# E-cadherin endocytosis promotes non-canonical EGFR:STAT signalling to induce cell death and inhibit heterochromatinisation

## TL;DR

This study shows how E-cadherin controls the balance between two non-canonical STAT signaling pathways, leading to cell death and potentially acting as a tumor suppressor.

## Contribution

The paper reveals a novel mechanism where E-cadherin endocytosis regulates competing non-canonical STAT signaling pathways in Drosophila.

## Key findings

- E-cadherin endocytosis promotes EGFR:STAT signaling, leading to apoptosis.
- EGFR:STAT and HP1:STAT pathways compete for unphosphorylated STAT.
- Blocking E-cadherin internalization prevents cell death induced by EGFR:STAT signaling.

## Abstract

Signalling molecules often contribute to several downstream pathways that produce distinct transcriptional outputs and cellular phenotypes. One of the major unanswered questions in cell biology is how multiple activities of signalling molecules are coordinated in space and time in vivo. Here, we focus on the Signal Transducer and Activator of Transcription (STAT) protein as a paradigm of signalling molecules involved in several independent signalling pathways. Using Drosophila wing discs as an in vivo model, we demonstrate an interplay of at least three STAT activities in this tissue. In addition to the ‘canonical’ pathways, in which STAT is phosphorylated and activated by Janus Kinases, STAT is involved in two ‘non-canonical’ pathways. In one pathway, STAT is activated by the Epidermal Growth Factor Receptor (EGFR), promoting apoptosis. In the other, it binds the Heterochromatin Protein 1 (HP1) to enhance heterochromatin formation. We provide evidence that while the ‘canonical’ STAT signalling is dominant over ‘non-canonical’ pathways, EGFR:STAT and HP1:STAT pathways compete for the availability of unphosphorylated STAT. We also describe a central role for the cell-cell adhesion protein E-cadherin, with both EGFR and STAT colocalising with E-cadherin at cell-cell junctions and on intracellular vesicles. We show that elevated intracellular E-cadherin promotes EGFR:STAT pathway leading to apoptosis, which is prevented by inhibiting E-cad endocytosis. Taken together, we conclude that E-cadherin controls the balance between two non-canonical STAT activities. We hypothesise that this balance represents a tumour-suppressive mechanism, in which junctional disassembly in dysregulated epithelial-to-mesenchymal transitions would shift this balance towards the EGFR:STAT signalling to promote apoptosis.

One of the central questions of modern biology is how a small number of highly conserved signal transduction pathways act and interact to generate the cellular and transcriptional diversity required for complex multicellular life. Here, we utilise the low complexity Drosophila model organism to study the Signal Transducer and Activator of Transcription (STAT) protein signalling in a developing in vivo tissue using developmental genetics approaches. We demonstrate the existence of a hierarchy of at least three STAT-related signalling pathways, with one pathway dominating the other two, which, in turn, compete for STAT availability. Additionally, we show that this competition is controlled by the protein E-cadherin, which is crucial for adhesion between cells in epithelial tissues and for maintaining tissue integrity. In particular, E-cadherin internalisation from the cell surface into the cell enhances one of the STAT downstream pathways that leads to cell death. Consequently, experimentally blocking E-cadherin internalisation prevents this cell death. We hypothesise that balancing the competition between two STAT downstream pathways through E-cadherin serves a tumour-suppressive role; specifically, the disassembly of adhesion by cells undergoing cancerous transformation shifts this balance towards the pathway that induces cell death.