In the present study we found that the angiogenic MiR-126 may distinguish between malignant and benign forms, such as SIP and NIP (cf Fig.
1). In particular, MiR-126, expressed at low levels in ITACs due to changes in the methylation status of the host gene
EGFL7 T2 promoter (cf Fig.
2), was found up-regulated in SIP and even more in NIP. Methylation-associated regulation of MiR-126 and its host gene
EGFL7 has been described in other tumours [
24‐
26]. Although epigenetic modification is a crucial mechanism for controlling the expression of miR-126, we did not find a strict relationship between MiR-126 expression and methylation status of the host EGFL7 gene in SNC tissues (cf Fig.
2). There are other factors linked to cancer metabolism or exosomal delivery from neighbouring cells that may affect MiR-126 expression [
14,
22,
23]. It was suggested that a cancer stroma cross-talk induced repression of MiR-126 to facilitate angiogenesis and invasion [
27]. MiR-126 is enriched in ECs, modulating the level of diverse transcripts that control angiogenesis. Expression analysis documents increased levels of MiR-126 in highly vascularized tissues, such as the heart, liver, and lung. On the other hand, MiR-126 has been shown to be a negative modulator of angiogenesis in the eye [
28]. There is evidence that MiR-126 contributes to vascular homeostasis by inhibiting angiogenesis and maintaining the quiescent endothelial phenotype associated with increased vascular integrity and inhibited proliferation and motility. MiR-126 up-regulation activates endothelial progenitor cells (EPCs) and ECs in the case of vascular injury and/or hypoxia, contributing to vascular healing and formation of new blood vessels [
29]. High level of MiR-126 induces angiogenesis in non-malignant tissues by a mechanism that involves MiR-126-induced inhibition of the tumour suppressor SPRED1 [
30]. Conversely, low levels of MiR-126 contribute to the deregulation of blood vessel formation in tumours, probably by enhancing VEGF expression [
31]. These dual pro- and anti-angiogenic properties make MiR-126 a plausible biomarker to detect transition from the benign to the malignant phenotype. This is the case of SIP, which is characterized by a relatively strong potential for local tissue destruction, high rate of recurrence, and a risk of carcinomas. The follow-up of patients is critical to diagnose local relapse, which often occurs as an early event. The serious nature of this pathology is linked to its association with carcinomas, which may be diagnosed at the onset or the recurrence during the follow-up. It is therefore imperative to diagnose recurrence of the pathology in early stages in order to initiate early treatment, especially in the case of SIP-associated carcinomas [
32,
33].
As previously reported, low levels of MiR-126 in cancer [
12], including SNC, highlight its role in tumour formation and development. In this context, restoration of MiR-126 induced metabolic changes and inhibited cell growth and tumorigenesis of cultured MNSC cells (cf Additional file
1: Figure S1 and Additional file
2: Figure S2). Cancer cells prefer glycolysis and circumvent OXPHOS for ATP generation; consistent with this, we found that MiR-126 inhibited glucose uptake and glycolysis, thus restoring OXPHOS associated with a decrease in the mitochondrial redox activity (cf Fig.
3). Taking into account the possibility that exosomes released by cancer cells present a major factor driving the cancer phenotype, we can hypothesize that exosomes from normal cells can be used as a vehicle for delivery of anti-tumour factors. Since MiR-126 is secreted by ECs by means of exosomes and since it can be readily internalised by recipient cells, we used exosomes as carriers to deliver MiR-126 to cancer cells. Exosomal MiR-126 derived from HUVECs was found to interact with MNSC cells, leading to increased MiR-126 levels, whereby inducing anti-tumour responses such as inhibition of cell proliferation and target gene modulation including IRS1 and VEGF (cf Fig.
4).
To mimic the tumour stroma, a triple co-culture model was performed. As previously reported [
27], we found that down-regulation of MiR-126 was related to cancer stroma. This down-regulation can be attributed to a cross-talk among cancer cells and the adjacent fibroblasts and angiogenic endothelial cells. Thus, targeting MiR-126 can be an innovative means to normalize the aberrant vasculature in cancer environment. In this context, exosomal MiR-126 significantly reduced VEGF gene expression associated with reduced colony formation, and inhibition of cell migration and proliferation (cf Fig.
5).
Involvement of MiR-126 in cancer biology is not limited to modulation of angiogenesis. Our results indicate that this miRNA plays a role in cancer by altering several cellular mechanisms of cancer pathogenesis. Exosomes containing miRNAs represent a promising new therapeutic approach, since they play an important natural role in cellular processes combined with high stability, tissue-specific expression, and secretion into body fluids. The half-life of exosomes in the circulation is greater than that of liposomes due to their endogenous origin and unique surface composition. This enables them to specifically bind to recipient cell receptors, pointing to the possibility of utilising exosomes that specifically target a relevant cell type. Moreover, exosomes are not immunogenic and can carry diverse cargo that will not be ‘destroyed’ prior to delivery.