HDAC inhibitors induce transcriptional activation of Hippo pathway signaling and is associated with improved outcomes in Hormone Receptor positive breast cancer

Breast cancer has constantly been the leading causes of death in women, and hormone receptor (HR) positive, HER2 negative is the majority subtype. Treatment modalities for HR (+) metastatic breast cancer (mBC) includes endocrine therapy (tamoxifen, fulvestrant, aromatase inhibitors) in addition to targeted therapies (CDK4/6 inhibitors, PI3K/AKT/mTOR inhibitors, histone deacetylase (HDAC) inhibitors, and others. The hippo pathway LATS1/2-YAP/TAZ-TEAD signaling cascade is a fundamentally important pathway that governs multiple essential biological functions in cell biology and cancer biology. YAP/TAZ are often viewed as pro-tumorogenic, however, recent studies support a role of YAP as a tumor suppressor in HR (+) breast cancer due to its inhibition of estrogen signaling. Few studies have investigated the link between HDAC inhibitors and the Hippo pathway. In our study, we demonstrate that HDAC inhibitors induce transcriptional downregulation of YAP expression, while conversely activating a TEAD mediated transcriptional program with upregulation of many Hippo pathway canonical genes. We further identified 4 genes (CCDC80, GADD45A, F3, TGFB2) that were upregulated by HDAC inhibitors and associated with significantly improved survival in a HR (+) breast cancer cohort. We also correlate in patients samples from a clinical cohort of HR (+) metastatic breast cancer that upregulation of Hippo downstream genes are correlated with improved outcomes. Our study provide a novel mechanistic explanation for the clinical benefit of HDAC inhibitors, while providing further experimental support that Hippo-TEAD transcriptional activation profile is associated with better outcomes in HR (+) breast cancer.