E-cadherin as an indicator of mesenchymal to epithelial reverting transitions during the metastatic seeding of disseminated carcinomas

Abstract

Cancer metastasis follows a sequential series of events, and many of the critical steps are distinctly similar to EMT-like transformations that occur during normal embryonic development. A current area of focus is the similarities between how cancer cells interact with the ectopic parenchyma after metastatic spread, and secondary developmental MET events that occur in epithelial tissues that have re-assembled within the embryo from mesenchymal cells. Accumulating evidence suggests a critical role for these secondary events, termed mesenchymal-epithelial transitions (MET) in development and mesenchymal-epithelial reverting transitions (MErT) in cancer. In this situation, metastatic seed cancer cells may inertly become part of the ectopic tissue and therefore surmount the metastatic inefficiencies to which most disseminated cancer cells succumb. Just as a critical EMT event is the downregulation or silencing of E-cadherin, we discuss the role of E-cadherin in cancer-associated MErT at distant metastatic sites and speculate on the implications for the fate of micrometastases that undergo a transition to being E-cadherin positive.

Fig. 1

Carcinomagenesis and progression entails multiple phenotypic changes. Epithelial tissues consist of sheets of normal cells (stippled black) linked together by homotypic binding of E-cadherin (thick black bars). This establishes a polarity that segregates apically secreted factors, such as EGF, from their basolaterally-presented receptors that normally are utilized by stromally-derived factors, such as TGFα. Due to genetic and epigenetic events, the E-cadherins are lost during carcinoma development (lined cells), allowing for autocrine signaling. This ‘dedifferentiation’ is the carcinoma-associated EMT. However, we propose that during metastatic seeding to other epithelial organs E-cadherin is re-expressed enabling linkages to, and signaling from normal parenchymal cells (gray cells). This is concurrent with downregulation of signaling from the previous autocrine factor/receptor loops. This characterizes the mesenchymal-to-epithelial reverting transition (MErT). Adapted from Kim et al. [23]

Fig. 2

E-cadherin sequesters catenins and controls their signaling in addition to forming cell–cell adhesions. (a) E-cadherin sequesters β-and p120-catenins on its intracellular catenin binding domains. In an untransformed cell, p120 is thought to stabilize E-cadherin at the surface, β-catenin is sequestered from forming a complex with axin and, in this location also functions as an adaptor protein for a-catenin, which in turn anchors E-cadherin to the cytoskeleton. (b) In many carcinomas, E-cadherin is silenced by promoter methylation allowing β-catenin to translocate to the nucleus and p120 to promote an epithelial phenotype. However the mechanisms of how these catenins act and wether they act individually or in concert are not settled. (c) E-cadherin dependent adhesion in itself is not a dominant stop mechanism to inhibit invasion. In studies where the β-catenin binding domain was deleted from the E-cadherin intracellular domain, but E-cadherin was still able to mediate adhesion through direct crosslinking with α-catenin and therefore interaction with the actin cytoskeleton, β-catenin was free to signal in the cell cytoplasm and led to an invasive phenotype (though this phenotype was independent of its TCF-mediated transcriptional activity). Therefore, cytoplasmic localization of β-catenin is thought to contribute to the mesenchymal nature of cells. (d) In studies of cell that had low levels of E-cadherin and cytoplasmic localization of p120, tyrosine phosphorylation on p120’s amino-terminal by the pro-oncogene Src was thought to contribute to modulate its contribution to cell migration. When p120 is knocked down, the equilibrium shift to Rho-GDP promotes actin polymerization, stress fiber formation, a flattened morphology and less invasive phenotype. Therefore, cytoplasmic localization of p120 is thought to contribute to the mesenchymal nature of cells. These four scenarios provide data that show the critical role of E-cadherin as a signal modulation molecule by sequestering catenins, primarily p120 and β-catenin. In the absence of E-cadherin homotypic binding, this plaque is unstable and the catenins are now free to relocalize

Fig. 3

Carcinomagenesis and metastatic dissemination requires diverse behaviors and phenotypes at different stages. Initially, the primary tumor cells need to lose their cell–cell adhesion (mainly E-cadherin mediated) to break from the tumor mass and enter vascular conduits via active migration. Survival in the vascaulature through at least one capillary bed requires distinct behaviors of architectural malleability and resistance to anoikis. Arrest at the target organ is usually via physical arrest, but the tumor cells must then recognize the endothelial lining and transit this barrier. We propose that establishment and longer term survival entails re-expression of E-cadherin along with some redifferentiation at least initially. It is possible and even likely that when the micrometastasis progress to clinically-relevant lesions and even seed other sites, that this transient MErT itself reverts as the metastatic lesion becomes more aggressive with autocrine signaling reminiscent of the primary aggressive lesion

Fig. 4

Breast cancer metastases to the liver can subsume a hepatocyte morphology. Not the organ morphology flows seamlessly from the liver cords (right) through the breast cancer metastases (left). From Stessels et al. [39]

Fig. 5

E-cadherin expression is regulated on multiple levels. The plastic nature of these controls allows for EMT defined and enabled by loss of E-cadherin and MErT when E-cadherin is re-expressed. Methylation is reported to be responsible for E-cadherin silencing in the majority of aggressive and metastatic carcinomas. These epigenetic changes are at the base of the pyramid, as access to the gene in the first place is necessary for its ultimate expression. On the transcriptional level, overexpression of SLUG/SNAIL is responsible for for transcription downregulation of E-cadherin, and is noted in a large subset of cancers. On the effector level, receptor tyrosine kinase phosphorylation causes internalization of E-cadherin from the cell-surface by phosphorylation of E-cadherin effector catenins and modulation of the ubiquitinase Hakai, resulting in either trafficking of E-cadherin for endosomal recycling or degredation. As all of these mechanisms are prevalent in invasive cancers, the pathway to MErT may be diverse as each suppression mechanism is controlled individually