Cells undergoing EMT display a downregulation of E cadherin

Cells undergoing EMT display a downregulation of E-cadherin and dissociation of their junctional complexes that associates cell-cell contacts with the BMS-345541 mg cytoskeleton, known as the adherens junction (Berx and van Roy, 2009, Birchmeier and Behrens, 1994). Adherens junctions are specialized cell-cell adhesion complexes which help maintain apicobasal polarity in epithelial cells. In animal models with a loss of E-cadherin, epithelial tumor cells transform into highly invasive cells, suggesting that the loss of E-cadherin is a causal factor in metastasis (Derksen et al., 2006). Besides E-cadherin, the adherens junctional complex consists of several proteins involved in anchoring the transmembrane E-cadherin homodimers to the actin cytoskeleton, most notably the catenin family members β-catenin, α-catenin and p120-catenin. Rho family members are both crucial in junctional assembly and disassembly (Akhtar and Hotchin, 2001, Shen et al., 2008). The role of Rho proteins in the establishment and maintenance of epithelial cell polarity and junctional assembly has been the focus of a recent review, so will not be discussed in detail here (Mack & Georgiou, 2014). Regarding disassembly of the adherens junctions, a crucial step during EMT, studies in pancreatic carcinoma cells demonstrate that overexpression of constitutively active Rac1 reduces E-cadherin expression, while conversely, dominant negative Rac1 increases E-cadherin levels (Hage, Meinel, Baum, Giehl, & Menke, 2009). Additionally, activation of Rac1 by the Rac GEF Tiam1 has been found to be required for adherens junctional disassembly (Knezevic et al., 2009, Rios-Doria et al., 2004). One mechanism by which Rac1 regulates stability of the adherens junction is through interaction with the Rac1 effector IQGAP1 (Fig. 2). Although IQGAP has high homology to other GAPs, it does not exhibit intrinsic GAP activity but binds with high affinity to GTP-bound Rac1. IQGAP1 associates with several proteins that constitute adherens junctions. In several carcinomas, IQGAP1 exhibits a high expression level with close correlation to metastatic potential, and increased cell membrane localization due to its association with adherens junctions (Jadeski et al., 2008, Nabeshima et al., 2002, Nakamura et al., 2005). IQGAP1 expression is implicated in tumor progression via decreasing E-cadherin-mediated cell adhesion (Hage et al., 2009, Kuroda et al., 1998). In the absence of active Rac1, IQGAP1 stabilizes the interaction between E-cadherin and β-catenin and thereby the junctional complex. Strikingly, transformation of pancreatic carcinoma cells with active Rac1, results in altered subcellular distribution of both E-cadherin and β-catenin and attenuated junctional stability (Hage et al., 2009). However, the effect of Rac1 on adherens junctional stability and the effect on cell-migration are still under debate and recent findings support the idea that activation of Rac1 might actually stabilize E-cadherin-mediated cell-cell contacts. The effect Rac1 has on junctional stability appears to be dependent on the mechanism of Rac1 activation. For instance, phosphorylation of the Rac GEF Tiam1, by the oncoproterin sarcoma-family kinase (Src), results in subsequent degradation of Tiam1 at adherens junctions, a step that is required for Src-mediated adherens junction disassembly and enhanced cell migration (Woodcock et al., 2009), possibly indicating that there is a requirement of suppressed Rac1 activity at adherens junctions during junctional disassembly. A recent study supports this idea by showing that expression of Rac1 enhanced actin and E-cadherin localization at the adherens junctions, thereby promoting cellular aggregation with reduced cell migration (Marei et al., 2016). Intriguingly, while earlier studies have suggested IQGAP1 to dirupt the adherens junction via binding to β-catenin and thereby destabilizing the E-cadherin junctional complex (Hage et al., 2009, Kuroda et al., 1998), here, the authors suggest that Tiam1-mediated Rac1 activation results in IQGAP1 binding to Rac1 and Tiam1, in a complex in which Tiam1 acts as a scaffolding protein. This increased binding of active Rac1 to IQGAP1 inhibits its binding to β-catenin, thereby stabilizing E-cadherin-mediated cell-cell contacts (Fukata et al., 1999, Marei et al., 2016). In sharp contrast, the same study showed that expression of another Rac GEF, P-Rex1, is associated with cell-cell contact dissociation and a marked increase in cell migration (Marei et al., 2016). Here, the authors suggest that the underlying mechanism is the interaction between P-Rex1-activated Rac1 and a novel Rac1 interacting protein, FLII. Importantly, Tiam1-mediated Rac1 activation did not enhance this interaction, demonstrating that, beyond Rac1 activation, there are additional levels of regulation.