by ecancer reporter Clare Sansom

Melanoma is by far the most aggressive type of skin cancer: about 80% of skin cancer deaths are from melanoma, and its 5-year survival rate is only about 15-20%.

This tumour arises in the melanocytes, pigment-producing cells that are found towards the base of the epidermis (the upper layer of skin).

The pigment melanin is stored in these cells in organelles termed melanosomes, which gain more pigment as they mature.

Melanosomes are secreted and transferred to neighbouring keratinocytes; this process occurs in response to ultra-violet radiation in normal skin but is dysregulated in melanoma.

The first stage of melanoma progression is from the basal epidermis into the upper epidermis, forming melanoma in situ; it then invades the lower skin layer, or dermis, and this stage is necessary for metastasis to occur.

As melanoma cells invade the dermis, fibroblasts there are re-programmed into cancer-associated fibroblasts (CAFs), which are able to proliferate at enhanced rates and to migrate.

The exact mechanisms through which melanoma cells interact with neighbouring fibroblasts in the dermis to form the CAF phenotype is still poorly understood, but is known to involve the secretion of cytokines, growth factors and small RNA molecules that are involved in the suppression of gene expression and termed microRNAs.

A large, international group of researchers led by Carmit Levy from Tel Aviv University, Tel Aviv, Israel has investigated the role of microRNA in this process.

Firstly, the researchers observed that fibroblasts accumulate in the dermis before melanoma cells invade there; they stained skin samples from melanoma patients with markers of melanosomes and fibroblasts, and showed that fibroblasts take up large numbers of melanosomes even before the tumour invades the dermis.

This implicates melanosome trafficking in the process of cell-cell communication and thus, very likely, in CAF formation.

Electron microscopy also showed that fibroblasts exposed to culture medium from melanoma cells also took up large numbers of both early and mature melanosomes.

Fibroblasts that had taken up melanosomes form melanoma cells showed increased expression levels of genes associated with proliferation and migration and an actual increase in these properties.

The expression levels of genes associated with inflammation and matrix adhesion, two other properties of proliferative cells, also increased.

Fibroblasts exposed to melanosomes from normal melanocytes showed no increase in proliferation and migration, but their expression of pro-inflammatory genes increased slightly.

Next, the researchers investigated whether fibroblasts that had been treated with melanoma-associated melanosomes were able to promote tumour development, and found that this occurred both in cell co-culture and in a mouse model of melanoma.

They then examined the role of microRNAs in cell trafficking and melanoma progression, firstly by cataloguing the population of microRNA molecules found in melanosomes.

This showed that five miRNAs that had previously been associated with melanoma were expressed at high levels in mature melanosomes: miR-149, miR-211, miR-23, miR-let7a and miR-let7b.

All melanosomes transferred to fibroblasts were shown to contain RNA, and fluorescent labelling and quantification of microRNA in the same fibroblasts indicated that the concentration of miR-211 increased most significantly after melanosome transfer.

The researchers therefore focused their further studies on this microRNA, using miR-320c as a control.

Fibroblasts were treated with of miR-211 or miR-320c and their gene expression profiles compared with that of similar cells treated with a mixture of miRNAs.

Treatment with miR-211 but not miR-320c caused enhanced expression of genes associated with cell growth and migration, and of pro-inflammatory genes.

Preventing miR-211 from binding to its target mRNA in fibroblasts with a specific synthetic oligonucleotide (an antagomir) caused a reduction in the expression of these gene sets.

Expression of this miRNA was also found to be higher in fibroblasts from clinical melanoma samples than in normal fibroblasts.

This suggests that miR-211 is transferred to fibroblasts in the melanoma micro-environment and thus contributes to the development of the CAF phenotype.

Gene expression profiling further showed that miR-211 targets and down-regulates the tumoursuppressor gene insulin-like growth factor 2 receptor (IGF2R).

Reducing the interaction between insulin-like growth factor 2 (IGF2) and its receptor increases its binding to the insulin-like growth factor 1 receptor (IGF1R), which stimulates cell growth via the MAPK signalling pathway.

Levy and her co-workers showed that inhibition of MAPK signalling with an ERK inhibitor, U0126, prevented the development of the CAF phenotype.

Taken together, these results suggest that transfer of miR-211 into fibroblasts in the dermis promotes the development of a tumour-promoting micro-environment there, and that this might be prevented by blocking the MAPK signalling pathway.

Reference
Dror, S., Sander, L., Schwartz, H. and 23 others (2016). Melanoma miRNA trafficking controls tumour primary niche formation. Nature Cell Biology, published online ahead of print 22 August 2016