E-Cadherin is a key molecule for adhesion between cells, being a fundamental protein regulating normal epithelia architecture and homeostasis. E-cadherin loss of expression and/or function, occurring in more than 70% of all carcinomas, leads to increasedinvasion which is a hallmark of cancer cells. However, the molecular and cellular mechanisms by which cells with dysfunctional E-cadherin invade instead of being eliminated into the epithelial lumen of the organ are not known.

We hypothesize that interaction of E-cadherin mutant cells with normal cells results in the activation of a specific molecular program that forces mutant cells to invade the stroma in a process referred to as basal cell extrusion.

Cell extrusion is a critical process in epithelia homeostasis by controlling cell number and renewal. In this process, cells are apically extruded into the lumen to be eliminated, constituting a self-defense mechanism. Many cancer cells impair apical extrusion switching the direction of extrusion from apical to basal. As many cancer cells escape cell death and epithelial elimination, basal extrusion could boost tumor cells to invade.

We propose that carcinomas caused by E-cadherin loss recapitulate this extrusion switch, initiating invasion.

E-cadherin mutations are causative events in 50% of hereditary diffuse gastric cancer (HDGC) which is a highly invasive cancer syndrome with a lethal outcome. In this syndrome, the pre-malignant lesions described show abnormal E-cadherin cells spread along and below wild-type cells (pagetoid lesions). Therefore, we suggest that HDGC pagetoid lesions could result from the flip from apical to basal extrusion, as such phenotypes are likely to represent the first steps of invasion. Thus, this cancer constitutes an exceptionally appropriate model in which to characterize the molecules involved in this process.

We have established at IPATIMUP a panel of 54 cell lines expressing germline mutations of E-cadherin found in HDGC patients. All cell lines are functionally characterized and constitute powerful tools to study how mutant and normal E-cadherin cells interact and signal to trigger cell extrusion and consequently invasion.

We will focus on identifying the signaling involved in E-cadherin-deficient mediated basal cell extrusion. Our preliminary data already show that E-cadherin defective cells surrounded by wild-type neighbors are extruded basally from an epithelial monolayer. Drosophila in vivo studies demonstrate that basal cell extrusion only occurs at the interface of wild-type and E-cadherin defective cells and is accompanied by overexpression of a matrix component that allows mutant cells to invade.

Thus, we will investigate 1) if E-cadherin deficient cells basally extrude only when surrounded by wild type cells; and 2) whether signaling and/or changes in mechanical tensions between E-cadherin mutant and wild-type cells regulate cytoskeletal reorganization, cell shape changes and cell-matrix interactions that could drive invasion.

To test our hypothesis, our specific aims are:
1-To determine the fate of E-cadherin mutant cells enclosed by wild-type or by similar mutant cells. We will take advantage of our E-cadherin mutant cell lines for in vitro assays. In collaboration with J. Rosenblatt, a Zebrafish model will be used to follow the fate of E-cadherin defective cells in a normal live tissue context.
2-To test how the surrounding cellular context influences mutant E-cadherin cell behavior by altering actomyosin regulation, cell morphology, extracellular matrix remodeling, or aberrant intracellular signaling. We will then test if molecules involved in these changes also occur in primary gastric samples. With this objective we will use a series of 246 primary gastric cancers (sporadic and hereditary) to validate our results.
3-To investigate ways of blocking basal extrusion and, consequently, block cell invasion. For this purpose, cell behavior and fate will be monitored upon inhibition or overexpression of deregulated molecules, with or without concomitant modulation of the extracellular matrix. A drug screening approach in Zebrafish will also be used to test potential therapies in physiological conditions.

As opposed to previous studies focusing on cancer cells alone, which have been clinically disappointing, our approach will explore the relevance of the surrounding cellular microenvironment of cancer cells in E-cadherin mediated invasion. Overall, we expect that this project using a more holistic approach will provide novel insights in order to stop the early stages of cancer cell invasion.