Cell Migration and Tumor Progression
p120-YFP expressing MDA-231 cell.
p120 role in migration and invasiveness
A primary focus of our laboratory is to elucidate the role of cadherins and catenins in regulating cell migration and promoting invasion and metastasis (for a review see Anastasiadis and Reynolds, Curr Opin Cell Biol 2001.). In particular, we are investigating E-cadherin-mediated signaling events that suppress cell motility and invasiveness and promote the reorganization of the actin cytoskeleton. Our preliminary data indicates that endogenous p120 promotes the motility and invasiveness of E-cadherin-deficient cells. Association of cytoplasmic p120 in these cells with ectopically expressed E-cadherin blocks the p120 pro-migratory effects and provides a potential explanation for E-cadherin-mediated suppression of invasiveness. Current studies on this project are focused on:
p120 overexpression (green) induces lamellipodia formation in MDCK cells and promotes a mesenchymal-like morphology.
- Testing the hypothesis that p120 binding mediates the pro-migratory, pro-metastatic functions of mesenchymal cadherins.
- Elucidating the role of Rho GTPases in p120-induced motility/invasiveness
- Validating cell culture data in animal models of tumor invasion and metastasis
- Mapping of p120 domains that mediate increased motility/invasiveness
- Use of proteomics to identify p120 binding partners involved in motility and invasiveness
- Following up on microarray data supporting the involvement of particular signaling pathways in the p120 effects
- Determining the role of p120 isoform expression, as well as phosphorylation status, in adhesion and invasiveness
Receptor tyrosine kinase signaling
Receptor tyrosine kinases (RTKs) are often overactivated in human cancer. Their activation induces signaling cascades that control cell growth, survival and cell motility. We postulate that intact E-cadherin complexes actively repress RTK pro-migratory functions, while dissolution of cadherin complexes, or loss of E-cadherin expression during tumor progression relieves this repression (Fig. 1).
We postulate that the normal interaction of E-cadherin complexes with RTKs promotes cell adhesion and maturation of cell-cell junctions. It also represses the pro-migratory function of RTKs, possibly by sequestering p120ctn, and/or protein phosphatases. Upon sustained activation, RTKs induce tyrosine phosphorylation of the cadherin/catenin complex (particularly β-catenin) causing its dissolution. E-cadherin-uncoupled p120 promotes RTK-mediated motility/invasion, possibly by affecting the activity of Rho GTPases. Loss of E-cadherin expression in metastatic tumors has a similar effect, as cadherin-uncoupled p120 accumulates under these conditions.
Fig. 2. Hypothetical model
Wound healing migration assay. MDA-231 cells moving in response to HGF treatment.
Several studies suggest that preserving E-cadherin function prevents invasiveness. We are currently testing the hypothesis that the association of E-cadherin complexes with RTKs is required for cadherin-mediated induction of Rho GTPases, maturation of cell junctions and suppression of invasiveness.
Given recent evidence that β-catenin and p120 mediate the pro-migratory functions of RTKs, we postulate that small cadherin-catenin fragments could be identified that block the dissolution of junctions after sustained activation of RTKs, thereby preventing invasiveness. Our preliminary data indicate that overexpression of such fragments blocks the invasiveness of MDA-MB-231 breast cancer cells, in vitro. We are now testing the efficacy of these fragments in preventing invasiveness and metastasis in nude mice, using whole-animal imaging techniques that allow the visualization of tumor growth and metastasis in live animals.
Finally, we are investigating the mechanism by which p120 mediates HGF/c-Met signaling, and testing the hypothesis that p120 is also required for signaling downstream of other RTKs, including Her2.
p120-depleted (shRNA) epithelial tumor cells are unable to grow in soft agar. Re-introduction of murine p120 rescues anchorage-independent growth (rescue).
p120 and cell transformation
While β-catenin signaling is known to affect cell growth, little is known about p120's effects on tumor growth, cell transformation, apoptosis, or chemosensitivity. One project in the lab is focused on elucidating p120's effects on tumor growth and Ras-mediated cell transformation. The mechanism by which p120 affects these processes and the potential involvement of Rho GTPases are also under investigation. Finally, the involvement of the p120-binding transcription factor Kaiso is also being investigated as recent data indicates that p120-uncoupled Kaiso can repress classical β-catenin signaling targets, including c-myc and cyclin D1.