Section of lung showing metastasis by lung metastatic breast cancer cells

From Nature 451, 147–152 (2008).

Metastasis is one of the major causes of mortality for cancer patients, but its underlying molecular and cellular mechanisms are still poorly understood. It is largely unknown which upstream regulatory networks lead to the gene misexpression that is correlated with the metastatic phenotype. Joan Massagué and colleagues now report in Nature that two endogenous human microRNAs, miR-335 and miR-126, suppress breast cancer metastasis by regulating expression of the SRY-box containing transcription factor SOX4 and extracellular matrix (ECM) component tenascin C (TNC).

The authors used array-based microRNA profiling to identify eight microRNAs exhibiting decreased expression in breast cancer cell derivatives that metastasize to bone or lung. The authors focused on miR-335 and miR-126, because restoring their expression significantly suppressed cell dissemination to bones and lungs. Loss of miR-335 and miR-126 expression was also found in other metastatic cell lines, showing that these microRNAs are downregulated in multiple, independently-derived breast cancer samples.

Whereas miR-126 inhibited cell proliferation to suppress overall tumour growth; miR-335 had no effect on either the proliferation or apoptotic rates of cancer cells. Instead, miR-335 prevented the formation of elongated cells — an alteration in shape that is associated with both decreased cell motility and invasive capacity.

The authors then showed that miR-335 loss could promote metastasis: miR-335 inhibition with a specific antagomir increased the ability of cells to colonize the lungs. Moreover, both miR-335 and miR-126 expression levels were low in archived primary breast tumours from patients that had suffered metastatic relapse, suggesting a strong association between the loss of microRNAs and future distal metastasis.

Combining transcription profiling results with existing data sets yielded six genes that were suppressed by miR-335 as well as being highly expressed in lung and bone metastatic cells. These included the transcription factor SOX4, which is known to regulate cell migration, and the ECM component TNC, which is expressed at the invasive front of human carcinomas. The 3' untranslated regions of SOX4 and TNC were important for their expression and showed sequence complementarity to miR-335, suggesting these genes are direct targets of miR-335.

In support of this, knockdown of SOX4 reduced cell elongation, similar to the restoration of miR-335 expression, and cell migration and invasive ability were reduced in cells lacking SOX4 or TNC. Thus, miR-335 regulates metastasis by suppressing target genes, including SOX4 and TNC.

Previous findings have shown that the microRNA miR-10b promotes lung metastasis when overexpressed in breast tumours. Here, Joan Massagué and colleagues provide evidence that microRNAs can also suppress metastasis, and show that identifying the loss of miR-335 and miR126 may be of potential value in breast cancer patients' prognosis.

Author: Kim Baumann