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

<b>Background/Objectives</b>: 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. <b>Methods</b>: 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. <b>Results</b>: We identified consistent downregulation of <i>STK4</i>, <i>RASSF6</i>, and <i>FGF1</i>, alongside overexpression of <i>BIRC5</i> and <i>SERPINE1</i>. miRNA analysis revealed that <i>STK4</i> is potentially regulated by miR-522-3p, <i>SERPINE1</i> by miR-199b-5p and miR-30a-3p, whereas <i>RASSF6</i>, <i>FGF1</i>, and <i>BIRC5</i> 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. <b>Conclusions</b>: 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.