RFP-labelled fibroblast (red) followed by GFP-labeled SCC cells (green) invading into naïve matrix.

From Nat. Cell Biol.9, 1392-1400 (2007)

The development of metastatic tumours depends upon the capacity of tumour cells to penetrate into surrounding tissues. In Nature Cell Biology, Erik Sahai and colleagues now report that collectively invading squamous cell carcinoma (SCC) cells use tracks in the extracellular matrix (ECM) that are generated by leading stromal fibroblasts.

The authors cultured SCC cells on top of a dense 3D matrix that mimicked physiological conditions. Although motile, SCC cells alone were unable to invade the matrix. Introducing fibroblasts isolated from oral or vulval SCCs into the matrix enabled SCC cells to penetrate the matrix. Differential cell labelling revealed that invasion occurred in chains - the leading cell was always a fibroblast and SCC cells followed behind and were able to switch order.

When a thin layer of ECM separated the SCC cells from the fibroblasts, invasion was blocked; however, matrix remodelled by fibroblasts that were subsequently removed supported SCC invasion. This suggested that neither soluble factors nor direct contact between SCC cells and fibroblasts are required for fibroblast-induced invasion, but rather the physical modification of the matrix by fibroblasts is the key factor. As observed by transmission electron microscopy, leading fibroblasts deform the ECM to generate channels that SCC cells use as tracks to invade surrounding tissue.

Sahai and colleagues went on to study the molecular pathways involved in ECM remodelling and collective invasion. Inhibition of matrix metalloproteinases (MMPs) prevented the formation of tracks and fibroblast invasion. Rho-ROCK signalling is required for force-mediated matrix remodelling. Although blocking Rho-ROCK signalling did not affect fibroblast invasion it did inhibit the formation of tracks behind the fibroblasts, thus uncoupling the invasion of fibroblast from their ability to enable SCC cells to follow them. Rho-ROCK function was required only in fibroblasts, as specific blockade of Rho or ROCK in SCC cells did not affect their ability to invade in the presence of fibroblasts. Once fibroblasts had created the tracks, SCC cells no longer required MMP or Rho function to invade. Furthermore, Sahai and colleagues found that depletion of integrin 3 and 5 in stromal fibroblasts, but not in SCC cells, resulted in reduced force-mediated matrix remodelling by fibroblasts leading to defects in track formation and prevented SCC cells from following fibroblasts lacking integrin 3 or 5.

Whereas following SCC cells do not require matrix remodelling pathways, Sahai and colleagues found that these cells do require the activity of Cdc42 and the effector MRCK (myotonic dystrophy kinase-related CDC42-binding protein kinases) to regulate actomyosin organization at the cell cortex, and to generate the force needed to follow fibroblasts.

This work shows that fibroblasts lead chains of invading SCC cells by creating tracks into the ECM through a combination of protease and force-mediated matrix remodelling. Unlike epithelial cells, mesenchymal cells are capable of remodelling the surrounding ECM. In the model presented here, physical matrix remodelling by fibroblasts enables the collective invasion of carcinoma cells that do not undergo epithelial-mesenchymal transition.

Author: Kim Baumann