCL1, CXCL2, CXCL3, CXCL5, CXCL8, Arg-1, iNOS, VEGF, MMP2, MMP9, TYRP1, IL-1β, MITF-M,	Regulation of cell proliferation, migration, immunosuppression, inhibition of anti-tumor immunity	Melanoma immune evasion, tumor growth, and angiogenesis	89,94,99, 102–103, 107–108
miR-122-5p	NOP14, LPAR3, Wnt1, GSK3, SH3, PPARGC1A/PGC1	Regulation of cell proliferation and cell cycle	Tumor growth	110, 113
miR-182	FOXO3, MITF-M, FOXO1, TGFβ, NF-kB	Regulation of cell invasion, apoptosis, epigenetic modulation	Tumor growth and metastasis	114–117
miR-149*/miR-149	GSK3α, Mcl-1, LRIG2	Regulation of cell proliferation/survival and apoptosis	Tumor growth	120–121
miR-182	MITF	Regulation of melanogenesis	Melanogenesis	53
miR-34a, miR-100 & miR-125b	CCL-2	Regulation of cell proliferation, apoptosis	Therapeutic resistance to BRAF inhibitors	122
miR-1246	ERK	Regulation of cell cycle, autophagy, apoptosis	Therapeutic resistance to BRAF inhibitors	123

Table 2:

Summary of downregulated miRNAs

miRNA	Target(s)	Function(s)	Cellular response(s)	References
miR-211	MITF, TRPM1, KCNMA1, IGF2R, TGFBR2, NFAT5, NUAK1	Regulation of cell proliferation, migration, and invasion	Inhibition of melanoma progression and metastasis	124–127
miR-193b	Cyclin D1, Mcl-1	Regulation of cell proliferation, cycle regulation	Tumor suppressor	128–129
miR-126/miR-126*	ADAM9, MMP7, MAPK, PI3K/AKT, KIT, MITF, TYR, AP2α	Regulation of cell proliferation, invasion, migration, chemotaxis, apoptosis, and metastasis	Tumor suppressor	133–134
miR-145	c-MYC, FASCIN1, NRAS, MAPK, PI3K/AKT	Regulation of cell proliferation, invasion, migration and apoptosis	Tumor suppressor	135, 137
miR-200 family members	E-cadherin	Regulation of tumor progression	Tumor suppressor	138
miR-365	NRP1, BCL2, Cyclin D1	Regulation of cell cycle, proliferation, and apoptosis	Tumor suppressor	140–141
miR-205	E2F1, E2F5, Akt, BCL2, VEGF	Regulation of cell proliferation/survival and apoptosis	Tumor suppressor	142–143
miR-206	CDK4, cyclin D1, cyclin C, TNF-α	Regulation of cell cycle, tumor growth	Tumor suppressor	144–145
miR-196a	HOX-B7, bFGF, ETS-1, BMP-4, HOX-C8, Cadherin-11, Calponin-1, Osteopontin	Regulation of cell proliferation, invasiveness and tumor progression	Tumor growth	146–147
miR-143–3p	COX-2	Regulation of cell proliferation, invasion, migration, and apoptosis	Tumor suppressor	148

miR-21

Of several key miRNAs, miR-21 has been extensively studied for its crucial roles in modulating the tumorigenicity of malignant melanoma or overall prognostic responses in melanoma patients [65–71]. Importantly, miR-21 has been shown to target multiple oncogenic and tumor suppressive pathways involved in genetic or cellular modifications governing melanomagenesis in response to environmental factors such as UV radiation [reviewed in ref. 72]. Regarding miR-21 effects in melanoma development and metastasis, Yang et al., characterized the role of miR-21 using B16 mouse melanoma model [66]. The authors demonstrated that knockdown of miR-21 or induction of interferon (IFN) alone, inhibited the in vitro cell proliferation and migration and that these effects were enhanced with the combination of miR-21 and IFN treatments. In this study, authors observed that IFN induced enhanced apoptosis in miR-21 knockdown B16 cells, and oncogenic STAT-3, tumor suppressor PTEN and PDCD4, and antiproliferative BTG2 proteins were found to be the targets of these miR-21-mediated effects [66]. Moreover, decreased tumor metastasis, and increased mean body weight or mice survival were noted in syngeneic mice injected with miR-21 knockdown B16 cells [66]. These finding indicated that miR-21 regulates the metastatic behavior of B16 melanoma cells via enhancing cell proliferation, migration/invasion, and survival as well as suppressing IFN actions.

Notably, miR-21 expression was found to be significantly upregulated in primary and malignant melanoma tissues as well as melanoma cells compared to benign and dysplastic nevi, normal skin or melanocytic cell preparations [66,67–71,73]. Of significance, high expression of miR-21 was correlated with advanced clinical stage and a poorer 5-year disease-free survival (DFS) or overall survival (OS) of cutaneous melanoma patients [67]. To that end, miR-21 downregulation in human HTB-67 and A375 cutaneous melanoma cells has been shown to result in reduced growth and enhanced apoptosis as well as increased sensitivity of melanoma cells to a chemotherapeutic agent, cisplatin and radiotherapy [67]. These effects were mediated via increased expression of PTEN protein and inhibition of phosphorylated Akt, which in turn resulted in increased levels of pro-apoptotic Bax and decreased levels of anti-apoptotic Bcl-2 proteins [67]. In another report, while the levels of miR-21 in melanoma patients were not significantly correlated with either recurrence-free survival (RFS) or overall survival (OS) [68], the in-vitro studies were supported by similar findings by Jiang et al., studies [67]. In this study, the authors demonstrated that downregulation of miR-21 in WM35 and WM951 melanoma cells with high endogenous miR-21 expression resulted in a significant reduction in the cell proliferation as assessed via MTT assay, and induced apoptosis as measured via Annexin V staining and TUNEL assay [68]. However, when cell proliferation was assessed via ³H-Thymidine uptake assay, a slight reduction in WM35 cell proliferation and no difference in WM951 cell proliferation were noted by miR-21 downregulation, and that this discrepancy was explained due to the transfection procedure with miR-21 inhibitor [68]. On the other hand, significantly increased cell proliferation and reduced apoptosis was noted with miR-21 upregulation in melanocytic preparation M1 expressing low levels of endogenous miR-21. Nevertheless, neither the cell proliferation nor apoptosis was altered with miR-21 upregulation in MEWO melanoma cells with low endogenous miR-21 expression [68]. These data indicated that miR-21 could serve as a potential target in a subset of melanomas.

Of importance, increased miR-21 expression was significantly correlated with reduced expression of tumor suppressor gene, programmed cell death 4 (PDCD4) in malignant melanoma samples compared to the normal skin [69]. In addition, the levels of elevated miR-21 versus reduced PDCD4 were significantly correlated with increased tumor size, higher Clark classification and lymph node metastases of malignant melanoma [69]. These data indicated that miR-21/PDCD4 could serve as potential biomarkers as well as therapeutic targets for malignant melanoma. These findings were supported by another study, where increased miR-21 expression in clinical melanoma samples was inversely correlated with the loss of nuclear PTEN but not cytosolic PTEN expression as compared to naevi samples [70]. The downregulation of miR-21 in melanoma cells resulted in altered nuclear expression of PTEN. Notably, BRAF or NRAS mutation status exhibited no significant effects on melanoma miR-21 expression [70]. Along similar lines, Babapoor and colleagues using next-generation sequencing defined a comprehensive repertoire of distinct sets of miRNAs involved in early melanoma invasion [71]. Of 765 distinct mature miRNAs, a set of 40 differentially expressed miRNAs were associated with melanomas, and among these, the expression of miR-21–5p (along with miR-424–5p) were significantly increased in invasive melanomas compared to in situ melanomas. Importantly, increased levels of miR-21–5p were found to be significantly associated with several prognostic parameters including invasive depth, tumor mitotic index and lymphovascular invasion [71]. In addition to malignant melanomas, the miR-21 expression has also been reported to serve as a marker for indeterminate melanocytic lesions with malignant potential [73]. Moreover, miRNA expression profiling of benign nevi, benign Spitz tumors, indeterminate Spitz tumors and Spitzoid melanomas in adult and children has documented upregulation of miR-21 in Spitzoid melanomas compared to indeterminate Spitz lesions, benign Spitz tumors and benign nevi [73]. Importantly, indeterminate Spitz lesions with low-risk pathologic features had significantly lower expression of miR-21 compared to Spitzoid melanoma tumors in adults [73]. These studies provided the impetus of evaluating miR profiling including miR-21 in the diagnosis and treatment of indeterminate Spitz lesions.

Multiple cellular signaling pathways have been shown to be regulated by miR-21 or vice-versa in melanoma models with or without therapeutic interventions [74–80]. Among multiple factors affecting tumorigenesis, matrix metalloproteinases (MMPs) play critical roles in various aspects of tumor development including tumor progression to angiogenesis and metastases [74–76]. Upon activation, MMPs release pro-angiogenic factors such as vascular endothelial growth factor (VEGF) and fibroblast growth factor-2 (FGF-2). These growth factors bind to their receptors on endothelial cells and trigger signaling cascades via interacting with tyrosine kinases receptors [74–76]. These interactions initiate angiogenic switch, which results in cellular orchestrations including endothelial cell reorganization and matrix and tissue remodeling leading to angiogenesis [74–76]. To that end, modulation of MMPs via mechanisms including tissue inhibitor of metalloproteinases (TIMPs) or by miRNAs such as miR-21 has been explored as a promising approach for inhibiting cancer growth including melanoma [75–76]. In one such report, the authors demonstrated that overexpression of miR-21 in melanoma cell lines resulted in reduced expression of TIMP3 compared to scrambled miR-treated control cells [76]. Importantly, this decreased expression of TIMP3 resulted in increased invasiveness of the radial growth phase WM1552c cells and vertical growth phase WM793b cells, but not in metastatic A375 and MEL39 melanoma cell lines. However, the proliferation and migration of melanoma cells were not affected by miR-21 overexpression. TIMP3 silencing of melanoma cells mimicked the effects of miR-21 overexpression in terms of enhancing the invasiveness of melanoma cells [76]. Inhibition of miR-21 via miR-21 antagomir or intratumoral injections of anti-miR-21 increased tumoral expression of TIMP3 and reduced the in vivo growth of melanoma tumors in athymic NCr-nu/nu mice [76]. These findings indicated that miR-21 targets TIMP3 to regulate the growth and invasiveness of melanoma tumors. However, in another report, inhibition of MMP-9 and MMP-13 did not exert tumor growth inhibitory effects [77]. Nevertheless, simultaneous inhibition of MMP-9 and MMP-13 resulted in the stimulation of tumor-infiltrating lymphocytes into primary melanoma tumor nodules. These effects were mediated via decreased expression of miR-21 along with miR-let-7b [77]. These findings provided the rationale of exploring concomitant inhibition of MMP-9 and MMP-13 for the treatment of melanoma.

To identify miR-21 target genes in melanoma, Yang and colleagues have utilized microarray and quantitative PCR analysis of miR-21 knockdown B16 melanoma cells and lung tumors isolated from mice of miR-21 knockdown B16 cells injected via tail-vein injection [78]. In this study, the authors identified and validated a member of the F-box subfamily lacking a distinct unifying domain (FBXO11) as a direct target of miR-21 [78]. Additional studies demonstrating the increased expression of the known targets of miR-21 such as PDCD4, BTG2, and PTEN along with FBXO11 in miR-21 knockdown B16 cells and lung tumors, confirmed FBXO11 as a direct target of miR-21. Moreover, loss and gain of function studies demonstrated that FBXO11 functions as a tumor suppressor gene, which induces apoptosis via targeting the degradation of BCL6 oncogene. The in vivo tumorigenesis studies with FBXO11 overexpressing B16 cells demonstrated that FBXO11 suppressed the growth of melanoma tumors and its metastasis to lung as well as prolonged mice survival compared to the mice injected with B16 cells harboring empty vector. Importantly, these effects mediated via FBXO11 overexpression were reversed by FBXO11 silencing [78]. Of significance, decreased tumor expression of FBXO11 and increased tumor expression of miR-21 was identified in skin cancer patients, which were consistent with the higher tumor grades. This increased tumor expression of miR-21 was correlated with poor survival and high tumor FBXO11 expression with better survival of skin cancer patients [78]. These data indicated that miR-21 targets tumor suppressor FBXO11 to promote the development of skin cancer.

In another report, the effects of high intensity focused ultrasound (HIFU) were investigated in the murine B16F10 melanoma model [79]. The authors demonstrated that while HIFU minimally affected the in vitro cell viability/survival or apoptosis of B16F10 cells, it suppressed the in vitro cell migration as well as in vivo tumor metastasis, associated with increased mouse survival compared to the untreated control group [79]. HIFU exposure to B16F10 cells resulted in decreased expression of miR-21 and p-Akt and increased expression of PTEN. MiR-21 inhibition also increased PTEN expression, and inhibited B16F10 cell migration after HIFU exposure, indicating that HIFU-induced anti-metastatic effects were mediated via decreased expression/activity of miR-21 and Akt as well as increased PTEN expression [79]. Another study by Mao et al., validated that miR-21 expression was higher whereas SPRY1, PDCD4, and PTEN expressions were lower in melanoma tissues compared to the adjacent tissues [80]. Inhibition of miR-21 in A375 human melanoma cells resulted in decreased expressions of ERK and NF-kBp65 activation, and increased expressions of SPRY1, PDCD4, and PTEN in the manner abrogated via the inhibition of SPRY1, PDCD4, and PTEN. In addition, miR-21 inhibition resulted in decreased proliferation, migration, and invasion as well as increased apoptosis of B16F10 cells in a process reversed by SPRY1, PDCD4 and PTEN silencing [80]. These findings indicated that miR-21 could promote cell growth by targeting oncogenic ERK/NF-kB and inhibiting tumor suppressor SPRY1, PDCD4, and PTEN pathways.

Naturally occurring dietary agents have been shown to possess promising anti-melanoma activity [81–83]. Importantly, recent studies have demonstrated that dietary agents and their active analogs target miR-21 and associated signaling cascades to exert anti-melanoma activity [84–85]. In one such report, curcumin analog EF24 has been shown to induce apoptosis in B16 melanoma cells via inhibiting miR-21 expression and enhancing the expression of tumor suppressor genes such as PTEN and PDCD4, the known targets of miR-21 [84]. Further studies exploiting miRNAs profiling revealed that EF24 treatment resulted in reduced B16 tumor metastasis to lungs as well as increased mice survival. These effects were mediated via enhanced expression of tumor suppressor genes and decreased expression of oncogenic miRNAs including miR-21 as well as the inhibition of NF-kB signaling pathway [84]. In another study, Mu et al. demonstrated that an active component of blister beetle known as cantharidin (CTD) suppressed the in vitro proliferation and colony formation as well as induced apoptosis of A375 human melanoma cells [85]. These effects were mediated via CTD-induced reduced expression of miR-21 and enhanced expression of PTEN protein. The in vivo studies demonstrating reduced growth of subcutaneous tumor xenografts further supported the anti-tumor activity of CTD [85]. Moreover, miR-21 overexpression in A375 cells blocked these in vitro and in vivo effects of CTD [85]. These findings indicated that CTD targets miR-21-PTEN signaling pathway to inhibit the growth of melanoma cells, and therefore could be used as a novel anti-proliferative agent against human melanoma. The summary of miR-21 targeted signaling pathways is depicted in Figure 1.

Figure 1.

The schematic representation of miR-21 targeted signaling pathways. The signs denote inhibition, upregulation and induction.

miR-155

Similar to miR-21, miRNA profiling studies have documented significant upregulation of miR-155 in primary, malignant and metastatic melanoma tissues as well as melanoma cells compared to benign and dysplastic nevi or normal skin [66,73,86–87]. Importantly, serum miR-155 levels along with other miRNAs have been shown to be the sensitive diagnostic tools to distinguish metastatic melanoma patients with non-metastatic melanoma patients [88]. In contrast, a group has identified that miR-155 levels in melanoma cells were downregulated compared to the normal melanocytes, and that miR-155 ectopically expressing melanoma cells exhibited impaired proliferation and apoptosis induction [89]. As downregulation of SKI protein, a transcriptional coregulator overexpressed in melanoma cells, resulted in reduced in vitro and in vivo growth of melanoma cells, later this group determined the effects of miR-155 in SKI-mediated inhibition of melanoma growth [89]. Using luciferase reporter assay, the authors have shown that miR-155 via binding to the 3’UTR region of SKI gene, impaired its expression. Overexpression of miR-155 downregulated SKI expression and inhibition of miR-155 endogenous levels resulted in the upregulation of SKI expression [89]. Further studies demonstrated that SKI gene silencing or overexpression of a 3’UTR-deleted SKI gene in endogenous miR-155 expressing melanoma cells resulted in the inhibition of cell proliferation. These findings indicated that miR-155 targets SKI in melanoma.

Tumor microenvironment associated immune cells play crucial roles in favoring tumor growth and metastasis via mechanisms facilitating immune evasion. To that end, suppressive immunophenotypes including regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSC) have been shown to impact the growth of tumors as well as the efficacy of therapeutic agents via mechanisms involving miRNAs [5,90–94]. Importantly, miR-155 has been shown to promote tumor growth by regulating host anti-tumor immune responses [92]. In this regard, a study demonstrated that miR-155 knockout (mir-155−/−) mice resulted in increased growth of B16F10 melanoma and Lewis lung carcinoma tumors accompanied by an increase in intratumoral MDSC and decreased CD8+ T cells compared to the wild type (WT) mice, in a process mimicked by the transplantation of miR-155 deficient bone marrow cells [94]. Functional analysis revealed that these tumor-infiltrating MDSC possessed greater migration ability and expressed increased levels of chemokines such as CXCL1, CXCL2, CXCL3, CXCL5, CXCL6, CXCL7, and CXCL8. Increased expression of HIF-1α was identified in the tumor and splenic microenvironment of miR-155−/−mice compared to WT mice [94]. HIF-1α silencing resulted in decreased expression of CXCL1, CXCL2, CXCL3, CXCL5 and CXCL8 chemokines in the bone marrow of miR-155−/−mice, in the process reversed by culturing the bone marrow cells from WT and miR-155 mice under hypoxic condition compared to the normoxic condition [94]. Furthermore, tumor-infiltrating MDSC from miR-155−/−mice possessed immunosuppressive properties via their abilities to suppress CD4+ and CD8+ T cell proliferation. In addition, such MDSC exhibited increased arginase-1 and iNOS expression as well as proangiogenic factors such as VEGF, MMP2, and MMP9 [94]. These findings indicated that miR-155 deficiency recruits tumor-infiltrating immunosuppressive MDSC that possessed tumor growth enhancing effects. Therefore, the upregulation of miR-155 expression could be developed as a therapeutic approach to suppress melanoma growth.

The genetic events leading to a nucleotide change known as single nucleotide polymorphisms (SNPs) have been shown to result in transcriptional and translational alterations regulating mRNA and protein expressions [95–99]. Such genetic alterations affect the cellular behavior of malignant tumor cells including melanoma [95–99]. Notably, SNPs in miRNAs including miR-155 have been shown to impact the incidence, risk, and prognosis of human malignancies including melanoma [95–99]. In this regard, studies by El et al. determined the role of miR-155 in the regulation of tyrosinase-related protein 1 (TYRP1) mRNA and protein expression via SNPs at the 3’UTR region of TYRP1 [99]. The authors demonstrated that miR-155 transfection resulted in a dose-dependent degradation of TYRP1 mRNA and protein expression in ME1402 melanoma cells with “CC/AA” match genotype compared to MM031 with “AA/CC” mismatch genotype [99]. The evaluation of TYRP1 mRNA and protein expression, TYRP1 SNPs and miR-155 expression in the skin and lymph node melanoma metastases revealed that TYRP1 mRNA expression was inversely correlated with miR-155 expression [99]. In contrast, TYRP1 SNPs in the match group was positively correlated with TYRP1 protein expression, but not with miR-155 in the mismatched group. Interestingly, TYRP1 mRNA expression was positively correlated, whereas TYRP1 protein expression was inversely correlated with the overall survival of melanoma patients [99]. These data indicated that SNPs affect TYRP1 mRNA regulation and subsequent protein translation by miR-155 and explained the discrepancy between the prognostic values of TYRP1 mRNA and protein expression [99]. Thus, such SNPs affecting TYRP1 along with miR-155 could be used as melanogenic markers to refine/improve the prognosis of advanced stage melanoma patients.

Similarly, microphthalmia-associated transcription factor (MITF-M) expressed exclusively in melanocytes have been shown to regulate melanocyte fate [100–101]. MITF-M regulate the expression of several genes such as tyrosinase (TYR), Melan-A/Mart-1 and gp100 involved in the survival, proliferation, and differentiation of melanocytes. In addition, the protein expression of such genes at the surface of tumor cells can be recognized by cytotoxic T lymphocytes and result in tumor eradication [100–101]. Initial studies by De Plaen group have shown that interleukin-1β (IL-1β) can downregulate MITF-M levels in melanoma cells [102]. Later this group identified that IL-1β-mediated downregulation of MITF-M was accompanied by a miR-155 upregulation in melanoma cells [103]. miR-155 also downregulated MITF-M and that MITF-M repression by IL-1β was abrogated by miR-155 downregulation via specific antagomir. Notably, qPCR studies in mouse melanoma tumor samples confirmed a negative correlation between MITF-M/tyrosinase and miR-155/IL-1β levels [103]. These results suggested that miR-155 targets MITF-M in an inflammatory microenvironment, and highlighted a novel immune evasion mechanism that represents a therapeutic approach to chemotherapy and immunotherapy for melanoma treatment.

Several other targets of miR-155 have been identified in melanoma models [5, Figure 2]. Since tumoral miR-155 expression levels have been validated reproducibly with patient outcome, it has been proposed that miR-155 expression could be used as prognostic signatures for metastatic melanoma in clinical settings [104]. Of significance, targeting cancer-associated fibroblasts (CAFs) within the tumor microenvironment have been explored as promising approaches to modulate the rate of tumor cell proliferation, angiogenesis, and metastases [105–106]. Studies by El et al. determined the role of miR-155–5p and underlying mechanisms in the proangiogenic switch of cancer-associated fibroblasts (CAFs) in melanoma model [107]. The authors demonstrated that exosomes secreted from B16 and B16F10 melanoma cells transformed fibroblasts into CAFs as confirmed by increased expression of CAFs markers such as fibroblast activation protein (FAP) and α-smooth muscle actin (α-SMA) [107]. Additional studies confirmed that these CAFs possessed proangiogenic properties as measured by increased expression of FGF2, VEGFa, and MMP9. This proangiogenic switch of CAFs was regulated via the activation of JAK2/STAT3 signaling and inhibition of SOCS1 expression [107]. Importantly, bioinformatics and sequence analysis revealed that miR-155 targets the 3’UTR of SOCS1. B16-and B16F10-secreted exosomes carried abundant amounts of miR-155, which can be delivered to CAFs. This exosomal miR-155 regulated the proangiogenic phenotype of CAFs in vitro assay and in vivo melanoma xenografts models [107]. These findings elucidated a new mechanism of miR-155-mediated tumor angiogenesis, which could be targeted for melanoma prevention.

Figure 2.

The schematic representation of miR-155 targeted signaling cascades. The signs denote inhibition and induction.

In another report, decreased miR-155 expression was documented in tumor samples of the majority of melanoma patients, and WEE1 protein kinase was found to be a novel target of miR-155 [108]. In addition, increased expression of miR-155 and WEE1 silencing resulted in decreased metastasis in experimental melanoma model [108]. These studies indicated that increased WEE1 expression and loss of miR-155 could result in metastatic phenotype in melanoma patients. The summary of miR-155 targeted signaling cascades is depicted in Figure 2.

Other commonly upregulated miRNAs in melanoma

The whole miRNome and custom qPCR array analysis of miRNA expression profiling of whole blood and tissues samples from healthy subjects and melanoma patients have identified signature miRNAs exhibiting prognostic scores for the late stage, but not early-stage melanoma patients [109]. Among several miRNAs, miR-30b-5p and miR-374a-5p and others were exclusively expressed in melanoma tissues, whereas miR-3201 and miR-122–5p were highly expressed in matching serums samples of melanoma patients [109]. Overall, these data implicated the use of cell-free miRNAs as biomarkers that significantly changes at the later stages of melanoma progression. Regarding miR-122–5p, another report has shown that its expression was higher in human melanoma tissues compared to human pigmented nevus tissues [110]. Inhibition of endogenous miR-122–5p expression in SK-LEL-110 and A375 human melanoma cells resulted in the inhibition of cell proliferation and G1 cell cycle arrest without inducing apoptosis [110]. NOP14 was identified as a direct target of miR-122–5p by luciferase reporter assay and miR-122–5p inhibitor only increased NOP14 protein expression but did not affect NOP14 mRNA expression [110]. Overexpression of miR-122–5p resulted in decreased luciferase activity compared to control cells. These findings indicated that miR-122–5p targets NOP14 to regulate the proliferation and development of melanoma.

Notably, the secretion of extracellular vesicles such as microvesicle particles and exosomes from tumor cells are being increasingly recognized for their diverse roles in cancer and cancer therapy effectiveness [111–113]. These secreted extracellular vesicles carry bioactive components including lipids and miRNAs and have been proposed to be involved in mediating various activities including cell-to-cell communications [111–112]. In a recent report, Byrnes and colleagues have demonstrated that the lysophosphatidic acid receptor-3 (LPAR3) signaling in human melanoma cells induced the expression of miR-122–5p intracellularly and its secretion into exosomes [113]. Several cellular mechanisms/signaling pathways such as Wnt1, GSK3, Src homology 3 (SH3) ligand-binding motif, and peroxisome proliferator-activated receptor gamma coactivator 1-α (PPARGC1A/PGC1) were identified to be involved in increased miR-122–5p expression/transcription in LPAR3-dependent or independent manner [113]. The clinical studies also explained the possible mechanisms of increased miR-122–5p in the serum of cancer patients and indicated the implications of LPAR3 in the production and secretion of miR-122–5p in the serum in clinical settings of cancer patients.

Similarly, miR-182, a frequently amplified miRNA in melanoma tumors has been shown to promote melanoma metastasis via repressing FOXO3 gene and microphthalmia-associated transcription factor (MITF-M) [114]. Downregulation of miR-182 in melanoma cells inhibited the invasion via inducing apoptosis, and that enhanced expression of FOXO3 or MITF-M blocked these miR-182-induced proinvasive effects [114]. The findings that miR-182 directly targets the FOXO3 gene product was supported by Hanniford and Hernando studies [115]. In another report, Liu et al., have demonstrated that the expression of miR-182 in melanoma cells is likely mediated by the methylation of CpG islands as this effect was not observed in human melanocytes, skin or peripheral blood mononuclear cells (PBMCs) [116]. In clinical studies, increased expression of miR-182 has been linked to poor prognosis of cancer patients [117]. Among other miR-182 targets, FoxO1, transforming growth factor beta (TGFβ) and NF-kB have been identified as possible mechanisms to induce tumor growth and metastasis as activation/expression of these cascades have been shown to be blocked by targeting miR-182 [117].

Of significance, the aberrant expressions of several other miRNAs such as let-7a/let-7a-5p and let-7b/let-7b-5p, miR-148, miR-149, miR-199b-5p, miR-200c, miR-205–5p, miR-214, miR-221 and miR-222, miR-424–5p have been linked with the development and/or progression of melanoma in clinical settings [118–119]. In addition, the targets of these miRNAs including NRAS, receptor tyrosine kinase c-KIT, and AP2 transcription factor have also been documented in tumor samples of melanoma patients [118–119]. Among these, miR-149* has been shown to function as an oncogenic regulator in melanoma, which is directly regulated by p53, and targets glycogen synthase kinase-3α (GSK3α) and anti-apoptotic Mcl-1 protein [120]. To that end, miR-149* deficiency resulted in reduced melanoma cell survival or the growth of melanoma tumor xenografts, indicating that miR-149* induces resistance to apoptosis by decreasing GSK3α and increasing the expression of Mcl-1 [120]. Importantly, elevated levels of miR-149* in human metastatic melanoma isolates were found to be associated with decreased GSK3α and increased Mcl-1 [120]. On the other hand, miR-149–5p has been shown to directly regulate the expression of oncogene LRIG2 expression in melanoma cells and tissues [121]. Overexpression of miR-149–5p resulted in the suppression of melanoma cell proliferation and colony formation as well as induction of apoptosis mediated via the downregulation of LRIG2 expression [121].

In addition to the above-mentioned miRNAs, affecting the growth and proliferation of melanoma cells, several other miRNAs including miR-506–514 (a cluster of 14 miRNAs on the X chromosome) and miR-218 have been demonstrated to play crucial roles in initiating melanocyte transformation (melanomagenesis) or their progression to melanoma [13,52]. The detailed profiling of miR-506–514 cluster expression was performed from tumor punch biopsies of melanoma patients and compared them to normal donors followed by the detection of this cluster in a panel of 5 melanoma cell lines [13]. The authors divided the cluster in sub-cluster A (miR-506, miR-507, miR-508 and miR-513(a,b,1,2)) and sub-cluster B (miR-509(1,2,3), miR-510, miR-514(1,2,3) and found that all members of this complete cluster (miR-506–514) are needed to initiate carcinogenesis. However, the cancer growth and prevention of apoptosis was found to be regulated under the control of sub-cluster A [13]. Similarly, melanogenesis has been shown to be regulated by miR-218 and that this effect was directly mediated via the inhibition of MITF, which controls the transcription of tyrosinase (TYR) [52]. MITF is the upstream regulator of melanogenesis a complex multistep process in which TYR, a key enzyme regulates the rate-limiting step of melanin synthesis. This study used bioinformatics analysis to predict common miRNAs targeting MITF. Of 8 candidate miRNAs, miR-218 was found to be a novel promising candidate that suppressed melanogenesis via inhibiting TYR activity or its mRNA expression [52].

Importantly, some miRNAs have also been shown to limit the therapeutic benefits of BRAF inhibitors (BRAFi) in melanoma via developing resistance. In one such study, the authors demonstrated increased expression of miR-34a, miR-100, and miR-125b in BRAFi resistant melanoma cells, as well as tumor samples from BRAFi, treated patients [122]. However, in this study, low levels of CCL2, an autocrine growth factor crucial for increased cell proliferation and resistance to apoptosis was found to be associated with the long term clinical outcomes [122]. In addition, increased levels of CCL2 was detected in tumors and plasma samples of melanoma patients following vemurafenib treatment [122]. The inhibition of these miRNAs and CCL2 resulted in increased melanoma cell apoptosis and improved efficacy of vemurafenib [122]. In another study, the authors have demonstrated that among five miRNAs (miR-3617, miR92a-1, miR1246, miR-193b-3p and miR-17–3p), miR-1246 mimic was able to reduce the antiproliferative effects of BRAFi, PLX4720 in multi-drug resistant, A375P melanoma (A375P/Mdr) cells [123]. Furthermore, this effect was mediated by G2/M arrest and reduced phosphorylation of p-ERK, suggesting miR-1246 to be a potential target in BRAF inhibitors resistant melanoma cells [123].

miR-211

The miRNA expression profiling in melanocytes, human melanoma cell lines and melanoma samples from patients revealed that miR-211 expression levels were significantly downregulated in several melanoma cell lines and most of the melanoma samples compared to melanocytes [124]. Overexpression of miR-211 resulted in reduced melanoma cell proliferation and invasiveness in a process blocked by miR-211 inhibitor. The Ca⁺⁺ regulated K⁺ channel protein, KCNMA1 was identified as the direct target of miR-211 [124]. As the TRPM1 (melastatin) gene, which encodes a Ca⁺⁺ channel protein, contains miR-211 sequences within the sixth intron, and MITF regulates TRPM1 expression and need for increased expression of miR-211. The authors speculated that both TRPM1 and miR-211 regulation by MITF could exert similar effects on melanoma invasiveness separately via mechanisms mediated by targeting TRPM1 and KCNMA1 [124]. These data indicated that miR-211 expression is dependent on MITF activity and its ability to directly regulate the posttranscriptional activity of KCNMA1 establishes its regulatory role in human melanoma.

The findings of Mazar et al., studies [124] were supported by another report, where reduced miR-211 expression in A375M human melanoma cells was identified via genome-wide miRNA screening [125]. The authors have shown that miR-211 overexpression reduced the migration and invasiveness of human melanoma cells having low expression of TRPM1 and miR-211 [125]. The gene network analysis identified that miR-211 targets three central node genes regulating metastasis namely insulin-like growth factor 2 receptor (IGF2R), TGF-beta receptor 2 (TGFBR2) and nuclear factor of activated T cells 5 (NFAT5), respectively [125]. As miR-211 repressed the endogenous expression of IGF2R, TGFBR2 and NFAT5 in melanoma cells, miR-211 inhibition de-repressed, and its ectopic expression further repressed the expression of these target genes compared to melanoma cells transfected with control miRNA antagomir or mimic [125]. These findings implicated that miR-211 functions as a tumor suppressor in melanoma, and miR-211 or its target genes could be exploited for the development of new therapies against metastatic melanoma.

Notably, the microarray analysis of formalin-fixed and paraffin-embedded (FFPE) human samples has also identified differential expression of miR-211 in different stages of melanomagenesis and documented significantly reduced miR-211 expression in primary and metastatic melanoma tumor tissues compared to nevi [126]. These findings were validated using melanocyte and melanoma cell lines, which demonstrated reduced expression of miR-211 in melanoma cells compared to melanocyte cell lines [126]. Ectopic expression of miR-211 resulted in reduced anchorage-independent colony formation and melanoma cell invasion compared to the control cells. These findings indicated that miR-211 functions as a tumor suppressor and its expression could be used as a diagnostic or prognostic marker for melanoma [126]. Another report characterizing the transcription factor-miRNA relationship in modulating the proliferative and invasive programs of melanoma by using gene expression profiling of normal melanocytes and melanoma has identified two sets of miRNAs including miR-211 involved in mediating these events [127]. This study identified that miR-211 directly targets the NUAK1 gene to induce melanoma adhesion and that inhibition of miR-211 decreased melanoma cell adhesion and increased NUAK1 expression and vice versa [127]. As MITF has been shown to be a known target of miR-211, the authors demonstrated that MITF/miR-211 axis inhibited melanoma cell invasiveness via blocking melanoma cell adhesion [127]. The summary of miR-211 targeted signaling pathways is depicted in Figure 3.

Figure 3.

The schematic representation of miR-211 targeted signaling pathways. The signs denote inhibition and induction.

miR-193

Of significance, the analysis of miRNA expression profiling of 470 miRNAs in benign nevi and metastatic melanoma tissues identified 31 differentially expressed miRNAs, of which miR-193b was found to be significantly downregulated in melanoma tissues compared to benign nevi [128]. The overexpression of miR-193b in Malme-3M melanoma cells resulted in reduced cell proliferation. Among 314 downregulated genes identified by miR-196b overexpression, 18 genes including cyclin D1 (CCND1) were found to be the direct targets of miR-196b [128]. These findings indicated that miR-196b represses cell proliferation via targeting CCND1, and thus, acts as a tumor suppressor in melanoma development. Later this group identified that miR-193b also downregulated myeloid cell leukemia sequence 1 (Mcl-1) in melanoma cells [129]. The low expression of miR-193b was correlated with high expression of Mcl-1 in malignant melanoma tissues compared to benign nevi [129]. Importantly, miR-193b overexpression suppressed Mcl-1 expression and restored the sensitivity of ABT-737-resistant melanoma cells to ABT-737, a small-molecule inhibitor of anti-apoptotic proteins, Bcl-2, Bcl-XL and Bcl-w [129].

Another report supported these clinical findings of Chen et al., studies [129] that low expression of miR-193a was detected in melanoma samples, particularly those harboring BRAF^V600 mutation, which exhibited a correlation with patients’ survival [130]. Along similar lines, Pinto and colleagues have demonstrated low expression of miR-193b* along with other miRNAs in BRAF^V600-mutated metastatic melanoma patients compared to BRAF wild type patients [131]. Importantly, high miR-193b* expression along with miR-192 and low expression of miR-132 in BRAF-mutated melanoma patients treated with vemurafenib was significantly associated with shorter progression [131]. These studies implicated the usefulness of such miRNAs in predicting the clinical outcomes for patients with BRAF-mutated melanomas.

miR-126

In clinical settings of melanoma patients, significant downregulation in miR-126 expression was noted in metastatic melanoma tissues compared to the primary cutaneous melanoma and dysplastic nevi [132]. Importantly, low miR-126 expression was associated with a shorter 5-year survival rate compared to the patients with high miR-126 expression. However, the status of miR-126 was predicted to be an independent prognostic factor for evaluating the overall survival of melanoma patients [132]. Similar findings were shown by Felli et al., studies, were markedly reduced expression of miR-126&126* were documented in metastatic melanoma cell lines compared to primary melanoma cell lines and normal melanocytes [133]. In addition, overexpression of miR-126&126* in metastatic melanoma cell lines resulted in decreased in vitro proliferation, invasion, and chemotaxis as well as in vivo growth and metastasis [133]. Furthermore, ADAM9 and MMP7 were identified as direct targets of miR-126&126*, and among the oncogenic signaling pathways, inactivation of PI3K/AKT and MAPK, as well as induction of KIT/MITF/TYR, were observed [133]. These findings validated the role of miR-126&126* as a tumor suppressor in melanoma. Later this group determined the circuitry connection between oncomiRs miR-122&222 and miR-126&126* and found an inverse correlation between the endogenous levels of these two sets of miRNAs and their targets in melanoma [134]. Notably, AP2α was identified as a central player with dual regulation that was directly targeted by miR-221&222 and on the other hand involved as a transcriptional activator of miR-126&126* [134]. These findings implicated that AP2α plays a central role in orchestrating melanoma development and/or progression, and is required as a circuitry mechanism between miR-221&222 and miR-126&126* in melanoma.

miR-145

miR-145 has been studied for its critical roles in cancer progression. A study by Noguchi et al. has identified significantly reduced expression of miR-145 in canine and human melanoma cell lines [135]. The ectopic expression of miR-145 resulted in the inhibition of growth and migration of human melanoma cells via suppressing c-MYC and the actin-bundling protein FASCIN1 levels that regulate the behavioral activities of cancer cells including cell migration and invasion [135]. Notably, the differential regulatory effects of miR-145 expression have been shown on the migration and invasion of primary and non-invasive melanoma cells as well as the invasive potential of metastatic melanoma cells [136].

Of significance, an inverse correlation between miR-145–5p and NRAS expression has been observed in melanoma tumor tissues (low miR-145–5p and high NRAS expression) compared to adjacent tissues [137]. Similar trends in miR-145–5p and NRAS expression were found in human melanoma cells compared to the normal human epidermal melanocytes [137]. Importantly, miR-145–5p has been shown to directly target NRAS to inhibit the in vitro proliferation, invasion, and migration and induce apoptosis of melanoma cells as well as suppress the in vivo growth of melanoma xenografts by inhibiting MAPK and PI3K/AKT pathways [137]. However, these miR-145–5p mediated in vitro and in vivo effects were not observed at the same degree/potential in BRAF-mutant melanoma cells [137]. These findings may explain that the low expression of miR-145–5p in BRAFV600 mutated melanoma cells could possibly result in developing resistance to single BRAFi and implicated that miR-145–5p could be a potential target for the clinical intervention of melanoma.

miR-200 family members

The miRNAs of miR-200 family have been identified as important mediators of cutaneous carcinogenesis including melanoma. In clinical settings of melanoma, loss of miR-200a, miR-200c, and miR-203 expression were found to be associated with increased tumor thickness or disease progression and correlated with reduced E-cadherin expression [138]. These findings indicated that miR-200 and miR-203 regulate E-cadherin during melanoma progression [138]. Along similar lines, a recent study evaluated intratumoral miRNAs expression in primary vs metastatic melanomas for their ability to be used as prognostic biomarkers and correlated with clinicopathological factors and clinical outcomes [139]. The intratumoral expression of several miRNAs including miR-200c and miR-205 were found to be downregulated in primary and distant metastatic melanomas and correlated as independent factors associated with shorter survival and prognostic biomarkers for the diagnosis of high-risk melanoma patients [139].

Other commonly downregulated miRNAs in melanoma

The recent studies have demonstrated that miR-365 expression was downregulated in melanoma cells as well as malignant melanoma tissues from patients. In addition, the overexpression of miR-365 inhibited the growth/proliferation, metastasis or invasion of malignant melanoma cells via their abilities to target distinct signaling pathways such as neuropilin 1 (NRP1), and also resulted in cell cycle arrest and apoptosis by targeting BCL2 and Cyclin D1 [140–141]. Similarly, reduced miR-205 expression was identified in melanoma cells compared to benign nevi [142]. The authors demonstrated an inverse correlation between elevated levels of miR-205 with diminished E2F1 and E2F5 protein levels [142]. These changes resulted in reduced melanoma cell proliferation, colony formation and induced apoptotic cell death mediated through E2F directed phosphorylation of Akt signaling [142]. In addition, studies by Noguchi et al., have shown that chemically modified miR-205 was able to reduce the in vitro and in vivo growth melanoma cells by targeting BCL2 and VEGF, which were transcribed by E2F1 [143]. The antitumor effect of chemically modified synthetic miR-205 was mediated via its ability to function as a tumor suppressor in human melanoma cells [143]. Furthermore, in vitro experiments demonstrated that E2F1 and VEGF levels were diminished by miR-205, which regulated cell proliferation and survival [143]. Along similar lines, miR-206 has also been demonstrated to act as a tumor suppressor in melanoma via its ability to target cell cycle genes such as CDK4, cyclin D1 and cyclin C [144]. Of significance, decreased levels of circulating miR-206 have been found in serum samples of melanoma patients compared to healthy controls [57]. Importantly, studies in wild type and tumor necrosis factor alpha (TNF-α) knock out mice exposed to UVR indicated the role of miR-206 in the development of squamous cell carcinoma (SCC) [145]. The authors have shown that UVR induced formation of SCC tumors exhibited reduced miR-206–3p expression [145].

Among other miRNAs downregulated in melanoma cells, Braig et al. studies defined that miR-196a expression was strongly reduced in melanoma cells compared to healthy melanocytes [146]. This reduced miR-196a expression caused enhanced mRNA and protein expression of HOX-B7, which consequently stimulated basic fibroblast growth factor (bFGF) signaling. This bFGF signaling upregulated the expressions of ETS-1 transcription factor and bone morphogenetic protein 4 (BMP-4), which are critical factors in melanoma progression [146]. Later, this group using high-throughput miRNA expression profiling approach has shown that miR-196a expression was significantly reduced in malignant melanoma cells and tissue samples [147]. Using stable miR-196a expression approach and luciferase reporter assays, the authors determined that miR-196a directly interacts and downregulates HOX-C8 gene. These changes resulted in significantly reduced melanoma cell invasiveness via targeting cadherin-11, calponin-1, and osteopontin genes [147]. Along similar lines, miR-143–3p expression was found to be downregulated in human melanoma cells and clinical melanoma specimens compared to the normal human melanocytes [148]. The overexpression of miR-143–3p in melanoma cells resulted in decreased proliferation, migration, invasion and increased apoptosis mediated in part via the inhibition of cyclooxygenase-2 (COX-2) gene [148]. These data suggested that miR-143–3p acts as a tumor suppressor, and thus, could be exploited as a new early diagnostic marker and therapeutic target for melanoma.

In sum, given the heterogeneity of miRNAs in modulating the growth as well as the effectiveness of cancer therapies, the detailed analysis of miRNAs and their mechanistic insights impacting drug sensitivity/efficacy, would open new areas of investigation to overcome drug resistance and/or improve the clinical management of cancers. Importantly, several miRNAs have been identified that could assist in the diagnosis or early detection of melanoma recurrence. To that end, such miRNAs could be exploited as diagnostic or prognostic biomarkers in predicting patients’ outcomes/responses to therapeutic interventions. Most importantly, as miRNAs role in analyzing treatment responses could reproducibly be utilized in diverse settings of melanoma patients, according to the personalized medicine model, miRNAs-associated molecular taxonomy may be helpful to predict the likelihood of patients developing resistance against a given treatment. Therefore, it is possible that definitive sets of miRNA could aid in devising particular treatment option for individual patient to achieve overall improved responses with a long-term survival benefits.

Abbreviations:

miRNAs/miRs

microRNAs

3’UTR

3’ untranslated region

PDCD4

programmed cell death 4

PTEN

phosphatase and tensin homolog

STAT-3

signal transducer and activator of transcription 3

BTG2

B-cell translocation gene 2

NF-kB

nuclear factor-kappa B

MAPK

mitogen-activated protein kinase

ERK

extracellular signal-regulated kinase

PI3K

phosphatidylinositol 3-kinases or phosphoinositide 3-kinases

Akt

protein kinase B

Tregs

regulatory T cells

MDSC

myeloid-derived suppressor cells

TAM

tumor-associated macrophages

HIF-1α

hypoxia-inducible factor 1-alpha

CXCL

chemokine (C-X-C) ligand

Arg-1

arginase 1

iNOS

inducible nitric oxide synthase

VEGF

vascular endothelial growth factor

MMPs

matrix metalloproteinases

TIMPs

tissue inhibitor of metalloproteinases

TYRP1

tyrosinase related protein 1

IL-1β

interleukin-1β

MITF

microphthalmia-associated transcription factor

TYR

tyrosinase

LPAR3

lysophosphatidic acid receptor-3

GSK3α

glycogen synthase kinase-3α

PPARGC1A/PGC1

peroxisome proliferator-activated receptor gamma coactivator 1-α

TGFβ

transforming growth factor beta

bFGF

basic fibroblast growth factor

BMP-4

bone morphogenetic protein 4

TRPM1

Transcriptional regulation of the melanoma prognostic marker melastatin

KCNMA1

the Ca⁺⁺ regulated K⁺ channel protein

IGF2R

insulin-like growth factor 2 receptor

TGFBR2

TGF-beta receptor 2

NFAT5

nuclear factor of activated T cells 5

FASCIN1

fascin actin-bundling protein 1

NRP1

neuropilin 1

CDK

cyclin-dependent kinase

TNF-α

tumor necrosis factor alpha

COX-2

cyclooxygenase-2