by ecancer reporter Clare Sansom

Cancer metastasis is a complex process, and one that is very difficult to tackle clinically: any solid tumour is harder to treat and has a worse prognosis if it has spread from its organ of origin into other parts of the body. Understanding the biochemical processes that underlie metastasis will be important if we are to develop more effective drugs for advanced cancer.

Micro RNAs (miRNAs) are small RNA sequences that bind to messenger RNA molecules to down-regulate or “switch off” the expression of certain genes. Recently, a set of these miRNAs have been discovered that suppress the development of breast cancer metastases; expression levels of these miRNAs in breast cancer patients seem to be inversely correlated with the extent of metastasis. Sohail Tavazoie and his colleagues at Rockefeller University, New York, USA have now investigated the function and mechanism of one of these, miR-126, which is known to be silenced in several human tumour types.

First, Tavazoie and his co-workers injected immunodeficient mice with breast cancer cells that expressed either a hairpin miRNA that knocked down miR-126 or a control miRNA hairpin. Mice injected with cancer cells in which miR-126 had been silenced developed more lung, brain and bone metastases than those injected with cells with functional miR-126. The former mice had only a slight increase in primary tumour volume compared to those injected with control cells, and the greatest increase was in the number of small metastases, implying that miR-126 blocks the initiation of metastases rather then the growth of established ones or more general tumour growth. This micro-RNA was also found to inhibit the recruitment of endothelial cells to tumours independent of the tumour location.

The researchers observed that tumour metastases with miR-126 silenced contained higher blood vessel densities than those with functional miR-126, implying that this micro-RNA might suppress the recruitment of endothelial cells to developing metastases, and thus angiogenesis.  They analysed mRNA expression patterns in the tumour cells and identified a set of eight genes that were down-regulated in tumours containing functional miR-126. Breast cancer patients whose tumours over-expressed these genes were found to develop metastases more rapidly than those without this over-expression pattern, whatever their subtype.

This gene set includes insulin-like growth factor binding protein 2 (IGFBP2); the phosphatidylinositol transfer protein PITPNC1; a proto-oncogene tyrosine kinase, MERTK; and the hydroxymethyltransferase SHMT2. Knockdown of the first three of these genes significantly suppressed the recruitment of endothelial cells by breast cancer cells, confirming their involvement in this process.  However, cancer cell proliferation was not decreased by any of these gene knockdowns.

Expression levels of miR-126 were found to correlate inversely with levels of the protein IGFBP2 secreted from cells, but not with levels of the vascular endothelial growth factor VEGF. Antibody-mediated inhibition of IGFBP2 reduced recruitment of endothelial cells to metastases, whereas recombinant IGFBP2 restored this phenotype in cancer cells over-expressing miR-126. This protein binds the insulin-like growth factors IGF1 and IGF2; antibody-mediated inhibition of IGF1, but not IGF2, was found to inhibit endothelial cell recruitment in miR-126 knockdown models. Further experiments showed the insulin-like growth factor receptor IGF1R to be part of a signalling network that promotes endothelial cell recruitment and that is inhibited by miR-126. 

The researchers then explored possible roles of PITPNC1 and MERTK in regulating the pathway involving IGFBP2 secretion. The protein PITPNC1 was found to be upstream of IGFBP2 in the same signalling pathway, whereas the function of MERTK was independent of IGFBP2. However, adding GAS6, a protein ligand of MERTK, to tumour cells expressing this kinase reduced miR-126 dependent endothelial cell recruitment, indicating that this protein acts as a MERTK inhibitor.

Taken together, the results obtained by Tavazoie and his co-workers implicate two separate signalling pathways, IGFBP2/IGF1/IGF1R and GAS6/MERTK, in the initiation of tumour metastases via endothelial cell recruitment and angiogenesis in breast cancer. Either of these pathways may provide targets for the development of novel drugs to prevent the progression of this disease.

Reference

Png, K.J., Halsberg, N.,  Yoshida, M. and Tavazoie, S.F. (2011). A microRNA regulon that mediates endothelial cell recruitment and metastasis by cancer cells. Nature, published online ahead of print 14 December 2011. doi: 10.1038/nature10661