YAP activation and Hippo signaling suppression by PKC eta promote triple-negative breast cancer metastasis

Abstract Breast cancer is the leading cause of cancer-related deaths in women, with metastasis being the primary reason for mortality. Patients with triple-negative breast cancer (TNBC) show an increased risk of metastatic dissemination. Protein kinase C eta (PKCη), an anti-apoptotic kinase of the novel PKC subfamily, is associated with poor prognosis in breast cancer patients. Here, we demonstrate that PKCη promotes metastasis in TNBC cells and show that this is mediated by the PKCƞ-YAP signaling axis. Knockout of PKCη (PKCη KO ) in the TNBC cells, 4T1 and MDA-MB-231, markedly inhibited their invasion and migration capability. Furthermore, orthotopic xenografts of the latter PKCη KO cells in NSG mice reduced tumor growth and lung metastasis compared to PKCη-intact tumors. Mechanistically, we show that PKCη regulates epithelial-to-mesenchymal transition (EMT), as knockout of PKCη in TNBC cell lines increased expression of the EMT markers E-cadherin, EpCAM, and slug, and decreased expression of vimentin, ZEB1. Further profiling of the Hippo-YAP axis showed that PKCη is a negative regulator of the Hippo pathway that leads to YAP stabilization and its phosphorylation at Ser128, which allows YAP to translocate to the nucleus and contribute to metastasis of TNBC cells. We further show that PKCη directly interacts with YAP in silico and TNBC cells. Lastly, we demonstrate that treatment of TNBC cells with uPEP2, a recently discovered PKCη kinase inhibitory peptide (encoded by a uORF upstream of PKCη coding sequence), activates the Hippo pathway and YAP degradation. In summary, our results highlight the impact of PKCη in TNBC metastasis and offer a novel avenue for therapeutic intervention in this aggressive and fatal disease.