Getting the green light for metastasis
In their habitual zeal for the next hot story, the press have done something of a disservice to the important research of a couple of scientists at the University of East Anglia in the UK. The work of Soond and Chantry that appears in the January 24th issue of Oncogene is an important step forward in our understanding of the path that a cell takes from normality to neoplasia. The preliminary nature of these published findings however, did not inhibit some in the popular press to claim that it heralded an imminent cure for many types of cancer, based upon the notion that this research has succeeded in isolating "the rogue gene" that is responsible for the spread of cancer in the body. Having heard this kind of hype from the press before (remember how in 2000, the Human Genome Project was being predicted to yield a cure for most cancers within two years?) it is easy for such fanfare to have the opposite effect and to foster cynicism. Somewhere between fanfare and cynicism however, lies the kind of cautious optimism with which most researchers in the field would view these findings and this would seem to be an appropriate response to them. What the authors describe is indeed a fundamental pathway by which cells make the transition from the kind of neighborly and cooperative way of life that is necessary to maintain the integrity of tissues and organs, to the kind of every-man-for-himself maverick cell that wants to blaze its own trail, regardless of the consequences to the organism that it was hitherto a working part of. Through their communications with their surroundings and with neighboring cells, most cells in the body have a pretty good sense of where they are at all times, and will continue to do their thing so long as these signals are all present. Remove such a cell from its normal context however and it will invariably pursue a new course of action, often involving its own self destruction through apoptosis. For example, the signaling pathways involving cell-to-cell adhesion are generally inhibitory to apoptosis when their receptor ligands (furnished by contacts with neighboring cells) are present. Remove these contacts however, and the cell will rapidly initiate a program that leads to apoptosis. Sometimes however, it is necessary for a cell to be able to migrate from one place to another (as in wound healing for example), thereby requiring it to have (at least temporarily) the ability to exist as a free-standing cell. The carefully regulated process by which this occurs is know as the epidermal to mesenchymal transition or EMT.
So why are we talking so much about EMT? Because EMT plays a pivotal role in the progression of many cancers. A cancer cell that can bypass the regulatory controls that maintain the epidermal state and undergo EMT, becomes a cell that is capable of metastasis and the invasion of other tissues - the hallmark of a malignant tumor cell.
Soond and Chantry focused their research on a family of ubiquitin ligase proteins that modulate the cellular machinery that degrades proteins, and which itself has far-reaching consequences for cell metabolism. In particular, they studied the protein WWP2 whose function is to tag other proteins in the cell with ubiquitin, thereby targeting them for degradation in the proteosome. WWP2 turns out to be one of those proteins that is produced as isoforms of different lengths, there being a full length isoform WWP2-FL and two shorter variants WWP2-N and WWP2-C. The authors were interested in understanding how the interactions of these various forms of WWP2 with other cellular proteins could influence the ability of a cell to migrate - a hallmark of cells that have undergone EMT.
One big "needle-in-the-haystack" question that confronted the authors was to figure out which other proteins in the cell interact with the various isoforms of WWP2. An immunoprecipitation experiment showed that WWP2 interacts with the SMAD family of proteins - a finding of particular relevance for the regulation of EMT. Under the control of TGF-β, the SMAD2 and SMAD3 proteins act in a stimultory fashion with regard to EMT, pushing the cell towards a migratory phenotype, whereas SMAD7 is currently believed to be an inhibitor of EMT. Interestingly, the authors discovered that WWP2-FL, the full length form of WWP2, interacts with all 3 SMAD proteins whereas WWP2-N interacts only with SMAD3 and WWP2-C interacts only with SMAD7. Furthermore, they found that the presence of the WWP2-N variant in the cell, increased the activity of the WWP2-FL protein.
So what was happening?
The overarching finding was that increasing the amount of WWP2 in cancer cell lines known to undergo EMT, inhibited a cell's progression towards the mesenchymal state. Increasing the amount of WWP2-FL reduced the ability of TGF-β to switch on the SMAD2 and SMAD3 genes. As a control for this experiment, they were also able to show that silencing the WWP2-FL gene using siRNA reversed this effect in the treated cells, restoring the ability of TGF-β to switch on SMAD2 and SMAD3 once more. WWP2-FL was also shown to enhance the ubiquitination of SMAD2 and SMAD3, thereby targeting them for degradation and reducing their levels in the cell. This ubiquitinating activity of WWP2-FL was further shown to be enhanced by the presence of the WWP2-N isoform. And while the presence of WWP2-FL also increased the removal rate of the EMT-inhibitory SMAD7, it was found that WWP2-C seemed to be able to act in opposition, increasing the activity of SMAD7 in the cell.
While these findings are preliminary, what they point to is an interconnected network of enhancers and inhibitors that are able to subtly modulate the equilibrium of the cell between the epidermal and mesenchymal states, based upon the relative abundances of the 3 WWP2 isoforms. This network includes interactions between these WWP2 isoforms and the SMAD family of proteins that are under the control of TGF-β, but there are also interactions between the WWP2 isoforms themselves that modify their activity. This is a system that cries out for a good computer model, although it may be necessary to do some more quantitative experiments in the lab first, in order to be able to provide a realistic set of parameters for such a model.
The system of protein interactions described in Sood and Chantry's article are an important step forward in our understanding of EMT and with further research and accumulated knowledge, a fuller understanding of them could well lead to new approaches for combating invasive cancers. Such a system is also a potential poster child for a digital biology approach to understanding the complicated equilibrium between rival cell phenotypes and perhaps even for helping us along the road to new cancer therapies.
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