Dysregulation of histone modifications promotes carcinogenesis by altering transcription. Breast cancers frequently overexpress the histone methyltransferase EZH2, the catalytic subunit of Polycomb Repressor Complex 2 (PRC2). However, the role of EZH2 in this setting is unclear due to the context-dependent functions of PRC2 and the heterogeneity of breast cancer. Moreover, the mechanisms underlying PRC2 overexpression in cancer are obscure. Here, using multiple models of breast cancer driven by the oncogene ErbB2, we show that the tyrosine kinase c-Src links energy sufficiency with PRC2 overexpression via control of mRNA translation. By stimulating mitochondrial ATP production, c-Src suppresses energy stress, permitting sustained activation of the mammalian/mechanistic target of rapamycin complex 1 (mTORC1), which increases the translation of mRNAs encoding the PRC2 subunits Ezh2 and Suz12. We show that Ezh2 overexpression and activity are pivotal in ErbB2-mediated mammary tumourigenesis. These results reveal the hitherto unknown c-Src/mTORC1/PRC2 axis, which is essential for ErbB2-driven carcinogenesis.

Activation of the tyrosine kinase c-Src promotes breast cancer progression and poor outcome, yet the underlying mechanisms are incompletely understood. Here, we have shown that deleting c-Src in a genetically engineered model mimicking the Luminal B molecular subtype of breast cancer abrogates the activity of Forkhead Box M1 (FOXM1), a master transcriptional regulator of the cell cycle. We determined that c-Src phosphorylates FOXM1 on two tyrosine residues to stimulate its nuclear localization and target gene expression. These included key regulators of G2-M cell cycle progression as well as c-Src itself, forming a positive feedback loop that drove proliferation in genetically engineered and patient-derived models of Luminal B-like breast cancer. Using genetic approaches and small molecules that destabilize the FOXM1 protein, we found that targeting this mechanism induced G2-M cell cycle arrest and apoptosis, blocked tumor progression and impaired metastasis. We identified a positive correlation between FOXM1 and c-Src expression in human breast cancer and showed that the expression of FOXM1 target genes predicts poor outcome and associates with the Luminal B subtype, which responds poorly to approved therapies. These findings have revealed a regulatory network centered on c-Src and FOXM1 that is a targetable vulnerability in aggressive luminal breast cancers.

PTEN loss-of-function contributes to hyperactivation of the PI3K pathway and to drug resistance in breast cancer. Unchecked PI3K pathway signaling increases activation of the mechanistic target of rapamycin complex 1 (mTORC1), which promotes tumorigenicity. Several studies have suggested that vacuolar (H+)–ATPase (V–ATPase) complex activity is regulated by PI3K signaling. In this study, we showed that loss of PTEN elevated V–ATPase activity. Enhanced V–ATPase activity was mediated by increased expression of the ATPase H+ transporting accessory protein 2 (ATP6AP2), also known as the prorenin receptor (PRR). PRR is cleaved into a secreted extracellular fragment (sPRR) and an intracellular fragment (M8.9) that remains associated with the V–ATPase complex. Reduced PTEN expression increased V–ATPase complex activity in a PRR-dependent manner. Breast cancer cell lines with reduced PTEN expression demonstrated increased PRR expression. Similarly, PRR expression became elevated upon PTEN deletion in a mouse model of breast cancer. Interestingly, concentration of sPRR was elevated in the plasma of patients with breast cancer and correlated with tumor burden in HER2-enriched cancers. Moreover, PRR was essential for proper HER2 receptor expression, localization, and signaling. PRR knockdown attenuated HER2 signaling and resulted in reduced Akt and ERK 1/2 phosphorylation, and in lower mTORC1 activity. Overall, our study demonstrates a mechanism by which PTEN loss in breast cancer can potentiate multiple signaling pathways through upregulation of the V–ATPase complex. Implications: Our study contributed to the understanding of the role of the V–ATPase complex in breast cancer cell tumorigenesis and provided a potential biomarker in breast cancer.

<p>Clinical parameters of subjects from whom plasma was acquired for ELISA analysis.</p>