In some cell lines, an epithelial–mesenchymal transition (EMT) arises as the result of a joint effort between Hras and transforming growth factor-β (Tgf-β). How relevant this is to the multistage nature of in vivo tumour progression, though, is a burning question.

So, Allan Balmain's group studied whether changes in the levels of Hras and Tgf-β affect tumour progression, using a series of well-characterized tumour cell lines that arise from initiated cells that carry activating mutations in the Hras1 gene. And, as they now report in Nature Cell Biology, Smad2 (a downstream target of Tgf-β signalling) and Hras surpass discrete thresholds during progression from early-stage papillomas, through squamous carcinomas, to late-stage undifferentiated spindle-cell tumours.

First, the authors studied the molecular changes that cause squamous carcinomas to be converted into spindle-cell tumours. Tgf-β-mediated transcriptional activity was very high in the spindle cells, and phosphorylated Smad2 accumulated in the nucleus, which indicated that the Tgf-β pathway was activated in these cells. Furthermore, in primary material from spindle tumour cells, but not from differentiated tumours or squamous carcinomas, Smad2 was phosphorylated and predominantly localized in the cytoplasm.

Although Smad2 alone induced changes in the migration of squamous carcinoma cells, only in the presence of increased levels of mutated Hras did changes in cell shape and the expression of genes such as α-smooth muscle actin (a mesenchymal marker) occur, resulting in EMT.

The authors then investigated whether, once this stage has been reached, Tgf-β signalling by Smad2 is still necessary for tumour progression. Expression of a dominant-negative form of Smad2 showed that this is indeed the case; spindle cells that expressed this construct reverted to a more epithelial phenotype and took on many features of epithelial gene expression. Notably, surface expression of αvβ3 integrin was lost, and this was coincident with the loss of collagen-matrix invasion. In vivo, this correlated with an inability to form tumours. By contrast, parental spindle cells or spindle cells that express a dominant-active form of Smad2 formed tumours, and those formed by dominant-active Smad2 were particularly invasive. Expression of dominant-active Smad2 also promoted extravasation into the target tissue, and a subsequent increase in lung metastases.

As the ability of a tumour to metastasize is the main determinant of whether or not patients with cancer die, these findings that different thresholds of Hras and Tgf-β activity — intermediate levels of Smad2 co-operating with Hras to induce EMT and invasiveness, and even higher levels of Smad2 being required for metastasis — are crucial for metastasis offer the opportunity for the design of small-molecule inhibitors to prevent the spread of tumours.