Hippo Signaling Dysregulation in Breast Cancer: Subtype-Independent Gene and miRNA Signatures

Background/Objectives: Breast cancer represents a diverse group of malignancies and continues to rank among the leading causes of cancer-related deaths in women. Altered Hippo pathway signaling has been increasingly recognized as a contributor to tumor growth, therapeutic resistance, and metastatic spread. This study aimed to identify miRNAs targeting Hippo pathway-related genes that are consistently dysregulated across all five breast cancer subtypes. Methods: The study cohort included patients representing five breast cancer subtypes: 130 luminal A, 96 HER2-positive luminal B, 100 HER2-negative luminal B, 36 non-luminal HER2-positive, and 43 triple-negative breast cancer (TNBC). Tumor samples were collected during surgery, along with adjacent healthy tissue that served as controls. Expression of Hippo-related genes was analyzed using mRNA microarrays and validated with reverse transcription quantitative polymerase chain reaction (RT-qPCR). Protein levels were assessed via enzyme-linked immunosorbent assay (ELISA), while miRNA expression profiling was performed with miRNA microarrays. Potential mRNA targets were predicted using the miRDB database. Results: We identified consistent downregulation of STK4, RASSF6, and FGF1, alongside overexpression of BIRC5 and SERPINE1. miRNA analysis revealed that STK4 is potentially regulated by miR-522-3p, SERPINE1 by miR-199b-5p and miR-30a-3p, whereas RASSF6, FGF1, and BIRC5 appeared to be predominantly regulated at the transcriptional level. These alterations reflect both the suppression of upstream Hippo activation and activation of downstream oncogenic effectors across all subtypes. Conclusions: Our findings reveal a conserved Hippo dysregulation program in breast cancer, highlighting subtype-independent Hippo-related genes and their miRNA regulators as potential universal biomarkers and therapeutic targets, complementing subtype-specific treatment strategies.