Signals that encourage the delinquent behaviour of tumour cells can originate from the surrounding environs as well as from cancer cells themselves. Stromal matrix metalloproteinases (MMPs), including MMP3/stromelysin-1, are upregulated in many breast cancers and can induce transformation in cultured mammary epithelial cells and in transgenic mice. Here, Radisky et al. reveal the molecular details by which this occurs, and implicate Rac1b as the main culprit in this process.
MMP3 brings about epithelial–mesenchymal transition (EMT), which increases cellular motility and invasiveness, a key trait of tumour cells. The process requires changes in the actin cytoskeleton and, although the activities of RhoA and Cdc42 were unaffected by MMP3, the authors found that a previously identified splice variant of Rac1 — Rac1b — was upregulated and, indeed, was necessary for MMP-induced motility.
Could Rac1b mediate the pleiotropic effects that are caused by MMP3 and, if so, how? Previous studies have shown that active Rac can stimulate the production and release of mitochondrial superoxide into the cytoplasm, which can damage cells, potentiate tumour progression and be further converted into other destructive reactive oxygen species (ROS). Treating cells with MMP3 or expressing Rac1b increased cellular ROS levels, and this increase was inhibited by expressing dominant-negative Rac1. Transfecting cells with superoxide dismutase-2 (SOD2), a mitochondrial enzyme that reduces superoxide levels, inhibited MMP3-induced cell scattering, which indicates that MMP3- or Rac1b-induced mitochondrial superoxide production is required for EMT. Similarly, N-acetyl cysteine (NAC), a ROS quencher, inhibited the MMP3-induced downregulation of an epithelial marker and the upregulation of a mesenchymal marker and, consequently, blocked motility and invasion. Radisky et al. saw that MMP3-induced ROS also caused cellular DNA damage and genomic instability, which, again, could be induced independently by the ROS hydrogen peroxide and inhibited by NAC.
The expression of many genes, including the transcription factor Snail, changes during EMT. Snail expression was induced by MMP3, but was also stimulated independently by increasing ROS levels or by expressing Rac1b. Conversely, NAC blocked Snail induction. Snail, when expressed in mouse mammary epithelial cells, was sufficient to induce EMT and cell scattering. So MMP3 induces Rac1b expression, which, in turn, increases cellular ROS levels, upregulating Snail and causing EMT. So, by inducing Rac1b, MMP3 can lead normal cells astray.