Tiam1/Rac signalling and the regulation of cell-cell adhesion
The skin carcinogenesis model revealed an additional role for Tiam1 in tumourigenesis. The few skin tumours arising in Tiam1-deficient mice progressed more frequently to malignancy than those in wild-type mice, suggesting that Tiam1 deficiency promotes malignant conversion (Malliri et al., Nature 2002; 417: 867). Analysis of Tiam1 expression in skin tumours of wild-type mice revealed that benign papillomas maintained high levels of Tiam1 expression, whereas expression was reduced in squamous cell carcinomas and was completely lost in highly invasive spindle cell carcinomas. The increased Ras signalling associated with advanced skin malignancies (resulting from amplification of the mutated Ras allele) seems to be responsible for the reduction or loss of Tiam1 expression in the later stages of tumour progression, as demonstrated in vitro for Ras-transformed MDCK cells (Zondag et al., J Cell Biol 2000; 149: 775). Thus, while Tiam1/Rac co-operate with Ras in establishing tumours, they antagonize Ras during tumour invasion.
One probable mechanism by which Tiam1 and Rac suppress malignant progression is through their ability to stimulate cell–cell adhesion. In vitro studies have shown that over-expression of activated Rac or Tiam1 can promote the formation of adherens junctions (AJs) and the accompanying induction of an epithelial-like phenotype in a number of mesenchymal cell lines (Malliri & Collard, Curr Opin Cell Biol 2003; 15: 583). Moreover, using both RNA interference and cells derived from Tiam1-deficient mice, it has been shown that endogenous Tiam1 is required for both the formation as well as the maintenance of cadherin-based adhesions (Malliri et al., J Biol Chem 2004; 279: 30092).
The oncoprotein Src, a non-receptor tyrosine kinase implicated in malignant progression, potently induces epithelial–mesenchymal transition (EMT) by targeting AJs for dissassembly. We recently showed that direct phosphorylation of Tiam1 by Src is required for Src-induced EMT. Moreover, we identified a novel post-translational mechanism of regulating Tiam1 levels. We showed that Src phosphorylates Tiam1 on tyrosine 384 (Y384). This occurs predominantly at AJs during the initial stages of Src-induced EMT and creates a docking site on Tiam1 for Grb2. We found that Tiam1 is constitutively associated with extracellular signal-regulated kinase (ERK). Following recruitment of the Grb2-Sos1 complex, ERK becomes activated and triggers the localised degradation of Tiam1 at AJs through, in turn, activating calpain proteases. Significantly, we demonstrated that in human lung, colon, and head and neck cancers phosphorylation of Y384 of Tiam1 positively correlated with Src activity, while total levels of Tiam1 were inversely correlated with Src activity, consistent with the above-mentioned post-translational regulatory mechanism operating in malignancies. Abrogating Tiam1 phosphorylation and degradation suppressesed Src-induced AJ disassembly. As a consequence, cells expressing a non-phosphorylatable Tiam1 showed a marked decrease in wound closure in response to Src. These data establish a new paradigm for regulating local concentrations of Rho-GEFs, as well as linking Tiam1-Rac signalling with a further oncoprotein (Woodcock et al., Molecular Cell 2009, 33: 639).
Figure 3 Legend: Model for the regulation of Tiam1 by Src activation. Src directly phosphorylates Tiam1, preferentially at sites of cell-cell adhesions. Phosphorylated Tiam1 recruits the Grb2-Sos complex which, via MEK, increases activation of the ERK associated with Tiam1, and hence the local activation of calpain proteases at cell–cell adhesions. Calpain mediated proteolysis of Tiam1 results in its inactivation, reducing the activity of Rac that is necessary to maintain cadherin adhesions. The weakening of cell–cell adhesions in this way by Src would allow increased migration of cells.