miRNAs, ROS and Melanoma
Molecular mechanisms associated with melanoma tumour physiology in processes such as proliferation, growth, energy metabolism, migration, differentiation and cell death involve oxidative stress and thus genetic and epigenetic alterations of oncogenes and tumour suppressor genes [
10,
11,
12,
13]. Non-mutational epigenetic reprogramming is exerted by miRNAs, small non-coding RNAs of 18-22 nt, which play a role in the post-transcriptional regulation of up to 60% of mammalian protein-coding mRNAs [
14,
15,
16,
17]. Most miRNAs are derived from long intramolecular double-stranded RNAs; these RNAs are sequentially cleaved by type III RNases, first in the nucleus and then in the cytoplasm, to generate a miRNA duplex. In subsequent steps, one of the strands of the duplex associates with an RNA-induced silencing complex (RISC), which triggers a decrease in a specific mRNA by degrading the transcript or repressing mRNA-to-protein translation [
18,
19]. A single miRNA can target hundreds of mRNAs, and a single mRNA can target multiple miRNAs, so variations in the expression of thousands of mRNAs could be explained by the coordinated network expression patterns of a few miRNAs [
20]. There are miRBase records of 2600 corresponding sequences for mature miRNAs in humans [
16,
21,
22,
23,
24].
Potential relationships between ROS and miRNAs in cancer have been reported, e.g., miRNA-21 can inhibit the antioxidant pathway of superoxide dismutase (SOD), facilitating oxidative damage by superoxide anion (O
2-) and other ROS, as well as enhancing oncogenic and fibrotic processes associated with carcinoma [
24,
25]. Alterations in miRNA-5096 increase hydroxyl radical (OH
-) levels and suppress non-apoptotic cell proliferation in breast cancer cells; the mechanism of cell damage of this miRNA would be mediated by ROS and is reversible with the use of antioxidants such as N-acetyl cysteine (NAC) [
26]. Meanwhile, it has been shown that the increase of miRNA-34 in human glioma cells, together with the suppression of nitrogen dioxide radicals, supports the increase of apoptosis rate and the decrease of cell viability in a glioma model [
27].
In melanoma, miRNAs are also differentially expressed and their alteration may be related to changes in the oxidative environment and the activation of cancer imprints. For example, enrichment of extracellular vesicles with miRNA-214 secreted by melanoma cells stimulates over-activation of macrophages, which release nitric oxide (NO), a process that facilitates endothelial permeability and favors metastasis [
19]. Nitric oxide alone is not capable of causing DNA damage, but it can inhibit enzymes such as DNA ligase and thus indirectly cause double helix breaks [
28,
29]. It has also been suggested that silencing miRNA-517a in melanoma patient samples results in overexpression of CDKN1C (cyclin-dependent kinase inhibitor 1C) and suppression of the JNK kinase-mediated survival and proliferation pathway, leading to increased ROS [
8]. Furthermore, excessive levels of ROS, such as those caused by the administration of chemotherapeutic agents, induce the development of resistance mechanisms to the oxidative environment; such resistance has been studied in the human melanoma cell line A375, in which activation of the mitogen-activated protein kinase (MAPK) pathway (RAS/MEK/ERK) and the transcription factors hypoxia-inducible factor 1 (HIF-1α) and the master regulator of melanocyte differentiation, microphthalmia-associated transcription factor (MITF), were observed.
Indeed, other miRNAs with different molecular targets have been linked to the HIF-1α and MITF pathways. The miR-33a and miR-138 are thought to directly regulate HIF-1α in the transition from melanocyte to metastatic melanoma [
30,
31]. miR-182 induces increased invasion and metastasis by binding to the tumour suppressor FBXW, and its binding to FOXO3 and MITF appears to be associated with melanoma progression, and miR-182 is a target of epigenetic modulation with hypermethylated CpG islands in melanoma cells [
14]. On the other hand, miR-211 modulates the expression of genes involved in cell cycle regulation and tumour suppression, such as p16INK4A, BRN2 and MITF [
32,
33]. Meanwhile, miR-203 has been suggested to act as a tumour suppressor by regulating melanosome transport and tyrosinase enzyme (TYR) expression through the kinesin 5 (kif5b) superfamily of proteins and appears to negatively modulate one of the major signalling pathways active in melanoma cells, the CREB1/MITF/Rab27a pathway [
34].
In addition to promoting a tumorigenic environment, miRNAs can also act as regulators of p53-associated tumour suppressor conditions and molecular machinery [
35,
36,
37,
38]. Activation of p53, stimulated by cellular stresses such as ionizing radiation, hypoxia, carcinogens or oxidative stress, leads to cell cycle arrest and promotes DNA repair or induces apoptosis through various pathways [
39]. The expression of miR-18b is significantly reduced in patient-derived melanomas and cell lines due to hypermethylation of the p53 pathway, while its stable overexpression results in potent tumour suppressor activity as measured by cell viability, induction of apoptosis and reduction of tumour growth in xenograft assays [
40].
It is possible that oxidative stress in melanoma involves non-mutational epigenetic reprogramming in which networked sets of miRNAs, together with transcription factors such as MITF and HIF-1α, exert control over specific mRNAs whose protein products modulate the activity of various signalling pathways that ensure survival under these stress conditions, an aspect that will be discussed in the next section.
Expression Networks and Regulation under Oxidative Stress-Induced Melanoma
The study of signalling pathways provides insight into the molecular mechanisms involved in melanoma development and progression, including alterations in ROS-induced oxidative stress. However, more recent approaches integrate functional enrichment by downstream signalling pathways into the construction of gene regulatory networks between miRNAs, genes and transcription factors, facilitating the abstraction of information in a visual format that is often intuitive and interpretable of non-linear regulation exerted by miRNAs [
101,
102]. As an example of this approach, regulatory and co-expression networks were constructed from the GSE109378 dataset reported in the Gene Expression Omnibus - GEO [
103], in which changes in gene expression were evaluated by next generation sequencing (NGS) in the human melanoma cell line (SKMEL29), with silencing of miR-211 (SK-P8-2) and xenografts in each line, and their potential association with changes in ROS levels, mitochondrial respiration, and cell growth and invasion processes [
104].
From these data, differential expression analysis was performed using the SAM and LIMMA libraries (Bioconductor) in R code [
105,
106]. The data were normalized in the VSN library (Bioconductor) by stabilizing the means and making the samples comparable using a Log2 transformation. For log-fold change estimation, the expression of those mRNAs with a
p-value, corrected for multiple testing, of less than 5% was defined as differential [
106]. The comparison was first performed between the SKMEL29 and SK-P8-2 lines and between the respective xenografts, and then the differentially expressed mRNAs were compared between lines and xenografts. Initially, 1,300 mRNAs with differential expression were obtained and, after manual curation, 30 of them were related to oxidative stress processes; finally, 6 were found to be differentially expressed. The intensity values of the mRNAs were visualized in heatmaps using the “pheatmap” function in the R code, taking the average value for each of the selected mRNAs in the cell line samples (Cell line) and xenograft samples (Xeno). The color and intensity of the squares were used to represent changes (absolute values) in expression [
107].
As shown in
Figure 1, MAFF and TFEB mRNAs are upregulated in both cell line and Xeno samples. MAFF mRNA has been implicated in melanogenesis by forming a heterodimer with NRF3 in response to stress [
108], whereas the TFEB gene would cross-regulate with MITF in cellular clearance pathways [
109,
110] and induce melanoma growth by participating in metabolic regulation and ERK1/2 activation [
111]. ZEB1, NFE2 and DYRK1B mRNAs were found to be decreased in both conditions. The transcription factor ZEB1 is a known inducer of mesenchymal–epithelial transition and invasiveness; in melanoma, elevated ZEB1 levels are associated with resistance to treatment with MAPK inhibitors [
112], while the ZEB1/ZEB2 ratio would mediate phenotypic plasticity [
57,
112,
113]. The transcription factor NFE2 is involved in megakaryocyte production [
114], although it has not been reported to be altered in melanoma, its regulation by Nrf2 would promote the accumulation of ROS on megakaryocytes [
115].
Dual-specificity tyrosine phosphorylation-regulated kinase 1B-DYRK1B is activated by the RAF-MEK1/2-ERK1/2 signalling pathway in melanoma and would be related to the promotion of cell differentiation [
116]. The only gene that showed differential expression in the conditions studied was the T-cell intracellular antigen 1 gene, which encodes the TIA1 protein, an RNA-binding protein associated with nucleolytic activity against cytotoxic lymphocyte target cells, which has been proposed to be an oncogene localized in the cytoplasm of esophageal squamous cell carcinoma cells and to promote the expression of the progression-related genes SKP2 and CCNA2 [
117]. It is possible that the increase in TIA1 in xenografts derived from miR-211 deletion lines compared to deletion-only cell lines reflects the influence of media-influenced tumour growth on the expression of tumour progression-related genes.
Using the String platform, the co-expression network was constructed (
Figure 2), where the proteins encoded by the genes over- and under-expressed in both conditions are related to signalling pathways associated with oxidative stress, response to ROS, response to cytoplasmic stress and some of the pathways mentioned above, such as the PI3K-AKT-mTOR signalling pathway.
Figure 2. shows several distinct nodes. The node shown in blue/purple, formed by TIA1, G3BP1 and TIAL1, is associated with the formation of stress granules (SGs), cytoplasmic structures induced in response to environmental stress, mainly viral infection [
118]; the formation of SGs appears to play an important role in the progression of several types of cancer by inhibiting apoptosis in response to stress [
119,
120]; although in melanoma the presence of the retinoblastoma binding (RB) mRNA Rbfox2 in SGs was associated with progression and metastasis [
121], the presence of TIA1, G3BP1 and TIAL1 in melanoma-derived SGs has not been reported, so new experimental approaches would be needed.
Another highly interconnected node is the one shown in white, formed by DYRK1B, DCAF7 and CTBP1/2. DCAF7 and DYRK1B form a complex associated with the regulation of processes such as cell proliferation, differentiation and survival; it has been proposed that DCAF7 acts as an adaptor protein capable of mediating the binding and subsequent phosphorylation of E1A by DYRK1A/B, triggering a suppressive effect on proliferation through negative regulation of the transcriptional co-repressor CTBP [
122]. In addition, elevated levels of CTBP1 in melanoma have been associated with increased proliferation and DNA damage [
123], although by sequencing CTBP1 mRNA was not found to be differentially expressed in the two conditions evaluated, it is possible that by qPCR differences related to the decrease in DYRK1B could be found (
Figure 2.).
The most highly connected node contains the proteins NFE2, MAF and NFE2L2, which have been implicated in chaperone-mediated autophagy, a process of selective degradation of cytosolic proteins in response to oxidative stress [
124]; It has been reported that NRF2 has the function of regulating the antioxidant response, either by promoting tumor resistance to oxidative stress damage or by modulating carcinogenesis [
100], while MAFF has been evaluated as a tumor suppressor and its interaction with NRF2 may suggest a protective antioxidant response of the healthy melanocyte.
Finally, proteins associated with PI3K-AKT-mTOR are shown in pink. In 2015, Hambright et al. demonstrated in different cell lines the key role of PI3K/AKT/mTOR signalling and upregulation of the antioxidant system in ensuring the survival of melanoma cells. Altering redox homeostasis by increasing oxidative stress in melanoma cells inhibited PI3K/AKT/mTOR signalling by disrupting mTORC1 formation, thereby reducing colony formation and cell proliferation [
43]. However, the interaction between mTOR, ROS and the antioxidant response is not fully understood, as these ROS may play a dual role by possessing both activating and inhibitory functions [
125].
For the construction of regulatory network models, we used miRNet (
https://www.mirnet.ca/), a freely available network-based web tool that integrates several statistical tools, data mining and visualization systems for the integrated study of miRNA-molecular target interaction [
107,
108]. In addition to implementing a flexible interface for filtering, refining and customizing data during network construction, miRNet includes a network visualization system with the possibility of functional enrichment analysis. The tool was fed with the IDs of the over- and under-expressed mRNAs from
Figure 1 and for functional enrichment the KEGG and GO pathways were used with statistical analysis of a hypergeometric test.
In
Figure 3, the transcription factor ZEB1 (zinc-fingered E-box binding homeobox 1), best known for its involvement in mesenchymal epithelial transition processes, cancer cell differentiation, progression and metastasis [
56,
126] through the regulation of proteins such as SALL4 and GATA3 [
127,
128], which were also found to be related in the network, is the hub with the highest number of interactions. Its involvement in modulating the oxidative stress environment has been described in breast cancer models through the promotion of MCT4 and GPX4 [
129,
130]; in melanoma, it has been associated with increased resistance to MAPK inhibitors, promoting immune evasion [
112,
131]. It is possible that the highly defined DNA- and protein-binding domains explain the large number of interactions represented in the hub and the high modularity exerted. MAFF has been postulated as a possible tumor suppressor regulated by miRNA-224-5p in hepatocellular carcinoma [
132].
Among the miRNAs that potentially modulate ZEB1 in the proposed network, we can highlight some that have been reported in the literature to act on other molecular targets in melanoma. The miR-126, which inhibits invasion and migration in cervical cancer cells by binding to ZEB1 [
117]; the miR-126-3p isoform, which was found to be associated with the transcription factor DYRK1B, is involved in acquired resistance to dabrafenib in melanoma cells by regulating ADAM9 and VEGF-A [
133]. The miR-200a-3p has been reported as a potential regulator of melanogenesis through direct binding to MITF [
134]. miR-205-5p functions as a negative suppressor of the PI3K/AKT pathway in renal cancer [
133]. Another node described is miR-224-5p and MAFF, this miRNA has been reported as an oncomiR and biomarker in several types of cancer [
135,
136], additionally miR-224-5p binding to MAFF occurs in hepatocellular carcinoma [
132].
In the network, one of the miRNAs that interacts with multiple targets is mir 24-3p, which has been associated with negative regulation in metastatic cancer, in B16F10 melanoma cells ectopic expression could be generated in melanoma, which would generate suppression in cell migration, mir-24-3p generates the suppression of cancer cell growth, the overexpression of this mir-24-3p could generate a decrease in cell viability, as well as the inhibition of cell migration and invasion. mir 24-3p could control cell proliferation by regulating hydrofolate reductase [
137].
Meanwhile, exosomal mir-155-5p would be related to the pro-angiogenic onset of Caf (cancer-associated fibroblasts), as it could be delivered to fibroblasts and generate angiogenic factors such as vascular endothelial growth factor and fibroblast growth factor 2. The B16F10 cell line, which is a metastatic melanoma line, produces exosomal secretions with the ability to induce reprogramming of fibroblasts into Caf and the expression of tumour angiogenesis markers. These exosomes suppress SOSC1 expression, leading to activation of the JAK2/STAT3 pathway, which in turn regulates a pathogenic switch that increases vascular endothelial growth factor and fibroblast growth factor 2 expression in fibroblasts [
138].
Likewise, TIA1 is modulated by miR-27b, among others, which is widely associated with melanoma development, as described in 2021 by Yi Tian and collaborators, where they determined expression levels in melanoma cells and normal tissues by immunoelectrotransfer, correlating a negative regulation with MYC, a gene related to the Wnt-β catenin pathway, a pathway of importance in melanogenesis described previously, as a target in this case [
139]. Another modulator detected in the network is mir 20a- 5p, also described in recent years as a tumour suppressor in different tissues, demonstrated in 2019 by Ahred and his team in B16 melanoma cells, where a suppression of the same was detected in comparison with non-malignant keratinocytes [
140].