Loss of Par3 promotes breast cancer metastasis by compromising cell-cell cohesion.

The mechanisms by which tumour cells metastasize and the role that cell polarity proteins play in this process are not well understood. We report that partitioning defective protein 3 (Par3) is dysregulated in metastasis in human breast cancer, and is associated with a higher tumour grade and ErbB2-positive status. Downregulation of Par3 cooperated with ErbB2 to induce cell invasion and metastasis in vivo. Interestingly, the metastatic behaviour was not associated with an overt mesenchymal phenotype. However, loss of Par3 inhibited E-cadherin junction stability, disrupted membrane and actin dynamics at cell-cell junctions and decreased cell-cell cohesion in a manner dependent on the Tiam1/Rac-GTP pathway. Inhibition of this pathway restored E-cadherin junction stability and blocked invasive behaviour of cells lacking Par3, suggesting that loss of Par3 promotes metastatic behaviour of ErbB2-induced tumour epithelial cells by decreasing cell-cell cohesion.