Abstract PO5-06-07: Unravelling the role of PKC-eta in modulating the Hippo Pathway: a novel therapeutic strategy for triple-negative breast cancer metastasis
Authors: Belsamma Liju Vijaya Steltar , Muraleedharan Amitha , Jayaram Divya Ram , Waidha Kamran , Jagadeeshan Sankar , Sinay Rose , Eremenko Ekaterina , Berner Omer , Elkabets Moshe , Livneh Etta 
Published: May 2024

Abstract
Breast cancer (BC) is the leading cause of cancer-related deaths in women, with metastasis being the primary reason for BC mortality. Triple-Negative Breast Cancer (TNBC) is aggressive and highly metastatic, having fewer treatment options than other types of breast cancer. Protein kinase C eta (PKCη), an anti-apoptotic kinase of the novel PKC subfamily, is associated with poor prognosis in invasive BC patients. In this study, we show the role of PKCη in promoting cell migration and invasion, and we demonstrate that its expression is required to maintain the mesenchymal state during EMT. Furthermore, we show that PKCη activates YAP/TAZ, tumor aggressiveness, and metastasis using in vitro and in vivo models.
PKCη promotes metastasis in TNBC cells, showing that the PKCƞ-YAP signaling axis mediates this. Data analysis of BC patients with a high metastatic invasive propensity and low survival BC revealed a positive correlation between the elevated expression levels of PKCη and the expression of YAP. Knockout of PKCη (PKCηKO) in the TNBC cells 4T1 and MDA-MB-231 markedly inhibited their invasion and migration capability. We further show that PKCη enhances cell survival and anoikis prevention ability, which have crucial roles in metastasis formation, allowing cancer cells to circulate and home to distant sites in the body. Further substantiated by in vivo experiments with 4T1 xenografts in NSG mice, demonstrating that 4T1 PKCηKO cells depicted reduced primary tumor size and a significant decrease in size and number of tumor metastases in the lungs, with fewer tumor nodules. MDA-MB-231 PKCηKO xenografts also expressed reduced metastasis in the distant brain, lung, and liver organs.
Mechanistically, we show that PKCη regulates epithelial-to-mesenchymal transition (EMT) markers as knockout of PKCη in TNBC cell lines increased expression of E-cadherin, EpCAM, and slug, and decreased expression of vimentin and 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 is associated with phosphorylation at Ser128, which allows YAP to translocate to the nucleus and contributes to metastasis of TNBC cells. Moreover, our data support direct interaction between PKCη and YAP1 in TNBC cells. Finally, we show that treatment of TNBC cells with uPEP2 (a uORF-encoded peptide of the PKCη transcript) downregulates the PKCη expression, activates the Hippo signaling pathway and promotes YAP degradation. Our findings highlight the importance of PKCη in TNBC metastasis and provide a new avenue for therapeutic intervention in this aggressive and lethal disease.
In conclusion, our studies identify a novel role of PKCη as a negative regulator of the Hippo pathway and reveal its function in promoting EMT and metastasis by modulating YAP/TAZ activity in TNBC. Results suggest that PKCη may represent a therapeutic target for this highly lethal and metastatic TNBC.
Citation Format: Liju Vijaya Steltar Belsamma, Amitha Muraleedharan, Divya Ram Jayaram, Kamran Waidha, Sankar Jagadeeshan, Rose Sinay, Ekaterina Eremenko, Omer Berner, Moshe Elkabets, Etta Livneh. Unravelling the role of PKC-eta in modulating the Hippo Pathway: a novel therapeutic strategy for triple-negative breast cancer metastasis [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-06-07.