Novel Metastasis Suppressor PI3KC2β Is Mediated by mTORC1 Signaling in Breast Cancer.

HER2 amplification or mutation accounts for 25% of patients with breast cancer that can advance to metastatic disease. Therefore, it is important to identify novel genes that mediate metastasis in HER2+ breast cancer. In this study, we describe a new metastatic suppressor gene, class II phosphatidylinositol 3-kinase β (Pi3kc2β), through in vivo CRISPR-Cas9 library screening of a custom-designed library targeting genes implicated in autophagy using murine HER2+ breast cancer (N418) cells. We further showed that PI3KC2β knockout N418 cells increased their migration and invasion in vitro and lung metastasis in both spontaneous and experimental metastasis assays in vivo. Analysis of databases and tissue samples from patients with breast cancer correlated lower expression of PI3KC2β with decreased metastasis, overall survival, and relapse-free survival. Further, PI3KC2β deletion induced the activation of mTORC1 signaling, independent of affecting its kinase activity. Mechanistically, we found that PI3KC2β forms a complex with intersectin 1 (ITSN1) and raptor that could be decreasing the stability of raptor, and deletion of either PI3KC2β or ITSN1 led to increased raptor levels and mTORC1 signaling. Lastly, rapamycin treatment reduced the migration and invasion of PI3KC2β knockout tumor cells in vitro and their lung metastasis in vivo, supporting an important role of the mTORC1 pathway. Together, our results identify PI3KC2β as a suppressor of HER2+ breast cancer metastasis by negatively regulating mTORC1 signaling by affecting its complex formation with ITSN1 and raptor. IMPLICATIONS: Our findings revealed PI3KC2β as a new metastasis suppressor for HER2+ breast cancer, which might serve as a potential diagnostic and therapeutic target for the disease.