Activation of the actin/MRTF-A/SRF signalling pathway in pre-malignant mammary epithelial cells by P-cadherin is essential for transformation

Alterations in the expression or function of cell adhesion molecules have been implicated in all steps of tumour progression. Among those, P-cadherin expression is highly enriched in basal-like breast cancer, a molecular subset of triple-negative breast carcinomas, playing a central role in inducing cancer cell self-renewal, as well as collective cell migration and invasion capacity. To decipher the P-cadherin-dependent signalling network, we generated a humanised P-cadherin fly model, establishing a clinically relevant platform for functional exploration of P-cadherin effectors in vivo. We report that actin nucleators, MRTF and SRF are main effectors of P-cadherin functional effects. In addition, we validated these findings in a human mammary epithelial cell line with conditional activation of the Src oncogene, which recapitulates molecular events taking place during cellular transformation. We show that prior to triggering the gain of malignant phenotypes, Src induces a transient increase in P-cadherin expression levels, which correlates with MRTF-A accumulation, its nuclear translocation and the upregulation of SRF target genes. Moreover, knocking down P-cadherin, or preventing Factin polymerization with Latrunculin A, impairs SRF transcriptional activity. Furthermore, blocking MRTF-A nuclear translocation with CCG-203971 hampers proliferation, selfrenewal and invasion. Thus, in addition to sustaining malignant phenotypes, P-cadherin can also play a major role in the very early stages of breast carcinogenesis by promoting a transient boost of MRTF-A/SRF signalling through actin regulation.